1 \input texinfo @c -*- Texinfo -*-
4 @settitle GNU @code{make}
8 @c FSF publishers: format makebook.texi instead of using this file directly.
10 @set RCSID $Id: make.texi,v 1.21 2004/11/28 23:11:23 psmith Exp $
13 @set UPDATED 23 Feb 2003
14 @set UPDATE-MONTH Feb 2003
15 @c ISBN provided by Lisa M. Opus Goldstein <opus@gnu.org>, 5 May 2004
16 @set ISBN 1-882114-83-5
20 @c ISPELL CHECK: done, 10 June 1993 --roland
21 @c ISPELL CHECK: done, 2000-06-25 --Martin Buchholz
23 @c Combine the variable and function indices:
25 @c Combine the program and concept indices:
28 @dircategory GNU Packages
30 * Make: (make). Remake files automatically.
34 This file documents the GNU Make utility, which determines
35 automatically which pieces of a large program need to be recompiled,
36 and issues the commands to recompile them.
38 This is Edition @value{EDITION}, last updated @value{UPDATED},
39 of @cite{The GNU Make Manual}, for @code{make}, Version @value{VERSION}.
41 Copyright 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
42 1998, 1999, 2000, 2002, 2003, 2004
43 Free Software Foundation, Inc.
45 Permission is granted to copy, distribute and/or modify this document
46 under the terms of the GNU Free Documentation License, Version 1.1 or
47 any later version published by the Free Software Foundation; with no
48 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
49 Texts. A copy of the license is included in the section entitled
50 ``GNU Free Documentation License''.
54 @shorttitlepage GNU Make
58 @subtitle A Program for Directing Recompilation
59 @subtitle GNU @code{make} Version @value{VERSION}
60 @subtitle @value{UPDATE-MONTH}
61 @author Richard M. Stallman, Roland McGrath, Paul D. Smith
63 @vskip 0pt plus 1filll
64 Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995,
65 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004 Free Software Foundation, Inc.
67 Published by the Free Software Foundation @*
68 59 Temple Place -- Suite 330, @*
69 Boston, MA 02111-1307 USA @*
72 Permission is granted to copy, distribute and/or modify this document
73 under the terms of the GNU Free Documentation License, Version 1.1 or
74 any later version published by the Free Software Foundation; with the
75 Invariant Sections being ``GNU General Public License'', the Front-Cover
76 Texts being ``A GNU Manual'', and with the Back-Cover Texts being as in
77 (a) below. A copy of the license is included in the section entitled
78 ``GNU Free Documentation License''.
80 (a) The FSF's Back-Cover Text is:
83 You have freedom to copy and modify this GNU Manual, like GNU
84 software. Copies published by the Free Software Foundation raise
85 funds for GNU development.
88 Cover art by Etienne Suvasa.
93 @node Top, Overview, (dir), (dir)
96 The GNU @code{make} utility automatically determines which pieces of a
97 large program need to be recompiled, and issues the commands to
98 recompile them.@refill
100 This edition of the @cite{GNU Make Manual},
101 last updated @value{UPDATED},
102 documents GNU @code{make} Version @value{VERSION}.@refill
104 This manual describes @code{make} and contains the following chapters:@refill
108 * Overview:: Overview of @code{make}.
109 * Introduction:: An introduction to @code{make}.
110 * Makefiles:: Makefiles tell @code{make} what to do.
111 * Rules:: Rules describe when a file must be remade.
112 * Commands:: Commands say how to remake a file.
113 * Using Variables:: You can use variables to avoid repetition.
114 * Conditionals:: Use or ignore parts of the makefile based
115 on the values of variables.
116 * Functions:: Many powerful ways to manipulate text.
117 * Invoking make: Running. How to invoke @code{make} on the command line.
118 * Implicit Rules:: Use implicit rules to treat many files alike,
119 based on their file names.
120 * Archives:: How @code{make} can update library archives.
121 * Features:: Features GNU @code{make} has over other @code{make}s.
122 * Missing:: What GNU @code{make} lacks from other @code{make}s.
123 * Makefile Conventions:: Conventions for writing makefiles for
125 * Quick Reference:: A quick reference for experienced users.
126 * Error Messages:: A list of common errors generated by @code{make}.
127 * Complex Makefile:: A real example of a straightforward,
128 but nontrivial, makefile.
130 * GNU Free Documentation License:: License for copying this manual
131 * Concept Index:: Index of Concepts
132 * Name Index:: Index of Functions, Variables, & Directives
135 --- The Detailed Node Listing ---
137 Overview of @code{make}
139 * Preparing:: Preparing and Running Make
140 * Reading:: On Reading this Text
141 * Bugs:: Problems and Bugs
143 An Introduction to Makefiles
145 * Rule Introduction:: What a rule looks like.
146 * Simple Makefile:: A Simple Makefile
147 * How Make Works:: How @code{make} Processes This Makefile
148 * Variables Simplify:: Variables Make Makefiles Simpler
149 * make Deduces:: Letting @code{make} Deduce the Commands
150 * Combine By Prerequisite:: Another Style of Makefile
151 * Cleanup:: Rules for Cleaning the Directory
155 * Makefile Contents:: What makefiles contain.
156 * Makefile Names:: How to name your makefile.
157 * Include:: How one makefile can use another makefile.
158 * MAKEFILES Variable:: The environment can specify extra makefiles.
159 * MAKEFILE_LIST Variable:: Discover which makefiles have been read.
160 * Special Variables:: Other special variables.
161 * Remaking Makefiles:: How makefiles get remade.
162 * Overriding Makefiles:: How to override part of one makefile
163 with another makefile.
164 * Reading Makefiles:: How makefiles are parsed.
168 * Rule Example:: An example explained.
169 * Rule Syntax:: General syntax explained.
170 * Prerequisite Types:: There are two types of prerequisites.
171 * Wildcards:: Using wildcard characters such as `*'.
172 * Directory Search:: Searching other directories for source files.
173 * Phony Targets:: Using a target that is not a real file's name.
174 * Force Targets:: You can use a target without commands
175 or prerequisites to mark other
177 * Empty Targets:: When only the date matters and the
179 * Special Targets:: Targets with special built-in meanings.
180 * Multiple Targets:: When to make use of several targets in a rule.
181 * Multiple Rules:: How to use several rules with the same target.
182 * Static Pattern:: Static pattern rules apply to multiple targets
183 and can vary the prerequisites according to
185 * Double-Colon:: How to use a special kind of rule to allow
186 several independent rules for one target.
187 * Automatic Prerequisites:: How to automatically generate rules giving
188 prerequisites from source files themselves.
190 Using Wildcard Characters in File Names
192 * Wildcard Examples:: Several examples
193 * Wildcard Pitfall:: Problems to avoid.
194 * Wildcard Function:: How to cause wildcard expansion where
195 it does not normally take place.
197 Searching Directories for Prerequisites
199 * General Search:: Specifying a search path that applies
200 to every prerequisite.
201 * Selective Search:: Specifying a search path
202 for a specified class of names.
203 * Search Algorithm:: When and how search paths are applied.
204 * Commands/Search:: How to write shell commands that work together
206 * Implicit/Search:: How search paths affect implicit rules.
207 * Libraries/Search:: Directory search for link libraries.
211 * Static Usage:: The syntax of static pattern rules.
212 * Static versus Implicit:: When are they better than implicit rules?
214 Writing the Commands in Rules
216 * Echoing:: How to control when commands are echoed.
217 * Execution:: How commands are executed.
218 * Parallel:: How commands can be executed in parallel.
219 * Errors:: What happens after a command execution error.
220 * Interrupts:: What happens when a command is interrupted.
221 * Recursion:: Invoking @code{make} from makefiles.
222 * Sequences:: Defining canned sequences of commands.
223 * Empty Commands:: Defining useful, do-nothing commands.
225 Recursive Use of @code{make}
227 * MAKE Variable:: The special effects of using @samp{$(MAKE)}.
228 * Variables/Recursion:: How to communicate variables to a sub-@code{make}.
229 * Options/Recursion:: How to communicate options to a sub-@code{make}.
230 * -w Option:: How the @samp{-w} or @samp{--print-directory} option
231 helps debug use of recursive @code{make} commands.
235 * Reference:: How to use the value of a variable.
236 * Flavors:: Variables come in two flavors.
237 * Advanced:: Advanced features for referencing a variable.
238 * Values:: All the ways variables get their values.
239 * Setting:: How to set a variable in the makefile.
240 * Appending:: How to append more text to the old value
242 * Override Directive:: How to set a variable in the makefile even if
243 the user has set it with a command argument.
244 * Defining:: An alternate way to set a variable
245 to a verbatim string.
246 * Environment:: Variable values can come from the environment.
247 * Target-specific:: Variable values can be defined on a per-target
249 * Pattern-specific:: Target-specific variable values can be applied
250 to a group of targets that match a pattern.
252 Advanced Features for Reference to Variables
254 * Substitution Refs:: Referencing a variable with
255 substitutions on the value.
256 * Computed Names:: Computing the name of the variable to refer to.
258 Conditional Parts of Makefiles
260 * Conditional Example:: Example of a conditional
261 * Conditional Syntax:: The syntax of conditionals.
262 * Testing Flags:: Conditionals that test flags.
264 Functions for Transforming Text
266 * Syntax of Functions:: How to write a function call.
267 * Text Functions:: General-purpose text manipulation functions.
268 * File Name Functions:: Functions for manipulating file names.
269 * Foreach Function:: Repeat some text with controlled variation.
270 * If Function:: Conditionally expand a value.
271 * Call Function:: Expand a user-defined function.
272 * Value Function:: Return the un-expanded value of a variable.
273 * Eval Function:: Evaluate the arguments as makefile syntax.
274 * Origin Function:: Find where a variable got its value.
275 * Shell Function:: Substitute the output of a shell command.
276 * Make Control Functions:: Functions that control how make runs.
278 How to Run @code{make}
280 * Makefile Arguments:: How to specify which makefile to use.
281 * Goals:: How to use goal arguments to specify which
282 parts of the makefile to use.
283 * Instead of Execution:: How to use mode flags to specify what
284 kind of thing to do with the commands
285 in the makefile other than simply
287 * Avoiding Compilation:: How to avoid recompiling certain files.
288 * Overriding:: How to override a variable to specify
289 an alternate compiler and other things.
290 * Testing:: How to proceed past some errors, to
292 * Options Summary:: Summary of Options
296 * Using Implicit:: How to use an existing implicit rule
297 to get the commands for updating a file.
298 * Catalogue of Rules:: A list of built-in implicit rules.
299 * Implicit Variables:: How to change what predefined rules do.
300 * Chained Rules:: How to use a chain of implicit rules.
301 * Pattern Rules:: How to define new implicit rules.
302 * Last Resort:: How to defining commands for rules
303 which cannot find any.
304 * Suffix Rules:: The old-fashioned style of implicit rule.
305 * Implicit Rule Search:: The precise algorithm for applying
308 Defining and Redefining Pattern Rules
310 * Pattern Intro:: An introduction to pattern rules.
311 * Pattern Examples:: Examples of pattern rules.
312 * Automatic Variables:: How to use automatic variables in the
313 commands of implicit rules.
314 * Pattern Match:: How patterns match.
315 * Match-Anything Rules:: Precautions you should take prior to
316 defining rules that can match any
317 target file whatever.
318 * Canceling Rules:: How to override or cancel built-in rules.
320 Using @code{make} to Update Archive Files
322 * Archive Members:: Archive members as targets.
323 * Archive Update:: The implicit rule for archive member targets.
324 * Archive Pitfalls:: Dangers to watch out for when using archives.
325 * Archive Suffix Rules:: You can write a special kind of suffix rule
326 for updating archives.
328 Implicit Rule for Archive Member Targets
330 * Archive Symbols:: How to update archive symbol directories.
334 * Makefile Basics:: General Conventions for Makefiles
335 * Utilities in Makefiles:: Utilities in Makefiles
336 * Command Variables:: Variables for Specifying Commands
337 * Directory Variables:: Variables for Installation Directories
338 * Standard Targets:: Standard Targets for Users
339 * Install Command Categories:: Three categories of commands in the `install'
346 @node Overview, Introduction, Top, Top
347 @comment node-name, next, previous, up
348 @chapter Overview of @code{make}
350 The @code{make} utility automatically determines which pieces of a large
351 program need to be recompiled, and issues commands to recompile them.
352 This manual describes GNU @code{make}, which was implemented by Richard
353 Stallman and Roland McGrath. Development since Version 3.76 has been
354 handled by Paul D. Smith.
356 GNU @code{make} conforms to section 6.2 of @cite{IEEE Standard
357 1003.2-1992} (POSIX.2).
359 @cindex IEEE Standard 1003.2
360 @cindex standards conformance
362 Our examples show C programs, since they are most common, but you can use
363 @code{make} with any programming language whose compiler can be run with a
364 shell command. Indeed, @code{make} is not limited to programs. You can
365 use it to describe any task where some files must be updated automatically
366 from others whenever the others change.
369 * Preparing:: Preparing and Running Make
370 * Reading:: On Reading this Text
371 * Bugs:: Problems and Bugs
374 @node Preparing, Reading, Overview, Overview
376 @heading Preparing and Running Make
379 To prepare to use @code{make}, you must write a file called
380 the @dfn{makefile} that describes the relationships among files
381 in your program and provides commands for updating each file.
382 In a program, typically, the executable file is updated from object
383 files, which are in turn made by compiling source files.@refill
385 Once a suitable makefile exists, each time you change some source files,
386 this simple shell command:
393 suffices to perform all necessary recompilations. The @code{make} program
394 uses the makefile data base and the last-modification times of the files to
395 decide which of the files need to be updated. For each of those files, it
396 issues the commands recorded in the data base.
398 You can provide command line arguments to @code{make} to control which
399 files should be recompiled, or how. @xref{Running, ,How to Run
402 @node Reading, Bugs, Preparing, Overview
403 @section How to Read This Manual
405 If you are new to @code{make}, or are looking for a general
406 introduction, read the first few sections of each chapter, skipping the
407 later sections. In each chapter, the first few sections contain
408 introductory or general information and the later sections contain
409 specialized or technical information.
411 The exception is the second chapter, @ref{Introduction, ,An
412 Introduction to Makefiles}, all of which is introductory.
415 The exception is @ref{Introduction, ,An Introduction to Makefiles},
416 all of which is introductory.
419 If you are familiar with other @code{make} programs, see @ref{Features,
420 ,Features of GNU @code{make}}, which lists the enhancements GNU
421 @code{make} has, and @ref{Missing, ,Incompatibilities and Missing
422 Features}, which explains the few things GNU @code{make} lacks that
425 For a quick summary, see @ref{Options Summary}, @ref{Quick Reference},
426 and @ref{Special Targets}.
428 @node Bugs, , Reading, Overview
429 @section Problems and Bugs
430 @cindex reporting bugs
431 @cindex bugs, reporting
432 @cindex problems and bugs, reporting
434 If you have problems with GNU @code{make} or think you've found a bug,
435 please report it to the developers; we cannot promise to do anything but
436 we might well want to fix it.
438 Before reporting a bug, make sure you've actually found a real bug.
439 Carefully reread the documentation and see if it really says you can do
440 what you're trying to do. If it's not clear whether you should be able
441 to do something or not, report that too; it's a bug in the
444 Before reporting a bug or trying to fix it yourself, try to isolate it
445 to the smallest possible makefile that reproduces the problem. Then
446 send us the makefile and the exact results @code{make} gave you,
447 including any error or warning messages. Please don't paraphrase
448 these messages: it's best to cut and paste them into your report.
449 When generating this small makefile, be sure to not use any non-free
450 or unusual tools in your commands: you can almost always emulate what
451 such a tool would do with simple shell commands. Finally, be sure to
452 explain what you expected to occur; this will help us decide whether
453 the problem was really in the documentation.
455 Once you have a precise problem you can report it in one of two ways.
456 Either send electronic mail to:
463 or use our Web-based project management tool, at:
466 http://savannah.gnu.org/projects/make/
470 In addition to the information above, please be careful to include the
471 version number of @code{make} you are using. You can get this
472 information with the command @samp{make --version}. Be sure also to
473 include the type of machine and operating system you are using. One
474 way to obtain this information is by looking at the final lines of
475 output from the command @samp{make --help}.
477 @node Introduction, Makefiles, Overview, Top
478 @comment node-name, next, previous, up
479 @chapter An Introduction to Makefiles
481 You need a file called a @dfn{makefile} to tell @code{make} what to do.
482 Most often, the makefile tells @code{make} how to compile and link a
486 In this chapter, we will discuss a simple makefile that describes how to
487 compile and link a text editor which consists of eight C source files
488 and three header files. The makefile can also tell @code{make} how to
489 run miscellaneous commands when explicitly asked (for example, to remove
490 certain files as a clean-up operation). To see a more complex example
491 of a makefile, see @ref{Complex Makefile}.
493 When @code{make} recompiles the editor, each changed C source file
494 must be recompiled. If a header file has changed, each C source file
495 that includes the header file must be recompiled to be safe. Each
496 compilation produces an object file corresponding to the source file.
497 Finally, if any source file has been recompiled, all the object files,
498 whether newly made or saved from previous compilations, must be linked
499 together to produce the new executable editor.
500 @cindex recompilation
504 * Rule Introduction:: What a rule looks like.
505 * Simple Makefile:: A Simple Makefile
506 * How Make Works:: How @code{make} Processes This Makefile
507 * Variables Simplify:: Variables Make Makefiles Simpler
508 * make Deduces:: Letting @code{make} Deduce the Commands
509 * Combine By Prerequisite:: Another Style of Makefile
510 * Cleanup:: Rules for Cleaning the Directory
513 @node Rule Introduction, Simple Makefile, Introduction, Introduction
514 @comment node-name, next, previous, up
515 @section What a Rule Looks Like
516 @cindex rule, introduction to
517 @cindex makefile rule parts
518 @cindex parts of makefile rule
520 A simple makefile consists of ``rules'' with the following shape:
522 @cindex targets, introduction to
523 @cindex prerequisites, introduction to
524 @cindex commands, introduction to
527 @var{target} @dots{} : @var{prerequisites} @dots{}
534 A @dfn{target} is usually the name of a file that is generated by a
535 program; examples of targets are executable or object files. A target
536 can also be the name of an action to carry out, such as @samp{clean}
537 (@pxref{Phony Targets}).
539 A @dfn{prerequisite} is a file that is used as input to create the
540 target. A target often depends on several files.
542 @cindex tabs in rules
543 A @dfn{command} is an action that @code{make} carries out.
544 A rule may have more than one command, each on its own line.
545 @strong{Please note:} you need to put a tab character at the beginning of
546 every command line! This is an obscurity that catches the unwary.
548 Usually a command is in a rule with prerequisites and serves to create a
549 target file if any of the prerequisites change. However, the rule that
550 specifies commands for the target need not have prerequisites. For
551 example, the rule containing the delete command associated with the
552 target @samp{clean} does not have prerequisites.
554 A @dfn{rule}, then, explains how and when to remake certain files
555 which are the targets of the particular rule. @code{make} carries out
556 the commands on the prerequisites to create or update the target. A
557 rule can also explain how and when to carry out an action.
558 @xref{Rules, , Writing Rules}.
560 A makefile may contain other text besides rules, but a simple makefile
561 need only contain rules. Rules may look somewhat more complicated
562 than shown in this template, but all fit the pattern more or less.
564 @node Simple Makefile, How Make Works, Rule Introduction, Introduction
565 @section A Simple Makefile
566 @cindex simple makefile
567 @cindex makefile, simple
569 Here is a straightforward makefile that describes the way an
570 executable file called @code{edit} depends on eight object files
571 which, in turn, depend on eight C source and three header files.
573 In this example, all the C files include @file{defs.h}, but only those
574 defining editing commands include @file{command.h}, and only low
575 level files that change the editor buffer include @file{buffer.h}.
579 edit : main.o kbd.o command.o display.o \
580 insert.o search.o files.o utils.o
581 cc -o edit main.o kbd.o command.o display.o \
582 insert.o search.o files.o utils.o
584 main.o : main.c defs.h
586 kbd.o : kbd.c defs.h command.h
588 command.o : command.c defs.h command.h
590 display.o : display.c defs.h buffer.h
592 insert.o : insert.c defs.h buffer.h
594 search.o : search.c defs.h buffer.h
596 files.o : files.c defs.h buffer.h command.h
598 utils.o : utils.c defs.h
601 rm edit main.o kbd.o command.o display.o \
602 insert.o search.o files.o utils.o
607 We split each long line into two lines using backslash-newline; this is
608 like using one long line, but is easier to read.
609 @cindex continuation lines
610 @cindex @code{\} (backslash), for continuation lines
611 @cindex backslash (@code{\}), for continuation lines
612 @cindex quoting newline, in makefile
613 @cindex newline, quoting, in makefile
615 To use this makefile to create the executable file called @file{edit},
622 To use this makefile to delete the executable file and all the object
623 files from the directory, type:
629 In the example makefile, the targets include the executable file
630 @samp{edit}, and the object files @samp{main.o} and @samp{kbd.o}. The
631 prerequisites are files such as @samp{main.c} and @samp{defs.h}.
632 In fact, each @samp{.o} file is both a target and a prerequisite.
633 Commands include @w{@samp{cc -c main.c}} and @w{@samp{cc -c kbd.c}}.
635 When a target is a file, it needs to be recompiled or relinked if any
636 of its prerequisites change. In addition, any prerequisites that are
637 themselves automatically generated should be updated first. In this
638 example, @file{edit} depends on each of the eight object files; the
639 object file @file{main.o} depends on the source file @file{main.c} and
640 on the header file @file{defs.h}.
642 A shell command follows each line that contains a target and
643 prerequisites. These shell commands say how to update the target file.
644 A tab character must come at the beginning of every command line to
645 distinguish commands lines from other lines in the makefile. (Bear in
646 mind that @code{make} does not know anything about how the commands
647 work. It is up to you to supply commands that will update the target
648 file properly. All @code{make} does is execute the commands in the rule
649 you have specified when the target file needs to be updated.)
650 @cindex shell command
652 The target @samp{clean} is not a file, but merely the name of an
655 do not want to carry out the actions in this rule, @samp{clean} is not a prerequisite of any other rule.
656 Consequently, @code{make} never does anything with it unless you tell
657 it specifically. Note that this rule not only is not a prerequisite, it
658 also does not have any prerequisites, so the only purpose of the rule
659 is to run the specified commands. Targets that do not refer to files
660 but are just actions are called @dfn{phony targets}. @xref{Phony
661 Targets}, for information about this kind of target. @xref{Errors, ,
662 Errors in Commands}, to see how to cause @code{make} to ignore errors
663 from @code{rm} or any other command.
664 @cindex @code{clean} target
665 @cindex @code{rm} (shell command)
667 @node How Make Works, Variables Simplify, Simple Makefile, Introduction
668 @comment node-name, next, previous, up
669 @section How @code{make} Processes a Makefile
670 @cindex processing a makefile
671 @cindex makefile, how @code{make} processes
673 By default, @code{make} starts with the first target (not targets whose
674 names start with @samp{.}). This is called the @dfn{default goal}.
675 (@dfn{Goals} are the targets that @code{make} strives ultimately to
676 update. @xref{Goals, , Arguments to Specify the Goals}.)
678 @cindex goal, default
681 In the simple example of the previous section, the default goal is to
682 update the executable program @file{edit}; therefore, we put that rule
685 Thus, when you give the command:
692 @code{make} reads the makefile in the current directory and begins by
693 processing the first rule. In the example, this rule is for relinking
694 @file{edit}; but before @code{make} can fully process this rule, it
695 must process the rules for the files that @file{edit} depends on,
696 which in this case are the object files. Each of these files is
697 processed according to its own rule. These rules say to update each
698 @samp{.o} file by compiling its source file. The recompilation must
699 be done if the source file, or any of the header files named as
700 prerequisites, is more recent than the object file, or if the object
703 The other rules are processed because their targets appear as
704 prerequisites of the goal. If some other rule is not depended on by the
705 goal (or anything it depends on, etc.), that rule is not processed,
706 unless you tell @code{make} to do so (with a command such as
707 @w{@code{make clean}}).
709 Before recompiling an object file, @code{make} considers updating its
710 prerequisites, the source file and header files. This makefile does not
711 specify anything to be done for them---the @samp{.c} and @samp{.h} files
712 are not the targets of any rules---so @code{make} does nothing for these
713 files. But @code{make} would update automatically generated C programs,
714 such as those made by Bison or Yacc, by their own rules at this time.
716 After recompiling whichever object files need it, @code{make} decides
717 whether to relink @file{edit}. This must be done if the file
718 @file{edit} does not exist, or if any of the object files are newer than
719 it. If an object file was just recompiled, it is now newer than
720 @file{edit}, so @file{edit} is relinked.
723 Thus, if we change the file @file{insert.c} and run @code{make},
724 @code{make} will compile that file to update @file{insert.o}, and then
725 link @file{edit}. If we change the file @file{command.h} and run
726 @code{make}, @code{make} will recompile the object files @file{kbd.o},
727 @file{command.o} and @file{files.o} and then link the file @file{edit}.
729 @node Variables Simplify, make Deduces, How Make Works, Introduction
730 @section Variables Make Makefiles Simpler
732 @cindex simplifying with variables
734 In our example, we had to list all the object files twice in the rule for
735 @file{edit} (repeated here):
739 edit : main.o kbd.o command.o display.o \
740 insert.o search.o files.o utils.o
741 cc -o edit main.o kbd.o command.o display.o \
742 insert.o search.o files.o utils.o
746 @cindex @code{objects}
747 Such duplication is error-prone; if a new object file is added to the
748 system, we might add it to one list and forget the other. We can eliminate
749 the risk and simplify the makefile by using a variable. @dfn{Variables}
750 allow a text string to be defined once and substituted in multiple places
751 later (@pxref{Using Variables, ,How to Use Variables}).
753 @cindex @code{OBJECTS}
758 It is standard practice for every makefile to have a variable named
759 @code{objects}, @code{OBJECTS}, @code{objs}, @code{OBJS}, @code{obj},
760 or @code{OBJ} which is a list of all object file names. We would
761 define such a variable @code{objects} with a line like this in the
766 objects = main.o kbd.o command.o display.o \
767 insert.o search.o files.o utils.o
772 Then, each place we want to put a list of the object file names, we can
773 substitute the variable's value by writing @samp{$(objects)}
774 (@pxref{Using Variables, ,How to Use Variables}).
776 Here is how the complete simple makefile looks when you use a variable
777 for the object files:
781 objects = main.o kbd.o command.o display.o \
782 insert.o search.o files.o utils.o
785 cc -o edit $(objects)
786 main.o : main.c defs.h
788 kbd.o : kbd.c defs.h command.h
790 command.o : command.c defs.h command.h
792 display.o : display.c defs.h buffer.h
794 insert.o : insert.c defs.h buffer.h
796 search.o : search.c defs.h buffer.h
798 files.o : files.c defs.h buffer.h command.h
800 utils.o : utils.c defs.h
807 @node make Deduces, Combine By Prerequisite, Variables Simplify, Introduction
808 @section Letting @code{make} Deduce the Commands
809 @cindex deducing commands (implicit rules)
810 @cindex implicit rule, introduction to
811 @cindex rule, implicit, introduction to
813 It is not necessary to spell out the commands for compiling the individual
814 C source files, because @code{make} can figure them out: it has an
815 @dfn{implicit rule} for updating a @samp{.o} file from a correspondingly
816 named @samp{.c} file using a @samp{cc -c} command. For example, it will
817 use the command @samp{cc -c main.c -o main.o} to compile @file{main.c} into
818 @file{main.o}. We can therefore omit the commands from the rules for the
819 object files. @xref{Implicit Rules, ,Using Implicit Rules}.@refill
821 When a @samp{.c} file is used automatically in this way, it is also
822 automatically added to the list of prerequisites. We can therefore omit
823 the @samp{.c} files from the prerequisites, provided we omit the commands.
825 Here is the entire example, with both of these changes, and a variable
826 @code{objects} as suggested above:
830 objects = main.o kbd.o command.o display.o \
831 insert.o search.o files.o utils.o
834 cc -o edit $(objects)
837 kbd.o : defs.h command.h
838 command.o : defs.h command.h
839 display.o : defs.h buffer.h
840 insert.o : defs.h buffer.h
841 search.o : defs.h buffer.h
842 files.o : defs.h buffer.h command.h
852 This is how we would write the makefile in actual practice. (The
853 complications associated with @samp{clean} are described elsewhere.
854 See @ref{Phony Targets}, and @ref{Errors, ,Errors in Commands}.)
856 Because implicit rules are so convenient, they are important. You
857 will see them used frequently.@refill
859 @node Combine By Prerequisite, Cleanup, make Deduces, Introduction
860 @section Another Style of Makefile
861 @cindex combining rules by prerequisite
863 When the objects of a makefile are created only by implicit rules, an
864 alternative style of makefile is possible. In this style of makefile,
865 you group entries by their prerequisites instead of by their targets.
866 Here is what one looks like:
870 objects = main.o kbd.o command.o display.o \
871 insert.o search.o files.o utils.o
874 cc -o edit $(objects)
877 kbd.o command.o files.o : command.h
878 display.o insert.o search.o files.o : buffer.h
883 Here @file{defs.h} is given as a prerequisite of all the object files;
884 @file{command.h} and @file{buffer.h} are prerequisites of the specific
885 object files listed for them.
887 Whether this is better is a matter of taste: it is more compact, but some
888 people dislike it because they find it clearer to put all the information
889 about each target in one place.
891 @node Cleanup, , Combine By Prerequisite, Introduction
892 @section Rules for Cleaning the Directory
894 @cindex removing, to clean up
896 Compiling a program is not the only thing you might want to write rules
897 for. Makefiles commonly tell how to do a few other things besides
898 compiling a program: for example, how to delete all the object files
899 and executables so that the directory is @samp{clean}.
901 @cindex @code{clean} target
903 could write a @code{make} rule for cleaning our example editor:
912 In practice, we might want to write the rule in a somewhat more
913 complicated manner to handle unanticipated situations. We would do this:
924 This prevents @code{make} from getting confused by an actual file
925 called @file{clean} and causes it to continue in spite of errors from
926 @code{rm}. (See @ref{Phony Targets}, and @ref{Errors, ,Errors in
930 A rule such as this should not be placed at the beginning of the
931 makefile, because we do not want it to run by default! Thus, in the
932 example makefile, we want the rule for @code{edit}, which recompiles
933 the editor, to remain the default goal.
935 Since @code{clean} is not a prerequisite of @code{edit}, this rule will not
936 run at all if we give the command @samp{make} with no arguments. In
937 order to make the rule run, we have to type @samp{make clean}.
938 @xref{Running, ,How to Run @code{make}}.
940 @node Makefiles, Rules, Introduction, Top
941 @chapter Writing Makefiles
943 @cindex makefile, how to write
944 The information that tells @code{make} how to recompile a system comes from
945 reading a data base called the @dfn{makefile}.
948 * Makefile Contents:: What makefiles contain.
949 * Makefile Names:: How to name your makefile.
950 * Include:: How one makefile can use another makefile.
951 * MAKEFILES Variable:: The environment can specify extra makefiles.
952 * MAKEFILE_LIST Variable:: Discover which makefiles have been read.
953 * Special Variables:: Other special variables.
954 * Remaking Makefiles:: How makefiles get remade.
955 * Overriding Makefiles:: How to override part of one makefile
956 with another makefile.
957 * Reading Makefiles:: How makefiles are parsed.
960 @node Makefile Contents, Makefile Names, Makefiles, Makefiles
961 @section What Makefiles Contain
963 Makefiles contain five kinds of things: @dfn{explicit rules},
964 @dfn{implicit rules}, @dfn{variable definitions}, @dfn{directives},
965 and @dfn{comments}. Rules, variables, and directives are described at
966 length in later chapters.@refill
969 @cindex rule, explicit, definition of
970 @cindex explicit rule, definition of
972 An @dfn{explicit rule} says when and how to remake one or more files,
973 called the rule's @dfn{targets}. It lists the other files that the
974 targets depend on, called the @dfn{prerequisites} of the target, and
975 may also give commands to use to create or update the targets.
976 @xref{Rules, ,Writing Rules}.
978 @cindex rule, implicit, definition of
979 @cindex implicit rule, definition of
981 An @dfn{implicit rule} says when and how to remake a class of files
982 based on their names. It describes how a target may depend on a file
983 with a name similar to the target and gives commands to create or
984 update such a target. @xref{Implicit Rules, ,Using Implicit Rules}.
986 @cindex variable definition
988 A @dfn{variable definition} is a line that specifies a text string
989 value for a variable that can be substituted into the text later. The
990 simple makefile example shows a variable definition for @code{objects}
991 as a list of all object files (@pxref{Variables Simplify, , Variables
992 Make Makefiles Simpler}).
996 A @dfn{directive} is a command for @code{make} to do something special while
997 reading the makefile. These include:
1001 Reading another makefile (@pxref{Include, ,Including Other Makefiles}).
1004 Deciding (based on the values of variables) whether to use or
1005 ignore a part of the makefile (@pxref{Conditionals, ,Conditional Parts of Makefiles}).
1008 Defining a variable from a verbatim string containing multiple lines
1009 (@pxref{Defining, ,Defining Variables Verbatim}).
1012 @cindex comments, in makefile
1013 @cindex @code{#} (comments), in makefile
1015 @samp{#} in a line of a makefile starts a @dfn{comment}. It and the
1016 rest of the line are ignored, except that a trailing backslash not
1017 escaped by another backslash will continue the comment across multiple
1018 lines. A line containing just a comment (with perhaps spaces before
1019 it) is effectively blank, and is ignored. If you want a literal
1020 @code{#}, escape it with a backslash (e.g., @code{\#}). Comments may
1021 appear on any line in the makefile, although they are treated
1022 specially in certain situations.
1024 Within a command script (if the line begins with a TAB character) the
1025 entire line is passed to the shell, just as with any other line that
1026 begins with a TAB. The shell decides how to interpret the text:
1027 whether or not this is a comment is up to the shell.
1029 Within a @code{define} directive, comments are not ignored during the
1030 definition of the variable, but rather kept intact in the value of the
1031 variable. When the variable is expanded they will either be treated
1032 as @code{make} comments or as command script text, depending on the
1033 context in which the variable is evaluated.
1036 @node Makefile Names, Include, Makefile Contents, Makefiles
1037 @section What Name to Give Your Makefile
1038 @cindex makefile name
1039 @cindex name of makefile
1040 @cindex default makefile name
1041 @cindex file name of makefile
1043 @c following paragraph rewritten to avoid overfull hbox
1044 By default, when @code{make} looks for the makefile, it tries the
1045 following names, in order: @file{GNUmakefile}, @file{makefile}
1046 and @file{Makefile}.@refill
1051 @cindex @code{README}
1052 Normally you should call your makefile either @file{makefile} or
1053 @file{Makefile}. (We recommend @file{Makefile} because it appears
1054 prominently near the beginning of a directory listing, right near other
1055 important files such as @file{README}.) The first name checked,
1056 @file{GNUmakefile}, is not recommended for most makefiles. You should
1057 use this name if you have a makefile that is specific to GNU
1058 @code{make}, and will not be understood by other versions of
1059 @code{make}. Other @code{make} programs look for @file{makefile} and
1060 @file{Makefile}, but not @file{GNUmakefile}.
1062 If @code{make} finds none of these names, it does not use any makefile.
1063 Then you must specify a goal with a command argument, and @code{make}
1064 will attempt to figure out how to remake it using only its built-in
1065 implicit rules. @xref{Implicit Rules, ,Using Implicit Rules}.
1068 @cindex @code{--file}
1069 @cindex @code{--makefile}
1070 If you want to use a nonstandard name for your makefile, you can specify
1071 the makefile name with the @samp{-f} or @samp{--file} option. The
1072 arguments @w{@samp{-f @var{name}}} or @w{@samp{--file=@var{name}}} tell
1073 @code{make} to read the file @var{name} as the makefile. If you use
1074 more than one @samp{-f} or @samp{--file} option, you can specify several
1075 makefiles. All the makefiles are effectively concatenated in the order
1076 specified. The default makefile names @file{GNUmakefile},
1077 @file{makefile} and @file{Makefile} are not checked automatically if you
1078 specify @samp{-f} or @samp{--file}.@refill
1079 @cindex specifying makefile name
1080 @cindex makefile name, how to specify
1081 @cindex name of makefile, how to specify
1082 @cindex file name of makefile, how to specify
1084 @node Include, MAKEFILES Variable, Makefile Names, Makefiles
1085 @section Including Other Makefiles
1086 @cindex including other makefiles
1087 @cindex makefile, including
1090 The @code{include} directive tells @code{make} to suspend reading the
1091 current makefile and read one or more other makefiles before continuing.
1092 The directive is a line in the makefile that looks like this:
1095 include @var{filenames}@dots{}
1099 @var{filenames} can contain shell file name patterns.
1100 @cindex shell file name pattern (in @code{include})
1101 @cindex shell wildcards (in @code{include})
1102 @cindex wildcard, in @code{include}
1104 Extra spaces are allowed and ignored at the beginning of the line, but
1105 a tab is not allowed. (If the line begins with a tab, it will be
1106 considered a command line.) Whitespace is required between
1107 @code{include} and the file names, and between file names; extra
1108 whitespace is ignored there and at the end of the directive. A
1109 comment starting with @samp{#} is allowed at the end of the line. If
1110 the file names contain any variable or function references, they are
1111 expanded. @xref{Using Variables, ,How to Use Variables}.
1113 For example, if you have three @file{.mk} files, @file{a.mk},
1114 @file{b.mk}, and @file{c.mk}, and @code{$(bar)} expands to
1115 @code{bish bash}, then the following expression
1118 include foo *.mk $(bar)
1124 include foo a.mk b.mk c.mk bish bash
1127 When @code{make} processes an @code{include} directive, it suspends
1128 reading of the containing makefile and reads from each listed file in
1129 turn. When that is finished, @code{make} resumes reading the
1130 makefile in which the directive appears.
1132 One occasion for using @code{include} directives is when several programs,
1133 handled by individual makefiles in various directories, need to use a
1134 common set of variable definitions
1135 (@pxref{Setting, ,Setting Variables}) or pattern rules
1136 (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).
1138 Another such occasion is when you want to generate prerequisites from
1139 source files automatically; the prerequisites can be put in a file that
1140 is included by the main makefile. This practice is generally cleaner
1141 than that of somehow appending the prerequisites to the end of the main
1142 makefile as has been traditionally done with other versions of
1143 @code{make}. @xref{Automatic Prerequisites}.
1144 @cindex prerequisites, automatic generation
1145 @cindex automatic generation of prerequisites
1146 @cindex generating prerequisites automatically
1149 @cindex @code{--include-dir}
1150 @cindex included makefiles, default directories
1151 @cindex default directories for included makefiles
1152 @findex /usr/gnu/include
1153 @findex /usr/local/include
1154 @findex /usr/include
1155 If the specified name does not start with a slash, and the file is not
1156 found in the current directory, several other directories are searched.
1157 First, any directories you have specified with the @samp{-I} or
1158 @samp{--include-dir} option are searched
1159 (@pxref{Options Summary, ,Summary of Options}).
1160 Then the following directories (if they exist)
1161 are searched, in this order:
1162 @file{@var{prefix}/include} (normally @file{/usr/local/include}
1163 @footnote{GNU Make compiled for MS-DOS and MS-Windows behaves as if
1164 @var{prefix} has been defined to be the root of the DJGPP tree
1166 @file{/usr/gnu/include},
1167 @file{/usr/local/include}, @file{/usr/include}.
1169 If an included makefile cannot be found in any of these directories, a
1170 warning message is generated, but it is not an immediately fatal error;
1171 processing of the makefile containing the @code{include} continues.
1172 Once it has finished reading makefiles, @code{make} will try to remake
1173 any that are out of date or don't exist.
1174 @xref{Remaking Makefiles, ,How Makefiles Are Remade}.
1175 Only after it has tried to find a way to remake a makefile and failed,
1176 will @code{make} diagnose the missing makefile as a fatal error.
1178 If you want @code{make} to simply ignore a makefile which does not exist
1179 and cannot be remade, with no error message, use the @w{@code{-include}}
1180 directive instead of @code{include}, like this:
1183 -include @var{filenames}@dots{}
1186 This acts like @code{include} in every way except that there is no
1187 error (not even a warning) if any of the @var{filenames} do not exist.
1188 For compatibility with some other @code{make} implementations,
1189 @code{sinclude} is another name for @w{@code{-include}}.
1191 @node MAKEFILES Variable, MAKEFILE_LIST Variable, Include, Makefiles
1192 @section The Variable @code{MAKEFILES}
1193 @cindex makefile, and @code{MAKEFILES} variable
1194 @cindex including (@code{MAKEFILES} variable)
1197 If the environment variable @code{MAKEFILES} is defined, @code{make}
1198 considers its value as a list of names (separated by whitespace) of
1199 additional makefiles to be read before the others. This works much like
1200 the @code{include} directive: various directories are searched for those
1201 files (@pxref{Include, ,Including Other Makefiles}). In addition, the
1202 default goal is never taken from one of these makefiles and it is not an
1203 error if the files listed in @code{MAKEFILES} are not found.@refill
1205 @cindex recursion, and @code{MAKEFILES} variable
1206 The main use of @code{MAKEFILES} is in communication between recursive
1207 invocations of @code{make} (@pxref{Recursion, ,Recursive Use of
1208 @code{make}}). It usually is not desirable to set the environment
1209 variable before a top-level invocation of @code{make}, because it is
1210 usually better not to mess with a makefile from outside. However, if
1211 you are running @code{make} without a specific makefile, a makefile in
1212 @code{MAKEFILES} can do useful things to help the built-in implicit
1213 rules work better, such as defining search paths (@pxref{Directory Search}).
1215 Some users are tempted to set @code{MAKEFILES} in the environment
1216 automatically on login, and program makefiles to expect this to be done.
1217 This is a very bad idea, because such makefiles will fail to work if run by
1218 anyone else. It is much better to write explicit @code{include} directives
1219 in the makefiles. @xref{Include, , Including Other Makefiles}.
1221 @node MAKEFILE_LIST Variable, Special Variables, MAKEFILES Variable, Makefiles
1222 @comment node-name, next, previous, up
1223 @section The Variable @code{MAKEFILE_LIST}
1224 @cindex makefiles, and @code{MAKEFILE_LIST} variable
1225 @cindex including (@code{MAKEFILE_LIST} variable)
1227 As @code{make} reads various makefiles, including any obtained from the
1228 @code{MAKEFILES} variable, the command line, the default files, or
1229 from @code{include} directives, their names will be automatically
1230 appended to the @code{MAKEFILE_LIST} variable. They are added right
1231 before @code{make} begins to parse them.
1233 This means that if the first thing a makefile does is examine the last
1234 word in this variable, it will be the name of the current makefile.
1235 Once the current makefile has used @code{include}, however, the last
1236 word will be the just-included makefile.
1238 If a makefile named @code{Makefile} has this content:
1242 name1 := $(lastword $(MAKEFILE_LIST))
1246 name2 := $(lastword $(MAKEFILE_LIST))
1249 @@echo name1 = $(name1)
1250 @@echo name2 = $(name2)
1255 then you would expect to see this output:
1264 @xref{Text Functions}, for more information on the @code{word} and
1265 @code{words} functions used above. @xref{Flavors, The Two Flavors of
1266 Variables}, for more information on simply-expanded (@code{:=})
1267 variable definitions.
1269 @node Special Variables, Remaking Makefiles, MAKEFILE_LIST Variable, Makefiles
1270 @comment node-name, next, previous, up
1271 @section Other Special Variables
1272 @cindex makefiles, and special variables
1273 @cindex special variables
1275 GNU @code{make} also supports a special variable. Note that any value
1276 you assign to this variable will be ignored; it will always return its
1279 @vindex $(.VARIABLES)
1280 @vindex .VARIABLES @r{(list of variables)}
1281 The first special variable is @code{.VARIABLES}. When expanded, the
1282 value consists of a list of the @emph{names} of all global variables
1283 defined in all makefiles read up until that point. This includes
1284 variables which have empty values, as well as built-in variables
1285 (@pxref{Implicit Variables, , Variables Used by Implicit Rules}), but
1286 does not include any variables which are only defined in a
1287 target-specific context.
1289 @c @vindex $(.TARGETS)
1290 @c @vindex .TARGETS @r{(list of targets)}
1291 @c The second special variable is @code{.TARGETS}. When expanded, the
1292 @c value consists of a list of all targets defined in all makefiles read
1293 @c up until that point. Note it's not enough for a file to be simply
1294 @c mentioned in the makefile to be listed in this variable, even if it
1295 @c would match an implicit rule and become an ``implicit target''. The
1296 @c file must appear as a target, on the left-hand side of a ``:'', to be
1297 @c considered a target for the purposes of this variable.
1299 @node Remaking Makefiles, Overriding Makefiles, Special Variables, Makefiles
1300 @section How Makefiles Are Remade
1302 @cindex updating makefiles
1303 @cindex remaking makefiles
1304 @cindex makefile, remaking of
1305 Sometimes makefiles can be remade from other files, such as RCS or SCCS
1306 files. If a makefile can be remade from other files, you probably want
1307 @code{make} to get an up-to-date version of the makefile to read in.
1309 To this end, after reading in all makefiles, @code{make} will consider
1310 each as a goal target and attempt to update it. If a makefile has a
1311 rule which says how to update it (found either in that very makefile or
1312 in another one) or if an implicit rule applies to it (@pxref{Implicit
1313 Rules, ,Using Implicit Rules}), it will be updated if necessary. After
1314 all makefiles have been checked, if any have actually been changed,
1315 @code{make} starts with a clean slate and reads all the makefiles over
1316 again. (It will also attempt to update each of them over again, but
1317 normally this will not change them again, since they are already up to
1320 If you know that one or more of your makefiles cannot be remade and you
1321 want to keep @code{make} from performing an implicit rule search on
1322 them, perhaps for efficiency reasons, you can use any normal method of
1323 preventing implicit rule lookup to do so. For example, you can write an
1324 explicit rule with the makefile as the target, and an empty command
1325 string (@pxref{Empty Commands, ,Using Empty Commands}).
1327 If the makefiles specify a double-colon rule to remake a file with
1328 commands but no prerequisites, that file will always be remade
1329 (@pxref{Double-Colon}). In the case of makefiles, a makefile that has a
1330 double-colon rule with commands but no prerequisites will be remade every
1331 time @code{make} is run, and then again after @code{make} starts over
1332 and reads the makefiles in again. This would cause an infinite loop:
1333 @code{make} would constantly remake the makefile, and never do anything
1334 else. So, to avoid this, @code{make} will @strong{not} attempt to
1335 remake makefiles which are specified as targets of a double-colon rule
1336 with commands but no prerequisites.@refill
1338 If you do not specify any makefiles to be read with @samp{-f} or
1339 @samp{--file} options, @code{make} will try the default makefile names;
1340 @pxref{Makefile Names, ,What Name to Give Your Makefile}. Unlike
1341 makefiles explicitly requested with @samp{-f} or @samp{--file} options,
1342 @code{make} is not certain that these makefiles should exist. However,
1343 if a default makefile does not exist but can be created by running
1344 @code{make} rules, you probably want the rules to be run so that the
1345 makefile can be used.
1347 Therefore, if none of the default makefiles exists, @code{make} will try
1348 to make each of them in the same order in which they are searched for
1349 (@pxref{Makefile Names, ,What Name to Give Your Makefile})
1350 until it succeeds in making one, or it runs out of names to try. Note
1351 that it is not an error if @code{make} cannot find or make any makefile;
1352 a makefile is not always necessary.@refill
1354 When you use the @samp{-t} or @samp{--touch} option
1355 (@pxref{Instead of Execution, ,Instead of Executing the Commands}),
1356 you would not want to use an out-of-date makefile to decide which
1357 targets to touch. So the @samp{-t} option has no effect on updating
1358 makefiles; they are really updated even if @samp{-t} is specified.
1359 Likewise, @samp{-q} (or @samp{--question}) and @samp{-n} (or
1360 @samp{--just-print}) do not prevent updating of makefiles, because an
1361 out-of-date makefile would result in the wrong output for other targets.
1362 Thus, @samp{make -f mfile -n foo} will update @file{mfile}, read it in,
1363 and then print the commands to update @file{foo} and its prerequisites
1364 without running them. The commands printed for @file{foo} will be those
1365 specified in the updated contents of @file{mfile}.
1367 However, on occasion you might actually wish to prevent updating of even
1368 the makefiles. You can do this by specifying the makefiles as goals in
1369 the command line as well as specifying them as makefiles. When the
1370 makefile name is specified explicitly as a goal, the options @samp{-t}
1371 and so on do apply to them.
1373 Thus, @samp{make -f mfile -n mfile foo} would read the makefile
1374 @file{mfile}, print the commands needed to update it without actually
1375 running them, and then print the commands needed to update @file{foo}
1376 without running them. The commands for @file{foo} will be those
1377 specified by the existing contents of @file{mfile}.
1379 @node Overriding Makefiles, Reading Makefiles, Remaking Makefiles, Makefiles
1380 @section Overriding Part of Another Makefile
1382 @cindex overriding makefiles
1383 @cindex makefile, overriding
1384 Sometimes it is useful to have a makefile that is mostly just like
1385 another makefile. You can often use the @samp{include} directive to
1386 include one in the other, and add more targets or variable definitions.
1387 However, if the two makefiles give different commands for the same
1388 target, @code{make} will not let you just do this. But there is another way.
1390 @cindex match-anything rule, used to override
1391 In the containing makefile (the one that wants to include the other),
1392 you can use a match-anything pattern rule to say that to remake any
1393 target that cannot be made from the information in the containing
1394 makefile, @code{make} should look in another makefile.
1395 @xref{Pattern Rules}, for more information on pattern rules.
1397 For example, if you have a makefile called @file{Makefile} that says how
1398 to make the target @samp{foo} (and other targets), you can write a
1399 makefile called @file{GNUmakefile} that contains:
1406 @@$(MAKE) -f Makefile $@@
1410 If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
1411 read it, and see that to make @file{foo}, it needs to run the command
1412 @samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will
1413 find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
1414 commands from the pattern rule: @samp{make -f Makefile bar}. If
1415 @file{Makefile} provides a rule for updating @file{bar}, @code{make}
1416 will apply the rule. And likewise for any other target that
1417 @file{GNUmakefile} does not say how to make.
1419 The way this works is that the pattern rule has a pattern of just
1420 @samp{%}, so it matches any target whatever. The rule specifies a
1421 prerequisite @file{force}, to guarantee that the commands will be run even
1422 if the target file already exists. We give @file{force} target empty
1423 commands to prevent @code{make} from searching for an implicit rule to
1424 build it---otherwise it would apply the same match-anything rule to
1425 @file{force} itself and create a prerequisite loop!
1427 @node Reading Makefiles, , Overriding Makefiles, Makefiles
1428 @section How @code{make} Reads a Makefile
1429 @cindex reading makefiles
1430 @cindex makefile, parsing
1432 GNU @code{make} does its work in two distinct phases. During the first
1433 phase it reads all the makefiles, included makefiles, etc. and
1434 internalizes all the variables and their values, implicit and explicit
1435 rules, and constructs a dependency graph of all the targets and their
1436 prerequisites. During the second phase, @code{make} uses these internal
1437 structures to determine what targets will need to be rebuilt and to
1438 invoke the rules necessary to do so.
1440 It's important to understand this two-phase approach because it has a
1441 direct impact on how variable and function expansion happens; this is
1442 often a source of some confusion when writing makefiles. Here we will
1443 present a summary of the phases in which expansion happens for different
1444 constructs within the makefile. We say that expansion is
1445 @dfn{immediate} if it happens during the first phase: in this case
1446 @code{make} will expand any variables or functions in that section of a
1447 construct as the makefile is parsed. We say that expansion is
1448 @dfn{deferred} if expansion is not performed immediately. Expansion of
1449 deferred construct is not performed until either the construct appears
1450 later in an immediate context, or until the second phase.
1452 You may not be familiar with some of these constructs yet. You can
1453 reference this section as you become familiar with them, in later
1456 @subheading Variable Assignment
1457 @cindex +=, expansion
1458 @cindex =, expansion
1459 @cindex ?=, expansion
1460 @cindex +=, expansion
1461 @cindex define, expansion
1463 Variable definitions are parsed as follows:
1466 @var{immediate} = @var{deferred}
1467 @var{immediate} ?= @var{deferred}
1468 @var{immediate} := @var{immediate}
1469 @var{immediate} += @var{deferred} or @var{immediate}
1471 define @var{immediate}
1476 For the append operator, @samp{+=}, the right-hand side is considered
1477 immediate if the variable was previously set as a simple variable
1478 (@samp{:=}), and deferred otherwise.
1480 @subheading Conditional Statements
1481 @cindex ifdef, expansion
1482 @cindex ifeq, expansion
1483 @cindex ifndef, expansion
1484 @cindex ifneq, expansion
1486 All instances of conditional syntax are parsed immediately, in their
1487 entirety; this includes the @code{ifdef}, @code{ifeq}, @code{ifndef},
1488 and @code{ifneq} forms.
1490 @subheading Rule Definition
1491 @cindex target, expansion
1492 @cindex prerequisite, expansion
1493 @cindex implicit rule, expansion
1494 @cindex pattern rule, expansion
1495 @cindex explicit rule, expansion
1497 A rule is always expanded the same way, regardless of the form:
1500 @var{immediate} : @var{immediate} ; @var{deferred}
1504 That is, the target and prerequisite sections are expanded immediately,
1505 and the commands used to construct the target are always deferred. This
1506 general rule is true for explicit rules, pattern rules, suffix rules,
1507 static pattern rules, and simple prerequisite definitions.
1509 @node Rules, Commands, Makefiles, Top
1510 @chapter Writing Rules
1511 @cindex writing rules
1512 @cindex rule, how to write
1514 @cindex prerequisite
1516 A @dfn{rule} appears in the makefile and says when and how to remake
1517 certain files, called the rule's @dfn{targets} (most often only one per rule).
1518 It lists the other files that are the @dfn{prerequisites} of the target, and
1519 @dfn{commands} to use to create or update the target.
1521 @cindex default goal
1522 @cindex goal, default
1523 The order of rules is not significant, except for determining the
1524 @dfn{default goal}: the target for @code{make} to consider, if you do
1525 not otherwise specify one. The default goal is the target of the first
1526 rule in the first makefile. If the first rule has multiple targets,
1527 only the first target is taken as the default. There are two
1528 exceptions: a target starting with a period is not a default unless it
1529 contains one or more slashes, @samp{/}, as well; and, a target that
1530 defines a pattern rule has no effect on the default goal.
1531 (@xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.)
1533 Therefore, we usually write the makefile so that the first rule is the
1534 one for compiling the entire program or all the programs described by
1535 the makefile (often with a target called @samp{all}).
1536 @xref{Goals, ,Arguments to Specify the Goals}.
1539 * Rule Example:: An example explained.
1540 * Rule Syntax:: General syntax explained.
1541 * Prerequisite Types:: There are two types of prerequisites.
1542 * Wildcards:: Using wildcard characters such as `*'.
1543 * Directory Search:: Searching other directories for source files.
1544 * Phony Targets:: Using a target that is not a real file's name.
1545 * Force Targets:: You can use a target without commands
1546 or prerequisites to mark other
1548 * Empty Targets:: When only the date matters and the
1550 * Special Targets:: Targets with special built-in meanings.
1551 * Multiple Targets:: When to make use of several targets in a rule.
1552 * Multiple Rules:: How to use several rules with the same target.
1553 * Static Pattern:: Static pattern rules apply to multiple targets
1554 and can vary the prerequisites according to
1556 * Double-Colon:: How to use a special kind of rule to allow
1557 several independent rules for one target.
1558 * Automatic Prerequisites:: How to automatically generate rules giving
1559 prerequisites from source files themselves.
1563 @node Rule Example, Rule Syntax, Rules, Rules
1564 @section Rule Example
1566 Here is an example of a rule:
1569 foo.o : foo.c defs.h # module for twiddling the frobs
1573 Its target is @file{foo.o} and its prerequisites are @file{foo.c} and
1574 @file{defs.h}. It has one command, which is @samp{cc -c -g foo.c}.
1575 The command line starts with a tab to identify it as a command.
1577 This rule says two things:
1581 How to decide whether @file{foo.o} is out of date: it is out of date
1582 if it does not exist, or if either @file{foo.c} or @file{defs.h} is
1583 more recent than it.
1586 How to update the file @file{foo.o}: by running @code{cc} as stated.
1587 The command does not explicitly mention @file{defs.h}, but we presume
1588 that @file{foo.c} includes it, and that that is why @file{defs.h} was
1589 added to the prerequisites.
1593 @node Rule Syntax, Prerequisite Types, Rule Example, Rules
1594 @section Rule Syntax
1597 @cindex syntax of rules
1598 In general, a rule looks like this:
1601 @var{targets} : @var{prerequisites}
1610 @var{targets} : @var{prerequisites} ; @var{command}
1616 @cindex rule targets
1617 The @var{targets} are file names, separated by spaces. Wildcard
1618 characters may be used (@pxref{Wildcards, ,Using Wildcard Characters
1619 in File Names}) and a name of the form @file{@var{a}(@var{m})}
1620 represents member @var{m} in archive file @var{a}
1621 (@pxref{Archive Members, ,Archive Members as Targets}).
1622 Usually there is only one
1623 target per rule, but occasionally there is a reason to have more
1624 (@pxref{Multiple Targets, , Multiple Targets in a Rule}).@refill
1627 @cindex tab character (in commands)
1628 The @var{command} lines start with a tab character. The first command may
1629 appear on the line after the prerequisites, with a tab character, or may
1630 appear on the same line, with a semicolon. Either way, the effect is the
1631 same. @xref{Commands, ,Writing the Commands in Rules}.
1633 @cindex dollar sign (@code{$}), in rules
1634 @cindex @code{$}, in rules
1635 @cindex rule, and @code{$}
1636 Because dollar signs are used to start variable references, if you really
1637 want a dollar sign in a rule you must write two of them, @samp{$$}
1638 (@pxref{Using Variables, ,How to Use Variables}).
1639 You may split a long line by inserting a backslash
1640 followed by a newline, but this is not required, as @code{make} places no
1641 limit on the length of a line in a makefile.
1643 A rule tells @code{make} two things: when the targets are out of date,
1644 and how to update them when necessary.
1646 @cindex prerequisites
1647 @cindex rule prerequisites
1648 The criterion for being out of date is specified in terms of the
1649 @var{prerequisites}, which consist of file names separated by spaces.
1650 (Wildcards and archive members (@pxref{Archives}) are allowed here too.)
1651 A target is out of date if it does not exist or if it is older than any
1652 of the prerequisites (by comparison of last-modification times). The
1653 idea is that the contents of the target file are computed based on
1654 information in the prerequisites, so if any of the prerequisites changes,
1655 the contents of the existing target file are no longer necessarily
1658 How to update is specified by @var{commands}. These are lines to be
1659 executed by the shell (normally @samp{sh}), but with some extra features
1660 (@pxref{Commands, ,Writing the Commands in Rules}).
1662 @node Prerequisite Types, Wildcards, Rule Syntax, Rules
1663 @comment node-name, next, previous, up
1664 @section Types of Prerequisites
1665 @cindex prerequisite types
1666 @cindex types of prerequisites
1668 @cindex prerequisites, normal
1669 @cindex normal prerequisites
1670 @cindex prerequisites, order-only
1671 @cindex order-only prerequisites
1672 There are actually two different types of prerequisites understood by
1673 GNU @code{make}: normal prerequisites such as described in the
1674 previous section, and @dfn{order-only} prerequisites. A normal
1675 prerequisite makes two statements: first, it imposes an order of
1676 execution of build commands: any commands necessary to build any of a
1677 target's prerequisites will be fully executed before any commands
1678 necessary to build the target. Second, it imposes a dependency
1679 relationship: if any prerequisite is newer than the target, then the
1680 target is considered out-of-date and must be rebuilt.
1682 Normally, this is exactly what you want: if a target's prerequisite is
1683 updated, then the target should also be updated.
1685 Occasionally, however, you have a situation where you want to impose a
1686 specific ordering on the rules to be invoked @emph{without} forcing
1687 the target to be updated if one of those rules is executed. In that
1688 case, you want to define @dfn{order-only} prerequisites. Order-only
1689 prerequisites can be specified by placing a pipe symbol (@code{|})
1690 in the prerequisites list: any prerequisites to the left of the pipe
1691 symbol are normal; any prerequisites to the right are order-only:
1694 @var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites}
1697 The normal prerequisites section may of course be empty. Also, you
1698 may still declare multiple lines of prerequisites for the same target:
1699 they are appended appropriately. Note that if you declare the same
1700 file to be both a normal and an order-only prerequisite, the normal
1701 prerequisite takes precedence (since they are a strict superset of the
1702 behavior of an order-only prerequisite).
1704 @node Wildcards, Directory Search, Prerequisite Types, Rules
1705 @section Using Wildcard Characters in File Names
1707 @cindex file name with wildcards
1708 @cindex globbing (wildcards)
1710 @cindex @code{*} (wildcard character)
1711 @cindex @code{?} (wildcard character)
1712 @cindex @code{[@dots{}]} (wildcard characters)
1713 A single file name can specify many files using @dfn{wildcard characters}.
1714 The wildcard characters in @code{make} are @samp{*}, @samp{?} and
1715 @samp{[@dots{}]}, the same as in the Bourne shell. For example, @file{*.c}
1716 specifies a list of all the files (in the working directory) whose names
1717 end in @samp{.c}.@refill
1719 @cindex @code{~} (tilde)
1720 @cindex tilde (@code{~})
1721 @cindex home directory
1722 The character @samp{~} at the beginning of a file name also has special
1723 significance. If alone, or followed by a slash, it represents your home
1724 directory. For example @file{~/bin} expands to @file{/home/you/bin}.
1725 If the @samp{~} is followed by a word, the string represents the home
1726 directory of the user named by that word. For example @file{~john/bin}
1727 expands to @file{/home/john/bin}. On systems which don't have a home
1728 directory for each user (such as MS-DOS or MS-Windows), this
1729 functionality can be simulated by setting the environment variable
1732 Wildcard expansion happens automatically in targets, in prerequisites,
1733 and in commands (where the shell does the expansion). In other
1734 contexts, wildcard expansion happens only if you request it explicitly
1735 with the @code{wildcard} function.
1737 The special significance of a wildcard character can be turned off by
1738 preceding it with a backslash. Thus, @file{foo\*bar} would refer to a
1739 specific file whose name consists of @samp{foo}, an asterisk, and
1743 * Wildcard Examples:: Several examples
1744 * Wildcard Pitfall:: Problems to avoid.
1745 * Wildcard Function:: How to cause wildcard expansion where
1746 it does not normally take place.
1749 @node Wildcard Examples, Wildcard Pitfall, Wildcards, Wildcards
1750 @subsection Wildcard Examples
1752 Wildcards can be used in the commands of a rule, where they are expanded
1753 by the shell. For example, here is a rule to delete all the object files:
1761 @cindex @code{rm} (shell command)
1763 Wildcards are also useful in the prerequisites of a rule. With the
1764 following rule in the makefile, @samp{make print} will print all the
1765 @samp{.c} files that have changed since the last time you printed them:
1773 @cindex @code{print} target
1774 @cindex @code{lpr} (shell command)
1775 @cindex @code{touch} (shell command)
1777 This rule uses @file{print} as an empty target file; see @ref{Empty
1778 Targets, ,Empty Target Files to Record Events}. (The automatic variable
1779 @samp{$?} is used to print only those files that have changed; see
1780 @ref{Automatic Variables}.)@refill
1782 Wildcard expansion does not happen when you define a variable. Thus, if
1790 then the value of the variable @code{objects} is the actual string
1791 @samp{*.o}. However, if you use the value of @code{objects} in a target,
1792 prerequisite or command, wildcard expansion will take place at that time.
1793 To set @code{objects} to the expansion, instead use:
1796 objects := $(wildcard *.o)
1800 @xref{Wildcard Function}.
1802 @node Wildcard Pitfall, Wildcard Function, Wildcard Examples, Wildcards
1803 @subsection Pitfalls of Using Wildcards
1804 @cindex wildcard pitfalls
1805 @cindex pitfalls of wildcards
1806 @cindex mistakes with wildcards
1807 @cindex errors with wildcards
1808 @cindex problems with wildcards
1810 Now here is an example of a naive way of using wildcard expansion, that
1811 does not do what you would intend. Suppose you would like to say that the
1812 executable file @file{foo} is made from all the object files in the
1813 directory, and you write this:
1819 cc -o foo $(CFLAGS) $(objects)
1823 The value of @code{objects} is the actual string @samp{*.o}. Wildcard
1824 expansion happens in the rule for @file{foo}, so that each @emph{existing}
1825 @samp{.o} file becomes a prerequisite of @file{foo} and will be recompiled if
1828 But what if you delete all the @samp{.o} files? When a wildcard matches
1829 no files, it is left as it is, so then @file{foo} will depend on the
1830 oddly-named file @file{*.o}. Since no such file is likely to exist,
1831 @code{make} will give you an error saying it cannot figure out how to
1832 make @file{*.o}. This is not what you want!
1834 Actually it is possible to obtain the desired result with wildcard
1835 expansion, but you need more sophisticated techniques, including the
1836 @code{wildcard} function and string substitution.
1838 @xref{Wildcard Function, ,The Function @code{wildcard}}.
1841 These are described in the following section.
1844 @cindex wildcards and MS-DOS/MS-Windows backslashes
1845 @cindex backslashes in pathnames and wildcard expansion
1847 Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to
1848 separate directories in pathnames, like so:
1854 This is equivalent to the Unix-style @file{c:/foo/bar/baz.c} (the
1855 @file{c:} part is the so-called drive letter). When @code{make} runs on
1856 these systems, it supports backslashes as well as the Unix-style forward
1857 slashes in pathnames. However, this support does @emph{not} include the
1858 wildcard expansion, where backslash is a quote character. Therefore,
1859 you @emph{must} use Unix-style slashes in these cases.
1862 @node Wildcard Function, , Wildcard Pitfall, Wildcards
1863 @subsection The Function @code{wildcard}
1866 Wildcard expansion happens automatically in rules. But wildcard expansion
1867 does not normally take place when a variable is set, or inside the
1868 arguments of a function. If you want to do wildcard expansion in such
1869 places, you need to use the @code{wildcard} function, like this:
1872 $(wildcard @var{pattern}@dots{})
1876 This string, used anywhere in a makefile, is replaced by a
1877 space-separated list of names of existing files that match one of the
1878 given file name patterns. If no existing file name matches a pattern,
1879 then that pattern is omitted from the output of the @code{wildcard}
1880 function. Note that this is different from how unmatched wildcards
1881 behave in rules, where they are used verbatim rather than ignored
1882 (@pxref{Wildcard Pitfall}).
1884 One use of the @code{wildcard} function is to get a list of all the C source
1885 files in a directory, like this:
1891 We can change the list of C source files into a list of object files by
1892 replacing the @samp{.c} suffix with @samp{.o} in the result, like this:
1895 $(patsubst %.c,%.o,$(wildcard *.c))
1899 (Here we have used another function, @code{patsubst}.
1900 @xref{Text Functions, ,Functions for String Substitution and Analysis}.)@refill
1902 Thus, a makefile to compile all C source files in the directory and then
1903 link them together could be written as follows:
1906 objects := $(patsubst %.c,%.o,$(wildcard *.c))
1909 cc -o foo $(objects)
1913 (This takes advantage of the implicit rule for compiling C programs, so
1914 there is no need to write explicit rules for compiling the files.
1915 @xref{Flavors, ,The Two Flavors of Variables}, for an explanation of
1916 @samp{:=}, which is a variant of @samp{=}.)
1918 @node Directory Search, Phony Targets, Wildcards, Rules
1919 @section Searching Directories for Prerequisites
1923 @cindex search path for prerequisites (@code{VPATH})
1924 @cindex directory search (@code{VPATH})
1926 For large systems, it is often desirable to put sources in a separate
1927 directory from the binaries. The @dfn{directory search} features of
1928 @code{make} facilitate this by searching several directories
1929 automatically to find a prerequisite. When you redistribute the files
1930 among directories, you do not need to change the individual rules,
1931 just the search paths.
1934 * General Search:: Specifying a search path that applies
1935 to every prerequisite.
1936 * Selective Search:: Specifying a search path
1937 for a specified class of names.
1938 * Search Algorithm:: When and how search paths are applied.
1939 * Commands/Search:: How to write shell commands that work together
1941 * Implicit/Search:: How search paths affect implicit rules.
1942 * Libraries/Search:: Directory search for link libraries.
1945 @node General Search, Selective Search, Directory Search, Directory Search
1946 @subsection @code{VPATH}: Search Path for All Prerequisites
1949 The value of the @code{make} variable @code{VPATH} specifies a list of
1950 directories that @code{make} should search. Most often, the
1951 directories are expected to contain prerequisite files that are not in the
1952 current directory; however, @code{make} uses @code{VPATH} as a search
1953 list for both prerequisites and targets of rules.
1955 Thus, if a file that is listed as a target or prerequisite does not exist
1956 in the current directory, @code{make} searches the directories listed in
1957 @code{VPATH} for a file with that name. If a file is found in one of
1958 them, that file may become the prerequisite (see below). Rules may then
1959 specify the names of files in the prerequisite list as if they all
1960 existed in the current directory. @xref{Commands/Search, ,Writing Shell
1961 Commands with Directory Search}.
1963 In the @code{VPATH} variable, directory names are separated by colons or
1964 blanks. The order in which directories are listed is the order followed
1965 by @code{make} in its search. (On MS-DOS and MS-Windows, semi-colons
1966 are used as separators of directory names in @code{VPATH}, since the
1967 colon can be used in the pathname itself, after the drive letter.)
1972 VPATH = src:../headers
1976 specifies a path containing two directories, @file{src} and
1977 @file{../headers}, which @code{make} searches in that order.
1979 With this value of @code{VPATH}, the following rule,
1986 is interpreted as if it were written like this:
1993 assuming the file @file{foo.c} does not exist in the current directory but
1994 is found in the directory @file{src}.
1996 @node Selective Search, Search Algorithm, General Search, Directory Search
1997 @subsection The @code{vpath} Directive
2000 Similar to the @code{VPATH} variable, but more selective, is the
2001 @code{vpath} directive (note lower case), which allows you to specify a
2002 search path for a particular class of file names: those that match a
2003 particular pattern. Thus you can supply certain search directories for
2004 one class of file names and other directories (or none) for other file
2007 There are three forms of the @code{vpath} directive:
2010 @item vpath @var{pattern} @var{directories}
2011 Specify the search path @var{directories} for file names that match
2014 The search path, @var{directories}, is a list of directories to be
2015 searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or
2016 blanks, just like the search path used in the @code{VPATH} variable.
2018 @item vpath @var{pattern}
2019 Clear out the search path associated with @var{pattern}.
2021 @c Extra blank line makes sure this gets two lines.
2024 Clear all search paths previously specified with @code{vpath} directives.
2027 A @code{vpath} pattern is a string containing a @samp{%} character. The
2028 string must match the file name of a prerequisite that is being searched
2029 for, the @samp{%} character matching any sequence of zero or more
2030 characters (as in pattern rules; @pxref{Pattern Rules, ,Defining and
2031 Redefining Pattern Rules}). For example, @code{%.h} matches files that
2032 end in @code{.h}. (If there is no @samp{%}, the pattern must match the
2033 prerequisite exactly, which is not useful very often.)
2035 @cindex @code{%}, quoting in @code{vpath}
2036 @cindex @code{%}, quoting with @code{\} (backslash)
2037 @cindex @code{\} (backslash), to quote @code{%}
2038 @cindex backslash (@code{\}), to quote @code{%}
2039 @cindex quoting @code{%}, in @code{vpath}
2040 @samp{%} characters in a @code{vpath} directive's pattern can be quoted
2041 with preceding backslashes (@samp{\}). Backslashes that would otherwise
2042 quote @samp{%} characters can be quoted with more backslashes.
2043 Backslashes that quote @samp{%} characters or other backslashes are
2044 removed from the pattern before it is compared to file names. Backslashes
2045 that are not in danger of quoting @samp{%} characters go unmolested.@refill
2047 When a prerequisite fails to exist in the current directory, if the
2048 @var{pattern} in a @code{vpath} directive matches the name of the
2049 prerequisite file, then the @var{directories} in that directive are searched
2050 just like (and before) the directories in the @code{VPATH} variable.
2055 vpath %.h ../headers
2059 tells @code{make} to look for any prerequisite whose name ends in @file{.h}
2060 in the directory @file{../headers} if the file is not found in the current
2063 If several @code{vpath} patterns match the prerequisite file's name, then
2064 @code{make} processes each matching @code{vpath} directive one by one,
2065 searching all the directories mentioned in each directive. @code{make}
2066 handles multiple @code{vpath} directives in the order in which they
2067 appear in the makefile; multiple directives with the same pattern are
2068 independent of each other.
2082 will look for a file ending in @samp{.c} in @file{foo}, then
2083 @file{blish}, then @file{bar}, while
2093 will look for a file ending in @samp{.c} in @file{foo}, then
2094 @file{bar}, then @file{blish}.
2096 @node Search Algorithm, Commands/Search, Selective Search, Directory Search
2097 @subsection How Directory Searches are Performed
2098 @cindex algorithm for directory search
2099 @cindex directory search algorithm
2101 When a prerequisite is found through directory search, regardless of type
2102 (general or selective), the pathname located may not be the one that
2103 @code{make} actually provides you in the prerequisite list. Sometimes
2104 the path discovered through directory search is thrown away.
2106 The algorithm @code{make} uses to decide whether to keep or abandon a
2107 path found via directory search is as follows:
2111 If a target file does not exist at the path specified in the makefile,
2112 directory search is performed.
2115 If the directory search is successful, that path is kept and this file
2116 is tentatively stored as the target.
2119 All prerequisites of this target are examined using this same method.
2122 After processing the prerequisites, the target may or may not need to be
2127 If the target does @emph{not} need to be rebuilt, the path to the file
2128 found during directory search is used for any prerequisite lists which
2129 contain this target. In short, if @code{make} doesn't need to rebuild
2130 the target then you use the path found via directory search.
2133 If the target @emph{does} need to be rebuilt (is out-of-date), the
2134 pathname found during directory search is @emph{thrown away}, and the
2135 target is rebuilt using the file name specified in the makefile. In
2136 short, if @code{make} must rebuild, then the target is rebuilt locally,
2137 not in the directory found via directory search.
2141 This algorithm may seem complex, but in practice it is quite often
2142 exactly what you want.
2144 @cindex traditional directory search (GPATH)
2145 @cindex directory search, traditional (GPATH)
2146 Other versions of @code{make} use a simpler algorithm: if the file does
2147 not exist, and it is found via directory search, then that pathname is
2148 always used whether or not the target needs to be built. Thus, if the
2149 target is rebuilt it is created at the pathname discovered during
2153 If, in fact, this is the behavior you want for some or all of your
2154 directories, you can use the @code{GPATH} variable to indicate this to
2157 @code{GPATH} has the same syntax and format as @code{VPATH} (that is, a
2158 space- or colon-delimited list of pathnames). If an out-of-date target
2159 is found by directory search in a directory that also appears in
2160 @code{GPATH}, then that pathname is not thrown away. The target is
2161 rebuilt using the expanded path.
2163 @node Commands/Search, Implicit/Search, Search Algorithm, Directory Search
2164 @subsection Writing Shell Commands with Directory Search
2165 @cindex shell command, and directory search
2166 @cindex directory search (@code{VPATH}), and shell commands
2168 When a prerequisite is found in another directory through directory search,
2169 this cannot change the commands of the rule; they will execute as written.
2170 Therefore, you must write the commands with care so that they will look for
2171 the prerequisite in the directory where @code{make} finds it.
2173 This is done with the @dfn{automatic variables} such as @samp{$^}
2174 (@pxref{Automatic Variables}).
2175 For instance, the value of @samp{$^} is a
2176 list of all the prerequisites of the rule, including the names of
2177 the directories in which they were found, and the value of
2178 @samp{$@@} is the target. Thus:@refill
2182 cc -c $(CFLAGS) $^ -o $@@
2186 (The variable @code{CFLAGS} exists so you can specify flags for C
2187 compilation by implicit rules; we use it here for consistency so it will
2188 affect all C compilations uniformly;
2189 @pxref{Implicit Variables, ,Variables Used by Implicit Rules}.)
2191 Often the prerequisites include header files as well, which you do not
2192 want to mention in the commands. The automatic variable @samp{$<} is
2193 just the first prerequisite:
2196 VPATH = src:../headers
2197 foo.o : foo.c defs.h hack.h
2198 cc -c $(CFLAGS) $< -o $@@
2201 @node Implicit/Search, Libraries/Search, Commands/Search, Directory Search
2202 @subsection Directory Search and Implicit Rules
2203 @cindex @code{VPATH}, and implicit rules
2204 @cindex directory search (@code{VPATH}), and implicit rules
2205 @cindex search path for prerequisites (@code{VPATH}), and implicit rules
2206 @cindex implicit rule, and directory search
2207 @cindex implicit rule, and @code{VPATH}
2208 @cindex rule, implicit, and directory search
2209 @cindex rule, implicit, and @code{VPATH}
2211 The search through the directories specified in @code{VPATH} or with
2212 @code{vpath} also happens during consideration of implicit rules
2213 (@pxref{Implicit Rules, ,Using Implicit Rules}).
2215 For example, when a file @file{foo.o} has no explicit rule, @code{make}
2216 considers implicit rules, such as the built-in rule to compile
2217 @file{foo.c} if that file exists. If such a file is lacking in the
2218 current directory, the appropriate directories are searched for it. If
2219 @file{foo.c} exists (or is mentioned in the makefile) in any of the
2220 directories, the implicit rule for C compilation is applied.
2222 The commands of implicit rules normally use automatic variables as a
2223 matter of necessity; consequently they will use the file names found by
2224 directory search with no extra effort.
2226 @node Libraries/Search, , Implicit/Search, Directory Search
2227 @subsection Directory Search for Link Libraries
2228 @cindex link libraries, and directory search
2229 @cindex libraries for linking, directory search
2230 @cindex directory search (@code{VPATH}), and link libraries
2231 @cindex @code{VPATH}, and link libraries
2232 @cindex search path for prerequisites (@code{VPATH}), and link libraries
2233 @cindex @code{-l} (library search)
2234 @cindex link libraries, patterns matching
2235 @cindex @code{.LIBPATTERNS}, and link libraries
2236 @vindex .LIBPATTERNS
2238 Directory search applies in a special way to libraries used with the
2239 linker. This special feature comes into play when you write a prerequisite
2240 whose name is of the form @samp{-l@var{name}}. (You can tell something
2241 strange is going on here because the prerequisite is normally the name of a
2242 file, and the @emph{file name} of a library generally looks like
2243 @file{lib@var{name}.a}, not like @samp{-l@var{name}}.)@refill
2245 When a prerequisite's name has the form @samp{-l@var{name}}, @code{make}
2246 handles it specially by searching for the file @file{lib@var{name}.so} in
2247 the current directory, in directories specified by matching @code{vpath}
2248 search paths and the @code{VPATH} search path, and then in the
2249 directories @file{/lib}, @file{/usr/lib}, and @file{@var{prefix}/lib}
2250 (normally @file{/usr/local/lib}, but MS-DOS/MS-Windows versions of
2251 @code{make} behave as if @var{prefix} is defined to be the root of the
2252 DJGPP installation tree).
2254 If that file is not found, then the file @file{lib@var{name}.a} is
2255 searched for, in the same directories as above.
2257 For example, if there is a @file{/usr/lib/libcurses.a} library on your
2258 system (and no @file{/usr/lib/libcurses.so} file), then
2262 foo : foo.c -lcurses
2268 would cause the command @samp{cc foo.c /usr/lib/libcurses.a -o foo} to
2269 be executed when @file{foo} is older than @file{foo.c} or than
2270 @file{/usr/lib/libcurses.a}.@refill
2272 Although the default set of files to be searched for is
2273 @file{lib@var{name}.so} and @file{lib@var{name}.a}, this is customizable
2274 via the @code{.LIBPATTERNS} variable. Each word in the value of this
2275 variable is a pattern string. When a prerequisite like
2276 @samp{-l@var{name}} is seen, @code{make} will replace the percent in
2277 each pattern in the list with @var{name} and perform the above directory
2278 searches using that library filename. If no library is found, the next
2279 word in the list will be used.
2281 The default value for @code{.LIBPATTERNS} is ``@samp{lib%.so lib%.a}'',
2282 which provides the default behavior described above.
2284 You can turn off link library expansion completely by setting this
2285 variable to an empty value.
2287 @node Phony Targets, Force Targets, Directory Search, Rules
2288 @section Phony Targets
2289 @cindex phony targets
2290 @cindex targets, phony
2291 @cindex targets without a file
2293 A phony target is one that is not really the name of a file. It is just a
2294 name for some commands to be executed when you make an explicit request.
2295 There are two reasons to use a phony target: to avoid a conflict with
2296 a file of the same name, and to improve performance.
2298 If you write a rule whose commands will not create the target file, the
2299 commands will be executed every time the target comes up for remaking.
2310 Because the @code{rm} command does not create a file named @file{clean},
2311 probably no such file will ever exist. Therefore, the @code{rm} command
2312 will be executed every time you say @samp{make clean}.
2313 @cindex @code{rm} (shell command)
2316 The phony target will cease to work if anything ever does create a file
2317 named @file{clean} in this directory. Since it has no prerequisites, the
2318 file @file{clean} would inevitably be considered up to date, and its
2319 commands would not be executed. To avoid this problem, you can explicitly
2320 declare the target to be phony, using the special target @code{.PHONY}
2321 (@pxref{Special Targets, ,Special Built-in Target Names}) as follows:
2328 Once this is done, @samp{make clean} will run the commands regardless of
2329 whether there is a file named @file{clean}.
2331 Since it knows that phony targets do not name actual files that could be
2332 remade from other files, @code{make} skips the implicit rule search for
2333 phony targets (@pxref{Implicit Rules}). This is why declaring a target
2334 phony is good for performance, even if you are not worried about the
2335 actual file existing.
2337 Thus, you first write the line that states that @code{clean} is a
2338 phony target, then you write the rule, like this:
2348 Another example of the usefulness of phony targets is in conjunction
2349 with recursive invocations of @code{make} (for more information, see
2350 @ref{Recursion, ,Recursive Use of @code{make}}). In this case the
2351 makefile will often contain a variable which lists a number of
2352 subdirectories to be built. One way to handle this is with one rule
2353 whose command is a shell loop over the subdirectories, like this:
2357 SUBDIRS = foo bar baz
2360 for dir in $(SUBDIRS); do \
2366 There are a few problems with this method, however. First, any error
2367 detected in a submake is not noted by this rule, so it will continue to
2368 build the rest of the directories even when one fails. This can be
2369 overcome by adding shell commands to note the error and exit, but then
2370 it will do so even if @code{make} is invoked with the @code{-k} option,
2371 which is unfortunate. Second, and perhaps more importantly, you cannot
2372 take advantage of the parallel build capabilities of make using this
2373 method, since there is only one rule.
2375 By declaring the subdirectories as phony targets (you must do this as
2376 the subdirectory obviously always exists; otherwise it won't be built)
2377 you can remove these problems:
2381 SUBDIRS = foo bar baz
2383 .PHONY: subdirs $(SUBDIRS)
2394 Here we've also declared that the @file{foo} subdirectory cannot be
2395 built until after the @file{baz} subdirectory is complete; this kind of
2396 relationship declaration is particularly important when attempting
2399 A phony target should not be a prerequisite of a real target file; if it
2400 is, its commands are run every time @code{make} goes to update that
2401 file. As long as a phony target is never a prerequisite of a real
2402 target, the phony target commands will be executed only when the phony
2403 target is a specified goal (@pxref{Goals, ,Arguments to Specify the
2406 Phony targets can have prerequisites. When one directory contains multiple
2407 programs, it is most convenient to describe all of the programs in one
2408 makefile @file{./Makefile}. Since the target remade by default will be the
2409 first one in the makefile, it is common to make this a phony target named
2410 @samp{all} and give it, as prerequisites, all the individual programs. For
2414 all : prog1 prog2 prog3
2417 prog1 : prog1.o utils.o
2418 cc -o prog1 prog1.o utils.o
2423 prog3 : prog3.o sort.o utils.o
2424 cc -o prog3 prog3.o sort.o utils.o
2428 Now you can say just @samp{make} to remake all three programs, or specify
2429 as arguments the ones to remake (as in @samp{make prog1 prog3}).
2431 When one phony target is a prerequisite of another, it serves as a subroutine
2432 of the other. For example, here @samp{make cleanall} will delete the
2433 object files, the difference files, and the file @file{program}:
2436 .PHONY: cleanall cleanobj cleandiff
2438 cleanall : cleanobj cleandiff
2448 @node Force Targets, Empty Targets, Phony Targets, Rules
2449 @section Rules without Commands or Prerequisites
2450 @cindex force targets
2451 @cindex targets, force
2452 @cindex @code{FORCE}
2453 @cindex rule, no commands or prerequisites
2455 If a rule has no prerequisites or commands, and the target of the rule
2456 is a nonexistent file, then @code{make} imagines this target to have
2457 been updated whenever its rule is run. This implies that all targets
2458 depending on this one will always have their commands run.
2460 An example will illustrate this:
2470 Here the target @samp{FORCE} satisfies the special conditions, so the
2471 target @file{clean} that depends on it is forced to run its commands.
2472 There is nothing special about the name @samp{FORCE}, but that is one name
2473 commonly used this way.
2475 As you can see, using @samp{FORCE} this way has the same results as using
2476 @samp{.PHONY: clean}.
2478 Using @samp{.PHONY} is more explicit and more efficient. However,
2479 other versions of @code{make} do not support @samp{.PHONY}; thus
2480 @samp{FORCE} appears in many makefiles. @xref{Phony Targets}.
2482 @node Empty Targets, Special Targets, Force Targets, Rules
2483 @section Empty Target Files to Record Events
2484 @cindex empty targets
2485 @cindex targets, empty
2486 @cindex recording events with empty targets
2488 The @dfn{empty target} is a variant of the phony target; it is used to hold
2489 commands for an action that you request explicitly from time to time.
2490 Unlike a phony target, this target file can really exist; but the file's
2491 contents do not matter, and usually are empty.
2493 The purpose of the empty target file is to record, with its
2494 last-modification time, when the rule's commands were last executed. It
2495 does so because one of the commands is a @code{touch} command to update the
2498 The empty target file should have some prerequisites (otherwise it
2499 doesn't make sense). When you ask to remake the empty target, the
2500 commands are executed if any prerequisite is more recent than the target;
2501 in other words, if a prerequisite has changed since the last time you
2502 remade the target. Here is an example:
2509 @cindex @code{print} target
2510 @cindex @code{lpr} (shell command)
2511 @cindex @code{touch} (shell command)
2514 With this rule, @samp{make print} will execute the @code{lpr} command if
2515 either source file has changed since the last @samp{make print}. The
2516 automatic variable @samp{$?} is used to print only those files that have
2517 changed (@pxref{Automatic Variables}).
2519 @node Special Targets, Multiple Targets, Empty Targets, Rules
2520 @section Special Built-in Target Names
2521 @cindex special targets
2522 @cindex built-in special targets
2523 @cindex targets, built-in special
2525 Certain names have special meanings if they appear as targets.
2531 The prerequisites of the special target @code{.PHONY} are considered to
2532 be phony targets. When it is time to consider such a target,
2533 @code{make} will run its commands unconditionally, regardless of
2534 whether a file with that name exists or what its last-modification
2535 time is. @xref{Phony Targets, ,Phony Targets}.
2540 The prerequisites of the special target @code{.SUFFIXES} are the list
2541 of suffixes to be used in checking for suffix rules.
2542 @xref{Suffix Rules, , Old-Fashioned Suffix Rules}.
2547 The commands specified for @code{.DEFAULT} are used for any target for
2548 which no rules are found (either explicit rules or implicit rules).
2549 @xref{Last Resort}. If @code{.DEFAULT} commands are specified, every
2550 file mentioned as a prerequisite, but not as a target in a rule, will have
2551 these commands executed on its behalf. @xref{Implicit Rule Search,
2552 ,Implicit Rule Search Algorithm}.
2556 @cindex precious targets
2557 @cindex preserving with @code{.PRECIOUS}
2559 The targets which @code{.PRECIOUS} depends on are given the following
2560 special treatment: if @code{make} is killed or interrupted during the
2561 execution of their commands, the target is not deleted.
2562 @xref{Interrupts, ,Interrupting or Killing @code{make}}. Also, if the
2563 target is an intermediate file, it will not be deleted after it is no
2564 longer needed, as is normally done. @xref{Chained Rules, ,Chains of
2565 Implicit Rules}. In this latter respect it overlaps with the
2566 @code{.SECONDARY} special target.
2568 You can also list the target pattern of an implicit rule (such as
2569 @samp{%.o}) as a prerequisite file of the special target @code{.PRECIOUS}
2570 to preserve intermediate files created by rules whose target patterns
2571 match that file's name.
2573 @findex .INTERMEDIATE
2575 @cindex intermediate targets, explicit
2577 The targets which @code{.INTERMEDIATE} depends on are treated as
2578 intermediate files. @xref{Chained Rules, ,Chains of Implicit Rules}.
2579 @code{.INTERMEDIATE} with no prerequisites has no effect.
2583 @cindex secondary targets
2584 @cindex preserving with @code{.SECONDARY}
2586 The targets which @code{.SECONDARY} depends on are treated as
2587 intermediate files, except that they are never automatically deleted.
2588 @xref{Chained Rules, ,Chains of Implicit Rules}.
2590 @code{.SECONDARY} with no prerequisites causes all targets to be treated
2591 as secondary (i.e., no target is removed because it is considered
2594 @findex .DELETE_ON_ERROR
2595 @item .DELETE_ON_ERROR
2596 @cindex removing targets on failure
2598 If @code{.DELETE_ON_ERROR} is mentioned as a target anywhere in the
2599 makefile, then @code{make} will delete the target of a rule if it has
2600 changed and its commands exit with a nonzero exit status, just as it
2601 does when it receives a signal. @xref{Errors, ,Errors in Commands}.
2606 If you specify prerequisites for @code{.IGNORE}, then @code{make} will
2607 ignore errors in execution of the commands run for those particular
2608 files. The commands for @code{.IGNORE} are not meaningful.
2610 If mentioned as a target with no prerequisites, @code{.IGNORE} says to
2611 ignore errors in execution of commands for all files. This usage of
2612 @samp{.IGNORE} is supported only for historical compatibility. Since
2613 this affects every command in the makefile, it is not very useful; we
2614 recommend you use the more selective ways to ignore errors in specific
2615 commands. @xref{Errors, ,Errors in Commands}.
2617 @findex .LOW_RESOLUTION_TIME
2618 @item .LOW_RESOLUTION_TIME
2620 If you specify prerequisites for @code{.LOW_RESOLUTION_TIME},
2621 @command{make} assumes that these files are created by commands that
2622 generate low resolution time stamps. The commands for
2623 @code{.LOW_RESOLUTION_TIME} are not meaningful.
2625 The high resolution file time stamps of many modern hosts lessen the
2626 chance of @command{make} incorrectly concluding that a file is up to
2627 date. Unfortunately, these hosts provide no way to set a high
2628 resolution file time stamp, so commands like @samp{cp -p} that
2629 explicitly set a file's time stamp must discard its subsecond part. If
2630 a file is created by such a command, you should list it as a
2631 prerequisite of @code{.LOW_RESOLUTION_TIME} so that @command{make} does
2632 not mistakenly conclude that the file is out of date. For example:
2636 .LOW_RESOLUTION_TIME: dst
2642 Since @samp{cp -p} discards the subsecond part of @file{src}'s time
2643 stamp, @file{dst} is typically slightly older than @file{src} even when
2644 it is up to date. The @code{.LOW_RESOLUTION_TIME} line causes
2645 @command{make} to consider @file{dst} to be up to date if its time stamp
2646 is at the start of the same second that @file{src}'s time stamp is in.
2648 Due to a limitation of the archive format, archive member time stamps
2649 are always low resolution. You need not list archive members as
2650 prerequisites of @code{.LOW_RESOLUTION_TIME}, as @command{make} does this
2656 If you specify prerequisites for @code{.SILENT}, then @code{make} will
2657 not print the commands to remake those particular files before executing
2658 them. The commands for @code{.SILENT} are not meaningful.
2660 If mentioned as a target with no prerequisites, @code{.SILENT} says not
2661 to print any commands before executing them. This usage of
2662 @samp{.SILENT} is supported only for historical compatibility. We
2663 recommend you use the more selective ways to silence specific commands.
2664 @xref{Echoing, ,Command Echoing}. If you want to silence all commands
2665 for a particular run of @code{make}, use the @samp{-s} or
2666 @w{@samp{--silent}} option (@pxref{Options Summary}).
2668 @findex .EXPORT_ALL_VARIABLES
2669 @item .EXPORT_ALL_VARIABLES
2671 Simply by being mentioned as a target, this tells @code{make} to
2672 export all variables to child processes by default.
2673 @xref{Variables/Recursion, ,Communicating Variables to a
2676 @findex .NOTPARALLEL
2678 @cindex parallel execution, overriding
2680 If @code{.NOTPARALLEL} is mentioned as a target, then this invocation of
2681 @code{make} will be run serially, even if the @samp{-j} option is
2682 given. Any recursively invoked @code{make} command will still be run in
2683 parallel (unless its makefile contains this target). Any prerequisites
2684 on this target are ignored.
2687 Any defined implicit rule suffix also counts as a special target if it
2688 appears as a target, and so does the concatenation of two suffixes, such
2689 as @samp{.c.o}. These targets are suffix rules, an obsolete way of
2690 defining implicit rules (but a way still widely used). In principle, any
2691 target name could be special in this way if you break it in two and add
2692 both pieces to the suffix list. In practice, suffixes normally begin with
2693 @samp{.}, so these special target names also begin with @samp{.}.
2694 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
2696 @node Multiple Targets, Multiple Rules, Special Targets, Rules
2697 @section Multiple Targets in a Rule
2698 @cindex multiple targets
2699 @cindex several targets in a rule
2700 @cindex targets, multiple
2701 @cindex rule, with multiple targets
2703 A rule with multiple targets is equivalent to writing many rules, each with
2704 one target, and all identical aside from that. The same commands apply to
2705 all the targets, but their effects may vary because you can substitute the
2706 actual target name into the command using @samp{$@@}. The rule contributes
2707 the same prerequisites to all the targets also.
2709 This is useful in two cases.
2713 You want just prerequisites, no commands. For example:
2716 kbd.o command.o files.o: command.h
2720 gives an additional prerequisite to each of the three object files
2724 Similar commands work for all the targets. The commands do not need
2725 to be absolutely identical, since the automatic variable @samp{$@@}
2726 can be used to substitute the particular target to be remade into the
2727 commands (@pxref{Automatic Variables}). For example:
2731 bigoutput littleoutput : text.g
2732 generate text.g -$(subst output,,$@@) > $@@
2742 generate text.g -big > bigoutput
2743 littleoutput : text.g
2744 generate text.g -little > littleoutput
2748 Here we assume the hypothetical program @code{generate} makes two
2749 types of output, one if given @samp{-big} and one if given
2751 @xref{Text Functions, ,Functions for String Substitution and Analysis},
2752 for an explanation of the @code{subst} function.
2755 Suppose you would like to vary the prerequisites according to the target,
2756 much as the variable @samp{$@@} allows you to vary the commands.
2757 You cannot do this with multiple targets in an ordinary rule, but you can
2758 do it with a @dfn{static pattern rule}.
2759 @xref{Static Pattern, ,Static Pattern Rules}.
2761 @node Multiple Rules, Static Pattern, Multiple Targets, Rules
2762 @section Multiple Rules for One Target
2763 @cindex multiple rules for one target
2764 @cindex several rules for one target
2765 @cindex rule, multiple for one target
2766 @cindex target, multiple rules for one
2768 One file can be the target of several rules. All the prerequisites
2769 mentioned in all the rules are merged into one list of prerequisites for
2770 the target. If the target is older than any prerequisite from any rule,
2771 the commands are executed.
2773 There can only be one set of commands to be executed for a file. If
2774 more than one rule gives commands for the same file, @code{make} uses
2775 the last set given and prints an error message. (As a special case,
2776 if the file's name begins with a dot, no error message is printed.
2777 This odd behavior is only for compatibility with other implementations
2778 of @code{make}... you should avoid using it). Occasionally it is
2779 useful to have the same target invoke multiple commands which are
2780 defined in different parts of your makefile; you can use
2781 @dfn{double-colon rules} (@pxref{Double-Colon}) for this.
2783 An extra rule with just prerequisites can be used to give a few extra
2784 prerequisites to many files at once. For example, makefiles often
2785 have a variable, such as @code{objects}, containing a list of all the
2786 compiler output files in the system being made. An easy way to say
2787 that all of them must be recompiled if @file{config.h} changes is to
2788 write the following:
2791 objects = foo.o bar.o
2793 bar.o : defs.h test.h
2794 $(objects) : config.h
2797 This could be inserted or taken out without changing the rules that really
2798 specify how to make the object files, making it a convenient form to use if
2799 you wish to add the additional prerequisite intermittently.
2801 Another wrinkle is that the additional prerequisites could be specified with
2802 a variable that you set with a command argument to @code{make}
2803 (@pxref{Overriding, ,Overriding Variables}). For example,
2808 $(objects) : $(extradeps)
2813 means that the command @samp{make extradeps=foo.h} will consider
2814 @file{foo.h} as a prerequisite of each object file, but plain @samp{make}
2817 If none of the explicit rules for a target has commands, then @code{make}
2818 searches for an applicable implicit rule to find some commands
2819 @pxref{Implicit Rules, ,Using Implicit Rules}).
2821 @node Static Pattern, Double-Colon, Multiple Rules, Rules
2822 @section Static Pattern Rules
2823 @cindex static pattern rule
2824 @cindex rule, static pattern
2825 @cindex pattern rules, static (not implicit)
2826 @cindex varying prerequisites
2827 @cindex prerequisites, varying (static pattern)
2829 @dfn{Static pattern rules} are rules which specify multiple targets and
2830 construct the prerequisite names for each target based on the target name.
2831 They are more general than ordinary rules with multiple targets because the
2832 targets do not have to have identical prerequisites. Their prerequisites must
2833 be @emph{analogous}, but not necessarily @emph{identical}.
2836 * Static Usage:: The syntax of static pattern rules.
2837 * Static versus Implicit:: When are they better than implicit rules?
2840 @node Static Usage, Static versus Implicit, Static Pattern, Static Pattern
2841 @subsection Syntax of Static Pattern Rules
2842 @cindex static pattern rule, syntax of
2843 @cindex pattern rules, static, syntax of
2845 Here is the syntax of a static pattern rule:
2848 @var{targets} @dots{}: @var{target-pattern}: @var{prereq-patterns} @dots{}
2854 The @var{targets} list specifies the targets that the rule applies to.
2855 The targets can contain wildcard characters, just like the targets of
2856 ordinary rules (@pxref{Wildcards, ,Using Wildcard Characters in File
2859 @cindex target pattern, static (not implicit)
2861 The @var{target-pattern} and @var{prereq-patterns} say how to compute the
2862 prerequisites of each target. Each target is matched against the
2863 @var{target-pattern} to extract a part of the target name, called the
2864 @dfn{stem}. This stem is substituted into each of the @var{prereq-patterns}
2865 to make the prerequisite names (one from each @var{prereq-pattern}).
2867 Each pattern normally contains the character @samp{%} just once. When the
2868 @var{target-pattern} matches a target, the @samp{%} can match any part of
2869 the target name; this part is called the @dfn{stem}. The rest of the
2870 pattern must match exactly. For example, the target @file{foo.o} matches
2871 the pattern @samp{%.o}, with @samp{foo} as the stem. The targets
2872 @file{foo.c} and @file{foo.out} do not match that pattern.@refill
2874 @cindex prerequisite pattern, static (not implicit)
2875 The prerequisite names for each target are made by substituting the stem
2876 for the @samp{%} in each prerequisite pattern. For example, if one
2877 prerequisite pattern is @file{%.c}, then substitution of the stem
2878 @samp{foo} gives the prerequisite name @file{foo.c}. It is legitimate
2879 to write a prerequisite pattern that does not contain @samp{%}; then this
2880 prerequisite is the same for all targets.
2882 @cindex @code{%}, quoting in static pattern
2883 @cindex @code{%}, quoting with @code{\} (backslash)
2884 @cindex @code{\} (backslash), to quote @code{%}
2885 @cindex backslash (@code{\}), to quote @code{%}
2886 @cindex quoting @code{%}, in static pattern
2887 @samp{%} characters in pattern rules can be quoted with preceding
2888 backslashes (@samp{\}). Backslashes that would otherwise quote @samp{%}
2889 characters can be quoted with more backslashes. Backslashes that quote
2890 @samp{%} characters or other backslashes are removed from the pattern
2891 before it is compared to file names or has a stem substituted into it.
2892 Backslashes that are not in danger of quoting @samp{%} characters go
2893 unmolested. For example, the pattern @file{the\%weird\\%pattern\\} has
2894 @samp{the%weird\} preceding the operative @samp{%} character, and
2895 @samp{pattern\\} following it. The final two backslashes are left alone
2896 because they cannot affect any @samp{%} character.@refill
2898 Here is an example, which compiles each of @file{foo.o} and @file{bar.o}
2899 from the corresponding @file{.c} file:
2903 objects = foo.o bar.o
2907 $(objects): %.o: %.c
2908 $(CC) -c $(CFLAGS) $< -o $@@
2913 Here @samp{$<} is the automatic variable that holds the name of the
2914 prerequisite and @samp{$@@} is the automatic variable that holds the name
2915 of the target; see @ref{Automatic Variables}.
2917 Each target specified must match the target pattern; a warning is issued
2918 for each target that does not. If you have a list of files, only some of
2919 which will match the pattern, you can use the @code{filter} function to
2920 remove nonmatching file names (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):
2923 files = foo.elc bar.o lose.o
2925 $(filter %.o,$(files)): %.o: %.c
2926 $(CC) -c $(CFLAGS) $< -o $@@
2927 $(filter %.elc,$(files)): %.elc: %.el
2928 emacs -f batch-byte-compile $<
2932 In this example the result of @samp{$(filter %.o,$(files))} is
2933 @file{bar.o lose.o}, and the first static pattern rule causes each of
2934 these object files to be updated by compiling the corresponding C source
2935 file. The result of @w{@samp{$(filter %.elc,$(files))}} is
2936 @file{foo.elc}, so that file is made from @file{foo.el}.@refill
2938 Another example shows how to use @code{$*} in static pattern rules:
2939 @vindex $*@r{, and static pattern}
2943 bigoutput littleoutput : %output : text.g
2944 generate text.g -$* > $@@
2949 When the @code{generate} command is run, @code{$*} will expand to the
2950 stem, either @samp{big} or @samp{little}.
2952 @node Static versus Implicit, , Static Usage, Static Pattern
2953 @subsection Static Pattern Rules versus Implicit Rules
2954 @cindex rule, static pattern versus implicit
2955 @cindex static pattern rule, versus implicit
2957 A static pattern rule has much in common with an implicit rule defined as a
2958 pattern rule (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).
2959 Both have a pattern for the target and patterns for constructing the
2960 names of prerequisites. The difference is in how @code{make} decides
2961 @emph{when} the rule applies.
2963 An implicit rule @emph{can} apply to any target that matches its pattern,
2964 but it @emph{does} apply only when the target has no commands otherwise
2965 specified, and only when the prerequisites can be found. If more than one
2966 implicit rule appears applicable, only one applies; the choice depends on
2969 By contrast, a static pattern rule applies to the precise list of targets
2970 that you specify in the rule. It cannot apply to any other target and it
2971 invariably does apply to each of the targets specified. If two conflicting
2972 rules apply, and both have commands, that's an error.
2974 The static pattern rule can be better than an implicit rule for these
2979 You may wish to override the usual implicit rule for a few
2980 files whose names cannot be categorized syntactically but
2981 can be given in an explicit list.
2984 If you cannot be sure of the precise contents of the directories
2985 you are using, you may not be sure which other irrelevant files
2986 might lead @code{make} to use the wrong implicit rule. The choice
2987 might depend on the order in which the implicit rule search is done.
2988 With static pattern rules, there is no uncertainty: each rule applies
2989 to precisely the targets specified.
2992 @node Double-Colon, Automatic Prerequisites, Static Pattern, Rules
2993 @section Double-Colon Rules
2994 @cindex double-colon rules
2995 @cindex rule, double-colon (@code{::})
2996 @cindex multiple rules for one target (@code{::})
2997 @cindex @code{::} rules (double-colon)
2999 @dfn{Double-colon} rules are rules written with @samp{::} instead of
3000 @samp{:} after the target names. They are handled differently from
3001 ordinary rules when the same target appears in more than one rule.
3003 When a target appears in multiple rules, all the rules must be the same
3004 type: all ordinary, or all double-colon. If they are double-colon, each
3005 of them is independent of the others. Each double-colon rule's commands
3006 are executed if the target is older than any prerequisites of that rule.
3007 If there are no prerequisites for that rule, its commands are always
3008 executed (even if the target already exists). This can result in
3009 executing none, any, or all of the double-colon rules.
3011 Double-colon rules with the same target are in fact completely separate
3012 from one another. Each double-colon rule is processed individually, just
3013 as rules with different targets are processed.
3015 The double-colon rules for a target are executed in the order they appear
3016 in the makefile. However, the cases where double-colon rules really make
3017 sense are those where the order of executing the commands would not matter.
3019 Double-colon rules are somewhat obscure and not often very useful; they
3020 provide a mechanism for cases in which the method used to update a target
3021 differs depending on which prerequisite files caused the update, and such
3024 Each double-colon rule should specify commands; if it does not, an
3025 implicit rule will be used if one applies.
3026 @xref{Implicit Rules, ,Using Implicit Rules}.
3028 @node Automatic Prerequisites, , Double-Colon, Rules
3029 @section Generating Prerequisites Automatically
3030 @cindex prerequisites, automatic generation
3031 @cindex automatic generation of prerequisites
3032 @cindex generating prerequisites automatically
3034 In the makefile for a program, many of the rules you need to write often
3035 say only that some object file depends on some header
3036 file. For example, if @file{main.c} uses @file{defs.h} via an
3037 @code{#include}, you would write:
3044 You need this rule so that @code{make} knows that it must remake
3045 @file{main.o} whenever @file{defs.h} changes. You can see that for a
3046 large program you would have to write dozens of such rules in your
3047 makefile. And, you must always be very careful to update the makefile
3048 every time you add or remove an @code{#include}.
3049 @cindex @code{#include}
3051 @cindex @code{-M} (to compiler)
3052 To avoid this hassle, most modern C compilers can write these rules for
3053 you, by looking at the @code{#include} lines in the source files.
3054 Usually this is done with the @samp{-M} option to the compiler.
3055 For example, the command:
3062 generates the output:
3065 main.o : main.c defs.h
3069 Thus you no longer have to write all those rules yourself.
3070 The compiler will do it for you.
3072 Note that such a prerequisite constitutes mentioning @file{main.o} in a
3073 makefile, so it can never be considered an intermediate file by implicit
3074 rule search. This means that @code{make} won't ever remove the file
3075 after using it; @pxref{Chained Rules, ,Chains of Implicit Rules}.
3077 @cindex @code{make depend}
3078 With old @code{make} programs, it was traditional practice to use this
3079 compiler feature to generate prerequisites on demand with a command like
3080 @samp{make depend}. That command would create a file @file{depend}
3081 containing all the automatically-generated prerequisites; then the
3082 makefile could use @code{include} to read them in (@pxref{Include}).
3084 In GNU @code{make}, the feature of remaking makefiles makes this
3085 practice obsolete---you need never tell @code{make} explicitly to
3086 regenerate the prerequisites, because it always regenerates any makefile
3087 that is out of date. @xref{Remaking Makefiles}.
3089 The practice we recommend for automatic prerequisite generation is to have
3090 one makefile corresponding to each source file. For each source file
3091 @file{@var{name}.c} there is a makefile @file{@var{name}.d} which lists
3092 what files the object file @file{@var{name}.o} depends on. That way
3093 only the source files that have changed need to be rescanned to produce
3094 the new prerequisites.
3096 Here is the pattern rule to generate a file of prerequisites (i.e., a makefile)
3097 called @file{@var{name}.d} from a C source file called @file{@var{name}.c}:
3102 @set -e; rm -f $@@; \
3103 $(CC) -M $(CPPFLAGS) $< > $@@.$$$$; \
3104 sed 's,\($*\)\.o[ :]*,\1.o $@@ : ,g' < $@@.$$$$ > $@@; \
3110 @xref{Pattern Rules}, for information on defining pattern rules. The
3111 @samp{-e} flag to the shell causes it to exit immediately if the
3112 @code{$(CC)} command (or any other command) fails (exits with a
3114 @cindex @code{-e} (shell flag)
3116 @cindex @code{-MM} (to GNU compiler)
3117 With the GNU C compiler, you may wish to use the @samp{-MM} flag instead
3118 of @samp{-M}. This omits prerequisites on system header files.
3119 @xref{Preprocessor Options, , Options Controlling the Preprocessor,
3120 gcc.info, Using GNU CC}, for details.
3122 @cindex @code{sed} (shell command)
3123 The purpose of the @code{sed} command is to translate (for example):
3126 main.o : main.c defs.h
3133 main.o main.d : main.c defs.h
3138 This makes each @samp{.d} file depend on all the source and header files
3139 that the corresponding @samp{.o} file depends on. @code{make} then
3140 knows it must regenerate the prerequisites whenever any of the source or
3141 header files changes.
3143 Once you've defined the rule to remake the @samp{.d} files,
3144 you then use the @code{include} directive to read them all in.
3145 @xref{Include}. For example:
3149 sources = foo.c bar.c
3151 include $(sources:.c=.d)
3156 (This example uses a substitution variable reference to translate the
3157 list of source files @samp{foo.c bar.c} into a list of prerequisite
3158 makefiles, @samp{foo.d bar.d}. @xref{Substitution Refs}, for full
3159 information on substitution references.) Since the @samp{.d} files are
3160 makefiles like any others, @code{make} will remake them as necessary
3161 with no further work from you. @xref{Remaking Makefiles}.
3163 Note that the @samp{.d} files contain target definitions; you should
3164 be sure to place the @code{include} directive @emph{after} the first,
3165 default target in your makefiles or run the risk of having a random
3166 object file become the default target.
3167 @xref{How Make Works}.
3169 @node Commands, Using Variables, Rules, Top
3170 @chapter Writing the Commands in Rules
3171 @cindex commands, how to write
3172 @cindex rule commands
3173 @cindex writing rule commands
3175 The commands of a rule consist of shell command lines to be executed one
3176 by one. Each command line must start with a tab, except that the first
3177 command line may be attached to the target-and-prerequisites line with a
3178 semicolon in between. Blank lines and lines of just comments may appear
3179 among the command lines; they are ignored. (But beware, an apparently
3180 ``blank'' line that begins with a tab is @emph{not} blank! It is an
3181 empty command; @pxref{Empty Commands}.)
3183 Users use many different shell programs, but commands in makefiles are
3184 always interpreted by @file{/bin/sh} unless the makefile specifies
3185 otherwise. @xref{Execution, ,Command Execution}.
3187 @cindex comments, in commands
3188 @cindex commands, comments in
3189 @cindex @code{#} (comments), in commands
3190 The shell that is in use determines whether comments can be written on
3191 command lines, and what syntax they use. When the shell is
3192 @file{/bin/sh}, a @samp{#} starts a comment that extends to the end of
3193 the line. The @samp{#} does not have to be at the beginning of a line.
3194 Text on a line before a @samp{#} is not part of the comment.
3197 * Echoing:: How to control when commands are echoed.
3198 * Execution:: How commands are executed.
3199 * Parallel:: How commands can be executed in parallel.
3200 * Errors:: What happens after a command execution error.
3201 * Interrupts:: What happens when a command is interrupted.
3202 * Recursion:: Invoking @code{make} from makefiles.
3203 * Sequences:: Defining canned sequences of commands.
3204 * Empty Commands:: Defining useful, do-nothing commands.
3207 @node Echoing, Execution, Commands, Commands
3208 @section Command Echoing
3209 @cindex echoing of commands
3210 @cindex silent operation
3211 @cindex @code{@@} (in commands)
3212 @cindex commands, echoing
3213 @cindex printing of commands
3215 Normally @code{make} prints each command line before it is executed.
3216 We call this @dfn{echoing} because it gives the appearance that you
3217 are typing the commands yourself.
3219 When a line starts with @samp{@@}, the echoing of that line is suppressed.
3220 The @samp{@@} is discarded before the command is passed to the shell.
3221 Typically you would use this for a command whose only effect is to print
3222 something, such as an @code{echo} command to indicate progress through
3226 @@echo About to make distribution files
3230 @cindex @code{--just-print}
3231 @cindex @code{--dry-run}
3232 @cindex @code{--recon}
3233 When @code{make} is given the flag @samp{-n} or @samp{--just-print}
3234 it only echoes commands, it won't execute them. @xref{Options Summary,
3235 ,Summary of Options}. In this case and only this case, even the
3236 commands starting with @samp{@@} are printed. This flag is useful for
3237 finding out which commands @code{make} thinks are necessary without
3238 actually doing them.
3241 @cindex @code{--silent}
3242 @cindex @code{--quiet}
3244 The @samp{-s} or @samp{--silent}
3245 flag to @code{make} prevents all echoing, as if all commands
3246 started with @samp{@@}. A rule in the makefile for the special target
3247 @code{.SILENT} without prerequisites has the same effect
3248 (@pxref{Special Targets, ,Special Built-in Target Names}).
3249 @code{.SILENT} is essentially obsolete since @samp{@@} is more flexible.@refill
3251 @node Execution, Parallel, Echoing, Commands
3252 @section Command Execution
3253 @cindex commands, execution
3254 @cindex execution, of commands
3255 @cindex shell command, execution
3256 @vindex SHELL @r{(command execution)}
3258 When it is time to execute commands to update a target, they are executed
3259 by making a new subshell for each line. (In practice, @code{make} may
3260 take shortcuts that do not affect the results.)
3262 @cindex @code{cd} (shell command)
3263 @strong{Please note:} this implies that shell commands such as @code{cd}
3264 that set variables local to each process will not affect the following
3265 command lines. @footnote{On MS-DOS, the value of current working
3266 directory is @strong{global}, so changing it @emph{will} affect the
3267 following command lines on those systems.} If you want to use @code{cd}
3268 to affect the next command, put the two on a single line with a
3269 semicolon between them. Then @code{make} will consider them a single
3270 command and pass them, together, to a shell which will execute them in
3271 sequence. For example:
3275 cd bar; gobble lose > ../foo
3278 @cindex commands, backslash (@code{\}) in
3279 @cindex commands, quoting newlines in
3280 @cindex backslash (@code{\}), in commands
3281 @cindex @code{\} (backslash), in commands
3282 @cindex quoting newline, in commands
3283 @cindex newline, quoting, in commands
3284 If you would like to split a single shell command into multiple lines of
3285 text, you must use a backslash at the end of all but the last subline.
3286 Such a sequence of lines is combined into a single line, by deleting the
3287 backslash-newline sequences, before passing it to the shell. Thus, the
3288 following is equivalent to the preceding example:
3294 gobble lose > ../foo
3299 The program used as the shell is taken from the variable @code{SHELL}.
3300 By default, the program @file{/bin/sh} is used.
3303 On MS-DOS, if @code{SHELL} is not set, the value of the variable
3304 @code{COMSPEC} (which is always set) is used instead.
3306 @cindex @code{SHELL}, MS-DOS specifics
3307 The processing of lines that set the variable @code{SHELL} in Makefiles
3308 is different on MS-DOS. The stock shell, @file{command.com}, is
3309 ridiculously limited in its functionality and many users of @code{make}
3310 tend to install a replacement shell. Therefore, on MS-DOS, @code{make}
3311 examines the value of @code{SHELL}, and changes its behavior based on
3312 whether it points to a Unix-style or DOS-style shell. This allows
3313 reasonable functionality even if @code{SHELL} points to
3316 If @code{SHELL} points to a Unix-style shell, @code{make} on MS-DOS
3317 additionally checks whether that shell can indeed be found; if not, it
3318 ignores the line that sets @code{SHELL}. In MS-DOS, GNU @code{make}
3319 searches for the shell in the following places:
3323 In the precise place pointed to by the value of @code{SHELL}. For
3324 example, if the makefile specifies @samp{SHELL = /bin/sh}, @code{make}
3325 will look in the directory @file{/bin} on the current drive.
3328 In the current directory.
3331 In each of the directories in the @code{PATH} variable, in order.
3335 In every directory it examines, @code{make} will first look for the
3336 specific file (@file{sh} in the example above). If this is not found,
3337 it will also look in that directory for that file with one of the known
3338 extensions which identify executable files. For example @file{.exe},
3339 @file{.com}, @file{.bat}, @file{.btm}, @file{.sh}, and some others.
3341 If any of these attempts is successful, the value of @code{SHELL} will
3342 be set to the full pathname of the shell as found. However, if none of
3343 these is found, the value of @code{SHELL} will not be changed, and thus
3344 the line that sets it will be effectively ignored. This is so
3345 @code{make} will only support features specific to a Unix-style shell if
3346 such a shell is actually installed on the system where @code{make} runs.
3348 Note that this extended search for the shell is limited to the cases
3349 where @code{SHELL} is set from the Makefile; if it is set in the
3350 environment or command line, you are expected to set it to the full
3351 pathname of the shell, exactly as things are on Unix.
3353 The effect of the above DOS-specific processing is that a Makefile that
3354 says @samp{SHELL = /bin/sh} (as many Unix makefiles do), will work
3355 on MS-DOS unaltered if you have e.g. @file{sh.exe} installed in some
3356 directory along your @code{PATH}.
3358 @cindex environment, @code{SHELL} in
3359 Unlike most variables, the variable @code{SHELL} is never set from the
3360 environment. This is because the @code{SHELL} environment variable is
3361 used to specify your personal choice of shell program for interactive
3362 use. It would be very bad for personal choices like this to affect the
3363 functioning of makefiles. @xref{Environment, ,Variables from the
3364 Environment}. However, on MS-DOS and MS-Windows the value of
3365 @code{SHELL} in the environment @strong{is} used, since on those systems
3366 most users do not set this variable, and therefore it is most likely set
3367 specifically to be used by @code{make}. On MS-DOS, if the setting of
3368 @code{SHELL} is not suitable for @code{make}, you can set the variable
3369 @code{MAKESHELL} to the shell that @code{make} should use; this will
3370 override the value of @code{SHELL}.
3372 @node Parallel, Errors, Execution, Commands
3373 @section Parallel Execution
3374 @cindex commands, execution in parallel
3375 @cindex parallel execution
3376 @cindex execution, in parallel
3379 @cindex @code{--jobs}
3381 GNU @code{make} knows how to execute several commands at once.
3382 Normally, @code{make} will execute only one command at a time, waiting
3383 for it to finish before executing the next. However, the @samp{-j} or
3384 @samp{--jobs} option tells @code{make} to execute many commands
3385 simultaneously.@refill
3387 On MS-DOS, the @samp{-j} option has no effect, since that system doesn't
3388 support multi-processing.
3390 If the @samp{-j} option is followed by an integer, this is the number of
3391 commands to execute at once; this is called the number of @dfn{job slots}.
3392 If there is nothing looking like an integer after the @samp{-j} option,
3393 there is no limit on the number of job slots. The default number of job
3394 slots is one, which means serial execution (one thing at a time).
3396 One unpleasant consequence of running several commands simultaneously is
3397 that output generated by the commands appears whenever each command
3398 sends it, so messages from different commands may be interspersed.
3400 Another problem is that two processes cannot both take input from the
3401 same device; so to make sure that only one command tries to take input
3402 from the terminal at once, @code{make} will invalidate the standard
3403 input streams of all but one running command. This means that
3404 attempting to read from standard input will usually be a fatal error (a
3405 @samp{Broken pipe} signal) for most child processes if there are
3408 @cindex standard input
3410 It is unpredictable which command will have a valid standard input stream
3411 (which will come from the terminal, or wherever you redirect the standard
3412 input of @code{make}). The first command run will always get it first, and
3413 the first command started after that one finishes will get it next, and so
3416 We will change how this aspect of @code{make} works if we find a better
3417 alternative. In the mean time, you should not rely on any command using
3418 standard input at all if you are using the parallel execution feature; but
3419 if you are not using this feature, then standard input works normally in
3422 Finally, handling recursive @code{make} invocations raises issues. For
3423 more information on this, see
3424 @ref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
3426 If a command fails (is killed by a signal or exits with a nonzero
3427 status), and errors are not ignored for that command
3428 (@pxref{Errors, ,Errors in Commands}),
3429 the remaining command lines to remake the same target will not be run.
3430 If a command fails and the @samp{-k} or @samp{--keep-going}
3431 option was not given
3432 (@pxref{Options Summary, ,Summary of Options}),
3433 @code{make} aborts execution. If make
3434 terminates for any reason (including a signal) with child processes
3435 running, it waits for them to finish before actually exiting.@refill
3437 @cindex load average
3438 @cindex limiting jobs based on load
3439 @cindex jobs, limiting based on load
3440 @cindex @code{-l} (load average)
3441 @cindex @code{--max-load}
3442 @cindex @code{--load-average}
3443 When the system is heavily loaded, you will probably want to run fewer jobs
3444 than when it is lightly loaded. You can use the @samp{-l} option to tell
3445 @code{make} to limit the number of jobs to run at once, based on the load
3446 average. The @samp{-l} or @samp{--max-load}
3447 option is followed by a floating-point number. For
3455 will not let @code{make} start more than one job if the load average is
3456 above 2.5. The @samp{-l} option with no following number removes the
3457 load limit, if one was given with a previous @samp{-l} option.@refill
3459 More precisely, when @code{make} goes to start up a job, and it already has
3460 at least one job running, it checks the current load average; if it is not
3461 lower than the limit given with @samp{-l}, @code{make} waits until the load
3462 average goes below that limit, or until all the other jobs finish.
3464 By default, there is no load limit.
3466 @node Errors, Interrupts, Parallel, Commands
3467 @section Errors in Commands
3468 @cindex errors (in commands)
3469 @cindex commands, errors in
3470 @cindex exit status (errors)
3472 After each shell command returns, @code{make} looks at its exit status.
3473 If the command completed successfully, the next command line is executed
3474 in a new shell; after the last command line is finished, the rule is
3477 If there is an error (the exit status is nonzero), @code{make} gives up on
3478 the current rule, and perhaps on all rules.
3480 Sometimes the failure of a certain command does not indicate a problem.
3481 For example, you may use the @code{mkdir} command to ensure that a
3482 directory exists. If the directory already exists, @code{mkdir} will
3483 report an error, but you probably want @code{make} to continue regardless.
3485 @cindex @code{-} (in commands)
3486 To ignore errors in a command line, write a @samp{-} at the beginning of
3487 the line's text (after the initial tab). The @samp{-} is discarded before
3488 the command is passed to the shell for execution.
3498 @cindex @code{rm} (shell command)
3501 This causes @code{rm} to continue even if it is unable to remove a file.
3504 @cindex @code{--ignore-errors}
3506 When you run @code{make} with the @samp{-i} or @samp{--ignore-errors}
3507 flag, errors are ignored in all commands of all rules. A rule in the
3508 makefile for the special target @code{.IGNORE} has the same effect, if
3509 there are no prerequisites. These ways of ignoring errors are obsolete
3510 because @samp{-} is more flexible.
3512 When errors are to be ignored, because of either a @samp{-} or the
3513 @samp{-i} flag, @code{make} treats an error return just like success,
3514 except that it prints out a message that tells you the status code
3515 the command exited with, and says that the error has been ignored.
3517 When an error happens that @code{make} has not been told to ignore,
3518 it implies that the current target cannot be correctly remade, and neither
3519 can any other that depends on it either directly or indirectly. No further
3520 commands will be executed for these targets, since their preconditions
3521 have not been achieved.
3525 @cindex @code{--keep-going}
3526 Normally @code{make} gives up immediately in this circumstance, returning a
3527 nonzero status. However, if the @samp{-k} or @samp{--keep-going}
3528 flag is specified, @code{make}
3529 continues to consider the other prerequisites of the pending targets,
3530 remaking them if necessary, before it gives up and returns nonzero status.
3531 For example, after an error in compiling one object file, @samp{make -k}
3532 will continue compiling other object files even though it already knows
3533 that linking them will be impossible. @xref{Options Summary, ,Summary of Options}.
3535 The usual behavior assumes that your purpose is to get the specified
3536 targets up to date; once @code{make} learns that this is impossible, it
3537 might as well report the failure immediately. The @samp{-k} option says
3538 that the real purpose is to test as many of the changes made in the
3539 program as possible, perhaps to find several independent problems so
3540 that you can correct them all before the next attempt to compile. This
3541 is why Emacs' @code{compile} command passes the @samp{-k} flag by
3543 @cindex Emacs (@code{M-x compile})
3545 @findex .DELETE_ON_ERROR
3546 @cindex deletion of target files
3547 @cindex removal of target files
3548 @cindex target, deleting on error
3549 Usually when a command fails, if it has changed the target file at all,
3550 the file is corrupted and cannot be used---or at least it is not
3551 completely updated. Yet the file's time stamp says that it is now up to
3552 date, so the next time @code{make} runs, it will not try to update that
3553 file. The situation is just the same as when the command is killed by a
3554 signal; @pxref{Interrupts}. So generally the right thing to do is to
3555 delete the target file if the command fails after beginning to change
3556 the file. @code{make} will do this if @code{.DELETE_ON_ERROR} appears
3557 as a target. This is almost always what you want @code{make} to do, but
3558 it is not historical practice; so for compatibility, you must explicitly
3561 @node Interrupts, Recursion, Errors, Commands
3562 @section Interrupting or Killing @code{make}
3565 @cindex deletion of target files
3566 @cindex removal of target files
3567 @cindex target, deleting on interrupt
3568 @cindex killing (interruption)
3570 If @code{make} gets a fatal signal while a command is executing, it may
3571 delete the target file that the command was supposed to update. This is
3572 done if the target file's last-modification time has changed since
3573 @code{make} first checked it.
3575 The purpose of deleting the target is to make sure that it is remade from
3576 scratch when @code{make} is next run. Why is this? Suppose you type
3577 @kbd{Ctrl-c} while a compiler is running, and it has begun to write an
3578 object file @file{foo.o}. The @kbd{Ctrl-c} kills the compiler, resulting
3579 in an incomplete file whose last-modification time is newer than the source
3580 file @file{foo.c}. But @code{make} also receives the @kbd{Ctrl-c} signal
3581 and deletes this incomplete file. If @code{make} did not do this, the next
3582 invocation of @code{make} would think that @file{foo.o} did not require
3583 updating---resulting in a strange error message from the linker when it
3584 tries to link an object file half of which is missing.
3587 You can prevent the deletion of a target file in this way by making the
3588 special target @code{.PRECIOUS} depend on it. Before remaking a target,
3589 @code{make} checks to see whether it appears on the prerequisites of
3590 @code{.PRECIOUS}, and thereby decides whether the target should be deleted
3591 if a signal happens. Some reasons why you might do this are that the
3592 target is updated in some atomic fashion, or exists only to record a
3593 modification-time (its contents do not matter), or must exist at all
3594 times to prevent other sorts of trouble.
3596 @node Recursion, Sequences, Interrupts, Commands
3597 @section Recursive Use of @code{make}
3599 @cindex subdirectories, recursion for
3601 Recursive use of @code{make} means using @code{make} as a command in a
3602 makefile. This technique is useful when you want separate makefiles for
3603 various subsystems that compose a larger system. For example, suppose you
3604 have a subdirectory @file{subdir} which has its own makefile, and you would
3605 like the containing directory's makefile to run @code{make} on the
3606 subdirectory. You can do it by writing this:
3610 cd subdir && $(MAKE)
3614 or, equivalently, this (@pxref{Options Summary, ,Summary of Options}):
3621 @cindex @code{--directory}
3623 You can write recursive @code{make} commands just by copying this example,
3624 but there are many things to know about how they work and why, and about
3625 how the sub-@code{make} relates to the top-level @code{make}. You may
3626 also find it useful to declare targets that invoke recursive
3627 @code{make} commands as @samp{.PHONY} (for more discussion on when
3628 this is useful, see @ref{Phony Targets}).
3630 @vindex @code{CURDIR}
3631 For your convenience, when GNU @code{make} starts (after it has
3632 processed any @code{-C} options) it sets the variable @code{CURDIR} to
3633 the pathname of the current working directory. This value is never
3634 touched by @code{make} again: in particular note that if you include
3635 files from other directories the value of @code{CURDIR} does not
3636 change. The value has the same precedence it would have if it were
3637 set in the makefile (by default, an environment variable @code{CURDIR}
3638 will not override this value). Note that setting this variable has no
3639 impact on the operation of @code{make} (it does not cause @code{make}
3640 to change its working directory, for example).
3643 * MAKE Variable:: The special effects of using @samp{$(MAKE)}.
3644 * Variables/Recursion:: How to communicate variables to a sub-@code{make}.
3645 * Options/Recursion:: How to communicate options to a sub-@code{make}.
3646 * -w Option:: How the @samp{-w} or @samp{--print-directory} option
3647 helps debug use of recursive @code{make} commands.
3650 @node MAKE Variable, Variables/Recursion, Recursion, Recursion
3651 @subsection How the @code{MAKE} Variable Works
3653 @cindex recursion, and @code{MAKE} variable
3655 Recursive @code{make} commands should always use the variable @code{MAKE},
3656 not the explicit command name @samp{make}, as shown here:
3661 cd subdir && $(MAKE)
3665 The value of this variable is the file name with which @code{make} was
3666 invoked. If this file name was @file{/bin/make}, then the command executed
3667 is @samp{cd subdir && /bin/make}. If you use a special version of
3668 @code{make} to run the top-level makefile, the same special version will be
3669 executed for recursive invocations.
3670 @cindex @code{cd} (shell command)
3672 @cindex +, and commands
3673 As a special feature, using the variable @code{MAKE} in the commands of
3674 a rule alters the effects of the @samp{-t} (@samp{--touch}), @samp{-n}
3675 (@samp{--just-print}), or @samp{-q} (@w{@samp{--question}}) option.
3676 Using the @code{MAKE} variable has the same effect as using a @samp{+}
3677 character at the beginning of the command line. @xref{Instead of
3678 Execution, ,Instead of Executing the Commands}. This special feature
3679 is only enabled if the @code{MAKE} variable appears directly in the
3680 command script: it does not apply if the @code{MAKE} variable is
3681 referenced through expansion of another variable. In the latter case
3682 you must use the @samp{+} token to get these special effects.@refill
3684 Consider the command @samp{make -t} in the above example. (The
3685 @samp{-t} option marks targets as up to date without actually running
3686 any commands; see @ref{Instead of Execution}.) Following the usual
3687 definition of @samp{-t}, a @samp{make -t} command in the example would
3688 create a file named @file{subsystem} and do nothing else. What you
3689 really want it to do is run @samp{@w{cd subdir &&} @w{make -t}}; but that would
3690 require executing the command, and @samp{-t} says not to execute
3692 @cindex @code{-t}, and recursion
3693 @cindex recursion, and @code{-t}
3694 @cindex @code{--touch}, and recursion
3696 The special feature makes this do what you want: whenever a command
3697 line of a rule contains the variable @code{MAKE}, the flags @samp{-t},
3698 @samp{-n} and @samp{-q} do not apply to that line. Command lines
3699 containing @code{MAKE} are executed normally despite the presence of a
3700 flag that causes most commands not to be run. The usual
3701 @code{MAKEFLAGS} mechanism passes the flags to the sub-@code{make}
3702 (@pxref{Options/Recursion, ,Communicating Options to a
3703 Sub-@code{make}}), so your request to touch the files, or print the
3704 commands, is propagated to the subsystem.@refill
3706 @node Variables/Recursion, Options/Recursion, MAKE Variable, Recursion
3707 @subsection Communicating Variables to a Sub-@code{make}
3708 @cindex sub-@code{make}
3709 @cindex environment, and recursion
3710 @cindex exporting variables
3711 @cindex variables, environment
3712 @cindex variables, exporting
3713 @cindex recursion, and environment
3714 @cindex recursion, and variables
3716 Variable values of the top-level @code{make} can be passed to the
3717 sub-@code{make} through the environment by explicit request. These
3718 variables are defined in the sub-@code{make} as defaults, but do not
3719 override what is specified in the makefile used by the sub-@code{make}
3720 makefile unless you use the @samp{-e} switch (@pxref{Options Summary,
3721 ,Summary of Options}).@refill
3723 To pass down, or @dfn{export}, a variable, @code{make} adds the variable
3724 and its value to the environment for running each command. The
3725 sub-@code{make}, in turn, uses the environment to initialize its table
3726 of variable values. @xref{Environment, ,Variables from the
3729 Except by explicit request, @code{make} exports a variable only if it
3730 is either defined in the environment initially or set on the command
3731 line, and if its name consists only of letters, numbers, and underscores.
3732 Some shells cannot cope with environment variable names consisting of
3733 characters other than letters, numbers, and underscores.
3735 @cindex SHELL, exported value
3736 The value of the @code{make} variable @code{SHELL} is not exported.
3737 Instead, the value of the @code{SHELL} variable from the invoking
3738 environment is passed to the sub-@code{make}. You can force
3739 @code{make} to export its value for @code{SHELL} by using the
3740 @code{export} directive, described below.
3742 The special variable @code{MAKEFLAGS} is always exported (unless you
3743 unexport it). @code{MAKEFILES} is exported if you set it to anything.
3745 @code{make} automatically passes down variable values that were defined
3746 on the command line, by putting them in the @code{MAKEFLAGS} variable.
3748 See the next section.
3751 @xref{Options/Recursion}.
3754 Variables are @emph{not} normally passed down if they were created by
3755 default by @code{make} (@pxref{Implicit Variables, ,Variables Used by
3756 Implicit Rules}). The sub-@code{make} will define these for
3760 If you want to export specific variables to a sub-@code{make}, use the
3761 @code{export} directive, like this:
3764 export @var{variable} @dots{}
3769 If you want to @emph{prevent} a variable from being exported, use the
3770 @code{unexport} directive, like this:
3773 unexport @var{variable} @dots{}
3777 In both of these forms, the arguments to @code{export} and
3778 @code{unexport} are expanded, and so could be variables or functions
3779 which expand to a (list of) variable names to be (un)exported.
3781 As a convenience, you can define a variable and export it at the same
3785 export @var{variable} = value
3789 has the same result as:
3792 @var{variable} = value
3793 export @var{variable}
3800 export @var{variable} := value
3804 has the same result as:
3807 @var{variable} := value
3808 export @var{variable}
3814 export @var{variable} += value
3821 @var{variable} += value
3822 export @var{variable}
3826 @xref{Appending, ,Appending More Text to Variables}.
3828 You may notice that the @code{export} and @code{unexport} directives
3829 work in @code{make} in the same way they work in the shell, @code{sh}.
3831 If you want all variables to be exported by default, you can use
3832 @code{export} by itself:
3839 This tells @code{make} that variables which are not explicitly mentioned
3840 in an @code{export} or @code{unexport} directive should be exported.
3841 Any variable given in an @code{unexport} directive will still @emph{not}
3842 be exported. If you use @code{export} by itself to export variables by
3843 default, variables whose names contain characters other than
3844 alphanumerics and underscores will not be exported unless specifically
3845 mentioned in an @code{export} directive.@refill
3847 @findex .EXPORT_ALL_VARIABLES
3848 The behavior elicited by an @code{export} directive by itself was the
3849 default in older versions of GNU @code{make}. If your makefiles depend
3850 on this behavior and you want to be compatible with old versions of
3851 @code{make}, you can write a rule for the special target
3852 @code{.EXPORT_ALL_VARIABLES} instead of using the @code{export} directive.
3853 This will be ignored by old @code{make}s, while the @code{export}
3854 directive will cause a syntax error.@refill
3855 @cindex compatibility in exporting
3857 Likewise, you can use @code{unexport} by itself to tell @code{make}
3858 @emph{not} to export variables by default. Since this is the default
3859 behavior, you would only need to do this if @code{export} had been used
3860 by itself earlier (in an included makefile, perhaps). You
3861 @strong{cannot} use @code{export} and @code{unexport} by themselves to
3862 have variables exported for some commands and not for others. The last
3863 @code{export} or @code{unexport} directive that appears by itself
3864 determines the behavior for the entire run of @code{make}.@refill
3867 @cindex recursion, level of
3868 As a special feature, the variable @code{MAKELEVEL} is changed when it
3869 is passed down from level to level. This variable's value is a string
3870 which is the depth of the level as a decimal number. The value is
3871 @samp{0} for the top-level @code{make}; @samp{1} for a sub-@code{make},
3872 @samp{2} for a sub-sub-@code{make}, and so on. The incrementation
3873 happens when @code{make} sets up the environment for a command.@refill
3875 The main use of @code{MAKELEVEL} is to test it in a conditional
3876 directive (@pxref{Conditionals, ,Conditional Parts of Makefiles}); this
3877 way you can write a makefile that behaves one way if run recursively and
3878 another way if run directly by you.@refill
3881 You can use the variable @code{MAKEFILES} to cause all sub-@code{make}
3882 commands to use additional makefiles. The value of @code{MAKEFILES} is
3883 a whitespace-separated list of file names. This variable, if defined in
3884 the outer-level makefile, is passed down through the environment; then
3885 it serves as a list of extra makefiles for the sub-@code{make} to read
3886 before the usual or specified ones. @xref{MAKEFILES Variable, ,The
3887 Variable @code{MAKEFILES}}.@refill
3889 @node Options/Recursion, -w Option, Variables/Recursion, Recursion
3890 @subsection Communicating Options to a Sub-@code{make}
3891 @cindex options, and recursion
3892 @cindex recursion, and options
3895 Flags such as @samp{-s} and @samp{-k} are passed automatically to the
3896 sub-@code{make} through the variable @code{MAKEFLAGS}. This variable is
3897 set up automatically by @code{make} to contain the flag letters that
3898 @code{make} received. Thus, if you do @w{@samp{make -ks}} then
3899 @code{MAKEFLAGS} gets the value @samp{ks}.@refill
3901 As a consequence, every sub-@code{make} gets a value for @code{MAKEFLAGS}
3902 in its environment. In response, it takes the flags from that value and
3903 processes them as if they had been given as arguments.
3904 @xref{Options Summary, ,Summary of Options}.
3906 @cindex command line variable definitions, and recursion
3907 @cindex variables, command line, and recursion
3908 @cindex recursion, and command line variable definitions
3909 Likewise variables defined on the command line are passed to the
3910 sub-@code{make} through @code{MAKEFLAGS}. Words in the value of
3911 @code{MAKEFLAGS} that contain @samp{=}, @code{make} treats as variable
3912 definitions just as if they appeared on the command line.
3913 @xref{Overriding, ,Overriding Variables}.
3915 @cindex @code{-C}, and recursion
3916 @cindex @code{-f}, and recursion
3917 @cindex @code{-o}, and recursion
3918 @cindex @code{-W}, and recursion
3919 @cindex @code{--directory}, and recursion
3920 @cindex @code{--file}, and recursion
3921 @cindex @code{--old-file}, and recursion
3922 @cindex @code{--assume-old}, and recursion
3923 @cindex @code{--assume-new}, and recursion
3924 @cindex @code{--new-file}, and recursion
3925 @cindex recursion, and @code{-C}
3926 @cindex recursion, and @code{-f}
3927 @cindex recursion, and @code{-o}
3928 @cindex recursion, and @code{-W}
3929 The options @samp{-C}, @samp{-f}, @samp{-o}, and @samp{-W} are not put
3930 into @code{MAKEFLAGS}; these options are not passed down.@refill
3932 @cindex @code{-j}, and recursion
3933 @cindex @code{--jobs}, and recursion
3934 @cindex recursion, and @code{-j}
3935 @cindex job slots, and recursion
3936 The @samp{-j} option is a special case (@pxref{Parallel, ,Parallel Execution}).
3937 If you set it to some numeric value @samp{N} and your operating system
3938 supports it (most any UNIX system will; others typically won't), the
3939 parent @code{make} and all the sub-@code{make}s will communicate to
3940 ensure that there are only @samp{N} jobs running at the same time
3941 between them all. Note that any job that is marked recursive
3942 (@pxref{Instead of Execution, ,Instead of Executing the Commands})
3943 doesn't count against the total jobs (otherwise we could get @samp{N}
3944 sub-@code{make}s running and have no slots left over for any real work!)
3946 If your operating system doesn't support the above communication, then
3947 @samp{-j 1} is always put into @code{MAKEFLAGS} instead of the value you
3948 specified. This is because if the @w{@samp{-j}} option were passed down
3949 to sub-@code{make}s, you would get many more jobs running in parallel
3950 than you asked for. If you give @samp{-j} with no numeric argument,
3951 meaning to run as many jobs as possible in parallel, this is passed
3952 down, since multiple infinities are no more than one.@refill
3954 If you do not want to pass the other flags down, you must change the
3955 value of @code{MAKEFLAGS}, like this:
3959 cd subdir && $(MAKE) MAKEFLAGS=
3962 @vindex MAKEOVERRIDES
3963 The command line variable definitions really appear in the variable
3964 @code{MAKEOVERRIDES}, and @code{MAKEFLAGS} contains a reference to this
3965 variable. If you do want to pass flags down normally, but don't want to
3966 pass down the command line variable definitions, you can reset
3967 @code{MAKEOVERRIDES} to empty, like this:
3974 @cindex Arg list too long
3976 This is not usually useful to do. However, some systems have a small
3977 fixed limit on the size of the environment, and putting so much
3978 information into the value of @code{MAKEFLAGS} can exceed it. If you
3979 see the error message @samp{Arg list too long}, this may be the problem.
3982 (For strict compliance with POSIX.2, changing @code{MAKEOVERRIDES} does
3983 not affect @code{MAKEFLAGS} if the special target @samp{.POSIX} appears
3984 in the makefile. You probably do not care about this.)
3987 A similar variable @code{MFLAGS} exists also, for historical
3988 compatibility. It has the same value as @code{MAKEFLAGS} except that it
3989 does not contain the command line variable definitions, and it always
3990 begins with a hyphen unless it is empty (@code{MAKEFLAGS} begins with a
3991 hyphen only when it begins with an option that has no single-letter
3992 version, such as @samp{--warn-undefined-variables}). @code{MFLAGS} was
3993 traditionally used explicitly in the recursive @code{make} command, like
3998 cd subdir && $(MAKE) $(MFLAGS)
4002 but now @code{MAKEFLAGS} makes this usage redundant. If you want your
4003 makefiles to be compatible with old @code{make} programs, use this
4004 technique; it will work fine with more modern @code{make} versions too.
4006 @cindex setting options from environment
4007 @cindex options, setting from environment
4008 @cindex setting options in makefiles
4009 @cindex options, setting in makefiles
4010 The @code{MAKEFLAGS} variable can also be useful if you want to have
4011 certain options, such as @samp{-k} (@pxref{Options Summary, ,Summary of
4012 Options}), set each time you run @code{make}. You simply put a value for
4013 @code{MAKEFLAGS} in your environment. You can also set @code{MAKEFLAGS} in
4014 a makefile, to specify additional flags that should also be in effect for
4015 that makefile. (Note that you cannot use @code{MFLAGS} this way. That
4016 variable is set only for compatibility; @code{make} does not interpret a
4017 value you set for it in any way.)
4019 When @code{make} interprets the value of @code{MAKEFLAGS} (either from the
4020 environment or from a makefile), it first prepends a hyphen if the value
4021 does not already begin with one. Then it chops the value into words
4022 separated by blanks, and parses these words as if they were options given
4023 on the command line (except that @samp{-C}, @samp{-f}, @samp{-h},
4024 @samp{-o}, @samp{-W}, and their long-named versions are ignored; and there
4025 is no error for an invalid option).
4027 If you do put @code{MAKEFLAGS} in your environment, you should be sure not
4028 to include any options that will drastically affect the actions of
4029 @code{make} and undermine the purpose of makefiles and of @code{make}
4030 itself. For instance, the @samp{-t}, @samp{-n}, and @samp{-q} options, if
4031 put in one of these variables, could have disastrous consequences and would
4032 certainly have at least surprising and probably annoying effects.@refill
4034 @node -w Option, , Options/Recursion, Recursion
4035 @subsection The @samp{--print-directory} Option
4036 @cindex directories, printing them
4037 @cindex printing directories
4038 @cindex recursion, and printing directories
4040 If you use several levels of recursive @code{make} invocations, the
4041 @samp{-w} or @w{@samp{--print-directory}} option can make the output a
4042 lot easier to understand by showing each directory as @code{make}
4043 starts processing it and as @code{make} finishes processing it. For
4044 example, if @samp{make -w} is run in the directory @file{/u/gnu/make},
4045 @code{make} will print a line of the form:@refill
4048 make: Entering directory `/u/gnu/make'.
4052 before doing anything else, and a line of the form:
4055 make: Leaving directory `/u/gnu/make'.
4059 when processing is completed.
4061 @cindex @code{-C}, and @code{-w}
4062 @cindex @code{--directory}, and @code{--print-directory}
4063 @cindex recursion, and @code{-w}
4064 @cindex @code{-w}, and @code{-C}
4065 @cindex @code{-w}, and recursion
4066 @cindex @code{--print-directory}, and @code{--directory}
4067 @cindex @code{--print-directory}, and recursion
4068 @cindex @code{--no-print-directory}
4069 @cindex @code{--print-directory}, disabling
4070 @cindex @code{-w}, disabling
4071 Normally, you do not need to specify this option because @samp{make}
4072 does it for you: @samp{-w} is turned on automatically when you use the
4073 @samp{-C} option, and in sub-@code{make}s. @code{make} will not
4074 automatically turn on @samp{-w} if you also use @samp{-s}, which says to
4075 be silent, or if you use @samp{--no-print-directory} to explicitly
4078 @node Sequences, Empty Commands, Recursion, Commands
4079 @section Defining Canned Command Sequences
4080 @cindex sequences of commands
4081 @cindex commands, sequences of
4083 When the same sequence of commands is useful in making various targets, you
4084 can define it as a canned sequence with the @code{define} directive, and
4085 refer to the canned sequence from the rules for those targets. The canned
4086 sequence is actually a variable, so the name must not conflict with other
4089 Here is an example of defining a canned sequence of commands:
4093 yacc $(firstword $^)
4100 Here @code{run-yacc} is the name of the variable being defined;
4101 @code{endef} marks the end of the definition; the lines in between are the
4102 commands. The @code{define} directive does not expand variable references
4103 and function calls in the canned sequence; the @samp{$} characters,
4104 parentheses, variable names, and so on, all become part of the value of the
4105 variable you are defining.
4106 @xref{Defining, ,Defining Variables Verbatim},
4107 for a complete explanation of @code{define}.
4109 The first command in this example runs Yacc on the first prerequisite of
4110 whichever rule uses the canned sequence. The output file from Yacc is
4111 always named @file{y.tab.c}. The second command moves the output to the
4112 rule's target file name.
4114 To use the canned sequence, substitute the variable into the commands of a
4115 rule. You can substitute it like any other variable
4116 (@pxref{Reference, ,Basics of Variable References}).
4117 Because variables defined by @code{define} are recursively expanded
4118 variables, all the variable references you wrote inside the @code{define}
4119 are expanded now. For example:
4127 @samp{foo.y} will be substituted for the variable @samp{$^} when it occurs in
4128 @code{run-yacc}'s value, and @samp{foo.c} for @samp{$@@}.@refill
4130 This is a realistic example, but this particular one is not needed in
4131 practice because @code{make} has an implicit rule to figure out these
4132 commands based on the file names involved
4133 (@pxref{Implicit Rules, ,Using Implicit Rules}).
4135 @cindex @@, and @code{define}
4136 @cindex -, and @code{define}
4137 @cindex +, and @code{define}
4138 In command execution, each line of a canned sequence is treated just as
4139 if the line appeared on its own in the rule, preceded by a tab. In
4140 particular, @code{make} invokes a separate subshell for each line. You
4141 can use the special prefix characters that affect command lines
4142 (@samp{@@}, @samp{-}, and @samp{+}) on each line of a canned sequence.
4143 @xref{Commands, ,Writing the Commands in Rules}.
4144 For example, using this canned sequence:
4148 @@echo "frobnicating target $@@"
4149 frob-step-1 $< -o $@@-step-1
4150 frob-step-2 $@@-step-1 -o $@@
4155 @code{make} will not echo the first line, the @code{echo} command.
4156 But it @emph{will} echo the following two command lines.
4158 On the other hand, prefix characters on the command line that refers to
4159 a canned sequence apply to every line in the sequence. So the rule:
4167 does not echo @emph{any} commands.
4168 (@xref{Echoing, ,Command Echoing}, for a full explanation of @samp{@@}.)
4170 @node Empty Commands, , Sequences, Commands
4171 @section Using Empty Commands
4172 @cindex empty commands
4173 @cindex commands, empty
4175 It is sometimes useful to define commands which do nothing. This is done
4176 simply by giving a command that consists of nothing but whitespace. For
4184 defines an empty command string for @file{target}. You could also use a
4185 line beginning with a tab character to define an empty command string,
4186 but this would be confusing because such a line looks empty.
4188 @findex .DEFAULT@r{, and empty commands}
4189 You may be wondering why you would want to define a command string that
4190 does nothing. The only reason this is useful is to prevent a target
4191 from getting implicit commands (from implicit rules or the
4192 @code{.DEFAULT} special target; @pxref{Implicit Rules} and
4193 @pxref{Last Resort, ,Defining Last-Resort Default Rules}).@refill
4195 @c !!! another reason is for canonical stamp files:
4200 create foo frm foo.in
4205 You may be inclined to define empty command strings for targets that are
4206 not actual files, but only exist so that their prerequisites can be
4207 remade. However, this is not the best way to do that, because the
4208 prerequisites may not be remade properly if the target file actually does exist.
4209 @xref{Phony Targets, ,Phony Targets}, for a better way to do this.
4211 @node Using Variables, Conditionals, Commands, Top
4212 @chapter How to Use Variables
4215 @cindex recursive variable expansion
4216 @cindex simple variable expansion
4218 A @dfn{variable} is a name defined in a makefile to represent a string
4219 of text, called the variable's @dfn{value}. These values are
4220 substituted by explicit request into targets, prerequisites, commands,
4221 and other parts of the makefile. (In some other versions of @code{make},
4222 variables are called @dfn{macros}.)
4225 Variables and functions in all parts of a makefile are expanded when
4226 read, except for the shell commands in rules, the right-hand sides of
4227 variable definitions using @samp{=}, and the bodies of variable
4228 definitions using the @code{define} directive.@refill
4230 Variables can represent lists of file names, options to pass to compilers,
4231 programs to run, directories to look in for source files, directories to
4232 write output in, or anything else you can imagine.
4234 A variable name may be any sequence of characters not containing @samp{:},
4235 @samp{#}, @samp{=}, or leading or trailing whitespace. However,
4236 variable names containing characters other than letters, numbers, and
4237 underscores should be avoided, as they may be given special meanings in the
4238 future, and with some shells they cannot be passed through the environment to a
4240 (@pxref{Variables/Recursion, ,Communicating Variables to a Sub-@code{make}}).
4242 Variable names are case-sensitive. The names @samp{foo}, @samp{FOO},
4243 and @samp{Foo} all refer to different variables.
4245 It is traditional to use upper case letters in variable names, but we
4246 recommend using lower case letters for variable names that serve internal
4247 purposes in the makefile, and reserving upper case for parameters that
4248 control implicit rules or for parameters that the user should override with
4249 command options (@pxref{Overriding, ,Overriding Variables}).
4251 A few variables have names that are a single punctuation character or
4252 just a few characters. These are the @dfn{automatic variables}, and
4253 they have particular specialized uses. @xref{Automatic Variables}.
4256 * Reference:: How to use the value of a variable.
4257 * Flavors:: Variables come in two flavors.
4258 * Advanced:: Advanced features for referencing a variable.
4259 * Values:: All the ways variables get their values.
4260 * Setting:: How to set a variable in the makefile.
4261 * Appending:: How to append more text to the old value
4263 * Override Directive:: How to set a variable in the makefile even if
4264 the user has set it with a command argument.
4265 * Defining:: An alternate way to set a variable
4266 to a verbatim string.
4267 * Environment:: Variable values can come from the environment.
4268 * Target-specific:: Variable values can be defined on a per-target
4270 * Pattern-specific:: Target-specific variable values can be applied
4271 to a group of targets that match a pattern.
4274 @node Reference, Flavors, Using Variables, Using Variables
4275 @section Basics of Variable References
4276 @cindex variables, how to reference
4277 @cindex reference to variables
4278 @cindex @code{$}, in variable reference
4279 @cindex dollar sign (@code{$}), in variable reference
4281 To substitute a variable's value, write a dollar sign followed by the name
4282 of the variable in parentheses or braces: either @samp{$(foo)} or
4283 @samp{$@{foo@}} is a valid reference to the variable @code{foo}. This
4284 special significance of @samp{$} is why you must write @samp{$$} to have
4285 the effect of a single dollar sign in a file name or command.
4287 Variable references can be used in any context: targets, prerequisites,
4288 commands, most directives, and new variable values. Here is an
4289 example of a common case, where a variable holds the names of all the
4290 object files in a program:
4294 objects = program.o foo.o utils.o
4295 program : $(objects)
4296 cc -o program $(objects)
4302 Variable references work by strict textual substitution. Thus, the rule
4307 prog.o : prog.$(foo)
4308 $(foo)$(foo) -$(foo) prog.$(foo)
4313 could be used to compile a C program @file{prog.c}. Since spaces before
4314 the variable value are ignored in variable assignments, the value of
4315 @code{foo} is precisely @samp{c}. (Don't actually write your makefiles
4318 A dollar sign followed by a character other than a dollar sign,
4319 open-parenthesis or open-brace treats that single character as the
4320 variable name. Thus, you could reference the variable @code{x} with
4321 @samp{$x}. However, this practice is strongly discouraged, except in
4322 the case of the automatic variables (@pxref{Automatic Variables}).
4324 @node Flavors, Advanced, Reference, Using Variables
4325 @section The Two Flavors of Variables
4326 @cindex flavors of variables
4327 @cindex recursive variable expansion
4328 @cindex variables, flavors
4329 @cindex recursively expanded variables
4330 @cindex variables, recursively expanded
4332 There are two ways that a variable in GNU @code{make} can have a value;
4333 we call them the two @dfn{flavors} of variables. The two flavors are
4334 distinguished in how they are defined and in what they do when expanded.
4337 The first flavor of variable is a @dfn{recursively expanded} variable.
4338 Variables of this sort are defined by lines using @samp{=}
4339 (@pxref{Setting, ,Setting Variables}) or by the @code{define} directive
4340 (@pxref{Defining, ,Defining Variables Verbatim}). The value you specify
4341 is installed verbatim; if it contains references to other variables,
4342 these references are expanded whenever this variable is substituted (in
4343 the course of expanding some other string). When this happens, it is
4344 called @dfn{recursive expansion}.@refill
4357 will echo @samp{Huh?}: @samp{$(foo)} expands to @samp{$(bar)} which
4358 expands to @samp{$(ugh)} which finally expands to @samp{Huh?}.@refill
4360 This flavor of variable is the only sort supported by other versions of
4361 @code{make}. It has its advantages and its disadvantages. An advantage
4362 (most would say) is that:
4365 CFLAGS = $(include_dirs) -O
4366 include_dirs = -Ifoo -Ibar
4370 will do what was intended: when @samp{CFLAGS} is expanded in a command,
4371 it will expand to @samp{-Ifoo -Ibar -O}. A major disadvantage is that you
4372 cannot append something on the end of a variable, as in
4375 CFLAGS = $(CFLAGS) -O
4379 because it will cause an infinite loop in the variable expansion.
4380 (Actually @code{make} detects the infinite loop and reports an error.)
4381 @cindex loops in variable expansion
4382 @cindex variables, loops in expansion
4384 Another disadvantage is that any functions
4385 (@pxref{Functions, ,Functions for Transforming Text})
4386 referenced in the definition will be executed every time the variable is
4387 expanded. This makes @code{make} run slower; worse, it causes the
4388 @code{wildcard} and @code{shell} functions to give unpredictable results
4389 because you cannot easily control when they are called, or even how many
4392 To avoid all the problems and inconveniences of recursively expanded
4393 variables, there is another flavor: simply expanded variables.
4395 @cindex simply expanded variables
4396 @cindex variables, simply expanded
4398 @dfn{Simply expanded variables} are defined by lines using @samp{:=}
4399 (@pxref{Setting, ,Setting Variables}).
4400 The value of a simply expanded variable is scanned
4401 once and for all, expanding any references to other variables and
4402 functions, when the variable is defined. The actual value of the simply
4403 expanded variable is the result of expanding the text that you write.
4404 It does not contain any references to other variables; it contains their
4405 values @emph{as of the time this variable was defined}. Therefore,
4421 When a simply expanded variable is referenced, its value is substituted
4424 Here is a somewhat more complicated example, illustrating the use of
4425 @samp{:=} in conjunction with the @code{shell} function.
4426 (@xref{Shell Function, , The @code{shell} Function}.) This example
4427 also shows use of the variable @code{MAKELEVEL}, which is changed
4428 when it is passed down from level to level.
4429 (@xref{Variables/Recursion, , Communicating Variables to a
4430 Sub-@code{make}}, for information about @code{MAKELEVEL}.)
4436 ifeq (0,$@{MAKELEVEL@})
4437 cur-dir := $(shell pwd)
4438 whoami := $(shell whoami)
4439 host-type := $(shell arch)
4440 MAKE := $@{MAKE@} host-type=$@{host-type@} whoami=$@{whoami@}
4446 An advantage of this use of @samp{:=} is that a typical
4447 `descend into a directory' command then looks like this:
4452 $@{MAKE@} cur-dir=$@{cur-dir@}/$@@ -C $@@ all
4456 Simply expanded variables generally make complicated makefile programming
4457 more predictable because they work like variables in most programming
4458 languages. They allow you to redefine a variable using its own value (or
4459 its value processed in some way by one of the expansion functions) and to
4460 use the expansion functions much more efficiently
4461 (@pxref{Functions, ,Functions for Transforming Text}).
4463 @cindex spaces, in variable values
4464 @cindex whitespace, in variable values
4465 @cindex variables, spaces in values
4466 You can also use them to introduce controlled leading whitespace into
4467 variable values. Leading whitespace characters are discarded from your
4468 input before substitution of variable references and function calls;
4469 this means you can include leading spaces in a variable value by
4470 protecting them with variable references, like this:
4474 space := $(nullstring) # end of the line
4478 Here the value of the variable @code{space} is precisely one space. The
4479 comment @w{@samp{# end of the line}} is included here just for clarity.
4480 Since trailing space characters are @emph{not} stripped from variable
4481 values, just a space at the end of the line would have the same effect
4482 (but be rather hard to read). If you put whitespace at the end of a
4483 variable value, it is a good idea to put a comment like that at the end
4484 of the line to make your intent clear. Conversely, if you do @emph{not}
4485 want any whitespace characters at the end of your variable value, you
4486 must remember not to put a random comment on the end of the line after
4487 some whitespace, such as this:
4490 dir := /foo/bar # directory to put the frobs in
4494 Here the value of the variable @code{dir} is @w{@samp{/foo/bar }}
4495 (with four trailing spaces), which was probably not the intention.
4496 (Imagine something like @w{@samp{$(dir)/file}} with this definition!)
4498 @cindex conditional variable assignment
4499 @cindex variables, conditional assignment
4501 There is another assignment operator for variables, @samp{?=}. This
4502 is called a conditional variable assignment operator, because it only
4503 has an effect if the variable is not yet defined. This statement:
4510 is exactly equivalent to this
4511 (@pxref{Origin Function, ,The @code{origin} Function}):
4514 ifeq ($(origin FOO), undefined)
4519 Note that a variable set to an empty value is still defined, so
4520 @samp{?=} will not set that variable.
4522 @node Advanced, Values, Flavors, Using Variables
4523 @section Advanced Features for Reference to Variables
4524 @cindex reference to variables
4526 This section describes some advanced features you can use to reference
4527 variables in more flexible ways.
4530 * Substitution Refs:: Referencing a variable with
4531 substitutions on the value.
4532 * Computed Names:: Computing the name of the variable to refer to.
4535 @node Substitution Refs, Computed Names, Advanced, Advanced
4536 @subsection Substitution References
4537 @cindex modified variable reference
4538 @cindex substitution variable reference
4539 @cindex variables, modified reference
4540 @cindex variables, substitution reference
4542 @cindex variables, substituting suffix in
4543 @cindex suffix, substituting in variables
4544 A @dfn{substitution reference} substitutes the value of a variable with
4545 alterations that you specify. It has the form
4546 @samp{$(@var{var}:@var{a}=@var{b})} (or
4547 @samp{$@{@var{var}:@var{a}=@var{b}@}}) and its meaning is to take the value
4548 of the variable @var{var}, replace every @var{a} at the end of a word with
4549 @var{b} in that value, and substitute the resulting string.
4551 When we say ``at the end of a word'', we mean that @var{a} must appear
4552 either followed by whitespace or at the end of the value in order to be
4553 replaced; other occurrences of @var{a} in the value are unaltered. For
4562 sets @samp{bar} to @samp{a.c b.c c.c}. @xref{Setting, ,Setting Variables}.
4564 A substitution reference is actually an abbreviation for use of the
4565 @code{patsubst} expansion function (@pxref{Text Functions, ,Functions for String Substitution and Analysis}). We provide
4566 substitution references as well as @code{patsubst} for compatibility with
4567 other implementations of @code{make}.
4570 Another type of substitution reference lets you use the full power of
4571 the @code{patsubst} function. It has the same form
4572 @samp{$(@var{var}:@var{a}=@var{b})} described above, except that now
4573 @var{a} must contain a single @samp{%} character. This case is
4574 equivalent to @samp{$(patsubst @var{a},@var{b},$(@var{var}))}.
4575 @xref{Text Functions, ,Functions for String Substitution and Analysis},
4576 for a description of the @code{patsubst} function.@refill
4580 @exdent For example:
4583 bar := $(foo:%.o=%.c)
4588 sets @samp{bar} to @samp{a.c b.c c.c}.
4590 @node Computed Names, , Substitution Refs, Advanced
4591 @subsection Computed Variable Names
4592 @cindex nested variable reference
4593 @cindex computed variable name
4594 @cindex variables, computed names
4595 @cindex variables, nested references
4596 @cindex variables, @samp{$} in name
4597 @cindex @code{$}, in variable name
4598 @cindex dollar sign (@code{$}), in variable name
4600 Computed variable names are a complicated concept needed only for
4601 sophisticated makefile programming. For most purposes you need not
4602 consider them, except to know that making a variable with a dollar sign
4603 in its name might have strange results. However, if you are the type
4604 that wants to understand everything, or you are actually interested in
4605 what they do, read on.
4607 Variables may be referenced inside the name of a variable. This is
4608 called a @dfn{computed variable name} or a @dfn{nested variable
4609 reference}. For example,
4618 defines @code{a} as @samp{z}: the @samp{$(x)} inside @samp{$($(x))} expands
4619 to @samp{y}, so @samp{$($(x))} expands to @samp{$(y)} which in turn expands
4620 to @samp{z}. Here the name of the variable to reference is not stated
4621 explicitly; it is computed by expansion of @samp{$(x)}. The reference
4622 @samp{$(x)} here is nested within the outer variable reference.
4624 The previous example shows two levels of nesting, but any number of levels
4625 is possible. For example, here are three levels:
4635 Here the innermost @samp{$(x)} expands to @samp{y}, so @samp{$($(x))}
4636 expands to @samp{$(y)} which in turn expands to @samp{z}; now we have
4637 @samp{$(z)}, which becomes @samp{u}.
4639 References to recursively-expanded variables within a variable name are
4640 reexpanded in the usual fashion. For example:
4650 defines @code{a} as @samp{Hello}: @samp{$($(x))} becomes @samp{$($(y))}
4651 which becomes @samp{$(z)} which becomes @samp{Hello}.
4653 Nested variable references can also contain modified references and
4654 function invocations (@pxref{Functions, ,Functions for Transforming Text}),
4655 just like any other reference.
4656 For example, using the @code{subst} function
4657 (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):
4663 y = $(subst 1,2,$(x))
4670 eventually defines @code{a} as @samp{Hello}. It is doubtful that anyone
4671 would ever want to write a nested reference as convoluted as this one, but
4672 it works: @samp{$($($(z)))} expands to @samp{$($(y))} which becomes
4673 @samp{$($(subst 1,2,$(x)))}. This gets the value @samp{variable1} from
4674 @code{x} and changes it by substitution to @samp{variable2}, so that the
4675 entire string becomes @samp{$(variable2)}, a simple variable reference
4676 whose value is @samp{Hello}.@refill
4678 A computed variable name need not consist entirely of a single variable
4679 reference. It can contain several variable references, as well as some
4680 invariant text. For example,
4689 a_files := filea fileb
4690 1_files := file1 file2
4694 ifeq "$(use_a)" "yes"
4702 ifeq "$(use_dirs)" "yes"
4708 dirs := $($(a1)_$(df))
4713 will give @code{dirs} the same value as @code{a_dirs}, @code{1_dirs},
4714 @code{a_files} or @code{1_files} depending on the settings of @code{use_a}
4715 and @code{use_dirs}.@refill
4717 Computed variable names can also be used in substitution references:
4721 a_objects := a.o b.o c.o
4722 1_objects := 1.o 2.o 3.o
4724 sources := $($(a1)_objects:.o=.c)
4729 defines @code{sources} as either @samp{a.c b.c c.c} or @samp{1.c 2.c 3.c},
4730 depending on the value of @code{a1}.
4732 The only restriction on this sort of use of nested variable references
4733 is that they cannot specify part of the name of a function to be called.
4734 This is because the test for a recognized function name is done before
4735 the expansion of nested references. For example,
4751 foo := $($(func) $(bar))
4756 attempts to give @samp{foo} the value of the variable @samp{sort a d b g
4757 q c} or @samp{strip a d b g q c}, rather than giving @samp{a d b g q c}
4758 as the argument to either the @code{sort} or the @code{strip} function.
4759 This restriction could be removed in the future if that change is shown
4762 You can also use computed variable names in the left-hand side of a
4763 variable assignment, or in a @code{define} directive, as in:
4767 $(dir)_sources := $(wildcard $(dir)/*.c)
4769 lpr $($(dir)_sources)
4774 This example defines the variables @samp{dir}, @samp{foo_sources}, and
4777 Note that @dfn{nested variable references} are quite different from
4778 @dfn{recursively expanded variables}
4779 (@pxref{Flavors, ,The Two Flavors of Variables}), though both are
4780 used together in complex ways when doing makefile programming.@refill
4782 @node Values, Setting, Advanced, Using Variables
4783 @section How Variables Get Their Values
4784 @cindex variables, how they get their values
4785 @cindex value, how a variable gets it
4787 Variables can get values in several different ways:
4791 You can specify an overriding value when you run @code{make}.
4792 @xref{Overriding, ,Overriding Variables}.
4795 You can specify a value in the makefile, either
4796 with an assignment (@pxref{Setting, ,Setting Variables}) or with a
4797 verbatim definition (@pxref{Defining, ,Defining Variables Verbatim}).@refill
4800 Variables in the environment become @code{make} variables.
4801 @xref{Environment, ,Variables from the Environment}.
4804 Several @dfn{automatic} variables are given new values for each rule.
4805 Each of these has a single conventional use.
4806 @xref{Automatic Variables}.
4809 Several variables have constant initial values.
4810 @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
4813 @node Setting, Appending, Values, Using Variables
4814 @section Setting Variables
4815 @cindex setting variables
4816 @cindex variables, setting
4821 To set a variable from the makefile, write a line starting with the
4822 variable name followed by @samp{=} or @samp{:=}. Whatever follows the
4823 @samp{=} or @samp{:=} on the line becomes the value. For example,
4826 objects = main.o foo.o bar.o utils.o
4830 defines a variable named @code{objects}. Whitespace around the variable
4831 name and immediately after the @samp{=} is ignored.
4833 Variables defined with @samp{=} are @dfn{recursively expanded} variables.
4834 Variables defined with @samp{:=} are @dfn{simply expanded} variables; these
4835 definitions can contain variable references which will be expanded before
4836 the definition is made. @xref{Flavors, ,The Two Flavors of Variables}.
4838 The variable name may contain function and variable references, which
4839 are expanded when the line is read to find the actual variable name to use.
4841 There is no limit on the length of the value of a variable except the
4842 amount of swapping space on the computer. When a variable definition is
4843 long, it is a good idea to break it into several lines by inserting
4844 backslash-newline at convenient places in the definition. This will not
4845 affect the functioning of @code{make}, but it will make the makefile easier
4848 Most variable names are considered to have the empty string as a value if
4849 you have never set them. Several variables have built-in initial values
4850 that are not empty, but you can set them in the usual ways
4851 (@pxref{Implicit Variables, ,Variables Used by Implicit Rules}).
4852 Several special variables are set
4853 automatically to a new value for each rule; these are called the
4854 @dfn{automatic} variables (@pxref{Automatic Variables}).
4856 If you'd like a variable to be set to a value only if it's not already
4857 set, then you can use the shorthand operator @samp{?=} instead of
4858 @samp{=}. These two settings of the variable @samp{FOO} are identical
4859 (@pxref{Origin Function, ,The @code{origin} Function}):
4869 ifeq ($(origin FOO), undefined)
4874 @node Appending, Override Directive, Setting, Using Variables
4875 @section Appending More Text to Variables
4877 @cindex appending to variables
4878 @cindex variables, appending to
4880 Often it is useful to add more text to the value of a variable already defined.
4881 You do this with a line containing @samp{+=}, like this:
4884 objects += another.o
4888 This takes the value of the variable @code{objects}, and adds the text
4889 @samp{another.o} to it (preceded by a single space). Thus:
4892 objects = main.o foo.o bar.o utils.o
4893 objects += another.o
4897 sets @code{objects} to @samp{main.o foo.o bar.o utils.o another.o}.
4899 Using @samp{+=} is similar to:
4902 objects = main.o foo.o bar.o utils.o
4903 objects := $(objects) another.o
4907 but differs in ways that become important when you use more complex values.
4909 When the variable in question has not been defined before, @samp{+=}
4910 acts just like normal @samp{=}: it defines a recursively-expanded
4911 variable. However, when there @emph{is} a previous definition, exactly
4912 what @samp{+=} does depends on what flavor of variable you defined
4913 originally. @xref{Flavors, ,The Two Flavors of Variables}, for an
4914 explanation of the two flavors of variables.
4916 When you add to a variable's value with @samp{+=}, @code{make} acts
4917 essentially as if you had included the extra text in the initial
4918 definition of the variable. If you defined it first with @samp{:=},
4919 making it a simply-expanded variable, @samp{+=} adds to that
4920 simply-expanded definition, and expands the new text before appending it
4921 to the old value just as @samp{:=} does
4922 (@pxref{Setting, ,Setting Variables}, for a full explanation of @samp{:=}).
4931 is exactly equivalent to:
4936 variable := $(variable) more
4939 On the other hand, when you use @samp{+=} with a variable that you defined
4940 first to be recursively-expanded using plain @samp{=}, @code{make} does
4941 something a bit different. Recall that when you define a
4942 recursively-expanded variable, @code{make} does not expand the value you set
4943 for variable and function references immediately. Instead it stores the text
4944 verbatim, and saves these variable and function references to be expanded
4945 later, when you refer to the new variable (@pxref{Flavors, ,The Two Flavors
4946 of Variables}). When you use @samp{+=} on a recursively-expanded variable,
4947 it is this unexpanded text to which @code{make} appends the new text you
4958 is roughly equivalent to:
4963 variable = $(temp) more
4968 except that of course it never defines a variable called @code{temp}.
4969 The importance of this comes when the variable's old value contains
4970 variable references. Take this common example:
4973 CFLAGS = $(includes) -O
4975 CFLAGS += -pg # enable profiling
4979 The first line defines the @code{CFLAGS} variable with a reference to another
4980 variable, @code{includes}. (@code{CFLAGS} is used by the rules for C
4981 compilation; @pxref{Catalogue of Rules, ,Catalogue of Implicit Rules}.)
4982 Using @samp{=} for the definition makes @code{CFLAGS} a recursively-expanded
4983 variable, meaning @w{@samp{$(includes) -O}} is @emph{not} expanded when
4984 @code{make} processes the definition of @code{CFLAGS}. Thus, @code{includes}
4985 need not be defined yet for its value to take effect. It only has to be
4986 defined before any reference to @code{CFLAGS}. If we tried to append to the
4987 value of @code{CFLAGS} without using @samp{+=}, we might do it like this:
4990 CFLAGS := $(CFLAGS) -pg # enable profiling
4994 This is pretty close, but not quite what we want. Using @samp{:=}
4995 redefines @code{CFLAGS} as a simply-expanded variable; this means
4996 @code{make} expands the text @w{@samp{$(CFLAGS) -pg}} before setting the
4997 variable. If @code{includes} is not yet defined, we get @w{@samp{ -O
4998 -pg}}, and a later definition of @code{includes} will have no effect.
4999 Conversely, by using @samp{+=} we set @code{CFLAGS} to the
5000 @emph{unexpanded} value @w{@samp{$(includes) -O -pg}}. Thus we preserve
5001 the reference to @code{includes}, so if that variable gets defined at
5002 any later point, a reference like @samp{$(CFLAGS)} still uses its
5005 @node Override Directive, Defining, Appending, Using Variables
5006 @section The @code{override} Directive
5008 @cindex overriding with @code{override}
5009 @cindex variables, overriding
5011 If a variable has been set with a command argument
5012 (@pxref{Overriding, ,Overriding Variables}),
5013 then ordinary assignments in the makefile are ignored. If you want to set
5014 the variable in the makefile even though it was set with a command
5015 argument, you can use an @code{override} directive, which is a line that
5016 looks like this:@refill
5019 override @var{variable} = @var{value}
5026 override @var{variable} := @var{value}
5029 To append more text to a variable defined on the command line, use:
5032 override @var{variable} += @var{more text}
5036 @xref{Appending, ,Appending More Text to Variables}.
5038 The @code{override} directive was not invented for escalation in the war
5039 between makefiles and command arguments. It was invented so you can alter
5040 and add to values that the user specifies with command arguments.
5042 For example, suppose you always want the @samp{-g} switch when you run the
5043 C compiler, but you would like to allow the user to specify the other
5044 switches with a command argument just as usual. You could use this
5045 @code{override} directive:
5048 override CFLAGS += -g
5051 You can also use @code{override} directives with @code{define} directives.
5052 This is done as you might expect:
5062 See the next section for information about @code{define}.
5065 @xref{Defining, ,Defining Variables Verbatim}.
5068 @node Defining, Environment, Override Directive, Using Variables
5069 @section Defining Variables Verbatim
5072 @cindex verbatim variable definition
5073 @cindex defining variables verbatim
5074 @cindex variables, defining verbatim
5076 Another way to set the value of a variable is to use the @code{define}
5077 directive. This directive has an unusual syntax which allows newline
5078 characters to be included in the value, which is convenient for defining
5079 both canned sequences of commands
5080 (@pxref{Sequences, ,Defining Canned Command Sequences}), and also
5081 sections of makefile syntax to use with @code{eval} (@pxref{Eval Function}).
5083 The @code{define} directive is followed on the same line by the name of the
5084 variable and nothing more. The value to give the variable appears on the
5085 following lines. The end of the value is marked by a line containing just
5086 the word @code{endef}. Aside from this difference in syntax, @code{define}
5087 works just like @samp{=}: it creates a recursively-expanded variable
5088 (@pxref{Flavors, ,The Two Flavors of Variables}).
5089 The variable name may contain function and variable references, which
5090 are expanded when the directive is read to find the actual variable name
5093 You may nest @code{define} directives: @code{make} will keep track of
5094 nested directives and report an error if they are not all properly
5095 closed with @code{endef}. Note that lines beginning with tab
5096 characters are considered part of a command script, so any
5097 @code{define} or @code{endef} strings appearing on such a line will
5098 not be considered @code{make} operators.
5107 The value in an ordinary assignment cannot contain a newline; but the
5108 newlines that separate the lines of the value in a @code{define} become
5109 part of the variable's value (except for the final newline which precedes
5110 the @code{endef} and is not considered part of the value).@refill
5113 When used in a command script, the previous example is functionally
5117 two-lines = echo foo; echo $(bar)
5121 since two commands separated by semicolon behave much like two separate
5122 shell commands. However, note that using two separate lines means
5123 @code{make} will invoke the shell twice, running an independent subshell
5124 for each line. @xref{Execution, ,Command Execution}.
5126 If you want variable definitions made with @code{define} to take
5127 precedence over command-line variable definitions, you can use the
5128 @code{override} directive together with @code{define}:
5131 override define two-lines
5138 @xref{Override Directive, ,The @code{override} Directive}.
5140 @node Environment, Target-specific, Defining, Using Variables
5141 @section Variables from the Environment
5143 @cindex variables, environment
5145 Variables in @code{make} can come from the environment in which
5146 @code{make} is run. Every environment variable that @code{make} sees
5147 when it starts up is transformed into a @code{make} variable with the
5148 same name and value. However, an explicit assignment in the makefile,
5149 or with a command argument, overrides the environment. (If the
5150 @samp{-e} flag is specified, then values from the environment override
5151 assignments in the makefile. @xref{Options Summary, ,Summary of
5152 Options}. But this is not recommended practice.)
5154 Thus, by setting the variable @code{CFLAGS} in your environment, you can
5155 cause all C compilations in most makefiles to use the compiler switches you
5156 prefer. This is safe for variables with standard or conventional meanings
5157 because you know that no makefile will use them for other things. (Note
5158 this is not totally reliable; some makefiles set @code{CFLAGS} explicitly
5159 and therefore are not affected by the value in the environment.)
5161 When @code{make} runs a command script, variables defined in the
5162 makefile are placed into the environment of that command. This allows
5163 you to pass values to sub-@code{make} invocations. (@pxref{Recursion,
5164 ,Recursive Use of @code{make}}). By default, only variables that came
5165 from the environment or the command line are passed to recursive
5166 invocations. You can use the @code{export} directive to pass other
5167 variables. @xref{Variables/Recursion, , Communicating Variables to a
5168 Sub-@code{make}}, for full details.
5170 Other use of variables from the environment is not recommended. It is not
5171 wise for makefiles to depend for their functioning on environment variables
5172 set up outside their control, since this would cause different users to get
5173 different results from the same makefile. This is against the whole
5174 purpose of most makefiles.
5176 @cindex SHELL, import from environment
5177 Such problems would be especially likely with the variable @code{SHELL},
5178 which is normally present in the environment to specify the user's choice
5179 of interactive shell. It would be very undesirable for this choice to
5180 affect @code{make}. So @code{make} ignores the environment value of
5181 @code{SHELL} (except on MS-DOS and MS-Windows, where @code{SHELL} is
5182 usually not set. @xref{Execution, ,Special handling of SHELL on
5185 @cindex SHELL, export to environment
5186 The @code{SHELL} variable is special in another way: just as the value
5187 of the @code{make} variable @code{SHELL} is not taken from the
5188 environment, so also it is not placed into the environment of commands
5189 that @code{make} invokes. Instead, the value of @code{SHELL} from the
5190 invoking environment is provided to the command. You can use
5191 @code{export SHELL} to force the value of the @code{make} variable
5192 @code{SHELL} to be placed in the environment of commands.
5194 @node Target-specific, Pattern-specific, Environment, Using Variables
5195 @section Target-specific Variable Values
5196 @cindex target-specific variables
5197 @cindex variables, target-specific
5199 Variable values in @code{make} are usually global; that is, they are the
5200 same regardless of where they are evaluated (unless they're reset, of
5201 course). One exception to that is automatic variables
5202 (@pxref{Automatic Variables}).
5204 The other exception is @dfn{target-specific variable values}. This
5205 feature allows you to define different values for the same variable,
5206 based on the target that @code{make} is currently building. As with
5207 automatic variables, these values are only available within the context
5208 of a target's command script (and in other target-specific assignments).
5210 Set a target-specific variable value like this:
5213 @var{target} @dots{} : @var{variable-assignment}
5220 @var{target} @dots{} : override @var{variable-assignment}
5227 @var{target} @dots{} : export @var{variable-assignment}
5230 Multiple @var{target} values create a target-specific variable value for
5231 each member of the target list individually.
5233 The @var{variable-assignment} can be any valid form of assignment;
5234 recursive (@samp{=}), static (@samp{:=}), appending (@samp{+=}), or
5235 conditional (@samp{?=}). All variables that appear within the
5236 @var{variable-assignment} are evaluated within the context of the
5237 target: thus, any previously-defined target-specific variable values
5238 will be in effect. Note that this variable is actually distinct from
5239 any ``global'' value: the two variables do not have to have the same
5240 flavor (recursive vs. static).
5242 Target-specific variables have the same priority as any other makefile
5243 variable. Variables provided on the command-line (and in the
5244 environment if the @samp{-e} option is in force) will take precedence.
5245 Specifying the @code{override} directive will allow the target-specific
5246 variable value to be preferred.
5248 There is one more special feature of target-specific variables: when
5249 you define a target-specific variable that variable value is also in
5250 effect for all prerequisites of this target, and all their
5251 prerequisites, etc. (unless those prerequisites override that variable
5252 with their own target-specific variable value). So, for example, a
5253 statement like this:
5257 prog : prog.o foo.o bar.o
5261 will set @code{CFLAGS} to @samp{-g} in the command script for
5262 @file{prog}, but it will also set @code{CFLAGS} to @samp{-g} in the
5263 command scripts that create @file{prog.o}, @file{foo.o}, and
5264 @file{bar.o}, and any command scripts which create their
5267 Be aware that a given prerequisite will only be built once per
5268 invocation of make, at most. If the same file is a prerequisite of
5269 multiple targets, and each of those targets has a different value for
5270 the same target-specific variable, then the first target to be built
5271 will cause that prerequisite to be built and the prerequisite will
5272 inherit the target-specific value from the first target. It will
5273 ignore the target-specific values from any other targets.
5275 @node Pattern-specific, , Target-specific, Using Variables
5276 @section Pattern-specific Variable Values
5277 @cindex pattern-specific variables
5278 @cindex variables, pattern-specific
5280 In addition to target-specific variable values
5281 (@pxref{Target-specific, ,Target-specific Variable Values}), GNU
5282 @code{make} supports pattern-specific variable values. In this form,
5283 the variable is defined for any target that matches the pattern
5284 specified. If a target matches more than one pattern, all the
5285 matching pattern-specific variables are interpreted in the order in
5286 which they were defined in the makefile, and collected together into
5287 one set. Variables defined in this way are searched after any
5288 target-specific variables defined explicitly for that target, and
5289 before target-specific variables defined for the parent target.
5291 Set a pattern-specific variable value like this:
5294 @var{pattern} @dots{} : @var{variable-assignment}
5301 @var{pattern} @dots{} : override @var{variable-assignment}
5305 where @var{pattern} is a %-pattern. As with target-specific variable
5306 values, multiple @var{pattern} values create a pattern-specific variable
5307 value for each pattern individually. The @var{variable-assignment} can
5308 be any valid form of assignment. Any command-line variable setting will
5309 take precedence, unless @code{override} is specified.
5318 will assign @code{CFLAGS} the value of @samp{-O} for all targets
5319 matching the pattern @code{%.o}.
5321 @node Conditionals, Functions, Using Variables, Top
5322 @chapter Conditional Parts of Makefiles
5324 @cindex conditionals
5325 A @dfn{conditional} causes part of a makefile to be obeyed or ignored
5326 depending on the values of variables. Conditionals can compare the
5327 value of one variable to another, or the value of a variable to
5328 a constant string. Conditionals control what @code{make} actually
5329 ``sees'' in the makefile, so they @emph{cannot} be used to control shell
5330 commands at the time of execution.@refill
5333 * Conditional Example:: Example of a conditional
5334 * Conditional Syntax:: The syntax of conditionals.
5335 * Testing Flags:: Conditionals that test flags.
5338 @node Conditional Example, Conditional Syntax, Conditionals, Conditionals
5339 @section Example of a Conditional
5341 The following example of a conditional tells @code{make} to use one set
5342 of libraries if the @code{CC} variable is @samp{gcc}, and a different
5343 set of libraries otherwise. It works by controlling which of two
5344 command lines will be used as the command for a rule. The result is
5345 that @samp{CC=gcc} as an argument to @code{make} changes not only which
5346 compiler is used but also which libraries are linked.
5349 libs_for_gcc = -lgnu
5354 $(CC) -o foo $(objects) $(libs_for_gcc)
5356 $(CC) -o foo $(objects) $(normal_libs)
5360 This conditional uses three directives: one @code{ifeq}, one @code{else}
5361 and one @code{endif}.
5363 The @code{ifeq} directive begins the conditional, and specifies the
5364 condition. It contains two arguments, separated by a comma and surrounded
5365 by parentheses. Variable substitution is performed on both arguments and
5366 then they are compared. The lines of the makefile following the
5367 @code{ifeq} are obeyed if the two arguments match; otherwise they are
5370 The @code{else} directive causes the following lines to be obeyed if the
5371 previous conditional failed. In the example above, this means that the
5372 second alternative linking command is used whenever the first alternative
5373 is not used. It is optional to have an @code{else} in a conditional.
5375 The @code{endif} directive ends the conditional. Every conditional must
5376 end with an @code{endif}. Unconditional makefile text follows.
5378 As this example illustrates, conditionals work at the textual level:
5379 the lines of the conditional are treated as part of the makefile, or
5380 ignored, according to the condition. This is why the larger syntactic
5381 units of the makefile, such as rules, may cross the beginning or the
5382 end of the conditional.
5384 When the variable @code{CC} has the value @samp{gcc}, the above example has
5389 $(CC) -o foo $(objects) $(libs_for_gcc)
5393 When the variable @code{CC} has any other value, the effect is this:
5397 $(CC) -o foo $(objects) $(normal_libs)
5400 Equivalent results can be obtained in another way by conditionalizing a
5401 variable assignment and then using the variable unconditionally:
5404 libs_for_gcc = -lgnu
5408 libs=$(libs_for_gcc)
5414 $(CC) -o foo $(objects) $(libs)
5417 @node Conditional Syntax, Testing Flags, Conditional Example, Conditionals
5418 @section Syntax of Conditionals
5426 The syntax of a simple conditional with no @code{else} is as follows:
5429 @var{conditional-directive}
5435 The @var{text-if-true} may be any lines of text, to be considered as part
5436 of the makefile if the condition is true. If the condition is false, no
5437 text is used instead.
5439 The syntax of a complex conditional is as follows:
5442 @var{conditional-directive}
5450 If the condition is true, @var{text-if-true} is used; otherwise,
5451 @var{text-if-false} is used instead. The @var{text-if-false} can be any
5452 number of lines of text.
5454 The syntax of the @var{conditional-directive} is the same whether the
5455 conditional is simple or complex. There are four different directives that
5456 test different conditions. Here is a table of them:
5459 @item ifeq (@var{arg1}, @var{arg2})
5460 @itemx ifeq '@var{arg1}' '@var{arg2}'
5461 @itemx ifeq "@var{arg1}" "@var{arg2}"
5462 @itemx ifeq "@var{arg1}" '@var{arg2}'
5463 @itemx ifeq '@var{arg1}' "@var{arg2}"
5464 Expand all variable references in @var{arg1} and @var{arg2} and
5465 compare them. If they are identical, the @var{text-if-true} is
5466 effective; otherwise, the @var{text-if-false}, if any, is effective.
5468 Often you want to test if a variable has a non-empty value. When the
5469 value results from complex expansions of variables and functions,
5470 expansions you would consider empty may actually contain whitespace
5471 characters and thus are not seen as empty. However, you can use the
5472 @code{strip} function (@pxref{Text Functions}) to avoid interpreting
5473 whitespace as a non-empty value. For example:
5477 ifeq ($(strip $(foo)),)
5484 will evaluate @var{text-if-empty} even if the expansion of
5485 @code{$(foo)} contains whitespace characters.
5487 @item ifneq (@var{arg1}, @var{arg2})
5488 @itemx ifneq '@var{arg1}' '@var{arg2}'
5489 @itemx ifneq "@var{arg1}" "@var{arg2}"
5490 @itemx ifneq "@var{arg1}" '@var{arg2}'
5491 @itemx ifneq '@var{arg1}' "@var{arg2}"
5492 Expand all variable references in @var{arg1} and @var{arg2} and
5493 compare them. If they are different, the @var{text-if-true} is
5494 effective; otherwise, the @var{text-if-false}, if any, is effective.
5496 @item ifdef @var{variable-name}
5497 If the variable @var{variable-name} has a non-empty value, the
5498 @var{text-if-true} is effective; otherwise, the @var{text-if-false},
5499 if any, is effective. Variables that have never been defined have an
5500 empty value. The variable @var{variable-name} is itself expanded, so
5501 it could be a variable or function that expands to the name of a
5504 Note that @code{ifdef} only tests whether a variable has a value. It
5505 does not expand the variable to see if that value is nonempty.
5506 Consequently, tests using @code{ifdef} return true for all definitions
5507 except those like @code{foo =}. To test for an empty value, use
5508 @w{@code{ifeq ($(foo),)}}. For example,
5521 sets @samp{frobozz} to @samp{yes}, while:
5533 sets @samp{frobozz} to @samp{no}.
5535 @item ifndef @var{variable-name}
5536 If the variable @var{variable-name} has an empty value, the
5537 @var{text-if-true} is effective; otherwise, the @var{text-if-false},
5538 if any, is effective.
5541 Extra spaces are allowed and ignored at the beginning of the conditional
5542 directive line, but a tab is not allowed. (If the line begins with a tab,
5543 it will be considered a command for a rule.) Aside from this, extra spaces
5544 or tabs may be inserted with no effect anywhere except within the directive
5545 name or within an argument. A comment starting with @samp{#} may appear at
5546 the end of the line.
5548 The other two directives that play a part in a conditional are @code{else}
5549 and @code{endif}. Each of these directives is written as one word, with no
5550 arguments. Extra spaces are allowed and ignored at the beginning of the
5551 line, and spaces or tabs at the end. A comment starting with @samp{#} may
5552 appear at the end of the line.
5554 Conditionals affect which lines of the makefile @code{make} uses. If
5555 the condition is true, @code{make} reads the lines of the
5556 @var{text-if-true} as part of the makefile; if the condition is false,
5557 @code{make} ignores those lines completely. It follows that syntactic
5558 units of the makefile, such as rules, may safely be split across the
5559 beginning or the end of the conditional.@refill
5561 @code{make} evaluates conditionals when it reads a makefile.
5562 Consequently, you cannot use automatic variables in the tests of
5563 conditionals because they are not defined until commands are run
5564 (@pxref{Automatic Variables}).
5566 To prevent intolerable confusion, it is not permitted to start a
5567 conditional in one makefile and end it in another. However, you may
5568 write an @code{include} directive within a conditional, provided you do
5569 not attempt to terminate the conditional inside the included file.
5571 @node Testing Flags, , Conditional Syntax, Conditionals
5572 @section Conditionals that Test Flags
5574 You can write a conditional that tests @code{make} command flags such as
5575 @samp{-t} by using the variable @code{MAKEFLAGS} together with the
5576 @code{findstring} function
5577 (@pxref{Text Functions, , Functions for String Substitution and Analysis}).
5578 This is useful when @code{touch} is not enough to make a file appear up
5581 The @code{findstring} function determines whether one string appears as a
5582 substring of another. If you want to test for the @samp{-t} flag,
5583 use @samp{t} as the first string and the value of @code{MAKEFLAGS} as
5586 For example, here is how to arrange to use @samp{ranlib -t} to finish
5587 marking an archive file up to date:
5591 ifneq (,$(findstring t,$(MAKEFLAGS)))
5593 +ranlib -t archive.a
5600 The @samp{+} prefix marks those command lines as ``recursive'' so
5601 that they will be executed despite use of the @samp{-t} flag.
5602 @xref{Recursion, ,Recursive Use of @code{make}}.
5604 @node Functions, Running, Conditionals, Top
5605 @chapter Functions for Transforming Text
5608 @dfn{Functions} allow you to do text processing in the makefile to compute
5609 the files to operate on or the commands to use. You use a function in a
5610 @dfn{function call}, where you give the name of the function and some text
5611 (the @dfn{arguments}) for the function to operate on. The result of the
5612 function's processing is substituted into the makefile at the point of the
5613 call, just as a variable might be substituted.
5616 * Syntax of Functions:: How to write a function call.
5617 * Text Functions:: General-purpose text manipulation functions.
5618 * File Name Functions:: Functions for manipulating file names.
5619 * Foreach Function:: Repeat some text with controlled variation.
5620 * If Function:: Conditionally expand a value.
5621 * Call Function:: Expand a user-defined function.
5622 * Value Function:: Return the un-expanded value of a variable.
5623 * Eval Function:: Evaluate the arguments as makefile syntax.
5624 * Origin Function:: Find where a variable got its value.
5625 * Shell Function:: Substitute the output of a shell command.
5626 * Make Control Functions:: Functions that control how make runs.
5629 @node Syntax of Functions, Text Functions, Functions, Functions
5630 @section Function Call Syntax
5631 @cindex @code{$}, in function call
5632 @cindex dollar sign (@code{$}), in function call
5633 @cindex arguments of functions
5634 @cindex functions, syntax of
5636 A function call resembles a variable reference. It looks like this:
5639 $(@var{function} @var{arguments})
5646 $@{@var{function} @var{arguments}@}
5649 Here @var{function} is a function name; one of a short list of names
5650 that are part of @code{make}. You can also essentially create your own
5651 functions by using the @code{call} builtin function.
5653 The @var{arguments} are the arguments of the function. They are
5654 separated from the function name by one or more spaces or tabs, and if
5655 there is more than one argument, then they are separated by commas.
5656 Such whitespace and commas are not part of an argument's value. The
5657 delimiters which you use to surround the function call, whether
5658 parentheses or braces, can appear in an argument only in matching pairs;
5659 the other kind of delimiters may appear singly. If the arguments
5660 themselves contain other function calls or variable references, it is
5661 wisest to use the same kind of delimiters for all the references; write
5662 @w{@samp{$(subst a,b,$(x))}}, not @w{@samp{$(subst a,b,$@{x@})}}. This
5663 is because it is clearer, and because only one type of delimiter is
5664 matched to find the end of the reference.
5666 The text written for each argument is processed by substitution of
5667 variables and function calls to produce the argument value, which
5668 is the text on which the function acts. The substitution is done in the
5669 order in which the arguments appear.
5671 Commas and unmatched parentheses or braces cannot appear in the text of an
5672 argument as written; leading spaces cannot appear in the text of the first
5673 argument as written. These characters can be put into the argument value
5674 by variable substitution. First define variables @code{comma} and
5675 @code{space} whose values are isolated comma and space characters, then
5676 substitute these variables where such characters are wanted, like this:
5682 space:= $(empty) $(empty)
5684 bar:= $(subst $(space),$(comma),$(foo))
5685 # @r{bar is now `a,b,c'.}
5690 Here the @code{subst} function replaces each space with a comma, through
5691 the value of @code{foo}, and substitutes the result.
5693 @node Text Functions, File Name Functions, Syntax of Functions, Functions
5694 @section Functions for String Substitution and Analysis
5695 @cindex functions, for text
5697 Here are some functions that operate on strings:
5700 @item $(subst @var{from},@var{to},@var{text})
5702 Performs a textual replacement on the text @var{text}: each occurrence
5703 of @var{from} is replaced by @var{to}. The result is substituted for
5704 the function call. For example,
5707 $(subst ee,EE,feet on the street)
5710 substitutes the string @samp{fEEt on the strEEt}.
5712 @item $(patsubst @var{pattern},@var{replacement},@var{text})
5714 Finds whitespace-separated words in @var{text} that match
5715 @var{pattern} and replaces them with @var{replacement}. Here
5716 @var{pattern} may contain a @samp{%} which acts as a wildcard,
5717 matching any number of any characters within a word. If
5718 @var{replacement} also contains a @samp{%}, the @samp{%} is replaced
5719 by the text that matched the @samp{%} in @var{pattern}. Only the first
5720 @samp{%} in the @var{pattern} and @var{replacement} is treated this
5721 way; any subsequent @samp{%} is unchanged.@refill
5723 @cindex @code{%}, quoting in @code{patsubst}
5724 @cindex @code{%}, quoting with @code{\} (backslash)
5725 @cindex @code{\} (backslash), to quote @code{%}
5726 @cindex backslash (@code{\}), to quote @code{%}
5727 @cindex quoting @code{%}, in @code{patsubst}
5728 @samp{%} characters in @code{patsubst} function invocations can be
5729 quoted with preceding backslashes (@samp{\}). Backslashes that would
5730 otherwise quote @samp{%} characters can be quoted with more backslashes.
5731 Backslashes that quote @samp{%} characters or other backslashes are
5732 removed from the pattern before it is compared file names or has a stem
5733 substituted into it. Backslashes that are not in danger of quoting
5734 @samp{%} characters go unmolested. For example, the pattern
5735 @file{the\%weird\\%pattern\\} has @samp{the%weird\} preceding the
5736 operative @samp{%} character, and @samp{pattern\\} following it. The
5737 final two backslashes are left alone because they cannot affect any
5738 @samp{%} character.@refill
5740 Whitespace between words is folded into single space characters;
5741 leading and trailing whitespace is discarded.
5746 $(patsubst %.c,%.o,x.c.c bar.c)
5750 produces the value @samp{x.c.o bar.o}.
5752 Substitution references (@pxref{Substitution Refs, ,Substitution
5753 References}) are a simpler way to get the effect of the @code{patsubst}
5757 $(@var{var}:@var{pattern}=@var{replacement})
5764 $(patsubst @var{pattern},@var{replacement},$(@var{var}))
5767 The second shorthand simplifies one of the most common uses of
5768 @code{patsubst}: replacing the suffix at the end of file names.
5771 $(@var{var}:@var{suffix}=@var{replacement})
5778 $(patsubst %@var{suffix},%@var{replacement},$(@var{var}))
5782 For example, you might have a list of object files:
5785 objects = foo.o bar.o baz.o
5789 To get the list of corresponding source files, you could simply write:
5796 instead of using the general form:
5799 $(patsubst %.o,%.c,$(objects))
5802 @item $(strip @var{string})
5803 @cindex stripping whitespace
5804 @cindex whitespace, stripping
5805 @cindex spaces, stripping
5807 Removes leading and trailing whitespace from @var{string} and replaces
5808 each internal sequence of one or more whitespace characters with a
5809 single space. Thus, @samp{$(strip a b c )} results in @w{@samp{a b c}}.
5811 The function @code{strip} can be very useful when used in conjunction
5812 with conditionals. When comparing something with the empty string
5813 @samp{} using @code{ifeq} or @code{ifneq}, you usually want a string of
5814 just whitespace to match the empty string (@pxref{Conditionals}).
5816 Thus, the following may fail to have the desired results:
5820 ifneq "$(needs_made)" ""
5823 all:;@@echo 'Nothing to make!'
5828 Replacing the variable reference @w{@samp{$(needs_made)}} with the
5829 function call @w{@samp{$(strip $(needs_made))}} in the @code{ifneq}
5830 directive would make it more robust.@refill
5832 @item $(findstring @var{find},@var{in})
5834 @cindex searching for strings
5835 @cindex finding strings
5836 @cindex strings, searching for
5837 Searches @var{in} for an occurrence of @var{find}. If it occurs, the
5838 value is @var{find}; otherwise, the value is empty. You can use this
5839 function in a conditional to test for the presence of a specific
5840 substring in a given string. Thus, the two examples,
5843 $(findstring a,a b c)
5848 produce the values @samp{a} and @samp{} (the empty string),
5849 respectively. @xref{Testing Flags}, for a practical application of
5850 @code{findstring}.@refill
5854 @cindex filtering words
5855 @cindex words, filtering
5856 @item $(filter @var{pattern}@dots{},@var{text})
5857 Returns all whitespace-separated words in @var{text} that @emph{do} match
5858 any of the @var{pattern} words, removing any words that @emph{do not}
5859 match. The patterns are written using @samp{%}, just like the patterns
5860 used in the @code{patsubst} function above.@refill
5862 The @code{filter} function can be used to separate out different types
5863 of strings (such as file names) in a variable. For example:
5866 sources := foo.c bar.c baz.s ugh.h
5868 cc $(filter %.c %.s,$(sources)) -o foo
5872 says that @file{foo} depends of @file{foo.c}, @file{bar.c},
5873 @file{baz.s} and @file{ugh.h} but only @file{foo.c}, @file{bar.c} and
5874 @file{baz.s} should be specified in the command to the
5877 @item $(filter-out @var{pattern}@dots{},@var{text})
5879 @cindex filtering out words
5880 @cindex words, filtering out
5881 Returns all whitespace-separated words in @var{text} that @emph{do not}
5882 match any of the @var{pattern} words, removing the words that @emph{do}
5883 match one or more. This is the exact opposite of the @code{filter}
5890 objects=main1.o foo.o main2.o bar.o
5891 mains=main1.o main2.o
5896 the following generates a list which contains all the object files not
5900 $(filter-out $(mains),$(objects))
5905 @cindex sorting words
5906 @item $(sort @var{list})
5907 Sorts the words of @var{list} in lexical order, removing duplicate
5908 words. The output is a list of words separated by single spaces.
5912 $(sort foo bar lose)
5916 returns the value @samp{bar foo lose}.
5918 @cindex removing duplicate words
5919 @cindex duplicate words, removing
5920 @cindex words, removing duplicates
5921 Incidentally, since @code{sort} removes duplicate words, you can use
5922 it for this purpose even if you don't care about the sort order.
5924 @item $(word @var{n},@var{text})
5926 @cindex word, selecting a
5927 @cindex selecting a word
5928 Returns the @var{n}th word of @var{text}. The legitimate values of
5929 @var{n} start from 1. If @var{n} is bigger than the number of words
5930 in @var{text}, the value is empty. For example,
5933 $(word 2, foo bar baz)
5939 @item $(wordlist @var{s},@var{e},@var{text})
5941 @cindex words, selecting lists of
5942 @cindex selecting word lists
5943 Returns the list of words in @var{text} starting with word @var{s} and
5944 ending with word @var{e} (inclusive). The legitimate values of @var{s}
5945 and @var{e} start from 1. If @var{s} is bigger than the number of words
5946 in @var{text}, the value is empty. If @var{e} is bigger than the number
5947 of words in @var{text}, words up to the end of @var{text} are returned.
5948 If @var{s} is greater than @var{e}, nothing is returned. For example,
5951 $(wordlist 2, 3, foo bar baz)
5955 returns @samp{bar baz}.
5957 @c Following item phrased to prevent overfull hbox. --RJC 17 Jul 92
5958 @item $(words @var{text})
5960 @cindex words, finding number
5961 Returns the number of words in @var{text}.
5962 Thus, the last word of @var{text} is
5963 @w{@code{$(word $(words @var{text}),@var{text})}}.@refill
5965 @item $(firstword @var{names}@dots{})
5967 @cindex words, extracting first
5968 The argument @var{names} is regarded as a series of names, separated
5969 by whitespace. The value is the first name in the series. The rest
5970 of the names are ignored.
5975 $(firstword foo bar)
5979 produces the result @samp{foo}. Although @code{$(firstword
5980 @var{text})} is the same as @code{$(word 1,@var{text})}, the
5981 @code{firstword} function is retained for its simplicity.@refill
5984 @item $(lastword @var{names}@dots{})
5986 @cindex words, extracting last
5987 The argument @var{names} is regarded as a series of names, separated
5988 by whitespace. The value is the last name in the series.
5997 produces the result @samp{bar}. Although @code{$(lastword
5998 @var{text})} is the same as @code{$(word $(words @var{text}),@var{text})},
5999 the @code{lastword} function was added for its simplicity and better
6004 Here is a realistic example of the use of @code{subst} and
6005 @code{patsubst}. Suppose that a makefile uses the @code{VPATH} variable
6006 to specify a list of directories that @code{make} should search for
6008 (@pxref{General Search, , @code{VPATH} Search Path for All Prerequisites}).
6009 This example shows how to
6010 tell the C compiler to search for header files in the same list of
6013 The value of @code{VPATH} is a list of directories separated by colons,
6014 such as @samp{src:../headers}. First, the @code{subst} function is used to
6015 change the colons to spaces:
6018 $(subst :, ,$(VPATH))
6022 This produces @samp{src ../headers}. Then @code{patsubst} is used to turn
6023 each directory name into a @samp{-I} flag. These can be added to the
6024 value of the variable @code{CFLAGS}, which is passed automatically to the C
6025 compiler, like this:
6028 override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
6032 The effect is to append the text @samp{-Isrc -I../headers} to the
6033 previously given value of @code{CFLAGS}. The @code{override} directive is
6034 used so that the new value is assigned even if the previous value of
6035 @code{CFLAGS} was specified with a command argument (@pxref{Override
6036 Directive, , The @code{override} Directive}).
6038 @node File Name Functions, Foreach Function, Text Functions, Functions
6039 @section Functions for File Names
6040 @cindex functions, for file names
6041 @cindex file name functions
6043 Several of the built-in expansion functions relate specifically to
6044 taking apart file names or lists of file names.
6046 Each of the following functions performs a specific transformation on a
6047 file name. The argument of the function is regarded as a series of file
6048 names, separated by whitespace. (Leading and trailing whitespace is
6049 ignored.) Each file name in the series is transformed in the same way and
6050 the results are concatenated with single spaces between them.
6053 @item $(dir @var{names}@dots{})
6055 @cindex directory part
6056 @cindex file name, directory part
6057 Extracts the directory-part of each file name in @var{names}. The
6058 directory-part of the file name is everything up through (and
6059 including) the last slash in it. If the file name contains no slash,
6060 the directory part is the string @samp{./}. For example,
6063 $(dir src/foo.c hacks)
6067 produces the result @samp{src/ ./}.
6069 @item $(notdir @var{names}@dots{})
6071 @cindex file name, nondirectory part
6072 @cindex nondirectory part
6073 Extracts all but the directory-part of each file name in @var{names}.
6074 If the file name contains no slash, it is left unchanged. Otherwise,
6075 everything through the last slash is removed from it.
6077 A file name that ends with a slash becomes an empty string. This is
6078 unfortunate, because it means that the result does not always have the
6079 same number of whitespace-separated file names as the argument had;
6080 but we do not see any other valid alternative.
6085 $(notdir src/foo.c hacks)
6089 produces the result @samp{foo.c hacks}.
6091 @item $(suffix @var{names}@dots{})
6093 @cindex suffix, function to find
6094 @cindex file name suffix
6095 Extracts the suffix of each file name in @var{names}. If the file name
6096 contains a period, the suffix is everything starting with the last
6097 period. Otherwise, the suffix is the empty string. This frequently
6098 means that the result will be empty when @var{names} is not, and if
6099 @var{names} contains multiple file names, the result may contain fewer
6105 $(suffix src/foo.c src-1.0/bar.c hacks)
6109 produces the result @samp{.c .c}.
6111 @item $(basename @var{names}@dots{})
6114 @cindex file name, basename of
6115 Extracts all but the suffix of each file name in @var{names}. If the
6116 file name contains a period, the basename is everything starting up to
6117 (and not including) the last period. Periods in the directory part are
6118 ignored. If there is no period, the basename is the entire file name.
6122 $(basename src/foo.c src-1.0/bar hacks)
6126 produces the result @samp{src/foo src-1.0/bar hacks}.
6128 @c plural convention with dots (be consistent)
6129 @item $(addsuffix @var{suffix},@var{names}@dots{})
6131 @cindex suffix, adding
6132 @cindex file name suffix, adding
6133 The argument @var{names} is regarded as a series of names, separated
6134 by whitespace; @var{suffix} is used as a unit. The value of
6135 @var{suffix} is appended to the end of each individual name and the
6136 resulting larger names are concatenated with single spaces between
6140 $(addsuffix .c,foo bar)
6144 produces the result @samp{foo.c bar.c}.
6146 @item $(addprefix @var{prefix},@var{names}@dots{})
6148 @cindex prefix, adding
6149 @cindex file name prefix, adding
6150 The argument @var{names} is regarded as a series of names, separated
6151 by whitespace; @var{prefix} is used as a unit. The value of
6152 @var{prefix} is prepended to the front of each individual name and the
6153 resulting larger names are concatenated with single spaces between
6157 $(addprefix src/,foo bar)
6161 produces the result @samp{src/foo src/bar}.
6163 @item $(join @var{list1},@var{list2})
6165 @cindex joining lists of words
6166 @cindex words, joining lists
6167 Concatenates the two arguments word by word: the two first words (one
6168 from each argument) concatenated form the first word of the result, the
6169 two second words form the second word of the result, and so on. So the
6170 @var{n}th word of the result comes from the @var{n}th word of each
6171 argument. If one argument has more words that the other, the extra
6172 words are copied unchanged into the result.
6174 For example, @samp{$(join a b,.c .o)} produces @samp{a.c b.o}.
6176 Whitespace between the words in the lists is not preserved; it is
6177 replaced with a single space.
6179 This function can merge the results of the @code{dir} and
6180 @code{notdir} functions, to produce the original list of files which
6181 was given to those two functions.@refill
6183 @item $(wildcard @var{pattern})
6185 @cindex wildcard, function
6186 The argument @var{pattern} is a file name pattern, typically containing
6187 wildcard characters (as in shell file name patterns). The result of
6188 @code{wildcard} is a space-separated list of the names of existing files
6189 that match the pattern.
6190 @xref{Wildcards, ,Using Wildcard Characters in File Names}.
6193 @node Foreach Function, If Function, File Name Functions, Functions
6194 @section The @code{foreach} Function
6196 @cindex words, iterating over
6198 The @code{foreach} function is very different from other functions. It
6199 causes one piece of text to be used repeatedly, each time with a different
6200 substitution performed on it. It resembles the @code{for} command in the
6201 shell @code{sh} and the @code{foreach} command in the C-shell @code{csh}.
6203 The syntax of the @code{foreach} function is:
6206 $(foreach @var{var},@var{list},@var{text})
6210 The first two arguments, @var{var} and @var{list}, are expanded before
6211 anything else is done; note that the last argument, @var{text}, is
6212 @strong{not} expanded at the same time. Then for each word of the expanded
6213 value of @var{list}, the variable named by the expanded value of @var{var}
6214 is set to that word, and @var{text} is expanded. Presumably @var{text}
6215 contains references to that variable, so its expansion will be different
6218 The result is that @var{text} is expanded as many times as there are
6219 whitespace-separated words in @var{list}. The multiple expansions of
6220 @var{text} are concatenated, with spaces between them, to make the result
6223 This simple example sets the variable @samp{files} to the list of all files
6224 in the directories in the list @samp{dirs}:
6228 files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
6231 Here @var{text} is @samp{$(wildcard $(dir)/*)}. The first repetition
6232 finds the value @samp{a} for @code{dir}, so it produces the same result
6233 as @samp{$(wildcard a/*)}; the second repetition produces the result
6234 of @samp{$(wildcard b/*)}; and the third, that of @samp{$(wildcard c/*)}.
6236 This example has the same result (except for setting @samp{dirs}) as
6237 the following example:
6240 files := $(wildcard a/* b/* c/* d/*)
6243 When @var{text} is complicated, you can improve readability by giving it
6244 a name, with an additional variable:
6247 find_files = $(wildcard $(dir)/*)
6249 files := $(foreach dir,$(dirs),$(find_files))
6253 Here we use the variable @code{find_files} this way. We use plain @samp{=}
6254 to define a recursively-expanding variable, so that its value contains an
6255 actual function call to be reexpanded under the control of @code{foreach};
6256 a simply-expanded variable would not do, since @code{wildcard} would be
6257 called only once at the time of defining @code{find_files}.
6259 The @code{foreach} function has no permanent effect on the variable
6260 @var{var}; its value and flavor after the @code{foreach} function call are
6261 the same as they were beforehand. The other values which are taken from
6262 @var{list} are in effect only temporarily, during the execution of
6263 @code{foreach}. The variable @var{var} is a simply-expanded variable
6264 during the execution of @code{foreach}. If @var{var} was undefined
6265 before the @code{foreach} function call, it is undefined after the call.
6266 @xref{Flavors, ,The Two Flavors of Variables}.@refill
6268 You must take care when using complex variable expressions that result in
6269 variable names because many strange things are valid variable names, but
6270 are probably not what you intended. For example,
6273 files := $(foreach Esta escrito en espanol!,b c ch,$(find_files))
6277 might be useful if the value of @code{find_files} references the variable
6278 whose name is @samp{Esta escrito en espanol!} (es un nombre bastante largo,
6279 no?), but it is more likely to be a mistake.
6281 @node If Function, Call Function, Foreach Function, Functions
6282 @section The @code{if} Function
6284 @cindex conditional expansion
6286 The @code{if} function provides support for conditional expansion in a
6287 functional context (as opposed to the GNU @code{make} makefile
6288 conditionals such as @code{ifeq} (@pxref{Conditional Syntax, ,Syntax of
6291 An @code{if} function call can contain either two or three arguments:
6294 $(if @var{condition},@var{then-part}[,@var{else-part}])
6297 The first argument, @var{condition}, first has all preceding and
6298 trailing whitespace stripped, then is expanded. If it expands to any
6299 non-empty string, then the condition is considered to be true. If it
6300 expands to an empty string, the condition is considered to be false.
6302 If the condition is true then the second argument, @var{then-part}, is
6303 evaluated and this is used as the result of the evaluation of the entire
6306 If the condition is false then the third argument, @var{else-part}, is
6307 evaluated and this is the result of the @code{if} function. If there is
6308 no third argument, the @code{if} function evaluates to nothing (the
6311 Note that only one of the @var{then-part} or the @var{else-part} will be
6312 evaluated, never both. Thus, either can contain side-effects (such as
6313 @code{shell} function calls, etc.)
6315 @node Call Function, Value Function, If Function, Functions
6316 @section The @code{call} Function
6318 @cindex functions, user defined
6319 @cindex user defined functions
6321 The @code{call} function is unique in that it can be used to create new
6322 parameterized functions. You can write a complex expression as the
6323 value of a variable, then use @code{call} to expand it with different
6326 The syntax of the @code{call} function is:
6329 $(call @var{variable},@var{param},@var{param},@dots{})
6332 When @code{make} expands this function, it assigns each @var{param} to
6333 temporary variables @code{$(1)}, @code{$(2)}, etc. The variable
6334 @code{$(0)} will contain @var{variable}. There is no maximum number of
6335 parameter arguments. There is no minimum, either, but it doesn't make
6336 sense to use @code{call} with no parameters.
6338 Then @var{variable} is expanded as a @code{make} variable in the context
6339 of these temporary assignments. Thus, any reference to @code{$(1)} in
6340 the value of @var{variable} will resolve to the first @var{param} in the
6341 invocation of @code{call}.
6343 Note that @var{variable} is the @emph{name} of a variable, not a
6344 @emph{reference} to that variable. Therefore you would not normally use
6345 a @samp{$} or parentheses when writing it. (You can, however, use a
6346 variable reference in the name if you want the name not to be a
6349 If @var{variable} is the name of a builtin function, the builtin function
6350 is always invoked (even if a @code{make} variable by that name also
6353 The @code{call} function expands the @var{param} arguments before
6354 assigning them to temporary variables. This means that @var{variable}
6355 values containing references to builtin functions that have special
6356 expansion rules, like @code{foreach} or @code{if}, may not work as you
6359 Some examples may make this clearer.
6361 This macro simply reverses its arguments:
6366 foo = $(call reverse,a,b)
6370 Here @var{foo} will contain @samp{b a}.
6372 This one is slightly more interesting: it defines a macro to search for
6373 the first instance of a program in @code{PATH}:
6376 pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH)))))
6378 LS := $(call pathsearch,ls)
6382 Now the variable LS contains @code{/bin/ls} or similar.
6384 The @code{call} function can be nested. Each recursive invocation gets
6385 its own local values for @code{$(1)}, etc. that mask the values of
6386 higher-level @code{call}. For example, here is an implementation of a
6390 map = $(foreach a,$(2),$(call $(1),$(a)))
6393 Now you can @var{map} a function that normally takes only one argument,
6394 such as @code{origin}, to multiple values in one step:
6397 o = $(call map,origin,o map MAKE)
6400 and end up with @var{o} containing something like @samp{file file default}.
6402 A final caution: be careful when adding whitespace to the arguments to
6403 @code{call}. As with other functions, any whitespace contained in the
6404 second and subsequent arguments is kept; this can cause strange
6405 effects. It's generally safest to remove all extraneous whitespace when
6406 providing parameters to @code{call}.
6408 @node Value Function, Eval Function, Call Function, Functions
6409 @comment node-name, next, previous, up
6410 @section The @code{value} Function
6412 @cindex variables, unexpanded value
6414 The @code{value} function provides a way for you to use the value of a
6415 variable @emph{without} having it expanded. Please note that this
6416 does not undo expansions which have already occurred; for example if
6417 you create a simply expanded variable its value is expanded during the
6418 definition; in that case the @code{value} function will return the
6419 same result as using the variable directly.
6421 The syntax of the @code{value} function is:
6424 $(value @var{variable})
6427 Note that @var{variable} is the @emph{name} of a variable; not a
6428 @emph{reference} to that variable. Therefore you would not normally
6429 use a @samp{$} or parentheses when writing it. (You can, however, use
6430 a variable reference in the name if you want the name not to be a
6433 The result of this function is a string containing the value of
6434 @var{variable}, without any expansion occurring. For example, in this
6448 The first output line would be @code{ATH}, since the ``$P'' would be
6449 expanded as a @code{make} variable, while the second output line would
6450 be the current value of your @code{$PATH} environment variable, since
6451 the @code{value} function avoided the expansion.
6453 The @code{value} function is most often used in conjunction with the
6454 @code{eval} function (@pxref{Eval Function}).
6456 @node Eval Function, Origin Function, Value Function, Functions
6457 @comment node-name, next, previous, up
6458 @section The @code{eval} Function
6460 @cindex evaluating makefile syntax
6461 @cindex makefile syntax, evaluating
6463 The @code{eval} function is very special: it allows you to define new
6464 makefile constructs that are not constant; which are the result of
6465 evaluating other variables and functions. The argument to the
6466 @code{eval} function is expanded, then the results of that expansion
6467 are parsed as makefile syntax. The expanded results can define new
6468 @code{make} variables, targets, implicit or explicit rules, etc.
6470 The result of the @code{eval} function is always the empty string;
6471 thus, it can be placed virtually anywhere in a makefile without
6472 causing syntax errors.
6474 It's important to realize that the @code{eval} argument is expanded
6475 @emph{twice}; first by the @code{eval} function, then the results of
6476 that expansion are expanded again when they are parsed as makefile
6477 syntax. This means you may need to provide extra levels of escaping
6478 for ``$'' characters when using @code{eval}. The @code{value}
6479 function (@pxref{Value Function}) can sometimes be useful in these
6480 situations, to circumvent unwanted expansions.
6482 Here is an example of how @code{eval} can be used; this example
6483 combines a number of concepts and other functions. Although it might
6484 seem overly complex to use @code{eval} in this example, rather than
6485 just writing out the rules, consider two things: first, the template
6486 definition (in @code{PROGRAM_template}) could need to be much more
6487 complex than it is here; and second, you might put the complex,
6488 ``generic'' part of this example into another makefile, then include
6489 it in all the individual makefiles. Now your individual makefiles are
6490 quite straightforward.
6494 PROGRAMS = server client
6496 server_OBJS = server.o server_priv.o server_access.o
6497 server_LIBS = priv protocol
6499 client_OBJS = client.o client_api.o client_mem.o
6500 client_LIBS = protocol
6502 # Everything after this is generic
6507 define PROGRAM_template
6508 $(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
6509 ALL_OBJS += $$($(1)_OBJS)
6512 $(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))
6515 $(LINK.o) $^ $(LDLIBS) -o $@@
6518 rm -f $(ALL_OBJS) $(PROGRAMS)
6522 @node Origin Function, Shell Function, Eval Function, Functions
6523 @section The @code{origin} Function
6525 @cindex variables, origin of
6526 @cindex origin of variable
6528 The @code{origin} function is unlike most other functions in that it does
6529 not operate on the values of variables; it tells you something @emph{about}
6530 a variable. Specifically, it tells you where it came from.
6532 The syntax of the @code{origin} function is:
6535 $(origin @var{variable})
6538 Note that @var{variable} is the @emph{name} of a variable to inquire about;
6539 not a @emph{reference} to that variable. Therefore you would not normally
6540 use a @samp{$} or parentheses when writing it. (You can, however, use a
6541 variable reference in the name if you want the name not to be a constant.)
6543 The result of this function is a string telling you how the variable
6544 @var{variable} was defined:
6549 if @var{variable} was never defined.
6553 if @var{variable} has a default definition, as is usual with @code{CC}
6554 and so on. @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
6555 Note that if you have redefined a default variable, the @code{origin}
6556 function will return the origin of the later definition.
6560 if @var{variable} was defined as an environment variable and the
6561 @samp{-e} option is @emph{not} turned on (@pxref{Options Summary, ,Summary of Options}).
6563 @item environment override
6565 if @var{variable} was defined as an environment variable and the
6566 @w{@samp{-e}} option @emph{is} turned on (@pxref{Options Summary,
6567 ,Summary of Options}).@refill
6571 if @var{variable} was defined in a makefile.
6575 if @var{variable} was defined on the command line.
6579 if @var{variable} was defined with an @code{override} directive in a
6580 makefile (@pxref{Override Directive, ,The @code{override} Directive}).
6584 if @var{variable} is an automatic variable defined for the
6585 execution of the commands for each rule
6586 (@pxref{Automatic Variables}).
6589 This information is primarily useful (other than for your curiosity) to
6590 determine if you want to believe the value of a variable. For example,
6591 suppose you have a makefile @file{foo} that includes another makefile
6592 @file{bar}. You want a variable @code{bletch} to be defined in @file{bar}
6593 if you run the command @w{@samp{make -f bar}}, even if the environment contains
6594 a definition of @code{bletch}. However, if @file{foo} defined
6595 @code{bletch} before including @file{bar}, you do not want to override that
6596 definition. This could be done by using an @code{override} directive in
6597 @file{foo}, giving that definition precedence over the later definition in
6598 @file{bar}; unfortunately, the @code{override} directive would also
6599 override any command line definitions. So, @file{bar} could
6605 ifeq "$(origin bletch)" "environment"
6606 bletch = barf, gag, etc.
6613 If @code{bletch} has been defined from the environment, this will redefine
6616 If you want to override a previous definition of @code{bletch} if it came
6617 from the environment, even under @samp{-e}, you could instead write:
6621 ifneq "$(findstring environment,$(origin bletch))" ""
6622 bletch = barf, gag, etc.
6627 Here the redefinition takes place if @samp{$(origin bletch)} returns either
6628 @samp{environment} or @samp{environment override}.
6629 @xref{Text Functions, , Functions for String Substitution and Analysis}.
6631 @node Shell Function, Make Control Functions, Origin Function, Functions
6632 @section The @code{shell} Function
6634 @cindex commands, expansion
6636 @cindex shell command, function for
6638 The @code{shell} function is unlike any other function other than the
6639 @code{wildcard} function
6640 (@pxref{Wildcard Function, ,The Function @code{wildcard}}) in that it
6641 communicates with the world outside of @code{make}.
6643 The @code{shell} function performs the same function that backquotes
6644 (@samp{`}) perform in most shells: it does @dfn{command expansion}.
6645 This means that it takes as an argument a shell command and evaluates
6646 to the output of the command. The only processing @code{make} does on
6647 the result is to convert each newline (or carriage-return / newline
6648 pair) to a single space. If there is a trailing (carriage-return
6649 and) newline it will simply be removed.@refill
6651 The commands run by calls to the @code{shell} function are run when the
6652 function calls are expanded (@pxref{Reading Makefiles, , How
6653 @code{make} Reads a Makefile}). Because this function involves
6654 spawning a new shell, you should carefully consider the performance
6655 implications of using the @code{shell} function within recursively
6656 expanded variables vs. simply expanded variables (@pxref{Flavors, ,The
6657 Two Flavors of Variables}).
6659 Here are some examples of the use of the @code{shell} function:
6662 contents := $(shell cat foo)
6666 sets @code{contents} to the contents of the file @file{foo}, with a space
6667 (rather than a newline) separating each line.
6670 files := $(shell echo *.c)
6674 sets @code{files} to the expansion of @samp{*.c}. Unless @code{make} is
6675 using a very strange shell, this has the same result as
6676 @w{@samp{$(wildcard *.c)}} (as long as at least one @samp{.c} file
6679 @node Make Control Functions, , Shell Function, Functions
6680 @section Functions That Control Make
6681 @cindex functions, for controlling make
6682 @cindex controlling make
6684 These functions control the way make runs. Generally, they are used to
6685 provide information to the user of the makefile or to cause make to stop
6686 if some sort of environmental error is detected.
6689 @item $(error @var{text}@dots{})
6691 @cindex error, stopping on
6692 @cindex stopping make
6693 Generates a fatal error where the message is @var{text}. Note that the
6694 error is generated whenever this function is evaluated. So, if you put
6695 it inside a command script or on the right side of a recursive variable
6696 assignment, it won't be evaluated until later. The @var{text} will be
6697 expanded before the error is generated.
6703 $(error error is $(ERROR1))
6708 will generate a fatal error during the read of the makefile if the
6709 @code{make} variable @code{ERROR1} is defined. Or,
6712 ERR = $(error found an error!)
6719 will generate a fatal error while @code{make} is running, if the
6720 @code{err} target is invoked.
6722 @item $(warning @var{text}@dots{})
6724 @cindex warnings, printing
6725 @cindex printing user warnings
6726 This function works similarly to the @code{error} function, above,
6727 except that @code{make} doesn't exit. Instead, @var{text} is expanded
6728 and the resulting message is displayed, but processing of the makefile
6731 The result of the expansion of this function is the empty string.
6734 @node Running, Implicit Rules, Functions, Top
6735 @chapter How to Run @code{make}
6737 A makefile that says how to recompile a program can be used in more
6738 than one way. The simplest use is to recompile every file that is out
6739 of date. Usually, makefiles are written so that if you run
6740 @code{make} with no arguments, it does just that.
6742 But you might want to update only some of the files; you might want to use
6743 a different compiler or different compiler options; you might want just to
6744 find out which files are out of date without changing them.
6746 By giving arguments when you run @code{make}, you can do any of these
6747 things and many others.
6749 The exit status of @code{make} is always one of three values:
6752 The exit status is zero if @code{make} is successful.
6754 The exit status is two if @code{make} encounters any errors.
6755 It will print messages describing the particular errors.
6757 The exit status is one if you use the @samp{-q} flag and @code{make}
6758 determines that some target is not already up to date.
6759 @xref{Instead of Execution, ,Instead of Executing the Commands}.
6763 * Makefile Arguments:: How to specify which makefile to use.
6764 * Goals:: How to use goal arguments to specify which
6765 parts of the makefile to use.
6766 * Instead of Execution:: How to use mode flags to specify what
6767 kind of thing to do with the commands
6768 in the makefile other than simply
6770 * Avoiding Compilation:: How to avoid recompiling certain files.
6771 * Overriding:: How to override a variable to specify
6772 an alternate compiler and other things.
6773 * Testing:: How to proceed past some errors, to
6775 * Options Summary:: Summary of Options
6778 @node Makefile Arguments, Goals, Running, Running
6779 @section Arguments to Specify the Makefile
6780 @cindex @code{--file}
6781 @cindex @code{--makefile}
6784 The way to specify the name of the makefile is with the @samp{-f} or
6785 @samp{--file} option (@samp{--makefile} also works). For example,
6786 @samp{-f altmake} says to use the file @file{altmake} as the makefile.
6788 If you use the @samp{-f} flag several times and follow each @samp{-f}
6789 with an argument, all the specified files are used jointly as
6792 If you do not use the @samp{-f} or @samp{--file} flag, the default is
6793 to try @file{GNUmakefile}, @file{makefile}, and @file{Makefile}, in
6794 that order, and use the first of these three which exists or can be made
6795 (@pxref{Makefiles, ,Writing Makefiles}).@refill
6797 @node Goals, Instead of Execution, Makefile Arguments, Running
6798 @section Arguments to Specify the Goals
6799 @cindex goal, how to specify
6801 The @dfn{goals} are the targets that @code{make} should strive ultimately
6802 to update. Other targets are updated as well if they appear as
6803 prerequisites of goals, or prerequisites of prerequisites of goals, etc.
6805 By default, the goal is the first target in the makefile (not counting
6806 targets that start with a period). Therefore, makefiles are usually
6807 written so that the first target is for compiling the entire program or
6808 programs they describe. If the first rule in the makefile has several
6809 targets, only the first target in the rule becomes the default goal, not
6812 You can specify a different goal or goals with arguments to @code{make}.
6813 Use the name of the goal as an argument. If you specify several goals,
6814 @code{make} processes each of them in turn, in the order you name them.
6816 Any target in the makefile may be specified as a goal (unless it
6817 starts with @samp{-} or contains an @samp{=}, in which case it will be
6818 parsed as a switch or variable definition, respectively). Even
6819 targets not in the makefile may be specified, if @code{make} can find
6820 implicit rules that say how to make them.
6822 @cindex @code{MAKECMDGOALS}
6823 @vindex MAKECMDGOALS
6824 @code{Make} will set the special variable @code{MAKECMDGOALS} to the
6825 list of goals you specified on the command line. If no goals were given
6826 on the command line, this variable is empty. Note that this variable
6827 should be used only in special circumstances.
6829 An example of appropriate use is to avoid including @file{.d} files
6830 during @code{clean} rules (@pxref{Automatic Prerequisites}), so
6831 @code{make} won't create them only to immediately remove them
6836 sources = foo.c bar.c
6838 ifneq ($(MAKECMDGOALS),clean)
6839 include $(sources:.c=.d)
6844 One use of specifying a goal is if you want to compile only a part of
6845 the program, or only one of several programs. Specify as a goal each
6846 file that you wish to remake. For example, consider a directory containing
6847 several programs, with a makefile that starts like this:
6851 all: size nm ld ar as
6854 If you are working on the program @code{size}, you might want to say
6855 @w{@samp{make size}} so that only the files of that program are recompiled.
6857 Another use of specifying a goal is to make files that are not normally
6858 made. For example, there may be a file of debugging output, or a
6859 version of the program that is compiled specially for testing, which has
6860 a rule in the makefile but is not a prerequisite of the default goal.
6862 Another use of specifying a goal is to run the commands associated with
6863 a phony target (@pxref{Phony Targets}) or empty target (@pxref{Empty
6864 Targets, ,Empty Target Files to Record Events}). Many makefiles contain
6865 a phony target named @file{clean} which deletes everything except source
6866 files. Naturally, this is done only if you request it explicitly with
6867 @w{@samp{make clean}}. Following is a list of typical phony and empty
6868 target names. @xref{Standard Targets}, for a detailed list of all the
6869 standard target names which GNU software packages use.
6873 @cindex @code{all} @r{(standard target)}
6874 Make all the top-level targets the makefile knows about.
6877 @cindex @code{clean} @r{(standard target)}
6878 Delete all files that are normally created by running @code{make}.
6881 @cindex @code{mostlyclean} @r{(standard target)}
6882 Like @samp{clean}, but may refrain from deleting a few files that people
6883 normally don't want to recompile. For example, the @samp{mostlyclean}
6884 target for GCC does not delete @file{libgcc.a}, because recompiling it
6885 is rarely necessary and takes a lot of time.
6888 @cindex @code{distclean} @r{(standard target)}
6890 @cindex @code{realclean} @r{(standard target)}
6892 @cindex @code{clobber} @r{(standard target)}
6893 Any of these targets might be defined to delete @emph{more} files than
6894 @samp{clean} does. For example, this would delete configuration files
6895 or links that you would normally create as preparation for compilation,
6896 even if the makefile itself cannot create these files.
6899 @cindex @code{install} @r{(standard target)}
6900 Copy the executable file into a directory that users typically search
6901 for commands; copy any auxiliary files that the executable uses into
6902 the directories where it will look for them.
6905 @cindex @code{print} @r{(standard target)}
6906 Print listings of the source files that have changed.
6909 @cindex @code{tar} @r{(standard target)}
6910 Create a tar file of the source files.
6913 @cindex @code{shar} @r{(standard target)}
6914 Create a shell archive (shar file) of the source files.
6917 @cindex @code{dist} @r{(standard target)}
6918 Create a distribution file of the source files. This might
6919 be a tar file, or a shar file, or a compressed version of one of the
6920 above, or even more than one of the above.
6923 @cindex @code{TAGS} @r{(standard target)}
6924 Update a tags table for this program.
6927 @cindex @code{check} @r{(standard target)}
6929 @cindex @code{test} @r{(standard target)}
6930 Perform self tests on the program this makefile builds.
6933 @node Instead of Execution, Avoiding Compilation, Goals, Running
6934 @section Instead of Executing the Commands
6935 @cindex execution, instead of
6936 @cindex commands, instead of executing
6938 The makefile tells @code{make} how to tell whether a target is up to date,
6939 and how to update each target. But updating the targets is not always
6940 what you want. Certain options specify other activities for @code{make}.
6942 @comment Extra blank lines make it print better.
6948 @cindex @code{--just-print}
6949 @cindex @code{--dry-run}
6950 @cindex @code{--recon}
6953 ``No-op''. The activity is to print what commands would be used to make
6954 the targets up to date, but not actually execute them.
6958 @cindex @code{--touch}
6959 @cindex touching files
6960 @cindex target, touching
6963 ``Touch''. The activity is to mark the targets as up to date without
6964 actually changing them. In other words, @code{make} pretends to compile
6965 the targets but does not really change their contents.
6969 @cindex @code{--question}
6971 @cindex question mode
6973 ``Question''. The activity is to find out silently whether the targets
6974 are up to date already; but execute no commands in either case. In other
6975 words, neither compilation nor output will occur.
6978 @itemx --what-if=@var{file}
6979 @itemx --assume-new=@var{file}
6980 @itemx --new-file=@var{file}
6981 @cindex @code{--what-if}
6983 @cindex @code{--assume-new}
6984 @cindex @code{--new-file}
6986 @cindex files, assuming new
6988 ``What if''. Each @samp{-W} flag is followed by a file name. The given
6989 files' modification times are recorded by @code{make} as being the present
6990 time, although the actual modification times remain the same.
6991 You can use the @samp{-W} flag in conjunction with the @samp{-n} flag
6992 to see what would happen if you were to modify specific files.@refill
6995 With the @samp{-n} flag, @code{make} prints the commands that it would
6996 normally execute but does not execute them.
6998 With the @samp{-t} flag, @code{make} ignores the commands in the rules
6999 and uses (in effect) the command @code{touch} for each target that needs to
7000 be remade. The @code{touch} command is also printed, unless @samp{-s} or
7001 @code{.SILENT} is used. For speed, @code{make} does not actually invoke
7002 the program @code{touch}. It does the work directly.
7004 With the @samp{-q} flag, @code{make} prints nothing and executes no
7005 commands, but the exit status code it returns is zero if and only if the
7006 targets to be considered are already up to date. If the exit status is
7007 one, then some updating needs to be done. If @code{make} encounters an
7008 error, the exit status is two, so you can distinguish an error from a
7009 target that is not up to date.
7011 It is an error to use more than one of these three flags in the same
7012 invocation of @code{make}.
7014 @cindex +, and command execution
7015 The @samp{-n}, @samp{-t}, and @samp{-q} options do not affect command
7016 lines that begin with @samp{+} characters or contain the strings
7017 @samp{$(MAKE)} or @samp{$@{MAKE@}}. Note that only the line containing
7018 the @samp{+} character or the strings @samp{$(MAKE)} or @samp{$@{MAKE@}}
7019 is run regardless of these options. Other lines in the same rule are
7020 not run unless they too begin with @samp{+} or contain @samp{$(MAKE)} or
7021 @samp{$@{MAKE@}} (@xref{MAKE Variable, ,How the @code{MAKE} Variable Works}.)
7023 The @samp{-W} flag provides two features:
7027 If you also use the @samp{-n} or @samp{-q} flag, you can see what
7028 @code{make} would do if you were to modify some files.
7031 Without the @samp{-n} or @samp{-q} flag, when @code{make} is actually
7032 executing commands, the @samp{-W} flag can direct @code{make} to act
7033 as if some files had been modified, without actually modifying the
7037 Note that the options @samp{-p} and @samp{-v} allow you to obtain other
7038 information about @code{make} or about the makefiles in use
7039 (@pxref{Options Summary, ,Summary of Options}).@refill
7041 @node Avoiding Compilation, Overriding, Instead of Execution, Running
7042 @section Avoiding Recompilation of Some Files
7044 @cindex @code{--old-file}
7045 @cindex @code{--assume-old}
7046 @cindex files, assuming old
7047 @cindex files, avoiding recompilation of
7048 @cindex recompilation, avoiding
7050 Sometimes you may have changed a source file but you do not want to
7051 recompile all the files that depend on it. For example, suppose you add
7052 a macro or a declaration to a header file that many other files depend
7053 on. Being conservative, @code{make} assumes that any change in the
7054 header file requires recompilation of all dependent files, but you know
7055 that they do not need to be recompiled and you would rather not waste
7056 the time waiting for them to compile.
7058 If you anticipate the problem before changing the header file, you can
7059 use the @samp{-t} flag. This flag tells @code{make} not to run the
7060 commands in the rules, but rather to mark the target up to date by
7061 changing its last-modification date. You would follow this procedure:
7065 Use the command @samp{make} to recompile the source files that really
7066 need recompilation, ensuring that the object files are up-to-date
7070 Make the changes in the header files.
7073 Use the command @samp{make -t} to mark all the object files as
7074 up to date. The next time you run @code{make}, the changes in the
7075 header files will not cause any recompilation.
7078 If you have already changed the header file at a time when some files
7079 do need recompilation, it is too late to do this. Instead, you can
7080 use the @w{@samp{-o @var{file}}} flag, which marks a specified file as
7081 ``old'' (@pxref{Options Summary, ,Summary of Options}). This means
7082 that the file itself will not be remade, and nothing else will be
7083 remade on its account. Follow this procedure:
7087 Recompile the source files that need compilation for reasons independent
7088 of the particular header file, with @samp{make -o @var{headerfile}}.
7089 If several header files are involved, use a separate @samp{-o} option
7090 for each header file.
7093 Touch all the object files with @samp{make -t}.
7096 @node Overriding, Testing, Avoiding Compilation, Running
7097 @section Overriding Variables
7098 @cindex overriding variables with arguments
7099 @cindex variables, overriding with arguments
7100 @cindex command line variables
7101 @cindex variables, command line
7103 An argument that contains @samp{=} specifies the value of a variable:
7104 @samp{@var{v}=@var{x}} sets the value of the variable @var{v} to @var{x}.
7105 If you specify a value in this way, all ordinary assignments of the same
7106 variable in the makefile are ignored; we say they have been
7107 @dfn{overridden} by the command line argument.
7109 The most common way to use this facility is to pass extra flags to
7110 compilers. For example, in a properly written makefile, the variable
7111 @code{CFLAGS} is included in each command that runs the C compiler, so a
7112 file @file{foo.c} would be compiled something like this:
7115 cc -c $(CFLAGS) foo.c
7118 Thus, whatever value you set for @code{CFLAGS} affects each compilation
7119 that occurs. The makefile probably specifies the usual value for
7120 @code{CFLAGS}, like this:
7126 Each time you run @code{make}, you can override this value if you
7127 wish. For example, if you say @samp{make CFLAGS='-g -O'}, each C
7128 compilation will be done with @samp{cc -c -g -O}. (This also
7129 illustrates how you can use quoting in the shell to enclose spaces and
7130 other special characters in the value of a variable when you override
7133 The variable @code{CFLAGS} is only one of many standard variables that
7134 exist just so that you can change them this way. @xref{Implicit
7135 Variables, , Variables Used by Implicit Rules}, for a complete list.
7137 You can also program the makefile to look at additional variables of your
7138 own, giving the user the ability to control other aspects of how the
7139 makefile works by changing the variables.
7141 When you override a variable with a command argument, you can define either
7142 a recursively-expanded variable or a simply-expanded variable. The
7143 examples shown above make a recursively-expanded variable; to make a
7144 simply-expanded variable, write @samp{:=} instead of @samp{=}. But, unless
7145 you want to include a variable reference or function call in the
7146 @emph{value} that you specify, it makes no difference which kind of
7147 variable you create.
7149 There is one way that the makefile can change a variable that you have
7150 overridden. This is to use the @code{override} directive, which is a line
7151 that looks like this: @samp{override @var{variable} = @var{value}}
7152 (@pxref{Override Directive, ,The @code{override} Directive}).
7154 @node Testing, Options Summary, Overriding, Running
7155 @section Testing the Compilation of a Program
7156 @cindex testing compilation
7157 @cindex compilation, testing
7159 Normally, when an error happens in executing a shell command, @code{make}
7160 gives up immediately, returning a nonzero status. No further commands are
7161 executed for any target. The error implies that the goal cannot be
7162 correctly remade, and @code{make} reports this as soon as it knows.
7164 When you are compiling a program that you have just changed, this is not
7165 what you want. Instead, you would rather that @code{make} try compiling
7166 every file that can be tried, to show you as many compilation errors
7170 @cindex @code{--keep-going}
7171 On these occasions, you should use the @samp{-k} or
7172 @samp{--keep-going} flag. This tells @code{make} to continue to
7173 consider the other prerequisites of the pending targets, remaking them
7174 if necessary, before it gives up and returns nonzero status. For
7175 example, after an error in compiling one object file, @samp{make -k}
7176 will continue compiling other object files even though it already
7177 knows that linking them will be impossible. In addition to continuing
7178 after failed shell commands, @samp{make -k} will continue as much as
7179 possible after discovering that it does not know how to make a target
7180 or prerequisite file. This will always cause an error message, but
7181 without @samp{-k}, it is a fatal error (@pxref{Options Summary,
7182 ,Summary of Options}).@refill
7184 The usual behavior of @code{make} assumes that your purpose is to get the
7185 goals up to date; once @code{make} learns that this is impossible, it might
7186 as well report the failure immediately. The @samp{-k} flag says that the
7187 real purpose is to test as much as possible of the changes made in the
7188 program, perhaps to find several independent problems so that you can
7189 correct them all before the next attempt to compile. This is why Emacs'
7190 @kbd{M-x compile} command passes the @samp{-k} flag by default.
7192 @node Options Summary, , Testing, Running
7193 @section Summary of Options
7198 Here is a table of all the options @code{make} understands:
7205 These options are ignored for compatibility with other versions of @code{make}.
7209 @itemx --always-make
7210 @cindex @code{--always-make}
7211 Consider all targets out-of-date. GNU @code{make} proceeds to
7212 consider targets and their prerequisites using the normal algorithms;
7213 however, all these targets are remade, regardless of the status of
7214 their prerequisites.
7218 @itemx --directory=@var{dir}
7219 @cindex @code{--directory}
7220 Change to directory @var{dir} before reading the makefiles. If multiple
7221 @samp{-C} options are specified, each is interpreted relative to the
7222 previous one: @samp{-C / -C etc} is equivalent to @samp{-C /etc}.
7223 This is typically used with recursive invocations of @code{make}
7224 (@pxref{Recursion, ,Recursive Use of @code{make}}).
7228 @c Extra blank line here makes the table look better.
7230 Print debugging information in addition to normal processing. The
7231 debugging information says which files are being considered for
7232 remaking, which file-times are being compared and with what results,
7233 which files actually need to be remade, which implicit rules are
7234 considered and which are applied---everything interesting about how
7235 @code{make} decides what to do. The @code{-d} option is equivalent to
7236 @samp{--debug=a} (see below).
7238 @item --debug[=@var{options}]
7239 @cindex @code{--debug}
7240 @c Extra blank line here makes the table look better.
7242 Print debugging information in addition to normal processing. Various
7243 levels and types of output can be chosen. With no arguments, print the
7244 ``basic'' level of debugging. Possible arguments are below; only the
7245 first character is considered, and values must be comma- or
7250 All types of debugging output are enabled. This is equivalent to using
7254 Basic debugging prints each target that was found to be out-of-date, and
7255 whether the build was successful or not.
7257 @item v (@i{verbose})
7258 A level above @samp{basic}; includes messages about which makefiles were
7259 parsed, prerequisites that did not need to be rebuilt, etc. This option
7260 also enables @samp{basic} messages.
7262 @item i (@i{implicit})
7263 Prints messages describing the implicit rule searches for each target.
7264 This option also enables @samp{basic} messages.
7267 Prints messages giving details on the invocation of specific subcommands.
7269 @item m (@i{makefile})
7270 By default, the above messages are not enabled while trying to remake
7271 the makefiles. This option enables messages while rebuilding makefiles,
7272 too. Note that the @samp{all} option does enable this option. This
7273 option also enables @samp{basic} messages.
7278 @itemx --environment-overrides
7279 @cindex @code{--environment-overrides}
7280 Give variables taken from the environment precedence
7281 over variables from makefiles.
7282 @xref{Environment, ,Variables from the Environment}.
7286 @itemx --file=@var{file}
7287 @cindex @code{--file}
7288 @itemx --makefile=@var{file}
7289 @cindex @code{--makefile}
7290 Read the file named @var{file} as a makefile.
7291 @xref{Makefiles, ,Writing Makefiles}.
7296 @cindex @code{--help}
7297 @c Extra blank line here makes the table look better.
7299 Remind you of the options that @code{make} understands and then exit.
7303 @itemx --ignore-errors
7304 @cindex @code{--ignore-errors}
7305 Ignore all errors in commands executed to remake files.
7306 @xref{Errors, ,Errors in Commands}.
7310 @itemx --include-dir=@var{dir}
7311 @cindex @code{--include-dir}
7312 Specifies a directory @var{dir} to search for included makefiles.
7313 @xref{Include, ,Including Other Makefiles}. If several @samp{-I}
7314 options are used to specify several directories, the directories are
7315 searched in the order specified.
7317 @item -j [@var{jobs}]
7319 @itemx --jobs[=@var{jobs}]
7320 @cindex @code{--jobs}
7321 Specifies the number of jobs (commands) to run simultaneously. With no
7322 argument, @code{make} runs as many jobs simultaneously as possible. If
7323 there is more than one @samp{-j} option, the last one is effective.
7324 @xref{Parallel, ,Parallel Execution},
7325 for more information on how commands are run.
7326 Note that this option is ignored on MS-DOS.
7331 @cindex @code{--keep-going}
7332 Continue as much as possible after an error. While the target that
7333 failed, and those that depend on it, cannot be remade, the other
7334 prerequisites of these targets can be processed all the same.
7335 @xref{Testing, ,Testing the Compilation of a Program}.
7337 @item -l [@var{load}]
7339 @itemx --load-average[=@var{load}]
7340 @cindex @code{--load-average}
7341 @itemx --max-load[=@var{load}]
7342 @cindex @code{--max-load}
7343 Specifies that no new jobs (commands) should be started if there are
7344 other jobs running and the load average is at least @var{load} (a
7345 floating-point number). With no argument, removes a previous load
7346 limit. @xref{Parallel, ,Parallel Execution}.
7351 @cindex @code{--just-print}
7353 @cindex @code{--dry-run}
7355 @cindex @code{--recon}
7356 @c Extra blank line here makes the table look better.
7358 Print the commands that would be executed, but do not execute them.
7359 @xref{Instead of Execution, ,Instead of Executing the Commands}.
7363 @itemx --old-file=@var{file}
7364 @cindex @code{--old-file}
7365 @itemx --assume-old=@var{file}
7366 @cindex @code{--assume-old}
7367 Do not remake the file @var{file} even if it is older than its
7368 prerequisites, and do not remake anything on account of changes in
7369 @var{file}. Essentially the file is treated as very old and its rules
7370 are ignored. @xref{Avoiding Compilation, ,Avoiding Recompilation of
7375 @itemx --print-data-base
7376 @cindex @code{--print-data-base}
7377 @cindex data base of @code{make} rules
7378 @cindex predefined rules and variables, printing
7379 Print the data base (rules and variable values) that results from
7380 reading the makefiles; then execute as usual or as otherwise specified.
7381 This also prints the version information given by the @samp{-v} switch
7382 (see below). To print the data base without trying to remake any files,
7383 use @w{@samp{make -qp}}. To print the data base of predefined rules and
7384 variables, use @w{@samp{make -p -f /dev/null}}. The data base output
7385 contains filename and linenumber information for command and variable
7386 definitions, so it can be a useful debugging tool in complex environments.
7391 @cindex @code{--question}
7392 ``Question mode''. Do not run any commands, or print anything; just
7393 return an exit status that is zero if the specified targets are already
7394 up to date, one if any remaking is required, or two if an error is
7395 encountered. @xref{Instead of Execution, ,Instead of Executing the
7400 @itemx --no-builtin-rules
7401 @cindex @code{--no-builtin-rules}
7402 Eliminate use of the built-in implicit rules (@pxref{Implicit Rules,
7403 ,Using Implicit Rules}). You can still define your own by writing
7404 pattern rules (@pxref{Pattern Rules, ,Defining and Redefining Pattern
7405 Rules}). The @samp{-r} option also clears out the default list of
7406 suffixes for suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix
7407 Rules}). But you can still define your own suffixes with a rule for
7408 @code{.SUFFIXES}, and then define your own suffix rules. Note that only
7409 @emph{rules} are affected by the @code{-r} option; default variables
7410 remain in effect (@pxref{Implicit Variables, ,Variables Used by Implicit
7411 Rules}); see the @samp{-R} option below.
7415 @itemx --no-builtin-variables
7416 @cindex @code{--no-builtin-variables}
7417 Eliminate use of the built-in rule-specific variables (@pxref{Implicit
7418 Variables, ,Variables Used by Implicit Rules}). You can still define
7419 your own, of course. The @samp{-R} option also automatically enables
7420 the @samp{-r} option (see above), since it doesn't make sense to have
7421 implicit rules without any definitions for the variables that they use.
7426 @cindex @code{--silent}
7428 @cindex @code{--quiet}
7429 @c Extra blank line here makes the table look better.
7431 Silent operation; do not print the commands as they are executed.
7432 @xref{Echoing, ,Command Echoing}.
7436 @itemx --no-keep-going
7437 @cindex @code{--no-keep-going}
7439 @cindex @code{--stop}
7440 @c Extra blank line here makes the table look better.
7442 Cancel the effect of the @samp{-k} option. This is never necessary
7443 except in a recursive @code{make} where @samp{-k} might be inherited
7444 from the top-level @code{make} via @code{MAKEFLAGS}
7445 (@pxref{Recursion, ,Recursive Use of @code{make}})
7446 or if you set @samp{-k} in @code{MAKEFLAGS} in your environment.@refill
7451 @cindex @code{--touch}
7452 @c Extra blank line here makes the table look better.
7454 Touch files (mark them up to date without really changing them)
7455 instead of running their commands. This is used to pretend that the
7456 commands were done, in order to fool future invocations of
7457 @code{make}. @xref{Instead of Execution, ,Instead of Executing the Commands}.
7462 @cindex @code{--version}
7463 Print the version of the @code{make} program plus a copyright, a list
7464 of authors, and a notice that there is no warranty; then exit.
7468 @itemx --print-directory
7469 @cindex @code{--print-directory}
7470 Print a message containing the working directory both before and after
7471 executing the makefile. This may be useful for tracking down errors
7472 from complicated nests of recursive @code{make} commands.
7473 @xref{Recursion, ,Recursive Use of @code{make}}. (In practice, you
7474 rarely need to specify this option since @samp{make} does it for you;
7475 see @ref{-w Option, ,The @samp{--print-directory} Option}.)
7477 @itemx --no-print-directory
7478 @cindex @code{--no-print-directory}
7479 Disable printing of the working directory under @code{-w}.
7480 This option is useful when @code{-w} is turned on automatically,
7481 but you do not want to see the extra messages.
7482 @xref{-w Option, ,The @samp{--print-directory} Option}.
7486 @itemx --what-if=@var{file}
7487 @cindex @code{--what-if}
7488 @itemx --new-file=@var{file}
7489 @cindex @code{--new-file}
7490 @itemx --assume-new=@var{file}
7491 @cindex @code{--assume-new}
7492 Pretend that the target @var{file} has just been modified. When used
7493 with the @samp{-n} flag, this shows you what would happen if you were
7494 to modify that file. Without @samp{-n}, it is almost the same as
7495 running a @code{touch} command on the given file before running
7496 @code{make}, except that the modification time is changed only in the
7497 imagination of @code{make}.
7498 @xref{Instead of Execution, ,Instead of Executing the Commands}.
7500 @item --warn-undefined-variables
7501 @cindex @code{--warn-undefined-variables}
7502 @cindex variables, warning for undefined
7503 @cindex undefined variables, warning message
7504 Issue a warning message whenever @code{make} sees a reference to an
7505 undefined variable. This can be helpful when you are trying to debug
7506 makefiles which use variables in complex ways.
7509 @node Implicit Rules, Archives, Running, Top
7510 @chapter Using Implicit Rules
7511 @cindex implicit rule
7512 @cindex rule, implicit
7514 Certain standard ways of remaking target files are used very often. For
7515 example, one customary way to make an object file is from a C source file
7516 using the C compiler, @code{cc}.
7518 @dfn{Implicit rules} tell @code{make} how to use customary techniques so
7519 that you do not have to specify them in detail when you want to use
7520 them. For example, there is an implicit rule for C compilation. File
7521 names determine which implicit rules are run. For example, C
7522 compilation typically takes a @file{.c} file and makes a @file{.o} file.
7523 So @code{make} applies the implicit rule for C compilation when it sees
7524 this combination of file name endings.@refill
7526 A chain of implicit rules can apply in sequence; for example, @code{make}
7527 will remake a @file{.o} file from a @file{.y} file by way of a @file{.c} file.
7529 @xref{Chained Rules, ,Chains of Implicit Rules}.
7532 The built-in implicit rules use several variables in their commands so
7533 that, by changing the values of the variables, you can change the way the
7534 implicit rule works. For example, the variable @code{CFLAGS} controls the
7535 flags given to the C compiler by the implicit rule for C compilation.
7537 @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
7540 You can define your own implicit rules by writing @dfn{pattern rules}.
7542 @xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.
7545 @dfn{Suffix rules} are a more limited way to define implicit rules.
7546 Pattern rules are more general and clearer, but suffix rules are
7547 retained for compatibility.
7549 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
7553 * Using Implicit:: How to use an existing implicit rule
7554 to get the commands for updating a file.
7555 * Catalogue of Rules:: A list of built-in implicit rules.
7556 * Implicit Variables:: How to change what predefined rules do.
7557 * Chained Rules:: How to use a chain of implicit rules.
7558 * Pattern Rules:: How to define new implicit rules.
7559 * Last Resort:: How to define commands for rules which
7561 * Suffix Rules:: The old-fashioned style of implicit rule.
7562 * Implicit Rule Search:: The precise algorithm for applying
7566 @node Using Implicit, Catalogue of Rules, Implicit Rules, Implicit Rules
7567 @section Using Implicit Rules
7568 @cindex implicit rule, how to use
7569 @cindex rule, implicit, how to use
7571 To allow @code{make} to find a customary method for updating a target file,
7572 all you have to do is refrain from specifying commands yourself. Either
7573 write a rule with no command lines, or don't write a rule at all. Then
7574 @code{make} will figure out which implicit rule to use based on which
7575 kind of source file exists or can be made.
7577 For example, suppose the makefile looks like this:
7581 cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
7585 Because you mention @file{foo.o} but do not give a rule for it, @code{make}
7586 will automatically look for an implicit rule that tells how to update it.
7587 This happens whether or not the file @file{foo.o} currently exists.
7589 If an implicit rule is found, it can supply both commands and one or
7590 more prerequisites (the source files). You would want to write a rule
7591 for @file{foo.o} with no command lines if you need to specify additional
7592 prerequisites, such as header files, that the implicit rule cannot
7595 Each implicit rule has a target pattern and prerequisite patterns. There may
7596 be many implicit rules with the same target pattern. For example, numerous
7597 rules make @samp{.o} files: one, from a @samp{.c} file with the C compiler;
7598 another, from a @samp{.p} file with the Pascal compiler; and so on. The rule
7599 that actually applies is the one whose prerequisites exist or can be made.
7600 So, if you have a file @file{foo.c}, @code{make} will run the C compiler;
7601 otherwise, if you have a file @file{foo.p}, @code{make} will run the Pascal
7602 compiler; and so on.
7604 Of course, when you write the makefile, you know which implicit rule you
7605 want @code{make} to use, and you know it will choose that one because you
7606 know which possible prerequisite files are supposed to exist.
7607 @xref{Catalogue of Rules, ,Catalogue of Implicit Rules},
7608 for a catalogue of all the predefined implicit rules.
7610 Above, we said an implicit rule applies if the required prerequisites ``exist
7611 or can be made''. A file ``can be made'' if it is mentioned explicitly in
7612 the makefile as a target or a prerequisite, or if an implicit rule can be
7613 recursively found for how to make it. When an implicit prerequisite is the
7614 result of another implicit rule, we say that @dfn{chaining} is occurring.
7615 @xref{Chained Rules, ,Chains of Implicit Rules}.
7617 In general, @code{make} searches for an implicit rule for each target, and
7618 for each double-colon rule, that has no commands. A file that is mentioned
7619 only as a prerequisite is considered a target whose rule specifies nothing,
7620 so implicit rule search happens for it. @xref{Implicit Rule Search, ,Implicit Rule Search Algorithm}, for the
7621 details of how the search is done.
7623 Note that explicit prerequisites do not influence implicit rule search.
7624 For example, consider this explicit rule:
7631 The prerequisite on @file{foo.p} does not necessarily mean that
7632 @code{make} will remake @file{foo.o} according to the implicit rule to
7633 make an object file, a @file{.o} file, from a Pascal source file, a
7634 @file{.p} file. For example, if @file{foo.c} also exists, the implicit
7635 rule to make an object file from a C source file is used instead,
7636 because it appears before the Pascal rule in the list of predefined
7637 implicit rules (@pxref{Catalogue of Rules, , Catalogue of Implicit
7640 If you do not want an implicit rule to be used for a target that has no
7641 commands, you can give that target empty commands by writing a semicolon
7642 (@pxref{Empty Commands, ,Defining Empty Commands}).
7644 @node Catalogue of Rules, Implicit Variables, Using Implicit, Implicit Rules
7645 @section Catalogue of Implicit Rules
7646 @cindex implicit rule, predefined
7647 @cindex rule, implicit, predefined
7649 Here is a catalogue of predefined implicit rules which are always
7650 available unless the makefile explicitly overrides or cancels them.
7651 @xref{Canceling Rules, ,Canceling Implicit Rules}, for information on
7652 canceling or overriding an implicit rule. The @samp{-r} or
7653 @samp{--no-builtin-rules} option cancels all predefined rules.
7655 Not all of these rules will always be defined, even when the @samp{-r}
7656 option is not given. Many of the predefined implicit rules are
7657 implemented in @code{make} as suffix rules, so which ones will be
7658 defined depends on the @dfn{suffix list} (the list of prerequisites of
7659 the special target @code{.SUFFIXES}). The default suffix list is:
7660 @code{.out}, @code{.a}, @code{.ln}, @code{.o}, @code{.c}, @code{.cc},
7661 @code{.C}, @code{.p}, @code{.f}, @code{.F}, @code{.r}, @code{.y},
7662 @code{.l}, @code{.s}, @code{.S}, @code{.mod}, @code{.sym}, @code{.def},
7663 @code{.h}, @code{.info}, @code{.dvi}, @code{.tex}, @code{.texinfo},
7664 @code{.texi}, @code{.txinfo}, @code{.w}, @code{.ch} @code{.web},
7665 @code{.sh}, @code{.elc}, @code{.el}. All of the implicit rules
7666 described below whose prerequisites have one of these suffixes are
7667 actually suffix rules. If you modify the suffix list, the only
7668 predefined suffix rules in effect will be those named by one or two of
7669 the suffixes that are on the list you specify; rules whose suffixes fail
7670 to be on the list are disabled. @xref{Suffix Rules, ,Old-Fashioned
7671 Suffix Rules}, for full details on suffix rules.
7674 @item Compiling C programs
7675 @cindex C, rule to compile
7680 @file{@var{n}.o} is made automatically from @file{@var{n}.c} with
7681 a command of the form @samp{$(CC) -c $(CPPFLAGS) $(CFLAGS)}.@refill
7683 @item Compiling C++ programs
7684 @cindex C++, rule to compile
7688 @file{@var{n}.o} is made automatically from @file{@var{n}.cc} or
7689 @file{@var{n}.C} with a command of the form @samp{$(CXX) -c $(CPPFLAGS)
7690 $(CXXFLAGS)}. We encourage you to use the suffix @samp{.cc} for C++
7691 source files instead of @samp{.C}.@refill
7693 @item Compiling Pascal programs
7694 @cindex Pascal, rule to compile
7697 @file{@var{n}.o} is made automatically from @file{@var{n}.p}
7698 with the command @samp{$(PC) -c $(PFLAGS)}.@refill
7700 @item Compiling Fortran and Ratfor programs
7701 @cindex Fortran, rule to compile
7702 @cindex Ratfor, rule to compile
7707 @file{@var{n}.o} is made automatically from @file{@var{n}.r},
7708 @file{@var{n}.F} or @file{@var{n}.f} by running the
7709 Fortran compiler. The precise command used is as follows:@refill
7713 @samp{$(FC) -c $(FFLAGS)}.
7715 @samp{$(FC) -c $(FFLAGS) $(CPPFLAGS)}.
7717 @samp{$(FC) -c $(FFLAGS) $(RFLAGS)}.
7720 @item Preprocessing Fortran and Ratfor programs
7721 @file{@var{n}.f} is made automatically from @file{@var{n}.r} or
7722 @file{@var{n}.F}. This rule runs just the preprocessor to convert a
7723 Ratfor or preprocessable Fortran program into a strict Fortran
7724 program. The precise command used is as follows:@refill
7728 @samp{$(FC) -F $(CPPFLAGS) $(FFLAGS)}.
7730 @samp{$(FC) -F $(FFLAGS) $(RFLAGS)}.
7733 @item Compiling Modula-2 programs
7734 @cindex Modula-2, rule to compile
7739 @file{@var{n}.sym} is made from @file{@var{n}.def} with a command
7740 of the form @samp{$(M2C) $(M2FLAGS) $(DEFFLAGS)}. @file{@var{n}.o}
7741 is made from @file{@var{n}.mod}; the form is:
7742 @w{@samp{$(M2C) $(M2FLAGS) $(MODFLAGS)}}.@refill
7745 @item Assembling and preprocessing assembler programs
7746 @cindex assembly, rule to compile
7749 @file{@var{n}.o} is made automatically from @file{@var{n}.s} by
7750 running the assembler, @code{as}. The precise command is
7751 @samp{$(AS) $(ASFLAGS)}.@refill
7754 @file{@var{n}.s} is made automatically from @file{@var{n}.S} by
7755 running the C preprocessor, @code{cpp}. The precise command is
7756 @w{@samp{$(CPP) $(CPPFLAGS)}}.
7758 @item Linking a single object file
7759 @cindex linking, predefined rule for
7762 @file{@var{n}} is made automatically from @file{@var{n}.o} by running
7763 the linker (usually called @code{ld}) via the C compiler. The precise
7764 command used is @w{@samp{$(CC) $(LDFLAGS) @var{n}.o $(LOADLIBES) $(LDLIBS)}}.
7766 This rule does the right thing for a simple program with only one
7767 source file. It will also do the right thing if there are multiple
7768 object files (presumably coming from various other source files), one
7769 of which has a name matching that of the executable file. Thus,
7776 when @file{x.c}, @file{y.c} and @file{z.c} all exist will execute:
7791 In more complicated cases, such as when there is no object file whose
7792 name derives from the executable file name, you must write an explicit
7793 command for linking.
7795 Each kind of file automatically made into @samp{.o} object files will
7796 be automatically linked by using the compiler (@samp{$(CC)},
7797 @samp{$(FC)} or @samp{$(PC)}; the C compiler @samp{$(CC)} is used to
7798 assemble @samp{.s} files) without the @samp{-c} option. This could be
7799 done by using the @samp{.o} object files as intermediates, but it is
7800 faster to do the compiling and linking in one step, so that's how it's
7803 @item Yacc for C programs
7805 @cindex Yacc, rule to run
7807 @file{@var{n}.c} is made automatically from @file{@var{n}.y} by
7808 running Yacc with the command @samp{$(YACC) $(YFLAGS)}.
7810 @item Lex for C programs
7812 @cindex Lex, rule to run
7814 @file{@var{n}.c} is made automatically from @file{@var{n}.l} by
7815 running Lex. The actual command is @samp{$(LEX) $(LFLAGS)}.
7817 @item Lex for Ratfor programs
7818 @file{@var{n}.r} is made automatically from @file{@var{n}.l} by
7819 running Lex. The actual command is @samp{$(LEX) $(LFLAGS)}.
7821 The convention of using the same suffix @samp{.l} for all Lex files
7822 regardless of whether they produce C code or Ratfor code makes it
7823 impossible for @code{make} to determine automatically which of the two
7824 languages you are using in any particular case. If @code{make} is
7825 called upon to remake an object file from a @samp{.l} file, it must
7826 guess which compiler to use. It will guess the C compiler, because
7827 that is more common. If you are using Ratfor, make sure @code{make}
7828 knows this by mentioning @file{@var{n}.r} in the makefile. Or, if you
7829 are using Ratfor exclusively, with no C files, remove @samp{.c} from
7830 the list of implicit rule suffixes with:@refill
7835 .SUFFIXES: .o .r .f .l @dots{}
7839 @item Making Lint Libraries from C, Yacc, or Lex programs
7841 @cindex @code{lint}, rule to run
7843 @file{@var{n}.ln} is made from @file{@var{n}.c} by running @code{lint}.
7844 The precise command is @w{@samp{$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i}}.
7845 The same command is used on the C code produced from
7846 @file{@var{n}.y} or @file{@var{n}.l}.@refill
7848 @item @TeX{} and Web
7849 @cindex @TeX{}, rule to run
7850 @cindex Web, rule to run
7861 @file{@var{n}.dvi} is made from @file{@var{n}.tex} with the command
7862 @samp{$(TEX)}. @file{@var{n}.tex} is made from @file{@var{n}.web} with
7863 @samp{$(WEAVE)}, or from @file{@var{n}.w} (and from @file{@var{n}.ch} if
7864 it exists or can be made) with @samp{$(CWEAVE)}. @file{@var{n}.p} is
7865 made from @file{@var{n}.web} with @samp{$(TANGLE)} and @file{@var{n}.c}
7866 is made from @file{@var{n}.w} (and from @file{@var{n}.ch} if it exists
7867 or can be made) with @samp{$(CTANGLE)}.@refill
7869 @item Texinfo and Info
7870 @cindex Texinfo, rule to format
7871 @cindex Info, rule to format
7878 @file{@var{n}.dvi} is made from @file{@var{n}.texinfo},
7879 @file{@var{n}.texi}, or @file{@var{n}.txinfo}, with the command
7880 @w{@samp{$(TEXI2DVI) $(TEXI2DVI_FLAGS)}}. @file{@var{n}.info} is made from
7881 @file{@var{n}.texinfo}, @file{@var{n}.texi}, or @file{@var{n}.txinfo}, with
7882 the command @w{@samp{$(MAKEINFO) $(MAKEINFO_FLAGS)}}.
7885 @cindex RCS, rule to extract from
7887 @pindex ,v @r{(RCS file extension)}
7888 Any file @file{@var{n}} is extracted if necessary from an RCS file
7889 named either @file{@var{n},v} or @file{RCS/@var{n},v}. The precise
7890 command used is @w{@samp{$(CO) $(COFLAGS)}}. @file{@var{n}} will not be
7891 extracted from RCS if it already exists, even if the RCS file is
7892 newer. The rules for RCS are terminal
7893 (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
7894 so RCS files cannot be generated from another source; they must
7895 actually exist.@refill
7898 @cindex SCCS, rule to extract from
7900 @pindex s. @r{(SCCS file prefix)}
7901 Any file @file{@var{n}} is extracted if necessary from an SCCS file
7902 named either @file{s.@var{n}} or @file{SCCS/s.@var{n}}. The precise
7903 command used is @w{@samp{$(GET) $(GFLAGS)}}. The rules for SCCS are
7904 terminal (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
7905 so SCCS files cannot be generated from another source; they must
7906 actually exist.@refill
7909 For the benefit of SCCS, a file @file{@var{n}} is copied from
7910 @file{@var{n}.sh} and made executable (by everyone). This is for
7911 shell scripts that are checked into SCCS. Since RCS preserves the
7912 execution permission of a file, you do not need to use this feature
7915 We recommend that you avoid using of SCCS. RCS is widely held to be
7916 superior, and is also free. By choosing free software in place of
7917 comparable (or inferior) proprietary software, you support the free
7921 Usually, you want to change only the variables listed in the table
7922 above, which are documented in the following section.
7924 However, the commands in built-in implicit rules actually use
7925 variables such as @code{COMPILE.c}, @code{LINK.p}, and
7926 @code{PREPROCESS.S}, whose values contain the commands listed above.
7928 @code{make} follows the convention that the rule to compile a
7929 @file{.@var{x}} source file uses the variable @code{COMPILE.@var{x}}.
7930 Similarly, the rule to produce an executable from a @file{.@var{x}}
7931 file uses @code{LINK.@var{x}}; and the rule to preprocess a
7932 @file{.@var{x}} file uses @code{PREPROCESS.@var{x}}.
7934 @vindex OUTPUT_OPTION
7935 Every rule that produces an object file uses the variable
7936 @code{OUTPUT_OPTION}. @code{make} defines this variable either to
7937 contain @samp{-o $@@}, or to be empty, depending on a compile-time
7938 option. You need the @samp{-o} option to ensure that the output goes
7939 into the right file when the source file is in a different directory,
7940 as when using @code{VPATH} (@pxref{Directory Search}). However,
7941 compilers on some systems do not accept a @samp{-o} switch for object
7942 files. If you use such a system, and use @code{VPATH}, some
7943 compilations will put their output in the wrong place.
7944 A possible workaround for this problem is to give @code{OUTPUT_OPTION}
7945 the value @w{@samp{; mv $*.o $@@}}.
7947 @node Implicit Variables, Chained Rules, Catalogue of Rules, Implicit Rules
7948 @section Variables Used by Implicit Rules
7949 @cindex flags for compilers
7951 The commands in built-in implicit rules make liberal use of certain
7952 predefined variables. You can alter these variables in the makefile,
7953 with arguments to @code{make}, or in the environment to alter how the
7954 implicit rules work without redefining the rules themselves. You can
7955 cancel all variables used by implicit rules with the @samp{-R} or
7956 @samp{--no-builtin-variables} option.
7958 For example, the command used to compile a C source file actually says
7959 @samp{$(CC) -c $(CFLAGS) $(CPPFLAGS)}. The default values of the variables
7960 used are @samp{cc} and nothing, resulting in the command @samp{cc -c}. By
7961 redefining @samp{CC} to @samp{ncc}, you could cause @samp{ncc} to be
7962 used for all C compilations performed by the implicit rule. By redefining
7963 @samp{CFLAGS} to be @samp{-g}, you could pass the @samp{-g} option to
7964 each compilation. @emph{All} implicit rules that do C compilation use
7965 @samp{$(CC)} to get the program name for the compiler and @emph{all}
7966 include @samp{$(CFLAGS)} among the arguments given to the compiler.@refill
7968 The variables used in implicit rules fall into two classes: those that are
7969 names of programs (like @code{CC}) and those that contain arguments for the
7970 programs (like @code{CFLAGS}). (The ``name of a program'' may also contain
7971 some command arguments, but it must start with an actual executable program
7972 name.) If a variable value contains more than one argument, separate them
7975 Here is a table of variables used as names of programs in built-in rules:
7980 Archive-maintaining program; default @samp{ar}.
7985 Program for doing assembly; default @samp{as}.
7990 Program for compiling C programs; default @samp{cc}.
7995 Program for compiling C++ programs; default @samp{g++}.
8000 Program for extracting a file from RCS; default @samp{co}.
8005 Program for running the C preprocessor, with results to standard output;
8006 default @samp{$(CC) -E}.
8010 Program for compiling or preprocessing Fortran and Ratfor programs;
8016 Program for extracting a file from SCCS; default @samp{get}.
8021 Program to use to turn Lex grammars into C programs or Ratfor programs;
8027 Program for compiling Pascal programs; default @samp{pc}.
8032 Program to use to turn Yacc grammars into C programs; default @samp{yacc}.
8037 Program to use to turn Yacc grammars into Ratfor
8038 programs; default @samp{yacc -r}.
8042 Program to convert a Texinfo source file into an Info file; default
8048 Program to make @TeX{} @sc{dvi} files from @TeX{} source;
8054 Program to make @TeX{} @sc{dvi} files from Texinfo source;
8055 default @samp{texi2dvi}.
8060 Program to translate Web into @TeX{}; default @samp{weave}.
8065 Program to translate C Web into @TeX{}; default @samp{cweave}.
8070 Program to translate Web into Pascal; default @samp{tangle}.
8075 Program to translate C Web into C; default @samp{ctangle}.
8080 Command to remove a file; default @samp{rm -f}.
8084 Here is a table of variables whose values are additional arguments for the
8085 programs above. The default values for all of these is the empty
8086 string, unless otherwise noted.
8091 Flags to give the archive-maintaining program; default @samp{rv}.
8095 Extra flags to give to the assembler (when explicitly
8096 invoked on a @samp{.s} or @samp{.S} file).
8100 Extra flags to give to the C compiler.
8104 Extra flags to give to the C++ compiler.
8108 Extra flags to give to the RCS @code{co} program.
8112 Extra flags to give to the C preprocessor and programs
8113 that use it (the C and Fortran compilers).
8117 Extra flags to give to the Fortran compiler.
8121 Extra flags to give to the SCCS @code{get} program.
8125 Extra flags to give to compilers when they are
8126 supposed to invoke the linker, @samp{ld}.
8130 Extra flags to give to Lex.
8134 Extra flags to give to the Pascal compiler.
8138 Extra flags to give to the Fortran compiler for Ratfor programs.
8142 Extra flags to give to Yacc.
8145 @node Chained Rules, Pattern Rules, Implicit Variables, Implicit Rules
8146 @section Chains of Implicit Rules
8148 @cindex chains of rules
8149 @cindex rule, implicit, chains of
8150 Sometimes a file can be made by a sequence of implicit rules. For example,
8151 a file @file{@var{n}.o} could be made from @file{@var{n}.y} by running
8152 first Yacc and then @code{cc}. Such a sequence is called a @dfn{chain}.
8154 If the file @file{@var{n}.c} exists, or is mentioned in the makefile, no
8155 special searching is required: @code{make} finds that the object file can
8156 be made by C compilation from @file{@var{n}.c}; later on, when considering
8157 how to make @file{@var{n}.c}, the rule for running Yacc is
8158 used. Ultimately both @file{@var{n}.c} and @file{@var{n}.o} are
8161 @cindex intermediate files
8162 @cindex files, intermediate
8163 However, even if @file{@var{n}.c} does not exist and is not mentioned,
8164 @code{make} knows how to envision it as the missing link between
8165 @file{@var{n}.o} and @file{@var{n}.y}! In this case, @file{@var{n}.c} is
8166 called an @dfn{intermediate file}. Once @code{make} has decided to use the
8167 intermediate file, it is entered in the data base as if it had been
8168 mentioned in the makefile, along with the implicit rule that says how to
8171 Intermediate files are remade using their rules just like all other
8172 files. But intermediate files are treated differently in two ways.
8174 The first difference is what happens if the intermediate file does not
8175 exist. If an ordinary file @var{b} does not exist, and @code{make}
8176 considers a target that depends on @var{b}, it invariably creates
8177 @var{b} and then updates the target from @var{b}. But if @var{b} is an
8178 intermediate file, then @code{make} can leave well enough alone. It
8179 won't bother updating @var{b}, or the ultimate target, unless some
8180 prerequisite of @var{b} is newer than that target or there is some other
8181 reason to update that target.
8183 The second difference is that if @code{make} @emph{does} create @var{b}
8184 in order to update something else, it deletes @var{b} later on after it
8185 is no longer needed. Therefore, an intermediate file which did not
8186 exist before @code{make} also does not exist after @code{make}.
8187 @code{make} reports the deletion to you by printing a @samp{rm -f}
8188 command showing which file it is deleting.
8190 Ordinarily, a file cannot be intermediate if it is mentioned in the
8191 makefile as a target or prerequisite. However, you can explicitly mark a
8192 file as intermediate by listing it as a prerequisite of the special target
8193 @code{.INTERMEDIATE}. This takes effect even if the file is mentioned
8194 explicitly in some other way.
8196 @cindex intermediate files, preserving
8197 @cindex preserving intermediate files
8198 @cindex secondary files
8199 You can prevent automatic deletion of an intermediate file by marking it
8200 as a @dfn{secondary} file. To do this, list it as a prerequisite of the
8201 special target @code{.SECONDARY}. When a file is secondary, @code{make}
8202 will not create the file merely because it does not already exist, but
8203 @code{make} does not automatically delete the file. Marking a file as
8204 secondary also marks it as intermediate.
8206 You can list the target pattern of an implicit rule (such as @samp{%.o})
8207 as a prerequisite of the special target @code{.PRECIOUS} to preserve
8208 intermediate files made by implicit rules whose target patterns match
8209 that file's name; see @ref{Interrupts}.@refill
8210 @cindex preserving with @code{.PRECIOUS}
8211 @cindex @code{.PRECIOUS} intermediate files
8213 A chain can involve more than two implicit rules. For example, it is
8214 possible to make a file @file{foo} from @file{RCS/foo.y,v} by running RCS,
8215 Yacc and @code{cc}. Then both @file{foo.y} and @file{foo.c} are
8216 intermediate files that are deleted at the end.@refill
8218 No single implicit rule can appear more than once in a chain. This means
8219 that @code{make} will not even consider such a ridiculous thing as making
8220 @file{foo} from @file{foo.o.o} by running the linker twice. This
8221 constraint has the added benefit of preventing any infinite loop in the
8222 search for an implicit rule chain.
8224 There are some special implicit rules to optimize certain cases that would
8225 otherwise be handled by rule chains. For example, making @file{foo} from
8226 @file{foo.c} could be handled by compiling and linking with separate
8227 chained rules, using @file{foo.o} as an intermediate file. But what
8228 actually happens is that a special rule for this case does the compilation
8229 and linking with a single @code{cc} command. The optimized rule is used in
8230 preference to the step-by-step chain because it comes earlier in the
8233 @node Pattern Rules, Last Resort, Chained Rules, Implicit Rules
8234 @section Defining and Redefining Pattern Rules
8236 You define an implicit rule by writing a @dfn{pattern rule}. A pattern
8237 rule looks like an ordinary rule, except that its target contains the
8238 character @samp{%} (exactly one of them). The target is considered a
8239 pattern for matching file names; the @samp{%} can match any nonempty
8240 substring, while other characters match only themselves. The prerequisites
8241 likewise use @samp{%} to show how their names relate to the target name.
8243 Thus, a pattern rule @samp{%.o : %.c} says how to make any file
8244 @file{@var{stem}.o} from another file @file{@var{stem}.c}.@refill
8246 Note that expansion using @samp{%} in pattern rules occurs
8247 @strong{after} any variable or function expansions, which take place
8248 when the makefile is read. @xref{Using Variables, , How to Use
8249 Variables}, and @ref{Functions, ,Functions for Transforming Text}.
8252 * Pattern Intro:: An introduction to pattern rules.
8253 * Pattern Examples:: Examples of pattern rules.
8254 * Automatic Variables:: How to use automatic variables in the
8255 commands of implicit rules.
8256 * Pattern Match:: How patterns match.
8257 * Match-Anything Rules:: Precautions you should take prior to
8258 defining rules that can match any
8259 target file whatever.
8260 * Canceling Rules:: How to override or cancel built-in rules.
8263 @node Pattern Intro, Pattern Examples, Pattern Rules, Pattern Rules
8264 @subsection Introduction to Pattern Rules
8265 @cindex pattern rule
8266 @cindex rule, pattern
8268 A pattern rule contains the character @samp{%} (exactly one of them)
8269 in the target; otherwise, it looks exactly like an ordinary rule. The
8270 target is a pattern for matching file names; the @samp{%} matches any
8271 nonempty substring, while other characters match only themselves.
8272 @cindex target pattern, implicit
8273 @cindex @code{%}, in pattern rules
8275 For example, @samp{%.c} as a pattern matches any file name that ends in
8276 @samp{.c}. @samp{s.%.c} as a pattern matches any file name that starts
8277 with @samp{s.}, ends in @samp{.c} and is at least five characters long.
8278 (There must be at least one character to match the @samp{%}.) The substring
8279 that the @samp{%} matches is called the @dfn{stem}.@refill
8281 @samp{%} in a prerequisite of a pattern rule stands for the same stem
8282 that was matched by the @samp{%} in the target. In order for
8283 the pattern rule to apply, its target pattern must match the file name
8284 under consideration, and its prerequisite patterns must name files that
8285 exist or can be made. These files become prerequisites of the target.
8286 @cindex prerequisite pattern, implicit
8288 Thus, a rule of the form
8291 %.o : %.c ; @var{command}@dots{}
8295 specifies how to make a file @file{@var{n}.o}, with another file
8296 @file{@var{n}.c} as its prerequisite, provided that @file{@var{n}.c}
8297 exists or can be made.
8299 There may also be prerequisites that do not use @samp{%}; such a prerequisite
8300 attaches to every file made by this pattern rule. These unvarying
8301 prerequisites are useful occasionally.
8303 A pattern rule need not have any prerequisites that contain @samp{%}, or
8304 in fact any prerequisites at all. Such a rule is effectively a general
8305 wildcard. It provides a way to make any file that matches the target
8306 pattern. @xref{Last Resort}.
8308 @c !!! The end of of this paragraph should be rewritten. --bob
8309 Pattern rules may have more than one target. Unlike normal rules, this
8310 does not act as many different rules with the same prerequisites and
8311 commands. If a pattern rule has multiple targets, @code{make} knows that
8312 the rule's commands are responsible for making all of the targets. The
8313 commands are executed only once to make all the targets. When searching
8314 for a pattern rule to match a target, the target patterns of a rule other
8315 than the one that matches the target in need of a rule are incidental:
8316 @code{make} worries only about giving commands and prerequisites to the file
8317 presently in question. However, when this file's commands are run, the
8318 other targets are marked as having been updated themselves.
8319 @cindex multiple targets, in pattern rule
8320 @cindex target, multiple in pattern rule
8322 The order in which pattern rules appear in the makefile is important
8323 since this is the order in which they are considered.
8324 Of equally applicable
8325 rules, only the first one found is used. The rules you write take precedence
8326 over those that are built in. Note however, that a rule whose
8327 prerequisites actually exist or are mentioned always takes priority over a
8328 rule with prerequisites that must be made by chaining other implicit rules.
8329 @cindex pattern rules, order of
8330 @cindex order of pattern rules
8332 @node Pattern Examples, Automatic Variables, Pattern Intro, Pattern Rules
8333 @subsection Pattern Rule Examples
8335 Here are some examples of pattern rules actually predefined in
8336 @code{make}. First, the rule that compiles @samp{.c} files into @samp{.o}
8341 $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
8345 defines a rule that can make any file @file{@var{x}.o} from
8346 @file{@var{x}.c}. The command uses the automatic variables @samp{$@@} and
8347 @samp{$<} to substitute the names of the target file and the source file
8348 in each case where the rule applies (@pxref{Automatic Variables}).@refill
8350 Here is a second built-in rule:
8358 defines a rule that can make any file @file{@var{x}} whatsoever from a
8359 corresponding file @file{@var{x},v} in the subdirectory @file{RCS}. Since
8360 the target is @samp{%}, this rule will apply to any file whatever, provided
8361 the appropriate prerequisite file exists. The double colon makes the rule
8362 @dfn{terminal}, which means that its prerequisite may not be an intermediate
8363 file (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}).@refill
8366 This pattern rule has two targets:
8370 %.tab.c %.tab.h: %.y
8376 @c The following paragraph is rewritten to avoid overfull hboxes
8377 This tells @code{make} that the command @samp{bison -d @var{x}.y} will
8378 make both @file{@var{x}.tab.c} and @file{@var{x}.tab.h}. If the file
8379 @file{foo} depends on the files @file{parse.tab.o} and @file{scan.o}
8380 and the file @file{scan.o} depends on the file @file{parse.tab.h},
8381 when @file{parse.y} is changed, the command @samp{bison -d parse.y}
8382 will be executed only once, and the prerequisites of both
8383 @file{parse.tab.o} and @file{scan.o} will be satisfied. (Presumably
8384 the file @file{parse.tab.o} will be recompiled from @file{parse.tab.c}
8385 and the file @file{scan.o} from @file{scan.c}, while @file{foo} is
8386 linked from @file{parse.tab.o}, @file{scan.o}, and its other
8387 prerequisites, and it will execute happily ever after.)@refill
8389 @node Automatic Variables, Pattern Match, Pattern Examples, Pattern Rules
8390 @subsection Automatic Variables
8391 @cindex automatic variables
8392 @cindex variables, automatic
8393 @cindex variables, and implicit rule
8395 Suppose you are writing a pattern rule to compile a @samp{.c} file into a
8396 @samp{.o} file: how do you write the @samp{cc} command so that it operates
8397 on the right source file name? You cannot write the name in the command,
8398 because the name is different each time the implicit rule is applied.
8400 What you do is use a special feature of @code{make}, the @dfn{automatic
8401 variables}. These variables have values computed afresh for each rule that
8402 is executed, based on the target and prerequisites of the rule. In this
8403 example, you would use @samp{$@@} for the object file name and @samp{$<}
8404 for the source file name.
8406 @cindex automatic variables in prerequisites
8407 @cindex prerequisites, and automatic variables
8408 It's very important that you recognize the limited scope in which
8409 automatic variable values are available: they only have values within
8410 the command script. In particular, you cannot use them anywhere
8411 within the target or prerequisite lists of a rule; they have no value
8412 there and will expand to the empty string. A common mistake is
8413 attempting to use @code{$@@} within the prerequisites list in a rule;
8414 this will not work. However, see below for information on the
8415 SysV-style @code{$$@@} variables.
8417 Here is a table of automatic variables:
8421 @vindex @@ @r{(automatic variable)}
8423 The file name of the target of the rule. If the target is an archive
8424 member, then @samp{$@@} is the name of the archive file. In a pattern
8425 rule that has multiple targets (@pxref{Pattern Intro, ,Introduction to
8426 Pattern Rules}), @samp{$@@} is the name of whichever target caused the
8427 rule's commands to be run.
8430 @vindex % @r{(automatic variable)}
8432 The target member name, when the target is an archive member.
8433 @xref{Archives}. For example, if the target is @file{foo.a(bar.o)} then
8434 @samp{$%} is @file{bar.o} and @samp{$@@} is @file{foo.a}. @samp{$%} is
8435 empty when the target is not an archive member.
8438 @vindex < @r{(automatic variable)}
8440 The name of the first prerequisite. If the target got its commands from
8441 an implicit rule, this will be the first prerequisite added by the
8442 implicit rule (@pxref{Implicit Rules}).
8445 @vindex ? @r{(automatic variable)}
8447 The names of all the prerequisites that are newer than the target, with
8448 spaces between them. For prerequisites which are archive members, only
8449 the member named is used (@pxref{Archives}).
8450 @cindex prerequisites, list of changed
8451 @cindex list of changed prerequisites
8454 @vindex ^ @r{(automatic variable)}
8456 The names of all the prerequisites, with spaces between them. For
8457 prerequisites which are archive members, only the member named is used
8458 (@pxref{Archives}). A target has only one prerequisite on each other file
8459 it depends on, no matter how many times each file is listed as a
8460 prerequisite. So if you list a prerequisite more than once for a target,
8461 the value of @code{$^} contains just one copy of the name.
8462 @cindex prerequisites, list of all
8463 @cindex list of all prerequisites
8466 @vindex + @r{(automatic variable)}
8468 This is like @samp{$^}, but prerequisites listed more than once are
8469 duplicated in the order they were listed in the makefile. This is
8470 primarily useful for use in linking commands where it is meaningful to
8471 repeat library file names in a particular order.
8474 @vindex * @r{(automatic variable)}
8476 The stem with which an implicit rule matches (@pxref{Pattern Match, ,How
8477 Patterns Match}). If the target is @file{dir/a.foo.b} and the target
8478 pattern is @file{a.%.b} then the stem is @file{dir/foo}. The stem is
8479 useful for constructing names of related files.@refill
8480 @cindex stem, variable for
8482 In a static pattern rule, the stem is part of the file name that matched
8483 the @samp{%} in the target pattern.
8485 In an explicit rule, there is no stem; so @samp{$*} cannot be determined
8486 in that way. Instead, if the target name ends with a recognized suffix
8487 (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}), @samp{$*} is set to
8488 the target name minus the suffix. For example, if the target name is
8489 @samp{foo.c}, then @samp{$*} is set to @samp{foo}, since @samp{.c} is a
8490 suffix. GNU @code{make} does this bizarre thing only for compatibility
8491 with other implementations of @code{make}. You should generally avoid
8492 using @samp{$*} except in implicit rules or static pattern rules.@refill
8494 If the target name in an explicit rule does not end with a recognized
8495 suffix, @samp{$*} is set to the empty string for that rule.
8498 @samp{$?} is useful even in explicit rules when you wish to operate on only
8499 the prerequisites that have changed. For example, suppose that an archive
8500 named @file{lib} is supposed to contain copies of several object files.
8501 This rule copies just the changed object files into the archive:
8505 lib: foo.o bar.o lose.o win.o
8510 Of the variables listed above, four have values that are single file
8511 names, and three have values that are lists of file names. These seven
8512 have variants that get just the file's directory name or just the file
8513 name within the directory. The variant variables' names are formed by
8514 appending @samp{D} or @samp{F}, respectively. These variants are
8515 semi-obsolete in GNU @code{make} since the functions @code{dir} and
8516 @code{notdir} can be used to get a similar effect (@pxref{File Name
8517 Functions, , Functions for File Names}). Note, however, that the
8518 @samp{D} variants all omit the trailing slash which always appears in
8519 the output of the @code{dir} function. Here is a table of the variants:
8523 @vindex @@D @r{(automatic variable)}
8525 The directory part of the file name of the target, with the trailing
8526 slash removed. If the value of @samp{$@@} is @file{dir/foo.o} then
8527 @samp{$(@@D)} is @file{dir}. This value is @file{.} if @samp{$@@} does
8528 not contain a slash.
8531 @vindex @@F @r{(automatic variable)}
8533 The file-within-directory part of the file name of the target. If the
8534 value of @samp{$@@} is @file{dir/foo.o} then @samp{$(@@F)} is
8535 @file{foo.o}. @samp{$(@@F)} is equivalent to @samp{$(notdir $@@)}.
8538 @vindex *D @r{(automatic variable)}
8541 @vindex *F @r{(automatic variable)}
8543 The directory part and the file-within-directory
8544 part of the stem; @file{dir} and @file{foo} in this example.
8547 @vindex %D @r{(automatic variable)}
8550 @vindex %F @r{(automatic variable)}
8552 The directory part and the file-within-directory part of the target
8553 archive member name. This makes sense only for archive member targets
8554 of the form @file{@var{archive}(@var{member})} and is useful only when
8555 @var{member} may contain a directory name. (@xref{Archive Members,
8556 ,Archive Members as Targets}.)
8559 @vindex <D @r{(automatic variable)}
8562 @vindex <F @r{(automatic variable)}
8564 The directory part and the file-within-directory
8565 part of the first prerequisite.
8568 @vindex ^D @r{(automatic variable)}
8571 @vindex ^F @r{(automatic variable)}
8573 Lists of the directory parts and the file-within-directory
8574 parts of all prerequisites.
8577 @vindex +D @r{(automatic variable)}
8580 @vindex +F @r{(automatic variable)}
8582 Lists of the directory parts and the file-within-directory
8583 parts of all prerequisites, including multiple instances of duplicated
8587 @vindex ?D @r{(automatic variable)}
8590 @vindex ?F @r{(automatic variable)}
8592 Lists of the directory parts and the file-within-directory parts of
8593 all prerequisites that are newer than the target.
8596 Note that we use a special stylistic convention when we talk about these
8597 automatic variables; we write ``the value of @samp{$<}'', rather than
8598 @w{``the variable @code{<}''} as we would write for ordinary variables
8599 such as @code{objects} and @code{CFLAGS}. We think this convention
8600 looks more natural in this special case. Please do not assume it has a
8601 deep significance; @samp{$<} refers to the variable named @code{<} just
8602 as @samp{$(CFLAGS)} refers to the variable named @code{CFLAGS}.
8603 You could just as well use @samp{$(<)} in place of @samp{$<}.
8608 @cindex $$@@, support for
8609 GNU @code{make} provides support for the SysV @code{make} feature that
8610 allows special variable references @code{$$@@}, @code{$$(@@D)}, and
8611 @code{$$(@@F)} (note the required double-''$''!) to appear with the
8612 @emph{prerequisites list} (normal automatic variables are available
8613 only within a command script). When appearing in a prerequisites
8614 list, these variables are expanded to the name of the target, the
8615 directory component of the target, and the file component of the
8616 target, respectively.
8618 Note that these variables are available only within explicit and
8619 static pattern (@pxref{Static Pattern, ,Static Pattern Rules}) rules;
8620 they have no special significance within implicit (suffix or pattern)
8621 rules. Also note that while SysV @code{make} actually expands its
8622 entire prerequisite list @emph{twice}, GNU @code{make} does not behave
8623 this way: instead it simply expands these special variables without
8624 re-expanding any other part of the prerequisites list.
8626 This somewhat bizarre feature is included only to provide some
8627 compatibility with SysV makefiles. In a native GNU @code{make} file
8628 there are other ways to accomplish the same results. This feature is
8629 disabled if the special pseudo target @code{.POSIX} is defined.
8631 @node Pattern Match, Match-Anything Rules, Automatic Variables, Pattern Rules
8632 @subsection How Patterns Match
8635 A target pattern is composed of a @samp{%} between a prefix and a suffix,
8636 either or both of which may be empty. The pattern matches a file name only
8637 if the file name starts with the prefix and ends with the suffix, without
8638 overlap. The text between the prefix and the suffix is called the
8639 @dfn{stem}. Thus, when the pattern @samp{%.o} matches the file name
8640 @file{test.o}, the stem is @samp{test}. The pattern rule prerequisites are
8641 turned into actual file names by substituting the stem for the character
8642 @samp{%}. Thus, if in the same example one of the prerequisites is written
8643 as @samp{%.c}, it expands to @samp{test.c}.@refill
8645 When the target pattern does not contain a slash (and it usually does
8646 not), directory names in the file names are removed from the file name
8647 before it is compared with the target prefix and suffix. After the
8648 comparison of the file name to the target pattern, the directory
8649 names, along with the slash that ends them, are added on to the
8650 prerequisite file names generated from the pattern rule's prerequisite
8651 patterns and the file name. The directories are ignored only for the
8652 purpose of finding an implicit rule to use, not in the application of
8653 that rule. Thus, @samp{e%t} matches the file name @file{src/eat},
8654 with @samp{src/a} as the stem. When prerequisites are turned into file
8655 names, the directories from the stem are added at the front, while the
8656 rest of the stem is substituted for the @samp{%}. The stem
8657 @samp{src/a} with a prerequisite pattern @samp{c%r} gives the file name
8658 @file{src/car}.@refill
8660 @node Match-Anything Rules, Canceling Rules, Pattern Match, Pattern Rules
8661 @subsection Match-Anything Pattern Rules
8663 @cindex match-anything rule
8664 @cindex terminal rule
8665 When a pattern rule's target is just @samp{%}, it matches any file name
8666 whatever. We call these rules @dfn{match-anything} rules. They are very
8667 useful, but it can take a lot of time for @code{make} to think about them,
8668 because it must consider every such rule for each file name listed either
8669 as a target or as a prerequisite.
8671 Suppose the makefile mentions @file{foo.c}. For this target, @code{make}
8672 would have to consider making it by linking an object file @file{foo.c.o},
8673 or by C compilation-and-linking in one step from @file{foo.c.c}, or by
8674 Pascal compilation-and-linking from @file{foo.c.p}, and many other
8677 We know these possibilities are ridiculous since @file{foo.c} is a C source
8678 file, not an executable. If @code{make} did consider these possibilities,
8679 it would ultimately reject them, because files such as @file{foo.c.o} and
8680 @file{foo.c.p} would not exist. But these possibilities are so
8681 numerous that @code{make} would run very slowly if it had to consider
8684 To gain speed, we have put various constraints on the way @code{make}
8685 considers match-anything rules. There are two different constraints that
8686 can be applied, and each time you define a match-anything rule you must
8687 choose one or the other for that rule.
8689 One choice is to mark the match-anything rule as @dfn{terminal} by defining
8690 it with a double colon. When a rule is terminal, it does not apply unless
8691 its prerequisites actually exist. Prerequisites that could be made with
8692 other implicit rules are not good enough. In other words, no further
8693 chaining is allowed beyond a terminal rule.
8695 For example, the built-in implicit rules for extracting sources from RCS
8696 and SCCS files are terminal; as a result, if the file @file{foo.c,v} does
8697 not exist, @code{make} will not even consider trying to make it as an
8698 intermediate file from @file{foo.c,v.o} or from @file{RCS/SCCS/s.foo.c,v}.
8699 RCS and SCCS files are generally ultimate source files, which should not be
8700 remade from any other files; therefore, @code{make} can save time by not
8701 looking for ways to remake them.@refill
8703 If you do not mark the match-anything rule as terminal, then it is
8704 nonterminal. A nonterminal match-anything rule cannot apply to a file name
8705 that indicates a specific type of data. A file name indicates a specific
8706 type of data if some non-match-anything implicit rule target matches it.
8708 For example, the file name @file{foo.c} matches the target for the pattern
8709 rule @samp{%.c : %.y} (the rule to run Yacc). Regardless of whether this
8710 rule is actually applicable (which happens only if there is a file
8711 @file{foo.y}), the fact that its target matches is enough to prevent
8712 consideration of any nonterminal match-anything rules for the file
8713 @file{foo.c}. Thus, @code{make} will not even consider trying to make
8714 @file{foo.c} as an executable file from @file{foo.c.o}, @file{foo.c.c},
8715 @file{foo.c.p}, etc.@refill
8717 The motivation for this constraint is that nonterminal match-anything
8718 rules are used for making files containing specific types of data (such as
8719 executable files) and a file name with a recognized suffix indicates some
8720 other specific type of data (such as a C source file).
8722 Special built-in dummy pattern rules are provided solely to recognize
8723 certain file names so that nonterminal match-anything rules will not be
8724 considered. These dummy rules have no prerequisites and no commands, and
8725 they are ignored for all other purposes. For example, the built-in
8733 exists to make sure that Pascal source files such as @file{foo.p} match a
8734 specific target pattern and thereby prevent time from being wasted looking
8735 for @file{foo.p.o} or @file{foo.p.c}.
8737 Dummy pattern rules such as the one for @samp{%.p} are made for every
8738 suffix listed as valid for use in suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}).
8740 @node Canceling Rules, , Match-Anything Rules, Pattern Rules
8741 @subsection Canceling Implicit Rules
8743 You can override a built-in implicit rule (or one you have defined
8744 yourself) by defining a new pattern rule with the same target and
8745 prerequisites, but different commands. When the new rule is defined, the
8746 built-in one is replaced. The new rule's position in the sequence of
8747 implicit rules is determined by where you write the new rule.
8749 You can cancel a built-in implicit rule by defining a pattern rule with the
8750 same target and prerequisites, but no commands. For example, the following
8751 would cancel the rule that runs the assembler:
8757 @node Last Resort, Suffix Rules, Pattern Rules, Implicit Rules
8758 @section Defining Last-Resort Default Rules
8759 @cindex last-resort default rules
8760 @cindex default rules, last-resort
8762 You can define a last-resort implicit rule by writing a terminal
8763 match-anything pattern rule with no prerequisites (@pxref{Match-Anything
8764 Rules}). This is just like any other pattern rule; the only thing
8765 special about it is that it will match any target. So such a rule's
8766 commands are used for all targets and prerequisites that have no commands
8767 of their own and for which no other implicit rule applies.
8769 For example, when testing a makefile, you might not care if the source
8770 files contain real data, only that they exist. Then you might do this:
8778 to cause all the source files needed (as prerequisites) to be created
8782 You can instead define commands to be used for targets for which there
8783 are no rules at all, even ones which don't specify commands. You do
8784 this by writing a rule for the target @code{.DEFAULT}. Such a rule's
8785 commands are used for all prerequisites which do not appear as targets in
8786 any explicit rule, and for which no implicit rule applies. Naturally,
8787 there is no @code{.DEFAULT} rule unless you write one.
8789 If you use @code{.DEFAULT} with no commands or prerequisites:
8796 the commands previously stored for @code{.DEFAULT} are cleared.
8797 Then @code{make} acts as if you had never defined @code{.DEFAULT} at all.
8799 If you do not want a target to get the commands from a match-anything
8800 pattern rule or @code{.DEFAULT}, but you also do not want any commands
8801 to be run for the target, you can give it empty commands (@pxref{Empty
8802 Commands, ,Defining Empty Commands}).@refill
8804 You can use a last-resort rule to override part of another makefile.
8805 @xref{Overriding Makefiles, , Overriding Part of Another Makefile}.
8807 @node Suffix Rules, Implicit Rule Search, Last Resort, Implicit Rules
8808 @section Old-Fashioned Suffix Rules
8809 @cindex old-fashioned suffix rules
8812 @dfn{Suffix rules} are the old-fashioned way of defining implicit rules for
8813 @code{make}. Suffix rules are obsolete because pattern rules are more
8814 general and clearer. They are supported in GNU @code{make} for
8815 compatibility with old makefiles. They come in two kinds:
8816 @dfn{double-suffix} and @dfn{single-suffix}.@refill
8818 A double-suffix rule is defined by a pair of suffixes: the target suffix
8819 and the source suffix. It matches any file whose name ends with the
8820 target suffix. The corresponding implicit prerequisite is made by
8821 replacing the target suffix with the source suffix in the file name. A
8822 two-suffix rule whose target and source suffixes are @samp{.o} and
8823 @samp{.c} is equivalent to the pattern rule @samp{%.o : %.c}.
8825 A single-suffix rule is defined by a single suffix, which is the source
8826 suffix. It matches any file name, and the corresponding implicit
8827 prerequisite name is made by appending the source suffix. A single-suffix
8828 rule whose source suffix is @samp{.c} is equivalent to the pattern rule
8831 Suffix rule definitions are recognized by comparing each rule's target
8832 against a defined list of known suffixes. When @code{make} sees a rule
8833 whose target is a known suffix, this rule is considered a single-suffix
8834 rule. When @code{make} sees a rule whose target is two known suffixes
8835 concatenated, this rule is taken as a double-suffix rule.
8837 For example, @samp{.c} and @samp{.o} are both on the default list of
8838 known suffixes. Therefore, if you define a rule whose target is
8839 @samp{.c.o}, @code{make} takes it to be a double-suffix rule with source
8840 suffix @samp{.c} and target suffix @samp{.o}. Here is the old-fashioned
8841 way to define the rule for compiling a C source file:@refill
8845 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
8848 Suffix rules cannot have any prerequisites of their own. If they have any,
8849 they are treated as normal files with funny names, not as suffix rules.
8854 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
8858 tells how to make the file @file{.c.o} from the prerequisite file
8859 @file{foo.h}, and is not at all like the pattern rule:
8863 $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
8867 which tells how to make @samp{.o} files from @samp{.c} files, and makes all
8868 @samp{.o} files using this pattern rule also depend on @file{foo.h}.
8870 Suffix rules with no commands are also meaningless. They do not remove
8871 previous rules as do pattern rules with no commands (@pxref{Canceling
8872 Rules, , Canceling Implicit Rules}). They simply enter the suffix or pair of suffixes concatenated as
8873 a target in the data base.@refill
8876 The known suffixes are simply the names of the prerequisites of the special
8877 target @code{.SUFFIXES}. You can add your own suffixes by writing a rule
8878 for @code{.SUFFIXES} that adds more prerequisites, as in:
8881 .SUFFIXES: .hack .win
8885 which adds @samp{.hack} and @samp{.win} to the end of the list of suffixes.
8887 If you wish to eliminate the default known suffixes instead of just adding
8888 to them, write a rule for @code{.SUFFIXES} with no prerequisites. By
8889 special dispensation, this eliminates all existing prerequisites of
8890 @code{.SUFFIXES}. You can then write another rule to add the suffixes you
8895 .SUFFIXES: # @r{Delete the default suffixes}
8896 .SUFFIXES: .c .o .h # @r{Define our suffix list}
8900 The @samp{-r} or @samp{--no-builtin-rules} flag causes the default
8901 list of suffixes to be empty.
8904 The variable @code{SUFFIXES} is defined to the default list of suffixes
8905 before @code{make} reads any makefiles. You can change the list of suffixes
8906 with a rule for the special target @code{.SUFFIXES}, but that does not alter
8909 @node Implicit Rule Search, , Suffix Rules, Implicit Rules
8910 @section Implicit Rule Search Algorithm
8911 @cindex implicit rule, search algorithm
8912 @cindex search algorithm, implicit rule
8914 Here is the procedure @code{make} uses for searching for an implicit rule
8915 for a target @var{t}. This procedure is followed for each double-colon
8916 rule with no commands, for each target of ordinary rules none of which have
8917 commands, and for each prerequisite that is not the target of any rule. It
8918 is also followed recursively for prerequisites that come from implicit
8919 rules, in the search for a chain of rules.
8921 Suffix rules are not mentioned in this algorithm because suffix rules are
8922 converted to equivalent pattern rules once the makefiles have been read in.
8924 For an archive member target of the form
8925 @samp{@var{archive}(@var{member})}, the following algorithm is run
8926 twice, first using the entire target name @var{t}, and second using
8927 @samp{(@var{member})} as the target @var{t} if the first run found no
8932 Split @var{t} into a directory part, called @var{d}, and the rest,
8933 called @var{n}. For example, if @var{t} is @samp{src/foo.o}, then
8934 @var{d} is @samp{src/} and @var{n} is @samp{foo.o}.@refill
8937 Make a list of all the pattern rules one of whose targets matches
8938 @var{t} or @var{n}. If the target pattern contains a slash, it is
8939 matched against @var{t}; otherwise, against @var{n}.
8942 If any rule in that list is @emph{not} a match-anything rule, then
8943 remove all nonterminal match-anything rules from the list.
8946 Remove from the list all rules with no commands.
8949 For each pattern rule in the list:
8953 Find the stem @var{s}, which is the nonempty part of @var{t} or @var{n}
8954 matched by the @samp{%} in the target pattern.@refill
8957 Compute the prerequisite names by substituting @var{s} for @samp{%}; if
8958 the target pattern does not contain a slash, append @var{d} to
8959 the front of each prerequisite name.@refill
8962 Test whether all the prerequisites exist or ought to exist. (If a
8963 file name is mentioned in the makefile as a target or as an explicit
8964 prerequisite, then we say it ought to exist.)
8966 If all prerequisites exist or ought to exist, or there are no prerequisites,
8967 then this rule applies.
8971 If no pattern rule has been found so far, try harder.
8972 For each pattern rule in the list:
8976 If the rule is terminal, ignore it and go on to the next rule.
8979 Compute the prerequisite names as before.
8982 Test whether all the prerequisites exist or ought to exist.
8985 For each prerequisite that does not exist, follow this algorithm
8986 recursively to see if the prerequisite can be made by an implicit
8990 If all prerequisites exist, ought to exist, or can be
8991 made by implicit rules, then this rule applies.
8995 If no implicit rule applies, the rule for @code{.DEFAULT}, if any,
8996 applies. In that case, give @var{t} the same commands that
8997 @code{.DEFAULT} has. Otherwise, there are no commands for @var{t}.
9000 Once a rule that applies has been found, for each target pattern of the
9001 rule other than the one that matched @var{t} or @var{n}, the @samp{%} in
9002 the pattern is replaced with @var{s} and the resultant file name is stored
9003 until the commands to remake the target file @var{t} are executed. After
9004 these commands are executed, each of these stored file names are entered
9005 into the data base and marked as having been updated and having the same
9006 update status as the file @var{t}.
9008 When the commands of a pattern rule are executed for @var{t}, the automatic
9009 variables are set corresponding to the target and prerequisites.
9010 @xref{Automatic Variables}.
9012 @node Archives, Features, Implicit Rules, Top
9013 @chapter Using @code{make} to Update Archive Files
9016 @dfn{Archive files} are files containing named subfiles called
9017 @dfn{members}; they are maintained with the program @code{ar} and their
9018 main use is as subroutine libraries for linking.
9021 * Archive Members:: Archive members as targets.
9022 * Archive Update:: The implicit rule for archive member targets.
9023 * Archive Pitfalls:: Dangers to watch out for when using archives.
9024 * Archive Suffix Rules:: You can write a special kind of suffix rule
9025 for updating archives.
9028 @node Archive Members, Archive Update, Archives, Archives
9029 @section Archive Members as Targets
9030 @cindex archive member targets
9032 An individual member of an archive file can be used as a target or
9033 prerequisite in @code{make}. You specify the member named @var{member} in
9034 archive file @var{archive} as follows:
9037 @var{archive}(@var{member})
9041 This construct is available only in targets and prerequisites, not in
9042 commands! Most programs that you might use in commands do not support this
9043 syntax and cannot act directly on archive members. Only @code{ar} and
9044 other programs specifically designed to operate on archives can do so.
9045 Therefore, valid commands to update an archive member target probably must
9046 use @code{ar}. For example, this rule says to create a member
9047 @file{hack.o} in archive @file{foolib} by copying the file @file{hack.o}:
9050 foolib(hack.o) : hack.o
9054 In fact, nearly all archive member targets are updated in just this way
9055 and there is an implicit rule to do it for you. @strong{Note:} The
9056 @samp{c} flag to @code{ar} is required if the archive file does not
9059 To specify several members in the same archive, you can write all the
9060 member names together between the parentheses. For example:
9063 foolib(hack.o kludge.o)
9070 foolib(hack.o) foolib(kludge.o)
9073 @cindex wildcard, in archive member
9074 You can also use shell-style wildcards in an archive member reference.
9075 @xref{Wildcards, ,Using Wildcard Characters in File Names}. For
9076 example, @w{@samp{foolib(*.o)}} expands to all existing members of the
9077 @file{foolib} archive whose names end in @samp{.o}; perhaps
9078 @samp{@w{foolib(hack.o)} @w{foolib(kludge.o)}}.
9080 @node Archive Update, Archive Pitfalls, Archive Members, Archives
9081 @section Implicit Rule for Archive Member Targets
9083 Recall that a target that looks like @file{@var{a}(@var{m})} stands for the
9084 member named @var{m} in the archive file @var{a}.
9086 When @code{make} looks for an implicit rule for such a target, as a special
9087 feature it considers implicit rules that match @file{(@var{m})}, as well as
9088 those that match the actual target @file{@var{a}(@var{m})}.
9090 This causes one special rule whose target is @file{(%)} to match. This
9091 rule updates the target @file{@var{a}(@var{m})} by copying the file @var{m}
9092 into the archive. For example, it will update the archive member target
9093 @file{foo.a(bar.o)} by copying the @emph{file} @file{bar.o} into the
9094 archive @file{foo.a} as a @emph{member} named @file{bar.o}.
9096 When this rule is chained with others, the result is very powerful.
9097 Thus, @samp{make "foo.a(bar.o)"} (the quotes are needed to protect the
9098 @samp{(} and @samp{)} from being interpreted specially by the shell) in
9099 the presence of a file @file{bar.c} is enough to cause the following
9100 commands to be run, even without a makefile:
9103 cc -c bar.c -o bar.o
9109 Here @code{make} has envisioned the file @file{bar.o} as an intermediate
9110 file. @xref{Chained Rules, ,Chains of Implicit Rules}.
9112 Implicit rules such as this one are written using the automatic variable
9113 @samp{$%}. @xref{Automatic Variables}.
9115 An archive member name in an archive cannot contain a directory name, but
9116 it may be useful in a makefile to pretend that it does. If you write an
9117 archive member target @file{foo.a(dir/file.o)}, @code{make} will perform
9118 automatic updating with this command:
9121 ar r foo.a dir/file.o
9125 which has the effect of copying the file @file{dir/file.o} into a member
9126 named @file{file.o}. In connection with such usage, the automatic variables
9127 @code{%D} and @code{%F} may be useful.
9130 * Archive Symbols:: How to update archive symbol directories.
9133 @node Archive Symbols, , Archive Update, Archive Update
9134 @subsection Updating Archive Symbol Directories
9135 @cindex @code{__.SYMDEF}
9136 @cindex updating archive symbol directories
9137 @cindex archive symbol directory updating
9138 @cindex symbol directories, updating archive
9139 @cindex directories, updating archive symbol
9141 An archive file that is used as a library usually contains a special member
9142 named @file{__.SYMDEF} that contains a directory of the external symbol
9143 names defined by all the other members. After you update any other
9144 members, you need to update @file{__.SYMDEF} so that it will summarize the
9145 other members properly. This is done by running the @code{ranlib} program:
9148 ranlib @var{archivefile}
9151 Normally you would put this command in the rule for the archive file,
9152 and make all the members of the archive file prerequisites of that rule.
9156 libfoo.a: libfoo.a(x.o) libfoo.a(y.o) @dots{}
9161 The effect of this is to update archive members @file{x.o}, @file{y.o},
9162 etc., and then update the symbol directory member @file{__.SYMDEF} by
9163 running @code{ranlib}. The rules for updating the members are not shown
9164 here; most likely you can omit them and use the implicit rule which copies
9165 files into the archive, as described in the preceding section.
9167 This is not necessary when using the GNU @code{ar} program, which
9168 updates the @file{__.SYMDEF} member automatically.
9170 @node Archive Pitfalls, Archive Suffix Rules, Archive Update, Archives
9171 @section Dangers When Using Archives
9172 @cindex archive, and parallel execution
9173 @cindex parallel execution, and archive update
9174 @cindex archive, and @code{-j}
9175 @cindex @code{-j}, and archive update
9177 It is important to be careful when using parallel execution (the
9178 @code{-j} switch; @pxref{Parallel, ,Parallel Execution}) and archives.
9179 If multiple @code{ar} commands run at the same time on the same archive
9180 file, they will not know about each other and can corrupt the file.
9182 Possibly a future version of @code{make} will provide a mechanism to
9183 circumvent this problem by serializing all commands that operate on the
9184 same archive file. But for the time being, you must either write your
9185 makefiles to avoid this problem in some other way, or not use @code{-j}.
9187 @node Archive Suffix Rules, , Archive Pitfalls, Archives
9188 @section Suffix Rules for Archive Files
9189 @cindex suffix rule, for archive
9190 @cindex archive, suffix rule for
9191 @cindex library archive, suffix rule for
9192 @cindex @code{.a} (archives)
9194 You can write a special kind of suffix rule for dealing with archive
9195 files. @xref{Suffix Rules}, for a full explanation of suffix rules.
9196 Archive suffix rules are obsolete in GNU @code{make}, because pattern
9197 rules for archives are a more general mechanism (@pxref{Archive
9198 Update}). But they are retained for compatibility with other
9201 To write a suffix rule for archives, you simply write a suffix rule
9202 using the target suffix @samp{.a} (the usual suffix for archive files).
9203 For example, here is the old-fashioned suffix rule to update a library
9204 archive from C source files:
9209 $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
9216 This works just as if you had written the pattern rule:
9221 $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
9227 In fact, this is just what @code{make} does when it sees a suffix rule
9228 with @samp{.a} as the target suffix. Any double-suffix rule
9229 @w{@samp{.@var{x}.a}} is converted to a pattern rule with the target
9230 pattern @samp{(%.o)} and a prerequisite pattern of @samp{%.@var{x}}.
9232 Since you might want to use @samp{.a} as the suffix for some other kind
9233 of file, @code{make} also converts archive suffix rules to pattern rules
9234 in the normal way (@pxref{Suffix Rules}). Thus a double-suffix rule
9235 @w{@samp{.@var{x}.a}} produces two pattern rules: @samp{@w{(%.o):}
9236 @w{%.@var{x}}} and @samp{@w{%.a}: @w{%.@var{x}}}.@refill
9238 @node Features, Missing, Archives, Top
9239 @chapter Features of GNU @code{make}
9240 @cindex features of GNU @code{make}
9242 @cindex compatibility
9244 Here is a summary of the features of GNU @code{make}, for comparison
9245 with and credit to other versions of @code{make}. We consider the
9246 features of @code{make} in 4.2 BSD systems as a baseline. If you are
9247 concerned with writing portable makefiles, you should not use the
9248 features of @code{make} listed here, nor the ones in @ref{Missing}.
9250 Many features come from the version of @code{make} in System V.
9254 The @code{VPATH} variable and its special meaning.
9255 @xref{Directory Search, , Searching Directories for Prerequisites}.
9256 This feature exists in System V @code{make}, but is undocumented.
9257 It is documented in 4.3 BSD @code{make} (which says it mimics System V's
9258 @code{VPATH} feature).@refill
9261 Included makefiles. @xref{Include, ,Including Other Makefiles}.
9262 Allowing multiple files to be included with a single directive is a GNU
9266 Variables are read from and communicated via the environment.
9267 @xref{Environment, ,Variables from the Environment}.
9270 Options passed through the variable @code{MAKEFLAGS} to recursive
9271 invocations of @code{make}.
9272 @xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
9275 The automatic variable @code{$%} is set to the member name
9276 in an archive reference. @xref{Automatic Variables}.
9279 The automatic variables @code{$@@}, @code{$*}, @code{$<}, @code{$%},
9280 and @code{$?} have corresponding forms like @code{$(@@F)} and
9281 @code{$(@@D)}. We have generalized this to @code{$^} as an obvious
9282 extension. @xref{Automatic Variables}.@refill
9285 Substitution variable references.
9286 @xref{Reference, ,Basics of Variable References}.
9289 The command-line options @samp{-b} and @samp{-m}, accepted and
9290 ignored. In System V @code{make}, these options actually do something.
9293 Execution of recursive commands to run @code{make} via the variable
9294 @code{MAKE} even if @samp{-n}, @samp{-q} or @samp{-t} is specified.
9295 @xref{Recursion, ,Recursive Use of @code{make}}.
9298 Support for suffix @samp{.a} in suffix rules. @xref{Archive Suffix
9299 Rules}. This feature is obsolete in GNU @code{make}, because the
9300 general feature of rule chaining (@pxref{Chained Rules, ,Chains of
9301 Implicit Rules}) allows one pattern rule for installing members in an
9302 archive (@pxref{Archive Update}) to be sufficient.
9305 The arrangement of lines and backslash-newline combinations in
9306 commands is retained when the commands are printed, so they appear as
9307 they do in the makefile, except for the stripping of initial
9311 The following features were inspired by various other versions of
9312 @code{make}. In some cases it is unclear exactly which versions inspired
9317 Pattern rules using @samp{%}.
9318 This has been implemented in several versions of @code{make}.
9319 We're not sure who invented it first, but it's been spread around a bit.
9320 @xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.@refill
9323 Rule chaining and implicit intermediate files.
9324 This was implemented by Stu Feldman in his version of @code{make}
9325 for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
9326 AT&T Bell Labs in his @code{mk} program (where he terms it
9327 ``transitive closure''). We do not really know if
9328 we got this from either of them or thought it up ourselves at the
9329 same time. @xref{Chained Rules, ,Chains of Implicit Rules}.
9332 The automatic variable @code{$^} containing a list of all prerequisites
9333 of the current target. We did not invent this, but we have no idea who
9334 did. @xref{Automatic Variables}. The automatic variable
9335 @code{$+} is a simple extension of @code{$^}.
9338 The ``what if'' flag (@samp{-W} in GNU @code{make}) was (as far as we know)
9339 invented by Andrew Hume in @code{mk}.
9340 @xref{Instead of Execution, ,Instead of Executing the Commands}.
9343 The concept of doing several things at once (parallelism) exists in
9344 many incarnations of @code{make} and similar programs, though not in the
9345 System V or BSD implementations. @xref{Execution, ,Command Execution}.
9348 Modified variable references using pattern substitution come from
9349 SunOS 4. @xref{Reference, ,Basics of Variable References}.
9350 This functionality was provided in GNU @code{make} by the
9351 @code{patsubst} function before the alternate syntax was implemented
9352 for compatibility with SunOS 4. It is not altogether clear who
9353 inspired whom, since GNU @code{make} had @code{patsubst} before SunOS
9354 4 was released.@refill
9357 The special significance of @samp{+} characters preceding command lines
9358 (@pxref{Instead of Execution, ,Instead of Executing the Commands}) is
9360 @cite{IEEE Standard 1003.2-1992} (POSIX.2).
9363 The @samp{+=} syntax to append to the value of a variable comes from SunOS
9364 4 @code{make}. @xref{Appending, , Appending More Text to Variables}.
9367 The syntax @w{@samp{@var{archive}(@var{mem1} @var{mem2}@dots{})}} to list
9368 multiple members in a single archive file comes from SunOS 4 @code{make}.
9369 @xref{Archive Members}.
9372 The @code{-include} directive to include makefiles with no error for a
9373 nonexistent file comes from SunOS 4 @code{make}. (But note that SunOS 4
9374 @code{make} does not allow multiple makefiles to be specified in one
9375 @code{-include} directive.) The same feature appears with the name
9376 @code{sinclude} in SGI @code{make} and perhaps others.
9379 The remaining features are inventions new in GNU @code{make}:
9383 Use the @samp{-v} or @samp{--version} option to print version and
9384 copyright information.
9387 Use the @samp{-h} or @samp{--help} option to summarize the options to
9391 Simply-expanded variables. @xref{Flavors, ,The Two Flavors of Variables}.
9394 Pass command-line variable assignments automatically through the
9395 variable @code{MAKE} to recursive @code{make} invocations.
9396 @xref{Recursion, ,Recursive Use of @code{make}}.
9399 Use the @samp{-C} or @samp{--directory} command option to change
9400 directory. @xref{Options Summary, ,Summary of Options}.
9403 Make verbatim variable definitions with @code{define}.
9404 @xref{Defining, ,Defining Variables Verbatim}.
9407 Declare phony targets with the special target @code{.PHONY}.
9409 Andrew Hume of AT&T Bell Labs implemented a similar feature with a
9410 different syntax in his @code{mk} program. This seems to be a case of
9411 parallel discovery. @xref{Phony Targets, ,Phony Targets}.
9414 Manipulate text by calling functions.
9415 @xref{Functions, ,Functions for Transforming Text}.
9418 Use the @samp{-o} or @samp{--old-file}
9419 option to pretend a file's modification-time is old.
9420 @xref{Avoiding Compilation, ,Avoiding Recompilation of Some Files}.
9423 Conditional execution.
9425 This feature has been implemented numerous times in various versions
9426 of @code{make}; it seems a natural extension derived from the features
9427 of the C preprocessor and similar macro languages and is not a
9428 revolutionary concept. @xref{Conditionals, ,Conditional Parts of Makefiles}.
9431 Specify a search path for included makefiles.
9432 @xref{Include, ,Including Other Makefiles}.
9435 Specify extra makefiles to read with an environment variable.
9436 @xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.
9439 Strip leading sequences of @samp{./} from file names, so that
9440 @file{./@var{file}} and @file{@var{file}} are considered to be the
9444 Use a special search method for library prerequisites written in the
9445 form @samp{-l@var{name}}.
9446 @xref{Libraries/Search, ,Directory Search for Link Libraries}.
9449 Allow suffixes for suffix rules
9450 (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}) to contain any
9451 characters. In other versions of @code{make}, they must begin with
9452 @samp{.} and not contain any @samp{/} characters.
9455 Keep track of the current level of @code{make} recursion using the
9456 variable @code{MAKELEVEL}. @xref{Recursion, ,Recursive Use of @code{make}}.
9459 Provide any goals given on the command line in the variable
9460 @code{MAKECMDGOALS}. @xref{Goals, ,Arguments to Specify the Goals}.
9463 Specify static pattern rules. @xref{Static Pattern, ,Static Pattern Rules}.
9466 Provide selective @code{vpath} search.
9467 @xref{Directory Search, ,Searching Directories for Prerequisites}.
9470 Provide computed variable references.
9471 @xref{Reference, ,Basics of Variable References}.
9474 Update makefiles. @xref{Remaking Makefiles, ,How Makefiles Are Remade}.
9475 System V @code{make} has a very, very limited form of this
9476 functionality in that it will check out SCCS files for makefiles.
9479 Various new built-in implicit rules.
9480 @xref{Catalogue of Rules, ,Catalogue of Implicit Rules}.
9483 The built-in variable @samp{MAKE_VERSION} gives the version number of
9485 @vindex MAKE_VERSION
9488 @node Missing, Makefile Conventions, Features, Top
9489 @chapter Incompatibilities and Missing Features
9490 @cindex incompatibilities
9491 @cindex missing features
9492 @cindex features, missing
9494 The @code{make} programs in various other systems support a few features
9495 that are not implemented in GNU @code{make}. The POSIX.2 standard
9496 (@cite{IEEE Standard 1003.2-1992}) which specifies @code{make} does not
9497 require any of these features.@refill
9501 A target of the form @samp{@var{file}((@var{entry}))} stands for a member
9502 of archive file @var{file}. The member is chosen, not by name, but by
9503 being an object file which defines the linker symbol @var{entry}.@refill
9505 This feature was not put into GNU @code{make} because of the
9506 nonmodularity of putting knowledge into @code{make} of the internal
9507 format of archive file symbol tables.
9508 @xref{Archive Symbols, ,Updating Archive Symbol Directories}.
9511 Suffixes (used in suffix rules) that end with the character @samp{~}
9512 have a special meaning to System V @code{make};
9513 they refer to the SCCS file that corresponds
9514 to the file one would get without the @samp{~}. For example, the
9515 suffix rule @samp{.c~.o} would make the file @file{@var{n}.o} from
9516 the SCCS file @file{s.@var{n}.c}. For complete coverage, a whole
9517 series of such suffix rules is required.
9518 @xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
9520 In GNU @code{make}, this entire series of cases is handled by two
9521 pattern rules for extraction from SCCS, in combination with the
9522 general feature of rule chaining.
9523 @xref{Chained Rules, ,Chains of Implicit Rules}.
9526 In System V and 4.3 BSD @code{make}, files found by @code{VPATH} search
9527 (@pxref{Directory Search, ,Searching Directories for Prerequisites}) have their names changed inside command
9528 strings. We feel it is much cleaner to always use automatic variables
9529 and thus make this feature obsolete.@refill
9532 In some Unix @code{make}s, the automatic variable @code{$*} appearing in
9533 the prerequisites of a rule has the amazingly strange ``feature'' of
9534 expanding to the full name of the @emph{target of that rule}. We cannot
9535 imagine what went on in the minds of Unix @code{make} developers to do
9536 this; it is utterly inconsistent with the normal definition of @code{$*}.
9537 @vindex * @r{(automatic variable), unsupported bizarre usage}
9540 In some Unix @code{make}s, implicit rule search
9541 (@pxref{Implicit Rules, ,Using Implicit Rules}) is apparently done for
9542 @emph{all} targets, not just those without commands. This means you can
9553 and Unix @code{make} will intuit that @file{foo.o} depends on
9554 @file{foo.c}.@refill
9556 We feel that such usage is broken. The prerequisite properties of
9557 @code{make} are well-defined (for GNU @code{make}, at least),
9558 and doing such a thing simply does not fit the model.@refill
9561 GNU @code{make} does not include any built-in implicit rules for
9562 compiling or preprocessing EFL programs. If we hear of anyone who is
9563 using EFL, we will gladly add them.
9566 It appears that in SVR4 @code{make}, a suffix rule can be specified with
9567 no commands, and it is treated as if it had empty commands
9568 (@pxref{Empty Commands}). For example:
9575 will override the built-in @file{.c.a} suffix rule.
9577 We feel that it is cleaner for a rule without commands to always simply
9578 add to the prerequisite list for the target. The above example can be
9579 easily rewritten to get the desired behavior in GNU @code{make}:
9586 Some versions of @code{make} invoke the shell with the @samp{-e} flag,
9587 except under @samp{-k} (@pxref{Testing, ,Testing the Compilation of a
9588 Program}). The @samp{-e} flag tells the shell to exit as soon as any
9589 program it runs returns a nonzero status. We feel it is cleaner to
9590 write each shell command line to stand on its own and not require this
9594 @comment The makefile standards are in a separate file that is also
9595 @comment included by standards.texi.
9596 @include make-stds.texi
9598 @node Quick Reference, Error Messages, Makefile Conventions, Top
9599 @appendix Quick Reference
9601 This appendix summarizes the directives, text manipulation functions,
9602 and special variables which GNU @code{make} understands.
9603 @xref{Special Targets}, @ref{Catalogue of Rules, ,Catalogue of Implicit Rules},
9604 and @ref{Options Summary, ,Summary of Options},
9605 for other summaries.
9607 Here is a summary of the directives GNU @code{make} recognizes:
9610 @item define @var{variable}
9613 Define a multi-line, recursively-expanded variable.@*
9616 @item ifdef @var{variable}
9617 @itemx ifndef @var{variable}
9618 @itemx ifeq (@var{a},@var{b})
9619 @itemx ifeq "@var{a}" "@var{b}"
9620 @itemx ifeq '@var{a}' '@var{b}'
9621 @itemx ifneq (@var{a},@var{b})
9622 @itemx ifneq "@var{a}" "@var{b}"
9623 @itemx ifneq '@var{a}' '@var{b}'
9627 Conditionally evaluate part of the makefile.@*
9628 @xref{Conditionals}.
9630 @item include @var{file}
9631 @itemx -include @var{file}
9632 @itemx sinclude @var{file}
9634 Include another makefile.@*
9635 @xref{Include, ,Including Other Makefiles}.
9637 @item override @var{variable} = @var{value}
9638 @itemx override @var{variable} := @var{value}
9639 @itemx override @var{variable} += @var{value}
9640 @itemx override @var{variable} ?= @var{value}
9641 @itemx override define @var{variable}
9644 Define a variable, overriding any previous definition, even one from
9646 @xref{Override Directive, ,The @code{override} Directive}.
9650 Tell @code{make} to export all variables to child processes by default.@*
9651 @xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.
9653 @item export @var{variable}
9654 @itemx export @var{variable} = @var{value}
9655 @itemx export @var{variable} := @var{value}
9656 @itemx export @var{variable} += @var{value}
9657 @itemx export @var{variable} ?= @var{value}
9658 @itemx unexport @var{variable}
9659 Tell @code{make} whether or not to export a particular variable to child
9661 @xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.
9663 @item vpath @var{pattern} @var{path}
9664 Specify a search path for files matching a @samp{%} pattern.@*
9665 @xref{Selective Search, , The @code{vpath} Directive}.
9667 @item vpath @var{pattern}
9668 Remove all search paths previously specified for @var{pattern}.
9671 Remove all search paths previously specified in any @code{vpath}
9675 Here is a summary of the built-in functions (@pxref{Functions}):
9678 @item $(subst @var{from},@var{to},@var{text})
9679 Replace @var{from} with @var{to} in @var{text}.@*
9680 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9682 @item $(patsubst @var{pattern},@var{replacement},@var{text})
9683 Replace words matching @var{pattern} with @var{replacement} in @var{text}.@*
9684 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9686 @item $(strip @var{string})
9687 Remove excess whitespace characters from @var{string}.@*
9688 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9690 @item $(findstring @var{find},@var{text})
9691 Locate @var{find} in @var{text}.@*
9692 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9694 @item $(filter @var{pattern}@dots{},@var{text})
9695 Select words in @var{text} that match one of the @var{pattern} words.@*
9696 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9698 @item $(filter-out @var{pattern}@dots{},@var{text})
9699 Select words in @var{text} that @emph{do not} match any of the @var{pattern} words.@*
9700 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9702 @item $(sort @var{list})
9703 Sort the words in @var{list} lexicographically, removing duplicates.@*
9704 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9706 @item $(word @var{n},@var{text})
9707 Extract the @var{n}th word (one-origin) of @var{text}.@*
9708 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9710 @item $(words @var{text})
9711 Count the number of words in @var{text}.@*
9712 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9714 @item $(wordlist @var{s},@var{e},@var{text})
9715 Returns the list of words in @var{text} from @var{s} to @var{e}.@*
9716 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9718 @item $(firstword @var{names}@dots{})
9719 Extract the first word of @var{names}.@*
9720 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9722 @item $(lastword @var{names}@dots{})
9723 Extract the last word of @var{names}.@*
9724 @xref{Text Functions, , Functions for String Substitution and Analysis}.
9726 @item $(dir @var{names}@dots{})
9727 Extract the directory part of each file name.@*
9728 @xref{File Name Functions, ,Functions for File Names}.
9730 @item $(notdir @var{names}@dots{})
9731 Extract the non-directory part of each file name.@*
9732 @xref{File Name Functions, ,Functions for File Names}.
9734 @item $(suffix @var{names}@dots{})
9735 Extract the suffix (the last @samp{.} and following characters) of each file name.@*
9736 @xref{File Name Functions, ,Functions for File Names}.
9738 @item $(basename @var{names}@dots{})
9739 Extract the base name (name without suffix) of each file name.@*
9740 @xref{File Name Functions, ,Functions for File Names}.
9742 @item $(addsuffix @var{suffix},@var{names}@dots{})
9743 Append @var{suffix} to each word in @var{names}.@*
9744 @xref{File Name Functions, ,Functions for File Names}.
9746 @item $(addprefix @var{prefix},@var{names}@dots{})
9747 Prepend @var{prefix} to each word in @var{names}.@*
9748 @xref{File Name Functions, ,Functions for File Names}.
9750 @item $(join @var{list1},@var{list2})
9751 Join two parallel lists of words.@*
9752 @xref{File Name Functions, ,Functions for File Names}.
9754 @item $(wildcard @var{pattern}@dots{})
9755 Find file names matching a shell file name pattern (@emph{not} a
9756 @samp{%} pattern).@*
9757 @xref{Wildcard Function, ,The Function @code{wildcard}}.
9759 @item $(error @var{text}@dots{})
9761 When this function is evaluated, @code{make} generates a fatal error
9762 with the message @var{text}.@*
9763 @xref{Make Control Functions, ,Functions That Control Make}.
9765 @item $(warning @var{text}@dots{})
9767 When this function is evaluated, @code{make} generates a warning with
9768 the message @var{text}.@*
9769 @xref{Make Control Functions, ,Functions That Control Make}.
9771 @item $(shell @var{command})
9773 Execute a shell command and return its output.@*
9774 @xref{Shell Function, , The @code{shell} Function}.
9776 @item $(origin @var{variable})
9778 Return a string describing how the @code{make} variable @var{variable} was
9780 @xref{Origin Function, , The @code{origin} Function}.
9782 @item $(foreach @var{var},@var{words},@var{text})
9784 Evaluate @var{text} with @var{var} bound to each word in @var{words},
9785 and concatenate the results.@*
9786 @xref{Foreach Function, ,The @code{foreach} Function}.
9788 @item $(call @var{var},@var{param},@dots{})
9790 Evaluate the variable @var{var} replacing any references to @code{$(1)},
9791 @code{$(2)} with the first, second, etc. @var{param} values.@*
9792 @xref{Call Function, ,The @code{call} Function}.
9794 @item $(eval @var{text})
9796 Evaluate @var{text} then read the results as makefile commands.
9797 Expands to the empty string.@*
9798 @xref{Eval Function, ,The @code{eval} Function}.
9800 @item $(value @var{var})
9802 Evaluates to the contents of the variable @var{var}, with no expansion
9804 @xref{Value Function, ,The @code{value} Function}.
9807 Here is a summary of the automatic variables.
9808 @xref{Automatic Variables},
9809 for full information.
9813 The file name of the target.
9816 The target member name, when the target is an archive member.
9819 The name of the first prerequisite.
9822 The names of all the prerequisites that are
9823 newer than the target, with spaces between them.
9824 For prerequisites which are archive members, only
9825 the member named is used (@pxref{Archives}).
9829 The names of all the prerequisites, with spaces between them. For
9830 prerequisites which are archive members, only the member named is used
9831 (@pxref{Archives}). The value of @code{$^} omits duplicate
9832 prerequisites, while @code{$+} retains them and preserves their order.
9835 The stem with which an implicit rule matches
9836 (@pxref{Pattern Match, ,How Patterns Match}).
9840 The directory part and the file-within-directory part of @code{$@@}.
9844 The directory part and the file-within-directory part of @code{$*}.
9848 The directory part and the file-within-directory part of @code{$%}.
9852 The directory part and the file-within-directory part of @code{$<}.
9856 The directory part and the file-within-directory part of @code{$^}.
9860 The directory part and the file-within-directory part of @code{$+}.
9864 The directory part and the file-within-directory part of @code{$?}.
9867 These variables are used specially by GNU @code{make}:
9872 Makefiles to be read on every invocation of @code{make}.@*
9873 @xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.
9877 Directory search path for files not found in the current directory.@*
9878 @xref{General Search, , @code{VPATH} Search Path for All Prerequisites}.
9882 The name of the system default command interpreter, usually @file{/bin/sh}.
9883 You can set @code{SHELL} in the makefile to change the shell used to run
9884 commands. @xref{Execution, ,Command Execution}. The @code{SHELL}
9885 variable is handled specially when importing from and exporting to the
9886 environment. @xref{Environment, ,Using Variable from the Environment}.
9890 On MS-DOS only, the name of the command interpreter that is to be used
9891 by @code{make}. This value takes precedence over the value of
9892 @code{SHELL}. @xref{Execution, ,MAKESHELL variable}.
9896 The name with which @code{make} was invoked.
9897 Using this variable in commands has special meaning.
9898 @xref{MAKE Variable, ,How the @code{MAKE} Variable Works}.
9902 The number of levels of recursion (sub-@code{make}s).@*
9903 @xref{Variables/Recursion}.
9907 The flags given to @code{make}. You can set this in the environment or
9908 a makefile to set flags.@*
9909 @xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.
9911 It is @emph{never} appropriate to use @code{MAKEFLAGS} directly on a
9912 command line: its contents may not be quoted correctly for use in the
9913 shell. Always allow recursive @code{make}'s to obtain these values
9914 through the environment from its parent.
9918 The targets given to @code{make} on the command line. Setting this
9919 variable has no effect on the operation of @code{make}.@*
9920 @xref{Goals, ,Arguments to Specify the Goals}.
9924 Set to the pathname of the current working directory (after all
9925 @code{-C} options are processed, if any). Setting this variable has no
9926 effect on the operation of @code{make}.@*
9927 @xref{Recursion, ,Recursive Use of @code{make}}.
9931 The default list of suffixes before @code{make} reads any makefiles.
9934 Defines the naming of the libraries @code{make} searches for, and their
9936 @xref{Libraries/Search, ,Directory Search for Link Libraries}.
9939 @node Error Messages, Complex Makefile, Quick Reference, Top
9940 @comment node-name, next, previous, up
9941 @appendix Errors Generated by Make
9943 Here is a list of the more common errors you might see generated by
9944 @code{make}, and some information about what they mean and how to fix
9947 Sometimes @code{make} errors are not fatal, especially in the presence
9948 of a @code{-} prefix on a command script line, or the @code{-k} command
9949 line option. Errors that are fatal are prefixed with the string
9952 Error messages are all either prefixed with the name of the program
9953 (usually @samp{make}), or, if the error is found in a makefile, the name
9954 of the file and linenumber containing the problem.
9956 In the table below, these common prefixes are left off.
9960 @item [@var{foo}] Error @var{NN}
9961 @itemx [@var{foo}] @var{signal description}
9962 These errors are not really @code{make} errors at all. They mean that a
9963 program that @code{make} invoked as part of a command script returned a
9964 non-0 error code (@samp{Error @var{NN}}), which @code{make} interprets
9965 as failure, or it exited in some other abnormal fashion (with a
9966 signal of some type). @xref{Errors, ,Errors in Commands}.
9968 If no @code{***} is attached to the message, then the subprocess failed
9969 but the rule in the makefile was prefixed with the @code{-} special
9970 character, so @code{make} ignored the error.
9972 @item missing separator. Stop.
9973 @itemx missing separator (did you mean TAB instead of 8 spaces?). Stop.
9974 This means that @code{make} could not understand much of anything about
9975 the command line it just read. GNU @code{make} looks for various kinds
9976 of separators (@code{:}, @code{=}, TAB characters, etc.) to help it
9977 decide what kind of commandline it's seeing. This means it couldn't
9980 One of the most common reasons for this message is that you (or perhaps
9981 your oh-so-helpful editor, as is the case with many MS-Windows editors)
9982 have attempted to indent your command scripts with spaces instead of a
9983 TAB character. In this case, @code{make} will use the second form of
9984 the error above. Remember that every line in the command script must
9985 begin with a TAB character. Eight spaces do not count. @xref{Rule
9988 @item commands commence before first target. Stop.
9989 @itemx missing rule before commands. Stop.
9990 This means the first thing in the makefile seems to be part of a command
9991 script: it begins with a TAB character and doesn't appear to be a legal
9992 @code{make} command (such as a variable assignment). Command scripts
9993 must always be associated with a target.
9995 The second form is generated if the line has a semicolon as the first
9996 non-whitespace character; @code{make} interprets this to mean you left
9997 out the "target: prerequisite" section of a rule. @xref{Rule Syntax}.
9999 @item No rule to make target `@var{xxx}'.
10000 @itemx No rule to make target `@var{xxx}', needed by `@var{yyy}'.
10001 This means that @code{make} decided it needed to build a target, but
10002 then couldn't find any instructions in the makefile on how to do that,
10003 either explicit or implicit (including in the default rules database).
10005 If you want that file to be built, you will need to add a rule to your
10006 makefile describing how that target can be built. Other possible
10007 sources of this problem are typos in the makefile (if that filename is
10008 wrong) or a corrupted source tree (if that file is not supposed to be
10009 built, but rather only a prerequisite).
10011 @item No targets specified and no makefile found. Stop.
10012 @itemx No targets. Stop.
10013 The former means that you didn't provide any targets to be built on the
10014 command line, and @code{make} couldn't find any makefiles to read in.
10015 The latter means that some makefile was found, but it didn't contain any
10016 default target and none was given on the command line. GNU @code{make}
10017 has nothing to do in these situations.
10018 @xref{Makefile Arguments, ,Arguments to Specify the Makefile}.@refill
10020 @item Makefile `@var{xxx}' was not found.
10021 @itemx Included makefile `@var{xxx}' was not found.
10022 A makefile specified on the command line (first form) or included
10023 (second form) was not found.
10025 @item warning: overriding commands for target `@var{xxx}'
10026 @itemx warning: ignoring old commands for target `@var{xxx}'
10027 GNU @code{make} allows commands to be specified only once per target
10028 (except for double-colon rules). If you give commands for a target
10029 which already has been defined to have commands, this warning is issued
10030 and the second set of commands will overwrite the first set.
10031 @xref{Multiple Rules, ,Multiple Rules for One Target}.
10033 @item Circular @var{xxx} <- @var{yyy} dependency dropped.
10034 This means that @code{make} detected a loop in the dependency graph:
10035 after tracing the prerequisite @var{yyy} of target @var{xxx}, and its
10036 prerequisites, etc., one of them depended on @var{xxx} again.
10038 @item Recursive variable `@var{xxx}' references itself (eventually). Stop.
10039 This means you've defined a normal (recursive) @code{make} variable
10040 @var{xxx} that, when it's expanded, will refer to itself (@var{xxx}).
10041 This is not allowed; either use simply-expanded variables (@code{:=}) or
10042 use the append operator (@code{+=}). @xref{Using Variables, ,How to Use
10045 @item Unterminated variable reference. Stop.
10046 This means you forgot to provide the proper closing parenthesis
10047 or brace in your variable or function reference.
10049 @item insufficient arguments to function `@var{xxx}'. Stop.
10050 This means you haven't provided the requisite number of arguments for
10051 this function. See the documentation of the function for a description
10052 of its arguments. @xref{Functions, ,Functions for Transforming Text}.
10054 @item missing target pattern. Stop.
10055 @itemx multiple target patterns. Stop.
10056 @itemx target pattern contains no `%'. Stop.
10057 @itemx mixed implicit and static pattern rules. Stop.
10058 These are generated for malformed static pattern rules. The first means
10059 there's no pattern in the target section of the rule; the second means
10060 there are multiple patterns in the target section; the third means
10061 the target doesn't contain a pattern character (@code{%}); and the
10062 fourth means that all three parts of the static pattern rule contain
10063 pattern characters (@code{%})--only the first two parts should.
10064 @xref{Static Usage, ,Syntax of Static Pattern Rules}.
10066 @item warning: -jN forced in submake: disabling jobserver mode.
10067 This warning and the next are generated if @code{make} detects error
10068 conditions related to parallel processing on systems where
10069 sub-@code{make}s can communicate (@pxref{Options/Recursion,
10070 ,Communicating Options to a Sub-@code{make}}). This warning is
10071 generated if a recursive invocation of a @code{make} process is forced
10072 to have @samp{-j@var{N}} in its argument list (where @var{N} is greater
10073 than one). This could happen, for example, if you set the @code{MAKE}
10074 environment variable to @samp{make -j2}. In this case, the
10075 sub-@code{make} doesn't communicate with other @code{make} processes and
10076 will simply pretend it has two jobs of its own.
10078 @item warning: jobserver unavailable: using -j1. Add `+' to parent make rule.
10079 In order for @code{make} processes to communicate, the parent will pass
10080 information to the child. Since this could result in problems if the
10081 child process isn't actually a @code{make}, the parent will only do this
10082 if it thinks the child is a @code{make}. The parent uses the normal
10083 algorithms to determine this (@pxref{MAKE Variable, ,How the @code{MAKE}
10084 Variable Works}). If the makefile is constructed such that the parent
10085 doesn't know the child is a @code{make} process, then the child will
10086 receive only part of the information necessary. In this case, the child
10087 will generate this warning message and proceed with its build in a
10092 @node Complex Makefile, GNU Free Documentation License, Error Messages, Top
10093 @appendix Complex Makefile Example
10095 Here is the makefile for the GNU @code{tar} program. This is a
10096 moderately complex makefile.
10098 Because it is the first target, the default goal is @samp{all}. An
10099 interesting feature of this makefile is that @file{testpad.h} is a
10100 source file automatically created by the @code{testpad} program,
10101 itself compiled from @file{testpad.c}.
10103 If you type @samp{make} or @samp{make all}, then @code{make} creates
10104 the @file{tar} executable, the @file{rmt} daemon that provides
10105 remote tape access, and the @file{tar.info} Info file.
10107 If you type @samp{make install}, then @code{make} not only creates
10108 @file{tar}, @file{rmt}, and @file{tar.info}, but also installs
10111 If you type @samp{make clean}, then @code{make} removes the @samp{.o}
10112 files, and the @file{tar}, @file{rmt}, @file{testpad},
10113 @file{testpad.h}, and @file{core} files.
10115 If you type @samp{make distclean}, then @code{make} not only removes
10116 the same files as does @samp{make clean} but also the
10117 @file{TAGS}, @file{Makefile}, and @file{config.status} files.
10118 (Although it is not evident, this makefile (and
10119 @file{config.status}) is generated by the user with the
10120 @code{configure} program, which is provided in the @code{tar}
10121 distribution, but is not shown here.)
10123 If you type @samp{make realclean}, then @code{make} removes the same
10124 files as does @samp{make distclean} and also removes the Info files
10125 generated from @file{tar.texinfo}.
10127 In addition, there are targets @code{shar} and @code{dist} that create
10132 # Generated automatically from Makefile.in by configure.
10133 # Un*x Makefile for GNU tar program.
10134 # Copyright (C) 1991 Free Software Foundation, Inc.
10138 # This program is free software; you can redistribute
10139 # it and/or modify it under the terms of the GNU
10140 # General Public License @dots{}
10147 #### Start of system configuration section. ####
10152 # If you use gcc, you should either run the
10153 # fixincludes script that comes with it or else use
10154 # gcc with the -traditional option. Otherwise ioctl
10155 # calls will be compiled incorrectly on some systems.
10158 INSTALL = /usr/local/bin/install -c
10159 INSTALLDATA = /usr/local/bin/install -c -m 644
10162 # Things you might add to DEFS:
10163 # -DSTDC_HEADERS If you have ANSI C headers and
10165 # -DPOSIX If you have POSIX.1 headers and
10167 # -DBSD42 If you have sys/dir.h (unless
10168 # you use -DPOSIX), sys/file.h,
10169 # and st_blocks in `struct stat'.
10170 # -DUSG If you have System V/ANSI C
10171 # string and memory functions
10172 # and headers, sys/sysmacros.h,
10173 # fcntl.h, getcwd, no valloc,
10174 # and ndir.h (unless
10175 # you use -DDIRENT).
10176 # -DNO_MEMORY_H If USG or STDC_HEADERS but do not
10177 # include memory.h.
10178 # -DDIRENT If USG and you have dirent.h
10179 # instead of ndir.h.
10180 # -DSIGTYPE=int If your signal handlers
10181 # return int, not void.
10182 # -DNO_MTIO If you lack sys/mtio.h
10183 # (magtape ioctls).
10184 # -DNO_REMOTE If you do not have a remote shell
10186 # -DUSE_REXEC To use rexec for remote tape
10187 # operations instead of
10188 # forking rsh or remsh.
10189 # -DVPRINTF_MISSING If you lack vprintf function
10190 # (but have _doprnt).
10191 # -DDOPRNT_MISSING If you lack _doprnt function.
10192 # Also need to define
10193 # -DVPRINTF_MISSING.
10194 # -DFTIME_MISSING If you lack ftime system call.
10195 # -DSTRSTR_MISSING If you lack strstr function.
10196 # -DVALLOC_MISSING If you lack valloc function.
10197 # -DMKDIR_MISSING If you lack mkdir and
10198 # rmdir system calls.
10199 # -DRENAME_MISSING If you lack rename system call.
10200 # -DFTRUNCATE_MISSING If you lack ftruncate
10202 # -DV7 On Version 7 Unix (not
10203 # tested in a long time).
10204 # -DEMUL_OPEN3 If you lack a 3-argument version
10205 # of open, and want to emulate it
10206 # with system calls you do have.
10207 # -DNO_OPEN3 If you lack the 3-argument open
10208 # and want to disable the tar -k
10209 # option instead of emulating open.
10210 # -DXENIX If you have sys/inode.h
10211 # and need it 94 to be included.
10213 DEFS = -DSIGTYPE=int -DDIRENT -DSTRSTR_MISSING \
10214 -DVPRINTF_MISSING -DBSD42
10215 # Set this to rtapelib.o unless you defined NO_REMOTE,
10216 # in which case make it empty.
10217 RTAPELIB = rtapelib.o
10219 DEF_AR_FILE = /dev/rmt8
10224 CFLAGS = $(CDEBUG) -I. -I$(srcdir) $(DEFS) \
10225 -DDEF_AR_FILE=\"$(DEF_AR_FILE)\" \
10226 -DDEFBLOCKING=$(DEFBLOCKING)
10231 prefix = /usr/local
10232 # Prefix for each installed program,
10233 # normally empty or `g'.
10236 # The directory to install tar in.
10237 bindir = $(prefix)/bin
10239 # The directory to install the info files in.
10240 infodir = $(prefix)/info
10243 #### End of system configuration section. ####
10245 SRC1 = tar.c create.c extract.c buffer.c \
10246 getoldopt.c update.c gnu.c mangle.c
10247 SRC2 = version.c list.c names.c diffarch.c \
10248 port.c wildmat.c getopt.c
10249 SRC3 = getopt1.c regex.c getdate.y
10250 SRCS = $(SRC1) $(SRC2) $(SRC3)
10251 OBJ1 = tar.o create.o extract.o buffer.o \
10252 getoldopt.o update.o gnu.o mangle.o
10253 OBJ2 = version.o list.o names.o diffarch.o \
10254 port.o wildmat.o getopt.o
10255 OBJ3 = getopt1.o regex.o getdate.o $(RTAPELIB)
10256 OBJS = $(OBJ1) $(OBJ2) $(OBJ3)
10258 AUX = README COPYING ChangeLog Makefile.in \
10259 makefile.pc configure configure.in \
10260 tar.texinfo tar.info* texinfo.tex \
10261 tar.h port.h open3.h getopt.h regex.h \
10262 rmt.h rmt.c rtapelib.c alloca.c \
10263 msd_dir.h msd_dir.c tcexparg.c \
10264 level-0 level-1 backup-specs testpad.c
10268 all: tar rmt tar.info
10273 $(CC) $(LDFLAGS) -o $@@ $(OBJS) $(LIBS)
10278 $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ rmt.c
10282 tar.info: tar.texinfo
10283 makeinfo tar.texinfo
10289 $(INSTALL) tar $(bindir)/$(binprefix)tar
10290 -test ! -f rmt || $(INSTALL) rmt /etc/rmt
10291 $(INSTALLDATA) $(srcdir)/tar.info* $(infodir)
10295 $(OBJS): tar.h port.h testpad.h
10296 regex.o buffer.o tar.o: regex.h
10297 # getdate.y has 8 shift/reduce conflicts.
10307 $(CC) -o $@@ testpad.o
10318 rm -f *.o tar rmt testpad testpad.h core
10324 rm -f TAGS Makefile config.status
10329 realclean: distclean
10335 shar: $(SRCS) $(AUX)
10336 shar $(SRCS) $(AUX) | compress \
10337 > tar-`sed -e '/version_string/!d' \
10338 -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
10345 dist: $(SRCS) $(AUX)
10347 -e '/version_string/!d' \
10348 -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
10350 version.c` > .fname
10351 -rm -rf `cat .fname`
10353 ln $(SRCS) $(AUX) `cat .fname`
10354 tar chZf `cat .fname`.tar.Z `cat .fname`
10355 -rm -rf `cat .fname` .fname
10359 tar.zoo: $(SRCS) $(AUX)
10363 for X in $(SRCS) $(AUX) ; do \
10365 sed 's/$$/^M/' $$X \
10366 > tmp.dir/$$X ; done
10367 cd tmp.dir ; zoo aM ../tar.zoo *
10376 @node Concept Index, Name Index, GNU Free Documentation License, Top
10377 @unnumbered Index of Concepts
10381 @node Name Index, , Concept Index, Top
10382 @unnumbered Index of Functions, Variables, & Directives