1 \input texinfo @c -*- Texinfo -*-
7 @set RCSID $Id: make.texi,v 1.61 2009/09/28 23:08:49 psmith Exp $
9 @settitle GNU @code{make}
10 @setchapternewpage odd
11 @c Combine the variable and function indices:
13 @c Combine the program and concept indices:
15 @c FSF publishers: format makebook.texi instead of using this file directly.
16 @c ISBN confirmed by Jasimin Huang <jasimin@fsf.org> on 25 Mar 2009
17 @set ISBN 1-882114-83-3
21 This file documents the GNU @code{make} utility, which determines
22 automatically which pieces of a large program need to be recompiled,
23 and issues the commands to recompile them.
25 This is Edition @value{EDITION}, last updated @value{UPDATED},
26 of @cite{The GNU Make Manual}, for GNU @code{make} version @value{VERSION}.
28 Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
29 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009
30 Free Software Foundation, Inc.
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.2 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
37 and with the Back-Cover Texts as in (a) below. A copy of the
38 license is included in the section entitled ``GNU Free Documentation
41 (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
42 modify this GNU manual. Buying copies from the FSF supports it in
43 developing GNU and promoting software freedom.''
49 @c ISPELL CHECK: done, 10 June 1993 --roland
50 @c ISPELL CHECK: done, 2000-06-25 --Martin Buchholz
53 @dircategory Software development
55 * Make: (make). Remake files automatically.
59 @shorttitlepage GNU Make
63 @subtitle A Program for Directing Recompilation
64 @subtitle GNU @code{make} Version @value{VERSION}
65 @subtitle @value{UPDATED-MONTH}
66 @author Richard M. Stallman, Roland McGrath, Paul D. Smith
68 @vskip 0pt plus 1filll
71 Published by the Free Software Foundation @*
72 51 Franklin St. -- Fifth Floor @*
73 Boston, MA 02110-1301 USA @*
76 Cover art by Etienne Suvasa.
83 @node Top, Overview, (dir), (dir)
90 * Overview:: Overview of @code{make}.
91 * Introduction:: An introduction to @code{make}.
92 * Makefiles:: Makefiles tell @code{make} what to do.
93 * Rules:: Rules describe when a file must be remade.
94 * Recipes:: Recipes say how to remake a file.
95 * Using Variables:: You can use variables to avoid repetition.
96 * Conditionals:: Use or ignore parts of the makefile based
97 on the values of variables.
98 * Functions:: Many powerful ways to manipulate text.
99 * Invoking make: Running. How to invoke @code{make} on the command line.
100 * Implicit Rules:: Use implicit rules to treat many files alike,
101 based on their file names.
102 * Archives:: How @code{make} can update library archives.
103 * Features:: Features GNU @code{make} has over other @code{make}s.
104 * Missing:: What GNU @code{make} lacks from other @code{make}s.
105 * Makefile Conventions:: Conventions for writing makefiles for
107 * Quick Reference:: A quick reference for experienced users.
108 * Error Messages:: A list of common errors generated by @code{make}.
109 * Complex Makefile:: A real example of a straightforward,
110 but nontrivial, makefile.
112 * GNU Free Documentation License:: License for copying this manual
113 * Concept Index:: Index of Concepts
114 * Name Index:: Index of Functions, Variables, & Directives
117 --- The Detailed Node Listing ---
119 Overview of @code{make}
121 * Preparing:: Preparing and running make
122 * Reading:: On reading this text
123 * Bugs:: Problems and bugs
125 An Introduction to Makefiles
127 * Rule Introduction:: What a rule looks like.
128 * Simple Makefile:: A simple makefile
129 * How Make Works:: How @code{make} processes this makefile
130 * Variables Simplify:: Variables make makefiles simpler
131 * make Deduces:: Letting @code{make} deduce the recipe
132 * Combine By Prerequisite:: Another style of makefile
133 * Cleanup:: Rules for cleaning the directory
137 * Makefile Contents:: What makefiles contain.
138 * Makefile Names:: How to name your makefile.
139 * Include:: How one makefile can use another makefile.
140 * MAKEFILES Variable:: The environment can specify extra makefiles.
141 * Remaking Makefiles:: How makefiles get remade.
142 * Overriding Makefiles:: How to override part of one makefile
143 with another makefile.
144 * Reading Makefiles:: How makefiles are parsed.
145 * Secondary Expansion:: How and when secondary expansion is performed.
149 * Rule Example:: An example explained.
150 * Rule Syntax:: General syntax explained.
151 * Prerequisite Types:: There are two types of prerequisites.
152 * Wildcards:: Using wildcard characters such as `*'.
153 * Directory Search:: Searching other directories for source files.
154 * Phony Targets:: Using a target that is not a real file's name.
155 * Force Targets:: You can use a target without a recipe
156 or prerequisites to mark other targets
158 * Empty Targets:: When only the date matters and the
160 * Special Targets:: Targets with special built-in meanings.
161 * Multiple Targets:: When to make use of several targets in a rule.
162 * Multiple Rules:: How to use several rules with the same target.
163 * Static Pattern:: Static pattern rules apply to multiple targets
164 and can vary the prerequisites according to
166 * Double-Colon:: How to use a special kind of rule to allow
167 several independent rules for one target.
168 * Automatic Prerequisites:: How to automatically generate rules giving
169 prerequisites from source files themselves.
171 Using Wildcard Characters in File Names
173 * Wildcard Examples:: Several examples
174 * Wildcard Pitfall:: Problems to avoid.
175 * Wildcard Function:: How to cause wildcard expansion where
176 it does not normally take place.
178 Searching Directories for Prerequisites
180 * General Search:: Specifying a search path that applies
181 to every prerequisite.
182 * Selective Search:: Specifying a search path
183 for a specified class of names.
184 * Search Algorithm:: When and how search paths are applied.
185 * Recipes/Search:: How to write recipes that work together
187 * Implicit/Search:: How search paths affect implicit rules.
188 * Libraries/Search:: Directory search for link libraries.
192 * Static Usage:: The syntax of static pattern rules.
193 * Static versus Implicit:: When are they better than implicit rules?
195 Writing Recipes in Rules
197 * Recipe Syntax:: Recipe syntax features and pitfalls.
198 * Echoing:: How to control when recipes are echoed.
199 * Execution:: How recipes are executed.
200 * Parallel:: How recipes can be executed in parallel.
201 * Errors:: What happens after a recipe execution error.
202 * Interrupts:: What happens when a recipe is interrupted.
203 * Recursion:: Invoking @code{make} from makefiles.
204 * Canned Recipes:: Defining canned recipes.
205 * Empty Recipes:: Defining useful, do-nothing recipes.
209 * Splitting Lines:: Breaking long recipe lines for readability.
210 * Variables in Recipes:: Using @code{make} variables in recipes.
214 * Choosing the Shell:: How @code{make} chooses the shell used
217 Recursive Use of @code{make}
219 * MAKE Variable:: The special effects of using @samp{$(MAKE)}.
220 * Variables/Recursion:: How to communicate variables to a sub-@code{make}.
221 * Options/Recursion:: How to communicate options to a sub-@code{make}.
222 * -w Option:: How the @samp{-w} or @samp{--print-directory} option
223 helps debug use of recursive @code{make} commands.
227 * Reference:: How to use the value of a variable.
228 * Flavors:: Variables come in two flavors.
229 * Advanced:: Advanced features for referencing a variable.
230 * Values:: All the ways variables get their values.
231 * Setting:: How to set a variable in the makefile.
232 * Appending:: How to append more text to the old value
234 * Override Directive:: How to set a variable in the makefile even if
235 the user has set it with a command argument.
236 * Multi-Line:: An alternate way to set a variable
237 to a multi-line string.
238 * Environment:: Variable values can come from the environment.
239 * Target-specific:: Variable values can be defined on a per-target
241 * Pattern-specific:: Target-specific variable values can be applied
242 to a group of targets that match a pattern.
243 * Suppressing Inheritance:: Suppress inheritance of variables.
244 * Special Variables:: Variables with special meaning or behavior.
246 Advanced Features for Reference to Variables
248 * Substitution Refs:: Referencing a variable with
249 substitutions on the value.
250 * Computed Names:: Computing the name of the variable to refer to.
252 Conditional Parts of Makefiles
254 * Conditional Example:: Example of a conditional
255 * Conditional Syntax:: The syntax of conditionals.
256 * Testing Flags:: Conditionals that test flags.
258 Functions for Transforming Text
260 * Syntax of Functions:: How to write a function call.
261 * Text Functions:: General-purpose text manipulation functions.
262 * File Name Functions:: Functions for manipulating file names.
263 * Conditional Functions:: Functions that implement conditions.
264 * Foreach Function:: Repeat some text with controlled variation.
265 * Call Function:: Expand a user-defined function.
266 * Value Function:: Return the un-expanded value of a variable.
267 * Eval Function:: Evaluate the arguments as makefile syntax.
268 * Origin Function:: Find where a variable got its value.
269 * Flavor Function:: Find out the flavor of a variable.
270 * Shell Function:: Substitute the output of a shell command.
271 * Make Control Functions:: Functions that control how make runs.
273 How to Run @code{make}
275 * Makefile Arguments:: How to specify which makefile to use.
276 * Goals:: How to use goal arguments to specify which
277 parts of the makefile to use.
278 * Instead of Execution:: How to use mode flags to specify what
279 kind of thing to do with the recipes
280 in the makefile other than simply
282 * Avoiding Compilation:: How to avoid recompiling certain files.
283 * Overriding:: How to override a variable to specify
284 an alternate compiler and other things.
285 * Testing:: How to proceed past some errors, to
287 * Options Summary:: Summary of Options
291 * Using Implicit:: How to use an existing implicit rule
292 to get the recipe for updating a file.
293 * Catalogue of Rules:: A list of built-in implicit rules.
294 * Implicit Variables:: How to change what predefined rules do.
295 * Chained Rules:: How to use a chain of implicit rules.
296 * Pattern Rules:: How to define new implicit rules.
297 * Last Resort:: How to define a recipe for rules which
299 * Suffix Rules:: The old-fashioned style of implicit rule.
300 * Implicit Rule Search:: The precise algorithm for applying
303 Defining and Redefining Pattern Rules
305 * Pattern Intro:: An introduction to pattern rules.
306 * Pattern Examples:: Examples of pattern rules.
307 * Automatic Variables:: How to use automatic variables in the
308 recipe of implicit rules.
309 * Pattern Match:: How patterns match.
310 * Match-Anything Rules:: Precautions you should take prior to
311 defining rules that can match any
312 target file whatever.
313 * Canceling Rules:: How to override or cancel built-in rules.
315 Using @code{make} to Update Archive Files
317 * Archive Members:: Archive members as targets.
318 * Archive Update:: The implicit rule for archive member targets.
319 * Archive Pitfalls:: Dangers to watch out for when using archives.
320 * Archive Suffix Rules:: You can write a special kind of suffix rule
321 for updating archives.
323 Implicit Rule for Archive Member Targets
325 * Archive Symbols:: How to update archive symbol directories.
330 @node Overview, Introduction, Top, Top
331 @comment node-name, next, previous, up
332 @chapter Overview of @code{make}
334 The @code{make} utility automatically determines which pieces of a large
335 program need to be recompiled, and issues commands to recompile them.
336 This manual describes GNU @code{make}, which was implemented by Richard
337 Stallman and Roland McGrath. Development since Version 3.76 has been
338 handled by Paul D. Smith.
340 GNU @code{make} conforms to section 6.2 of @cite{IEEE Standard
341 1003.2-1992} (POSIX.2).
343 @cindex IEEE Standard 1003.2
344 @cindex standards conformance
346 Our examples show C programs, since they are most common, but you can use
347 @code{make} with any programming language whose compiler can be run with a
348 shell command. Indeed, @code{make} is not limited to programs. You can
349 use it to describe any task where some files must be updated automatically
350 from others whenever the others change.
353 * Preparing:: Preparing and Running Make
354 * Reading:: On Reading this Text
355 * Bugs:: Problems and Bugs
358 @node Preparing, Reading, Overview, Overview
360 @heading Preparing and Running Make
363 To prepare to use @code{make}, you must write a file called
364 the @dfn{makefile} that describes the relationships among files
365 in your program and provides commands for updating each file.
366 In a program, typically, the executable file is updated from object
367 files, which are in turn made by compiling source files.@refill
369 Once a suitable makefile exists, each time you change some source files,
370 this simple shell command:
377 suffices to perform all necessary recompilations. The @code{make} program
378 uses the makefile data base and the last-modification times of the files to
379 decide which of the files need to be updated. For each of those files, it
380 issues the recipes recorded in the data base.
382 You can provide command line arguments to @code{make} to control which
383 files should be recompiled, or how. @xref{Running, ,How to Run
386 @node Reading, Bugs, Preparing, Overview
387 @section How to Read This Manual
389 If you are new to @code{make}, or are looking for a general
390 introduction, read the first few sections of each chapter, skipping the
391 later sections. In each chapter, the first few sections contain
392 introductory or general information and the later sections contain
393 specialized or technical information.
395 The exception is the second chapter, @ref{Introduction, ,An
396 Introduction to Makefiles}, all of which is introductory.
399 The exception is @ref{Introduction, ,An Introduction to Makefiles},
400 all of which is introductory.
403 If you are familiar with other @code{make} programs, see @ref{Features,
404 ,Features of GNU @code{make}}, which lists the enhancements GNU
405 @code{make} has, and @ref{Missing, ,Incompatibilities and Missing
406 Features}, which explains the few things GNU @code{make} lacks that
409 For a quick summary, see @ref{Options Summary}, @ref{Quick Reference},
410 and @ref{Special Targets}.
412 @node Bugs, , Reading, Overview
413 @section Problems and Bugs
414 @cindex reporting bugs
415 @cindex bugs, reporting
416 @cindex problems and bugs, reporting
418 If you have problems with GNU @code{make} or think you've found a bug,
419 please report it to the developers; we cannot promise to do anything but
420 we might well want to fix it.
422 Before reporting a bug, make sure you've actually found a real bug.
423 Carefully reread the documentation and see if it really says you can do
424 what you're trying to do. If it's not clear whether you should be able
425 to do something or not, report that too; it's a bug in the
428 Before reporting a bug or trying to fix it yourself, try to isolate it
429 to the smallest possible makefile that reproduces the problem. Then
430 send us the makefile and the exact results @code{make} gave you,
431 including any error or warning messages. Please don't paraphrase
432 these messages: it's best to cut and paste them into your report.
433 When generating this small makefile, be sure to not use any non-free
434 or unusual tools in your recipes: you can almost always emulate what
435 such a tool would do with simple shell commands. Finally, be sure to
436 explain what you expected to occur; this will help us decide whether
437 the problem was really in the documentation.
439 Once you have a precise problem you can report it in one of two ways.
440 Either send electronic mail to:
447 or use our Web-based project management tool, at:
450 http://savannah.gnu.org/projects/make/
454 In addition to the information above, please be careful to include the
455 version number of @code{make} you are using. You can get this
456 information with the command @samp{make --version}. Be sure also to
457 include the type of machine and operating system you are using. One
458 way to obtain this information is by looking at the final lines of
459 output from the command @samp{make --help}.
461 @node Introduction, Makefiles, Overview, Top
462 @comment node-name, next, previous, up
463 @chapter An Introduction to Makefiles
465 You need a file called a @dfn{makefile} to tell @code{make} what to do.
466 Most often, the makefile tells @code{make} how to compile and link a
470 In this chapter, we will discuss a simple makefile that describes how to
471 compile and link a text editor which consists of eight C source files
472 and three header files. The makefile can also tell @code{make} how to
473 run miscellaneous commands when explicitly asked (for example, to remove
474 certain files as a clean-up operation). To see a more complex example
475 of a makefile, see @ref{Complex Makefile}.
477 When @code{make} recompiles the editor, each changed C source file
478 must be recompiled. If a header file has changed, each C source file
479 that includes the header file must be recompiled to be safe. Each
480 compilation produces an object file corresponding to the source file.
481 Finally, if any source file has been recompiled, all the object files,
482 whether newly made or saved from previous compilations, must be linked
483 together to produce the new executable editor.
484 @cindex recompilation
488 * Rule Introduction:: What a rule looks like.
489 * Simple Makefile:: A Simple Makefile
490 * How Make Works:: How @code{make} Processes This Makefile
491 * Variables Simplify:: Variables Make Makefiles Simpler
492 * make Deduces:: Letting @code{make} Deduce the Recipes
493 * Combine By Prerequisite:: Another Style of Makefile
494 * Cleanup:: Rules for Cleaning the Directory
497 @node Rule Introduction, Simple Makefile, Introduction, Introduction
498 @comment node-name, next, previous, up
499 @section What a Rule Looks Like
500 @cindex rule, introduction to
501 @cindex makefile rule parts
502 @cindex parts of makefile rule
504 A simple makefile consists of ``rules'' with the following shape:
506 @cindex targets, introduction to
507 @cindex prerequisites, introduction to
508 @cindex recipes, introduction to
511 @var{target} @dots{} : @var{prerequisites} @dots{}
518 A @dfn{target} is usually the name of a file that is generated by a
519 program; examples of targets are executable or object files. A target
520 can also be the name of an action to carry out, such as @samp{clean}
521 (@pxref{Phony Targets}).
523 A @dfn{prerequisite} is a file that is used as input to create the
524 target. A target often depends on several files.
526 @cindex tabs in rules
527 A @dfn{recipe} is an action that @code{make} carries out.
528 A recipe may have more than one command, each on its own line.
529 @strong{Please note:} you need to put a tab character at the beginning of
530 every command line! This is an obscurity that catches the unwary. If
531 you prefer to prefix your recipes with a character other than tab,
532 you can set the @code{.CMDPREFIX} variable to an alternate character
533 (@pxref{Special Variables}).
535 Usually a recipe is in a rule with prerequisites and serves to create a
536 target file if any of the prerequisites change. However, the rule that
537 specifies a recipe for the target need not have prerequisites. For
538 example, the rule containing the delete command associated with the
539 target @samp{clean} does not have prerequisites.
541 A @dfn{rule}, then, explains how and when to remake certain files
542 which are the targets of the particular rule. @code{make} carries out
543 the recipe on the prerequisites to create or update the target. A
544 rule can also explain how and when to carry out an action.
545 @xref{Rules, , Writing Rules}.
547 A makefile may contain other text besides rules, but a simple makefile
548 need only contain rules. Rules may look somewhat more complicated
549 than shown in this template, but all fit the pattern more or less.
551 @node Simple Makefile, How Make Works, Rule Introduction, Introduction
552 @section A Simple Makefile
553 @cindex simple makefile
554 @cindex makefile, simple
556 Here is a straightforward makefile that describes the way an
557 executable file called @code{edit} depends on eight object files
558 which, in turn, depend on eight C source and three header files.
560 In this example, all the C files include @file{defs.h}, but only those
561 defining editing commands include @file{command.h}, and only low
562 level files that change the editor buffer include @file{buffer.h}.
566 edit : main.o kbd.o command.o display.o \
567 insert.o search.o files.o utils.o
568 cc -o edit main.o kbd.o command.o display.o \
569 insert.o search.o files.o utils.o
571 main.o : main.c defs.h
573 kbd.o : kbd.c defs.h command.h
575 command.o : command.c defs.h command.h
577 display.o : display.c defs.h buffer.h
579 insert.o : insert.c defs.h buffer.h
581 search.o : search.c defs.h buffer.h
583 files.o : files.c defs.h buffer.h command.h
585 utils.o : utils.c defs.h
588 rm edit main.o kbd.o command.o display.o \
589 insert.o search.o files.o utils.o
594 We split each long line into two lines using backslash-newline; this is
595 like using one long line, but is easier to read.
596 @cindex continuation lines
597 @cindex @code{\} (backslash), for continuation lines
598 @cindex backslash (@code{\}), for continuation lines
599 @cindex quoting newline, in makefile
600 @cindex newline, quoting, in makefile
602 To use this makefile to create the executable file called @file{edit},
609 To use this makefile to delete the executable file and all the object
610 files from the directory, type:
616 In the example makefile, the targets include the executable file
617 @samp{edit}, and the object files @samp{main.o} and @samp{kbd.o}. The
618 prerequisites are files such as @samp{main.c} and @samp{defs.h}.
619 In fact, each @samp{.o} file is both a target and a prerequisite.
620 Recipes include @w{@samp{cc -c main.c}} and @w{@samp{cc -c kbd.c}}.
622 When a target is a file, it needs to be recompiled or relinked if any
623 of its prerequisites change. In addition, any prerequisites that are
624 themselves automatically generated should be updated first. In this
625 example, @file{edit} depends on each of the eight object files; the
626 object file @file{main.o} depends on the source file @file{main.c} and
627 on the header file @file{defs.h}.
629 A recipe may follow each line that contains a target and
630 prerequisites. These recipes say how to update the target file. A
631 tab character (or whatever character is specified by the
632 @code{.CMDPREFIX} variable; @pxref{Special Variables}) must come at
633 the beginning of every line in the recipe to distinguish recipes from
634 other lines in the makefile. (Bear in mind that @code{make} does not
635 know anything about how the recipes work. It is up to you to supply
636 recipes that will update the target file properly. All @code{make}
637 does is execute the commands in the recipe you have specified when the
638 target file needs to be updated.)@refill
641 The target @samp{clean} is not a file, but merely the name of an
642 action. Since you normally do not want to carry out the actions in
643 this rule, @samp{clean} is not a prerequisite of any other rule.
644 Consequently, @code{make} never does anything with it unless you tell
645 it specifically. Note that this rule not only is not a prerequisite,
646 it also does not have any prerequisites, so the only purpose of the
647 rule is to run the specified recipe. Targets that do not refer to
648 files but are just actions are called @dfn{phony targets}.
649 @xref{Phony Targets}, for information about this kind of target.
650 @xref{Errors, , Errors in Recipes}, to see how to cause @code{make}
651 to ignore errors from @code{rm} or any other command.
652 @cindex @code{clean} target
653 @cindex @code{rm} (shell command)
655 @node How Make Works, Variables Simplify, Simple Makefile, Introduction
656 @comment node-name, next, previous, up
657 @section How @code{make} Processes a Makefile
658 @cindex processing a makefile
659 @cindex makefile, how @code{make} processes
661 By default, @code{make} starts with the first target (not targets whose
662 names start with @samp{.}). This is called the @dfn{default goal}.
663 (@dfn{Goals} are the targets that @code{make} strives ultimately to
664 update. You can override this behavior using the command line
665 (@pxref{Goals, , Arguments to Specify the Goals}) or with the
666 @code{.DEFAULT_GOAL} special variable (@pxref{Special Variables, ,
667 Other Special Variables}).
669 @cindex goal, default
672 In the simple example of the previous section, the default goal is to
673 update the executable program @file{edit}; therefore, we put that rule
676 Thus, when you give the command:
683 @code{make} reads the makefile in the current directory and begins by
684 processing the first rule. In the example, this rule is for relinking
685 @file{edit}; but before @code{make} can fully process this rule, it
686 must process the rules for the files that @file{edit} depends on,
687 which in this case are the object files. Each of these files is
688 processed according to its own rule. These rules say to update each
689 @samp{.o} file by compiling its source file. The recompilation must
690 be done if the source file, or any of the header files named as
691 prerequisites, is more recent than the object file, or if the object
694 The other rules are processed because their targets appear as
695 prerequisites of the goal. If some other rule is not depended on by the
696 goal (or anything it depends on, etc.), that rule is not processed,
697 unless you tell @code{make} to do so (with a command such as
698 @w{@code{make clean}}).
700 Before recompiling an object file, @code{make} considers updating its
701 prerequisites, the source file and header files. This makefile does not
702 specify anything to be done for them---the @samp{.c} and @samp{.h} files
703 are not the targets of any rules---so @code{make} does nothing for these
704 files. But @code{make} would update automatically generated C programs,
705 such as those made by Bison or Yacc, by their own rules at this time.
707 After recompiling whichever object files need it, @code{make} decides
708 whether to relink @file{edit}. This must be done if the file
709 @file{edit} does not exist, or if any of the object files are newer than
710 it. If an object file was just recompiled, it is now newer than
711 @file{edit}, so @file{edit} is relinked.
714 Thus, if we change the file @file{insert.c} and run @code{make},
715 @code{make} will compile that file to update @file{insert.o}, and then
716 link @file{edit}. If we change the file @file{command.h} and run
717 @code{make}, @code{make} will recompile the object files @file{kbd.o},
718 @file{command.o} and @file{files.o} and then link the file @file{edit}.
720 @node Variables Simplify, make Deduces, How Make Works, Introduction
721 @section Variables Make Makefiles Simpler
723 @cindex simplifying with variables
725 In our example, we had to list all the object files twice in the rule for
726 @file{edit} (repeated here):
730 edit : main.o kbd.o command.o display.o \
731 insert.o search.o files.o utils.o
732 cc -o edit main.o kbd.o command.o display.o \
733 insert.o search.o files.o utils.o
737 @cindex @code{objects}
738 Such duplication is error-prone; if a new object file is added to the
739 system, we might add it to one list and forget the other. We can eliminate
740 the risk and simplify the makefile by using a variable. @dfn{Variables}
741 allow a text string to be defined once and substituted in multiple places
742 later (@pxref{Using Variables, ,How to Use Variables}).
744 @cindex @code{OBJECTS}
749 It is standard practice for every makefile to have a variable named
750 @code{objects}, @code{OBJECTS}, @code{objs}, @code{OBJS}, @code{obj},
751 or @code{OBJ} which is a list of all object file names. We would
752 define such a variable @code{objects} with a line like this in the
757 objects = main.o kbd.o command.o display.o \
758 insert.o search.o files.o utils.o
763 Then, each place we want to put a list of the object file names, we can
764 substitute the variable's value by writing @samp{$(objects)}
765 (@pxref{Using Variables, ,How to Use Variables}).
767 Here is how the complete simple makefile looks when you use a variable
768 for the object files:
772 objects = main.o kbd.o command.o display.o \
773 insert.o search.o files.o utils.o
776 cc -o edit $(objects)
777 main.o : main.c defs.h
779 kbd.o : kbd.c defs.h command.h
781 command.o : command.c defs.h command.h
783 display.o : display.c defs.h buffer.h
785 insert.o : insert.c defs.h buffer.h
787 search.o : search.c defs.h buffer.h
789 files.o : files.c defs.h buffer.h command.h
791 utils.o : utils.c defs.h
798 @node make Deduces, Combine By Prerequisite, Variables Simplify, Introduction
799 @section Letting @code{make} Deduce the Recipes
800 @cindex deducing recipes (implicit rules)
801 @cindex implicit rule, introduction to
802 @cindex rule, implicit, introduction to
804 It is not necessary to spell out the recipes for compiling the individual
805 C source files, because @code{make} can figure them out: it has an
806 @dfn{implicit rule} for updating a @samp{.o} file from a correspondingly
807 named @samp{.c} file using a @samp{cc -c} command. For example, it will
808 use the recipe @samp{cc -c main.c -o main.o} to compile @file{main.c} into
809 @file{main.o}. We can therefore omit the recipes from the rules for the
810 object files. @xref{Implicit Rules, ,Using Implicit Rules}.@refill
812 When a @samp{.c} file is used automatically in this way, it is also
813 automatically added to the list of prerequisites. We can therefore omit
814 the @samp{.c} files from the prerequisites, provided we omit the recipe.
816 Here is the entire example, with both of these changes, and a variable
817 @code{objects} as suggested above:
821 objects = main.o kbd.o command.o display.o \
822 insert.o search.o files.o utils.o
825 cc -o edit $(objects)
828 kbd.o : defs.h command.h
829 command.o : defs.h command.h
830 display.o : defs.h buffer.h
831 insert.o : defs.h buffer.h
832 search.o : defs.h buffer.h
833 files.o : defs.h buffer.h command.h
843 This is how we would write the makefile in actual practice. (The
844 complications associated with @samp{clean} are described elsewhere.
845 See @ref{Phony Targets}, and @ref{Errors, ,Errors in Recipes}.)
847 Because implicit rules are so convenient, they are important. You
848 will see them used frequently.@refill
850 @node Combine By Prerequisite, Cleanup, make Deduces, Introduction
851 @section Another Style of Makefile
852 @cindex combining rules by prerequisite
854 When the objects of a makefile are created only by implicit rules, an
855 alternative style of makefile is possible. In this style of makefile,
856 you group entries by their prerequisites instead of by their targets.
857 Here is what one looks like:
861 objects = main.o kbd.o command.o display.o \
862 insert.o search.o files.o utils.o
865 cc -o edit $(objects)
868 kbd.o command.o files.o : command.h
869 display.o insert.o search.o files.o : buffer.h
874 Here @file{defs.h} is given as a prerequisite of all the object files;
875 @file{command.h} and @file{buffer.h} are prerequisites of the specific
876 object files listed for them.
878 Whether this is better is a matter of taste: it is more compact, but some
879 people dislike it because they find it clearer to put all the information
880 about each target in one place.
882 @node Cleanup, , Combine By Prerequisite, Introduction
883 @section Rules for Cleaning the Directory
885 @cindex removing, to clean up
887 Compiling a program is not the only thing you might want to write rules
888 for. Makefiles commonly tell how to do a few other things besides
889 compiling a program: for example, how to delete all the object files
890 and executables so that the directory is @samp{clean}.
892 @cindex @code{clean} target
894 could write a @code{make} rule for cleaning our example editor:
903 In practice, we might want to write the rule in a somewhat more
904 complicated manner to handle unanticipated situations. We would do this:
915 This prevents @code{make} from getting confused by an actual file
916 called @file{clean} and causes it to continue in spite of errors from
917 @code{rm}. (See @ref{Phony Targets}, and @ref{Errors, ,Errors in
921 A rule such as this should not be placed at the beginning of the
922 makefile, because we do not want it to run by default! Thus, in the
923 example makefile, we want the rule for @code{edit}, which recompiles
924 the editor, to remain the default goal.
926 Since @code{clean} is not a prerequisite of @code{edit}, this rule will not
927 run at all if we give the command @samp{make} with no arguments. In
928 order to make the rule run, we have to type @samp{make clean}.
929 @xref{Running, ,How to Run @code{make}}.
931 @node Makefiles, Rules, Introduction, Top
932 @chapter Writing Makefiles
934 @cindex makefile, how to write
935 The information that tells @code{make} how to recompile a system comes from
936 reading a data base called the @dfn{makefile}.
939 * Makefile Contents:: What makefiles contain.
940 * Makefile Names:: How to name your makefile.
941 * Include:: How one makefile can use another makefile.
942 * MAKEFILES Variable:: The environment can specify extra makefiles.
943 * Remaking Makefiles:: How makefiles get remade.
944 * Overriding Makefiles:: How to override part of one makefile
945 with another makefile.
946 * Reading Makefiles:: How makefiles are parsed.
947 * Secondary Expansion:: How and when secondary expansion is performed.
950 @node Makefile Contents, Makefile Names, Makefiles, Makefiles
951 @section What Makefiles Contain
953 Makefiles contain five kinds of things: @dfn{explicit rules},
954 @dfn{implicit rules}, @dfn{variable definitions}, @dfn{directives},
955 and @dfn{comments}. Rules, variables, and directives are described at
956 length in later chapters.@refill
959 @cindex rule, explicit, definition of
960 @cindex explicit rule, definition of
962 An @dfn{explicit rule} says when and how to remake one or more files,
963 called the rule's @dfn{targets}. It lists the other files that the
964 targets depend on, called the @dfn{prerequisites} of the target, and
965 may also give a recipe to use to create or update the targets.
966 @xref{Rules, ,Writing Rules}.
968 @cindex rule, implicit, definition of
969 @cindex implicit rule, definition of
971 An @dfn{implicit rule} says when and how to remake a class of files
972 based on their names. It describes how a target may depend on a file
973 with a name similar to the target and gives a recipe to create or
974 update such a target. @xref{Implicit Rules, ,Using Implicit Rules}.
976 @cindex variable definition
978 A @dfn{variable definition} is a line that specifies a text string
979 value for a variable that can be substituted into the text later. The
980 simple makefile example shows a variable definition for @code{objects}
981 as a list of all object files (@pxref{Variables Simplify, , Variables
982 Make Makefiles Simpler}).
986 A @dfn{directive} is an instruction for @code{make} to do something
987 special while reading the makefile. These include:
991 Reading another makefile (@pxref{Include, ,Including Other Makefiles}).
994 Deciding (based on the values of variables) whether to use or
995 ignore a part of the makefile (@pxref{Conditionals, ,Conditional Parts of Makefiles}).
998 Defining a variable from a verbatim string containing multiple lines
999 (@pxref{Multi-Line, ,Defining Multi-Line Variables}).
1002 @cindex comments, in makefile
1003 @cindex @code{#} (comments), in makefile
1005 @samp{#} in a line of a makefile starts a @dfn{comment}. It and the
1006 rest of the line are ignored, except that a trailing backslash not
1007 escaped by another backslash will continue the comment across multiple
1008 lines. A line containing just a comment (with perhaps spaces before
1009 it) is effectively blank, and is ignored. If you want a literal
1010 @code{#}, escape it with a backslash (e.g., @code{\#}). Comments may
1011 appear on any line in the makefile, although they are treated
1012 specially in certain situations.
1014 You cannot use comments within variable references or function calls:
1015 any instance of @code{#} will be treated literally (rather than as the
1016 start of a comment) inside a variable reference or function call.
1018 Comments within a recipe are passed to the shell, just as with any
1019 other recipe text. The shell decides how to interpret it: whether or
1020 not this is a comment is up to the shell.
1022 Within a @code{define} directive, comments are not ignored during the
1023 definition of the variable, but rather kept intact in the value of the
1024 variable. When the variable is expanded they will either be treated
1025 as @code{make} comments or as recipe text, depending on the context in
1026 which the variable is evaluated.
1029 @node Makefile Names, Include, Makefile Contents, Makefiles
1030 @section What Name to Give Your Makefile
1031 @cindex makefile name
1032 @cindex name of makefile
1033 @cindex default makefile name
1034 @cindex file name of makefile
1036 @c following paragraph rewritten to avoid overfull hbox
1037 By default, when @code{make} looks for the makefile, it tries the
1038 following names, in order: @file{GNUmakefile}, @file{makefile}
1039 and @file{Makefile}.@refill
1044 @cindex @code{README}
1045 Normally you should call your makefile either @file{makefile} or
1046 @file{Makefile}. (We recommend @file{Makefile} because it appears
1047 prominently near the beginning of a directory listing, right near other
1048 important files such as @file{README}.) The first name checked,
1049 @file{GNUmakefile}, is not recommended for most makefiles. You should
1050 use this name if you have a makefile that is specific to GNU
1051 @code{make}, and will not be understood by other versions of
1052 @code{make}. Other @code{make} programs look for @file{makefile} and
1053 @file{Makefile}, but not @file{GNUmakefile}.
1055 If @code{make} finds none of these names, it does not use any makefile.
1056 Then you must specify a goal with a command argument, and @code{make}
1057 will attempt to figure out how to remake it using only its built-in
1058 implicit rules. @xref{Implicit Rules, ,Using Implicit Rules}.
1061 @cindex @code{--file}
1062 @cindex @code{--makefile}
1063 If you want to use a nonstandard name for your makefile, you can specify
1064 the makefile name with the @samp{-f} or @samp{--file} option. The
1065 arguments @w{@samp{-f @var{name}}} or @w{@samp{--file=@var{name}}} tell
1066 @code{make} to read the file @var{name} as the makefile. If you use
1067 more than one @samp{-f} or @samp{--file} option, you can specify several
1068 makefiles. All the makefiles are effectively concatenated in the order
1069 specified. The default makefile names @file{GNUmakefile},
1070 @file{makefile} and @file{Makefile} are not checked automatically if you
1071 specify @samp{-f} or @samp{--file}.@refill
1072 @cindex specifying makefile name
1073 @cindex makefile name, how to specify
1074 @cindex name of makefile, how to specify
1075 @cindex file name of makefile, how to specify
1077 @node Include, MAKEFILES Variable, Makefile Names, Makefiles
1078 @section Including Other Makefiles
1079 @cindex including other makefiles
1080 @cindex makefile, including
1083 The @code{include} directive tells @code{make} to suspend reading the
1084 current makefile and read one or more other makefiles before continuing.
1085 The directive is a line in the makefile that looks like this:
1088 include @var{filenames}@dots{}
1092 @var{filenames} can contain shell file name patterns. If
1093 @var{filenames} is empty, nothing is included and no error is printed.
1094 @cindex shell file name pattern (in @code{include})
1095 @cindex shell wildcards (in @code{include})
1096 @cindex wildcard, in @code{include}
1098 Extra spaces are allowed and ignored at the beginning of the line, but
1099 the first character must not be a tab (or the value of
1100 @code{.CMDPREFIX})---if the line begins with a tab, it will be
1101 considered a recipe line. Whitespace is required between
1102 @code{include} and the file names, and between file names; extra
1103 whitespace is ignored there and at the end of the directive. A
1104 comment starting with @samp{#} is allowed at the end of the line. If
1105 the file names contain any variable or function references, they are
1106 expanded. @xref{Using Variables, ,How to Use Variables}.
1108 For example, if you have three @file{.mk} files, @file{a.mk},
1109 @file{b.mk}, and @file{c.mk}, and @code{$(bar)} expands to
1110 @code{bish bash}, then the following expression
1113 include foo *.mk $(bar)
1119 include foo a.mk b.mk c.mk bish bash
1122 When @code{make} processes an @code{include} directive, it suspends
1123 reading of the containing makefile and reads from each listed file in
1124 turn. When that is finished, @code{make} resumes reading the
1125 makefile in which the directive appears.
1127 One occasion for using @code{include} directives is when several programs,
1128 handled by individual makefiles in various directories, need to use a
1129 common set of variable definitions
1130 (@pxref{Setting, ,Setting Variables}) or pattern rules
1131 (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).
1133 Another such occasion is when you want to generate prerequisites from
1134 source files automatically; the prerequisites can be put in a file that
1135 is included by the main makefile. This practice is generally cleaner
1136 than that of somehow appending the prerequisites to the end of the main
1137 makefile as has been traditionally done with other versions of
1138 @code{make}. @xref{Automatic Prerequisites}.
1139 @cindex prerequisites, automatic generation
1140 @cindex automatic generation of prerequisites
1141 @cindex generating prerequisites automatically
1144 @cindex @code{--include-dir}
1145 @cindex included makefiles, default directories
1146 @cindex default directories for included makefiles
1147 @findex /usr/gnu/include
1148 @findex /usr/local/include
1149 @findex /usr/include
1150 If the specified name does not start with a slash, and the file is not
1151 found in the current directory, several other directories are searched.
1152 First, any directories you have specified with the @samp{-I} or
1153 @samp{--include-dir} option are searched
1154 (@pxref{Options Summary, ,Summary of Options}).
1155 Then the following directories (if they exist)
1156 are searched, in this order:
1157 @file{@var{prefix}/include} (normally @file{/usr/local/include}
1158 @footnote{GNU Make compiled for MS-DOS and MS-Windows behaves as if
1159 @var{prefix} has been defined to be the root of the DJGPP tree
1161 @file{/usr/gnu/include},
1162 @file{/usr/local/include}, @file{/usr/include}.
1164 If an included makefile cannot be found in any of these directories, a
1165 warning message is generated, but it is not an immediately fatal error;
1166 processing of the makefile containing the @code{include} continues.
1167 Once it has finished reading makefiles, @code{make} will try to remake
1168 any that are out of date or don't exist.
1169 @xref{Remaking Makefiles, ,How Makefiles Are Remade}.
1170 Only after it has tried to find a way to remake a makefile and failed,
1171 will @code{make} diagnose the missing makefile as a fatal error.
1173 If you want @code{make} to simply ignore a makefile which does not exist
1174 or cannot be remade, with no error message, use the @w{@code{-include}}
1175 directive instead of @code{include}, like this:
1178 -include @var{filenames}@dots{}
1181 This acts like @code{include} in every way except that there is no
1182 error (not even a warning) if any of the @var{filenames} (or any
1183 prerequisites of any of the @var{filenames}) do not exist or cannot be
1186 For compatibility with some other @code{make} implementations,
1187 @code{sinclude} is another name for @w{@code{-include}}.
1189 @node MAKEFILES Variable, Remaking Makefiles, Include, Makefiles
1190 @section The Variable @code{MAKEFILES}
1191 @cindex makefile, and @code{MAKEFILES} variable
1192 @cindex including (@code{MAKEFILES} variable)
1195 If the environment variable @code{MAKEFILES} is defined, @code{make}
1196 considers its value as a list of names (separated by whitespace) of
1197 additional makefiles to be read before the others. This works much
1198 like the @code{include} directive: various directories are searched
1199 for those files (@pxref{Include, ,Including Other Makefiles}). In
1200 addition, the default goal is never taken from one of these makefiles
1201 (or any makefile included by them) and it is not an error if the files
1202 listed in @code{MAKEFILES} are not found.@refill
1204 @cindex recursion, and @code{MAKEFILES} variable
1205 The main use of @code{MAKEFILES} is in communication between recursive
1206 invocations of @code{make} (@pxref{Recursion, ,Recursive Use of
1207 @code{make}}). It usually is not desirable to set the environment
1208 variable before a top-level invocation of @code{make}, because it is
1209 usually better not to mess with a makefile from outside. However, if
1210 you are running @code{make} without a specific makefile, a makefile in
1211 @code{MAKEFILES} can do useful things to help the built-in implicit
1212 rules work better, such as defining search paths (@pxref{Directory Search}).
1214 Some users are tempted to set @code{MAKEFILES} in the environment
1215 automatically on login, and program makefiles to expect this to be done.
1216 This is a very bad idea, because such makefiles will fail to work if run by
1217 anyone else. It is much better to write explicit @code{include} directives
1218 in the makefiles. @xref{Include, , Including Other Makefiles}.
1220 @node Remaking Makefiles, Overriding Makefiles, MAKEFILES Variable, Makefiles
1221 @section How Makefiles Are Remade
1223 @cindex updating makefiles
1224 @cindex remaking makefiles
1225 @cindex makefile, remaking of
1226 Sometimes makefiles can be remade from other files, such as RCS or SCCS
1227 files. If a makefile can be remade from other files, you probably want
1228 @code{make} to get an up-to-date version of the makefile to read in.
1230 To this end, after reading in all makefiles, @code{make} will consider
1231 each as a goal target and attempt to update it. If a makefile has a
1232 rule which says how to update it (found either in that very makefile or
1233 in another one) or if an implicit rule applies to it (@pxref{Implicit
1234 Rules, ,Using Implicit Rules}), it will be updated if necessary. After
1235 all makefiles have been checked, if any have actually been changed,
1236 @code{make} starts with a clean slate and reads all the makefiles over
1237 again. (It will also attempt to update each of them over again, but
1238 normally this will not change them again, since they are already up to
1241 If you know that one or more of your makefiles cannot be remade and
1242 you want to keep @code{make} from performing an implicit rule search
1243 on them, perhaps for efficiency reasons, you can use any normal method
1244 of preventing implicit rule lookup to do so. For example, you can
1245 write an explicit rule with the makefile as the target, and an empty
1246 recipe (@pxref{Empty Recipes, ,Using Empty Recipes}).
1248 If the makefiles specify a double-colon rule to remake a file with
1249 a recipe but no prerequisites, that file will always be remade
1250 (@pxref{Double-Colon}). In the case of makefiles, a makefile that has a
1251 double-colon rule with a recipe but no prerequisites will be remade every
1252 time @code{make} is run, and then again after @code{make} starts over
1253 and reads the makefiles in again. This would cause an infinite loop:
1254 @code{make} would constantly remake the makefile, and never do anything
1255 else. So, to avoid this, @code{make} will @strong{not} attempt to
1256 remake makefiles which are specified as targets of a double-colon rule
1257 with a recipe but no prerequisites.@refill
1259 If you do not specify any makefiles to be read with @samp{-f} or
1260 @samp{--file} options, @code{make} will try the default makefile names;
1261 @pxref{Makefile Names, ,What Name to Give Your Makefile}. Unlike
1262 makefiles explicitly requested with @samp{-f} or @samp{--file} options,
1263 @code{make} is not certain that these makefiles should exist. However,
1264 if a default makefile does not exist but can be created by running
1265 @code{make} rules, you probably want the rules to be run so that the
1266 makefile can be used.
1268 Therefore, if none of the default makefiles exists, @code{make} will try
1269 to make each of them in the same order in which they are searched for
1270 (@pxref{Makefile Names, ,What Name to Give Your Makefile})
1271 until it succeeds in making one, or it runs out of names to try. Note
1272 that it is not an error if @code{make} cannot find or make any makefile;
1273 a makefile is not always necessary.@refill
1275 When you use the @samp{-t} or @samp{--touch} option
1276 (@pxref{Instead of Execution, ,Instead of Executing Recipes}),
1277 you would not want to use an out-of-date makefile to decide which
1278 targets to touch. So the @samp{-t} option has no effect on updating
1279 makefiles; they are really updated even if @samp{-t} is specified.
1280 Likewise, @samp{-q} (or @samp{--question}) and @samp{-n} (or
1281 @samp{--just-print}) do not prevent updating of makefiles, because an
1282 out-of-date makefile would result in the wrong output for other targets.
1283 Thus, @samp{make -f mfile -n foo} will update @file{mfile}, read it in,
1284 and then print the recipe to update @file{foo} and its prerequisites
1285 without running it. The recipe printed for @file{foo} will be the one
1286 specified in the updated contents of @file{mfile}.
1288 However, on occasion you might actually wish to prevent updating of even
1289 the makefiles. You can do this by specifying the makefiles as goals in
1290 the command line as well as specifying them as makefiles. When the
1291 makefile name is specified explicitly as a goal, the options @samp{-t}
1292 and so on do apply to them.
1294 Thus, @samp{make -f mfile -n mfile foo} would read the makefile
1295 @file{mfile}, print the recipe needed to update it without actually
1296 running it, and then print the recipe needed to update @file{foo}
1297 without running that. The recipe for @file{foo} will be the one
1298 specified by the existing contents of @file{mfile}.
1300 @node Overriding Makefiles, Reading Makefiles, Remaking Makefiles, Makefiles
1301 @section Overriding Part of Another Makefile
1303 @cindex overriding makefiles
1304 @cindex makefile, overriding
1305 Sometimes it is useful to have a makefile that is mostly just like
1306 another makefile. You can often use the @samp{include} directive to
1307 include one in the other, and add more targets or variable definitions.
1308 However, it is illegal for two makefiles to give different recipes for
1309 the same target. But there is another way.
1311 @cindex match-anything rule, used to override
1312 In the containing makefile (the one that wants to include the other),
1313 you can use a match-anything pattern rule to say that to remake any
1314 target that cannot be made from the information in the containing
1315 makefile, @code{make} should look in another makefile.
1316 @xref{Pattern Rules}, for more information on pattern rules.
1318 For example, if you have a makefile called @file{Makefile} that says how
1319 to make the target @samp{foo} (and other targets), you can write a
1320 makefile called @file{GNUmakefile} that contains:
1327 @@$(MAKE) -f Makefile $@@
1331 If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
1332 read it, and see that to make @file{foo}, it needs to run the recipe
1333 @samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will
1334 find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
1335 recipe from the pattern rule: @samp{make -f Makefile bar}. If
1336 @file{Makefile} provides a rule for updating @file{bar}, @code{make}
1337 will apply the rule. And likewise for any other target that
1338 @file{GNUmakefile} does not say how to make.
1340 The way this works is that the pattern rule has a pattern of just
1341 @samp{%}, so it matches any target whatever. The rule specifies a
1342 prerequisite @file{force}, to guarantee that the recipe will be run even
1343 if the target file already exists. We give the @file{force} target an
1344 empty recipe to prevent @code{make} from searching for an implicit rule to
1345 build it---otherwise it would apply the same match-anything rule to
1346 @file{force} itself and create a prerequisite loop!
1348 @node Reading Makefiles, Secondary Expansion, Overriding Makefiles, Makefiles
1349 @section How @code{make} Reads a Makefile
1350 @cindex reading makefiles
1351 @cindex makefile, parsing
1353 GNU @code{make} does its work in two distinct phases. During the first
1354 phase it reads all the makefiles, included makefiles, etc. and
1355 internalizes all the variables and their values, implicit and explicit
1356 rules, and constructs a dependency graph of all the targets and their
1357 prerequisites. During the second phase, @code{make} uses these internal
1358 structures to determine what targets will need to be rebuilt and to
1359 invoke the rules necessary to do so.
1361 It's important to understand this two-phase approach because it has a
1362 direct impact on how variable and function expansion happens; this is
1363 often a source of some confusion when writing makefiles. Here we will
1364 present a summary of the phases in which expansion happens for different
1365 constructs within the makefile. We say that expansion is
1366 @dfn{immediate} if it happens during the first phase: in this case
1367 @code{make} will expand any variables or functions in that section of a
1368 construct as the makefile is parsed. We say that expansion is
1369 @dfn{deferred} if expansion is not performed immediately. Expansion of
1370 a deferred construct is not performed until either the construct appears
1371 later in an immediate context, or until the second phase.
1373 You may not be familiar with some of these constructs yet. You can
1374 reference this section as you become familiar with them, in later
1377 @subheading Variable Assignment
1378 @cindex +=, expansion
1379 @cindex =, expansion
1380 @cindex ?=, expansion
1381 @cindex +=, expansion
1382 @cindex define, expansion
1384 Variable definitions are parsed as follows:
1387 @var{immediate} = @var{deferred}
1388 @var{immediate} ?= @var{deferred}
1389 @var{immediate} := @var{immediate}
1390 @var{immediate} += @var{deferred} or @var{immediate}
1392 define @var{immediate}
1396 define @var{immediate} =
1400 define @var{immediate} ?=
1404 define @var{immediate} :=
1408 define @var{immediate} +=
1409 @var{deferred} or @var{immediate}
1413 For the append operator, @samp{+=}, the right-hand side is considered
1414 immediate if the variable was previously set as a simple variable
1415 (@samp{:=}), and deferred otherwise.
1417 @subheading Conditional Directives
1418 @cindex ifdef, expansion
1419 @cindex ifeq, expansion
1420 @cindex ifndef, expansion
1421 @cindex ifneq, expansion
1423 Conditional directives are parsed immediately. This means, for
1424 example, that automatic variables cannot be used in conditional
1425 directives, as automatic variables are not set until the recipe for
1426 that rule is invoked. If you need to use automatic variables in a
1427 conditional directive you @emph{must} move the condition into the
1428 recipe and use shell conditional syntax instead.
1430 @subheading Rule Definition
1431 @cindex target, expansion
1432 @cindex prerequisite, expansion
1433 @cindex implicit rule, expansion
1434 @cindex pattern rule, expansion
1435 @cindex explicit rule, expansion
1437 A rule is always expanded the same way, regardless of the form:
1440 @var{immediate} : @var{immediate} ; @var{deferred}
1444 That is, the target and prerequisite sections are expanded immediately,
1445 and the recipe used to construct the target is always deferred. This
1446 general rule is true for explicit rules, pattern rules, suffix rules,
1447 static pattern rules, and simple prerequisite definitions.
1449 @node Secondary Expansion, , Reading Makefiles, Makefiles
1450 @section Secondary Expansion
1451 @cindex secondary expansion
1452 @cindex expansion, secondary
1454 @findex .SECONDEXPANSION
1455 In the previous section we learned that GNU @code{make} works in two
1456 distinct phases: a read-in phase and a target-update phase
1457 (@pxref{Reading Makefiles, , How @code{make} Reads a Makefile}). GNU
1458 make also has the ability to enable a @emph{second expansion} of the
1459 prerequisites (only) for some or all targets defined in the makefile.
1460 In order for this second expansion to occur, the special target
1461 @code{.SECONDEXPANSION} must be defined before the first prerequisite
1462 list that makes use of this feature.
1464 If that special target is defined then in between the two phases
1465 mentioned above, right at the end of the read-in phase, all the
1466 prerequisites of the targets defined after the special target are
1467 expanded a @emph{second time}. In most circumstances this secondary
1468 expansion will have no effect, since all variable and function
1469 references will have been expanded during the initial parsing of the
1470 makefiles. In order to take advantage of the secondary expansion
1471 phase of the parser, then, it's necessary to @emph{escape} the
1472 variable or function reference in the makefile. In this case the
1473 first expansion merely un-escapes the reference but doesn't expand it,
1474 and expansion is left to the secondary expansion phase. For example,
1475 consider this makefile:
1481 myfile: $(ONEVAR) $$(TWOVAR)
1484 After the first expansion phase the prerequisites list of the
1485 @file{myfile} target will be @code{onefile} and @code{$(TWOVAR)}; the
1486 first (unescaped) variable reference to @var{ONEVAR} is expanded,
1487 while the second (escaped) variable reference is simply unescaped,
1488 without being recognized as a variable reference. Now during the
1489 secondary expansion the first word is expanded again but since it
1490 contains no variable or function references it remains the static
1491 value @file{onefile}, while the second word is now a normal reference
1492 to the variable @var{TWOVAR}, which is expanded to the value
1493 @file{twofile}. The final result is that there are two prerequisites,
1494 @file{onefile} and @file{twofile}.
1496 Obviously, this is not a very interesting case since the same result
1497 could more easily have been achieved simply by having both variables
1498 appear, unescaped, in the prerequisites list. One difference becomes
1499 apparent if the variables are reset; consider this example:
1509 Here the prerequisite of @file{onefile} will be expanded immediately,
1510 and resolve to the value @file{top}, while the prerequisite of
1511 @file{twofile} will not be full expanded until the secondary expansion
1512 and yield a value of @file{bottom}.
1514 This is marginally more exciting, but the true power of this feature
1515 only becomes apparent when you discover that secondary expansions
1516 always take place within the scope of the automatic variables for that
1517 target. This means that you can use variables such as @code{$@@},
1518 @code{$*}, etc. during the second expansion and they will have their
1519 expected values, just as in the recipe. All you have to do is defer
1520 the expansion by escaping the @code{$}. Also, secondary expansion
1521 occurs for both explicit and implicit (pattern) rules. Knowing this,
1522 the possible uses for this feature increase dramatically. For
1527 main_OBJS := main.o try.o test.o
1528 lib_OBJS := lib.o api.o
1530 main lib: $$($$@@_OBJS)
1533 Here, after the initial expansion the prerequisites of both the
1534 @file{main} and @file{lib} targets will be @code{$($@@_OBJS)}. During
1535 the secondary expansion, the @code{$@@} variable is set to the name of
1536 the target and so the expansion for the @file{main} target will yield
1537 @code{$(main_OBJS)}, or @code{main.o try.o test.o}, while the
1538 secondary expansion for the @file{lib} target will yield
1539 @code{$(lib_OBJS)}, or @code{lib.o api.o}.
1541 You can also mix in functions here, as long as they are properly escaped:
1544 main_SRCS := main.c try.c test.c
1545 lib_SRCS := lib.c api.c
1548 main lib: $$(patsubst %.c,%.o,$$($$@@_SRCS))
1551 This version allows users to specify source files rather than object
1552 files, but gives the same resulting prerequisites list as the previous
1555 Evaluation of automatic variables during the secondary expansion
1556 phase, especially of the target name variable @code{$$@@}, behaves
1557 similarly to evaluation within recipes. However, there are some
1558 subtle differences and ``corner cases'' which come into play for the
1559 different types of rule definitions that @code{make} understands. The
1560 subtleties of using the different automatic variables are described
1563 @subheading Secondary Expansion of Explicit Rules
1564 @cindex secondary expansion and explicit rules
1565 @cindex explicit rules, secondary expansion of
1567 During the secondary expansion of explicit rules, @code{$$@@} and
1568 @code{$$%} evaluate, respectively, to the file name of the target and,
1569 when the target is an archive member, the target member name. The
1570 @code{$$<} variable evaluates to the first prerequisite in the first
1571 rule for this target. @code{$$^} and @code{$$+} evaluate to the list
1572 of all prerequisites of rules @emph{that have already appeared} for
1573 the same target (@code{$$+} with repetitions and @code{$$^}
1574 without). The following example will help illustrate these behaviors:
1579 foo: foo.1 bar.1 $$< $$^ $$+ # line #1
1581 foo: foo.2 bar.2 $$< $$^ $$+ # line #2
1583 foo: foo.3 bar.3 $$< $$^ $$+ # line #3
1586 In the first prerequisite list, all three variables (@code{$$<},
1587 @code{$$^}, and @code{$$+}) expand to the empty string. In the
1588 second, they will have values @code{foo.1}, @code{foo.1 bar.1}, and
1589 @code{foo.1 bar.1} respectively. In the third they will have values
1590 @code{foo.1}, @code{foo.1 bar.1 foo.2 bar.2}, and @code{foo.1 bar.1
1591 foo.2 bar.2 foo.1 foo.1 bar.1 foo.1 bar.1} respectively.
1593 Rules undergo secondary expansion in makefile order, except that
1594 the rule with the recipe is always evaluated last.
1596 The variables @code{$$?} and @code{$$*} are not available and expand
1597 to the empty string.
1599 @subheading Secondary Expansion of Static Pattern Rules
1600 @cindex secondary expansion and static pattern rules
1601 @cindex static pattern rules, secondary expansion of
1603 Rules for secondary expansion of static pattern rules are identical to
1604 those for explicit rules, above, with one exception: for static
1605 pattern rules the @code{$$*} variable is set to the pattern stem. As
1606 with explicit rules, @code{$$?} is not available and expands to the
1609 @subheading Secondary Expansion of Implicit Rules
1610 @cindex secondary expansion and implicit rules
1611 @cindex implicit rules, secondary expansion of
1613 As @code{make} searches for an implicit rule, it substitutes the stem
1614 and then performs secondary expansion for every rule with a matching
1615 target pattern. The value of the automatic variables is derived in
1616 the same fashion as for static pattern rules. As an example:
1625 %oo: $$< $$^ $$+ $$*
1628 When the implicit rule is tried for target @file{foo}, @code{$$<}
1629 expands to @file{bar}, @code{$$^} expands to @file{bar boo},
1630 @code{$$+} also expands to @file{bar boo}, and @code{$$*} expands to
1633 Note that the directory prefix (D), as described in @ref{Implicit Rule
1634 Search, ,Implicit Rule Search Algorithm}, is appended (after
1635 expansion) to all the patterns in the prerequisites list. As an
1643 %.o: $$(addsuffix /%.c,foo bar) foo.h
1646 The prerequisite list after the secondary expansion and directory
1647 prefix reconstruction will be @file{/tmp/foo/foo.c /tmp/var/bar/foo.c
1648 foo.h}. If you are not interested in this reconstruction, you can use
1649 @code{$$*} instead of @code{%} in the prerequisites list.
1651 @node Rules, Recipes, Makefiles, Top
1652 @chapter Writing Rules
1653 @cindex writing rules
1654 @cindex rule, how to write
1656 @cindex prerequisite
1658 A @dfn{rule} appears in the makefile and says when and how to remake
1659 certain files, called the rule's @dfn{targets} (most often only one per rule).
1660 It lists the other files that are the @dfn{prerequisites} of the target, and
1661 the @dfn{recipe} to use to create or update the target.
1663 @cindex default goal
1664 @cindex goal, default
1665 The order of rules is not significant, except for determining the
1666 @dfn{default goal}: the target for @code{make} to consider, if you do
1667 not otherwise specify one. The default goal is the target of the first
1668 rule in the first makefile. If the first rule has multiple targets,
1669 only the first target is taken as the default. There are two
1670 exceptions: a target starting with a period is not a default unless it
1671 contains one or more slashes, @samp{/}, as well; and, a target that
1672 defines a pattern rule has no effect on the default goal.
1673 (@xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.)
1675 Therefore, we usually write the makefile so that the first rule is the
1676 one for compiling the entire program or all the programs described by
1677 the makefile (often with a target called @samp{all}).
1678 @xref{Goals, ,Arguments to Specify the Goals}.
1681 * Rule Example:: An example explained.
1682 * Rule Syntax:: General syntax explained.
1683 * Prerequisite Types:: There are two types of prerequisites.
1684 * Wildcards:: Using wildcard characters such as `*'.
1685 * Directory Search:: Searching other directories for source files.
1686 * Phony Targets:: Using a target that is not a real file's name.
1687 * Force Targets:: You can use a target without recipes
1688 or prerequisites to mark other targets
1690 * Empty Targets:: When only the date matters and the
1692 * Special Targets:: Targets with special built-in meanings.
1693 * Multiple Targets:: When to make use of several targets in a rule.
1694 * Multiple Rules:: How to use several rules with the same target.
1695 * Static Pattern:: Static pattern rules apply to multiple targets
1696 and can vary the prerequisites according to
1698 * Double-Colon:: How to use a special kind of rule to allow
1699 several independent rules for one target.
1700 * Automatic Prerequisites:: How to automatically generate rules giving
1701 prerequisites from source files themselves.
1705 @node Rule Example, Rule Syntax, Rules, Rules
1706 @section Rule Example
1708 Here is an example of a rule:
1711 foo.o : foo.c defs.h # module for twiddling the frobs
1715 Its target is @file{foo.o} and its prerequisites are @file{foo.c} and
1716 @file{defs.h}. It has one command in the recipe: @samp{cc -c -g foo.c}.
1717 The recipe starts with a tab to identify it as a recipe.
1719 This rule says two things:
1723 How to decide whether @file{foo.o} is out of date: it is out of date
1724 if it does not exist, or if either @file{foo.c} or @file{defs.h} is
1725 more recent than it.
1728 How to update the file @file{foo.o}: by running @code{cc} as stated.
1729 The recipe does not explicitly mention @file{defs.h}, but we presume
1730 that @file{foo.c} includes it, and that that is why @file{defs.h} was
1731 added to the prerequisites.
1735 @node Rule Syntax, Prerequisite Types, Rule Example, Rules
1736 @section Rule Syntax
1739 @cindex syntax of rules
1740 In general, a rule looks like this:
1743 @var{targets} : @var{prerequisites}
1752 @var{targets} : @var{prerequisites} ; @var{recipe}
1758 @cindex rule targets
1759 The @var{targets} are file names, separated by spaces. Wildcard
1760 characters may be used (@pxref{Wildcards, ,Using Wildcard Characters
1761 in File Names}) and a name of the form @file{@var{a}(@var{m})}
1762 represents member @var{m} in archive file @var{a}
1763 (@pxref{Archive Members, ,Archive Members as Targets}).
1764 Usually there is only one
1765 target per rule, but occasionally there is a reason to have more
1766 (@pxref{Multiple Targets, , Multiple Targets in a Rule}).@refill
1769 @cindex tab character (in commands)
1770 The @var{recipe} lines start with a tab character (or the first
1771 character in the value of the @code{.CMDPREFIX} variable;
1772 @pxref{Special Variables}). The first recipe line may appear on the line
1773 after the prerequisites, with a tab character, or may appear on the
1774 same line, with a semicolon. Either way, the effect is the same.
1775 There are other differences in the syntax of recipes.
1776 @xref{Recipes, ,Writing Recipes in Rules}.
1778 @cindex dollar sign (@code{$}), in rules
1779 @cindex @code{$}, in rules
1780 @cindex rules, and @code{$}
1781 Because dollar signs are used to start @code{make} variable
1782 references, if you really want a dollar sign in a target or
1783 prerequisite you must write two of them, @samp{$$} (@pxref{Using
1784 Variables, ,How to Use Variables}). If you have enabled secondary
1785 expansion (@pxref{Secondary Expansion}) and you want a literal dollar
1786 sign in the prerequisites list, you must actually write @emph{four}
1787 dollar signs (@samp{$$$$}).
1789 You may split a long line by inserting a backslash followed by a
1790 newline, but this is not required, as @code{make} places no limit on
1791 the length of a line in a makefile.
1793 A rule tells @code{make} two things: when the targets are out of date,
1794 and how to update them when necessary.
1796 @cindex prerequisites
1797 @cindex rule prerequisites
1798 The criterion for being out of date is specified in terms of the
1799 @var{prerequisites}, which consist of file names separated by spaces.
1800 (Wildcards and archive members (@pxref{Archives}) are allowed here too.)
1801 A target is out of date if it does not exist or if it is older than any
1802 of the prerequisites (by comparison of last-modification times). The
1803 idea is that the contents of the target file are computed based on
1804 information in the prerequisites, so if any of the prerequisites changes,
1805 the contents of the existing target file are no longer necessarily
1808 How to update is specified by a @var{recipe}. This is one or more
1809 lines to be executed by the shell (normally @samp{sh}), but with some
1810 extra features (@pxref{Recipes, ,Writing Recipes in Rules}).
1812 @node Prerequisite Types, Wildcards, Rule Syntax, Rules
1813 @comment node-name, next, previous, up
1814 @section Types of Prerequisites
1815 @cindex prerequisite types
1816 @cindex types of prerequisites
1818 @cindex prerequisites, normal
1819 @cindex normal prerequisites
1820 @cindex prerequisites, order-only
1821 @cindex order-only prerequisites
1822 There are actually two different types of prerequisites understood by
1823 GNU @code{make}: normal prerequisites such as described in the
1824 previous section, and @dfn{order-only} prerequisites. A normal
1825 prerequisite makes two statements: first, it imposes an order of
1826 execution of recipes: any recipes necessary to build any of a
1827 target's prerequisites will be fully executed before any recipe
1828 necessary to build the target. Second, it imposes a dependency
1829 relationship: if any prerequisite is newer than the target, then the
1830 target is considered out-of-date and must be rebuilt.
1832 Normally, this is exactly what you want: if a target's prerequisite is
1833 updated, then the target should also be updated.
1835 Occasionally, however, you have a situation where you want to impose a
1836 specific ordering on the rules to be invoked @emph{without} forcing
1837 the target to be updated if one of those rules is executed. In that
1838 case, you want to define @dfn{order-only} prerequisites. Order-only
1839 prerequisites can be specified by placing a pipe symbol (@code{|})
1840 in the prerequisites list: any prerequisites to the left of the pipe
1841 symbol are normal; any prerequisites to the right are order-only:
1844 @var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites}
1847 The normal prerequisites section may of course be empty. Also, you
1848 may still declare multiple lines of prerequisites for the same target:
1849 they are appended appropriately (normal prerequisites are appended to
1850 the list of normal prerequisites; order-only prerequisites are
1851 appended to the list of order-only prerequisites). Note that if you
1852 declare the same file to be both a normal and an order-only
1853 prerequisite, the normal prerequisite takes precedence (since they
1854 have a strict superset of the behavior of an order-only prerequisite).
1856 Consider an example where your targets are to be placed in a separate
1857 directory, and that directory might not exist before @code{make} is
1858 run. In this situation, you want the directory to be created before
1859 any targets are placed into it but, because the timestamps on
1860 directories change whenever a file is added, removed, or renamed, we
1861 certainly don't want to rebuild all the targets whenever the
1862 directory's timestamp changes. One way to manage this is with
1863 order-only prerequisites: make the directory an order-only
1864 prerequisite on all the targets:
1868 OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)
1871 $(COMPILE.c) $(OUTPUT_OPTION) $<
1875 $(OBJS): | $(OBJDIR)
1881 Now the rule to create the @file{objdir} directory will be run, if
1882 needed, before any @samp{.o} is built, but no @samp{.o} will be built
1883 because the @file{objdir} directory timestamp changed.
1885 @node Wildcards, Directory Search, Prerequisite Types, Rules
1886 @section Using Wildcard Characters in File Names
1888 @cindex file name with wildcards
1889 @cindex globbing (wildcards)
1891 @cindex @code{*} (wildcard character)
1892 @cindex @code{?} (wildcard character)
1893 @cindex @code{[@dots{}]} (wildcard characters)
1894 A single file name can specify many files using @dfn{wildcard characters}.
1895 The wildcard characters in @code{make} are @samp{*}, @samp{?} and
1896 @samp{[@dots{}]}, the same as in the Bourne shell. For example, @file{*.c}
1897 specifies a list of all the files (in the working directory) whose names
1898 end in @samp{.c}.@refill
1900 @cindex @code{~} (tilde)
1901 @cindex tilde (@code{~})
1902 @cindex home directory
1903 The character @samp{~} at the beginning of a file name also has special
1904 significance. If alone, or followed by a slash, it represents your home
1905 directory. For example @file{~/bin} expands to @file{/home/you/bin}.
1906 If the @samp{~} is followed by a word, the string represents the home
1907 directory of the user named by that word. For example @file{~john/bin}
1908 expands to @file{/home/john/bin}. On systems which don't have a home
1909 directory for each user (such as MS-DOS or MS-Windows), this
1910 functionality can be simulated by setting the environment variable
1913 Wildcard expansion is performed by @code{make} automatically in
1914 targets and in prerequisites. In recipes, the shell is responsible
1915 for wildcard expansion. In other contexts, wildcard expansion happens
1916 only if you request it explicitly with the @code{wildcard} function.
1918 The special significance of a wildcard character can be turned off by
1919 preceding it with a backslash. Thus, @file{foo\*bar} would refer to a
1920 specific file whose name consists of @samp{foo}, an asterisk, and
1924 * Wildcard Examples:: Several examples
1925 * Wildcard Pitfall:: Problems to avoid.
1926 * Wildcard Function:: How to cause wildcard expansion where
1927 it does not normally take place.
1930 @node Wildcard Examples, Wildcard Pitfall, Wildcards, Wildcards
1931 @subsection Wildcard Examples
1933 Wildcards can be used in the recipe of a rule, where they are expanded
1934 by the shell. For example, here is a rule to delete all the object files:
1942 @cindex @code{rm} (shell command)
1944 Wildcards are also useful in the prerequisites of a rule. With the
1945 following rule in the makefile, @samp{make print} will print all the
1946 @samp{.c} files that have changed since the last time you printed them:
1954 @cindex @code{print} target
1955 @cindex @code{lpr} (shell command)
1956 @cindex @code{touch} (shell command)
1958 This rule uses @file{print} as an empty target file; see @ref{Empty
1959 Targets, ,Empty Target Files to Record Events}. (The automatic variable
1960 @samp{$?} is used to print only those files that have changed; see
1961 @ref{Automatic Variables}.)@refill
1963 Wildcard expansion does not happen when you define a variable. Thus, if
1971 then the value of the variable @code{objects} is the actual string
1972 @samp{*.o}. However, if you use the value of @code{objects} in a
1973 target or prerequisite, wildcard expansion will take place there. If
1974 you use the value of @code{objects} in a recipe, the shell may perform
1975 wildcard expansion when the recipe runs. To set @code{objects} to the
1976 expansion, instead use:
1979 objects := $(wildcard *.o)
1983 @xref{Wildcard Function}.
1985 @node Wildcard Pitfall, Wildcard Function, Wildcard Examples, Wildcards
1986 @subsection Pitfalls of Using Wildcards
1987 @cindex wildcard pitfalls
1988 @cindex pitfalls of wildcards
1989 @cindex mistakes with wildcards
1990 @cindex errors with wildcards
1991 @cindex problems with wildcards
1993 Now here is an example of a naive way of using wildcard expansion, that
1994 does not do what you would intend. Suppose you would like to say that the
1995 executable file @file{foo} is made from all the object files in the
1996 directory, and you write this:
2002 cc -o foo $(CFLAGS) $(objects)
2006 The value of @code{objects} is the actual string @samp{*.o}. Wildcard
2007 expansion happens in the rule for @file{foo}, so that each @emph{existing}
2008 @samp{.o} file becomes a prerequisite of @file{foo} and will be recompiled if
2011 But what if you delete all the @samp{.o} files? When a wildcard matches
2012 no files, it is left as it is, so then @file{foo} will depend on the
2013 oddly-named file @file{*.o}. Since no such file is likely to exist,
2014 @code{make} will give you an error saying it cannot figure out how to
2015 make @file{*.o}. This is not what you want!
2017 Actually it is possible to obtain the desired result with wildcard
2018 expansion, but you need more sophisticated techniques, including the
2019 @code{wildcard} function and string substitution.
2021 @xref{Wildcard Function, ,The Function @code{wildcard}}.
2024 These are described in the following section.
2027 @cindex wildcards and MS-DOS/MS-Windows backslashes
2028 @cindex backslashes in pathnames and wildcard expansion
2030 Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to
2031 separate directories in pathnames, like so:
2037 This is equivalent to the Unix-style @file{c:/foo/bar/baz.c} (the
2038 @file{c:} part is the so-called drive letter). When @code{make} runs on
2039 these systems, it supports backslashes as well as the Unix-style forward
2040 slashes in pathnames. However, this support does @emph{not} include the
2041 wildcard expansion, where backslash is a quote character. Therefore,
2042 you @emph{must} use Unix-style slashes in these cases.
2045 @node Wildcard Function, , Wildcard Pitfall, Wildcards
2046 @subsection The Function @code{wildcard}
2049 Wildcard expansion happens automatically in rules. But wildcard expansion
2050 does not normally take place when a variable is set, or inside the
2051 arguments of a function. If you want to do wildcard expansion in such
2052 places, you need to use the @code{wildcard} function, like this:
2055 $(wildcard @var{pattern}@dots{})
2059 This string, used anywhere in a makefile, is replaced by a
2060 space-separated list of names of existing files that match one of the
2061 given file name patterns. If no existing file name matches a pattern,
2062 then that pattern is omitted from the output of the @code{wildcard}
2063 function. Note that this is different from how unmatched wildcards
2064 behave in rules, where they are used verbatim rather than ignored
2065 (@pxref{Wildcard Pitfall}).
2067 One use of the @code{wildcard} function is to get a list of all the C source
2068 files in a directory, like this:
2074 We can change the list of C source files into a list of object files by
2075 replacing the @samp{.c} suffix with @samp{.o} in the result, like this:
2078 $(patsubst %.c,%.o,$(wildcard *.c))
2082 (Here we have used another function, @code{patsubst}.
2083 @xref{Text Functions, ,Functions for String Substitution and Analysis}.)@refill
2085 Thus, a makefile to compile all C source files in the directory and then
2086 link them together could be written as follows:
2089 objects := $(patsubst %.c,%.o,$(wildcard *.c))
2092 cc -o foo $(objects)
2096 (This takes advantage of the implicit rule for compiling C programs, so
2097 there is no need to write explicit rules for compiling the files.
2098 @xref{Flavors, ,The Two Flavors of Variables}, for an explanation of
2099 @samp{:=}, which is a variant of @samp{=}.)
2101 @node Directory Search, Phony Targets, Wildcards, Rules
2102 @section Searching Directories for Prerequisites
2106 @cindex search path for prerequisites (@code{VPATH})
2107 @cindex directory search (@code{VPATH})
2109 For large systems, it is often desirable to put sources in a separate
2110 directory from the binaries. The @dfn{directory search} features of
2111 @code{make} facilitate this by searching several directories
2112 automatically to find a prerequisite. When you redistribute the files
2113 among directories, you do not need to change the individual rules,
2114 just the search paths.
2117 * General Search:: Specifying a search path that applies
2118 to every prerequisite.
2119 * Selective Search:: Specifying a search path
2120 for a specified class of names.
2121 * Search Algorithm:: When and how search paths are applied.
2122 * Recipes/Search:: How to write recipes that work together
2124 * Implicit/Search:: How search paths affect implicit rules.
2125 * Libraries/Search:: Directory search for link libraries.
2128 @node General Search, Selective Search, Directory Search, Directory Search
2129 @subsection @code{VPATH}: Search Path for All Prerequisites
2132 The value of the @code{make} variable @code{VPATH} specifies a list of
2133 directories that @code{make} should search. Most often, the
2134 directories are expected to contain prerequisite files that are not in the
2135 current directory; however, @code{make} uses @code{VPATH} as a search
2136 list for both prerequisites and targets of rules.
2138 Thus, if a file that is listed as a target or prerequisite does not exist
2139 in the current directory, @code{make} searches the directories listed in
2140 @code{VPATH} for a file with that name. If a file is found in one of
2141 them, that file may become the prerequisite (see below). Rules may then
2142 specify the names of files in the prerequisite list as if they all
2143 existed in the current directory. @xref{Recipes/Search, ,Writing Recipes with Directory Search}.
2145 In the @code{VPATH} variable, directory names are separated by colons or
2146 blanks. The order in which directories are listed is the order followed
2147 by @code{make} in its search. (On MS-DOS and MS-Windows, semi-colons
2148 are used as separators of directory names in @code{VPATH}, since the
2149 colon can be used in the pathname itself, after the drive letter.)
2154 VPATH = src:../headers
2158 specifies a path containing two directories, @file{src} and
2159 @file{../headers}, which @code{make} searches in that order.
2161 With this value of @code{VPATH}, the following rule,
2168 is interpreted as if it were written like this:
2175 assuming the file @file{foo.c} does not exist in the current directory but
2176 is found in the directory @file{src}.
2178 @node Selective Search, Search Algorithm, General Search, Directory Search
2179 @subsection The @code{vpath} Directive
2182 Similar to the @code{VPATH} variable, but more selective, is the
2183 @code{vpath} directive (note lower case), which allows you to specify a
2184 search path for a particular class of file names: those that match a
2185 particular pattern. Thus you can supply certain search directories for
2186 one class of file names and other directories (or none) for other file
2189 There are three forms of the @code{vpath} directive:
2192 @item vpath @var{pattern} @var{directories}
2193 Specify the search path @var{directories} for file names that match
2196 The search path, @var{directories}, is a list of directories to be
2197 searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or
2198 blanks, just like the search path used in the @code{VPATH} variable.
2200 @item vpath @var{pattern}
2201 Clear out the search path associated with @var{pattern}.
2203 @c Extra blank line makes sure this gets two lines.
2206 Clear all search paths previously specified with @code{vpath} directives.
2209 A @code{vpath} pattern is a string containing a @samp{%} character. The
2210 string must match the file name of a prerequisite that is being searched
2211 for, the @samp{%} character matching any sequence of zero or more
2212 characters (as in pattern rules; @pxref{Pattern Rules, ,Defining and
2213 Redefining Pattern Rules}). For example, @code{%.h} matches files that
2214 end in @code{.h}. (If there is no @samp{%}, the pattern must match the
2215 prerequisite exactly, which is not useful very often.)
2217 @cindex @code{%}, quoting in @code{vpath}
2218 @cindex @code{%}, quoting with @code{\} (backslash)
2219 @cindex @code{\} (backslash), to quote @code{%}
2220 @cindex backslash (@code{\}), to quote @code{%}
2221 @cindex quoting @code{%}, in @code{vpath}
2222 @samp{%} characters in a @code{vpath} directive's pattern can be quoted
2223 with preceding backslashes (@samp{\}). Backslashes that would otherwise
2224 quote @samp{%} characters can be quoted with more backslashes.
2225 Backslashes that quote @samp{%} characters or other backslashes are
2226 removed from the pattern before it is compared to file names. Backslashes
2227 that are not in danger of quoting @samp{%} characters go unmolested.@refill
2229 When a prerequisite fails to exist in the current directory, if the
2230 @var{pattern} in a @code{vpath} directive matches the name of the
2231 prerequisite file, then the @var{directories} in that directive are searched
2232 just like (and before) the directories in the @code{VPATH} variable.
2237 vpath %.h ../headers
2241 tells @code{make} to look for any prerequisite whose name ends in @file{.h}
2242 in the directory @file{../headers} if the file is not found in the current
2245 If several @code{vpath} patterns match the prerequisite file's name, then
2246 @code{make} processes each matching @code{vpath} directive one by one,
2247 searching all the directories mentioned in each directive. @code{make}
2248 handles multiple @code{vpath} directives in the order in which they
2249 appear in the makefile; multiple directives with the same pattern are
2250 independent of each other.
2264 will look for a file ending in @samp{.c} in @file{foo}, then
2265 @file{blish}, then @file{bar}, while
2275 will look for a file ending in @samp{.c} in @file{foo}, then
2276 @file{bar}, then @file{blish}.
2278 @node Search Algorithm, Recipes/Search, Selective Search, Directory Search
2279 @subsection How Directory Searches are Performed
2280 @cindex algorithm for directory search
2281 @cindex directory search algorithm
2283 When a prerequisite is found through directory search, regardless of type
2284 (general or selective), the pathname located may not be the one that
2285 @code{make} actually provides you in the prerequisite list. Sometimes
2286 the path discovered through directory search is thrown away.
2288 The algorithm @code{make} uses to decide whether to keep or abandon a
2289 path found via directory search is as follows:
2293 If a target file does not exist at the path specified in the makefile,
2294 directory search is performed.
2297 If the directory search is successful, that path is kept and this file
2298 is tentatively stored as the target.
2301 All prerequisites of this target are examined using this same method.
2304 After processing the prerequisites, the target may or may not need to be
2309 If the target does @emph{not} need to be rebuilt, the path to the file
2310 found during directory search is used for any prerequisite lists which
2311 contain this target. In short, if @code{make} doesn't need to rebuild
2312 the target then you use the path found via directory search.
2315 If the target @emph{does} need to be rebuilt (is out-of-date), the
2316 pathname found during directory search is @emph{thrown away}, and the
2317 target is rebuilt using the file name specified in the makefile. In
2318 short, if @code{make} must rebuild, then the target is rebuilt locally,
2319 not in the directory found via directory search.
2323 This algorithm may seem complex, but in practice it is quite often
2324 exactly what you want.
2326 @cindex traditional directory search (GPATH)
2327 @cindex directory search, traditional (GPATH)
2328 Other versions of @code{make} use a simpler algorithm: if the file does
2329 not exist, and it is found via directory search, then that pathname is
2330 always used whether or not the target needs to be built. Thus, if the
2331 target is rebuilt it is created at the pathname discovered during
2335 If, in fact, this is the behavior you want for some or all of your
2336 directories, you can use the @code{GPATH} variable to indicate this to
2339 @code{GPATH} has the same syntax and format as @code{VPATH} (that is, a
2340 space- or colon-delimited list of pathnames). If an out-of-date target
2341 is found by directory search in a directory that also appears in
2342 @code{GPATH}, then that pathname is not thrown away. The target is
2343 rebuilt using the expanded path.
2345 @node Recipes/Search, Implicit/Search, Search Algorithm, Directory Search
2346 @subsection Writing Recipes with Directory Search
2347 @cindex recipes, and directory search
2348 @cindex directory search (@code{VPATH}), and recipes
2350 When a prerequisite is found in another directory through directory search,
2351 this cannot change the recipe of the rule; they will execute as written.
2352 Therefore, you must write the recipe with care so that it will look for
2353 the prerequisite in the directory where @code{make} finds it.
2355 This is done with the @dfn{automatic variables} such as @samp{$^}
2356 (@pxref{Automatic Variables}).
2357 For instance, the value of @samp{$^} is a
2358 list of all the prerequisites of the rule, including the names of
2359 the directories in which they were found, and the value of
2360 @samp{$@@} is the target. Thus:@refill
2364 cc -c $(CFLAGS) $^ -o $@@
2368 (The variable @code{CFLAGS} exists so you can specify flags for C
2369 compilation by implicit rules; we use it here for consistency so it will
2370 affect all C compilations uniformly;
2371 @pxref{Implicit Variables, ,Variables Used by Implicit Rules}.)
2373 Often the prerequisites include header files as well, which you do not
2374 want to mention in the recipe. The automatic variable @samp{$<} is
2375 just the first prerequisite:
2378 VPATH = src:../headers
2379 foo.o : foo.c defs.h hack.h
2380 cc -c $(CFLAGS) $< -o $@@
2383 @node Implicit/Search, Libraries/Search, Recipes/Search, Directory Search
2384 @subsection Directory Search and Implicit Rules
2385 @cindex @code{VPATH}, and implicit rules
2386 @cindex directory search (@code{VPATH}), and implicit rules
2387 @cindex search path for prerequisites (@code{VPATH}), and implicit rules
2388 @cindex implicit rule, and directory search
2389 @cindex implicit rule, and @code{VPATH}
2390 @cindex rule, implicit, and directory search
2391 @cindex rule, implicit, and @code{VPATH}
2393 The search through the directories specified in @code{VPATH} or with
2394 @code{vpath} also happens during consideration of implicit rules
2395 (@pxref{Implicit Rules, ,Using Implicit Rules}).
2397 For example, when a file @file{foo.o} has no explicit rule, @code{make}
2398 considers implicit rules, such as the built-in rule to compile
2399 @file{foo.c} if that file exists. If such a file is lacking in the
2400 current directory, the appropriate directories are searched for it. If
2401 @file{foo.c} exists (or is mentioned in the makefile) in any of the
2402 directories, the implicit rule for C compilation is applied.
2404 The recipes of implicit rules normally use automatic variables as a
2405 matter of necessity; consequently they will use the file names found by
2406 directory search with no extra effort.
2408 @node Libraries/Search, , Implicit/Search, Directory Search
2409 @subsection Directory Search for Link Libraries
2410 @cindex link libraries, and directory search
2411 @cindex libraries for linking, directory search
2412 @cindex directory search (@code{VPATH}), and link libraries
2413 @cindex @code{VPATH}, and link libraries
2414 @cindex search path for prerequisites (@code{VPATH}), and link libraries
2415 @cindex @code{-l} (library search)
2416 @cindex link libraries, patterns matching
2417 @cindex @code{.LIBPATTERNS}, and link libraries
2418 @vindex .LIBPATTERNS
2420 Directory search applies in a special way to libraries used with the
2421 linker. This special feature comes into play when you write a prerequisite
2422 whose name is of the form @samp{-l@var{name}}. (You can tell something
2423 strange is going on here because the prerequisite is normally the name of a
2424 file, and the @emph{file name} of a library generally looks like
2425 @file{lib@var{name}.a}, not like @samp{-l@var{name}}.)@refill
2427 When a prerequisite's name has the form @samp{-l@var{name}}, @code{make}
2428 handles it specially by searching for the file @file{lib@var{name}.so} in
2429 the current directory, in directories specified by matching @code{vpath}
2430 search paths and the @code{VPATH} search path, and then in the
2431 directories @file{/lib}, @file{/usr/lib}, and @file{@var{prefix}/lib}
2432 (normally @file{/usr/local/lib}, but MS-DOS/MS-Windows versions of
2433 @code{make} behave as if @var{prefix} is defined to be the root of the
2434 DJGPP installation tree).
2436 If that file is not found, then the file @file{lib@var{name}.a} is
2437 searched for, in the same directories as above.
2439 For example, if there is a @file{/usr/lib/libcurses.a} library on your
2440 system (and no @file{/usr/lib/libcurses.so} file), then
2444 foo : foo.c -lcurses
2450 would cause the command @samp{cc foo.c /usr/lib/libcurses.a -o foo} to
2451 be executed when @file{foo} is older than @file{foo.c} or than
2452 @file{/usr/lib/libcurses.a}.@refill
2454 Although the default set of files to be searched for is
2455 @file{lib@var{name}.so} and @file{lib@var{name}.a}, this is customizable
2456 via the @code{.LIBPATTERNS} variable. Each word in the value of this
2457 variable is a pattern string. When a prerequisite like
2458 @samp{-l@var{name}} is seen, @code{make} will replace the percent in
2459 each pattern in the list with @var{name} and perform the above directory
2460 searches using that library filename. If no library is found, the next
2461 word in the list will be used.
2463 The default value for @code{.LIBPATTERNS} is @samp{lib%.so lib%.a},
2464 which provides the default behavior described above.
2466 You can turn off link library expansion completely by setting this
2467 variable to an empty value.
2469 @node Phony Targets, Force Targets, Directory Search, Rules
2470 @section Phony Targets
2471 @cindex phony targets
2472 @cindex targets, phony
2473 @cindex targets without a file
2475 A phony target is one that is not really the name of a file; rather it
2476 is just a name for a recipe to be executed when you make an explicit
2477 request. There are two reasons to use a phony target: to avoid a
2478 conflict with a file of the same name, and to improve performance.
2480 If you write a rule whose recipe will not create the target file, the
2481 recipe will be executed every time the target comes up for remaking.
2492 Because the @code{rm} command does not create a file named @file{clean},
2493 probably no such file will ever exist. Therefore, the @code{rm} command
2494 will be executed every time you say @samp{make clean}.
2495 @cindex @code{rm} (shell command)
2498 The phony target will cease to work if anything ever does create a file
2499 named @file{clean} in this directory. Since it has no prerequisites, the
2500 file @file{clean} would inevitably be considered up to date, and its
2501 recipe would not be executed. To avoid this problem, you can explicitly
2502 declare the target to be phony, using the special target @code{.PHONY}
2503 (@pxref{Special Targets, ,Special Built-in Target Names}) as follows:
2510 Once this is done, @samp{make clean} will run the recipe regardless of
2511 whether there is a file named @file{clean}.
2513 Since it knows that phony targets do not name actual files that could be
2514 remade from other files, @code{make} skips the implicit rule search for
2515 phony targets (@pxref{Implicit Rules}). This is why declaring a target
2516 phony is good for performance, even if you are not worried about the
2517 actual file existing.
2519 Thus, you first write the line that states that @code{clean} is a
2520 phony target, then you write the rule, like this:
2530 Another example of the usefulness of phony targets is in conjunction
2531 with recursive invocations of @code{make} (for more information, see
2532 @ref{Recursion, ,Recursive Use of @code{make}}). In this case the
2533 makefile will often contain a variable which lists a number of
2534 subdirectories to be built. One way to handle this is with one rule
2535 whose recipe is a shell loop over the subdirectories, like this:
2539 SUBDIRS = foo bar baz
2542 for dir in $(SUBDIRS); do \
2548 There are problems with this method, however. First, any error
2549 detected in a submake is ignored by this rule, so it will continue
2550 to build the rest of the directories even when one fails. This can be
2551 overcome by adding shell commands to note the error and exit, but then
2552 it will do so even if @code{make} is invoked with the @code{-k}
2553 option, which is unfortunate. Second, and perhaps more importantly,
2554 you cannot take advantage of @code{make}'s ability to build targets in
2555 parallel (@pxref{Parallel, ,Parallel Execution}), since there is only
2558 By declaring the subdirectories as phony targets (you must do this as
2559 the subdirectory obviously always exists; otherwise it won't be built)
2560 you can remove these problems:
2564 SUBDIRS = foo bar baz
2566 .PHONY: subdirs $(SUBDIRS)
2577 Here we've also declared that the @file{foo} subdirectory cannot be
2578 built until after the @file{baz} subdirectory is complete; this kind of
2579 relationship declaration is particularly important when attempting
2582 A phony target should not be a prerequisite of a real target file; if it
2583 is, its recipe will be run every time @code{make} goes to update that
2584 file. As long as a phony target is never a prerequisite of a real
2585 target, the phony target recipe will be executed only when the phony
2586 target is a specified goal (@pxref{Goals, ,Arguments to Specify the
2589 Phony targets can have prerequisites. When one directory contains multiple
2590 programs, it is most convenient to describe all of the programs in one
2591 makefile @file{./Makefile}. Since the target remade by default will be the
2592 first one in the makefile, it is common to make this a phony target named
2593 @samp{all} and give it, as prerequisites, all the individual programs. For
2597 all : prog1 prog2 prog3
2600 prog1 : prog1.o utils.o
2601 cc -o prog1 prog1.o utils.o
2606 prog3 : prog3.o sort.o utils.o
2607 cc -o prog3 prog3.o sort.o utils.o
2611 Now you can say just @samp{make} to remake all three programs, or
2612 specify as arguments the ones to remake (as in @samp{make prog1
2613 prog3}). Phoniness is not inherited: the prerequisites of a phony
2614 target are not themselves phony, unless explicitly declared to be so.
2616 When one phony target is a prerequisite of another, it serves as a subroutine
2617 of the other. For example, here @samp{make cleanall} will delete the
2618 object files, the difference files, and the file @file{program}:
2621 .PHONY: cleanall cleanobj cleandiff
2623 cleanall : cleanobj cleandiff
2633 @node Force Targets, Empty Targets, Phony Targets, Rules
2634 @section Rules without Recipes or Prerequisites
2635 @cindex force targets
2636 @cindex targets, force
2637 @cindex @code{FORCE}
2638 @cindex rule, no recipe or prerequisites
2640 If a rule has no prerequisites or recipe, and the target of the rule
2641 is a nonexistent file, then @code{make} imagines this target to have
2642 been updated whenever its rule is run. This implies that all targets
2643 depending on this one will always have their recipe run.
2645 An example will illustrate this:
2655 Here the target @samp{FORCE} satisfies the special conditions, so the
2656 target @file{clean} that depends on it is forced to run its recipe.
2657 There is nothing special about the name @samp{FORCE}, but that is one
2658 name commonly used this way.
2660 As you can see, using @samp{FORCE} this way has the same results as using
2661 @samp{.PHONY: clean}.
2663 Using @samp{.PHONY} is more explicit and more efficient. However,
2664 other versions of @code{make} do not support @samp{.PHONY}; thus
2665 @samp{FORCE} appears in many makefiles. @xref{Phony Targets}.
2667 @node Empty Targets, Special Targets, Force Targets, Rules
2668 @section Empty Target Files to Record Events
2669 @cindex empty targets
2670 @cindex targets, empty
2671 @cindex recording events with empty targets
2673 The @dfn{empty target} is a variant of the phony target; it is used to hold
2674 recipes for an action that you request explicitly from time to time.
2675 Unlike a phony target, this target file can really exist; but the file's
2676 contents do not matter, and usually are empty.
2678 The purpose of the empty target file is to record, with its
2679 last-modification time, when the rule's recipe was last executed. It
2680 does so because one of the commands in the recipe is a @code{touch}
2681 command to update the target file.
2683 The empty target file should have some prerequisites (otherwise it
2684 doesn't make sense). When you ask to remake the empty target, the
2685 recipe is executed if any prerequisite is more recent than the target;
2686 in other words, if a prerequisite has changed since the last time you
2687 remade the target. Here is an example:
2694 @cindex @code{print} target
2695 @cindex @code{lpr} (shell command)
2696 @cindex @code{touch} (shell command)
2699 With this rule, @samp{make print} will execute the @code{lpr} command if
2700 either source file has changed since the last @samp{make print}. The
2701 automatic variable @samp{$?} is used to print only those files that have
2702 changed (@pxref{Automatic Variables}).
2704 @node Special Targets, Multiple Targets, Empty Targets, Rules
2705 @section Special Built-in Target Names
2706 @cindex special targets
2707 @cindex built-in special targets
2708 @cindex targets, built-in special
2710 Certain names have special meanings if they appear as targets.
2716 The prerequisites of the special target @code{.PHONY} are considered to
2717 be phony targets. When it is time to consider such a target,
2718 @code{make} will run its recipe unconditionally, regardless of
2719 whether a file with that name exists or what its last-modification
2720 time is. @xref{Phony Targets, ,Phony Targets}.
2725 The prerequisites of the special target @code{.SUFFIXES} are the list
2726 of suffixes to be used in checking for suffix rules.
2727 @xref{Suffix Rules, , Old-Fashioned Suffix Rules}.
2732 The recipe specified for @code{.DEFAULT} is used for any target for
2733 which no rules are found (either explicit rules or implicit rules).
2734 @xref{Last Resort}. If a @code{.DEFAULT} recipe is specified, every
2735 file mentioned as a prerequisite, but not as a target in a rule, will have
2736 that recipe executed on its behalf. @xref{Implicit Rule Search,
2737 ,Implicit Rule Search Algorithm}.
2741 @cindex precious targets
2742 @cindex preserving with @code{.PRECIOUS}
2744 The targets which @code{.PRECIOUS} depends on are given the following
2745 special treatment: if @code{make} is killed or interrupted during the
2746 execution of their recipes, the target is not deleted.
2747 @xref{Interrupts, ,Interrupting or Killing @code{make}}. Also, if the
2748 target is an intermediate file, it will not be deleted after it is no
2749 longer needed, as is normally done. @xref{Chained Rules, ,Chains of
2750 Implicit Rules}. In this latter respect it overlaps with the
2751 @code{.SECONDARY} special target.
2753 You can also list the target pattern of an implicit rule (such as
2754 @samp{%.o}) as a prerequisite file of the special target @code{.PRECIOUS}
2755 to preserve intermediate files created by rules whose target patterns
2756 match that file's name.
2758 @findex .INTERMEDIATE
2760 @cindex intermediate targets, explicit
2762 The targets which @code{.INTERMEDIATE} depends on are treated as
2763 intermediate files. @xref{Chained Rules, ,Chains of Implicit Rules}.
2764 @code{.INTERMEDIATE} with no prerequisites has no effect.
2768 @cindex secondary targets
2769 @cindex preserving with @code{.SECONDARY}
2771 The targets which @code{.SECONDARY} depends on are treated as
2772 intermediate files, except that they are never automatically deleted.
2773 @xref{Chained Rules, ,Chains of Implicit Rules}.
2775 @code{.SECONDARY} with no prerequisites causes all targets to be treated
2776 as secondary (i.e., no target is removed because it is considered
2779 @findex .SECONDEXPANSION
2780 @item .SECONDEXPANSION
2782 If @code{.SECONDEXPANSION} is mentioned as a target anywhere in the
2783 makefile, then all prerequisite lists defined @emph{after} it appears
2784 will be expanded a second time after all makefiles have been read in.
2785 @xref{Secondary Expansion, ,Secondary Expansion}.
2787 @findex .DELETE_ON_ERROR
2788 @item .DELETE_ON_ERROR
2789 @cindex removing targets on failure
2791 If @code{.DELETE_ON_ERROR} is mentioned as a target anywhere in the
2792 makefile, then @code{make} will delete the target of a rule if it has
2793 changed and its recipe exits with a nonzero exit status, just as it
2794 does when it receives a signal. @xref{Errors, ,Errors in Recipes}.
2799 If you specify prerequisites for @code{.IGNORE}, then @code{make} will
2800 ignore errors in execution of the recipe for those particular files.
2801 The recipe for @code{.IGNORE} (if any) is ignored.
2803 If mentioned as a target with no prerequisites, @code{.IGNORE} says to
2804 ignore errors in execution of recipes for all files. This usage of
2805 @samp{.IGNORE} is supported only for historical compatibility. Since
2806 this affects every recipe in the makefile, it is not very useful; we
2807 recommend you use the more selective ways to ignore errors in specific
2808 recipes. @xref{Errors, ,Errors in Recipes}.
2810 @findex .LOW_RESOLUTION_TIME
2811 @item .LOW_RESOLUTION_TIME
2813 If you specify prerequisites for @code{.LOW_RESOLUTION_TIME},
2814 @command{make} assumes that these files are created by commands that
2815 generate low resolution time stamps. The recipe for the
2816 @code{.LOW_RESOLUTION_TIME} target are ignored.
2818 The high resolution file time stamps of many modern file systems
2819 lessen the chance of @command{make} incorrectly concluding that a file
2820 is up to date. Unfortunately, some hosts do not provide a way to set a
2821 high resolution file time stamp, so commands like @samp{cp -p} that
2822 explicitly set a file's time stamp must discard its subsecond part.
2823 If a file is created by such a command, you should list it as a
2824 prerequisite of @code{.LOW_RESOLUTION_TIME} so that @command{make}
2825 does not mistakenly conclude that the file is out of date. For
2830 .LOW_RESOLUTION_TIME: dst
2836 Since @samp{cp -p} discards the subsecond part of @file{src}'s time
2837 stamp, @file{dst} is typically slightly older than @file{src} even when
2838 it is up to date. The @code{.LOW_RESOLUTION_TIME} line causes
2839 @command{make} to consider @file{dst} to be up to date if its time stamp
2840 is at the start of the same second that @file{src}'s time stamp is in.
2842 Due to a limitation of the archive format, archive member time stamps
2843 are always low resolution. You need not list archive members as
2844 prerequisites of @code{.LOW_RESOLUTION_TIME}, as @command{make} does this
2850 If you specify prerequisites for @code{.SILENT}, then @code{make} will
2851 not print the recipe used to remake those particular files before
2852 executing them. The recipe for @code{.SILENT} is ignored.
2854 If mentioned as a target with no prerequisites, @code{.SILENT} says not
2855 to print any recipes before executing them. This usage of
2856 @samp{.SILENT} is supported only for historical compatibility. We
2857 recommend you use the more selective ways to silence specific recipes.
2858 @xref{Echoing, ,Recipe Echoing}. If you want to silence all recipes
2859 for a particular run of @code{make}, use the @samp{-s} or
2860 @w{@samp{--silent}} option (@pxref{Options Summary}).
2862 @findex .EXPORT_ALL_VARIABLES
2863 @item .EXPORT_ALL_VARIABLES
2865 Simply by being mentioned as a target, this tells @code{make} to
2866 export all variables to child processes by default.
2867 @xref{Variables/Recursion, ,Communicating Variables to a
2870 @findex .NOTPARALLEL
2872 @cindex parallel execution, overriding
2874 If @code{.NOTPARALLEL} is mentioned as a target, then this invocation
2875 of @code{make} will be run serially, even if the @samp{-j} option is
2876 given. Any recursively invoked @code{make} command will still run
2877 recipes in parallel (unless its makefile also contains this target).
2878 Any prerequisites on this target are ignored.
2881 Any defined implicit rule suffix also counts as a special target if it
2882 appears as a target, and so does the concatenation of two suffixes, such
2883 as @samp{.c.o}. These targets are suffix rules, an obsolete way of
2884 defining implicit rules (but a way still widely used). In principle, any
2885 target name could be special in this way if you break it in two and add
2886 both pieces to the suffix list. In practice, suffixes normally begin with
2887 @samp{.}, so these special target names also begin with @samp{.}.
2888 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
2890 @node Multiple Targets, Multiple Rules, Special Targets, Rules
2891 @section Multiple Targets in a Rule
2892 @cindex multiple targets
2893 @cindex several targets in a rule
2894 @cindex targets, multiple
2895 @cindex rule, with multiple targets
2897 A rule with multiple targets is equivalent to writing many rules, each with
2898 one target, and all identical aside from that. The same recipe applies to
2899 all the targets, but its effect may vary because you can substitute the
2900 actual target name into the recipe using @samp{$@@}. The rule contributes
2901 the same prerequisites to all the targets also.
2903 This is useful in two cases.
2907 You want just prerequisites, no recipe. For example:
2910 kbd.o command.o files.o: command.h
2914 gives an additional prerequisite to each of the three object files
2918 Similar recipes work for all the targets. The recipes do not need
2919 to be absolutely identical, since the automatic variable @samp{$@@}
2920 can be used to substitute the particular target to be remade into the
2921 commands (@pxref{Automatic Variables}). For example:
2925 bigoutput littleoutput : text.g
2926 generate text.g -$(subst output,,$@@) > $@@
2936 generate text.g -big > bigoutput
2937 littleoutput : text.g
2938 generate text.g -little > littleoutput
2942 Here we assume the hypothetical program @code{generate} makes two
2943 types of output, one if given @samp{-big} and one if given
2945 @xref{Text Functions, ,Functions for String Substitution and Analysis},
2946 for an explanation of the @code{subst} function.
2949 Suppose you would like to vary the prerequisites according to the
2950 target, much as the variable @samp{$@@} allows you to vary the recipe.
2951 You cannot do this with multiple targets in an ordinary rule, but you
2952 can do it with a @dfn{static pattern rule}. @xref{Static Pattern,
2953 ,Static Pattern Rules}.
2955 @node Multiple Rules, Static Pattern, Multiple Targets, Rules
2956 @section Multiple Rules for One Target
2957 @cindex multiple rules for one target
2958 @cindex several rules for one target
2959 @cindex rule, multiple for one target
2960 @cindex target, multiple rules for one
2962 One file can be the target of several rules. All the prerequisites
2963 mentioned in all the rules are merged into one list of prerequisites for
2964 the target. If the target is older than any prerequisite from any rule,
2965 the recipe is executed.
2967 There can only be one recipe to be executed for a file. If more than
2968 one rule gives a recipe for the same file, @code{make} uses the last
2969 one given and prints an error message. (As a special case, if the
2970 file's name begins with a dot, no error message is printed. This odd
2971 behavior is only for compatibility with other implementations of
2972 @code{make}@dots{} you should avoid using it). Occasionally it is
2973 useful to have the same target invoke multiple recipes which are
2974 defined in different parts of your makefile; you can use
2975 @dfn{double-colon rules} (@pxref{Double-Colon}) for this.
2977 An extra rule with just prerequisites can be used to give a few extra
2978 prerequisites to many files at once. For example, makefiles often
2979 have a variable, such as @code{objects}, containing a list of all the
2980 compiler output files in the system being made. An easy way to say
2981 that all of them must be recompiled if @file{config.h} changes is to
2982 write the following:
2985 objects = foo.o bar.o
2987 bar.o : defs.h test.h
2988 $(objects) : config.h
2991 This could be inserted or taken out without changing the rules that really
2992 specify how to make the object files, making it a convenient form to use if
2993 you wish to add the additional prerequisite intermittently.
2995 Another wrinkle is that the additional prerequisites could be
2996 specified with a variable that you set with a command line argument to
2997 @code{make} (@pxref{Overriding, ,Overriding Variables}). For example,
3002 $(objects) : $(extradeps)
3007 means that the command @samp{make extradeps=foo.h} will consider
3008 @file{foo.h} as a prerequisite of each object file, but plain @samp{make}
3011 If none of the explicit rules for a target has a recipe, then @code{make}
3012 searches for an applicable implicit rule to find one
3013 @pxref{Implicit Rules, ,Using Implicit Rules}).
3015 @node Static Pattern, Double-Colon, Multiple Rules, Rules
3016 @section Static Pattern Rules
3017 @cindex static pattern rule
3018 @cindex rule, static pattern
3019 @cindex pattern rules, static (not implicit)
3020 @cindex varying prerequisites
3021 @cindex prerequisites, varying (static pattern)
3023 @dfn{Static pattern rules} are rules which specify multiple targets and
3024 construct the prerequisite names for each target based on the target name.
3025 They are more general than ordinary rules with multiple targets because the
3026 targets do not have to have identical prerequisites. Their prerequisites must
3027 be @emph{analogous}, but not necessarily @emph{identical}.
3030 * Static Usage:: The syntax of static pattern rules.
3031 * Static versus Implicit:: When are they better than implicit rules?
3034 @node Static Usage, Static versus Implicit, Static Pattern, Static Pattern
3035 @subsection Syntax of Static Pattern Rules
3036 @cindex static pattern rule, syntax of
3037 @cindex pattern rules, static, syntax of
3039 Here is the syntax of a static pattern rule:
3042 @var{targets} @dots{}: @var{target-pattern}: @var{prereq-patterns} @dots{}
3048 The @var{targets} list specifies the targets that the rule applies to.
3049 The targets can contain wildcard characters, just like the targets of
3050 ordinary rules (@pxref{Wildcards, ,Using Wildcard Characters in File
3053 @cindex target pattern, static (not implicit)
3055 The @var{target-pattern} and @var{prereq-patterns} say how to compute the
3056 prerequisites of each target. Each target is matched against the
3057 @var{target-pattern} to extract a part of the target name, called the
3058 @dfn{stem}. This stem is substituted into each of the @var{prereq-patterns}
3059 to make the prerequisite names (one from each @var{prereq-pattern}).
3061 Each pattern normally contains the character @samp{%} just once. When the
3062 @var{target-pattern} matches a target, the @samp{%} can match any part of
3063 the target name; this part is called the @dfn{stem}. The rest of the
3064 pattern must match exactly. For example, the target @file{foo.o} matches
3065 the pattern @samp{%.o}, with @samp{foo} as the stem. The targets
3066 @file{foo.c} and @file{foo.out} do not match that pattern.@refill
3068 @cindex prerequisite pattern, static (not implicit)
3069 The prerequisite names for each target are made by substituting the stem
3070 for the @samp{%} in each prerequisite pattern. For example, if one
3071 prerequisite pattern is @file{%.c}, then substitution of the stem
3072 @samp{foo} gives the prerequisite name @file{foo.c}. It is legitimate
3073 to write a prerequisite pattern that does not contain @samp{%}; then this
3074 prerequisite is the same for all targets.
3076 @cindex @code{%}, quoting in static pattern
3077 @cindex @code{%}, quoting with @code{\} (backslash)
3078 @cindex @code{\} (backslash), to quote @code{%}
3079 @cindex backslash (@code{\}), to quote @code{%}
3080 @cindex quoting @code{%}, in static pattern
3081 @samp{%} characters in pattern rules can be quoted with preceding
3082 backslashes (@samp{\}). Backslashes that would otherwise quote @samp{%}
3083 characters can be quoted with more backslashes. Backslashes that quote
3084 @samp{%} characters or other backslashes are removed from the pattern
3085 before it is compared to file names or has a stem substituted into it.
3086 Backslashes that are not in danger of quoting @samp{%} characters go
3087 unmolested. For example, the pattern @file{the\%weird\\%pattern\\} has
3088 @samp{the%weird\} preceding the operative @samp{%} character, and
3089 @samp{pattern\\} following it. The final two backslashes are left alone
3090 because they cannot affect any @samp{%} character.@refill
3092 Here is an example, which compiles each of @file{foo.o} and @file{bar.o}
3093 from the corresponding @file{.c} file:
3097 objects = foo.o bar.o
3101 $(objects): %.o: %.c
3102 $(CC) -c $(CFLAGS) $< -o $@@
3107 Here @samp{$<} is the automatic variable that holds the name of the
3108 prerequisite and @samp{$@@} is the automatic variable that holds the name
3109 of the target; see @ref{Automatic Variables}.
3111 Each target specified must match the target pattern; a warning is issued
3112 for each target that does not. If you have a list of files, only some of
3113 which will match the pattern, you can use the @code{filter} function to
3114 remove nonmatching file names (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):
3117 files = foo.elc bar.o lose.o
3119 $(filter %.o,$(files)): %.o: %.c
3120 $(CC) -c $(CFLAGS) $< -o $@@
3121 $(filter %.elc,$(files)): %.elc: %.el
3122 emacs -f batch-byte-compile $<
3126 In this example the result of @samp{$(filter %.o,$(files))} is
3127 @file{bar.o lose.o}, and the first static pattern rule causes each of
3128 these object files to be updated by compiling the corresponding C source
3129 file. The result of @w{@samp{$(filter %.elc,$(files))}} is
3130 @file{foo.elc}, so that file is made from @file{foo.el}.@refill
3132 Another example shows how to use @code{$*} in static pattern rules:
3133 @vindex $*@r{, and static pattern}
3137 bigoutput littleoutput : %output : text.g
3138 generate text.g -$* > $@@
3143 When the @code{generate} command is run, @code{$*} will expand to the
3144 stem, either @samp{big} or @samp{little}.
3146 @node Static versus Implicit, , Static Usage, Static Pattern
3147 @subsection Static Pattern Rules versus Implicit Rules
3148 @cindex rule, static pattern versus implicit
3149 @cindex static pattern rule, versus implicit
3151 A static pattern rule has much in common with an implicit rule defined as a
3152 pattern rule (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).
3153 Both have a pattern for the target and patterns for constructing the
3154 names of prerequisites. The difference is in how @code{make} decides
3155 @emph{when} the rule applies.
3157 An implicit rule @emph{can} apply to any target that matches its pattern,
3158 but it @emph{does} apply only when the target has no recipe otherwise
3159 specified, and only when the prerequisites can be found. If more than one
3160 implicit rule appears applicable, only one applies; the choice depends on
3163 By contrast, a static pattern rule applies to the precise list of targets
3164 that you specify in the rule. It cannot apply to any other target and it
3165 invariably does apply to each of the targets specified. If two conflicting
3166 rules apply, and both have recipes, that's an error.
3168 The static pattern rule can be better than an implicit rule for these
3173 You may wish to override the usual implicit rule for a few
3174 files whose names cannot be categorized syntactically but
3175 can be given in an explicit list.
3178 If you cannot be sure of the precise contents of the directories
3179 you are using, you may not be sure which other irrelevant files
3180 might lead @code{make} to use the wrong implicit rule. The choice
3181 might depend on the order in which the implicit rule search is done.
3182 With static pattern rules, there is no uncertainty: each rule applies
3183 to precisely the targets specified.
3186 @node Double-Colon, Automatic Prerequisites, Static Pattern, Rules
3187 @section Double-Colon Rules
3188 @cindex double-colon rules
3189 @cindex rule, double-colon (@code{::})
3190 @cindex multiple rules for one target (@code{::})
3191 @cindex @code{::} rules (double-colon)
3193 @dfn{Double-colon} rules are explicit rules written with @samp{::}
3194 instead of @samp{:} after the target names. They are handled
3195 differently from ordinary rules when the same target appears in more
3196 than one rule. Pattern rules with double-colons have an entirely
3197 different meaning (@pxref{Match-Anything Rules}).
3199 When a target appears in multiple rules, all the rules must be the same
3200 type: all ordinary, or all double-colon. If they are double-colon, each
3201 of them is independent of the others. Each double-colon rule's recipe
3202 is executed if the target is older than any prerequisites of that rule.
3203 If there are no prerequisites for that rule, its recipe is always
3204 executed (even if the target already exists). This can result in
3205 executing none, any, or all of the double-colon rules.
3207 Double-colon rules with the same target are in fact completely separate
3208 from one another. Each double-colon rule is processed individually, just
3209 as rules with different targets are processed.
3211 The double-colon rules for a target are executed in the order they appear
3212 in the makefile. However, the cases where double-colon rules really make
3213 sense are those where the order of executing the recipes would not matter.
3215 Double-colon rules are somewhat obscure and not often very useful; they
3216 provide a mechanism for cases in which the method used to update a target
3217 differs depending on which prerequisite files caused the update, and such
3220 Each double-colon rule should specify a recipe; if it does not, an
3221 implicit rule will be used if one applies.
3222 @xref{Implicit Rules, ,Using Implicit Rules}.
3224 @node Automatic Prerequisites, , Double-Colon, Rules
3225 @section Generating Prerequisites Automatically
3226 @cindex prerequisites, automatic generation
3227 @cindex automatic generation of prerequisites
3228 @cindex generating prerequisites automatically
3230 In the makefile for a program, many of the rules you need to write often
3231 say only that some object file depends on some header
3232 file. For example, if @file{main.c} uses @file{defs.h} via an
3233 @code{#include}, you would write:
3240 You need this rule so that @code{make} knows that it must remake
3241 @file{main.o} whenever @file{defs.h} changes. You can see that for a
3242 large program you would have to write dozens of such rules in your
3243 makefile. And, you must always be very careful to update the makefile
3244 every time you add or remove an @code{#include}.
3245 @cindex @code{#include}
3247 @cindex @code{-M} (to compiler)
3248 To avoid this hassle, most modern C compilers can write these rules for
3249 you, by looking at the @code{#include} lines in the source files.
3250 Usually this is done with the @samp{-M} option to the compiler.
3251 For example, the command:
3258 generates the output:
3261 main.o : main.c defs.h
3265 Thus you no longer have to write all those rules yourself.
3266 The compiler will do it for you.
3268 Note that such a prerequisite constitutes mentioning @file{main.o} in a
3269 makefile, so it can never be considered an intermediate file by implicit
3270 rule search. This means that @code{make} won't ever remove the file
3271 after using it; @pxref{Chained Rules, ,Chains of Implicit Rules}.
3273 @cindex @code{make depend}
3274 With old @code{make} programs, it was traditional practice to use this
3275 compiler feature to generate prerequisites on demand with a command like
3276 @samp{make depend}. That command would create a file @file{depend}
3277 containing all the automatically-generated prerequisites; then the
3278 makefile could use @code{include} to read them in (@pxref{Include}).
3280 In GNU @code{make}, the feature of remaking makefiles makes this
3281 practice obsolete---you need never tell @code{make} explicitly to
3282 regenerate the prerequisites, because it always regenerates any makefile
3283 that is out of date. @xref{Remaking Makefiles}.
3285 The practice we recommend for automatic prerequisite generation is to have
3286 one makefile corresponding to each source file. For each source file
3287 @file{@var{name}.c} there is a makefile @file{@var{name}.d} which lists
3288 what files the object file @file{@var{name}.o} depends on. That way
3289 only the source files that have changed need to be rescanned to produce
3290 the new prerequisites.
3292 Here is the pattern rule to generate a file of prerequisites (i.e., a makefile)
3293 called @file{@var{name}.d} from a C source file called @file{@var{name}.c}:
3298 @@set -e; rm -f $@@; \
3299 $(CC) -M $(CPPFLAGS) $< > $@@.$$$$; \
3300 sed 's,\($*\)\.o[ :]*,\1.o $@@ : ,g' < $@@.$$$$ > $@@; \
3306 @xref{Pattern Rules}, for information on defining pattern rules. The
3307 @samp{-e} flag to the shell causes it to exit immediately if the
3308 @code{$(CC)} command (or any other command) fails (exits with a
3310 @cindex @code{-e} (shell flag)
3312 @cindex @code{-MM} (to GNU compiler)
3313 With the GNU C compiler, you may wish to use the @samp{-MM} flag instead
3314 of @samp{-M}. This omits prerequisites on system header files.
3315 @xref{Preprocessor Options, , Options Controlling the Preprocessor,
3316 gcc.info, Using GNU CC}, for details.
3318 @cindex @code{sed} (shell command)
3319 The purpose of the @code{sed} command is to translate (for example):
3322 main.o : main.c defs.h
3329 main.o main.d : main.c defs.h
3334 This makes each @samp{.d} file depend on all the source and header files
3335 that the corresponding @samp{.o} file depends on. @code{make} then
3336 knows it must regenerate the prerequisites whenever any of the source or
3337 header files changes.
3339 Once you've defined the rule to remake the @samp{.d} files,
3340 you then use the @code{include} directive to read them all in.
3341 @xref{Include}. For example:
3345 sources = foo.c bar.c
3347 include $(sources:.c=.d)
3352 (This example uses a substitution variable reference to translate the
3353 list of source files @samp{foo.c bar.c} into a list of prerequisite
3354 makefiles, @samp{foo.d bar.d}. @xref{Substitution Refs}, for full
3355 information on substitution references.) Since the @samp{.d} files are
3356 makefiles like any others, @code{make} will remake them as necessary
3357 with no further work from you. @xref{Remaking Makefiles}.
3359 Note that the @samp{.d} files contain target definitions; you should
3360 be sure to place the @code{include} directive @emph{after} the first,
3361 default goal in your makefiles or run the risk of having a random
3362 object file become the default goal.
3363 @xref{How Make Works}.
3365 @node Recipes, Using Variables, Rules, Top
3366 @chapter Writing Recipes in Rules
3368 @cindex recipes, how to write
3369 @cindex writing recipes
3371 The recipe of a rule consists of one or more shell command lines to
3372 be executed, one at a time, in the order they appear. Typically, the
3373 result of executing these commands is that the target of the rule is
3376 Users use many different shell programs, but recipes in makefiles are
3377 always interpreted by @file{/bin/sh} unless the makefile specifies
3378 otherwise. @xref{Execution, ,Command Execution}.
3381 * Recipe Syntax:: Recipe syntax features and pitfalls.
3382 * Echoing:: How to control when recipes are echoed.
3383 * Execution:: How recipes are executed.
3384 * Parallel:: How recipes can be executed in parallel.
3385 * Errors:: What happens after a recipe execution error.
3386 * Interrupts:: What happens when a recipe is interrupted.
3387 * Recursion:: Invoking @code{make} from makefiles.
3388 * Canned Recipes:: Defining canned recipes.
3389 * Empty Recipes:: Defining useful, do-nothing recipes.
3392 @node Recipe Syntax, Echoing, Recipes, Recipes
3393 @section Recipe Syntax
3394 @cindex recipe syntax
3395 @cindex syntax of recipe
3397 Makefiles have the unusual property that there are really two distinct
3398 syntaxes in one file. Most of the makefile uses @code{make} syntax
3399 (@pxref{Makefiles, ,Writing Makefiles}). However, recipes are meant
3400 to be interpreted by the shell and so they are written using shell
3401 syntax. The @code{make} program does not try to understand shell
3402 syntax: it performs only a very few specific translations on the
3403 content of the recipe before handing it to the shell.
3405 Each line in the recipe must start with a tab (or the first character
3406 in the value of the @code{.CMDPREFIX} variable; @pxref{Special
3407 Variables}), except that the first recipe line may be attached to the
3408 target-and-prerequisites line with a semicolon in between. @emph{Any}
3409 line in the makefile that begins with a tab and appears in a ``rule
3410 context'' (that is, after a rule has been started until another rule
3411 or variable definition) will be considered part of a recipe for that
3412 rule. Blank lines and lines of just comments may appear among the
3413 recipe lines; they are ignored.
3415 Some consequences of these rules include:
3419 A blank line that begins with a tab is not blank: it's an empty
3420 recipe (@pxref{Empty Recipes}).
3422 @cindex comments, in recipes
3423 @cindex recipes, comments in
3424 @cindex @code{#} (comments), in recipes
3426 A comment in a recipe is not a @code{make} comment; it will be
3427 passed to the shell as-is. Whether the shell treats it as a comment
3428 or not depends on your shell.
3431 A variable definition in a ``rule context'' which is indented by a tab
3432 as the first character on the line, will be considered part of a
3433 recipe, not a @code{make} variable definition, and passed to the
3437 A conditional expression (@code{ifdef}, @code{ifeq},
3438 etc. @pxref{Conditional Syntax, ,Syntax of Conditionals}) in a ``rule
3439 context'' which is indented by a tab as the first character on the
3440 line, will be considered part of a recipe and be passed to the shell.
3445 * Splitting Lines:: Breaking long recipe lines for readability.
3446 * Variables in Recipes:: Using @code{make} variables in recipes.
3449 @node Splitting Lines, Variables in Recipes, Recipe Syntax, Recipe Syntax
3450 @subsection Splitting Recipe Lines
3451 @cindex recipes, splitting
3452 @cindex splitting recipes
3453 @cindex recipes, backslash (@code{\}) in
3454 @cindex recipes, quoting newlines in
3455 @cindex backslash (@code{\}), in recipes
3456 @cindex @code{\} (backslash), in recipes
3457 @cindex quoting newline, in recipes
3458 @cindex newline, quoting, in recipes
3460 One of the few ways in which @code{make} does interpret recipes is
3461 checking for a backslash just before the newline. As in normal
3462 makefile syntax, a single logical recipe line can be split into
3463 multiple physical lines in the makefile by placing a backslash before
3464 each newline. A sequence of lines like this is considered a single
3465 recipe line, and one instance of the shell will be invoked to run it.
3467 However, in contrast to how they are treated in other places in a
3468 makefile, backslash-newline pairs are @emph{not} removed from the
3469 recipe. Both the backslash and the newline characters are preserved
3470 and passed to the shell. How the backslash-newline is interpreted
3471 depends on your shell. If the first character of the next line after
3472 the backslash-newline is the recipe prefix character (a tab by
3473 default; @pxref{Special Variables}), then that character (and only
3474 that character) is removed. Whitespace is never added to the recipe.
3476 For example, the recipe for the all target in this makefile:
3493 consists of four separate shell commands where the output is:
3504 As a more complex example, this makefile:
3508 all : ; @@echo 'hello \
3509 world' ; echo "hello \
3515 will invoke one shell with a command of:
3520 world' ; echo "hello \
3526 which, according to shell quoting rules, will yield the following output:
3537 Notice how the backslash/newline pair was removed inside the string
3538 quoted with double quotes (@code{"@dots{}"}), but not from the string
3539 quoted with single quotes (@code{'@dots{}'}). This is the way the
3540 default shell (@file{/bin/sh}) handles backslash/newline pairs. If
3541 you specify a different shell in your makefiles it may treat them
3544 Sometimes you want to split a long line inside of single quotes, but
3545 you don't want the backslash-newline to appear in the quoted content.
3546 This is often the case when passing scripts to languages such as Perl,
3547 where extraneous backslashes inside the script can change its meaning
3548 or even be a syntax error. One simple way of handling this is to
3549 place the quoted string, or even the entire command, into a
3550 @code{make} variable then use the variable in the recipe. In this
3551 situation the newline quoting rules for makefiles will be used, and
3552 the backslash-newline will be removed. If we rewrite our example
3553 above using this method:
3560 all : ; @@echo $(HELLO)
3565 we will get output like this:
3573 If you like, you can also use target-specific variables
3574 (@pxref{Target-specific, ,Target-specific Variable Values}) to obtain
3575 a tighter correspondence between the variable and the recipe that
3578 @node Variables in Recipes, , Splitting Lines, Recipe Syntax
3579 @subsection Using Variables in Recipes
3580 @cindex variable references in recipes
3581 @cindex recipes, using variables in
3583 The other way in which @code{make} processes recipes is by expanding
3584 any variable references in them (@pxref{Reference,Basics of Variable
3585 References}). This occurs after make has finished reading all the
3586 makefiles and the target is determined to be out of date; so, the
3587 recipes for targets which are not rebuilt are never expanded.
3589 Variable and function references in recipes have identical syntax and
3590 semantics to references elsewhere in the makefile. They also have the
3591 same quoting rules: if you want a dollar sign to appear in your
3592 recipe, you must double it (@samp{$$}). For shells like the default
3593 shell, that use dollar signs to introduce variables, it's important to
3594 keep clear in your mind whether the variable you want to reference is
3595 a @code{make} variable (use a single dollar sign) or a shell variable
3596 (use two dollar signs). For example:
3600 LIST = one two three
3602 for i in $(LIST); do \
3609 results in the following command being passed to the shell:
3613 for i in one two three; do \
3620 which generates the expected result:
3630 @node Echoing, Execution, Recipe Syntax, Recipes
3631 @section Recipe Echoing
3632 @cindex echoing of recipes
3633 @cindex silent operation
3634 @cindex @code{@@} (in recipes)
3635 @cindex recipes, echoing
3636 @cindex printing of recipes
3638 Normally @code{make} prints each line of the recipe before it is
3639 executed. We call this @dfn{echoing} because it gives the appearance
3640 that you are typing the lines yourself.
3642 When a line starts with @samp{@@}, the echoing of that line is suppressed.
3643 The @samp{@@} is discarded before the line is passed to the shell.
3644 Typically you would use this for a command whose only effect is to print
3645 something, such as an @code{echo} command to indicate progress through
3649 @@echo About to make distribution files
3653 @cindex @code{--just-print}
3654 @cindex @code{--dry-run}
3655 @cindex @code{--recon}
3656 When @code{make} is given the flag @samp{-n} or @samp{--just-print} it
3657 only echoes most recipes, without executing them. @xref{Options
3658 Summary, ,Summary of Options}. In this case even the recipe lines
3659 starting with @samp{@@} are printed. This flag is useful for finding
3660 out which recipes @code{make} thinks are necessary without actually
3664 @cindex @code{--silent}
3665 @cindex @code{--quiet}
3667 The @samp{-s} or @samp{--silent}
3668 flag to @code{make} prevents all echoing, as if all recipes
3669 started with @samp{@@}. A rule in the makefile for the special target
3670 @code{.SILENT} without prerequisites has the same effect
3671 (@pxref{Special Targets, ,Special Built-in Target Names}).
3672 @code{.SILENT} is essentially obsolete since @samp{@@} is more flexible.@refill
3674 @node Execution, Parallel, Echoing, Recipes
3675 @section Recipe Execution
3676 @cindex recipe, execution
3677 @cindex execution, of recipes
3678 @vindex @code{SHELL} @r{(recipe execution)}
3680 When it is time to execute recipes to update a target, they are
3681 executed by invoking a new subshell for each line of the recipe. (In
3682 practice, @code{make} may take shortcuts that do not affect the
3685 @cindex @code{cd} (shell command)
3686 @cindex shell variables, setting in recipes
3687 @cindex recipes setting shell variables
3688 @strong{Please note:} this implies that setting shell variables and
3689 invoking shell commands such as @code{cd} that set a context local to
3690 each process will not affect the following lines in the recipe.@footnote{On
3691 MS-DOS, the value of current working directory is @strong{global}, so
3692 changing it @emph{will} affect the following recipe lines on those
3693 systems.} If you want to use @code{cd} to affect the next statement,
3694 put both statements in a single recipe line. Then @code{make} will
3695 invoke one shell to run the entire line, and the shell will execute
3696 the statements in sequence. For example:
3700 cd $(@@D) && gobble $(@@F) > ../$@@
3704 Here we use the shell AND operator (@code{&&}) so that if the
3705 @code{cd} command fails, the script will fail without trying to invoke
3706 the @code{gobble} command in the wrong directory, which could cause
3707 problems (in this case it would certainly cause @file{../foo} to be
3708 truncated, at least).
3711 * Choosing the Shell:: How @code{make} chooses the shell used
3715 @node Choosing the Shell, , Execution, Execution
3716 @subsection Choosing the Shell
3717 @cindex shell, choosing the
3718 @cindex @code{SHELL}, value of
3721 The program used as the shell is taken from the variable @code{SHELL}.
3722 If this variable is not set in your makefile, the program
3723 @file{/bin/sh} is used as the shell.
3725 @cindex environment, @code{SHELL} in
3726 Unlike most variables, the variable @code{SHELL} is never set from the
3727 environment. This is because the @code{SHELL} environment variable is
3728 used to specify your personal choice of shell program for interactive
3729 use. It would be very bad for personal choices like this to affect the
3730 functioning of makefiles. @xref{Environment, ,Variables from the
3733 Furthermore, when you do set @code{SHELL} in your makefile that value
3734 is @emph{not} exported in the environment to recipe lines that
3735 @code{make} invokes. Instead, the value inherited from the user's
3736 environment, if any, is exported. You can override this behavior by
3737 explicitly exporting @code{SHELL} (@pxref{Variables/Recursion,
3738 ,Communicating Variables to a Sub-@code{make}}), forcing it to be
3739 passed in the environment to recipe lines.
3741 @vindex @code{MAKESHELL} @r{(MS-DOS alternative to @code{SHELL})}
3742 However, on MS-DOS and MS-Windows the value of @code{SHELL} in the
3743 environment @strong{is} used, since on those systems most users do not
3744 set this variable, and therefore it is most likely set specifically to
3745 be used by @code{make}. On MS-DOS, if the setting of @code{SHELL} is
3746 not suitable for @code{make}, you can set the variable
3747 @code{MAKESHELL} to the shell that @code{make} should use; if set it
3748 will be used as the shell instead of the value of @code{SHELL}.
3750 @subsubheading Choosing a Shell in DOS and Windows
3751 @cindex shell, in DOS and Windows
3752 @cindex DOS, choosing a shell in
3753 @cindex Windows, choosing a shell in
3755 Choosing a shell in MS-DOS and MS-Windows is much more complex than on
3759 On MS-DOS, if @code{SHELL} is not set, the value of the variable
3760 @code{COMSPEC} (which is always set) is used instead.
3762 @cindex @code{SHELL}, MS-DOS specifics
3763 The processing of lines that set the variable @code{SHELL} in Makefiles
3764 is different on MS-DOS. The stock shell, @file{command.com}, is
3765 ridiculously limited in its functionality and many users of @code{make}
3766 tend to install a replacement shell. Therefore, on MS-DOS, @code{make}
3767 examines the value of @code{SHELL}, and changes its behavior based on
3768 whether it points to a Unix-style or DOS-style shell. This allows
3769 reasonable functionality even if @code{SHELL} points to
3772 If @code{SHELL} points to a Unix-style shell, @code{make} on MS-DOS
3773 additionally checks whether that shell can indeed be found; if not, it
3774 ignores the line that sets @code{SHELL}. In MS-DOS, GNU @code{make}
3775 searches for the shell in the following places:
3779 In the precise place pointed to by the value of @code{SHELL}. For
3780 example, if the makefile specifies @samp{SHELL = /bin/sh}, @code{make}
3781 will look in the directory @file{/bin} on the current drive.
3784 In the current directory.
3787 In each of the directories in the @code{PATH} variable, in order.
3791 In every directory it examines, @code{make} will first look for the
3792 specific file (@file{sh} in the example above). If this is not found,
3793 it will also look in that directory for that file with one of the known
3794 extensions which identify executable files. For example @file{.exe},
3795 @file{.com}, @file{.bat}, @file{.btm}, @file{.sh}, and some others.
3797 If any of these attempts is successful, the value of @code{SHELL} will
3798 be set to the full pathname of the shell as found. However, if none of
3799 these is found, the value of @code{SHELL} will not be changed, and thus
3800 the line that sets it will be effectively ignored. This is so
3801 @code{make} will only support features specific to a Unix-style shell if
3802 such a shell is actually installed on the system where @code{make} runs.
3804 Note that this extended search for the shell is limited to the cases
3805 where @code{SHELL} is set from the Makefile; if it is set in the
3806 environment or command line, you are expected to set it to the full
3807 pathname of the shell, exactly as things are on Unix.
3809 The effect of the above DOS-specific processing is that a Makefile that
3810 contains @samp{SHELL = /bin/sh} (as many Unix makefiles do), will work
3811 on MS-DOS unaltered if you have e.g.@: @file{sh.exe} installed in some
3812 directory along your @code{PATH}.
3814 @node Parallel, Errors, Execution, Recipes
3815 @section Parallel Execution
3816 @cindex recipes, execution in parallel
3817 @cindex parallel execution
3818 @cindex execution, in parallel
3821 @cindex @code{--jobs}
3823 GNU @code{make} knows how to execute several recipes at once.
3824 Normally, @code{make} will execute only one recipe at a time, waiting
3825 for it to finish before executing the next. However, the @samp{-j} or
3826 @samp{--jobs} option tells @code{make} to execute many recipes
3827 simultaneously. You can inhibit parallelism in a particular makefile
3828 with the @code{.NOTPARALLEL} pseudo-target (@pxref{Special
3829 Targets,Special Built-in Target Names}).@refill
3831 On MS-DOS, the @samp{-j} option has no effect, since that system doesn't
3832 support multi-processing.
3834 If the @samp{-j} option is followed by an integer, this is the number of
3835 recipes to execute at once; this is called the number of @dfn{job slots}.
3836 If there is nothing looking like an integer after the @samp{-j} option,
3837 there is no limit on the number of job slots. The default number of job
3838 slots is one, which means serial execution (one thing at a time).
3840 One unpleasant consequence of running several recipes simultaneously is
3841 that output generated by the recipes appears whenever each recipe
3842 sends it, so messages from different recipes may be interspersed.
3844 Another problem is that two processes cannot both take input from the
3845 same device; so to make sure that only one recipe tries to take input
3846 from the terminal at once, @code{make} will invalidate the standard
3847 input streams of all but one running recipe. This means that
3848 attempting to read from standard input will usually be a fatal error (a
3849 @samp{Broken pipe} signal) for most child processes if there are
3852 @cindex standard input
3854 It is unpredictable which recipe will have a valid standard input stream
3855 (which will come from the terminal, or wherever you redirect the standard
3856 input of @code{make}). The first recipe run will always get it first, and
3857 the first recipe started after that one finishes will get it next, and so
3860 We will change how this aspect of @code{make} works if we find a better
3861 alternative. In the mean time, you should not rely on any recipe using
3862 standard input at all if you are using the parallel execution feature; but
3863 if you are not using this feature, then standard input works normally in
3866 Finally, handling recursive @code{make} invocations raises issues. For
3867 more information on this, see
3868 @ref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
3870 If a recipe fails (is killed by a signal or exits with a nonzero
3871 status), and errors are not ignored for that recipe
3872 (@pxref{Errors, ,Errors in Recipes}),
3873 the remaining recipe lines to remake the same target will not be run.
3874 If a recipe fails and the @samp{-k} or @samp{--keep-going}
3875 option was not given
3876 (@pxref{Options Summary, ,Summary of Options}),
3877 @code{make} aborts execution. If make
3878 terminates for any reason (including a signal) with child processes
3879 running, it waits for them to finish before actually exiting.@refill
3881 @cindex load average
3882 @cindex limiting jobs based on load
3883 @cindex jobs, limiting based on load
3884 @cindex @code{-l} (load average)
3885 @cindex @code{--max-load}
3886 @cindex @code{--load-average}
3887 When the system is heavily loaded, you will probably want to run fewer jobs
3888 than when it is lightly loaded. You can use the @samp{-l} option to tell
3889 @code{make} to limit the number of jobs to run at once, based on the load
3890 average. The @samp{-l} or @samp{--max-load}
3891 option is followed by a floating-point number. For
3899 will not let @code{make} start more than one job if the load average is
3900 above 2.5. The @samp{-l} option with no following number removes the
3901 load limit, if one was given with a previous @samp{-l} option.@refill
3903 More precisely, when @code{make} goes to start up a job, and it already has
3904 at least one job running, it checks the current load average; if it is not
3905 lower than the limit given with @samp{-l}, @code{make} waits until the load
3906 average goes below that limit, or until all the other jobs finish.
3908 By default, there is no load limit.
3910 @node Errors, Interrupts, Parallel, Recipes
3911 @section Errors in Recipes
3912 @cindex errors (in recipes)
3913 @cindex recipes, errors in
3914 @cindex exit status (errors)
3916 After each shell invocation returns, @code{make} looks at its exit
3917 status. If the shell completed successfully (the exit status is
3918 zero), the next line in the recipe is executed in a new shell; after
3919 the last line is finished, the rule is finished.
3921 If there is an error (the exit status is nonzero), @code{make} gives up on
3922 the current rule, and perhaps on all rules.
3924 Sometimes the failure of a certain recipe line does not indicate a problem.
3925 For example, you may use the @code{mkdir} command to ensure that a
3926 directory exists. If the directory already exists, @code{mkdir} will
3927 report an error, but you probably want @code{make} to continue regardless.
3929 @cindex @code{-} (in recipes)
3930 To ignore errors in a recipe line, write a @samp{-} at the beginning
3931 of the line's text (after the initial tab). The @samp{-} is discarded
3932 before the line is passed to the shell for execution.
3942 @cindex @code{rm} (shell command)
3945 This causes @code{make} to continue even if @code{rm} is unable to
3949 @cindex @code{--ignore-errors}
3951 When you run @code{make} with the @samp{-i} or @samp{--ignore-errors}
3952 flag, errors are ignored in all recipes of all rules. A rule in the
3953 makefile for the special target @code{.IGNORE} has the same effect, if
3954 there are no prerequisites. These ways of ignoring errors are obsolete
3955 because @samp{-} is more flexible.
3957 When errors are to be ignored, because of either a @samp{-} or the
3958 @samp{-i} flag, @code{make} treats an error return just like success,
3959 except that it prints out a message that tells you the status code
3960 the shell exited with, and says that the error has been ignored.
3962 When an error happens that @code{make} has not been told to ignore,
3963 it implies that the current target cannot be correctly remade, and neither
3964 can any other that depends on it either directly or indirectly. No further
3965 recipes will be executed for these targets, since their preconditions
3966 have not been achieved.
3970 @cindex @code{--keep-going}
3971 Normally @code{make} gives up immediately in this circumstance, returning a
3972 nonzero status. However, if the @samp{-k} or @samp{--keep-going}
3973 flag is specified, @code{make}
3974 continues to consider the other prerequisites of the pending targets,
3975 remaking them if necessary, before it gives up and returns nonzero status.
3976 For example, after an error in compiling one object file, @samp{make -k}
3977 will continue compiling other object files even though it already knows
3978 that linking them will be impossible. @xref{Options Summary, ,Summary of Options}.
3980 The usual behavior assumes that your purpose is to get the specified
3981 targets up to date; once @code{make} learns that this is impossible, it
3982 might as well report the failure immediately. The @samp{-k} option says
3983 that the real purpose is to test as many of the changes made in the
3984 program as possible, perhaps to find several independent problems so
3985 that you can correct them all before the next attempt to compile. This
3986 is why Emacs' @code{compile} command passes the @samp{-k} flag by
3988 @cindex Emacs (@code{M-x compile})
3990 @findex .DELETE_ON_ERROR
3991 @cindex deletion of target files
3992 @cindex removal of target files
3993 @cindex target, deleting on error
3994 Usually when a recipe line fails, if it has changed the target file at all,
3995 the file is corrupted and cannot be used---or at least it is not
3996 completely updated. Yet the file's time stamp says that it is now up to
3997 date, so the next time @code{make} runs, it will not try to update that
3998 file. The situation is just the same as when the shell is killed by a
3999 signal; @pxref{Interrupts}. So generally the right thing to do is to
4000 delete the target file if the recipe fails after beginning to change
4001 the file. @code{make} will do this if @code{.DELETE_ON_ERROR} appears
4002 as a target. This is almost always what you want @code{make} to do, but
4003 it is not historical practice; so for compatibility, you must explicitly
4006 @node Interrupts, Recursion, Errors, Recipes
4007 @section Interrupting or Killing @code{make}
4010 @cindex deletion of target files
4011 @cindex removal of target files
4012 @cindex target, deleting on interrupt
4013 @cindex killing (interruption)
4015 If @code{make} gets a fatal signal while a shell is executing, it may
4016 delete the target file that the recipe was supposed to update. This is
4017 done if the target file's last-modification time has changed since
4018 @code{make} first checked it.
4020 The purpose of deleting the target is to make sure that it is remade from
4021 scratch when @code{make} is next run. Why is this? Suppose you type
4022 @kbd{Ctrl-c} while a compiler is running, and it has begun to write an
4023 object file @file{foo.o}. The @kbd{Ctrl-c} kills the compiler, resulting
4024 in an incomplete file whose last-modification time is newer than the source
4025 file @file{foo.c}. But @code{make} also receives the @kbd{Ctrl-c} signal
4026 and deletes this incomplete file. If @code{make} did not do this, the next
4027 invocation of @code{make} would think that @file{foo.o} did not require
4028 updating---resulting in a strange error message from the linker when it
4029 tries to link an object file half of which is missing.
4032 You can prevent the deletion of a target file in this way by making the
4033 special target @code{.PRECIOUS} depend on it. Before remaking a target,
4034 @code{make} checks to see whether it appears on the prerequisites of
4035 @code{.PRECIOUS}, and thereby decides whether the target should be deleted
4036 if a signal happens. Some reasons why you might do this are that the
4037 target is updated in some atomic fashion, or exists only to record a
4038 modification-time (its contents do not matter), or must exist at all
4039 times to prevent other sorts of trouble.
4041 @node Recursion, Canned Recipes, Interrupts, Recipes
4042 @section Recursive Use of @code{make}
4044 @cindex subdirectories, recursion for
4046 Recursive use of @code{make} means using @code{make} as a command in a
4047 makefile. This technique is useful when you want separate makefiles for
4048 various subsystems that compose a larger system. For example, suppose you
4049 have a subdirectory @file{subdir} which has its own makefile, and you would
4050 like the containing directory's makefile to run @code{make} on the
4051 subdirectory. You can do it by writing this:
4055 cd subdir && $(MAKE)
4059 or, equivalently, this (@pxref{Options Summary, ,Summary of Options}):
4066 @cindex @code{--directory}
4068 You can write recursive @code{make} commands just by copying this example,
4069 but there are many things to know about how they work and why, and about
4070 how the sub-@code{make} relates to the top-level @code{make}. You may
4071 also find it useful to declare targets that invoke recursive
4072 @code{make} commands as @samp{.PHONY} (for more discussion on when
4073 this is useful, see @ref{Phony Targets}).
4075 @vindex @code{CURDIR}
4076 For your convenience, when GNU @code{make} starts (after it has
4077 processed any @code{-C} options) it sets the variable @code{CURDIR} to
4078 the pathname of the current working directory. This value is never
4079 touched by @code{make} again: in particular note that if you include
4080 files from other directories the value of @code{CURDIR} does not
4081 change. The value has the same precedence it would have if it were
4082 set in the makefile (by default, an environment variable @code{CURDIR}
4083 will not override this value). Note that setting this variable has no
4084 impact on the operation of @code{make} (it does not cause @code{make}
4085 to change its working directory, for example).
4088 * MAKE Variable:: The special effects of using @samp{$(MAKE)}.
4089 * Variables/Recursion:: How to communicate variables to a sub-@code{make}.
4090 * Options/Recursion:: How to communicate options to a sub-@code{make}.
4091 * -w Option:: How the @samp{-w} or @samp{--print-directory} option
4092 helps debug use of recursive @code{make} commands.
4095 @node MAKE Variable, Variables/Recursion, Recursion, Recursion
4096 @subsection How the @code{MAKE} Variable Works
4098 @cindex recursion, and @code{MAKE} variable
4100 Recursive @code{make} commands should always use the variable @code{MAKE},
4101 not the explicit command name @samp{make}, as shown here:
4106 cd subdir && $(MAKE)
4110 The value of this variable is the file name with which @code{make} was
4111 invoked. If this file name was @file{/bin/make}, then the recipe executed
4112 is @samp{cd subdir && /bin/make}. If you use a special version of
4113 @code{make} to run the top-level makefile, the same special version will be
4114 executed for recursive invocations.
4115 @cindex @code{cd} (shell command)
4117 @cindex +, and recipes
4118 As a special feature, using the variable @code{MAKE} in the recipe of
4119 a rule alters the effects of the @samp{-t} (@samp{--touch}), @samp{-n}
4120 (@samp{--just-print}), or @samp{-q} (@w{@samp{--question}}) option.
4121 Using the @code{MAKE} variable has the same effect as using a @samp{+}
4122 character at the beginning of the recipe line. @xref{Instead of
4123 Execution, ,Instead of Executing the Recipes}. This special feature
4124 is only enabled if the @code{MAKE} variable appears directly in the
4125 recipe: it does not apply if the @code{MAKE} variable is referenced
4126 through expansion of another variable. In the latter case you must
4127 use the @samp{+} token to get these special effects.@refill
4129 Consider the command @samp{make -t} in the above example. (The
4130 @samp{-t} option marks targets as up to date without actually running
4131 any recipes; see @ref{Instead of Execution}.) Following the usual
4132 definition of @samp{-t}, a @samp{make -t} command in the example would
4133 create a file named @file{subsystem} and do nothing else. What you
4134 really want it to do is run @samp{@w{cd subdir &&} @w{make -t}}; but
4135 that would require executing the recipe, and @samp{-t} says not to
4136 execute recipes.@refill
4137 @cindex @code{-t}, and recursion
4138 @cindex recursion, and @code{-t}
4139 @cindex @code{--touch}, and recursion
4141 The special feature makes this do what you want: whenever a recipe
4142 line of a rule contains the variable @code{MAKE}, the flags @samp{-t},
4143 @samp{-n} and @samp{-q} do not apply to that line. Recipe lines
4144 containing @code{MAKE} are executed normally despite the presence of a
4145 flag that causes most recipes not to be run. The usual
4146 @code{MAKEFLAGS} mechanism passes the flags to the sub-@code{make}
4147 (@pxref{Options/Recursion, ,Communicating Options to a
4148 Sub-@code{make}}), so your request to touch the files, or print the
4149 recipes, is propagated to the subsystem.@refill
4151 @node Variables/Recursion, Options/Recursion, MAKE Variable, Recursion
4152 @subsection Communicating Variables to a Sub-@code{make}
4153 @cindex sub-@code{make}
4154 @cindex environment, and recursion
4155 @cindex exporting variables
4156 @cindex variables, environment
4157 @cindex variables, exporting
4158 @cindex recursion, and environment
4159 @cindex recursion, and variables
4161 Variable values of the top-level @code{make} can be passed to the
4162 sub-@code{make} through the environment by explicit request. These
4163 variables are defined in the sub-@code{make} as defaults, but do not
4164 override what is specified in the makefile used by the sub-@code{make}
4165 makefile unless you use the @samp{-e} switch (@pxref{Options Summary,
4166 ,Summary of Options}).@refill
4168 To pass down, or @dfn{export}, a variable, @code{make} adds the
4169 variable and its value to the environment for running each line of the
4170 recipe. The sub-@code{make}, in turn, uses the environment to
4171 initialize its table of variable values. @xref{Environment,
4172 ,Variables from the Environment}.
4174 Except by explicit request, @code{make} exports a variable only if it
4175 is either defined in the environment initially or set on the command
4176 line, and if its name consists only of letters, numbers, and underscores.
4177 Some shells cannot cope with environment variable names consisting of
4178 characters other than letters, numbers, and underscores.
4180 @cindex SHELL, exported value
4181 The value of the @code{make} variable @code{SHELL} is not exported.
4182 Instead, the value of the @code{SHELL} variable from the invoking
4183 environment is passed to the sub-@code{make}. You can force
4184 @code{make} to export its value for @code{SHELL} by using the
4185 @code{export} directive, described below. @xref{Choosing the Shell}.
4187 The special variable @code{MAKEFLAGS} is always exported (unless you
4188 unexport it). @code{MAKEFILES} is exported if you set it to anything.
4190 @code{make} automatically passes down variable values that were defined
4191 on the command line, by putting them in the @code{MAKEFLAGS} variable.
4193 See the next section.
4196 @xref{Options/Recursion}.
4199 Variables are @emph{not} normally passed down if they were created by
4200 default by @code{make} (@pxref{Implicit Variables, ,Variables Used by
4201 Implicit Rules}). The sub-@code{make} will define these for
4205 If you want to export specific variables to a sub-@code{make}, use the
4206 @code{export} directive, like this:
4209 export @var{variable} @dots{}
4214 If you want to @emph{prevent} a variable from being exported, use the
4215 @code{unexport} directive, like this:
4218 unexport @var{variable} @dots{}
4222 In both of these forms, the arguments to @code{export} and
4223 @code{unexport} are expanded, and so could be variables or functions
4224 which expand to a (list of) variable names to be (un)exported.
4226 As a convenience, you can define a variable and export it at the same
4230 export @var{variable} = value
4234 has the same result as:
4237 @var{variable} = value
4238 export @var{variable}
4245 export @var{variable} := value
4249 has the same result as:
4252 @var{variable} := value
4253 export @var{variable}
4259 export @var{variable} += value
4266 @var{variable} += value
4267 export @var{variable}
4271 @xref{Appending, ,Appending More Text to Variables}.
4273 You may notice that the @code{export} and @code{unexport} directives
4274 work in @code{make} in the same way they work in the shell, @code{sh}.
4276 If you want all variables to be exported by default, you can use
4277 @code{export} by itself:
4284 This tells @code{make} that variables which are not explicitly mentioned
4285 in an @code{export} or @code{unexport} directive should be exported.
4286 Any variable given in an @code{unexport} directive will still @emph{not}
4287 be exported. If you use @code{export} by itself to export variables by
4288 default, variables whose names contain characters other than
4289 alphanumerics and underscores will not be exported unless specifically
4290 mentioned in an @code{export} directive.@refill
4292 @findex .EXPORT_ALL_VARIABLES
4293 The behavior elicited by an @code{export} directive by itself was the
4294 default in older versions of GNU @code{make}. If your makefiles depend
4295 on this behavior and you want to be compatible with old versions of
4296 @code{make}, you can write a rule for the special target
4297 @code{.EXPORT_ALL_VARIABLES} instead of using the @code{export} directive.
4298 This will be ignored by old @code{make}s, while the @code{export}
4299 directive will cause a syntax error.@refill
4300 @cindex compatibility in exporting
4302 Likewise, you can use @code{unexport} by itself to tell @code{make}
4303 @emph{not} to export variables by default. Since this is the default
4304 behavior, you would only need to do this if @code{export} had been used
4305 by itself earlier (in an included makefile, perhaps). You
4306 @strong{cannot} use @code{export} and @code{unexport} by themselves to
4307 have variables exported for some recipes and not for others. The last
4308 @code{export} or @code{unexport} directive that appears by itself
4309 determines the behavior for the entire run of @code{make}.@refill
4312 @cindex recursion, level of
4313 As a special feature, the variable @code{MAKELEVEL} is changed when it
4314 is passed down from level to level. This variable's value is a string
4315 which is the depth of the level as a decimal number. The value is
4316 @samp{0} for the top-level @code{make}; @samp{1} for a sub-@code{make},
4317 @samp{2} for a sub-sub-@code{make}, and so on. The incrementation
4318 happens when @code{make} sets up the environment for a recipe.@refill
4320 The main use of @code{MAKELEVEL} is to test it in a conditional
4321 directive (@pxref{Conditionals, ,Conditional Parts of Makefiles}); this
4322 way you can write a makefile that behaves one way if run recursively and
4323 another way if run directly by you.@refill
4326 You can use the variable @code{MAKEFILES} to cause all sub-@code{make}
4327 commands to use additional makefiles. The value of @code{MAKEFILES} is
4328 a whitespace-separated list of file names. This variable, if defined in
4329 the outer-level makefile, is passed down through the environment; then
4330 it serves as a list of extra makefiles for the sub-@code{make} to read
4331 before the usual or specified ones. @xref{MAKEFILES Variable, ,The
4332 Variable @code{MAKEFILES}}.@refill
4334 @node Options/Recursion, -w Option, Variables/Recursion, Recursion
4335 @subsection Communicating Options to a Sub-@code{make}
4336 @cindex options, and recursion
4337 @cindex recursion, and options
4340 Flags such as @samp{-s} and @samp{-k} are passed automatically to the
4341 sub-@code{make} through the variable @code{MAKEFLAGS}. This variable is
4342 set up automatically by @code{make} to contain the flag letters that
4343 @code{make} received. Thus, if you do @w{@samp{make -ks}} then
4344 @code{MAKEFLAGS} gets the value @samp{ks}.@refill
4346 As a consequence, every sub-@code{make} gets a value for @code{MAKEFLAGS}
4347 in its environment. In response, it takes the flags from that value and
4348 processes them as if they had been given as arguments.
4349 @xref{Options Summary, ,Summary of Options}.
4351 @cindex command line variable definitions, and recursion
4352 @cindex variables, command line, and recursion
4353 @cindex recursion, and command line variable definitions
4354 Likewise variables defined on the command line are passed to the
4355 sub-@code{make} through @code{MAKEFLAGS}. Words in the value of
4356 @code{MAKEFLAGS} that contain @samp{=}, @code{make} treats as variable
4357 definitions just as if they appeared on the command line.
4358 @xref{Overriding, ,Overriding Variables}.
4360 @cindex @code{-C}, and recursion
4361 @cindex @code{-f}, and recursion
4362 @cindex @code{-o}, and recursion
4363 @cindex @code{-W}, and recursion
4364 @cindex @code{--directory}, and recursion
4365 @cindex @code{--file}, and recursion
4366 @cindex @code{--old-file}, and recursion
4367 @cindex @code{--assume-old}, and recursion
4368 @cindex @code{--assume-new}, and recursion
4369 @cindex @code{--new-file}, and recursion
4370 @cindex recursion, and @code{-C}
4371 @cindex recursion, and @code{-f}
4372 @cindex recursion, and @code{-o}
4373 @cindex recursion, and @code{-W}
4374 The options @samp{-C}, @samp{-f}, @samp{-o}, and @samp{-W} are not put
4375 into @code{MAKEFLAGS}; these options are not passed down.@refill
4377 @cindex @code{-j}, and recursion
4378 @cindex @code{--jobs}, and recursion
4379 @cindex recursion, and @code{-j}
4380 @cindex job slots, and recursion
4381 The @samp{-j} option is a special case (@pxref{Parallel, ,Parallel Execution}).
4382 If you set it to some numeric value @samp{N} and your operating system
4383 supports it (most any UNIX system will; others typically won't), the
4384 parent @code{make} and all the sub-@code{make}s will communicate to
4385 ensure that there are only @samp{N} jobs running at the same time
4386 between them all. Note that any job that is marked recursive
4387 (@pxref{Instead of Execution, ,Instead of Executing Recipes})
4388 doesn't count against the total jobs (otherwise we could get @samp{N}
4389 sub-@code{make}s running and have no slots left over for any real work!)
4391 If your operating system doesn't support the above communication, then
4392 @samp{-j 1} is always put into @code{MAKEFLAGS} instead of the value you
4393 specified. This is because if the @w{@samp{-j}} option were passed down
4394 to sub-@code{make}s, you would get many more jobs running in parallel
4395 than you asked for. If you give @samp{-j} with no numeric argument,
4396 meaning to run as many jobs as possible in parallel, this is passed
4397 down, since multiple infinities are no more than one.@refill
4399 If you do not want to pass the other flags down, you must change the
4400 value of @code{MAKEFLAGS}, like this:
4404 cd subdir && $(MAKE) MAKEFLAGS=
4407 @vindex MAKEOVERRIDES
4408 The command line variable definitions really appear in the variable
4409 @code{MAKEOVERRIDES}, and @code{MAKEFLAGS} contains a reference to this
4410 variable. If you do want to pass flags down normally, but don't want to
4411 pass down the command line variable definitions, you can reset
4412 @code{MAKEOVERRIDES} to empty, like this:
4419 @cindex Arg list too long
4421 This is not usually useful to do. However, some systems have a small
4422 fixed limit on the size of the environment, and putting so much
4423 information into the value of @code{MAKEFLAGS} can exceed it. If you
4424 see the error message @samp{Arg list too long}, this may be the problem.
4427 (For strict compliance with POSIX.2, changing @code{MAKEOVERRIDES} does
4428 not affect @code{MAKEFLAGS} if the special target @samp{.POSIX} appears
4429 in the makefile. You probably do not care about this.)
4432 A similar variable @code{MFLAGS} exists also, for historical
4433 compatibility. It has the same value as @code{MAKEFLAGS} except that it
4434 does not contain the command line variable definitions, and it always
4435 begins with a hyphen unless it is empty (@code{MAKEFLAGS} begins with a
4436 hyphen only when it begins with an option that has no single-letter
4437 version, such as @samp{--warn-undefined-variables}). @code{MFLAGS} was
4438 traditionally used explicitly in the recursive @code{make} command, like
4443 cd subdir && $(MAKE) $(MFLAGS)
4447 but now @code{MAKEFLAGS} makes this usage redundant. If you want your
4448 makefiles to be compatible with old @code{make} programs, use this
4449 technique; it will work fine with more modern @code{make} versions too.
4451 @cindex setting options from environment
4452 @cindex options, setting from environment
4453 @cindex setting options in makefiles
4454 @cindex options, setting in makefiles
4455 The @code{MAKEFLAGS} variable can also be useful if you want to have
4456 certain options, such as @samp{-k} (@pxref{Options Summary, ,Summary of
4457 Options}), set each time you run @code{make}. You simply put a value for
4458 @code{MAKEFLAGS} in your environment. You can also set @code{MAKEFLAGS} in
4459 a makefile, to specify additional flags that should also be in effect for
4460 that makefile. (Note that you cannot use @code{MFLAGS} this way. That
4461 variable is set only for compatibility; @code{make} does not interpret a
4462 value you set for it in any way.)
4464 When @code{make} interprets the value of @code{MAKEFLAGS} (either from the
4465 environment or from a makefile), it first prepends a hyphen if the value
4466 does not already begin with one. Then it chops the value into words
4467 separated by blanks, and parses these words as if they were options given
4468 on the command line (except that @samp{-C}, @samp{-f}, @samp{-h},
4469 @samp{-o}, @samp{-W}, and their long-named versions are ignored; and there
4470 is no error for an invalid option).
4472 If you do put @code{MAKEFLAGS} in your environment, you should be sure not
4473 to include any options that will drastically affect the actions of
4474 @code{make} and undermine the purpose of makefiles and of @code{make}
4475 itself. For instance, the @samp{-t}, @samp{-n}, and @samp{-q} options, if
4476 put in one of these variables, could have disastrous consequences and would
4477 certainly have at least surprising and probably annoying effects.@refill
4479 @node -w Option, , Options/Recursion, Recursion
4480 @subsection The @samp{--print-directory} Option
4481 @cindex directories, printing them
4482 @cindex printing directories
4483 @cindex recursion, and printing directories
4485 If you use several levels of recursive @code{make} invocations, the
4486 @samp{-w} or @w{@samp{--print-directory}} option can make the output a
4487 lot easier to understand by showing each directory as @code{make}
4488 starts processing it and as @code{make} finishes processing it. For
4489 example, if @samp{make -w} is run in the directory @file{/u/gnu/make},
4490 @code{make} will print a line of the form:@refill
4493 make: Entering directory `/u/gnu/make'.
4497 before doing anything else, and a line of the form:
4500 make: Leaving directory `/u/gnu/make'.
4504 when processing is completed.
4506 @cindex @code{-C}, and @code{-w}
4507 @cindex @code{--directory}, and @code{--print-directory}
4508 @cindex recursion, and @code{-w}
4509 @cindex @code{-w}, and @code{-C}
4510 @cindex @code{-w}, and recursion
4511 @cindex @code{--print-directory}, and @code{--directory}
4512 @cindex @code{--print-directory}, and recursion
4513 @cindex @code{--no-print-directory}
4514 @cindex @code{--print-directory}, disabling
4515 @cindex @code{-w}, disabling
4516 Normally, you do not need to specify this option because @samp{make}
4517 does it for you: @samp{-w} is turned on automatically when you use the
4518 @samp{-C} option, and in sub-@code{make}s. @code{make} will not
4519 automatically turn on @samp{-w} if you also use @samp{-s}, which says to
4520 be silent, or if you use @samp{--no-print-directory} to explicitly
4523 @node Canned Recipes, Empty Recipes, Recursion, Recipes
4524 @section Defining Canned Recipes
4525 @cindex canned recipes
4526 @cindex recipes, canned
4527 @cindex sequences of commands
4528 @cindex commands, sequences of
4530 When the same sequence of commands is useful in making various
4531 targets, you can define it as a canned sequence with the @code{define}
4532 directive, and refer to the canned sequence from the recipes for those
4533 targets. The canned sequence is actually a variable, so the name must
4534 not conflict with other variable names.
4536 Here is an example of defining a canned recipe:
4540 yacc $(firstword $^)
4547 Here @code{run-yacc} is the name of the variable being defined;
4548 @code{endef} marks the end of the definition; the lines in between are the
4549 commands. The @code{define} directive does not expand variable references
4550 and function calls in the canned sequence; the @samp{$} characters,
4551 parentheses, variable names, and so on, all become part of the value of the
4552 variable you are defining.
4553 @xref{Multi-Line, ,Defining Multi-Line Variables},
4554 for a complete explanation of @code{define}.
4556 The first command in this example runs Yacc on the first prerequisite of
4557 whichever rule uses the canned sequence. The output file from Yacc is
4558 always named @file{y.tab.c}. The second command moves the output to the
4559 rule's target file name.
4561 To use the canned sequence, substitute the variable into the recipe of a
4562 rule. You can substitute it like any other variable
4563 (@pxref{Reference, ,Basics of Variable References}).
4564 Because variables defined by @code{define} are recursively expanded
4565 variables, all the variable references you wrote inside the @code{define}
4566 are expanded now. For example:
4574 @samp{foo.y} will be substituted for the variable @samp{$^} when it occurs in
4575 @code{run-yacc}'s value, and @samp{foo.c} for @samp{$@@}.@refill
4577 This is a realistic example, but this particular one is not needed in
4578 practice because @code{make} has an implicit rule to figure out these
4579 commands based on the file names involved
4580 (@pxref{Implicit Rules, ,Using Implicit Rules}).
4582 @cindex @@, and @code{define}
4583 @cindex -, and @code{define}
4584 @cindex +, and @code{define}
4585 In recipe execution, each line of a canned sequence is treated just as
4586 if the line appeared on its own in the rule, preceded by a tab. In
4587 particular, @code{make} invokes a separate subshell for each line. You
4588 can use the special prefix characters that affect command lines
4589 (@samp{@@}, @samp{-}, and @samp{+}) on each line of a canned sequence.
4590 @xref{Recipes, ,Writing Recipes in Rules}.
4591 For example, using this canned sequence:
4595 @@echo "frobnicating target $@@"
4596 frob-step-1 $< -o $@@-step-1
4597 frob-step-2 $@@-step-1 -o $@@
4602 @code{make} will not echo the first line, the @code{echo} command.
4603 But it @emph{will} echo the following two recipe lines.
4605 On the other hand, prefix characters on the recipe line that refers to
4606 a canned sequence apply to every line in the sequence. So the rule:
4614 does not echo @emph{any} recipe lines.
4615 (@xref{Echoing, ,Recipe Echoing}, for a full explanation of @samp{@@}.)
4617 @node Empty Recipes, , Canned Recipes, Recipes
4618 @section Using Empty Recipes
4619 @cindex empty recipes
4620 @cindex recipes, empty
4622 It is sometimes useful to define recipes which do nothing. This is done
4623 simply by giving a recipe that consists of nothing but whitespace. For
4631 defines an empty recipe for @file{target}. You could also use a line
4632 beginning with a recipe prefix character to define an empty recipe,
4633 but this would be confusing because such a line looks empty.
4635 @findex .DEFAULT@r{, and empty recipes}
4636 You may be wondering why you would want to define a recipe that
4637 does nothing. The only reason this is useful is to prevent a target
4638 from getting implicit recipes (from implicit rules or the
4639 @code{.DEFAULT} special target; @pxref{Implicit Rules} and
4640 @pxref{Last Resort, ,Defining Last-Resort Default Rules}).@refill
4642 @c !!! another reason is for canonical stamp files:
4647 create foo frm foo.in
4652 You may be inclined to define empty recipes for targets that are
4653 not actual files, but only exist so that their prerequisites can be
4654 remade. However, this is not the best way to do that, because the
4655 prerequisites may not be remade properly if the target file actually does exist.
4656 @xref{Phony Targets, ,Phony Targets}, for a better way to do this.
4658 @node Using Variables, Conditionals, Recipes, Top
4659 @chapter How to Use Variables
4662 @cindex recursive variable expansion
4663 @cindex simple variable expansion
4665 A @dfn{variable} is a name defined in a makefile to represent a string
4666 of text, called the variable's @dfn{value}. These values are
4667 substituted by explicit request into targets, prerequisites, recipes,
4668 and other parts of the makefile. (In some other versions of @code{make},
4669 variables are called @dfn{macros}.)
4672 Variables and functions in all parts of a makefile are expanded when
4673 read, except for in recipes, the right-hand sides of variable
4674 definitions using @samp{=}, and the bodies of variable definitions
4675 using the @code{define} directive.@refill
4677 Variables can represent lists of file names, options to pass to compilers,
4678 programs to run, directories to look in for source files, directories to
4679 write output in, or anything else you can imagine.
4681 A variable name may be any sequence of characters not containing @samp{:},
4682 @samp{#}, @samp{=}, or leading or trailing whitespace. However,
4683 variable names containing characters other than letters, numbers, and
4684 underscores should be avoided, as they may be given special meanings in the
4685 future, and with some shells they cannot be passed through the environment to a
4687 (@pxref{Variables/Recursion, ,Communicating Variables to a Sub-@code{make}}).
4689 Variable names are case-sensitive. The names @samp{foo}, @samp{FOO},
4690 and @samp{Foo} all refer to different variables.
4692 It is traditional to use upper case letters in variable names, but we
4693 recommend using lower case letters for variable names that serve internal
4694 purposes in the makefile, and reserving upper case for parameters that
4695 control implicit rules or for parameters that the user should override with
4696 command options (@pxref{Overriding, ,Overriding Variables}).
4698 A few variables have names that are a single punctuation character or
4699 just a few characters. These are the @dfn{automatic variables}, and
4700 they have particular specialized uses. @xref{Automatic Variables}.
4703 * Reference:: How to use the value of a variable.
4704 * Flavors:: Variables come in two flavors.
4705 * Advanced:: Advanced features for referencing a variable.
4706 * Values:: All the ways variables get their values.
4707 * Setting:: How to set a variable in the makefile.
4708 * Appending:: How to append more text to the old value
4710 * Override Directive:: How to set a variable in the makefile even if
4711 the user has set it with a command argument.
4712 * Multi-Line:: An alternate way to set a variable
4713 to a multi-line string.
4714 * Environment:: Variable values can come from the environment.
4715 * Target-specific:: Variable values can be defined on a per-target
4717 * Pattern-specific:: Target-specific variable values can be applied
4718 to a group of targets that match a pattern.
4719 * Suppressing Inheritance:: Suppress inheritance of variables.
4720 * Special Variables:: Variables with special meaning or behavior.
4723 @node Reference, Flavors, Using Variables, Using Variables
4724 @section Basics of Variable References
4725 @cindex variables, how to reference
4726 @cindex reference to variables
4727 @cindex @code{$}, in variable reference
4728 @cindex dollar sign (@code{$}), in variable reference
4730 To substitute a variable's value, write a dollar sign followed by the name
4731 of the variable in parentheses or braces: either @samp{$(foo)} or
4732 @samp{$@{foo@}} is a valid reference to the variable @code{foo}. This
4733 special significance of @samp{$} is why you must write @samp{$$} to have
4734 the effect of a single dollar sign in a file name or recipe.
4736 Variable references can be used in any context: targets, prerequisites,
4737 recipes, most directives, and new variable values. Here is an
4738 example of a common case, where a variable holds the names of all the
4739 object files in a program:
4743 objects = program.o foo.o utils.o
4744 program : $(objects)
4745 cc -o program $(objects)
4751 Variable references work by strict textual substitution. Thus, the rule
4756 prog.o : prog.$(foo)
4757 $(foo)$(foo) -$(foo) prog.$(foo)
4762 could be used to compile a C program @file{prog.c}. Since spaces before
4763 the variable value are ignored in variable assignments, the value of
4764 @code{foo} is precisely @samp{c}. (Don't actually write your makefiles
4767 A dollar sign followed by a character other than a dollar sign,
4768 open-parenthesis or open-brace treats that single character as the
4769 variable name. Thus, you could reference the variable @code{x} with
4770 @samp{$x}. However, this practice is strongly discouraged, except in
4771 the case of the automatic variables (@pxref{Automatic Variables}).
4773 @node Flavors, Advanced, Reference, Using Variables
4774 @section The Two Flavors of Variables
4775 @cindex flavors of variables
4776 @cindex recursive variable expansion
4777 @cindex variables, flavors
4778 @cindex recursively expanded variables
4779 @cindex variables, recursively expanded
4781 There are two ways that a variable in GNU @code{make} can have a value;
4782 we call them the two @dfn{flavors} of variables. The two flavors are
4783 distinguished in how they are defined and in what they do when expanded.
4786 The first flavor of variable is a @dfn{recursively expanded} variable.
4787 Variables of this sort are defined by lines using @samp{=}
4788 (@pxref{Setting, ,Setting Variables}) or by the @code{define} directive
4789 (@pxref{Multi-Line, ,Defining Multi-Line Variables}). The value you specify
4790 is installed verbatim; if it contains references to other variables,
4791 these references are expanded whenever this variable is substituted (in
4792 the course of expanding some other string). When this happens, it is
4793 called @dfn{recursive expansion}.@refill
4806 will echo @samp{Huh?}: @samp{$(foo)} expands to @samp{$(bar)} which
4807 expands to @samp{$(ugh)} which finally expands to @samp{Huh?}.@refill
4809 This flavor of variable is the only sort supported by other versions of
4810 @code{make}. It has its advantages and its disadvantages. An advantage
4811 (most would say) is that:
4814 CFLAGS = $(include_dirs) -O
4815 include_dirs = -Ifoo -Ibar
4819 will do what was intended: when @samp{CFLAGS} is expanded in a recipe,
4820 it will expand to @samp{-Ifoo -Ibar -O}. A major disadvantage is that you
4821 cannot append something on the end of a variable, as in
4824 CFLAGS = $(CFLAGS) -O
4828 because it will cause an infinite loop in the variable expansion.
4829 (Actually @code{make} detects the infinite loop and reports an error.)
4830 @cindex loops in variable expansion
4831 @cindex variables, loops in expansion
4833 Another disadvantage is that any functions
4834 (@pxref{Functions, ,Functions for Transforming Text})
4835 referenced in the definition will be executed every time the variable is
4836 expanded. This makes @code{make} run slower; worse, it causes the
4837 @code{wildcard} and @code{shell} functions to give unpredictable results
4838 because you cannot easily control when they are called, or even how many
4841 To avoid all the problems and inconveniences of recursively expanded
4842 variables, there is another flavor: simply expanded variables.
4844 @cindex simply expanded variables
4845 @cindex variables, simply expanded
4847 @dfn{Simply expanded variables} are defined by lines using @samp{:=}
4848 (@pxref{Setting, ,Setting Variables}).
4849 The value of a simply expanded variable is scanned
4850 once and for all, expanding any references to other variables and
4851 functions, when the variable is defined. The actual value of the simply
4852 expanded variable is the result of expanding the text that you write.
4853 It does not contain any references to other variables; it contains their
4854 values @emph{as of the time this variable was defined}. Therefore,
4870 When a simply expanded variable is referenced, its value is substituted
4873 Here is a somewhat more complicated example, illustrating the use of
4874 @samp{:=} in conjunction with the @code{shell} function.
4875 (@xref{Shell Function, , The @code{shell} Function}.) This example
4876 also shows use of the variable @code{MAKELEVEL}, which is changed
4877 when it is passed down from level to level.
4878 (@xref{Variables/Recursion, , Communicating Variables to a
4879 Sub-@code{make}}, for information about @code{MAKELEVEL}.)
4885 ifeq (0,$@{MAKELEVEL@})
4886 whoami := $(shell whoami)
4887 host-type := $(shell arch)
4888 MAKE := $@{MAKE@} host-type=$@{host-type@} whoami=$@{whoami@}
4894 An advantage of this use of @samp{:=} is that a typical
4895 `descend into a directory' recipe then looks like this:
4900 $@{MAKE@} -C $@@ all
4904 Simply expanded variables generally make complicated makefile programming
4905 more predictable because they work like variables in most programming
4906 languages. They allow you to redefine a variable using its own value (or
4907 its value processed in some way by one of the expansion functions) and to
4908 use the expansion functions much more efficiently
4909 (@pxref{Functions, ,Functions for Transforming Text}).
4911 @cindex spaces, in variable values
4912 @cindex whitespace, in variable values
4913 @cindex variables, spaces in values
4914 You can also use them to introduce controlled leading whitespace into
4915 variable values. Leading whitespace characters are discarded from your
4916 input before substitution of variable references and function calls;
4917 this means you can include leading spaces in a variable value by
4918 protecting them with variable references, like this:
4922 space := $(nullstring) # end of the line
4926 Here the value of the variable @code{space} is precisely one space. The
4927 comment @w{@samp{# end of the line}} is included here just for clarity.
4928 Since trailing space characters are @emph{not} stripped from variable
4929 values, just a space at the end of the line would have the same effect
4930 (but be rather hard to read). If you put whitespace at the end of a
4931 variable value, it is a good idea to put a comment like that at the end
4932 of the line to make your intent clear. Conversely, if you do @emph{not}
4933 want any whitespace characters at the end of your variable value, you
4934 must remember not to put a random comment on the end of the line after
4935 some whitespace, such as this:
4938 dir := /foo/bar # directory to put the frobs in
4942 Here the value of the variable @code{dir} is @w{@samp{/foo/bar }}
4943 (with four trailing spaces), which was probably not the intention.
4944 (Imagine something like @w{@samp{$(dir)/file}} with this definition!)
4946 @cindex conditional variable assignment
4947 @cindex variables, conditional assignment
4949 There is another assignment operator for variables, @samp{?=}. This
4950 is called a conditional variable assignment operator, because it only
4951 has an effect if the variable is not yet defined. This statement:
4958 is exactly equivalent to this
4959 (@pxref{Origin Function, ,The @code{origin} Function}):
4962 ifeq ($(origin FOO), undefined)
4967 Note that a variable set to an empty value is still defined, so
4968 @samp{?=} will not set that variable.
4970 @node Advanced, Values, Flavors, Using Variables
4971 @section Advanced Features for Reference to Variables
4972 @cindex reference to variables
4974 This section describes some advanced features you can use to reference
4975 variables in more flexible ways.
4978 * Substitution Refs:: Referencing a variable with
4979 substitutions on the value.
4980 * Computed Names:: Computing the name of the variable to refer to.
4983 @node Substitution Refs, Computed Names, Advanced, Advanced
4984 @subsection Substitution References
4985 @cindex modified variable reference
4986 @cindex substitution variable reference
4987 @cindex variables, modified reference
4988 @cindex variables, substitution reference
4990 @cindex variables, substituting suffix in
4991 @cindex suffix, substituting in variables
4992 A @dfn{substitution reference} substitutes the value of a variable with
4993 alterations that you specify. It has the form
4994 @samp{$(@var{var}:@var{a}=@var{b})} (or
4995 @samp{$@{@var{var}:@var{a}=@var{b}@}}) and its meaning is to take the value
4996 of the variable @var{var}, replace every @var{a} at the end of a word with
4997 @var{b} in that value, and substitute the resulting string.
4999 When we say ``at the end of a word'', we mean that @var{a} must appear
5000 either followed by whitespace or at the end of the value in order to be
5001 replaced; other occurrences of @var{a} in the value are unaltered. For
5010 sets @samp{bar} to @samp{a.c b.c c.c}. @xref{Setting, ,Setting Variables}.
5012 A substitution reference is actually an abbreviation for use of the
5013 @code{patsubst} expansion function (@pxref{Text Functions, ,Functions for String Substitution and Analysis}). We provide
5014 substitution references as well as @code{patsubst} for compatibility with
5015 other implementations of @code{make}.
5018 Another type of substitution reference lets you use the full power of
5019 the @code{patsubst} function. It has the same form
5020 @samp{$(@var{var}:@var{a}=@var{b})} described above, except that now
5021 @var{a} must contain a single @samp{%} character. This case is
5022 equivalent to @samp{$(patsubst @var{a},@var{b},$(@var{var}))}.
5023 @xref{Text Functions, ,Functions for String Substitution and Analysis},
5024 for a description of the @code{patsubst} function.@refill
5028 @exdent For example:
5031 bar := $(foo:%.o=%.c)
5036 sets @samp{bar} to @samp{a.c b.c c.c}.
5038 @node Computed Names, , Substitution Refs, Advanced
5039 @subsection Computed Variable Names
5040 @cindex nested variable reference
5041 @cindex computed variable name
5042 @cindex variables, computed names
5043 @cindex variables, nested references
5044 @cindex variables, @samp{$} in name
5045 @cindex @code{$}, in variable name
5046 @cindex dollar sign (@code{$}), in variable name
5048 Computed variable names are a complicated concept needed only for
5049 sophisticated makefile programming. For most purposes you need not
5050 consider them, except to know that making a variable with a dollar sign
5051 in its name might have strange results. However, if you are the type
5052 that wants to understand everything, or you are actually interested in
5053 what they do, read on.
5055 Variables may be referenced inside the name of a variable. This is
5056 called a @dfn{computed variable name} or a @dfn{nested variable
5057 reference}. For example,
5066 defines @code{a} as @samp{z}: the @samp{$(x)} inside @samp{$($(x))} expands
5067 to @samp{y}, so @samp{$($(x))} expands to @samp{$(y)} which in turn expands
5068 to @samp{z}. Here the name of the variable to reference is not stated
5069 explicitly; it is computed by expansion of @samp{$(x)}. The reference
5070 @samp{$(x)} here is nested within the outer variable reference.
5072 The previous example shows two levels of nesting, but any number of levels
5073 is possible. For example, here are three levels:
5083 Here the innermost @samp{$(x)} expands to @samp{y}, so @samp{$($(x))}
5084 expands to @samp{$(y)} which in turn expands to @samp{z}; now we have
5085 @samp{$(z)}, which becomes @samp{u}.
5087 References to recursively-expanded variables within a variable name are
5088 reexpanded in the usual fashion. For example:
5098 defines @code{a} as @samp{Hello}: @samp{$($(x))} becomes @samp{$($(y))}
5099 which becomes @samp{$(z)} which becomes @samp{Hello}.
5101 Nested variable references can also contain modified references and
5102 function invocations (@pxref{Functions, ,Functions for Transforming Text}),
5103 just like any other reference.
5104 For example, using the @code{subst} function
5105 (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):
5111 y = $(subst 1,2,$(x))
5118 eventually defines @code{a} as @samp{Hello}. It is doubtful that anyone
5119 would ever want to write a nested reference as convoluted as this one, but
5120 it works: @samp{$($($(z)))} expands to @samp{$($(y))} which becomes
5121 @samp{$($(subst 1,2,$(x)))}. This gets the value @samp{variable1} from
5122 @code{x} and changes it by substitution to @samp{variable2}, so that the
5123 entire string becomes @samp{$(variable2)}, a simple variable reference
5124 whose value is @samp{Hello}.@refill
5126 A computed variable name need not consist entirely of a single variable
5127 reference. It can contain several variable references, as well as some
5128 invariant text. For example,
5137 a_files := filea fileb
5138 1_files := file1 file2
5142 ifeq "$(use_a)" "yes"
5150 ifeq "$(use_dirs)" "yes"
5156 dirs := $($(a1)_$(df))
5161 will give @code{dirs} the same value as @code{a_dirs}, @code{1_dirs},
5162 @code{a_files} or @code{1_files} depending on the settings of @code{use_a}
5163 and @code{use_dirs}.@refill
5165 Computed variable names can also be used in substitution references:
5169 a_objects := a.o b.o c.o
5170 1_objects := 1.o 2.o 3.o
5172 sources := $($(a1)_objects:.o=.c)
5177 defines @code{sources} as either @samp{a.c b.c c.c} or @samp{1.c 2.c 3.c},
5178 depending on the value of @code{a1}.
5180 The only restriction on this sort of use of nested variable references
5181 is that they cannot specify part of the name of a function to be called.
5182 This is because the test for a recognized function name is done before
5183 the expansion of nested references. For example,
5199 foo := $($(func) $(bar))
5204 attempts to give @samp{foo} the value of the variable @samp{sort a d b g
5205 q c} or @samp{strip a d b g q c}, rather than giving @samp{a d b g q c}
5206 as the argument to either the @code{sort} or the @code{strip} function.
5207 This restriction could be removed in the future if that change is shown
5210 You can also use computed variable names in the left-hand side of a
5211 variable assignment, or in a @code{define} directive, as in:
5215 $(dir)_sources := $(wildcard $(dir)/*.c)
5216 define $(dir)_print =
5217 lpr $($(dir)_sources)
5222 This example defines the variables @samp{dir}, @samp{foo_sources}, and
5225 Note that @dfn{nested variable references} are quite different from
5226 @dfn{recursively expanded variables}
5227 (@pxref{Flavors, ,The Two Flavors of Variables}), though both are
5228 used together in complex ways when doing makefile programming.@refill
5230 @node Values, Setting, Advanced, Using Variables
5231 @section How Variables Get Their Values
5232 @cindex variables, how they get their values
5233 @cindex value, how a variable gets it
5235 Variables can get values in several different ways:
5239 You can specify an overriding value when you run @code{make}.
5240 @xref{Overriding, ,Overriding Variables}.
5243 You can specify a value in the makefile, either
5244 with an assignment (@pxref{Setting, ,Setting Variables}) or with a
5245 verbatim definition (@pxref{Multi-Line, ,Defining Multi-Line Variables}).@refill
5248 Variables in the environment become @code{make} variables.
5249 @xref{Environment, ,Variables from the Environment}.
5252 Several @dfn{automatic} variables are given new values for each rule.
5253 Each of these has a single conventional use.
5254 @xref{Automatic Variables}.
5257 Several variables have constant initial values.
5258 @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
5261 @node Setting, Appending, Values, Using Variables
5262 @section Setting Variables
5263 @cindex setting variables
5264 @cindex variables, setting
5269 To set a variable from the makefile, write a line starting with the
5270 variable name followed by @samp{=} or @samp{:=}. Whatever follows the
5271 @samp{=} or @samp{:=} on the line becomes the value. For example,
5274 objects = main.o foo.o bar.o utils.o
5278 defines a variable named @code{objects}. Whitespace around the variable
5279 name and immediately after the @samp{=} is ignored.
5281 Variables defined with @samp{=} are @dfn{recursively expanded} variables.
5282 Variables defined with @samp{:=} are @dfn{simply expanded} variables; these
5283 definitions can contain variable references which will be expanded before
5284 the definition is made. @xref{Flavors, ,The Two Flavors of Variables}.
5286 The variable name may contain function and variable references, which
5287 are expanded when the line is read to find the actual variable name to use.
5289 There is no limit on the length of the value of a variable except the
5290 amount of swapping space on the computer. When a variable definition is
5291 long, it is a good idea to break it into several lines by inserting
5292 backslash-newline at convenient places in the definition. This will not
5293 affect the functioning of @code{make}, but it will make the makefile easier
5296 Most variable names are considered to have the empty string as a value if
5297 you have never set them. Several variables have built-in initial values
5298 that are not empty, but you can set them in the usual ways
5299 (@pxref{Implicit Variables, ,Variables Used by Implicit Rules}).
5300 Several special variables are set
5301 automatically to a new value for each rule; these are called the
5302 @dfn{automatic} variables (@pxref{Automatic Variables}).
5304 If you'd like a variable to be set to a value only if it's not already
5305 set, then you can use the shorthand operator @samp{?=} instead of
5306 @samp{=}. These two settings of the variable @samp{FOO} are identical
5307 (@pxref{Origin Function, ,The @code{origin} Function}):
5317 ifeq ($(origin FOO), undefined)
5322 @node Appending, Override Directive, Setting, Using Variables
5323 @section Appending More Text to Variables
5325 @cindex appending to variables
5326 @cindex variables, appending to
5328 Often it is useful to add more text to the value of a variable already defined.
5329 You do this with a line containing @samp{+=}, like this:
5332 objects += another.o
5336 This takes the value of the variable @code{objects}, and adds the text
5337 @samp{another.o} to it (preceded by a single space). Thus:
5340 objects = main.o foo.o bar.o utils.o
5341 objects += another.o
5345 sets @code{objects} to @samp{main.o foo.o bar.o utils.o another.o}.
5347 Using @samp{+=} is similar to:
5350 objects = main.o foo.o bar.o utils.o
5351 objects := $(objects) another.o
5355 but differs in ways that become important when you use more complex values.
5357 When the variable in question has not been defined before, @samp{+=}
5358 acts just like normal @samp{=}: it defines a recursively-expanded
5359 variable. However, when there @emph{is} a previous definition, exactly
5360 what @samp{+=} does depends on what flavor of variable you defined
5361 originally. @xref{Flavors, ,The Two Flavors of Variables}, for an
5362 explanation of the two flavors of variables.
5364 When you add to a variable's value with @samp{+=}, @code{make} acts
5365 essentially as if you had included the extra text in the initial
5366 definition of the variable. If you defined it first with @samp{:=},
5367 making it a simply-expanded variable, @samp{+=} adds to that
5368 simply-expanded definition, and expands the new text before appending it
5369 to the old value just as @samp{:=} does
5370 (see @ref{Setting, ,Setting Variables}, for a full explanation of @samp{:=}).
5379 is exactly equivalent to:
5384 variable := $(variable) more
5387 On the other hand, when you use @samp{+=} with a variable that you defined
5388 first to be recursively-expanded using plain @samp{=}, @code{make} does
5389 something a bit different. Recall that when you define a
5390 recursively-expanded variable, @code{make} does not expand the value you set
5391 for variable and function references immediately. Instead it stores the text
5392 verbatim, and saves these variable and function references to be expanded
5393 later, when you refer to the new variable (@pxref{Flavors, ,The Two Flavors
5394 of Variables}). When you use @samp{+=} on a recursively-expanded variable,
5395 it is this unexpanded text to which @code{make} appends the new text you
5406 is roughly equivalent to:
5411 variable = $(temp) more
5416 except that of course it never defines a variable called @code{temp}.
5417 The importance of this comes when the variable's old value contains
5418 variable references. Take this common example:
5421 CFLAGS = $(includes) -O
5423 CFLAGS += -pg # enable profiling
5427 The first line defines the @code{CFLAGS} variable with a reference to another
5428 variable, @code{includes}. (@code{CFLAGS} is used by the rules for C
5429 compilation; @pxref{Catalogue of Rules, ,Catalogue of Implicit Rules}.)
5430 Using @samp{=} for the definition makes @code{CFLAGS} a recursively-expanded
5431 variable, meaning @w{@samp{$(includes) -O}} is @emph{not} expanded when
5432 @code{make} processes the definition of @code{CFLAGS}. Thus, @code{includes}
5433 need not be defined yet for its value to take effect. It only has to be
5434 defined before any reference to @code{CFLAGS}. If we tried to append to the
5435 value of @code{CFLAGS} without using @samp{+=}, we might do it like this:
5438 CFLAGS := $(CFLAGS) -pg # enable profiling
5442 This is pretty close, but not quite what we want. Using @samp{:=}
5443 redefines @code{CFLAGS} as a simply-expanded variable; this means
5444 @code{make} expands the text @w{@samp{$(CFLAGS) -pg}} before setting the
5445 variable. If @code{includes} is not yet defined, we get @w{@samp{ -O
5446 -pg}}, and a later definition of @code{includes} will have no effect.
5447 Conversely, by using @samp{+=} we set @code{CFLAGS} to the
5448 @emph{unexpanded} value @w{@samp{$(includes) -O -pg}}. Thus we preserve
5449 the reference to @code{includes}, so if that variable gets defined at
5450 any later point, a reference like @samp{$(CFLAGS)} still uses its
5453 @node Override Directive, Multi-Line, Appending, Using Variables
5454 @section The @code{override} Directive
5456 @cindex overriding with @code{override}
5457 @cindex variables, overriding
5459 If a variable has been set with a command argument
5460 (@pxref{Overriding, ,Overriding Variables}),
5461 then ordinary assignments in the makefile are ignored. If you want to set
5462 the variable in the makefile even though it was set with a command
5463 argument, you can use an @code{override} directive, which is a line that
5464 looks like this:@refill
5467 override @var{variable} = @var{value}
5474 override @var{variable} := @var{value}
5477 To append more text to a variable defined on the command line, use:
5480 override @var{variable} += @var{more text}
5484 @xref{Appending, ,Appending More Text to Variables}.
5486 Variable assignments marked with the @code{override} flag have a
5487 higher priority than all other assignments, except another
5488 @code{override}. Subsequent assignments or appends to this variable
5489 which are not marked @code{override} will be ignored.
5491 The @code{override} directive was not invented for escalation in the war
5492 between makefiles and command arguments. It was invented so you can alter
5493 and add to values that the user specifies with command arguments.
5495 For example, suppose you always want the @samp{-g} switch when you run the
5496 C compiler, but you would like to allow the user to specify the other
5497 switches with a command argument just as usual. You could use this
5498 @code{override} directive:
5501 override CFLAGS += -g
5504 You can also use @code{override} directives with @code{define} directives.
5505 This is done as you might expect:
5508 override define foo =
5515 See the next section for information about @code{define}.
5518 @xref{Multi-Line, ,Defining Multi-Line Variables}.
5521 @node Multi-Line, Environment, Override Directive, Using Variables
5522 @section Defining Multi-Line Variables
5525 @cindex multi-line variable definition
5526 @cindex variables, multi-line
5527 @cindex verbatim variable definition
5528 @cindex defining variables verbatim
5529 @cindex variables, defining verbatim
5531 Another way to set the value of a variable is to use the @code{define}
5532 directive. This directive has an unusual syntax which allows newline
5533 characters to be included in the value, which is convenient for
5534 defining both canned sequences of commands (@pxref{Canned Recipes,
5535 ,Defining Canned Recipes}), and also sections of makefile syntax to
5536 use with @code{eval} (@pxref{Eval Function}).@refill
5538 The @code{define} directive is followed on the same line by the name
5539 of the variable being defined and an (optional) assignment operator,
5540 and nothing more. The value to give the variable appears on the
5541 following lines. The end of the value is marked by a line containing
5542 just the word @code{endef}. Aside from this difference in syntax,
5543 @code{define} works just like any other variable definition. The
5544 variable name may contain function and variable references, which are
5545 expanded when the directive is read to find the actual variable name
5548 You may omit the variable assignment operator if you prefer. If
5549 omitted, @code{make} assumes it to be @samp{=} and creates a
5550 recursively-expanded variable (@pxref{Flavors, ,The Two Flavors of Variables}).
5551 When using a @samp{+=} operator, the value is appended to the previous
5552 value as with any other append operation: with a single space
5553 separating the old and new values.
5555 You may nest @code{define} directives: @code{make} will keep track of
5556 nested directives and report an error if they are not all properly
5557 closed with @code{endef}. Note that lines beginning with the recipe
5558 prefix character are considered part of a recipe, so any @code{define}
5559 or @code{endef} strings appearing on such a line will not be
5560 considered @code{make} directives.
5569 The value in an ordinary assignment cannot contain a newline; but the
5570 newlines that separate the lines of the value in a @code{define} become
5571 part of the variable's value (except for the final newline which precedes
5572 the @code{endef} and is not considered part of the value).@refill
5575 When used in a recipe, the previous example is functionally equivalent
5579 two-lines = echo foo; echo $(bar)
5583 since two commands separated by semicolon behave much like two separate
5584 shell commands. However, note that using two separate lines means
5585 @code{make} will invoke the shell twice, running an independent subshell
5586 for each line. @xref{Execution, ,Recipe Execution}.
5588 If you want variable definitions made with @code{define} to take
5589 precedence over command-line variable definitions, you can use the
5590 @code{override} directive together with @code{define}:
5593 override define two-lines =
5600 @xref{Override Directive, ,The @code{override} Directive}.
5602 @node Environment, Target-specific, Multi-Line, Using Variables
5603 @section Variables from the Environment
5605 @cindex variables, environment
5607 Variables in @code{make} can come from the environment in which
5608 @code{make} is run. Every environment variable that @code{make} sees
5609 when it starts up is transformed into a @code{make} variable with the
5610 same name and value. However, an explicit assignment in the makefile,
5611 or with a command argument, overrides the environment. (If the
5612 @samp{-e} flag is specified, then values from the environment override
5613 assignments in the makefile. @xref{Options Summary, ,Summary of
5614 Options}. But this is not recommended practice.)
5616 Thus, by setting the variable @code{CFLAGS} in your environment, you can
5617 cause all C compilations in most makefiles to use the compiler switches you
5618 prefer. This is safe for variables with standard or conventional meanings
5619 because you know that no makefile will use them for other things. (Note
5620 this is not totally reliable; some makefiles set @code{CFLAGS} explicitly
5621 and therefore are not affected by the value in the environment.)
5623 When @code{make} runs a recipe, variables defined in the
5624 makefile are placed into the environment of each shell. This allows
5625 you to pass values to sub-@code{make} invocations (@pxref{Recursion,
5626 ,Recursive Use of @code{make}}). By default, only variables that came
5627 from the environment or the command line are passed to recursive
5628 invocations. You can use the @code{export} directive to pass other
5629 variables. @xref{Variables/Recursion, , Communicating Variables to a
5630 Sub-@code{make}}, for full details.
5632 Other use of variables from the environment is not recommended. It is not
5633 wise for makefiles to depend for their functioning on environment variables
5634 set up outside their control, since this would cause different users to get
5635 different results from the same makefile. This is against the whole
5636 purpose of most makefiles.
5638 @cindex SHELL, import from environment
5639 Such problems would be especially likely with the variable
5640 @code{SHELL}, which is normally present in the environment to specify
5641 the user's choice of interactive shell. It would be very undesirable
5642 for this choice to affect @code{make}; so, @code{make} handles the
5643 @code{SHELL} environment variable in a special way; see @ref{Choosing
5646 @node Target-specific, Pattern-specific, Environment, Using Variables
5647 @section Target-specific Variable Values
5648 @cindex target-specific variables
5649 @cindex variables, target-specific
5651 Variable values in @code{make} are usually global; that is, they are the
5652 same regardless of where they are evaluated (unless they're reset, of
5653 course). One exception to that is automatic variables
5654 (@pxref{Automatic Variables}).
5656 The other exception is @dfn{target-specific variable values}. This
5657 feature allows you to define different values for the same variable,
5658 based on the target that @code{make} is currently building. As with
5659 automatic variables, these values are only available within the context
5660 of a target's recipe (and in other target-specific assignments).
5662 Set a target-specific variable value like this:
5665 @var{target} @dots{} : @var{variable-assignment}
5668 Target-specific variable assignments can be prefixed with any or all of the
5669 special keywords @code{export}, @code{override}, or @code{private};
5670 these apply their normal behavior to this instance of the variable only.
5672 Multiple @var{target} values create a target-specific variable value for
5673 each member of the target list individually.
5675 The @var{variable-assignment} can be any valid form of assignment;
5676 recursive (@samp{=}), static (@samp{:=}), appending (@samp{+=}), or
5677 conditional (@samp{?=}). All variables that appear within the
5678 @var{variable-assignment} are evaluated within the context of the
5679 target: thus, any previously-defined target-specific variable values
5680 will be in effect. Note that this variable is actually distinct from
5681 any ``global'' value: the two variables do not have to have the same
5682 flavor (recursive vs.@: static).
5684 Target-specific variables have the same priority as any other makefile
5685 variable. Variables provided on the command line (and in the
5686 environment if the @samp{-e} option is in force) will take precedence.
5687 Specifying the @code{override} directive will allow the target-specific
5688 variable value to be preferred.
5690 There is one more special feature of target-specific variables: when
5691 you define a target-specific variable that variable value is also in
5692 effect for all prerequisites of this target, and all their
5693 prerequisites, etc.@: (unless those prerequisites override that variable
5694 with their own target-specific variable value). So, for example, a
5695 statement like this:
5699 prog : prog.o foo.o bar.o
5703 will set @code{CFLAGS} to @samp{-g} in the recipe for @file{prog}, but
5704 it will also set @code{CFLAGS} to @samp{-g} in the recipes that create
5705 @file{prog.o}, @file{foo.o}, and @file{bar.o}, and any recipes which
5706 create their prerequisites.
5708 Be aware that a given prerequisite will only be built once per
5709 invocation of make, at most. If the same file is a prerequisite of
5710 multiple targets, and each of those targets has a different value for
5711 the same target-specific variable, then the first target to be built
5712 will cause that prerequisite to be built and the prerequisite will
5713 inherit the target-specific value from the first target. It will
5714 ignore the target-specific values from any other targets.
5716 @node Pattern-specific, Suppressing Inheritance, Target-specific, Using Variables
5717 @section Pattern-specific Variable Values
5718 @cindex pattern-specific variables
5719 @cindex variables, pattern-specific
5721 In addition to target-specific variable values
5722 (@pxref{Target-specific, ,Target-specific Variable Values}), GNU
5723 @code{make} supports pattern-specific variable values. In this form,
5724 the variable is defined for any target that matches the pattern
5727 Set a pattern-specific variable value like this:
5730 @var{pattern} @dots{} : @var{variable-assignment}
5732 where @var{pattern} is a %-pattern. As with target-specific variable
5733 values, multiple @var{pattern} values create a pattern-specific variable
5734 value for each pattern individually. The @var{variable-assignment} can
5735 be any valid form of assignment. Any command line variable setting will
5736 take precedence, unless @code{override} is specified.
5745 will assign @code{CFLAGS} the value of @samp{-O} for all targets
5746 matching the pattern @code{%.o}.
5748 If a target matches more than one pattern, the matching pattern-specific
5749 variables with longer stems are interpreted first. This results in more
5750 specific variables taking precedence over the more generic ones, for
5755 $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
5757 lib/%.o: CFLAGS := -fPIC -g
5760 all: foo.o lib/bar.o
5763 In this example the first definition of the @code{CFLAGS} variable
5764 will be used to update @file{lib/bar.o} even though the second one
5765 also applies to this target. Pattern-specific variables which result
5766 in the same stem length are considered in the order in which they
5767 were defined in the makefile.
5769 Pattern-specific variables are searched after any target-specific
5770 variables defined explicitly for that target, and before target-specific
5771 variables defined for the parent target.
5773 @node Suppressing Inheritance, Special Variables, Pattern-specific, Using Variables
5774 @section Suppressing Inheritance
5776 @cindex suppressing inheritance
5777 @cindex inheritance, suppressing
5779 As described in previous sections, @code{make} variables are inherited
5780 by prerequisites. This capability allows you to modify the behavior
5781 of a prerequisite based on which targets caused it to be rebuilt. For
5782 example, you might set a target-specific variable on a @code{debug}
5783 target, then running @samp{make debug} will cause that variable to be
5784 inherited by all prerequisites of @code{debug}, while just running
5785 @samp{make all} (for example) would not have that assignment.
5787 Sometimes, however, you may not want a variable to be inherited. For
5788 these situations, @code{make} provides the @code{private} modifier.
5789 Although this modifier can be used with any variable assignment, it
5790 makes the most sense with target- and pattern-specific variables. Any
5791 variable marked @code{private} will be visible to its local target but
5792 will not be inherited by prerequisites of that target. A global
5793 variable marked @code{private} will be visible in the global scope but
5794 will not be inherited by any target, and hence will not be visible
5797 As an example, consider this makefile:
5801 prog: private EXTRA_CFLAGS = -L/usr/local/lib
5805 Due to the @code{private} modifier, @code{a.o} and @code{b.o} will not
5806 inherit the @code{EXTRA_CFLAGS} variable assignment from the
5807 @code{progs} target.
5809 @node Special Variables, , Suppressing Inheritance, Using Variables
5810 @comment node-name, next, previous, up
5811 @section Other Special Variables
5812 @cindex makefiles, and special variables
5813 @cindex special variables
5815 GNU @code{make} supports some variables that have special properties.
5819 @vindex MAKEFILE_LIST @r{(list of parsed makefiles)}
5820 @cindex makefiles, and @code{MAKEFILE_LIST} variable
5821 @cindex including (@code{MAKEFILE_LIST} variable)
5823 Contains the name of each makefile that is parsed by @code{make}, in
5824 the order in which it was parsed. The name is appended just
5825 before @code{make} begins to parse the makefile. Thus, if the first
5826 thing a makefile does is examine the last word in this variable, it
5827 will be the name of the current makefile. Once the current makefile
5828 has used @code{include}, however, the last word will be the
5829 just-included makefile.
5831 If a makefile named @code{Makefile} has this content:
5835 name1 := $(lastword $(MAKEFILE_LIST))
5839 name2 := $(lastword $(MAKEFILE_LIST))
5842 @@echo name1 = $(name1)
5843 @@echo name2 = $(name2)
5848 then you would expect to see this output:
5857 @vindex .DEFAULT_GOAL @r{(define default goal)}
5859 Sets the default goal to be used if no targets were specified on the
5860 command line (@pxref{Goals, , Arguments to Specify the Goals}). The
5861 @code{.DEFAULT_GOAL} variable allows you to discover the current
5862 default goal, restart the default goal selection algorithm by clearing
5863 its value, or to explicitly set the default goal. The following
5864 example illustrates these cases:
5868 # Query the default goal.
5869 ifeq ($(.DEFAULT_GOAL),)
5870 $(warning no default goal is set)
5876 $(warning default goal is $(.DEFAULT_GOAL))
5878 # Reset the default goal.
5884 $(warning default goal is $(.DEFAULT_GOAL))
5887 .DEFAULT_GOAL := foo
5891 This makefile prints:
5895 no default goal is set
5902 Note that assigning more than one target name to @code{.DEFAULT_GOAL} is
5903 illegal and will result in an error.
5905 @vindex MAKE_RESTARTS @r{(number of times @code{make} has restarted)}
5907 This variable is set only if this instance of @code{make} has
5908 restarted (@pxref{Remaking Makefiles, , How Makefiles Are Remade}): it
5909 will contain the number of times this instance has restarted. Note
5910 this is not the same as recursion (counted by the @code{MAKELEVEL}
5911 variable). You should not set, modify, or export this variable.
5913 @vindex .CMDPREFIX @r{(change the recipe prefix character)}
5915 The first character of the value of this variable is used as the
5916 character make assumes is introducing a recipe line. If the variable
5917 is empty (as it is by default) that character is the standard tab
5918 character. For example, this is a valid makefile:
5924 > @@echo Hello, world
5928 The value of @code{.CMDPREFIX} can be changed multiple times; once set
5929 it stays in effect for all rules parsed until it is modified.
5931 @vindex .VARIABLES @r{(list of variables)}
5933 Expands to a list of the @emph{names} of all global variables defined
5934 so far. This includes variables which have empty values, as well as
5935 built-in variables (@pxref{Implicit Variables, , Variables Used by
5936 Implicit Rules}), but does not include any variables which are only
5937 defined in a target-specific context. Note that any value you assign
5938 to this variable will be ignored; it will always return its special
5941 @c @vindex .TARGETS @r{(list of targets)}
5943 @c The second special variable is @code{.TARGETS}. When expanded, the
5944 @c value consists of a list of all targets defined in all makefiles read
5945 @c up until that point. Note it's not enough for a file to be simply
5946 @c mentioned in the makefile to be listed in this variable, even if it
5947 @c would match an implicit rule and become an ``implicit target''. The
5948 @c file must appear as a target, on the left-hand side of a ``:'', to be
5949 @c considered a target for the purposes of this variable.
5951 @vindex .FEATURES @r{(list of supported features)}
5953 Expands to a list of special features supported by this version of
5954 @code{make}. Possible values include:
5959 Supports @code{ar} (archive) files using special filename syntax.
5960 @xref{Archives, ,Using @code{make} to Update Archive Files}.
5963 Supports the @code{-L} (@code{--check-symlink-times}) flag.
5964 @xref{Options Summary, ,Summary of Options}.
5967 Supports ``else if'' non-nested conditionals. @xref{Conditional
5968 Syntax, ,Syntax of Conditionals}.
5971 Supports ``job server'' enhanced parallel builds. @xref{Parallel,
5972 ,Parallel Execution}.
5974 @item second-expansion
5975 Supports secondary expansion of prerequisite lists.
5978 Supports order-only prerequisites. @xref{Prerequisite Types, ,Types
5981 @item target-specific
5982 Supports target-specific and pattern-specific variable assignments.
5983 @xref{Target-specific, ,Target-specific Variable Values}.
5987 @vindex .INCLUDE_DIRS @r{(list of include directories)}
5989 Expands to a list of directories that @code{make} searches for
5990 included makefiles (@pxref{Include, , Including Other Makefiles}).
5994 @node Conditionals, Functions, Using Variables, Top
5995 @chapter Conditional Parts of Makefiles
5997 @cindex conditionals
5998 A @dfn{conditional} directive causes part of a makefile to be obeyed
5999 or ignored depending on the values of variables. Conditionals can
6000 compare the value of one variable to another, or the value of a
6001 variable to a constant string. Conditionals control what @code{make}
6002 actually ``sees'' in the makefile, so they @emph{cannot} be used to
6003 control recipes at the time of execution.@refill
6006 * Conditional Example:: Example of a conditional
6007 * Conditional Syntax:: The syntax of conditionals.
6008 * Testing Flags:: Conditionals that test flags.
6011 @node Conditional Example, Conditional Syntax, Conditionals, Conditionals
6012 @section Example of a Conditional
6014 The following example of a conditional tells @code{make} to use one
6015 set of libraries if the @code{CC} variable is @samp{gcc}, and a
6016 different set of libraries otherwise. It works by controlling which
6017 of two recipe lines will be used for the rule. The result is that
6018 @samp{CC=gcc} as an argument to @code{make} changes not only which
6019 compiler is used but also which libraries are linked.
6022 libs_for_gcc = -lgnu
6027 $(CC) -o foo $(objects) $(libs_for_gcc)
6029 $(CC) -o foo $(objects) $(normal_libs)
6033 This conditional uses three directives: one @code{ifeq}, one @code{else}
6034 and one @code{endif}.
6036 The @code{ifeq} directive begins the conditional, and specifies the
6037 condition. It contains two arguments, separated by a comma and surrounded
6038 by parentheses. Variable substitution is performed on both arguments and
6039 then they are compared. The lines of the makefile following the
6040 @code{ifeq} are obeyed if the two arguments match; otherwise they are
6043 The @code{else} directive causes the following lines to be obeyed if the
6044 previous conditional failed. In the example above, this means that the
6045 second alternative linking command is used whenever the first alternative
6046 is not used. It is optional to have an @code{else} in a conditional.
6048 The @code{endif} directive ends the conditional. Every conditional must
6049 end with an @code{endif}. Unconditional makefile text follows.
6051 As this example illustrates, conditionals work at the textual level:
6052 the lines of the conditional are treated as part of the makefile, or
6053 ignored, according to the condition. This is why the larger syntactic
6054 units of the makefile, such as rules, may cross the beginning or the
6055 end of the conditional.
6057 When the variable @code{CC} has the value @samp{gcc}, the above example has
6062 $(CC) -o foo $(objects) $(libs_for_gcc)
6066 When the variable @code{CC} has any other value, the effect is this:
6070 $(CC) -o foo $(objects) $(normal_libs)
6073 Equivalent results can be obtained in another way by conditionalizing a
6074 variable assignment and then using the variable unconditionally:
6077 libs_for_gcc = -lgnu
6081 libs=$(libs_for_gcc)
6087 $(CC) -o foo $(objects) $(libs)
6090 @node Conditional Syntax, Testing Flags, Conditional Example, Conditionals
6091 @section Syntax of Conditionals
6099 The syntax of a simple conditional with no @code{else} is as follows:
6102 @var{conditional-directive}
6108 The @var{text-if-true} may be any lines of text, to be considered as part
6109 of the makefile if the condition is true. If the condition is false, no
6110 text is used instead.
6112 The syntax of a complex conditional is as follows:
6115 @var{conditional-directive}
6125 @var{conditional-directive}
6126 @var{text-if-one-is-true}
6127 else @var{conditional-directive}
6135 There can be as many ``@code{else} @var{conditional-directive}''
6136 clauses as necessary. Once a given condition is true,
6137 @var{text-if-true} is used and no other clause is used; if no
6138 condition is true then @var{text-if-false} is used. The
6139 @var{text-if-true} and @var{text-if-false} can be any number of lines
6142 The syntax of the @var{conditional-directive} is the same whether the
6143 conditional is simple or complex; after an @code{else} or not. There
6144 are four different directives that test different conditions. Here is
6148 @item ifeq (@var{arg1}, @var{arg2})
6149 @itemx ifeq '@var{arg1}' '@var{arg2}'
6150 @itemx ifeq "@var{arg1}" "@var{arg2}"
6151 @itemx ifeq "@var{arg1}" '@var{arg2}'
6152 @itemx ifeq '@var{arg1}' "@var{arg2}"
6153 Expand all variable references in @var{arg1} and @var{arg2} and
6154 compare them. If they are identical, the @var{text-if-true} is
6155 effective; otherwise, the @var{text-if-false}, if any, is effective.
6157 Often you want to test if a variable has a non-empty value. When the
6158 value results from complex expansions of variables and functions,
6159 expansions you would consider empty may actually contain whitespace
6160 characters and thus are not seen as empty. However, you can use the
6161 @code{strip} function (@pxref{Text Functions}) to avoid interpreting
6162 whitespace as a non-empty value. For example:
6166 ifeq ($(strip $(foo)),)
6173 will evaluate @var{text-if-empty} even if the expansion of
6174 @code{$(foo)} contains whitespace characters.
6176 @item ifneq (@var{arg1}, @var{arg2})
6177 @itemx ifneq '@var{arg1}' '@var{arg2}'
6178 @itemx ifneq "@var{arg1}" "@var{arg2}"
6179 @itemx ifneq "@var{arg1}" '@var{arg2}'
6180 @itemx ifneq '@var{arg1}' "@var{arg2}"
6181 Expand all variable references in @var{arg1} and @var{arg2} and
6182 compare them. If they are different, the @var{text-if-true} is
6183 effective; otherwise, the @var{text-if-false}, if any, is effective.
6185 @item ifdef @var{variable-name}
6186 The @code{ifdef} form takes the @emph{name} of a variable as its
6187 argument, not a reference to a variable. The value of that variable
6188 has a non-empty value, the @var{text-if-true} is effective; otherwise,
6189 the @var{text-if-false}, if any, is effective. Variables that have
6190 never been defined have an empty value. The text @var{variable-name}
6191 is expanded, so it could be a variable or function that expands
6192 to the name of a variable. For example:
6202 The variable reference @code{$(foo)} is expanded, yielding @code{bar},
6203 which is considered to be the name of a variable. The variable
6204 @code{bar} is not expanded, but its value is examined to determine if
6207 Note that @code{ifdef} only tests whether a variable has a value. It
6208 does not expand the variable to see if that value is nonempty.
6209 Consequently, tests using @code{ifdef} return true for all definitions
6210 except those like @code{foo =}. To test for an empty value, use
6211 @w{@code{ifeq ($(foo),)}}. For example,
6224 sets @samp{frobozz} to @samp{yes}, while:
6236 sets @samp{frobozz} to @samp{no}.
6238 @item ifndef @var{variable-name}
6239 If the variable @var{variable-name} has an empty value, the
6240 @var{text-if-true} is effective; otherwise, the @var{text-if-false},
6241 if any, is effective. The rules for expansion and testing of
6242 @var{variable-name} are identical to the @code{ifdef} directive.
6245 Extra spaces are allowed and ignored at the beginning of the
6246 conditional directive line, but a tab is not allowed. (If the line
6247 begins with a tab, it will be considered part of a recipe for a rule.)
6248 Aside from this, extra spaces or tabs may be inserted with no effect
6249 anywhere except within the directive name or within an argument. A
6250 comment starting with @samp{#} may appear at the end of the line.
6252 The other two directives that play a part in a conditional are @code{else}
6253 and @code{endif}. Each of these directives is written as one word, with no
6254 arguments. Extra spaces are allowed and ignored at the beginning of the
6255 line, and spaces or tabs at the end. A comment starting with @samp{#} may
6256 appear at the end of the line.
6258 Conditionals affect which lines of the makefile @code{make} uses. If
6259 the condition is true, @code{make} reads the lines of the
6260 @var{text-if-true} as part of the makefile; if the condition is false,
6261 @code{make} ignores those lines completely. It follows that syntactic
6262 units of the makefile, such as rules, may safely be split across the
6263 beginning or the end of the conditional.@refill
6265 @code{make} evaluates conditionals when it reads a makefile.
6266 Consequently, you cannot use automatic variables in the tests of
6267 conditionals because they are not defined until recipes are run
6268 (@pxref{Automatic Variables}).
6270 To prevent intolerable confusion, it is not permitted to start a
6271 conditional in one makefile and end it in another. However, you may
6272 write an @code{include} directive within a conditional, provided you do
6273 not attempt to terminate the conditional inside the included file.
6275 @node Testing Flags, , Conditional Syntax, Conditionals
6276 @section Conditionals that Test Flags
6278 You can write a conditional that tests @code{make} command flags such as
6279 @samp{-t} by using the variable @code{MAKEFLAGS} together with the
6280 @code{findstring} function
6281 (@pxref{Text Functions, , Functions for String Substitution and Analysis}).
6282 This is useful when @code{touch} is not enough to make a file appear up
6285 The @code{findstring} function determines whether one string appears as a
6286 substring of another. If you want to test for the @samp{-t} flag,
6287 use @samp{t} as the first string and the value of @code{MAKEFLAGS} as
6290 For example, here is how to arrange to use @samp{ranlib -t} to finish
6291 marking an archive file up to date:
6295 ifneq (,$(findstring t,$(MAKEFLAGS)))
6297 +ranlib -t archive.a
6304 The @samp{+} prefix marks those recipe lines as ``recursive'' so that
6305 they will be executed despite use of the @samp{-t} flag.
6306 @xref{Recursion, ,Recursive Use of @code{make}}.
6308 @node Functions, Running, Conditionals, Top
6309 @chapter Functions for Transforming Text
6312 @dfn{Functions} allow you to do text processing in the makefile to
6313 compute the files to operate on or the commands to use in recipes.
6314 You use a function in a @dfn{function call}, where you give the name
6315 of the function and some text (the @dfn{arguments}) for the function
6316 to operate on. The result of the function's processing is substituted
6317 into the makefile at the point of the call, just as a variable might
6321 * Syntax of Functions:: How to write a function call.
6322 * Text Functions:: General-purpose text manipulation functions.
6323 * File Name Functions:: Functions for manipulating file names.
6324 * Conditional Functions:: Functions that implement conditions.
6325 * Foreach Function:: Repeat some text with controlled variation.
6326 * Call Function:: Expand a user-defined function.
6327 * Value Function:: Return the un-expanded value of a variable.
6328 * Eval Function:: Evaluate the arguments as makefile syntax.
6329 * Origin Function:: Find where a variable got its value.
6330 * Flavor Function:: Find out the flavor of a variable.
6331 * Shell Function:: Substitute the output of a shell command.
6332 * Make Control Functions:: Functions that control how make runs.
6335 @node Syntax of Functions, Text Functions, Functions, Functions
6336 @section Function Call Syntax
6337 @cindex @code{$}, in function call
6338 @cindex dollar sign (@code{$}), in function call
6339 @cindex arguments of functions
6340 @cindex functions, syntax of
6342 A function call resembles a variable reference. It looks like this:
6345 $(@var{function} @var{arguments})
6352 $@{@var{function} @var{arguments}@}
6355 Here @var{function} is a function name; one of a short list of names
6356 that are part of @code{make}. You can also essentially create your own
6357 functions by using the @code{call} builtin function.
6359 The @var{arguments} are the arguments of the function. They are
6360 separated from the function name by one or more spaces or tabs, and if
6361 there is more than one argument, then they are separated by commas.
6362 Such whitespace and commas are not part of an argument's value. The
6363 delimiters which you use to surround the function call, whether
6364 parentheses or braces, can appear in an argument only in matching pairs;
6365 the other kind of delimiters may appear singly. If the arguments
6366 themselves contain other function calls or variable references, it is
6367 wisest to use the same kind of delimiters for all the references; write
6368 @w{@samp{$(subst a,b,$(x))}}, not @w{@samp{$(subst a,b,$@{x@})}}. This
6369 is because it is clearer, and because only one type of delimiter is
6370 matched to find the end of the reference.
6372 The text written for each argument is processed by substitution of
6373 variables and function calls to produce the argument value, which
6374 is the text on which the function acts. The substitution is done in the
6375 order in which the arguments appear.
6377 Commas and unmatched parentheses or braces cannot appear in the text of an
6378 argument as written; leading spaces cannot appear in the text of the first
6379 argument as written. These characters can be put into the argument value
6380 by variable substitution. First define variables @code{comma} and
6381 @code{space} whose values are isolated comma and space characters, then
6382 substitute these variables where such characters are wanted, like this:
6388 space:= $(empty) $(empty)
6390 bar:= $(subst $(space),$(comma),$(foo))
6391 # @r{bar is now `a,b,c'.}
6396 Here the @code{subst} function replaces each space with a comma, through
6397 the value of @code{foo}, and substitutes the result.
6399 @node Text Functions, File Name Functions, Syntax of Functions, Functions
6400 @section Functions for String Substitution and Analysis
6401 @cindex functions, for text
6403 Here are some functions that operate on strings:
6406 @item $(subst @var{from},@var{to},@var{text})
6408 Performs a textual replacement on the text @var{text}: each occurrence
6409 of @var{from} is replaced by @var{to}. The result is substituted for
6410 the function call. For example,
6413 $(subst ee,EE,feet on the street)
6416 substitutes the string @samp{fEEt on the strEEt}.
6418 @item $(patsubst @var{pattern},@var{replacement},@var{text})
6420 Finds whitespace-separated words in @var{text} that match
6421 @var{pattern} and replaces them with @var{replacement}. Here
6422 @var{pattern} may contain a @samp{%} which acts as a wildcard,
6423 matching any number of any characters within a word. If
6424 @var{replacement} also contains a @samp{%}, the @samp{%} is replaced
6425 by the text that matched the @samp{%} in @var{pattern}. Only the first
6426 @samp{%} in the @var{pattern} and @var{replacement} is treated this
6427 way; any subsequent @samp{%} is unchanged.@refill
6429 @cindex @code{%}, quoting in @code{patsubst}
6430 @cindex @code{%}, quoting with @code{\} (backslash)
6431 @cindex @code{\} (backslash), to quote @code{%}
6432 @cindex backslash (@code{\}), to quote @code{%}
6433 @cindex quoting @code{%}, in @code{patsubst}
6434 @samp{%} characters in @code{patsubst} function invocations can be
6435 quoted with preceding backslashes (@samp{\}). Backslashes that would
6436 otherwise quote @samp{%} characters can be quoted with more backslashes.
6437 Backslashes that quote @samp{%} characters or other backslashes are
6438 removed from the pattern before it is compared file names or has a stem
6439 substituted into it. Backslashes that are not in danger of quoting
6440 @samp{%} characters go unmolested. For example, the pattern
6441 @file{the\%weird\\%pattern\\} has @samp{the%weird\} preceding the
6442 operative @samp{%} character, and @samp{pattern\\} following it. The
6443 final two backslashes are left alone because they cannot affect any
6444 @samp{%} character.@refill
6446 Whitespace between words is folded into single space characters;
6447 leading and trailing whitespace is discarded.
6452 $(patsubst %.c,%.o,x.c.c bar.c)
6456 produces the value @samp{x.c.o bar.o}.
6458 Substitution references (@pxref{Substitution Refs, ,Substitution
6459 References}) are a simpler way to get the effect of the @code{patsubst}
6463 $(@var{var}:@var{pattern}=@var{replacement})
6470 $(patsubst @var{pattern},@var{replacement},$(@var{var}))
6473 The second shorthand simplifies one of the most common uses of
6474 @code{patsubst}: replacing the suffix at the end of file names.
6477 $(@var{var}:@var{suffix}=@var{replacement})
6484 $(patsubst %@var{suffix},%@var{replacement},$(@var{var}))
6488 For example, you might have a list of object files:
6491 objects = foo.o bar.o baz.o
6495 To get the list of corresponding source files, you could simply write:
6502 instead of using the general form:
6505 $(patsubst %.o,%.c,$(objects))
6508 @item $(strip @var{string})
6509 @cindex stripping whitespace
6510 @cindex whitespace, stripping
6511 @cindex spaces, stripping
6513 Removes leading and trailing whitespace from @var{string} and replaces
6514 each internal sequence of one or more whitespace characters with a
6515 single space. Thus, @samp{$(strip a b c )} results in @w{@samp{a b c}}.
6517 The function @code{strip} can be very useful when used in conjunction
6518 with conditionals. When comparing something with the empty string
6519 @samp{} using @code{ifeq} or @code{ifneq}, you usually want a string of
6520 just whitespace to match the empty string (@pxref{Conditionals}).
6522 Thus, the following may fail to have the desired results:
6526 ifneq "$(needs_made)" ""
6529 all:;@@echo 'Nothing to make!'
6534 Replacing the variable reference @w{@samp{$(needs_made)}} with the
6535 function call @w{@samp{$(strip $(needs_made))}} in the @code{ifneq}
6536 directive would make it more robust.@refill
6538 @item $(findstring @var{find},@var{in})
6540 @cindex searching for strings
6541 @cindex finding strings
6542 @cindex strings, searching for
6543 Searches @var{in} for an occurrence of @var{find}. If it occurs, the
6544 value is @var{find}; otherwise, the value is empty. You can use this
6545 function in a conditional to test for the presence of a specific
6546 substring in a given string. Thus, the two examples,
6549 $(findstring a,a b c)
6554 produce the values @samp{a} and @samp{} (the empty string),
6555 respectively. @xref{Testing Flags}, for a practical application of
6556 @code{findstring}.@refill
6560 @cindex filtering words
6561 @cindex words, filtering
6562 @item $(filter @var{pattern}@dots{},@var{text})
6563 Returns all whitespace-separated words in @var{text} that @emph{do} match
6564 any of the @var{pattern} words, removing any words that @emph{do not}
6565 match. The patterns are written using @samp{%}, just like the patterns
6566 used in the @code{patsubst} function above.@refill
6568 The @code{filter} function can be used to separate out different types
6569 of strings (such as file names) in a variable. For example:
6572 sources := foo.c bar.c baz.s ugh.h
6574 cc $(filter %.c %.s,$(sources)) -o foo
6578 says that @file{foo} depends of @file{foo.c}, @file{bar.c},
6579 @file{baz.s} and @file{ugh.h} but only @file{foo.c}, @file{bar.c} and
6580 @file{baz.s} should be specified in the command to the
6583 @item $(filter-out @var{pattern}@dots{},@var{text})
6585 @cindex filtering out words
6586 @cindex words, filtering out
6587 Returns all whitespace-separated words in @var{text} that @emph{do not}
6588 match any of the @var{pattern} words, removing the words that @emph{do}
6589 match one or more. This is the exact opposite of the @code{filter}
6596 objects=main1.o foo.o main2.o bar.o
6597 mains=main1.o main2.o
6602 the following generates a list which contains all the object files not
6606 $(filter-out $(mains),$(objects))
6611 @cindex sorting words
6612 @item $(sort @var{list})
6613 Sorts the words of @var{list} in lexical order, removing duplicate
6614 words. The output is a list of words separated by single spaces.
6618 $(sort foo bar lose)
6622 returns the value @samp{bar foo lose}.
6624 @cindex removing duplicate words
6625 @cindex duplicate words, removing
6626 @cindex words, removing duplicates
6627 Incidentally, since @code{sort} removes duplicate words, you can use
6628 it for this purpose even if you don't care about the sort order.
6630 @item $(word @var{n},@var{text})
6632 @cindex word, selecting a
6633 @cindex selecting a word
6634 Returns the @var{n}th word of @var{text}. The legitimate values of
6635 @var{n} start from 1. If @var{n} is bigger than the number of words
6636 in @var{text}, the value is empty. For example,
6639 $(word 2, foo bar baz)
6645 @item $(wordlist @var{s},@var{e},@var{text})
6647 @cindex words, selecting lists of
6648 @cindex selecting word lists
6649 Returns the list of words in @var{text} starting with word @var{s} and
6650 ending with word @var{e} (inclusive). The legitimate values of @var{s}
6651 start from 1; @var{e} may start from 0. If @var{s} is bigger than the
6652 number of words in @var{text}, the value is empty. If @var{e} is
6653 bigger than the number of words in @var{text}, words up to the end of
6654 @var{text} are returned. If @var{s} is greater than @var{e}, nothing
6655 is returned. For example,
6658 $(wordlist 2, 3, foo bar baz)
6662 returns @samp{bar baz}.
6664 @c Following item phrased to prevent overfull hbox. --RJC 17 Jul 92
6665 @item $(words @var{text})
6667 @cindex words, finding number
6668 Returns the number of words in @var{text}.
6669 Thus, the last word of @var{text} is
6670 @w{@code{$(word $(words @var{text}),@var{text})}}.@refill
6672 @item $(firstword @var{names}@dots{})
6674 @cindex words, extracting first
6675 The argument @var{names} is regarded as a series of names, separated
6676 by whitespace. The value is the first name in the series. The rest
6677 of the names are ignored.
6682 $(firstword foo bar)
6686 produces the result @samp{foo}. Although @code{$(firstword
6687 @var{text})} is the same as @code{$(word 1,@var{text})}, the
6688 @code{firstword} function is retained for its simplicity.@refill
6691 @item $(lastword @var{names}@dots{})
6693 @cindex words, extracting last
6694 The argument @var{names} is regarded as a series of names, separated
6695 by whitespace. The value is the last name in the series.
6704 produces the result @samp{bar}. Although @code{$(lastword
6705 @var{text})} is the same as @code{$(word $(words @var{text}),@var{text})},
6706 the @code{lastword} function was added for its simplicity and better
6711 Here is a realistic example of the use of @code{subst} and
6712 @code{patsubst}. Suppose that a makefile uses the @code{VPATH} variable
6713 to specify a list of directories that @code{make} should search for
6715 (@pxref{General Search, , @code{VPATH} Search Path for All Prerequisites}).
6716 This example shows how to
6717 tell the C compiler to search for header files in the same list of
6720 The value of @code{VPATH} is a list of directories separated by colons,
6721 such as @samp{src:../headers}. First, the @code{subst} function is used to
6722 change the colons to spaces:
6725 $(subst :, ,$(VPATH))
6729 This produces @samp{src ../headers}. Then @code{patsubst} is used to turn
6730 each directory name into a @samp{-I} flag. These can be added to the
6731 value of the variable @code{CFLAGS}, which is passed automatically to the C
6732 compiler, like this:
6735 override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
6739 The effect is to append the text @samp{-Isrc -I../headers} to the
6740 previously given value of @code{CFLAGS}. The @code{override} directive is
6741 used so that the new value is assigned even if the previous value of
6742 @code{CFLAGS} was specified with a command argument (@pxref{Override
6743 Directive, , The @code{override} Directive}).
6745 @node File Name Functions, Conditional Functions, Text Functions, Functions
6746 @section Functions for File Names
6747 @cindex functions, for file names
6748 @cindex file name functions
6750 Several of the built-in expansion functions relate specifically to
6751 taking apart file names or lists of file names.
6753 Each of the following functions performs a specific transformation on a
6754 file name. The argument of the function is regarded as a series of file
6755 names, separated by whitespace. (Leading and trailing whitespace is
6756 ignored.) Each file name in the series is transformed in the same way and
6757 the results are concatenated with single spaces between them.
6760 @item $(dir @var{names}@dots{})
6762 @cindex directory part
6763 @cindex file name, directory part
6764 Extracts the directory-part of each file name in @var{names}. The
6765 directory-part of the file name is everything up through (and
6766 including) the last slash in it. If the file name contains no slash,
6767 the directory part is the string @samp{./}. For example,
6770 $(dir src/foo.c hacks)
6774 produces the result @samp{src/ ./}.
6776 @item $(notdir @var{names}@dots{})
6778 @cindex file name, nondirectory part
6779 @cindex nondirectory part
6780 Extracts all but the directory-part of each file name in @var{names}.
6781 If the file name contains no slash, it is left unchanged. Otherwise,
6782 everything through the last slash is removed from it.
6784 A file name that ends with a slash becomes an empty string. This is
6785 unfortunate, because it means that the result does not always have the
6786 same number of whitespace-separated file names as the argument had;
6787 but we do not see any other valid alternative.
6792 $(notdir src/foo.c hacks)
6796 produces the result @samp{foo.c hacks}.
6798 @item $(suffix @var{names}@dots{})
6800 @cindex suffix, function to find
6801 @cindex file name suffix
6802 Extracts the suffix of each file name in @var{names}. If the file name
6803 contains a period, the suffix is everything starting with the last
6804 period. Otherwise, the suffix is the empty string. This frequently
6805 means that the result will be empty when @var{names} is not, and if
6806 @var{names} contains multiple file names, the result may contain fewer
6812 $(suffix src/foo.c src-1.0/bar.c hacks)
6816 produces the result @samp{.c .c}.
6818 @item $(basename @var{names}@dots{})
6821 @cindex file name, basename of
6822 Extracts all but the suffix of each file name in @var{names}. If the
6823 file name contains a period, the basename is everything starting up to
6824 (and not including) the last period. Periods in the directory part are
6825 ignored. If there is no period, the basename is the entire file name.
6829 $(basename src/foo.c src-1.0/bar hacks)
6833 produces the result @samp{src/foo src-1.0/bar hacks}.
6835 @c plural convention with dots (be consistent)
6836 @item $(addsuffix @var{suffix},@var{names}@dots{})
6838 @cindex suffix, adding
6839 @cindex file name suffix, adding
6840 The argument @var{names} is regarded as a series of names, separated
6841 by whitespace; @var{suffix} is used as a unit. The value of
6842 @var{suffix} is appended to the end of each individual name and the
6843 resulting larger names are concatenated with single spaces between
6847 $(addsuffix .c,foo bar)
6851 produces the result @samp{foo.c bar.c}.
6853 @item $(addprefix @var{prefix},@var{names}@dots{})
6855 @cindex prefix, adding
6856 @cindex file name prefix, adding
6857 The argument @var{names} is regarded as a series of names, separated
6858 by whitespace; @var{prefix} is used as a unit. The value of
6859 @var{prefix} is prepended to the front of each individual name and the
6860 resulting larger names are concatenated with single spaces between
6864 $(addprefix src/,foo bar)
6868 produces the result @samp{src/foo src/bar}.
6870 @item $(join @var{list1},@var{list2})
6872 @cindex joining lists of words
6873 @cindex words, joining lists
6874 Concatenates the two arguments word by word: the two first words (one
6875 from each argument) concatenated form the first word of the result, the
6876 two second words form the second word of the result, and so on. So the
6877 @var{n}th word of the result comes from the @var{n}th word of each
6878 argument. If one argument has more words that the other, the extra
6879 words are copied unchanged into the result.
6881 For example, @samp{$(join a b,.c .o)} produces @samp{a.c b.o}.
6883 Whitespace between the words in the lists is not preserved; it is
6884 replaced with a single space.
6886 This function can merge the results of the @code{dir} and
6887 @code{notdir} functions, to produce the original list of files which
6888 was given to those two functions.@refill
6890 @item $(wildcard @var{pattern})
6892 @cindex wildcard, function
6893 The argument @var{pattern} is a file name pattern, typically containing
6894 wildcard characters (as in shell file name patterns). The result of
6895 @code{wildcard} is a space-separated list of the names of existing files
6896 that match the pattern.
6897 @xref{Wildcards, ,Using Wildcard Characters in File Names}.
6899 @item $(realpath @var{names}@dots{})
6902 @cindex file name, realpath of
6903 For each file name in @var{names} return the canonical absolute name.
6904 A canonical name does not contain any @code{.} or @code{..} components,
6905 nor any repeated path separators (@code{/}) or symlinks. In case of a
6906 failure the empty string is returned. Consult the @code{realpath(3)}
6907 documentation for a list of possible failure causes.
6909 @item $(abspath @var{names}@dots{})
6912 @cindex file name, abspath of
6913 For each file name in @var{names} return an absolute name that does
6914 not contain any @code{.} or @code{..} components, nor any repeated path
6915 separators (@code{/}). Note that, in contrast to @code{realpath}
6916 function, @code{abspath} does not resolve symlinks and does not require
6917 the file names to refer to an existing file or directory. Use the
6918 @code{wildcard} function to test for existence.
6921 @node Conditional Functions, Foreach Function, File Name Functions, Functions
6922 @section Functions for Conditionals
6924 @cindex conditional expansion
6925 There are three functions that provide conditional expansion. A key
6926 aspect of these functions is that not all of the arguments are
6927 expanded initially. Only those arguments which need to be expanded,
6931 @item $(if @var{condition},@var{then-part}[,@var{else-part}])
6933 The @code{if} function provides support for conditional expansion in a
6934 functional context (as opposed to the GNU @code{make} makefile
6935 conditionals such as @code{ifeq} (@pxref{Conditional Syntax, ,Syntax of
6938 The first argument, @var{condition}, first has all preceding and
6939 trailing whitespace stripped, then is expanded. If it expands to any
6940 non-empty string, then the condition is considered to be true. If it
6941 expands to an empty string, the condition is considered to be false.
6943 If the condition is true then the second argument, @var{then-part}, is
6944 evaluated and this is used as the result of the evaluation of the entire
6947 If the condition is false then the third argument, @var{else-part}, is
6948 evaluated and this is the result of the @code{if} function. If there is
6949 no third argument, the @code{if} function evaluates to nothing (the
6952 Note that only one of the @var{then-part} or the @var{else-part} will be
6953 evaluated, never both. Thus, either can contain side-effects (such as
6954 @code{shell} function calls, etc.)
6956 @item $(or @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
6958 The @code{or} function provides a ``short-circuiting'' OR operation.
6959 Each argument is expanded, in order. If an argument expands to a
6960 non-empty string the processing stops and the result of the expansion
6961 is that string. If, after all arguments are expanded, all of them are
6962 false (empty), then the result of the expansion is the empty string.
6964 @item $(and @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
6966 The @code{and} function provides a ``short-circuiting'' AND operation.
6967 Each argument is expanded, in order. If an argument expands to an
6968 empty string the processing stops and the result of the expansion is
6969 the empty string. If all arguments expand to a non-empty string then
6970 the result of the expansion is the expansion of the last argument.
6974 @node Foreach Function, Call Function, Conditional Functions, Functions
6975 @section The @code{foreach} Function
6977 @cindex words, iterating over
6979 The @code{foreach} function is very different from other functions. It
6980 causes one piece of text to be used repeatedly, each time with a different
6981 substitution performed on it. It resembles the @code{for} command in the
6982 shell @code{sh} and the @code{foreach} command in the C-shell @code{csh}.
6984 The syntax of the @code{foreach} function is:
6987 $(foreach @var{var},@var{list},@var{text})
6991 The first two arguments, @var{var} and @var{list}, are expanded before
6992 anything else is done; note that the last argument, @var{text}, is
6993 @strong{not} expanded at the same time. Then for each word of the expanded
6994 value of @var{list}, the variable named by the expanded value of @var{var}
6995 is set to that word, and @var{text} is expanded. Presumably @var{text}
6996 contains references to that variable, so its expansion will be different
6999 The result is that @var{text} is expanded as many times as there are
7000 whitespace-separated words in @var{list}. The multiple expansions of
7001 @var{text} are concatenated, with spaces between them, to make the result
7004 This simple example sets the variable @samp{files} to the list of all files
7005 in the directories in the list @samp{dirs}:
7009 files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
7012 Here @var{text} is @samp{$(wildcard $(dir)/*)}. The first repetition
7013 finds the value @samp{a} for @code{dir}, so it produces the same result
7014 as @samp{$(wildcard a/*)}; the second repetition produces the result
7015 of @samp{$(wildcard b/*)}; and the third, that of @samp{$(wildcard c/*)}.
7017 This example has the same result (except for setting @samp{dirs}) as
7018 the following example:
7021 files := $(wildcard a/* b/* c/* d/*)
7024 When @var{text} is complicated, you can improve readability by giving it
7025 a name, with an additional variable:
7028 find_files = $(wildcard $(dir)/*)
7030 files := $(foreach dir,$(dirs),$(find_files))
7034 Here we use the variable @code{find_files} this way. We use plain @samp{=}
7035 to define a recursively-expanding variable, so that its value contains an
7036 actual function call to be reexpanded under the control of @code{foreach};
7037 a simply-expanded variable would not do, since @code{wildcard} would be
7038 called only once at the time of defining @code{find_files}.
7040 The @code{foreach} function has no permanent effect on the variable
7041 @var{var}; its value and flavor after the @code{foreach} function call are
7042 the same as they were beforehand. The other values which are taken from
7043 @var{list} are in effect only temporarily, during the execution of
7044 @code{foreach}. The variable @var{var} is a simply-expanded variable
7045 during the execution of @code{foreach}. If @var{var} was undefined
7046 before the @code{foreach} function call, it is undefined after the call.
7047 @xref{Flavors, ,The Two Flavors of Variables}.@refill
7049 You must take care when using complex variable expressions that result in
7050 variable names because many strange things are valid variable names, but
7051 are probably not what you intended. For example,
7054 files := $(foreach Esta escrito en espanol!,b c ch,$(find_files))
7058 might be useful if the value of @code{find_files} references the variable
7059 whose name is @samp{Esta escrito en espanol!} (es un nombre bastante largo,
7060 no?), but it is more likely to be a mistake.
7062 @node Call Function, Value Function, Foreach Function, Functions
7063 @section The @code{call} Function
7065 @cindex functions, user defined
7066 @cindex user defined functions
7068 The @code{call} function is unique in that it can be used to create new
7069 parameterized functions. You can write a complex expression as the
7070 value of a variable, then use @code{call} to expand it with different
7073 The syntax of the @code{call} function is:
7076 $(call @var{variable},@var{param},@var{param},@dots{})
7079 When @code{make} expands this function, it assigns each @var{param} to
7080 temporary variables @code{$(1)}, @code{$(2)}, etc. The variable
7081 @code{$(0)} will contain @var{variable}. There is no maximum number of
7082 parameter arguments. There is no minimum, either, but it doesn't make
7083 sense to use @code{call} with no parameters.
7085 Then @var{variable} is expanded as a @code{make} variable in the context
7086 of these temporary assignments. Thus, any reference to @code{$(1)} in
7087 the value of @var{variable} will resolve to the first @var{param} in the
7088 invocation of @code{call}.
7090 Note that @var{variable} is the @emph{name} of a variable, not a
7091 @emph{reference} to that variable. Therefore you would not normally use
7092 a @samp{$} or parentheses when writing it. (You can, however, use a
7093 variable reference in the name if you want the name not to be a
7096 If @var{variable} is the name of a builtin function, the builtin function
7097 is always invoked (even if a @code{make} variable by that name also
7100 The @code{call} function expands the @var{param} arguments before
7101 assigning them to temporary variables. This means that @var{variable}
7102 values containing references to builtin functions that have special
7103 expansion rules, like @code{foreach} or @code{if}, may not work as you
7106 Some examples may make this clearer.
7108 This macro simply reverses its arguments:
7113 foo = $(call reverse,a,b)
7117 Here @var{foo} will contain @samp{b a}.
7119 This one is slightly more interesting: it defines a macro to search for
7120 the first instance of a program in @code{PATH}:
7123 pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH)))))
7125 LS := $(call pathsearch,ls)
7129 Now the variable LS contains @code{/bin/ls} or similar.
7131 The @code{call} function can be nested. Each recursive invocation gets
7132 its own local values for @code{$(1)}, etc.@: that mask the values of
7133 higher-level @code{call}. For example, here is an implementation of a
7137 map = $(foreach a,$(2),$(call $(1),$(a)))
7140 Now you can @var{map} a function that normally takes only one argument,
7141 such as @code{origin}, to multiple values in one step:
7144 o = $(call map,origin,o map MAKE)
7147 and end up with @var{o} containing something like @samp{file file default}.
7149 A final caution: be careful when adding whitespace to the arguments to
7150 @code{call}. As with other functions, any whitespace contained in the
7151 second and subsequent arguments is kept; this can cause strange
7152 effects. It's generally safest to remove all extraneous whitespace when
7153 providing parameters to @code{call}.
7155 @node Value Function, Eval Function, Call Function, Functions
7156 @comment node-name, next, previous, up
7157 @section The @code{value} Function
7159 @cindex variables, unexpanded value
7161 The @code{value} function provides a way for you to use the value of a
7162 variable @emph{without} having it expanded. Please note that this
7163 does not undo expansions which have already occurred; for example if
7164 you create a simply expanded variable its value is expanded during the
7165 definition; in that case the @code{value} function will return the
7166 same result as using the variable directly.
7168 The syntax of the @code{value} function is:
7171 $(value @var{variable})
7174 Note that @var{variable} is the @emph{name} of a variable; not a
7175 @emph{reference} to that variable. Therefore you would not normally
7176 use a @samp{$} or parentheses when writing it. (You can, however, use
7177 a variable reference in the name if you want the name not to be a
7180 The result of this function is a string containing the value of
7181 @var{variable}, without any expansion occurring. For example, in this
7195 The first output line would be @code{ATH}, since the ``$P'' would be
7196 expanded as a @code{make} variable, while the second output line would
7197 be the current value of your @code{$PATH} environment variable, since
7198 the @code{value} function avoided the expansion.
7200 The @code{value} function is most often used in conjunction with the
7201 @code{eval} function (@pxref{Eval Function}).
7203 @node Eval Function, Origin Function, Value Function, Functions
7204 @comment node-name, next, previous, up
7205 @section The @code{eval} Function
7207 @cindex evaluating makefile syntax
7208 @cindex makefile syntax, evaluating
7210 The @code{eval} function is very special: it allows you to define new
7211 makefile constructs that are not constant; which are the result of
7212 evaluating other variables and functions. The argument to the
7213 @code{eval} function is expanded, then the results of that expansion
7214 are parsed as makefile syntax. The expanded results can define new
7215 @code{make} variables, targets, implicit or explicit rules, etc.
7217 The result of the @code{eval} function is always the empty string;
7218 thus, it can be placed virtually anywhere in a makefile without
7219 causing syntax errors.
7221 It's important to realize that the @code{eval} argument is expanded
7222 @emph{twice}; first by the @code{eval} function, then the results of
7223 that expansion are expanded again when they are parsed as makefile
7224 syntax. This means you may need to provide extra levels of escaping
7225 for ``$'' characters when using @code{eval}. The @code{value}
7226 function (@pxref{Value Function}) can sometimes be useful in these
7227 situations, to circumvent unwanted expansions.
7229 Here is an example of how @code{eval} can be used; this example
7230 combines a number of concepts and other functions. Although it might
7231 seem overly complex to use @code{eval} in this example, rather than
7232 just writing out the rules, consider two things: first, the template
7233 definition (in @code{PROGRAM_template}) could need to be much more
7234 complex than it is here; and second, you might put the complex,
7235 ``generic'' part of this example into another makefile, then include
7236 it in all the individual makefiles. Now your individual makefiles are
7237 quite straightforward.
7241 PROGRAMS = server client
7243 server_OBJS = server.o server_priv.o server_access.o
7244 server_LIBS = priv protocol
7246 client_OBJS = client.o client_api.o client_mem.o
7247 client_LIBS = protocol
7249 # Everything after this is generic
7254 define PROGRAM_template =
7255 $(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
7256 ALL_OBJS += $$($(1)_OBJS)
7259 $(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))
7262 $(LINK.o) $^ $(LDLIBS) -o $@@
7265 rm -f $(ALL_OBJS) $(PROGRAMS)
7269 @node Origin Function, Flavor Function, Eval Function, Functions
7270 @section The @code{origin} Function
7272 @cindex variables, origin of
7273 @cindex origin of variable
7275 The @code{origin} function is unlike most other functions in that it does
7276 not operate on the values of variables; it tells you something @emph{about}
7277 a variable. Specifically, it tells you where it came from.
7279 The syntax of the @code{origin} function is:
7282 $(origin @var{variable})
7285 Note that @var{variable} is the @emph{name} of a variable to inquire about;
7286 not a @emph{reference} to that variable. Therefore you would not normally
7287 use a @samp{$} or parentheses when writing it. (You can, however, use a
7288 variable reference in the name if you want the name not to be a constant.)
7290 The result of this function is a string telling you how the variable
7291 @var{variable} was defined:
7296 if @var{variable} was never defined.
7300 if @var{variable} has a default definition, as is usual with @code{CC}
7301 and so on. @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
7302 Note that if you have redefined a default variable, the @code{origin}
7303 function will return the origin of the later definition.
7307 if @var{variable} was inherited from the environment provided to
7310 @item environment override
7312 if @var{variable} was inherited from the environment provided to
7313 @code{make}, and is overriding a setting for @var{variable} in the
7314 makefile as a result of the @w{@samp{-e}} option (@pxref{Options
7315 Summary, ,Summary of Options}).@refill
7319 if @var{variable} was defined in a makefile.
7323 if @var{variable} was defined on the command line.
7327 if @var{variable} was defined with an @code{override} directive in a
7328 makefile (@pxref{Override Directive, ,The @code{override} Directive}).
7332 if @var{variable} is an automatic variable defined for the execution
7333 of the recipe for each rule (@pxref{Automatic Variables}).
7336 This information is primarily useful (other than for your curiosity) to
7337 determine if you want to believe the value of a variable. For example,
7338 suppose you have a makefile @file{foo} that includes another makefile
7339 @file{bar}. You want a variable @code{bletch} to be defined in @file{bar}
7340 if you run the command @w{@samp{make -f bar}}, even if the environment contains
7341 a definition of @code{bletch}. However, if @file{foo} defined
7342 @code{bletch} before including @file{bar}, you do not want to override that
7343 definition. This could be done by using an @code{override} directive in
7344 @file{foo}, giving that definition precedence over the later definition in
7345 @file{bar}; unfortunately, the @code{override} directive would also
7346 override any command line definitions. So, @file{bar} could
7352 ifeq "$(origin bletch)" "environment"
7353 bletch = barf, gag, etc.
7360 If @code{bletch} has been defined from the environment, this will redefine
7363 If you want to override a previous definition of @code{bletch} if it came
7364 from the environment, even under @samp{-e}, you could instead write:
7368 ifneq "$(findstring environment,$(origin bletch))" ""
7369 bletch = barf, gag, etc.
7374 Here the redefinition takes place if @samp{$(origin bletch)} returns either
7375 @samp{environment} or @samp{environment override}.
7376 @xref{Text Functions, , Functions for String Substitution and Analysis}.
7378 @node Flavor Function, Shell Function, Origin Function, Functions
7379 @section The @code{flavor} Function
7381 @cindex variables, flavor of
7382 @cindex flavor of variable
7384 The @code{flavor} function is unlike most other functions (and like
7385 @code{origin} function) in that it does not operate on the values of
7386 variables; it tells you something @emph{about} a variable.
7387 Specifically, it tells you the flavor of a variable (@pxref{Flavors,
7388 ,The Two Flavors of Variables}).
7390 The syntax of the @code{flavor} function is:
7393 $(flavor @var{variable})
7396 Note that @var{variable} is the @emph{name} of a variable to inquire about;
7397 not a @emph{reference} to that variable. Therefore you would not normally
7398 use a @samp{$} or parentheses when writing it. (You can, however, use a
7399 variable reference in the name if you want the name not to be a constant.)
7401 The result of this function is a string that identifies the flavor of the
7402 variable @var{variable}:
7407 if @var{variable} was never defined.
7411 if @var{variable} is a recursively expanded variable.
7415 if @var{variable} is a simply expanded variable.
7420 @node Shell Function, Make Control Functions, Flavor Function, Functions
7421 @section The @code{shell} Function
7423 @cindex command expansion
7425 @cindex shell command, function for
7427 The @code{shell} function is unlike any other function other than the
7428 @code{wildcard} function
7429 (@pxref{Wildcard Function, ,The Function @code{wildcard}}) in that it
7430 communicates with the world outside of @code{make}.
7432 The @code{shell} function performs the same function that backquotes
7433 (@samp{`}) perform in most shells: it does @dfn{command expansion}.
7434 This means that it takes as an argument a shell command and evaluates
7435 to the output of the command. The only processing @code{make} does on
7436 the result is to convert each newline (or carriage-return / newline
7437 pair) to a single space. If there is a trailing (carriage-return
7438 and) newline it will simply be removed.@refill
7440 The commands run by calls to the @code{shell} function are run when the
7441 function calls are expanded (@pxref{Reading Makefiles, , How
7442 @code{make} Reads a Makefile}). Because this function involves
7443 spawning a new shell, you should carefully consider the performance
7444 implications of using the @code{shell} function within recursively
7445 expanded variables vs.@: simply expanded variables (@pxref{Flavors, ,The
7446 Two Flavors of Variables}).
7448 Here are some examples of the use of the @code{shell} function:
7451 contents := $(shell cat foo)
7455 sets @code{contents} to the contents of the file @file{foo}, with a space
7456 (rather than a newline) separating each line.
7459 files := $(shell echo *.c)
7463 sets @code{files} to the expansion of @samp{*.c}. Unless @code{make} is
7464 using a very strange shell, this has the same result as
7465 @w{@samp{$(wildcard *.c)}} (as long as at least one @samp{.c} file
7468 @node Make Control Functions, , Shell Function, Functions
7469 @section Functions That Control Make
7470 @cindex functions, for controlling make
7471 @cindex controlling make
7473 These functions control the way make runs. Generally, they are used to
7474 provide information to the user of the makefile or to cause make to stop
7475 if some sort of environmental error is detected.
7478 @item $(error @var{text}@dots{})
7480 @cindex error, stopping on
7481 @cindex stopping make
7482 Generates a fatal error where the message is @var{text}. Note that
7483 the error is generated whenever this function is evaluated. So, if
7484 you put it inside a recipe or on the right side of a recursive
7485 variable assignment, it won't be evaluated until later. The
7486 @var{text} will be expanded before the error is generated.
7492 $(error error is $(ERROR1))
7497 will generate a fatal error during the read of the makefile if the
7498 @code{make} variable @code{ERROR1} is defined. Or,
7501 ERR = $(error found an error!)
7508 will generate a fatal error while @code{make} is running, if the
7509 @code{err} target is invoked.
7511 @item $(warning @var{text}@dots{})
7513 @cindex warnings, printing
7514 @cindex printing user warnings
7515 This function works similarly to the @code{error} function, above,
7516 except that @code{make} doesn't exit. Instead, @var{text} is expanded
7517 and the resulting message is displayed, but processing of the makefile
7520 The result of the expansion of this function is the empty string.
7522 @item $(info @var{text}@dots{})
7524 @cindex printing messages
7525 This function does nothing more than print its (expanded) argument(s)
7526 to standard output. No makefile name or line number is added. The
7527 result of the expansion of this function is the empty string.
7530 @node Running, Implicit Rules, Functions, Top
7531 @chapter How to Run @code{make}
7533 A makefile that says how to recompile a program can be used in more
7534 than one way. The simplest use is to recompile every file that is out
7535 of date. Usually, makefiles are written so that if you run
7536 @code{make} with no arguments, it does just that.
7538 But you might want to update only some of the files; you might want to use
7539 a different compiler or different compiler options; you might want just to
7540 find out which files are out of date without changing them.
7542 By giving arguments when you run @code{make}, you can do any of these
7543 things and many others.
7545 @cindex exit status of make
7546 The exit status of @code{make} is always one of three values:
7549 The exit status is zero if @code{make} is successful.
7551 The exit status is two if @code{make} encounters any errors.
7552 It will print messages describing the particular errors.
7554 The exit status is one if you use the @samp{-q} flag and @code{make}
7555 determines that some target is not already up to date.
7556 @xref{Instead of Execution, ,Instead of Executing Recipes}.
7560 * Makefile Arguments:: How to specify which makefile to use.
7561 * Goals:: How to use goal arguments to specify which
7562 parts of the makefile to use.
7563 * Instead of Execution:: How to use mode flags to specify what
7564 kind of thing to do with the recipes
7565 in the makefile other than simply
7567 * Avoiding Compilation:: How to avoid recompiling certain files.
7568 * Overriding:: How to override a variable to specify
7569 an alternate compiler and other things.
7570 * Testing:: How to proceed past some errors, to
7572 * Options Summary:: Summary of Options
7575 @node Makefile Arguments, Goals, Running, Running
7576 @section Arguments to Specify the Makefile
7577 @cindex @code{--file}
7578 @cindex @code{--makefile}
7581 The way to specify the name of the makefile is with the @samp{-f} or
7582 @samp{--file} option (@samp{--makefile} also works). For example,
7583 @samp{-f altmake} says to use the file @file{altmake} as the makefile.
7585 If you use the @samp{-f} flag several times and follow each @samp{-f}
7586 with an argument, all the specified files are used jointly as
7589 If you do not use the @samp{-f} or @samp{--file} flag, the default is
7590 to try @file{GNUmakefile}, @file{makefile}, and @file{Makefile}, in
7591 that order, and use the first of these three which exists or can be made
7592 (@pxref{Makefiles, ,Writing Makefiles}).@refill
7594 @node Goals, Instead of Execution, Makefile Arguments, Running
7595 @section Arguments to Specify the Goals
7596 @cindex goal, how to specify
7598 The @dfn{goals} are the targets that @code{make} should strive ultimately
7599 to update. Other targets are updated as well if they appear as
7600 prerequisites of goals, or prerequisites of prerequisites of goals, etc.
7602 By default, the goal is the first target in the makefile (not counting
7603 targets that start with a period). Therefore, makefiles are usually
7604 written so that the first target is for compiling the entire program or
7605 programs they describe. If the first rule in the makefile has several
7606 targets, only the first target in the rule becomes the default goal, not
7607 the whole list. You can manage the selection of the default goal from
7608 within your makefile using the @code{.DEFAULT_GOAL} variable
7609 (@pxref{Special Variables, , Other Special Variables}).
7611 You can also specify a different goal or goals with command line
7612 arguments to @code{make}. Use the name of the goal as an argument.
7613 If you specify several goals, @code{make} processes each of them in
7614 turn, in the order you name them.
7616 Any target in the makefile may be specified as a goal (unless it
7617 starts with @samp{-} or contains an @samp{=}, in which case it will be
7618 parsed as a switch or variable definition, respectively). Even
7619 targets not in the makefile may be specified, if @code{make} can find
7620 implicit rules that say how to make them.
7622 @vindex MAKECMDGOALS
7623 @code{Make} will set the special variable @code{MAKECMDGOALS} to the
7624 list of goals you specified on the command line. If no goals were given
7625 on the command line, this variable is empty. Note that this variable
7626 should be used only in special circumstances.
7628 An example of appropriate use is to avoid including @file{.d} files
7629 during @code{clean} rules (@pxref{Automatic Prerequisites}), so
7630 @code{make} won't create them only to immediately remove them
7635 sources = foo.c bar.c
7637 ifneq ($(MAKECMDGOALS),clean)
7638 include $(sources:.c=.d)
7643 One use of specifying a goal is if you want to compile only a part of
7644 the program, or only one of several programs. Specify as a goal each
7645 file that you wish to remake. For example, consider a directory containing
7646 several programs, with a makefile that starts like this:
7650 all: size nm ld ar as
7653 If you are working on the program @code{size}, you might want to say
7654 @w{@samp{make size}} so that only the files of that program are recompiled.
7656 Another use of specifying a goal is to make files that are not normally
7657 made. For example, there may be a file of debugging output, or a
7658 version of the program that is compiled specially for testing, which has
7659 a rule in the makefile but is not a prerequisite of the default goal.
7661 Another use of specifying a goal is to run the recipe associated with
7662 a phony target (@pxref{Phony Targets}) or empty target (@pxref{Empty
7663 Targets, ,Empty Target Files to Record Events}). Many makefiles contain
7664 a phony target named @file{clean} which deletes everything except source
7665 files. Naturally, this is done only if you request it explicitly with
7666 @w{@samp{make clean}}. Following is a list of typical phony and empty
7667 target names. @xref{Standard Targets}, for a detailed list of all the
7668 standard target names which GNU software packages use.
7672 @cindex @code{all} @r{(standard target)}
7673 Make all the top-level targets the makefile knows about.
7676 @cindex @code{clean} @r{(standard target)}
7677 Delete all files that are normally created by running @code{make}.
7680 @cindex @code{mostlyclean} @r{(standard target)}
7681 Like @samp{clean}, but may refrain from deleting a few files that people
7682 normally don't want to recompile. For example, the @samp{mostlyclean}
7683 target for GCC does not delete @file{libgcc.a}, because recompiling it
7684 is rarely necessary and takes a lot of time.
7687 @cindex @code{distclean} @r{(standard target)}
7689 @cindex @code{realclean} @r{(standard target)}
7691 @cindex @code{clobber} @r{(standard target)}
7692 Any of these targets might be defined to delete @emph{more} files than
7693 @samp{clean} does. For example, this would delete configuration files
7694 or links that you would normally create as preparation for compilation,
7695 even if the makefile itself cannot create these files.
7698 @cindex @code{install} @r{(standard target)}
7699 Copy the executable file into a directory that users typically search
7700 for commands; copy any auxiliary files that the executable uses into
7701 the directories where it will look for them.
7704 @cindex @code{print} @r{(standard target)}
7705 Print listings of the source files that have changed.
7708 @cindex @code{tar} @r{(standard target)}
7709 Create a tar file of the source files.
7712 @cindex @code{shar} @r{(standard target)}
7713 Create a shell archive (shar file) of the source files.
7716 @cindex @code{dist} @r{(standard target)}
7717 Create a distribution file of the source files. This might
7718 be a tar file, or a shar file, or a compressed version of one of the
7719 above, or even more than one of the above.
7722 @cindex @code{TAGS} @r{(standard target)}
7723 Update a tags table for this program.
7726 @cindex @code{check} @r{(standard target)}
7728 @cindex @code{test} @r{(standard target)}
7729 Perform self tests on the program this makefile builds.
7732 @node Instead of Execution, Avoiding Compilation, Goals, Running
7733 @section Instead of Executing Recipes
7734 @cindex execution, instead of
7735 @cindex recipes, instead of executing
7737 The makefile tells @code{make} how to tell whether a target is up to date,
7738 and how to update each target. But updating the targets is not always
7739 what you want. Certain options specify other activities for @code{make}.
7741 @comment Extra blank lines make it print better.
7747 @cindex @code{--just-print}
7748 @cindex @code{--dry-run}
7749 @cindex @code{--recon}
7752 ``No-op''. The activity is to print what recipe would be used to make
7753 the targets up to date, but not actually execute it. Some recipes are
7754 still executed, even with this flag (@pxref{MAKE Variable, ,How the @code{MAKE} Variable Works}).
7758 @cindex @code{--touch}
7759 @cindex touching files
7760 @cindex target, touching
7763 ``Touch''. The activity is to mark the targets as up to date without
7764 actually changing them. In other words, @code{make} pretends to compile
7765 the targets but does not really change their contents.
7769 @cindex @code{--question}
7771 @cindex question mode
7773 ``Question''. The activity is to find out silently whether the targets
7774 are up to date already; but execute no recipe in either case. In other
7775 words, neither compilation nor output will occur.
7778 @itemx --what-if=@var{file}
7779 @itemx --assume-new=@var{file}
7780 @itemx --new-file=@var{file}
7781 @cindex @code{--what-if}
7783 @cindex @code{--assume-new}
7784 @cindex @code{--new-file}
7786 @cindex files, assuming new
7788 ``What if''. Each @samp{-W} flag is followed by a file name. The given
7789 files' modification times are recorded by @code{make} as being the present
7790 time, although the actual modification times remain the same.
7791 You can use the @samp{-W} flag in conjunction with the @samp{-n} flag
7792 to see what would happen if you were to modify specific files.@refill
7795 With the @samp{-n} flag, @code{make} prints the recipe that it would
7796 normally execute but usually does not execute it.
7798 With the @samp{-t} flag, @code{make} ignores the recipes in the rules
7799 and uses (in effect) the command @code{touch} for each target that needs to
7800 be remade. The @code{touch} command is also printed, unless @samp{-s} or
7801 @code{.SILENT} is used. For speed, @code{make} does not actually invoke
7802 the program @code{touch}. It does the work directly.
7804 With the @samp{-q} flag, @code{make} prints nothing and executes no
7805 recipes, but the exit status code it returns is zero if and only if the
7806 targets to be considered are already up to date. If the exit status is
7807 one, then some updating needs to be done. If @code{make} encounters an
7808 error, the exit status is two, so you can distinguish an error from a
7809 target that is not up to date.
7811 It is an error to use more than one of these three flags in the same
7812 invocation of @code{make}.
7814 @cindex +, and recipe execution
7815 The @samp{-n}, @samp{-t}, and @samp{-q} options do not affect recipe
7816 lines that begin with @samp{+} characters or contain the strings
7817 @samp{$(MAKE)} or @samp{$@{MAKE@}}. Note that only the line containing
7818 the @samp{+} character or the strings @samp{$(MAKE)} or @samp{$@{MAKE@}}
7819 is run regardless of these options. Other lines in the same rule are
7820 not run unless they too begin with @samp{+} or contain @samp{$(MAKE)} or
7821 @samp{$@{MAKE@}} (@xref{MAKE Variable, ,How the @code{MAKE} Variable Works}.)
7823 @cindex phony targets and recipe execution
7824 The @samp{-t} flag prevents phony targets (@pxref{Phony Targets}) from
7825 being updated, unless there are recipe lines beginning with @samp{+}
7826 or containing @samp{$(MAKE)} or @samp{$@{MAKE@}}.
7828 The @samp{-W} flag provides two features:
7832 If you also use the @samp{-n} or @samp{-q} flag, you can see what
7833 @code{make} would do if you were to modify some files.
7836 Without the @samp{-n} or @samp{-q} flag, when @code{make} is actually
7837 executing recipes, the @samp{-W} flag can direct @code{make} to act as
7838 if some files had been modified, without actually running the recipes
7839 for those files.@refill
7842 Note that the options @samp{-p} and @samp{-v} allow you to obtain other
7843 information about @code{make} or about the makefiles in use
7844 (@pxref{Options Summary, ,Summary of Options}).@refill
7846 @node Avoiding Compilation, Overriding, Instead of Execution, Running
7847 @section Avoiding Recompilation of Some Files
7849 @cindex @code{--old-file}
7850 @cindex @code{--assume-old}
7851 @cindex files, assuming old
7852 @cindex files, avoiding recompilation of
7853 @cindex recompilation, avoiding
7855 Sometimes you may have changed a source file but you do not want to
7856 recompile all the files that depend on it. For example, suppose you add
7857 a macro or a declaration to a header file that many other files depend
7858 on. Being conservative, @code{make} assumes that any change in the
7859 header file requires recompilation of all dependent files, but you know
7860 that they do not need to be recompiled and you would rather not waste
7861 the time waiting for them to compile.
7863 If you anticipate the problem before changing the header file, you can
7864 use the @samp{-t} flag. This flag tells @code{make} not to run the
7865 recipes in the rules, but rather to mark the target up to date by
7866 changing its last-modification date. You would follow this procedure:
7870 Use the command @samp{make} to recompile the source files that really
7871 need recompilation, ensuring that the object files are up-to-date
7875 Make the changes in the header files.
7878 Use the command @samp{make -t} to mark all the object files as
7879 up to date. The next time you run @code{make}, the changes in the
7880 header files will not cause any recompilation.
7883 If you have already changed the header file at a time when some files
7884 do need recompilation, it is too late to do this. Instead, you can
7885 use the @w{@samp{-o @var{file}}} flag, which marks a specified file as
7886 ``old'' (@pxref{Options Summary, ,Summary of Options}). This means
7887 that the file itself will not be remade, and nothing else will be
7888 remade on its account. Follow this procedure:
7892 Recompile the source files that need compilation for reasons independent
7893 of the particular header file, with @samp{make -o @var{headerfile}}.
7894 If several header files are involved, use a separate @samp{-o} option
7895 for each header file.
7898 Touch all the object files with @samp{make -t}.
7901 @node Overriding, Testing, Avoiding Compilation, Running
7902 @section Overriding Variables
7903 @cindex overriding variables with arguments
7904 @cindex variables, overriding with arguments
7905 @cindex command line variables
7906 @cindex variables, command line
7908 An argument that contains @samp{=} specifies the value of a variable:
7909 @samp{@var{v}=@var{x}} sets the value of the variable @var{v} to @var{x}.
7910 If you specify a value in this way, all ordinary assignments of the same
7911 variable in the makefile are ignored; we say they have been
7912 @dfn{overridden} by the command line argument.
7914 The most common way to use this facility is to pass extra flags to
7915 compilers. For example, in a properly written makefile, the variable
7916 @code{CFLAGS} is included in each recipe that runs the C compiler, so a
7917 file @file{foo.c} would be compiled something like this:
7920 cc -c $(CFLAGS) foo.c
7923 Thus, whatever value you set for @code{CFLAGS} affects each compilation
7924 that occurs. The makefile probably specifies the usual value for
7925 @code{CFLAGS}, like this:
7931 Each time you run @code{make}, you can override this value if you
7932 wish. For example, if you say @samp{make CFLAGS='-g -O'}, each C
7933 compilation will be done with @samp{cc -c -g -O}. (This also
7934 illustrates how you can use quoting in the shell to enclose spaces and
7935 other special characters in the value of a variable when you override
7938 The variable @code{CFLAGS} is only one of many standard variables that
7939 exist just so that you can change them this way. @xref{Implicit
7940 Variables, , Variables Used by Implicit Rules}, for a complete list.
7942 You can also program the makefile to look at additional variables of your
7943 own, giving the user the ability to control other aspects of how the
7944 makefile works by changing the variables.
7946 When you override a variable with a command line argument, you can
7947 define either a recursively-expanded variable or a simply-expanded
7948 variable. The examples shown above make a recursively-expanded
7949 variable; to make a simply-expanded variable, write @samp{:=} instead
7950 of @samp{=}. But, unless you want to include a variable reference or
7951 function call in the @emph{value} that you specify, it makes no
7952 difference which kind of variable you create.
7954 There is one way that the makefile can change a variable that you have
7955 overridden. This is to use the @code{override} directive, which is a line
7956 that looks like this: @samp{override @var{variable} = @var{value}}
7957 (@pxref{Override Directive, ,The @code{override} Directive}).
7959 @node Testing, Options Summary, Overriding, Running
7960 @section Testing the Compilation of a Program
7961 @cindex testing compilation
7962 @cindex compilation, testing
7964 Normally, when an error happens in executing a shell command, @code{make}
7965 gives up immediately, returning a nonzero status. No further recipes are
7966 executed for any target. The error implies that the goal cannot be
7967 correctly remade, and @code{make} reports this as soon as it knows.
7969 When you are compiling a program that you have just changed, this is not
7970 what you want. Instead, you would rather that @code{make} try compiling
7971 every file that can be tried, to show you as many compilation errors
7975 @cindex @code{--keep-going}
7976 On these occasions, you should use the @samp{-k} or
7977 @samp{--keep-going} flag. This tells @code{make} to continue to
7978 consider the other prerequisites of the pending targets, remaking them
7979 if necessary, before it gives up and returns nonzero status. For
7980 example, after an error in compiling one object file, @samp{make -k}
7981 will continue compiling other object files even though it already
7982 knows that linking them will be impossible. In addition to continuing
7983 after failed shell commands, @samp{make -k} will continue as much as
7984 possible after discovering that it does not know how to make a target
7985 or prerequisite file. This will always cause an error message, but
7986 without @samp{-k}, it is a fatal error (@pxref{Options Summary,
7987 ,Summary of Options}).@refill
7989 The usual behavior of @code{make} assumes that your purpose is to get the
7990 goals up to date; once @code{make} learns that this is impossible, it might
7991 as well report the failure immediately. The @samp{-k} flag says that the
7992 real purpose is to test as much as possible of the changes made in the
7993 program, perhaps to find several independent problems so that you can
7994 correct them all before the next attempt to compile. This is why Emacs'
7995 @kbd{M-x compile} command passes the @samp{-k} flag by default.
7997 @node Options Summary, , Testing, Running
7998 @section Summary of Options
8003 Here is a table of all the options @code{make} understands:
8010 These options are ignored for compatibility with other versions of @code{make}.
8014 @itemx --always-make
8015 @cindex @code{--always-make}
8016 Consider all targets out-of-date. GNU @code{make} proceeds to
8017 consider targets and their prerequisites using the normal algorithms;
8018 however, all targets so considered are always remade regardless of the
8019 status of their prerequisites. To avoid infinite recursion, if
8020 @code{MAKE_RESTARTS} (@pxref{Special Variables, , Other Special
8021 Variables}) is set to a number greater than 0 this option is disabled
8022 when considering whether to remake makefiles (@pxref{Remaking
8023 Makefiles, , How Makefiles Are Remade}).
8027 @itemx --directory=@var{dir}
8028 @cindex @code{--directory}
8029 Change to directory @var{dir} before reading the makefiles. If multiple
8030 @samp{-C} options are specified, each is interpreted relative to the
8031 previous one: @samp{-C / -C etc} is equivalent to @samp{-C /etc}.
8032 This is typically used with recursive invocations of @code{make}
8033 (@pxref{Recursion, ,Recursive Use of @code{make}}).
8037 @c Extra blank line here makes the table look better.
8039 Print debugging information in addition to normal processing. The
8040 debugging information says which files are being considered for
8041 remaking, which file-times are being compared and with what results,
8042 which files actually need to be remade, which implicit rules are
8043 considered and which are applied---everything interesting about how
8044 @code{make} decides what to do. The @code{-d} option is equivalent to
8045 @samp{--debug=a} (see below).
8047 @item --debug[=@var{options}]
8048 @cindex @code{--debug}
8049 @c Extra blank line here makes the table look better.
8051 Print debugging information in addition to normal processing. Various
8052 levels and types of output can be chosen. With no arguments, print the
8053 ``basic'' level of debugging. Possible arguments are below; only the
8054 first character is considered, and values must be comma- or
8059 All types of debugging output are enabled. This is equivalent to using
8063 Basic debugging prints each target that was found to be out-of-date, and
8064 whether the build was successful or not.
8066 @item v (@i{verbose})
8067 A level above @samp{basic}; includes messages about which makefiles were
8068 parsed, prerequisites that did not need to be rebuilt, etc. This option
8069 also enables @samp{basic} messages.
8071 @item i (@i{implicit})
8072 Prints messages describing the implicit rule searches for each target.
8073 This option also enables @samp{basic} messages.
8076 Prints messages giving details on the invocation of specific subcommands.
8078 @item m (@i{makefile})
8079 By default, the above messages are not enabled while trying to remake
8080 the makefiles. This option enables messages while rebuilding makefiles,
8081 too. Note that the @samp{all} option does enable this option. This
8082 option also enables @samp{basic} messages.
8087 @itemx --environment-overrides
8088 @cindex @code{--environment-overrides}
8089 Give variables taken from the environment precedence
8090 over variables from makefiles.
8091 @xref{Environment, ,Variables from the Environment}.
8095 @itemx --file=@var{file}
8096 @cindex @code{--file}
8097 @itemx --makefile=@var{file}
8098 @cindex @code{--makefile}
8099 Read the file named @var{file} as a makefile.
8100 @xref{Makefiles, ,Writing Makefiles}.
8105 @cindex @code{--help}
8106 @c Extra blank line here makes the table look better.
8108 Remind you of the options that @code{make} understands and then exit.
8112 @itemx --ignore-errors
8113 @cindex @code{--ignore-errors}
8114 Ignore all errors in recipes executed to remake files.
8115 @xref{Errors, ,Errors in Recipes}.
8119 @itemx --include-dir=@var{dir}
8120 @cindex @code{--include-dir}
8121 Specifies a directory @var{dir} to search for included makefiles.
8122 @xref{Include, ,Including Other Makefiles}. If several @samp{-I}
8123 options are used to specify several directories, the directories are
8124 searched in the order specified.
8126 @item -j [@var{jobs}]
8128 @itemx --jobs[=@var{jobs}]
8129 @cindex @code{--jobs}
8130 Specifies the number of recipes (jobs) to run simultaneously. With no
8131 argument, @code{make} runs as many recipes simultaneously as possible.
8132 If there is more than one @samp{-j} option, the last one is effective.
8133 @xref{Parallel, ,Parallel Execution}, for more information on how
8134 recipes are run. Note that this option is ignored on MS-DOS.
8139 @cindex @code{--keep-going}
8140 Continue as much as possible after an error. While the target that
8141 failed, and those that depend on it, cannot be remade, the other
8142 prerequisites of these targets can be processed all the same.
8143 @xref{Testing, ,Testing the Compilation of a Program}.
8145 @item -l [@var{load}]
8147 @itemx --load-average[=@var{load}]
8148 @cindex @code{--load-average}
8149 @itemx --max-load[=@var{load}]
8150 @cindex @code{--max-load}
8151 Specifies that no new recipes should be started if there are other
8152 recipes running and the load average is at least @var{load} (a
8153 floating-point number). With no argument, removes a previous load
8154 limit. @xref{Parallel, ,Parallel Execution}.
8158 @itemx --check-symlink-times
8159 @cindex @code{--check-symlink-times}
8160 On systems that support symbolic links, this option causes @code{make}
8161 to consider the timestamps on any symbolic links in addition to the
8162 timestamp on the file referenced by those links. When this option is
8163 provided, the most recent timestamp among the file and the symbolic
8164 links is taken as the modification time for this target file.
8169 @cindex @code{--just-print}
8171 @cindex @code{--dry-run}
8173 @cindex @code{--recon}
8174 @c Extra blank line here makes the table look better.
8176 Print the recipe that would be executed, but do not execute it (except
8177 in certain circumstances).
8178 @xref{Instead of Execution, ,Instead of Executing Recipes}.
8182 @itemx --old-file=@var{file}
8183 @cindex @code{--old-file}
8184 @itemx --assume-old=@var{file}
8185 @cindex @code{--assume-old}
8186 Do not remake the file @var{file} even if it is older than its
8187 prerequisites, and do not remake anything on account of changes in
8188 @var{file}. Essentially the file is treated as very old and its rules
8189 are ignored. @xref{Avoiding Compilation, ,Avoiding Recompilation of
8194 @itemx --print-data-base
8195 @cindex @code{--print-data-base}
8196 @cindex data base of @code{make} rules
8197 @cindex predefined rules and variables, printing
8198 Print the data base (rules and variable values) that results from
8199 reading the makefiles; then execute as usual or as otherwise
8200 specified. This also prints the version information given by the
8201 @samp{-v} switch (see below). To print the data base without trying
8202 to remake any files, use @w{@samp{make -qp}}. To print the data base
8203 of predefined rules and variables, use @w{@samp{make -p -f /dev/null}}.
8204 The data base output contains filename and linenumber information for
8205 recipe and variable definitions, so it can be a useful debugging tool
8206 in complex environments.
8211 @cindex @code{--question}
8212 ``Question mode''. Do not run any recipes, or print anything; just
8213 return an exit status that is zero if the specified targets are already
8214 up to date, one if any remaking is required, or two if an error is
8215 encountered. @xref{Instead of Execution, ,Instead of Executing
8220 @itemx --no-builtin-rules
8221 @cindex @code{--no-builtin-rules}
8222 Eliminate use of the built-in implicit rules (@pxref{Implicit Rules,
8223 ,Using Implicit Rules}). You can still define your own by writing
8224 pattern rules (@pxref{Pattern Rules, ,Defining and Redefining Pattern
8225 Rules}). The @samp{-r} option also clears out the default list of
8226 suffixes for suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix
8227 Rules}). But you can still define your own suffixes with a rule for
8228 @code{.SUFFIXES}, and then define your own suffix rules. Note that only
8229 @emph{rules} are affected by the @code{-r} option; default variables
8230 remain in effect (@pxref{Implicit Variables, ,Variables Used by Implicit
8231 Rules}); see the @samp{-R} option below.
8235 @itemx --no-builtin-variables
8236 @cindex @code{--no-builtin-variables}
8237 Eliminate use of the built-in rule-specific variables (@pxref{Implicit
8238 Variables, ,Variables Used by Implicit Rules}). You can still define
8239 your own, of course. The @samp{-R} option also automatically enables
8240 the @samp{-r} option (see above), since it doesn't make sense to have
8241 implicit rules without any definitions for the variables that they use.
8246 @cindex @code{--silent}
8248 @cindex @code{--quiet}
8249 @c Extra blank line here makes the table look better.
8251 Silent operation; do not print the recipes as they are executed.
8252 @xref{Echoing, ,Recipe Echoing}.
8256 @itemx --no-keep-going
8257 @cindex @code{--no-keep-going}
8259 @cindex @code{--stop}
8260 @c Extra blank line here makes the table look better.
8262 Cancel the effect of the @samp{-k} option. This is never necessary
8263 except in a recursive @code{make} where @samp{-k} might be inherited
8264 from the top-level @code{make} via @code{MAKEFLAGS}
8265 (@pxref{Recursion, ,Recursive Use of @code{make}})
8266 or if you set @samp{-k} in @code{MAKEFLAGS} in your environment.@refill
8271 @cindex @code{--touch}
8272 @c Extra blank line here makes the table look better.
8274 Touch files (mark them up to date without really changing them)
8275 instead of running their recipes. This is used to pretend that the
8276 recipes were done, in order to fool future invocations of
8277 @code{make}. @xref{Instead of Execution, ,Instead of Executing Recipes}.
8282 @cindex @code{--version}
8283 Print the version of the @code{make} program plus a copyright, a list
8284 of authors, and a notice that there is no warranty; then exit.
8288 @itemx --print-directory
8289 @cindex @code{--print-directory}
8290 Print a message containing the working directory both before and after
8291 executing the makefile. This may be useful for tracking down errors
8292 from complicated nests of recursive @code{make} commands.
8293 @xref{Recursion, ,Recursive Use of @code{make}}. (In practice, you
8294 rarely need to specify this option since @samp{make} does it for you;
8295 see @ref{-w Option, ,The @samp{--print-directory} Option}.)
8297 @itemx --no-print-directory
8298 @cindex @code{--no-print-directory}
8299 Disable printing of the working directory under @code{-w}.
8300 This option is useful when @code{-w} is turned on automatically,
8301 but you do not want to see the extra messages.
8302 @xref{-w Option, ,The @samp{--print-directory} Option}.
8306 @itemx --what-if=@var{file}
8307 @cindex @code{--what-if}
8308 @itemx --new-file=@var{file}
8309 @cindex @code{--new-file}
8310 @itemx --assume-new=@var{file}
8311 @cindex @code{--assume-new}
8312 Pretend that the target @var{file} has just been modified. When used
8313 with the @samp{-n} flag, this shows you what would happen if you were
8314 to modify that file. Without @samp{-n}, it is almost the same as
8315 running a @code{touch} command on the given file before running
8316 @code{make}, except that the modification time is changed only in the
8317 imagination of @code{make}.
8318 @xref{Instead of Execution, ,Instead of Executing Recipes}.
8320 @item --warn-undefined-variables
8321 @cindex @code{--warn-undefined-variables}
8322 @cindex variables, warning for undefined
8323 @cindex undefined variables, warning message
8324 Issue a warning message whenever @code{make} sees a reference to an
8325 undefined variable. This can be helpful when you are trying to debug
8326 makefiles which use variables in complex ways.
8329 @node Implicit Rules, Archives, Running, Top
8330 @chapter Using Implicit Rules
8331 @cindex implicit rule
8332 @cindex rule, implicit
8334 Certain standard ways of remaking target files are used very often. For
8335 example, one customary way to make an object file is from a C source file
8336 using the C compiler, @code{cc}.
8338 @dfn{Implicit rules} tell @code{make} how to use customary techniques so
8339 that you do not have to specify them in detail when you want to use
8340 them. For example, there is an implicit rule for C compilation. File
8341 names determine which implicit rules are run. For example, C
8342 compilation typically takes a @file{.c} file and makes a @file{.o} file.
8343 So @code{make} applies the implicit rule for C compilation when it sees
8344 this combination of file name endings.@refill
8346 A chain of implicit rules can apply in sequence; for example, @code{make}
8347 will remake a @file{.o} file from a @file{.y} file by way of a @file{.c} file.
8349 @xref{Chained Rules, ,Chains of Implicit Rules}.
8352 The built-in implicit rules use several variables in their recipes so
8353 that, by changing the values of the variables, you can change the way the
8354 implicit rule works. For example, the variable @code{CFLAGS} controls the
8355 flags given to the C compiler by the implicit rule for C compilation.
8357 @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
8360 You can define your own implicit rules by writing @dfn{pattern rules}.
8362 @xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.
8365 @dfn{Suffix rules} are a more limited way to define implicit rules.
8366 Pattern rules are more general and clearer, but suffix rules are
8367 retained for compatibility.
8369 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
8373 * Using Implicit:: How to use an existing implicit rule
8374 to get the recipes for updating a file.
8375 * Catalogue of Rules:: A list of built-in implicit rules.
8376 * Implicit Variables:: How to change what predefined rules do.
8377 * Chained Rules:: How to use a chain of implicit rules.
8378 * Pattern Rules:: How to define new implicit rules.
8379 * Last Resort:: How to define recipes for rules which
8381 * Suffix Rules:: The old-fashioned style of implicit rule.
8382 * Implicit Rule Search:: The precise algorithm for applying
8386 @node Using Implicit, Catalogue of Rules, Implicit Rules, Implicit Rules
8387 @section Using Implicit Rules
8388 @cindex implicit rule, how to use
8389 @cindex rule, implicit, how to use
8391 To allow @code{make} to find a customary method for updating a target
8392 file, all you have to do is refrain from specifying recipes yourself.
8393 Either write a rule with no recipe, or don't write a rule at all.
8394 Then @code{make} will figure out which implicit rule to use based on
8395 which kind of source file exists or can be made.
8397 For example, suppose the makefile looks like this:
8401 cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
8405 Because you mention @file{foo.o} but do not give a rule for it, @code{make}
8406 will automatically look for an implicit rule that tells how to update it.
8407 This happens whether or not the file @file{foo.o} currently exists.
8409 If an implicit rule is found, it can supply both a recipe and one or
8410 more prerequisites (the source files). You would want to write a rule
8411 for @file{foo.o} with no recipe if you need to specify additional
8412 prerequisites, such as header files, that the implicit rule cannot
8415 Each implicit rule has a target pattern and prerequisite patterns. There may
8416 be many implicit rules with the same target pattern. For example, numerous
8417 rules make @samp{.o} files: one, from a @samp{.c} file with the C compiler;
8418 another, from a @samp{.p} file with the Pascal compiler; and so on. The rule
8419 that actually applies is the one whose prerequisites exist or can be made.
8420 So, if you have a file @file{foo.c}, @code{make} will run the C compiler;
8421 otherwise, if you have a file @file{foo.p}, @code{make} will run the Pascal
8422 compiler; and so on.
8424 Of course, when you write the makefile, you know which implicit rule you
8425 want @code{make} to use, and you know it will choose that one because you
8426 know which possible prerequisite files are supposed to exist.
8427 @xref{Catalogue of Rules, ,Catalogue of Implicit Rules},
8428 for a catalogue of all the predefined implicit rules.
8430 Above, we said an implicit rule applies if the required prerequisites ``exist
8431 or can be made''. A file ``can be made'' if it is mentioned explicitly in
8432 the makefile as a target or a prerequisite, or if an implicit rule can be
8433 recursively found for how to make it. When an implicit prerequisite is the
8434 result of another implicit rule, we say that @dfn{chaining} is occurring.
8435 @xref{Chained Rules, ,Chains of Implicit Rules}.
8437 In general, @code{make} searches for an implicit rule for each target, and
8438 for each double-colon rule, that has no recipe. A file that is mentioned
8439 only as a prerequisite is considered a target whose rule specifies nothing,
8440 so implicit rule search happens for it. @xref{Implicit Rule Search, ,Implicit Rule Search Algorithm}, for the
8441 details of how the search is done.
8443 Note that explicit prerequisites do not influence implicit rule search.
8444 For example, consider this explicit rule:
8451 The prerequisite on @file{foo.p} does not necessarily mean that
8452 @code{make} will remake @file{foo.o} according to the implicit rule to
8453 make an object file, a @file{.o} file, from a Pascal source file, a
8454 @file{.p} file. For example, if @file{foo.c} also exists, the implicit
8455 rule to make an object file from a C source file is used instead,
8456 because it appears before the Pascal rule in the list of predefined
8457 implicit rules (@pxref{Catalogue of Rules, , Catalogue of Implicit
8460 If you do not want an implicit rule to be used for a target that has no
8461 recipe, you can give that target an empty recipe by writing a semicolon
8462 (@pxref{Empty Recipes, ,Defining Empty Recipes}).
8464 @node Catalogue of Rules, Implicit Variables, Using Implicit, Implicit Rules
8465 @section Catalogue of Implicit Rules
8466 @cindex implicit rule, predefined
8467 @cindex rule, implicit, predefined
8469 Here is a catalogue of predefined implicit rules which are always
8470 available unless the makefile explicitly overrides or cancels them.
8471 @xref{Canceling Rules, ,Canceling Implicit Rules}, for information on
8472 canceling or overriding an implicit rule. The @samp{-r} or
8473 @samp{--no-builtin-rules} option cancels all predefined rules.
8475 This manual only documents the default rules available on POSIX-based
8476 operating systems. Other operating systems, such as VMS, Windows,
8477 OS/2, etc. may have different sets of default rules. To see the full
8478 list of default rules and variables available in your version of GNU
8479 @code{make}, run @samp{make -p} in a directory with no makefile.
8481 Not all of these rules will always be defined, even when the @samp{-r}
8482 option is not given. Many of the predefined implicit rules are
8483 implemented in @code{make} as suffix rules, so which ones will be
8484 defined depends on the @dfn{suffix list} (the list of prerequisites of
8485 the special target @code{.SUFFIXES}). The default suffix list is:
8486 @code{.out}, @code{.a}, @code{.ln}, @code{.o}, @code{.c}, @code{.cc},
8487 @code{.C}, @code{.cpp}, @code{.p}, @code{.f}, @code{.F}, @code{.m},
8488 @code{.r}, @code{.y}, @code{.l}, @code{.ym}, @code{.lm}, @code{.s},
8489 @code{.S}, @code{.mod}, @code{.sym}, @code{.def}, @code{.h},
8490 @code{.info}, @code{.dvi}, @code{.tex}, @code{.texinfo}, @code{.texi},
8491 @code{.txinfo}, @code{.w}, @code{.ch} @code{.web}, @code{.sh},
8492 @code{.elc}, @code{.el}. All of the implicit rules described below
8493 whose prerequisites have one of these suffixes are actually suffix
8494 rules. If you modify the suffix list, the only predefined suffix
8495 rules in effect will be those named by one or two of the suffixes that
8496 are on the list you specify; rules whose suffixes fail to be on the
8497 list are disabled. @xref{Suffix Rules, ,Old-Fashioned Suffix Rules},
8498 for full details on suffix rules.
8501 @item Compiling C programs
8502 @cindex C, rule to compile
8507 @file{@var{n}.o} is made automatically from @file{@var{n}.c} with
8508 a recipe of the form @samp{$(CC) $(CPPFLAGS) $(CFLAGS) -c}.@refill
8510 @item Compiling C++ programs
8511 @cindex C++, rule to compile
8516 @file{@var{n}.o} is made automatically from @file{@var{n}.cc},
8517 @file{@var{n}.cpp}, or @file{@var{n}.C} with a recipe of the form
8518 @samp{$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c}. We encourage you to use the
8519 suffix @samp{.cc} for C++ source files instead of @samp{.C}.@refill
8521 @item Compiling Pascal programs
8522 @cindex Pascal, rule to compile
8525 @file{@var{n}.o} is made automatically from @file{@var{n}.p}
8526 with the recipe @samp{$(PC) $(PFLAGS) -c}.@refill
8528 @item Compiling Fortran and Ratfor programs
8529 @cindex Fortran, rule to compile
8530 @cindex Ratfor, rule to compile
8535 @file{@var{n}.o} is made automatically from @file{@var{n}.r},
8536 @file{@var{n}.F} or @file{@var{n}.f} by running the
8537 Fortran compiler. The precise recipe used is as follows:@refill
8541 @samp{$(FC) $(FFLAGS) -c}.
8543 @samp{$(FC) $(FFLAGS) $(CPPFLAGS) -c}.
8545 @samp{$(FC) $(FFLAGS) $(RFLAGS) -c}.
8548 @item Preprocessing Fortran and Ratfor programs
8549 @file{@var{n}.f} is made automatically from @file{@var{n}.r} or
8550 @file{@var{n}.F}. This rule runs just the preprocessor to convert a
8551 Ratfor or preprocessable Fortran program into a strict Fortran
8552 program. The precise recipe used is as follows:@refill
8556 @samp{$(FC) $(CPPFLAGS) $(FFLAGS) -F}.
8558 @samp{$(FC) $(FFLAGS) $(RFLAGS) -F}.
8561 @item Compiling Modula-2 programs
8562 @cindex Modula-2, rule to compile
8567 @file{@var{n}.sym} is made from @file{@var{n}.def} with a recipe
8568 of the form @samp{$(M2C) $(M2FLAGS) $(DEFFLAGS)}. @file{@var{n}.o}
8569 is made from @file{@var{n}.mod}; the form is:
8570 @w{@samp{$(M2C) $(M2FLAGS) $(MODFLAGS)}}.@refill
8573 @item Assembling and preprocessing assembler programs
8574 @cindex assembly, rule to compile
8577 @file{@var{n}.o} is made automatically from @file{@var{n}.s} by
8578 running the assembler, @code{as}. The precise recipe is
8579 @samp{$(AS) $(ASFLAGS)}.@refill
8582 @file{@var{n}.s} is made automatically from @file{@var{n}.S} by
8583 running the C preprocessor, @code{cpp}. The precise recipe is
8584 @w{@samp{$(CPP) $(CPPFLAGS)}}.
8586 @item Linking a single object file
8587 @cindex linking, predefined rule for
8590 @file{@var{n}} is made automatically from @file{@var{n}.o} by running
8591 the linker (usually called @code{ld}) via the C compiler. The precise
8592 recipe used is @w{@samp{$(CC) $(LDFLAGS) @var{n}.o $(LOADLIBES) $(LDLIBS)}}.
8594 This rule does the right thing for a simple program with only one
8595 source file. It will also do the right thing if there are multiple
8596 object files (presumably coming from various other source files), one
8597 of which has a name matching that of the executable file. Thus,
8604 when @file{x.c}, @file{y.c} and @file{z.c} all exist will execute:
8619 In more complicated cases, such as when there is no object file whose
8620 name derives from the executable file name, you must write an explicit
8623 Each kind of file automatically made into @samp{.o} object files will
8624 be automatically linked by using the compiler (@samp{$(CC)},
8625 @samp{$(FC)} or @samp{$(PC)}; the C compiler @samp{$(CC)} is used to
8626 assemble @samp{.s} files) without the @samp{-c} option. This could be
8627 done by using the @samp{.o} object files as intermediates, but it is
8628 faster to do the compiling and linking in one step, so that's how it's
8631 @item Yacc for C programs
8633 @cindex Yacc, rule to run
8635 @file{@var{n}.c} is made automatically from @file{@var{n}.y} by
8636 running Yacc with the recipe @samp{$(YACC) $(YFLAGS)}.
8638 @item Lex for C programs
8640 @cindex Lex, rule to run
8642 @file{@var{n}.c} is made automatically from @file{@var{n}.l} by
8643 running Lex. The actual recipe is @samp{$(LEX) $(LFLAGS)}.
8645 @item Lex for Ratfor programs
8646 @file{@var{n}.r} is made automatically from @file{@var{n}.l} by
8647 running Lex. The actual recipe is @samp{$(LEX) $(LFLAGS)}.
8649 The convention of using the same suffix @samp{.l} for all Lex files
8650 regardless of whether they produce C code or Ratfor code makes it
8651 impossible for @code{make} to determine automatically which of the two
8652 languages you are using in any particular case. If @code{make} is
8653 called upon to remake an object file from a @samp{.l} file, it must
8654 guess which compiler to use. It will guess the C compiler, because
8655 that is more common. If you are using Ratfor, make sure @code{make}
8656 knows this by mentioning @file{@var{n}.r} in the makefile. Or, if you
8657 are using Ratfor exclusively, with no C files, remove @samp{.c} from
8658 the list of implicit rule suffixes with:@refill
8663 .SUFFIXES: .o .r .f .l @dots{}
8667 @item Making Lint Libraries from C, Yacc, or Lex programs
8669 @cindex @code{lint}, rule to run
8671 @file{@var{n}.ln} is made from @file{@var{n}.c} by running @code{lint}.
8672 The precise recipe is @w{@samp{$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i}}.
8673 The same recipe is used on the C code produced from
8674 @file{@var{n}.y} or @file{@var{n}.l}.@refill
8676 @item @TeX{} and Web
8677 @cindex @TeX{}, rule to run
8678 @cindex Web, rule to run
8689 @file{@var{n}.dvi} is made from @file{@var{n}.tex} with the recipe
8690 @samp{$(TEX)}. @file{@var{n}.tex} is made from @file{@var{n}.web} with
8691 @samp{$(WEAVE)}, or from @file{@var{n}.w} (and from @file{@var{n}.ch} if
8692 it exists or can be made) with @samp{$(CWEAVE)}. @file{@var{n}.p} is
8693 made from @file{@var{n}.web} with @samp{$(TANGLE)} and @file{@var{n}.c}
8694 is made from @file{@var{n}.w} (and from @file{@var{n}.ch} if it exists
8695 or can be made) with @samp{$(CTANGLE)}.@refill
8697 @item Texinfo and Info
8698 @cindex Texinfo, rule to format
8699 @cindex Info, rule to format
8706 @file{@var{n}.dvi} is made from @file{@var{n}.texinfo},
8707 @file{@var{n}.texi}, or @file{@var{n}.txinfo}, with the recipe
8708 @w{@samp{$(TEXI2DVI) $(TEXI2DVI_FLAGS)}}. @file{@var{n}.info} is made from
8709 @file{@var{n}.texinfo}, @file{@var{n}.texi}, or @file{@var{n}.txinfo}, with
8710 the recipe @w{@samp{$(MAKEINFO) $(MAKEINFO_FLAGS)}}.
8713 @cindex RCS, rule to extract from
8715 @pindex ,v @r{(RCS file extension)}
8716 Any file @file{@var{n}} is extracted if necessary from an RCS file
8717 named either @file{@var{n},v} or @file{RCS/@var{n},v}. The precise
8718 recipe used is @w{@samp{$(CO) $(COFLAGS)}}. @file{@var{n}} will not be
8719 extracted from RCS if it already exists, even if the RCS file is
8720 newer. The rules for RCS are terminal
8721 (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
8722 so RCS files cannot be generated from another source; they must
8723 actually exist.@refill
8726 @cindex SCCS, rule to extract from
8728 @pindex s. @r{(SCCS file prefix)}
8729 Any file @file{@var{n}} is extracted if necessary from an SCCS file
8730 named either @file{s.@var{n}} or @file{SCCS/s.@var{n}}. The precise
8731 recipe used is @w{@samp{$(GET) $(GFLAGS)}}. The rules for SCCS are
8732 terminal (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
8733 so SCCS files cannot be generated from another source; they must
8734 actually exist.@refill
8737 For the benefit of SCCS, a file @file{@var{n}} is copied from
8738 @file{@var{n}.sh} and made executable (by everyone). This is for
8739 shell scripts that are checked into SCCS. Since RCS preserves the
8740 execution permission of a file, you do not need to use this feature
8743 We recommend that you avoid using of SCCS. RCS is widely held to be
8744 superior, and is also free. By choosing free software in place of
8745 comparable (or inferior) proprietary software, you support the free
8749 Usually, you want to change only the variables listed in the table
8750 above, which are documented in the following section.
8752 However, the recipes in built-in implicit rules actually use
8753 variables such as @code{COMPILE.c}, @code{LINK.p}, and
8754 @code{PREPROCESS.S}, whose values contain the recipes listed above.
8756 @code{make} follows the convention that the rule to compile a
8757 @file{.@var{x}} source file uses the variable @code{COMPILE.@var{x}}.
8758 Similarly, the rule to produce an executable from a @file{.@var{x}}
8759 file uses @code{LINK.@var{x}}; and the rule to preprocess a
8760 @file{.@var{x}} file uses @code{PREPROCESS.@var{x}}.
8762 @vindex OUTPUT_OPTION
8763 Every rule that produces an object file uses the variable
8764 @code{OUTPUT_OPTION}. @code{make} defines this variable either to
8765 contain @samp{-o $@@}, or to be empty, depending on a compile-time
8766 option. You need the @samp{-o} option to ensure that the output goes
8767 into the right file when the source file is in a different directory,
8768 as when using @code{VPATH} (@pxref{Directory Search}). However,
8769 compilers on some systems do not accept a @samp{-o} switch for object
8770 files. If you use such a system, and use @code{VPATH}, some
8771 compilations will put their output in the wrong place.
8772 A possible workaround for this problem is to give @code{OUTPUT_OPTION}
8773 the value @w{@samp{; mv $*.o $@@}}.
8775 @node Implicit Variables, Chained Rules, Catalogue of Rules, Implicit Rules
8776 @section Variables Used by Implicit Rules
8777 @cindex flags for compilers
8779 The recipes in built-in implicit rules make liberal use of certain
8780 predefined variables. You can alter the values of these variables in
8781 the makefile, with arguments to @code{make}, or in the environment to
8782 alter how the implicit rules work without redefining the rules
8783 themselves. You can cancel all variables used by implicit rules with
8784 the @samp{-R} or @samp{--no-builtin-variables} option.
8786 For example, the recipe used to compile a C source file actually says
8787 @samp{$(CC) -c $(CFLAGS) $(CPPFLAGS)}. The default values of the variables
8788 used are @samp{cc} and nothing, resulting in the command @samp{cc -c}. By
8789 redefining @samp{CC} to @samp{ncc}, you could cause @samp{ncc} to be
8790 used for all C compilations performed by the implicit rule. By redefining
8791 @samp{CFLAGS} to be @samp{-g}, you could pass the @samp{-g} option to
8792 each compilation. @emph{All} implicit rules that do C compilation use
8793 @samp{$(CC)} to get the program name for the compiler and @emph{all}
8794 include @samp{$(CFLAGS)} among the arguments given to the compiler.@refill
8796 The variables used in implicit rules fall into two classes: those that are
8797 names of programs (like @code{CC}) and those that contain arguments for the
8798 programs (like @code{CFLAGS}). (The ``name of a program'' may also contain
8799 some command arguments, but it must start with an actual executable program
8800 name.) If a variable value contains more than one argument, separate them
8803 The following tables describe of some of the more commonly-used predefined
8804 variables. This list is not exhaustive, and the default values shown here may
8805 not be what @code{make} selects for your environment. To see the
8806 complete list of predefined variables for your instance of GNU @code{make} you
8807 can run @samp{make -p} in a directory with no makefiles.
8809 Here is a table of some of the more common variables used as names of
8810 programs in built-in rules:
8816 Archive-maintaining program; default @samp{ar}.
8821 Program for compiling assembly files; default @samp{as}.
8826 Program for compiling C programs; default @samp{cc}.
8831 Program for compiling C++ programs; default @samp{g++}.
8836 Program for running the C preprocessor, with results to standard output;
8837 default @samp{$(CC) -E}.
8841 Program for compiling or preprocessing Fortran and Ratfor programs;
8847 Program to use to compile Modula-2 source code; default @samp{m2c}.
8852 Program for compiling Pascal programs; default @samp{pc}.
8857 Program for extracting a file from RCS; default @samp{co}.
8862 Program for extracting a file from SCCS; default @samp{get}.
8867 Program to use to turn Lex grammars into source code; default @samp{lex}.
8872 Program to use to turn Yacc grammars into source code; default @samp{yacc}.
8877 Program to use to run lint on source code; default @samp{lint}.
8882 Program to convert a Texinfo source file into an Info file; default
8888 Program to make @TeX{} @sc{dvi} files from @TeX{} source;
8894 Program to make @TeX{} @sc{dvi} files from Texinfo source;
8895 default @samp{texi2dvi}.
8900 Program to translate Web into @TeX{}; default @samp{weave}.
8905 Program to translate C Web into @TeX{}; default @samp{cweave}.
8910 Program to translate Web into Pascal; default @samp{tangle}.
8915 Program to translate C Web into C; default @samp{ctangle}.
8920 Command to remove a file; default @samp{rm -f}.
8924 Here is a table of variables whose values are additional arguments for the
8925 programs above. The default values for all of these is the empty
8926 string, unless otherwise noted.
8931 Flags to give the archive-maintaining program; default @samp{rv}.
8935 Extra flags to give to the assembler (when explicitly
8936 invoked on a @samp{.s} or @samp{.S} file).
8940 Extra flags to give to the C compiler.
8944 Extra flags to give to the C++ compiler.
8948 Extra flags to give to the RCS @code{co} program.
8952 Extra flags to give to the C preprocessor and programs
8953 that use it (the C and Fortran compilers).
8957 Extra flags to give to the Fortran compiler.
8961 Extra flags to give to the SCCS @code{get} program.
8965 Extra flags to give to compilers when they are supposed to invoke the linker,
8970 Extra flags to give to Lex.
8974 Extra flags to give to Yacc.
8978 Extra flags to give to the Pascal compiler.
8982 Extra flags to give to the Fortran compiler for Ratfor programs.
8986 Extra flags to give to lint.
8989 @node Chained Rules, Pattern Rules, Implicit Variables, Implicit Rules
8990 @section Chains of Implicit Rules
8992 @cindex chains of rules
8993 @cindex rule, implicit, chains of
8994 Sometimes a file can be made by a sequence of implicit rules. For example,
8995 a file @file{@var{n}.o} could be made from @file{@var{n}.y} by running
8996 first Yacc and then @code{cc}. Such a sequence is called a @dfn{chain}.
8998 If the file @file{@var{n}.c} exists, or is mentioned in the makefile, no
8999 special searching is required: @code{make} finds that the object file can
9000 be made by C compilation from @file{@var{n}.c}; later on, when considering
9001 how to make @file{@var{n}.c}, the rule for running Yacc is
9002 used. Ultimately both @file{@var{n}.c} and @file{@var{n}.o} are
9005 @cindex intermediate files
9006 @cindex files, intermediate
9007 However, even if @file{@var{n}.c} does not exist and is not mentioned,
9008 @code{make} knows how to envision it as the missing link between
9009 @file{@var{n}.o} and @file{@var{n}.y}! In this case, @file{@var{n}.c} is
9010 called an @dfn{intermediate file}. Once @code{make} has decided to use the
9011 intermediate file, it is entered in the data base as if it had been
9012 mentioned in the makefile, along with the implicit rule that says how to
9015 Intermediate files are remade using their rules just like all other
9016 files. But intermediate files are treated differently in two ways.
9018 The first difference is what happens if the intermediate file does not
9019 exist. If an ordinary file @var{b} does not exist, and @code{make}
9020 considers a target that depends on @var{b}, it invariably creates
9021 @var{b} and then updates the target from @var{b}. But if @var{b} is an
9022 intermediate file, then @code{make} can leave well enough alone. It
9023 won't bother updating @var{b}, or the ultimate target, unless some
9024 prerequisite of @var{b} is newer than that target or there is some other
9025 reason to update that target.
9027 The second difference is that if @code{make} @emph{does} create @var{b}
9028 in order to update something else, it deletes @var{b} later on after it
9029 is no longer needed. Therefore, an intermediate file which did not
9030 exist before @code{make} also does not exist after @code{make}.
9031 @code{make} reports the deletion to you by printing a @samp{rm -f}
9032 command showing which file it is deleting.
9034 Ordinarily, a file cannot be intermediate if it is mentioned in the
9035 makefile as a target or prerequisite. However, you can explicitly mark a
9036 file as intermediate by listing it as a prerequisite of the special target
9037 @code{.INTERMEDIATE}. This takes effect even if the file is mentioned
9038 explicitly in some other way.
9040 @cindex intermediate files, preserving
9041 @cindex preserving intermediate files
9042 @cindex secondary files
9043 You can prevent automatic deletion of an intermediate file by marking it
9044 as a @dfn{secondary} file. To do this, list it as a prerequisite of the
9045 special target @code{.SECONDARY}. When a file is secondary, @code{make}
9046 will not create the file merely because it does not already exist, but
9047 @code{make} does not automatically delete the file. Marking a file as
9048 secondary also marks it as intermediate.
9050 You can list the target pattern of an implicit rule (such as @samp{%.o})
9051 as a prerequisite of the special target @code{.PRECIOUS} to preserve
9052 intermediate files made by implicit rules whose target patterns match
9053 that file's name; see @ref{Interrupts}.@refill
9054 @cindex preserving with @code{.PRECIOUS}
9055 @cindex @code{.PRECIOUS} intermediate files
9057 A chain can involve more than two implicit rules. For example, it is
9058 possible to make a file @file{foo} from @file{RCS/foo.y,v} by running RCS,
9059 Yacc and @code{cc}. Then both @file{foo.y} and @file{foo.c} are
9060 intermediate files that are deleted at the end.@refill
9062 No single implicit rule can appear more than once in a chain. This means
9063 that @code{make} will not even consider such a ridiculous thing as making
9064 @file{foo} from @file{foo.o.o} by running the linker twice. This
9065 constraint has the added benefit of preventing any infinite loop in the
9066 search for an implicit rule chain.
9068 There are some special implicit rules to optimize certain cases that would
9069 otherwise be handled by rule chains. For example, making @file{foo} from
9070 @file{foo.c} could be handled by compiling and linking with separate
9071 chained rules, using @file{foo.o} as an intermediate file. But what
9072 actually happens is that a special rule for this case does the compilation
9073 and linking with a single @code{cc} command. The optimized rule is used in
9074 preference to the step-by-step chain because it comes earlier in the
9077 @node Pattern Rules, Last Resort, Chained Rules, Implicit Rules
9078 @section Defining and Redefining Pattern Rules
9080 You define an implicit rule by writing a @dfn{pattern rule}. A pattern
9081 rule looks like an ordinary rule, except that its target contains the
9082 character @samp{%} (exactly one of them). The target is considered a
9083 pattern for matching file names; the @samp{%} can match any nonempty
9084 substring, while other characters match only themselves. The prerequisites
9085 likewise use @samp{%} to show how their names relate to the target name.
9087 Thus, a pattern rule @samp{%.o : %.c} says how to make any file
9088 @file{@var{stem}.o} from another file @file{@var{stem}.c}.@refill
9090 Note that expansion using @samp{%} in pattern rules occurs
9091 @strong{after} any variable or function expansions, which take place
9092 when the makefile is read. @xref{Using Variables, , How to Use
9093 Variables}, and @ref{Functions, ,Functions for Transforming Text}.
9096 * Pattern Intro:: An introduction to pattern rules.
9097 * Pattern Examples:: Examples of pattern rules.
9098 * Automatic Variables:: How to use automatic variables in the
9099 recipes of implicit rules.
9100 * Pattern Match:: How patterns match.
9101 * Match-Anything Rules:: Precautions you should take prior to
9102 defining rules that can match any
9103 target file whatever.
9104 * Canceling Rules:: How to override or cancel built-in rules.
9107 @node Pattern Intro, Pattern Examples, Pattern Rules, Pattern Rules
9108 @subsection Introduction to Pattern Rules
9109 @cindex pattern rule
9110 @cindex rule, pattern
9112 A pattern rule contains the character @samp{%} (exactly one of them)
9113 in the target; otherwise, it looks exactly like an ordinary rule. The
9114 target is a pattern for matching file names; the @samp{%} matches any
9115 nonempty substring, while other characters match only themselves.
9116 @cindex target pattern, implicit
9117 @cindex @code{%}, in pattern rules
9119 For example, @samp{%.c} as a pattern matches any file name that ends in
9120 @samp{.c}. @samp{s.%.c} as a pattern matches any file name that starts
9121 with @samp{s.}, ends in @samp{.c} and is at least five characters long.
9122 (There must be at least one character to match the @samp{%}.) The substring
9123 that the @samp{%} matches is called the @dfn{stem}.@refill
9125 @samp{%} in a prerequisite of a pattern rule stands for the same stem
9126 that was matched by the @samp{%} in the target. In order for the
9127 pattern rule to apply, its target pattern must match the file name
9128 under consideration and all of its prerequisites (after pattern
9129 substitution) must name files that exist or can be made. These files
9130 become prerequisites of the target.
9131 @cindex prerequisite pattern, implicit
9133 Thus, a rule of the form
9136 %.o : %.c ; @var{recipe}@dots{}
9140 specifies how to make a file @file{@var{n}.o}, with another file
9141 @file{@var{n}.c} as its prerequisite, provided that @file{@var{n}.c}
9142 exists or can be made.
9144 There may also be prerequisites that do not use @samp{%}; such a prerequisite
9145 attaches to every file made by this pattern rule. These unvarying
9146 prerequisites are useful occasionally.
9148 A pattern rule need not have any prerequisites that contain @samp{%}, or
9149 in fact any prerequisites at all. Such a rule is effectively a general
9150 wildcard. It provides a way to make any file that matches the target
9151 pattern. @xref{Last Resort}.
9153 More than one pattern rule may match a target. In this case
9154 @code{make} will choose the ``best fit'' rule. @xref{Pattern Match,
9155 ,How Patterns Match}.
9157 @c !!! The end of of this paragraph should be rewritten. --bob
9158 Pattern rules may have more than one target. Unlike normal rules,
9159 this does not act as many different rules with the same prerequisites
9160 and recipe. If a pattern rule has multiple targets, @code{make} knows
9161 that the rule's recipe is responsible for making all of the targets.
9162 The recipe is executed only once to make all the targets. When
9163 searching for a pattern rule to match a target, the target patterns of
9164 a rule other than the one that matches the target in need of a rule
9165 are incidental: @code{make} worries only about giving a recipe and
9166 prerequisites to the file presently in question. However, when this
9167 file's recipe is run, the other targets are marked as having been
9169 @cindex multiple targets, in pattern rule
9170 @cindex target, multiple in pattern rule
9172 @node Pattern Examples, Automatic Variables, Pattern Intro, Pattern Rules
9173 @subsection Pattern Rule Examples
9175 Here are some examples of pattern rules actually predefined in
9176 @code{make}. First, the rule that compiles @samp{.c} files into @samp{.o}
9181 $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
9185 defines a rule that can make any file @file{@var{x}.o} from
9186 @file{@var{x}.c}. The recipe uses the automatic variables @samp{$@@} and
9187 @samp{$<} to substitute the names of the target file and the source file
9188 in each case where the rule applies (@pxref{Automatic Variables}).@refill
9190 Here is a second built-in rule:
9198 defines a rule that can make any file @file{@var{x}} whatsoever from a
9199 corresponding file @file{@var{x},v} in the subdirectory @file{RCS}. Since
9200 the target is @samp{%}, this rule will apply to any file whatever, provided
9201 the appropriate prerequisite file exists. The double colon makes the rule
9202 @dfn{terminal}, which means that its prerequisite may not be an intermediate
9203 file (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}).@refill
9206 This pattern rule has two targets:
9210 %.tab.c %.tab.h: %.y
9216 @c The following paragraph is rewritten to avoid overfull hboxes
9217 This tells @code{make} that the recipe @samp{bison -d @var{x}.y} will
9218 make both @file{@var{x}.tab.c} and @file{@var{x}.tab.h}. If the file
9219 @file{foo} depends on the files @file{parse.tab.o} and @file{scan.o}
9220 and the file @file{scan.o} depends on the file @file{parse.tab.h},
9221 when @file{parse.y} is changed, the recipe @samp{bison -d parse.y}
9222 will be executed only once, and the prerequisites of both
9223 @file{parse.tab.o} and @file{scan.o} will be satisfied. (Presumably
9224 the file @file{parse.tab.o} will be recompiled from @file{parse.tab.c}
9225 and the file @file{scan.o} from @file{scan.c}, while @file{foo} is
9226 linked from @file{parse.tab.o}, @file{scan.o}, and its other
9227 prerequisites, and it will execute happily ever after.)@refill
9229 @node Automatic Variables, Pattern Match, Pattern Examples, Pattern Rules
9230 @subsection Automatic Variables
9231 @cindex automatic variables
9232 @cindex variables, automatic
9233 @cindex variables, and implicit rule
9235 Suppose you are writing a pattern rule to compile a @samp{.c} file into a
9236 @samp{.o} file: how do you write the @samp{cc} command so that it operates
9237 on the right source file name? You cannot write the name in the recipe,
9238 because the name is different each time the implicit rule is applied.
9240 What you do is use a special feature of @code{make}, the @dfn{automatic
9241 variables}. These variables have values computed afresh for each rule that
9242 is executed, based on the target and prerequisites of the rule. In this
9243 example, you would use @samp{$@@} for the object file name and @samp{$<}
9244 for the source file name.
9246 @cindex automatic variables in prerequisites
9247 @cindex prerequisites, and automatic variables
9248 It's very important that you recognize the limited scope in which
9249 automatic variable values are available: they only have values within
9250 the recipe. In particular, you cannot use them anywhere
9251 within the target list of a rule; they have no value there and will
9252 expand to the empty string. Also, they cannot be accessed directly
9253 within the prerequisite list of a rule. A common mistake is
9254 attempting to use @code{$@@} within the prerequisites list; this will
9255 not work. However, there is a special feature of GNU @code{make},
9256 secondary expansion (@pxref{Secondary Expansion}), which will allow
9257 automatic variable values to be used in prerequisite lists.
9259 Here is a table of automatic variables:
9263 @vindex @@ @r{(automatic variable)}
9265 The file name of the target of the rule. If the target is an archive
9266 member, then @samp{$@@} is the name of the archive file. In a pattern
9267 rule that has multiple targets (@pxref{Pattern Intro, ,Introduction to
9268 Pattern Rules}), @samp{$@@} is the name of whichever target caused the
9269 rule's recipe to be run.
9272 @vindex % @r{(automatic variable)}
9274 The target member name, when the target is an archive member.
9275 @xref{Archives}. For example, if the target is @file{foo.a(bar.o)} then
9276 @samp{$%} is @file{bar.o} and @samp{$@@} is @file{foo.a}. @samp{$%} is
9277 empty when the target is not an archive member.
9280 @vindex < @r{(automatic variable)}
9282 The name of the first prerequisite. If the target got its recipe from
9283 an implicit rule, this will be the first prerequisite added by the
9284 implicit rule (@pxref{Implicit Rules}).
9287 @vindex ? @r{(automatic variable)}
9289 The names of all the prerequisites that are newer than the target, with
9290 spaces between them. For prerequisites which are archive members, only
9291 the named member is used (@pxref{Archives}).
9292 @cindex prerequisites, list of changed
9293 @cindex list of changed prerequisites
9296 @vindex ^ @r{(automatic variable)}
9298 The names of all the prerequisites, with spaces between them. For
9299 prerequisites which are archive members, only the named member is used
9300 (@pxref{Archives}). A target has only one prerequisite on each other file
9301 it depends on, no matter how many times each file is listed as a
9302 prerequisite. So if you list a prerequisite more than once for a target,
9303 the value of @code{$^} contains just one copy of the name. This list
9304 does @strong{not} contain any of the order-only prerequisites; for those
9305 see the @samp{$|} variable, below.
9306 @cindex prerequisites, list of all
9307 @cindex list of all prerequisites
9310 @vindex + @r{(automatic variable)}
9312 This is like @samp{$^}, but prerequisites listed more than once are
9313 duplicated in the order they were listed in the makefile. This is
9314 primarily useful for use in linking commands where it is meaningful to
9315 repeat library file names in a particular order.
9318 @vindex | @r{(automatic variable)}
9320 The names of all the order-only prerequisites, with spaces between
9324 @vindex * @r{(automatic variable)}
9326 The stem with which an implicit rule matches (@pxref{Pattern Match, ,How
9327 Patterns Match}). If the target is @file{dir/a.foo.b} and the target
9328 pattern is @file{a.%.b} then the stem is @file{dir/foo}. The stem is
9329 useful for constructing names of related files.@refill
9330 @cindex stem, variable for
9332 In a static pattern rule, the stem is part of the file name that matched
9333 the @samp{%} in the target pattern.
9335 In an explicit rule, there is no stem; so @samp{$*} cannot be determined
9336 in that way. Instead, if the target name ends with a recognized suffix
9337 (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}), @samp{$*} is set to
9338 the target name minus the suffix. For example, if the target name is
9339 @samp{foo.c}, then @samp{$*} is set to @samp{foo}, since @samp{.c} is a
9340 suffix. GNU @code{make} does this bizarre thing only for compatibility
9341 with other implementations of @code{make}. You should generally avoid
9342 using @samp{$*} except in implicit rules or static pattern rules.@refill
9344 If the target name in an explicit rule does not end with a recognized
9345 suffix, @samp{$*} is set to the empty string for that rule.
9348 @samp{$?} is useful even in explicit rules when you wish to operate on only
9349 the prerequisites that have changed. For example, suppose that an archive
9350 named @file{lib} is supposed to contain copies of several object files.
9351 This rule copies just the changed object files into the archive:
9355 lib: foo.o bar.o lose.o win.o
9360 Of the variables listed above, four have values that are single file
9361 names, and three have values that are lists of file names. These seven
9362 have variants that get just the file's directory name or just the file
9363 name within the directory. The variant variables' names are formed by
9364 appending @samp{D} or @samp{F}, respectively. These variants are
9365 semi-obsolete in GNU @code{make} since the functions @code{dir} and
9366 @code{notdir} can be used to get a similar effect (@pxref{File Name
9367 Functions, , Functions for File Names}). Note, however, that the
9368 @samp{D} variants all omit the trailing slash which always appears in
9369 the output of the @code{dir} function. Here is a table of the variants:
9373 @vindex @@D @r{(automatic variable)}
9375 The directory part of the file name of the target, with the trailing
9376 slash removed. If the value of @samp{$@@} is @file{dir/foo.o} then
9377 @samp{$(@@D)} is @file{dir}. This value is @file{.} if @samp{$@@} does
9378 not contain a slash.
9381 @vindex @@F @r{(automatic variable)}
9383 The file-within-directory part of the file name of the target. If the
9384 value of @samp{$@@} is @file{dir/foo.o} then @samp{$(@@F)} is
9385 @file{foo.o}. @samp{$(@@F)} is equivalent to @samp{$(notdir $@@)}.
9388 @vindex *D @r{(automatic variable)}
9391 @vindex *F @r{(automatic variable)}
9393 The directory part and the file-within-directory
9394 part of the stem; @file{dir} and @file{foo} in this example.
9397 @vindex %D @r{(automatic variable)}
9400 @vindex %F @r{(automatic variable)}
9402 The directory part and the file-within-directory part of the target
9403 archive member name. This makes sense only for archive member targets
9404 of the form @file{@var{archive}(@var{member})} and is useful only when
9405 @var{member} may contain a directory name. (@xref{Archive Members,
9406 ,Archive Members as Targets}.)
9409 @vindex <D @r{(automatic variable)}
9412 @vindex <F @r{(automatic variable)}
9414 The directory part and the file-within-directory
9415 part of the first prerequisite.
9418 @vindex ^D @r{(automatic variable)}
9421 @vindex ^F @r{(automatic variable)}
9423 Lists of the directory parts and the file-within-directory
9424 parts of all prerequisites.
9427 @vindex +D @r{(automatic variable)}
9430 @vindex +F @r{(automatic variable)}
9432 Lists of the directory parts and the file-within-directory
9433 parts of all prerequisites, including multiple instances of duplicated
9437 @vindex ?D @r{(automatic variable)}
9440 @vindex ?F @r{(automatic variable)}
9442 Lists of the directory parts and the file-within-directory parts of
9443 all prerequisites that are newer than the target.
9446 Note that we use a special stylistic convention when we talk about these
9447 automatic variables; we write ``the value of @samp{$<}'', rather than
9448 @w{``the variable @code{<}''} as we would write for ordinary variables
9449 such as @code{objects} and @code{CFLAGS}. We think this convention
9450 looks more natural in this special case. Please do not assume it has a
9451 deep significance; @samp{$<} refers to the variable named @code{<} just
9452 as @samp{$(CFLAGS)} refers to the variable named @code{CFLAGS}.
9453 You could just as well use @samp{$(<)} in place of @samp{$<}.
9455 @node Pattern Match, Match-Anything Rules, Automatic Variables, Pattern Rules
9456 @subsection How Patterns Match
9459 A target pattern is composed of a @samp{%} between a prefix and a suffix,
9460 either or both of which may be empty. The pattern matches a file name only
9461 if the file name starts with the prefix and ends with the suffix, without
9462 overlap. The text between the prefix and the suffix is called the
9463 @dfn{stem}. Thus, when the pattern @samp{%.o} matches the file name
9464 @file{test.o}, the stem is @samp{test}. The pattern rule prerequisites are
9465 turned into actual file names by substituting the stem for the character
9466 @samp{%}. Thus, if in the same example one of the prerequisites is written
9467 as @samp{%.c}, it expands to @samp{test.c}.@refill
9469 When the target pattern does not contain a slash (and it usually does
9470 not), directory names in the file names are removed from the file name
9471 before it is compared with the target prefix and suffix. After the
9472 comparison of the file name to the target pattern, the directory
9473 names, along with the slash that ends them, are added on to the
9474 prerequisite file names generated from the pattern rule's prerequisite
9475 patterns and the file name. The directories are ignored only for the
9476 purpose of finding an implicit rule to use, not in the application of
9477 that rule. Thus, @samp{e%t} matches the file name @file{src/eat},
9478 with @samp{src/a} as the stem. When prerequisites are turned into file
9479 names, the directories from the stem are added at the front, while the
9480 rest of the stem is substituted for the @samp{%}. The stem
9481 @samp{src/a} with a prerequisite pattern @samp{c%r} gives the file name
9482 @file{src/car}.@refill
9484 @cindex pattern rules, order of
9485 @cindex order of pattern rules
9486 A pattern rule can be used to build a given file only if there is a
9487 target pattern that matches the file name, @emph{and} all
9488 prerequisites in that rule either exist or can be built. The rules
9489 you write take precedence over those that are built in. Note however,
9490 that a rule whose prerequisites actually exist or are mentioned always
9491 takes priority over a rule with prerequisites that must be made by
9492 chaining other implicit rules.
9494 @cindex stem, shortest
9495 It is possible that more than one pattern rule will meet these
9496 criteria. In that case, @code{make} will choose the rule with the
9497 shortest stem (that is, the pattern that matches most specifically).
9498 If more than one pattern rule has the shortest stem, @code{make} will
9499 choose the first one found in the makefile.
9501 This algorithm results in more specific rules being preferred over
9502 more generic ones; for example:
9506 $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
9509 $(COMPILE.F) $(OUTPUT_OPTION) $<
9512 $(CC) -fPIC -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
9515 Given these rules and asked to build @file{bar.o} where both
9516 @file{bar.c} and @file{bar.f} exist, @code{make} will choose the first
9517 rule and compile @file{bar.c} into @file{bar.o}. In the same
9518 situation where @file{bar.c} does not exist, then @code{make} will
9519 choose the second rule and compile @file{bar.f} into @file{bar.o}.
9521 If @code{make} is asked to build @file{lib/bar.o} and both
9522 @file{lib/bar.c} and @file{lib/bar.f} exist, then the third rule will
9523 be chosen since the stem for this rule (@samp{bar}) is shorter than
9524 the stem for the first rule (@samp{lib/bar}). If @file{lib/bar.c}
9525 does not exist then the third rule is not eligible and the second rule
9526 will be used, even though the stem is longer.
9528 @node Match-Anything Rules, Canceling Rules, Pattern Match, Pattern Rules
9529 @subsection Match-Anything Pattern Rules
9531 @cindex match-anything rule
9532 @cindex terminal rule
9533 When a pattern rule's target is just @samp{%}, it matches any file name
9534 whatever. We call these rules @dfn{match-anything} rules. They are very
9535 useful, but it can take a lot of time for @code{make} to think about them,
9536 because it must consider every such rule for each file name listed either
9537 as a target or as a prerequisite.
9539 Suppose the makefile mentions @file{foo.c}. For this target, @code{make}
9540 would have to consider making it by linking an object file @file{foo.c.o},
9541 or by C compilation-and-linking in one step from @file{foo.c.c}, or by
9542 Pascal compilation-and-linking from @file{foo.c.p}, and many other
9545 We know these possibilities are ridiculous since @file{foo.c} is a C source
9546 file, not an executable. If @code{make} did consider these possibilities,
9547 it would ultimately reject them, because files such as @file{foo.c.o} and
9548 @file{foo.c.p} would not exist. But these possibilities are so
9549 numerous that @code{make} would run very slowly if it had to consider
9552 To gain speed, we have put various constraints on the way @code{make}
9553 considers match-anything rules. There are two different constraints that
9554 can be applied, and each time you define a match-anything rule you must
9555 choose one or the other for that rule.
9557 One choice is to mark the match-anything rule as @dfn{terminal} by defining
9558 it with a double colon. When a rule is terminal, it does not apply unless
9559 its prerequisites actually exist. Prerequisites that could be made with
9560 other implicit rules are not good enough. In other words, no further
9561 chaining is allowed beyond a terminal rule.
9563 For example, the built-in implicit rules for extracting sources from RCS
9564 and SCCS files are terminal; as a result, if the file @file{foo.c,v} does
9565 not exist, @code{make} will not even consider trying to make it as an
9566 intermediate file from @file{foo.c,v.o} or from @file{RCS/SCCS/s.foo.c,v}.
9567 RCS and SCCS files are generally ultimate source files, which should not be
9568 remade from any other files; therefore, @code{make} can save time by not
9569 looking for ways to remake them.@refill
9571 If you do not mark the match-anything rule as terminal, then it is
9572 nonterminal. A nonterminal match-anything rule cannot apply to a file name
9573 that indicates a specific type of data. A file name indicates a specific
9574 type of data if some non-match-anything implicit rule target matches it.
9576 For example, the file name @file{foo.c} matches the target for the pattern
9577 rule @samp{%.c : %.y} (the rule to run Yacc). Regardless of whether this
9578 rule is actually applicable (which happens only if there is a file
9579 @file{foo.y}), the fact that its target matches is enough to prevent
9580 consideration of any nonterminal match-anything rules for the file
9581 @file{foo.c}. Thus, @code{make} will not even consider trying to make
9582 @file{foo.c} as an executable file from @file{foo.c.o}, @file{foo.c.c},
9583 @file{foo.c.p}, etc.@refill
9585 The motivation for this constraint is that nonterminal match-anything
9586 rules are used for making files containing specific types of data (such as
9587 executable files) and a file name with a recognized suffix indicates some
9588 other specific type of data (such as a C source file).
9590 Special built-in dummy pattern rules are provided solely to recognize
9591 certain file names so that nonterminal match-anything rules will not be
9592 considered. These dummy rules have no prerequisites and no recipes, and
9593 they are ignored for all other purposes. For example, the built-in
9601 exists to make sure that Pascal source files such as @file{foo.p} match a
9602 specific target pattern and thereby prevent time from being wasted looking
9603 for @file{foo.p.o} or @file{foo.p.c}.
9605 Dummy pattern rules such as the one for @samp{%.p} are made for every
9606 suffix listed as valid for use in suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}).
9608 @node Canceling Rules, , Match-Anything Rules, Pattern Rules
9609 @subsection Canceling Implicit Rules
9611 You can override a built-in implicit rule (or one you have defined
9612 yourself) by defining a new pattern rule with the same target and
9613 prerequisites, but a different recipe. When the new rule is defined, the
9614 built-in one is replaced. The new rule's position in the sequence of
9615 implicit rules is determined by where you write the new rule.
9617 You can cancel a built-in implicit rule by defining a pattern rule with the
9618 same target and prerequisites, but no recipe. For example, the following
9619 would cancel the rule that runs the assembler:
9625 @node Last Resort, Suffix Rules, Pattern Rules, Implicit Rules
9626 @section Defining Last-Resort Default Rules
9627 @cindex last-resort default rules
9628 @cindex default rules, last-resort
9630 You can define a last-resort implicit rule by writing a terminal
9631 match-anything pattern rule with no prerequisites (@pxref{Match-Anything
9632 Rules}). This is just like any other pattern rule; the only thing
9633 special about it is that it will match any target. So such a rule's
9634 recipe is used for all targets and prerequisites that have no recipe
9635 of their own and for which no other implicit rule applies.
9637 For example, when testing a makefile, you might not care if the source
9638 files contain real data, only that they exist. Then you might do this:
9646 to cause all the source files needed (as prerequisites) to be created
9650 You can instead define a recipe to be used for targets for which there
9651 are no rules at all, even ones which don't specify recipes. You do
9652 this by writing a rule for the target @code{.DEFAULT}. Such a rule's
9653 recipe is used for all prerequisites which do not appear as targets in
9654 any explicit rule, and for which no implicit rule applies. Naturally,
9655 there is no @code{.DEFAULT} rule unless you write one.
9657 If you use @code{.DEFAULT} with no recipe or prerequisites:
9664 the recipe previously stored for @code{.DEFAULT} is cleared. Then
9665 @code{make} acts as if you had never defined @code{.DEFAULT} at all.
9667 If you do not want a target to get the recipe from a match-anything
9668 pattern rule or @code{.DEFAULT}, but you also do not want any recipe
9669 to be run for the target, you can give it an empty recipe
9670 (@pxref{Empty Recipes, ,Defining Empty Recipes}).@refill
9672 You can use a last-resort rule to override part of another makefile.
9673 @xref{Overriding Makefiles, , Overriding Part of Another Makefile}.
9675 @node Suffix Rules, Implicit Rule Search, Last Resort, Implicit Rules
9676 @section Old-Fashioned Suffix Rules
9677 @cindex old-fashioned suffix rules
9680 @dfn{Suffix rules} are the old-fashioned way of defining implicit rules for
9681 @code{make}. Suffix rules are obsolete because pattern rules are more
9682 general and clearer. They are supported in GNU @code{make} for
9683 compatibility with old makefiles. They come in two kinds:
9684 @dfn{double-suffix} and @dfn{single-suffix}.@refill
9686 A double-suffix rule is defined by a pair of suffixes: the target suffix
9687 and the source suffix. It matches any file whose name ends with the
9688 target suffix. The corresponding implicit prerequisite is made by
9689 replacing the target suffix with the source suffix in the file name. A
9690 two-suffix rule whose target and source suffixes are @samp{.o} and
9691 @samp{.c} is equivalent to the pattern rule @samp{%.o : %.c}.
9693 A single-suffix rule is defined by a single suffix, which is the source
9694 suffix. It matches any file name, and the corresponding implicit
9695 prerequisite name is made by appending the source suffix. A single-suffix
9696 rule whose source suffix is @samp{.c} is equivalent to the pattern rule
9699 Suffix rule definitions are recognized by comparing each rule's target
9700 against a defined list of known suffixes. When @code{make} sees a rule
9701 whose target is a known suffix, this rule is considered a single-suffix
9702 rule. When @code{make} sees a rule whose target is two known suffixes
9703 concatenated, this rule is taken as a double-suffix rule.
9705 For example, @samp{.c} and @samp{.o} are both on the default list of
9706 known suffixes. Therefore, if you define a rule whose target is
9707 @samp{.c.o}, @code{make} takes it to be a double-suffix rule with source
9708 suffix @samp{.c} and target suffix @samp{.o}. Here is the old-fashioned
9709 way to define the rule for compiling a C source file:@refill
9713 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
9716 Suffix rules cannot have any prerequisites of their own. If they have any,
9717 they are treated as normal files with funny names, not as suffix rules.
9722 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
9726 tells how to make the file @file{.c.o} from the prerequisite file
9727 @file{foo.h}, and is not at all like the pattern rule:
9731 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
9735 which tells how to make @samp{.o} files from @samp{.c} files, and makes all
9736 @samp{.o} files using this pattern rule also depend on @file{foo.h}.
9738 Suffix rules with no recipe are also meaningless. They do not remove
9739 previous rules as do pattern rules with no recipe (@pxref{Canceling
9740 Rules, , Canceling Implicit Rules}). They simply enter the suffix or
9741 pair of suffixes concatenated as a target in the data base.@refill
9744 The known suffixes are simply the names of the prerequisites of the special
9745 target @code{.SUFFIXES}. You can add your own suffixes by writing a rule
9746 for @code{.SUFFIXES} that adds more prerequisites, as in:
9749 .SUFFIXES: .hack .win
9753 which adds @samp{.hack} and @samp{.win} to the end of the list of suffixes.
9755 If you wish to eliminate the default known suffixes instead of just adding
9756 to them, write a rule for @code{.SUFFIXES} with no prerequisites. By
9757 special dispensation, this eliminates all existing prerequisites of
9758 @code{.SUFFIXES}. You can then write another rule to add the suffixes you
9763 .SUFFIXES: # @r{Delete the default suffixes}
9764 .SUFFIXES: .c .o .h # @r{Define our suffix list}
9768 The @samp{-r} or @samp{--no-builtin-rules} flag causes the default
9769 list of suffixes to be empty.
9772 The variable @code{SUFFIXES} is defined to the default list of suffixes
9773 before @code{make} reads any makefiles. You can change the list of suffixes
9774 with a rule for the special target @code{.SUFFIXES}, but that does not alter
9777 @node Implicit Rule Search, , Suffix Rules, Implicit Rules
9778 @section Implicit Rule Search Algorithm
9779 @cindex implicit rule, search algorithm
9780 @cindex search algorithm, implicit rule
9782 Here is the procedure @code{make} uses for searching for an implicit rule
9783 for a target @var{t}. This procedure is followed for each double-colon
9784 rule with no recipe, for each target of ordinary rules none of which have
9785 a recipe, and for each prerequisite that is not the target of any rule. It
9786 is also followed recursively for prerequisites that come from implicit
9787 rules, in the search for a chain of rules.
9789 Suffix rules are not mentioned in this algorithm because suffix rules are
9790 converted to equivalent pattern rules once the makefiles have been read in.
9792 For an archive member target of the form
9793 @samp{@var{archive}(@var{member})}, the following algorithm is run
9794 twice, first using the entire target name @var{t}, and second using
9795 @samp{(@var{member})} as the target @var{t} if the first run found no
9800 Split @var{t} into a directory part, called @var{d}, and the rest,
9801 called @var{n}. For example, if @var{t} is @samp{src/foo.o}, then
9802 @var{d} is @samp{src/} and @var{n} is @samp{foo.o}.@refill
9805 Make a list of all the pattern rules one of whose targets matches
9806 @var{t} or @var{n}. If the target pattern contains a slash, it is
9807 matched against @var{t}; otherwise, against @var{n}.
9810 If any rule in that list is @emph{not} a match-anything rule, then
9811 remove all nonterminal match-anything rules from the list.
9814 Remove from the list all rules with no recipe.
9817 For each pattern rule in the list:
9821 Find the stem @var{s}, which is the nonempty part of @var{t} or @var{n}
9822 matched by the @samp{%} in the target pattern.@refill
9825 Compute the prerequisite names by substituting @var{s} for @samp{%}; if
9826 the target pattern does not contain a slash, append @var{d} to
9827 the front of each prerequisite name.@refill
9830 Test whether all the prerequisites exist or ought to exist. (If a
9831 file name is mentioned in the makefile as a target or as an explicit
9832 prerequisite, then we say it ought to exist.)
9834 If all prerequisites exist or ought to exist, or there are no prerequisites,
9835 then this rule applies.
9839 If no pattern rule has been found so far, try harder.
9840 For each pattern rule in the list:
9844 If the rule is terminal, ignore it and go on to the next rule.
9847 Compute the prerequisite names as before.
9850 Test whether all the prerequisites exist or ought to exist.
9853 For each prerequisite that does not exist, follow this algorithm
9854 recursively to see if the prerequisite can be made by an implicit
9858 If all prerequisites exist, ought to exist, or can be
9859 made by implicit rules, then this rule applies.
9863 If no implicit rule applies, the rule for @code{.DEFAULT}, if any,
9864 applies. In that case, give @var{t} the same recipe that
9865 @code{.DEFAULT} has. Otherwise, there is no recipe for @var{t}.
9868 Once a rule that applies has been found, for each target pattern of
9869 the rule other than the one that matched @var{t} or @var{n}, the
9870 @samp{%} in the pattern is replaced with @var{s} and the resultant
9871 file name is stored until the recipe to remake the target file @var{t}
9872 is executed. After the recipe is executed, each of these stored file
9873 names are entered into the data base and marked as having been updated
9874 and having the same update status as the file @var{t}.
9876 When the recipe of a pattern rule is executed for @var{t}, the
9877 automatic variables are set corresponding to the target and
9878 prerequisites. @xref{Automatic Variables}.
9880 @node Archives, Features, Implicit Rules, Top
9881 @chapter Using @code{make} to Update Archive Files
9884 @dfn{Archive files} are files containing named subfiles called
9885 @dfn{members}; they are maintained with the program @code{ar} and their
9886 main use is as subroutine libraries for linking.
9889 * Archive Members:: Archive members as targets.
9890 * Archive Update:: The implicit rule for archive member targets.
9891 * Archive Pitfalls:: Dangers to watch out for when using archives.
9892 * Archive Suffix Rules:: You can write a special kind of suffix rule
9893 for updating archives.
9896 @node Archive Members, Archive Update, Archives, Archives
9897 @section Archive Members as Targets
9898 @cindex archive member targets
9900 An individual member of an archive file can be used as a target or
9901 prerequisite in @code{make}. You specify the member named @var{member} in
9902 archive file @var{archive} as follows:
9905 @var{archive}(@var{member})
9909 This construct is available only in targets and prerequisites, not in
9910 recipes! Most programs that you might use in recipes do not support
9911 this syntax and cannot act directly on archive members. Only
9912 @code{ar} and other programs specifically designed to operate on
9913 archives can do so. Therefore, valid recipes to update an archive
9914 member target probably must use @code{ar}. For example, this rule
9915 says to create a member @file{hack.o} in archive @file{foolib} by
9916 copying the file @file{hack.o}:
9919 foolib(hack.o) : hack.o
9923 In fact, nearly all archive member targets are updated in just this way
9924 and there is an implicit rule to do it for you. @strong{Please note:} The
9925 @samp{c} flag to @code{ar} is required if the archive file does not
9928 To specify several members in the same archive, you can write all the
9929 member names together between the parentheses. For example:
9932 foolib(hack.o kludge.o)
9939 foolib(hack.o) foolib(kludge.o)
9942 @cindex wildcard, in archive member
9943 You can also use shell-style wildcards in an archive member reference.
9944 @xref{Wildcards, ,Using Wildcard Characters in File Names}. For
9945 example, @w{@samp{foolib(*.o)}} expands to all existing members of the
9946 @file{foolib} archive whose names end in @samp{.o}; perhaps
9947 @samp{@w{foolib(hack.o)} @w{foolib(kludge.o)}}.
9949 @node Archive Update, Archive Pitfalls, Archive Members, Archives
9950 @section Implicit Rule for Archive Member Targets
9952 Recall that a target that looks like @file{@var{a}(@var{m})} stands for the
9953 member named @var{m} in the archive file @var{a}.
9955 When @code{make} looks for an implicit rule for such a target, as a special
9956 feature it considers implicit rules that match @file{(@var{m})}, as well as
9957 those that match the actual target @file{@var{a}(@var{m})}.
9959 This causes one special rule whose target is @file{(%)} to match. This
9960 rule updates the target @file{@var{a}(@var{m})} by copying the file @var{m}
9961 into the archive. For example, it will update the archive member target
9962 @file{foo.a(bar.o)} by copying the @emph{file} @file{bar.o} into the
9963 archive @file{foo.a} as a @emph{member} named @file{bar.o}.
9965 When this rule is chained with others, the result is very powerful.
9966 Thus, @samp{make "foo.a(bar.o)"} (the quotes are needed to protect the
9967 @samp{(} and @samp{)} from being interpreted specially by the shell) in
9968 the presence of a file @file{bar.c} is enough to cause the following
9969 recipe to be run, even without a makefile:
9972 cc -c bar.c -o bar.o
9978 Here @code{make} has envisioned the file @file{bar.o} as an intermediate
9979 file. @xref{Chained Rules, ,Chains of Implicit Rules}.
9981 Implicit rules such as this one are written using the automatic variable
9982 @samp{$%}. @xref{Automatic Variables}.
9984 An archive member name in an archive cannot contain a directory name, but
9985 it may be useful in a makefile to pretend that it does. If you write an
9986 archive member target @file{foo.a(dir/file.o)}, @code{make} will perform
9987 automatic updating with this recipe:
9990 ar r foo.a dir/file.o
9994 which has the effect of copying the file @file{dir/file.o} into a member
9995 named @file{file.o}. In connection with such usage, the automatic variables
9996 @code{%D} and @code{%F} may be useful.
9999 * Archive Symbols:: How to update archive symbol directories.
10002 @node Archive Symbols, , Archive Update, Archive Update
10003 @subsection Updating Archive Symbol Directories
10004 @cindex @code{__.SYMDEF}
10005 @cindex updating archive symbol directories
10006 @cindex archive symbol directory updating
10007 @cindex symbol directories, updating archive
10008 @cindex directories, updating archive symbol
10010 An archive file that is used as a library usually contains a special member
10011 named @file{__.SYMDEF} that contains a directory of the external symbol
10012 names defined by all the other members. After you update any other
10013 members, you need to update @file{__.SYMDEF} so that it will summarize the
10014 other members properly. This is done by running the @code{ranlib} program:
10017 ranlib @var{archivefile}
10020 Normally you would put this command in the rule for the archive file,
10021 and make all the members of the archive file prerequisites of that rule.
10025 libfoo.a: libfoo.a(x.o) libfoo.a(y.o) @dots{}
10030 The effect of this is to update archive members @file{x.o}, @file{y.o},
10031 etc., and then update the symbol directory member @file{__.SYMDEF} by
10032 running @code{ranlib}. The rules for updating the members are not shown
10033 here; most likely you can omit them and use the implicit rule which copies
10034 files into the archive, as described in the preceding section.
10036 This is not necessary when using the GNU @code{ar} program, which
10037 updates the @file{__.SYMDEF} member automatically.
10039 @node Archive Pitfalls, Archive Suffix Rules, Archive Update, Archives
10040 @section Dangers When Using Archives
10041 @cindex archive, and parallel execution
10042 @cindex parallel execution, and archive update
10043 @cindex archive, and @code{-j}
10044 @cindex @code{-j}, and archive update
10046 It is important to be careful when using parallel execution (the
10047 @code{-j} switch; @pxref{Parallel, ,Parallel Execution}) and archives.
10048 If multiple @code{ar} commands run at the same time on the same archive
10049 file, they will not know about each other and can corrupt the file.
10051 Possibly a future version of @code{make} will provide a mechanism to
10052 circumvent this problem by serializing all recipes that operate on the
10053 same archive file. But for the time being, you must either write your
10054 makefiles to avoid this problem in some other way, or not use @code{-j}.
10056 @node Archive Suffix Rules, , Archive Pitfalls, Archives
10057 @section Suffix Rules for Archive Files
10058 @cindex suffix rule, for archive
10059 @cindex archive, suffix rule for
10060 @cindex library archive, suffix rule for
10061 @cindex @code{.a} (archives)
10063 You can write a special kind of suffix rule for dealing with archive
10064 files. @xref{Suffix Rules}, for a full explanation of suffix rules.
10065 Archive suffix rules are obsolete in GNU @code{make}, because pattern
10066 rules for archives are a more general mechanism (@pxref{Archive
10067 Update}). But they are retained for compatibility with other
10070 To write a suffix rule for archives, you simply write a suffix rule
10071 using the target suffix @samp{.a} (the usual suffix for archive files).
10072 For example, here is the old-fashioned suffix rule to update a library
10073 archive from C source files:
10078 $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
10085 This works just as if you had written the pattern rule:
10090 $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
10096 In fact, this is just what @code{make} does when it sees a suffix rule
10097 with @samp{.a} as the target suffix. Any double-suffix rule
10098 @w{@samp{.@var{x}.a}} is converted to a pattern rule with the target
10099 pattern @samp{(%.o)} and a prerequisite pattern of @samp{%.@var{x}}.
10101 Since you might want to use @samp{.a} as the suffix for some other kind
10102 of file, @code{make} also converts archive suffix rules to pattern rules
10103 in the normal way (@pxref{Suffix Rules}). Thus a double-suffix rule
10104 @w{@samp{.@var{x}.a}} produces two pattern rules: @samp{@w{(%.o):}
10105 @w{%.@var{x}}} and @samp{@w{%.a}: @w{%.@var{x}}}.@refill
10107 @node Features, Missing, Archives, Top
10108 @chapter Features of GNU @code{make}
10109 @cindex features of GNU @code{make}
10110 @cindex portability
10111 @cindex compatibility
10113 Here is a summary of the features of GNU @code{make}, for comparison
10114 with and credit to other versions of @code{make}. We consider the
10115 features of @code{make} in 4.2 BSD systems as a baseline. If you are
10116 concerned with writing portable makefiles, you should not use the
10117 features of @code{make} listed here, nor the ones in @ref{Missing}.
10119 Many features come from the version of @code{make} in System V.
10123 The @code{VPATH} variable and its special meaning.
10124 @xref{Directory Search, , Searching Directories for Prerequisites}.
10125 This feature exists in System V @code{make}, but is undocumented.
10126 It is documented in 4.3 BSD @code{make} (which says it mimics System V's
10127 @code{VPATH} feature).@refill
10130 Included makefiles. @xref{Include, ,Including Other Makefiles}.
10131 Allowing multiple files to be included with a single directive is a GNU
10135 Variables are read from and communicated via the environment.
10136 @xref{Environment, ,Variables from the Environment}.
10139 Options passed through the variable @code{MAKEFLAGS} to recursive
10140 invocations of @code{make}.
10141 @xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
10144 The automatic variable @code{$%} is set to the member name
10145 in an archive reference. @xref{Automatic Variables}.
10148 The automatic variables @code{$@@}, @code{$*}, @code{$<}, @code{$%},
10149 and @code{$?} have corresponding forms like @code{$(@@F)} and
10150 @code{$(@@D)}. We have generalized this to @code{$^} as an obvious
10151 extension. @xref{Automatic Variables}.@refill
10154 Substitution variable references.
10155 @xref{Reference, ,Basics of Variable References}.
10158 The command line options @samp{-b} and @samp{-m}, accepted and
10159 ignored. In System V @code{make}, these options actually do something.
10162 Execution of recursive commands to run @code{make} via the variable
10163 @code{MAKE} even if @samp{-n}, @samp{-q} or @samp{-t} is specified.
10164 @xref{Recursion, ,Recursive Use of @code{make}}.
10167 Support for suffix @samp{.a} in suffix rules. @xref{Archive Suffix
10168 Rules}. This feature is obsolete in GNU @code{make}, because the
10169 general feature of rule chaining (@pxref{Chained Rules, ,Chains of
10170 Implicit Rules}) allows one pattern rule for installing members in an
10171 archive (@pxref{Archive Update}) to be sufficient.
10174 The arrangement of lines and backslash-newline combinations in
10175 recipes is retained when the recipes are printed, so they appear as
10176 they do in the makefile, except for the stripping of initial
10180 The following features were inspired by various other versions of
10181 @code{make}. In some cases it is unclear exactly which versions inspired
10186 Pattern rules using @samp{%}.
10187 This has been implemented in several versions of @code{make}.
10188 We're not sure who invented it first, but it's been spread around a bit.
10189 @xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.@refill
10192 Rule chaining and implicit intermediate files.
10193 This was implemented by Stu Feldman in his version of @code{make}
10194 for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
10195 AT&T Bell Labs in his @code{mk} program (where he terms it
10196 ``transitive closure''). We do not really know if
10197 we got this from either of them or thought it up ourselves at the
10198 same time. @xref{Chained Rules, ,Chains of Implicit Rules}.
10201 The automatic variable @code{$^} containing a list of all prerequisites
10202 of the current target. We did not invent this, but we have no idea who
10203 did. @xref{Automatic Variables}. The automatic variable
10204 @code{$+} is a simple extension of @code{$^}.
10207 The ``what if'' flag (@samp{-W} in GNU @code{make}) was (as far as we know)
10208 invented by Andrew Hume in @code{mk}.
10209 @xref{Instead of Execution, ,Instead of Executing Recipes}.
10212 The concept of doing several things at once (parallelism) exists in
10213 many incarnations of @code{make} and similar programs, though not in the
10214 System V or BSD implementations. @xref{Execution, ,Recipe Execution}.
10217 Modified variable references using pattern substitution come from
10218 SunOS 4. @xref{Reference, ,Basics of Variable References}.
10219 This functionality was provided in GNU @code{make} by the
10220 @code{patsubst} function before the alternate syntax was implemented
10221 for compatibility with SunOS 4. It is not altogether clear who
10222 inspired whom, since GNU @code{make} had @code{patsubst} before SunOS
10223 4 was released.@refill
10226 The special significance of @samp{+} characters preceding recipe lines
10227 (@pxref{Instead of Execution, ,Instead of Executing Recipes}) is
10228 mandated by @cite{IEEE Standard 1003.2-1992} (POSIX.2).
10231 The @samp{+=} syntax to append to the value of a variable comes from SunOS
10232 4 @code{make}. @xref{Appending, , Appending More Text to Variables}.
10235 The syntax @w{@samp{@var{archive}(@var{mem1} @var{mem2}@dots{})}} to list
10236 multiple members in a single archive file comes from SunOS 4 @code{make}.
10237 @xref{Archive Members}.
10240 The @code{-include} directive to include makefiles with no error for a
10241 nonexistent file comes from SunOS 4 @code{make}. (But note that SunOS 4
10242 @code{make} does not allow multiple makefiles to be specified in one
10243 @code{-include} directive.) The same feature appears with the name
10244 @code{sinclude} in SGI @code{make} and perhaps others.
10247 The remaining features are inventions new in GNU @code{make}:
10251 Use the @samp{-v} or @samp{--version} option to print version and
10252 copyright information.
10255 Use the @samp{-h} or @samp{--help} option to summarize the options to
10259 Simply-expanded variables. @xref{Flavors, ,The Two Flavors of Variables}.
10262 Pass command line variable assignments automatically through the
10263 variable @code{MAKE} to recursive @code{make} invocations.
10264 @xref{Recursion, ,Recursive Use of @code{make}}.
10267 Use the @samp{-C} or @samp{--directory} command option to change
10268 directory. @xref{Options Summary, ,Summary of Options}.
10271 Make verbatim variable definitions with @code{define}.
10272 @xref{Multi-Line, ,Defining Multi-Line Variables}.
10275 Declare phony targets with the special target @code{.PHONY}.
10277 Andrew Hume of AT&T Bell Labs implemented a similar feature with a
10278 different syntax in his @code{mk} program. This seems to be a case of
10279 parallel discovery. @xref{Phony Targets, ,Phony Targets}.
10282 Manipulate text by calling functions.
10283 @xref{Functions, ,Functions for Transforming Text}.
10286 Use the @samp{-o} or @samp{--old-file}
10287 option to pretend a file's modification-time is old.
10288 @xref{Avoiding Compilation, ,Avoiding Recompilation of Some Files}.
10291 Conditional execution.
10293 This feature has been implemented numerous times in various versions
10294 of @code{make}; it seems a natural extension derived from the features
10295 of the C preprocessor and similar macro languages and is not a
10296 revolutionary concept. @xref{Conditionals, ,Conditional Parts of Makefiles}.
10299 Specify a search path for included makefiles.
10300 @xref{Include, ,Including Other Makefiles}.
10303 Specify extra makefiles to read with an environment variable.
10304 @xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.
10307 Strip leading sequences of @samp{./} from file names, so that
10308 @file{./@var{file}} and @file{@var{file}} are considered to be the
10312 Use a special search method for library prerequisites written in the
10313 form @samp{-l@var{name}}.
10314 @xref{Libraries/Search, ,Directory Search for Link Libraries}.
10317 Allow suffixes for suffix rules
10318 (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}) to contain any
10319 characters. In other versions of @code{make}, they must begin with
10320 @samp{.} and not contain any @samp{/} characters.
10323 Keep track of the current level of @code{make} recursion using the
10324 variable @code{MAKELEVEL}. @xref{Recursion, ,Recursive Use of @code{make}}.
10327 Provide any goals given on the command line in the variable
10328 @code{MAKECMDGOALS}. @xref{Goals, ,Arguments to Specify the Goals}.
10331 Specify static pattern rules. @xref{Static Pattern, ,Static Pattern Rules}.
10334 Provide selective @code{vpath} search.
10335 @xref{Directory Search, ,Searching Directories for Prerequisites}.
10338 Provide computed variable references.
10339 @xref{Reference, ,Basics of Variable References}.
10342 Update makefiles. @xref{Remaking Makefiles, ,How Makefiles Are Remade}.
10343 System V @code{make} has a very, very limited form of this
10344 functionality in that it will check out SCCS files for makefiles.
10347 Various new built-in implicit rules.
10348 @xref{Catalogue of Rules, ,Catalogue of Implicit Rules}.
10351 The built-in variable @samp{MAKE_VERSION} gives the version number of
10353 @vindex MAKE_VERSION
10356 @node Missing, Makefile Conventions, Features, Top
10357 @chapter Incompatibilities and Missing Features
10358 @cindex incompatibilities
10359 @cindex missing features
10360 @cindex features, missing
10362 The @code{make} programs in various other systems support a few features
10363 that are not implemented in GNU @code{make}. The POSIX.2 standard
10364 (@cite{IEEE Standard 1003.2-1992}) which specifies @code{make} does not
10365 require any of these features.@refill
10369 A target of the form @samp{@var{file}((@var{entry}))} stands for a member
10370 of archive file @var{file}. The member is chosen, not by name, but by
10371 being an object file which defines the linker symbol @var{entry}.@refill
10373 This feature was not put into GNU @code{make} because of the
10374 nonmodularity of putting knowledge into @code{make} of the internal
10375 format of archive file symbol tables.
10376 @xref{Archive Symbols, ,Updating Archive Symbol Directories}.
10379 Suffixes (used in suffix rules) that end with the character @samp{~}
10380 have a special meaning to System V @code{make};
10381 they refer to the SCCS file that corresponds
10382 to the file one would get without the @samp{~}. For example, the
10383 suffix rule @samp{.c~.o} would make the file @file{@var{n}.o} from
10384 the SCCS file @file{s.@var{n}.c}. For complete coverage, a whole
10385 series of such suffix rules is required.
10386 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
10388 In GNU @code{make}, this entire series of cases is handled by two
10389 pattern rules for extraction from SCCS, in combination with the
10390 general feature of rule chaining.
10391 @xref{Chained Rules, ,Chains of Implicit Rules}.
10394 In System V and 4.3 BSD @code{make}, files found by @code{VPATH}
10395 search (@pxref{Directory Search, ,Searching Directories for
10396 Prerequisites}) have their names changed inside recipes. We feel it
10397 is much cleaner to always use automatic variables and thus make this
10398 feature obsolete.@refill
10401 In some Unix @code{make}s, the automatic variable @code{$*} appearing in
10402 the prerequisites of a rule has the amazingly strange ``feature'' of
10403 expanding to the full name of the @emph{target of that rule}. We cannot
10404 imagine what went on in the minds of Unix @code{make} developers to do
10405 this; it is utterly inconsistent with the normal definition of @code{$*}.
10406 @vindex * @r{(automatic variable), unsupported bizarre usage}
10409 In some Unix @code{make}s, implicit rule search (@pxref{Implicit
10410 Rules, ,Using Implicit Rules}) is apparently done for @emph{all}
10411 targets, not just those without recipes. This means you can
10422 and Unix @code{make} will intuit that @file{foo.o} depends on
10423 @file{foo.c}.@refill
10425 We feel that such usage is broken. The prerequisite properties of
10426 @code{make} are well-defined (for GNU @code{make}, at least),
10427 and doing such a thing simply does not fit the model.@refill
10430 GNU @code{make} does not include any built-in implicit rules for
10431 compiling or preprocessing EFL programs. If we hear of anyone who is
10432 using EFL, we will gladly add them.
10435 It appears that in SVR4 @code{make}, a suffix rule can be specified
10436 with no recipe, and it is treated as if it had an empty recipe
10437 (@pxref{Empty Recipes}). For example:
10444 will override the built-in @file{.c.a} suffix rule.
10446 We feel that it is cleaner for a rule without a recipe to always simply
10447 add to the prerequisite list for the target. The above example can be
10448 easily rewritten to get the desired behavior in GNU @code{make}:
10455 Some versions of @code{make} invoke the shell with the @samp{-e} flag,
10456 except under @samp{-k} (@pxref{Testing, ,Testing the Compilation of a
10457 Program}). The @samp{-e} flag tells the shell to exit as soon as any
10458 program it runs returns a nonzero status. We feel it is cleaner to
10459 write each line of the recipe to stand on its own and not require this
10463 @comment The makefile standards are in a separate file that is also
10464 @comment included by standards.texi.
10465 @include make-stds.texi
10467 @node Quick Reference, Error Messages, Makefile Conventions, Top
10468 @appendix Quick Reference
10470 This appendix summarizes the directives, text manipulation functions,
10471 and special variables which GNU @code{make} understands.
10472 @xref{Special Targets}, @ref{Catalogue of Rules, ,Catalogue of Implicit Rules},
10473 and @ref{Options Summary, ,Summary of Options},
10474 for other summaries.
10476 Here is a summary of the directives GNU @code{make} recognizes:
10479 @item define @var{variable}
10480 @itemx define @var{variable} =
10481 @itemx define @var{variable} :=
10482 @itemx define @var{variable} +=
10483 @itemx define @var{variable} ?=
10485 Define multi-line variables.@*
10488 @item ifdef @var{variable}
10489 @itemx ifndef @var{variable}
10490 @itemx ifeq (@var{a},@var{b})
10491 @itemx ifeq "@var{a}" "@var{b}"
10492 @itemx ifeq '@var{a}' '@var{b}'
10493 @itemx ifneq (@var{a},@var{b})
10494 @itemx ifneq "@var{a}" "@var{b}"
10495 @itemx ifneq '@var{a}' '@var{b}'
10498 Conditionally evaluate part of the makefile.@*
10499 @xref{Conditionals}.
10501 @item include @var{file}
10502 @itemx -include @var{file}
10503 @itemx sinclude @var{file}
10504 Include another makefile.@*
10505 @xref{Include, ,Including Other Makefiles}.
10507 @item override @var{variable-assignment}
10508 Define a variable, overriding any previous definition, even one from
10509 the command line.@*
10510 @xref{Override Directive, ,The @code{override} Directive}.
10513 Tell @code{make} to export all variables to child processes by default.@*
10514 @xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.
10516 @item export @var{variable}
10517 @itemx export @var{variable-assignment}
10518 @itemx unexport @var{variable}
10519 Tell @code{make} whether or not to export a particular variable to child
10521 @xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.
10523 @item private @var{variable-assignment}
10524 Do not allow this variable assignment to be inherited by prerequisites.@*
10525 @xref{Suppressing Inheritance}.
10527 @item vpath @var{pattern} @var{path}
10528 Specify a search path for files matching a @samp{%} pattern.@*
10529 @xref{Selective Search, , The @code{vpath} Directive}.
10531 @item vpath @var{pattern}
10532 Remove all search paths previously specified for @var{pattern}.
10535 Remove all search paths previously specified in any @code{vpath}
10539 Here is a summary of the built-in functions (@pxref{Functions}):
10542 @item $(subst @var{from},@var{to},@var{text})
10543 Replace @var{from} with @var{to} in @var{text}.@*
10544 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10546 @item $(patsubst @var{pattern},@var{replacement},@var{text})
10547 Replace words matching @var{pattern} with @var{replacement} in @var{text}.@*
10548 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10550 @item $(strip @var{string})
10551 Remove excess whitespace characters from @var{string}.@*
10552 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10554 @item $(findstring @var{find},@var{text})
10555 Locate @var{find} in @var{text}.@*
10556 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10558 @item $(filter @var{pattern}@dots{},@var{text})
10559 Select words in @var{text} that match one of the @var{pattern} words.@*
10560 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10562 @item $(filter-out @var{pattern}@dots{},@var{text})
10563 Select words in @var{text} that @emph{do not} match any of the @var{pattern} words.@*
10564 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10566 @item $(sort @var{list})
10567 Sort the words in @var{list} lexicographically, removing duplicates.@*
10568 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10570 @item $(word @var{n},@var{text})
10571 Extract the @var{n}th word (one-origin) of @var{text}.@*
10572 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10574 @item $(words @var{text})
10575 Count the number of words in @var{text}.@*
10576 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10578 @item $(wordlist @var{s},@var{e},@var{text})
10579 Returns the list of words in @var{text} from @var{s} to @var{e}.@*
10580 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10582 @item $(firstword @var{names}@dots{})
10583 Extract the first word of @var{names}.@*
10584 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10586 @item $(lastword @var{names}@dots{})
10587 Extract the last word of @var{names}.@*
10588 @xref{Text Functions, , Functions for String Substitution and Analysis}.
10590 @item $(dir @var{names}@dots{})
10591 Extract the directory part of each file name.@*
10592 @xref{File Name Functions, ,Functions for File Names}.
10594 @item $(notdir @var{names}@dots{})
10595 Extract the non-directory part of each file name.@*
10596 @xref{File Name Functions, ,Functions for File Names}.
10598 @item $(suffix @var{names}@dots{})
10599 Extract the suffix (the last @samp{.} and following characters) of each file name.@*
10600 @xref{File Name Functions, ,Functions for File Names}.
10602 @item $(basename @var{names}@dots{})
10603 Extract the base name (name without suffix) of each file name.@*
10604 @xref{File Name Functions, ,Functions for File Names}.
10606 @item $(addsuffix @var{suffix},@var{names}@dots{})
10607 Append @var{suffix} to each word in @var{names}.@*
10608 @xref{File Name Functions, ,Functions for File Names}.
10610 @item $(addprefix @var{prefix},@var{names}@dots{})
10611 Prepend @var{prefix} to each word in @var{names}.@*
10612 @xref{File Name Functions, ,Functions for File Names}.
10614 @item $(join @var{list1},@var{list2})
10615 Join two parallel lists of words.@*
10616 @xref{File Name Functions, ,Functions for File Names}.
10618 @item $(wildcard @var{pattern}@dots{})
10619 Find file names matching a shell file name pattern (@emph{not} a
10620 @samp{%} pattern).@*
10621 @xref{Wildcard Function, ,The Function @code{wildcard}}.
10623 @item $(realpath @var{names}@dots{})
10624 For each file name in @var{names}, expand to an absolute name that
10625 does not contain any @code{.}, @code{..}, nor symlinks.@*
10626 @xref{File Name Functions, ,Functions for File Names}.
10628 @item $(abspath @var{names}@dots{})
10629 For each file name in @var{names}, expand to an absolute name that
10630 does not contain any @code{.} or @code{..} components, but preserves
10632 @xref{File Name Functions, ,Functions for File Names}.
10634 @item $(error @var{text}@dots{})
10635 When this function is evaluated, @code{make} generates a fatal error
10636 with the message @var{text}.@*
10637 @xref{Make Control Functions, ,Functions That Control Make}.
10639 @item $(warning @var{text}@dots{})
10640 When this function is evaluated, @code{make} generates a warning with
10641 the message @var{text}.@*
10642 @xref{Make Control Functions, ,Functions That Control Make}.
10644 @item $(shell @var{command})
10645 Execute a shell command and return its output.@*
10646 @xref{Shell Function, , The @code{shell} Function}.
10648 @item $(origin @var{variable})
10649 Return a string describing how the @code{make} variable @var{variable} was
10651 @xref{Origin Function, , The @code{origin} Function}.
10653 @item $(flavor @var{variable})
10654 Return a string describing the flavor of the @code{make} variable
10656 @xref{Flavor Function, , The @code{flavor} Function}.
10658 @item $(foreach @var{var},@var{words},@var{text})
10659 Evaluate @var{text} with @var{var} bound to each word in @var{words},
10660 and concatenate the results.@*
10661 @xref{Foreach Function, ,The @code{foreach} Function}.
10663 @item $(if @var{condition},@var{then-part}[,@var{else-part}])
10664 Evaluate the condition @var{condition}; if it's non-empty substitute
10665 the expansion of the @var{then-part} otherwise substitute the
10666 expansion of the @var{else-part}.@*
10667 @xref{Conditional Functions, ,Functions for Conditionals}.
10669 @item $(or @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
10670 Evaluate each condition @var{conditionN} one at a time; substitute the
10671 first non-empty expansion. If all expansions are empty, substitute
10672 the empty string.@*
10673 @xref{Conditional Functions, ,Functions for Conditionals}.
10675 @item $(and @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
10676 Evaluate each condition @var{conditionN} one at a time; if any
10677 expansion results in the empty string substitute the empty string. If
10678 all expansions result in a non-empty string, substitute the expansion
10679 of the last @var{condition}.@*
10680 @xref{Conditional Functions, ,Functions for Conditionals}.
10682 @item $(call @var{var},@var{param},@dots{})
10683 Evaluate the variable @var{var} replacing any references to @code{$(1)},
10684 @code{$(2)} with the first, second, etc.@: @var{param} values.@*
10685 @xref{Call Function, ,The @code{call} Function}.
10687 @item $(eval @var{text})
10688 Evaluate @var{text} then read the results as makefile commands.
10689 Expands to the empty string.@*
10690 @xref{Eval Function, ,The @code{eval} Function}.
10692 @item $(value @var{var})
10693 Evaluates to the contents of the variable @var{var}, with no expansion
10695 @xref{Value Function, ,The @code{value} Function}.
10698 Here is a summary of the automatic variables.
10699 @xref{Automatic Variables},
10700 for full information.
10704 The file name of the target.
10707 The target member name, when the target is an archive member.
10710 The name of the first prerequisite.
10713 The names of all the prerequisites that are
10714 newer than the target, with spaces between them.
10715 For prerequisites which are archive members, only
10716 the named member is used (@pxref{Archives}).
10720 The names of all the prerequisites, with spaces between them. For
10721 prerequisites which are archive members, only the named member is used
10722 (@pxref{Archives}). The value of @code{$^} omits duplicate
10723 prerequisites, while @code{$+} retains them and preserves their order.
10726 The stem with which an implicit rule matches
10727 (@pxref{Pattern Match, ,How Patterns Match}).
10731 The directory part and the file-within-directory part of @code{$@@}.
10735 The directory part and the file-within-directory part of @code{$*}.
10739 The directory part and the file-within-directory part of @code{$%}.
10743 The directory part and the file-within-directory part of @code{$<}.
10747 The directory part and the file-within-directory part of @code{$^}.
10751 The directory part and the file-within-directory part of @code{$+}.
10755 The directory part and the file-within-directory part of @code{$?}.
10758 These variables are used specially by GNU @code{make}:
10763 Makefiles to be read on every invocation of @code{make}.@*
10764 @xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.
10768 Directory search path for files not found in the current directory.@*
10769 @xref{General Search, , @code{VPATH} Search Path for All Prerequisites}.
10773 The name of the system default command interpreter, usually @file{/bin/sh}.
10774 You can set @code{SHELL} in the makefile to change the shell used to run
10775 recipes. @xref{Execution, ,Recipe Execution}. The @code{SHELL}
10776 variable is handled specially when importing from and exporting to the
10777 environment. @xref{Choosing the Shell}.
10781 On MS-DOS only, the name of the command interpreter that is to be used
10782 by @code{make}. This value takes precedence over the value of
10783 @code{SHELL}. @xref{Execution, ,MAKESHELL variable}.
10787 The name with which @code{make} was invoked. Using this variable in
10788 recipes has special meaning. @xref{MAKE Variable, ,How the
10789 @code{MAKE} Variable Works}.
10793 The number of levels of recursion (sub-@code{make}s).@*
10794 @xref{Variables/Recursion}.
10798 The flags given to @code{make}. You can set this in the environment or
10799 a makefile to set flags.@*
10800 @xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
10802 It is @emph{never} appropriate to use @code{MAKEFLAGS} directly in a
10803 recipe line: its contents may not be quoted correctly for use in the
10804 shell. Always allow recursive @code{make}'s to obtain these values
10805 through the environment from its parent.
10809 The targets given to @code{make} on the command line. Setting this
10810 variable has no effect on the operation of @code{make}.@*
10811 @xref{Goals, ,Arguments to Specify the Goals}.
10815 Set to the pathname of the current working directory (after all
10816 @code{-C} options are processed, if any). Setting this variable has no
10817 effect on the operation of @code{make}.@*
10818 @xref{Recursion, ,Recursive Use of @code{make}}.
10822 The default list of suffixes before @code{make} reads any makefiles.
10825 Defines the naming of the libraries @code{make} searches for, and their
10827 @xref{Libraries/Search, ,Directory Search for Link Libraries}.
10830 @node Error Messages, Complex Makefile, Quick Reference, Top
10831 @comment node-name, next, previous, up
10832 @appendix Errors Generated by Make
10834 Here is a list of the more common errors you might see generated by
10835 @code{make}, and some information about what they mean and how to fix
10838 Sometimes @code{make} errors are not fatal, especially in the presence
10839 of a @code{-} prefix on a recipe line, or the @code{-k} command line
10840 option. Errors that are fatal are prefixed with the string
10843 Error messages are all either prefixed with the name of the program
10844 (usually @samp{make}), or, if the error is found in a makefile, the name
10845 of the file and linenumber containing the problem.
10847 In the table below, these common prefixes are left off.
10851 @item [@var{foo}] Error @var{NN}
10852 @itemx [@var{foo}] @var{signal description}
10853 These errors are not really @code{make} errors at all. They mean that a
10854 program that @code{make} invoked as part of a recipe returned a
10855 non-0 error code (@samp{Error @var{NN}}), which @code{make} interprets
10856 as failure, or it exited in some other abnormal fashion (with a
10857 signal of some type). @xref{Errors, ,Errors in Recipes}.
10859 If no @code{***} is attached to the message, then the subprocess failed
10860 but the rule in the makefile was prefixed with the @code{-} special
10861 character, so @code{make} ignored the error.
10863 @item missing separator. Stop.
10864 @itemx missing separator (did you mean TAB instead of 8 spaces?). Stop.
10865 This means that @code{make} could not understand much of anything
10866 about the makefile line it just read. GNU @code{make} looks for
10867 various separators (@code{:}, @code{=}, recipe prefix characters,
10868 etc.) to indicate what kind of line it's parsing. This message means
10869 it couldn't find a valid one.
10871 One of the most common reasons for this message is that you (or
10872 perhaps your oh-so-helpful editor, as is the case with many MS-Windows
10873 editors) have attempted to indent your recipe lines with spaces
10874 instead of a tab character. In this case, @code{make} will use the
10875 second form of the error above. Remember that every line in the
10876 recipe must begin with a tab character (unless you set
10877 @code{.CMDPREFIX}; @pxref{Special Variables}). Eight spaces do not
10878 count. @xref{Rule Syntax}.
10880 @item recipe commences before first target. Stop.
10881 @itemx missing rule before recipe. Stop.
10882 This means the first thing in the makefile seems to be part of a
10883 recipe: it begins with a recipe prefix character and doesn't appear to
10884 be a legal @code{make} directive (such as a variable assignment).
10885 Recipes must always be associated with a target.
10887 The second form is generated if the line has a semicolon as the first
10888 non-whitespace character; @code{make} interprets this to mean you left
10889 out the "target: prerequisite" section of a rule. @xref{Rule Syntax}.
10891 @item No rule to make target `@var{xxx}'.
10892 @itemx No rule to make target `@var{xxx}', needed by `@var{yyy}'.
10893 This means that @code{make} decided it needed to build a target, but
10894 then couldn't find any instructions in the makefile on how to do that,
10895 either explicit or implicit (including in the default rules database).
10897 If you want that file to be built, you will need to add a rule to your
10898 makefile describing how that target can be built. Other possible
10899 sources of this problem are typos in the makefile (if that filename is
10900 wrong) or a corrupted source tree (if that file is not supposed to be
10901 built, but rather only a prerequisite).
10903 @item No targets specified and no makefile found. Stop.
10904 @itemx No targets. Stop.
10905 The former means that you didn't provide any targets to be built on the
10906 command line, and @code{make} couldn't find any makefiles to read in.
10907 The latter means that some makefile was found, but it didn't contain any
10908 default goal and none was given on the command line. GNU @code{make}
10909 has nothing to do in these situations.
10910 @xref{Makefile Arguments, ,Arguments to Specify the Makefile}.@refill
10912 @item Makefile `@var{xxx}' was not found.
10913 @itemx Included makefile `@var{xxx}' was not found.
10914 A makefile specified on the command line (first form) or included
10915 (second form) was not found.
10917 @item warning: overriding recipe for target `@var{xxx}'
10918 @itemx warning: ignoring old recipe for target `@var{xxx}'
10919 GNU @code{make} allows only one recipe to be specified per target
10920 (except for double-colon rules). If you give a recipe for a target
10921 which already has been defined to have one, this warning is issued and
10922 the second recipe will overwrite the first. @xref{Multiple Rules,
10923 ,Multiple Rules for One Target}.
10925 @item Circular @var{xxx} <- @var{yyy} dependency dropped.
10926 This means that @code{make} detected a loop in the dependency graph:
10927 after tracing the prerequisite @var{yyy} of target @var{xxx}, and its
10928 prerequisites, etc., one of them depended on @var{xxx} again.
10930 @item Recursive variable `@var{xxx}' references itself (eventually). Stop.
10931 This means you've defined a normal (recursive) @code{make} variable
10932 @var{xxx} that, when it's expanded, will refer to itself (@var{xxx}).
10933 This is not allowed; either use simply-expanded variables (@code{:=}) or
10934 use the append operator (@code{+=}). @xref{Using Variables, ,How to Use
10937 @item Unterminated variable reference. Stop.
10938 This means you forgot to provide the proper closing parenthesis
10939 or brace in your variable or function reference.
10941 @item insufficient arguments to function `@var{xxx}'. Stop.
10942 This means you haven't provided the requisite number of arguments for
10943 this function. See the documentation of the function for a description
10944 of its arguments. @xref{Functions, ,Functions for Transforming Text}.
10946 @item missing target pattern. Stop.
10947 @itemx multiple target patterns. Stop.
10948 @itemx target pattern contains no `%'. Stop.
10949 @itemx mixed implicit and static pattern rules. Stop.
10950 These are generated for malformed static pattern rules. The first means
10951 there's no pattern in the target section of the rule; the second means
10952 there are multiple patterns in the target section; the third means
10953 the target doesn't contain a pattern character (@code{%}); and the
10954 fourth means that all three parts of the static pattern rule contain
10955 pattern characters (@code{%})--only the first two parts should.
10956 @xref{Static Usage, ,Syntax of Static Pattern Rules}.
10958 @item warning: -jN forced in submake: disabling jobserver mode.
10959 This warning and the next are generated if @code{make} detects error
10960 conditions related to parallel processing on systems where
10961 sub-@code{make}s can communicate (@pxref{Options/Recursion,
10962 ,Communicating Options to a Sub-@code{make}}). This warning is
10963 generated if a recursive invocation of a @code{make} process is forced
10964 to have @samp{-j@var{N}} in its argument list (where @var{N} is greater
10965 than one). This could happen, for example, if you set the @code{MAKE}
10966 environment variable to @samp{make -j2}. In this case, the
10967 sub-@code{make} doesn't communicate with other @code{make} processes and
10968 will simply pretend it has two jobs of its own.
10970 @item warning: jobserver unavailable: using -j1. Add `+' to parent make rule.
10971 In order for @code{make} processes to communicate, the parent will pass
10972 information to the child. Since this could result in problems if the
10973 child process isn't actually a @code{make}, the parent will only do this
10974 if it thinks the child is a @code{make}. The parent uses the normal
10975 algorithms to determine this (@pxref{MAKE Variable, ,How the @code{MAKE}
10976 Variable Works}). If the makefile is constructed such that the parent
10977 doesn't know the child is a @code{make} process, then the child will
10978 receive only part of the information necessary. In this case, the child
10979 will generate this warning message and proceed with its build in a
10984 @node Complex Makefile, GNU Free Documentation License, Error Messages, Top
10985 @appendix Complex Makefile Example
10987 Here is the makefile for the GNU @code{tar} program. This is a
10988 moderately complex makefile.
10990 Because it is the first target, the default goal is @samp{all}. An
10991 interesting feature of this makefile is that @file{testpad.h} is a
10992 source file automatically created by the @code{testpad} program,
10993 itself compiled from @file{testpad.c}.
10995 If you type @samp{make} or @samp{make all}, then @code{make} creates
10996 the @file{tar} executable, the @file{rmt} daemon that provides
10997 remote tape access, and the @file{tar.info} Info file.
10999 If you type @samp{make install}, then @code{make} not only creates
11000 @file{tar}, @file{rmt}, and @file{tar.info}, but also installs
11003 If you type @samp{make clean}, then @code{make} removes the @samp{.o}
11004 files, and the @file{tar}, @file{rmt}, @file{testpad},
11005 @file{testpad.h}, and @file{core} files.
11007 If you type @samp{make distclean}, then @code{make} not only removes
11008 the same files as does @samp{make clean} but also the
11009 @file{TAGS}, @file{Makefile}, and @file{config.status} files.
11010 (Although it is not evident, this makefile (and
11011 @file{config.status}) is generated by the user with the
11012 @code{configure} program, which is provided in the @code{tar}
11013 distribution, but is not shown here.)
11015 If you type @samp{make realclean}, then @code{make} removes the same
11016 files as does @samp{make distclean} and also removes the Info files
11017 generated from @file{tar.texinfo}.
11019 In addition, there are targets @code{shar} and @code{dist} that create
11024 # Generated automatically from Makefile.in by configure.
11025 # Un*x Makefile for GNU tar program.
11026 # Copyright (C) 1991 Free Software Foundation, Inc.
11030 # This program is free software; you can redistribute
11031 # it and/or modify it under the terms of the GNU
11032 # General Public License @dots{}
11039 #### Start of system configuration section. ####
11044 # If you use gcc, you should either run the
11045 # fixincludes script that comes with it or else use
11046 # gcc with the -traditional option. Otherwise ioctl
11047 # calls will be compiled incorrectly on some systems.
11050 INSTALL = /usr/local/bin/install -c
11051 INSTALLDATA = /usr/local/bin/install -c -m 644
11054 # Things you might add to DEFS:
11055 # -DSTDC_HEADERS If you have ANSI C headers and
11057 # -DPOSIX If you have POSIX.1 headers and
11059 # -DBSD42 If you have sys/dir.h (unless
11060 # you use -DPOSIX), sys/file.h,
11061 # and st_blocks in `struct stat'.
11062 # -DUSG If you have System V/ANSI C
11063 # string and memory functions
11064 # and headers, sys/sysmacros.h,
11065 # fcntl.h, getcwd, no valloc,
11066 # and ndir.h (unless
11067 # you use -DDIRENT).
11068 # -DNO_MEMORY_H If USG or STDC_HEADERS but do not
11069 # include memory.h.
11070 # -DDIRENT If USG and you have dirent.h
11071 # instead of ndir.h.
11072 # -DSIGTYPE=int If your signal handlers
11073 # return int, not void.
11074 # -DNO_MTIO If you lack sys/mtio.h
11075 # (magtape ioctls).
11076 # -DNO_REMOTE If you do not have a remote shell
11078 # -DUSE_REXEC To use rexec for remote tape
11079 # operations instead of
11080 # forking rsh or remsh.
11081 # -DVPRINTF_MISSING If you lack vprintf function
11082 # (but have _doprnt).
11083 # -DDOPRNT_MISSING If you lack _doprnt function.
11084 # Also need to define
11085 # -DVPRINTF_MISSING.
11086 # -DFTIME_MISSING If you lack ftime system call.
11087 # -DSTRSTR_MISSING If you lack strstr function.
11088 # -DVALLOC_MISSING If you lack valloc function.
11089 # -DMKDIR_MISSING If you lack mkdir and
11090 # rmdir system calls.
11091 # -DRENAME_MISSING If you lack rename system call.
11092 # -DFTRUNCATE_MISSING If you lack ftruncate
11094 # -DV7 On Version 7 Unix (not
11095 # tested in a long time).
11096 # -DEMUL_OPEN3 If you lack a 3-argument version
11097 # of open, and want to emulate it
11098 # with system calls you do have.
11099 # -DNO_OPEN3 If you lack the 3-argument open
11100 # and want to disable the tar -k
11101 # option instead of emulating open.
11102 # -DXENIX If you have sys/inode.h
11103 # and need it 94 to be included.
11105 DEFS = -DSIGTYPE=int -DDIRENT -DSTRSTR_MISSING \
11106 -DVPRINTF_MISSING -DBSD42
11107 # Set this to rtapelib.o unless you defined NO_REMOTE,
11108 # in which case make it empty.
11109 RTAPELIB = rtapelib.o
11111 DEF_AR_FILE = /dev/rmt8
11116 CFLAGS = $(CDEBUG) -I. -I$(srcdir) $(DEFS) \
11117 -DDEF_AR_FILE=\"$(DEF_AR_FILE)\" \
11118 -DDEFBLOCKING=$(DEFBLOCKING)
11123 prefix = /usr/local
11124 # Prefix for each installed program,
11125 # normally empty or `g'.
11128 # The directory to install tar in.
11129 bindir = $(prefix)/bin
11131 # The directory to install the info files in.
11132 infodir = $(prefix)/info
11135 #### End of system configuration section. ####
11137 SRC1 = tar.c create.c extract.c buffer.c \
11138 getoldopt.c update.c gnu.c mangle.c
11139 SRC2 = version.c list.c names.c diffarch.c \
11140 port.c wildmat.c getopt.c
11141 SRC3 = getopt1.c regex.c getdate.y
11142 SRCS = $(SRC1) $(SRC2) $(SRC3)
11143 OBJ1 = tar.o create.o extract.o buffer.o \
11144 getoldopt.o update.o gnu.o mangle.o
11145 OBJ2 = version.o list.o names.o diffarch.o \
11146 port.o wildmat.o getopt.o
11147 OBJ3 = getopt1.o regex.o getdate.o $(RTAPELIB)
11148 OBJS = $(OBJ1) $(OBJ2) $(OBJ3)
11150 AUX = README COPYING ChangeLog Makefile.in \
11151 makefile.pc configure configure.in \
11152 tar.texinfo tar.info* texinfo.tex \
11153 tar.h port.h open3.h getopt.h regex.h \
11154 rmt.h rmt.c rtapelib.c alloca.c \
11155 msd_dir.h msd_dir.c tcexparg.c \
11156 level-0 level-1 backup-specs testpad.c
11160 all: tar rmt tar.info
11164 $(CC) $(LDFLAGS) -o $@@ $(OBJS) $(LIBS)
11169 $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ rmt.c
11173 tar.info: tar.texinfo
11174 makeinfo tar.texinfo
11180 $(INSTALL) tar $(bindir)/$(binprefix)tar
11181 -test ! -f rmt || $(INSTALL) rmt /etc/rmt
11182 $(INSTALLDATA) $(srcdir)/tar.info* $(infodir)
11186 $(OBJS): tar.h port.h testpad.h
11187 regex.o buffer.o tar.o: regex.h
11188 # getdate.y has 8 shift/reduce conflicts.
11198 $(CC) -o $@@ testpad.o
11209 rm -f *.o tar rmt testpad testpad.h core
11215 rm -f TAGS Makefile config.status
11220 realclean: distclean
11226 shar: $(SRCS) $(AUX)
11227 shar $(SRCS) $(AUX) | compress \
11228 > tar-`sed -e '/version_string/!d' \
11229 -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
11236 dist: $(SRCS) $(AUX)
11238 -e '/version_string/!d' \
11239 -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
11241 version.c` > .fname
11242 -rm -rf `cat .fname`
11244 ln $(SRCS) $(AUX) `cat .fname`
11245 tar chZf `cat .fname`.tar.Z `cat .fname`
11246 -rm -rf `cat .fname` .fname
11250 tar.zoo: $(SRCS) $(AUX)
11254 for X in $(SRCS) $(AUX) ; do \
11256 sed 's/$$/^M/' $$X \
11257 > tmp.dir/$$X ; done
11258 cd tmp.dir ; zoo aM ../tar.zoo *
11263 @node GNU Free Documentation License, Concept Index, Complex Makefile, Top
11264 @appendixsec GNU Free Documentation License
11265 @cindex FDL, GNU Free Documentation License
11268 @node Concept Index, Name Index, GNU Free Documentation License, Top
11269 @unnumbered Index of Concepts
11273 @node Name Index, , Concept Index, Top
11274 @unnumbered Index of Functions, Variables, & Directives