Don't warn when alignment of global common data exceeds maximum alignment.

[official-gcc.git] / gcc / go / gccgo.texi

\input texinfo @c -*-texinfo-*-
@setfilename gccgo.info
@settitle The GNU Go Compiler

@c Merge the standard indexes into a single one.
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex tp cp

@include gcc-common.texi

@c Copyright years for this manual.
@set copyrights-go 2010-2021

@copying
@c man begin COPYRIGHT
Copyright @copyright{} @value{copyrights-go} Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, the Front-Cover Texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below).
A copy of the license is included in the
@c man end
section entitled ``GNU Free Documentation License''.
@ignore
@c man begin COPYRIGHT
man page gfdl(7).
@c man end
@end ignore

@c man begin COPYRIGHT

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
@c man end
@end copying

@ifinfo
@format
@dircategory Software development
@direntry
* Gccgo: (gccgo).           A GCC-based compiler for the Go language
@end direntry
@end format

@insertcopying
@end ifinfo

@titlepage
@title The GNU Go Compiler
@versionsubtitle
@author Ian Lance Taylor

@page
@vskip 0pt plus 1filll
Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor@*
Boston, MA 02110-1301, USA@*
@sp 1
@insertcopying
@end titlepage
@contents
@page

@node Top
@top Introduction

This manual describes how to use @command{gccgo}, the GNU compiler for
the Go programming language.  This manual is specifically about
@command{gccgo}.  For more information about the Go programming
language in general, including language specifications and standard
package documentation, see @uref{http://golang.org/}.

@menu
* Copying::                     The GNU General Public License.
* GNU Free Documentation License::
                                How you can share and copy this manual.
* Invoking gccgo::              How to run gccgo.
* Import and Export::           Importing and exporting package data.
* Compiler Directives::         Comments to control compilation.
* C Interoperability::          Calling C from Go and vice-versa.
* Index::                       Index.
@end menu


@include gpl_v3.texi

@include fdl.texi


@node Invoking gccgo
@chapter Invoking gccgo

@c man title gccgo A GCC-based compiler for the Go language

@ignore
@c man begin SYNOPSIS gccgo
gccgo [@option{-c}|@option{-S}]
      [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
      [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
      [@option{-o} @var{outfile}] @var{infile}@dots{}

Only the most useful options are listed here; see below for the
remainder.
@c man end
@c man begin SEEALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1)
and the Info entries for @file{gccgo} and @file{gcc}.
@c man end
@end ignore

@c man begin DESCRIPTION gccgo

The @command{gccgo} command is a frontend to @command{gcc} and
supports many of the same options.  @xref{Option Summary, , Option
Summary, gcc, Using the GNU Compiler Collection (GCC)}.  This manual
only documents the options specific to @command{gccgo}.

The @command{gccgo} command may be used to compile Go source code into
an object file, link a collection of object files together, or do both
in sequence.

Go source code is compiled as packages.  A package consists of one or
more Go source files.  All the files in a single package must be
compiled together, by passing all the files as arguments to
@command{gccgo}.  A single invocation of @command{gccgo} may only
compile a single package.

One Go package may @code{import} a different Go package.  The imported
package must have already been compiled; @command{gccgo} will read
the import data directly from the compiled package.  When this package
is later linked, the compiled form of the package must be included in
the link command.

Go programs must generally be compiled with debugging information, and
@option{-g1} is the default as described below.  Stripping a Go
program will generally cause it to misbehave or fail.

@c man end

@c man begin OPTIONS gccgo

@table @gcctabopt
@item -I@var{dir}
@cindex @option{-I}
Specify a directory to use when searching for an import package at
compile time.

@item -L@var{dir}
@cindex @option{-L}
When linking, specify a library search directory, as with
@command{gcc}.

@item -fgo-pkgpath=@var{string}
@cindex @option{-fgo-pkgpath}
Set the package path to use.  This sets the value returned by the
PkgPath method of reflect.Type objects.  It is also used for the names
of globally visible symbols.  The argument to this option should
normally be the string that will be used to import this package after
it has been installed; in other words, a pathname within the
directories specified by the @option{-I} option.

@item -fgo-prefix=@var{string}
@cindex @option{-fgo-prefix}
An alternative to @option{-fgo-pkgpath}.  The argument will be
combined with the package name from the source file to produce the
package path.  If @option{-fgo-pkgpath} is used, @option{-fgo-prefix}
will be ignored.

Go permits a single program to include more than one package with the
same name in the @code{package} clause in the source file, though
obviously the two packages must be imported using different pathnames.
In order for this to work with @command{gccgo}, either
@option{-fgo-pkgpath} or @option{-fgo-prefix} must be specified when
compiling a package.

Using either @option{-fgo-pkgpath} or @option{-fgo-prefix} disables
the special treatment of the @code{main} package and permits that
package to be imported like any other.

@item -fgo-relative-import-path=@var{dir}
@cindex @option{-fgo-relative-import-path}
A relative import is an import that starts with @file{./} or
@file{../}.  If this option is used, @command{gccgo} will use
@var{dir} as a prefix for the relative import when searching for it.

@item -frequire-return-statement
@itemx -fno-require-return-statement
@cindex @option{-frequire-return-statement}
@cindex @option{-fno-require-return-statement}
By default @command{gccgo} will warn about functions which have one or
more return parameters but lack an explicit @code{return} statement.
This warning may be disabled using
@option{-fno-require-return-statement}.

@item -fgo-check-divide-zero
@cindex @option{-fgo-check-divide-zero}
@cindex @option{-fno-go-check-divide-zero}
Add explicit checks for division by zero.  In Go a division (or
modulos) by zero causes a panic.  On Unix systems this is detected in
the runtime by catching the @code{SIGFPE} signal.  Some processors,
such as PowerPC, do not generate a SIGFPE on division by zero.  Some
runtimes do not generate a signal that can be caught.  On those
systems, this option may be used.  Or the checks may be removed via
@option{-fno-go-check-divide-zero}.  This option is currently on by
default, but in the future may be off by default on systems that do
not require it.

@item -fgo-check-divide-overflow
@cindex @option{-fgo-check-divide-overflow}
@cindex @option{-fno-go-check-divide-overflow}
Add explicit checks for division overflow.  For example, division
overflow occurs when computing @code{INT_MIN / -1}.  In Go this should
be wrapped, to produce @code{INT_MIN}.  Some processors, such as x86,
generate a trap on division overflow.  On those systems, this option
may be used.  Or the checks may be removed via
@option{-fno-go-check-divide-overflow}.  This option is currently on
by default, but in the future may be off by default on systems that do
not require it.

@item -fno-go-optimize-allocs
@cindex @option{-fno-go-optimize-allocs}
Disable escape analysis, which tries to allocate objects on the stack
rather than the heap.

@item -fgo-debug-escape@var{n}
@cindex @option{-fgo-debug-escape}
Output escape analysis debugging information.  Larger values of
@var{n} generate more information.

@item -fgo-debug-escape-hash=@var{n}
@cindex @option{-fgo-debug-escape-hash}
A hash value to debug escape analysis.  @var{n} is a binary string.
This runs escape analysis only on functions whose names hash to values
that match the given suffix @var{n}.  This can be used to binary
search across functions to uncover escape analysis bugs.

@item -fgo-debug-optimization
@cindex @option{-fgo-debug-optimization}
@cindex @option{-fno-go-debug-optimization}
Output optimization diagnostics.

@item -fgo-c-header=@var{file}
@cindex @option{-fgo-c-header}
Write top-level named Go struct definitions to @var{file} as C code.
This is used when compiling the runtime package.

@item -fgo-compiling-runtime
@cindex @option{-fgo-compiling-runtime}
Apply special rules for compiling the runtime package.  Implicit
memory allocation is forbidden.  Some additional compiler directives
are supported.

@item -fgo-embedcfg=@var{file}
@cindex @option{-fgo-embedcfg}
Identify a JSON file used to map patterns used with special
@code{//go:embed} comments to the files named by the patterns.  The
JSON file should have two components: @code{Patterns} maps each
pattern to a list of file names, and @code{Files} maps each file name
to a full path to the file.  This option is intended for use by the
@command{go} command to implement @code{//go:embed}.

@item -g
@cindex @option{-g for gccgo}
This is the standard @command{gcc} option (@pxref{Debugging Options, ,
Debugging Options, gcc, Using the GNU Compiler Collection (GCC)}).  It
is mentioned here because by default @command{gccgo} turns on
debugging information generation with the equivalent of the standard
option @option{-g1}.  This is because Go programs require debugging
information to be available in order to get backtrace information.  An
explicit @option{-g0} may be used to disable the generation of
debugging information, in which case certain standard library
functions, such as @code{runtime.Callers}, will not operate correctly.
@end table

@c man end

@node Import and Export
@chapter Import and Export

When @command{gccgo} compiles a package which exports anything, the
export information will be stored directly in the object file.  When a
package is imported, @command{gccgo} must be able to find the file.

@cindex @file{.gox}
When Go code imports the package @file{@var{gopackage}}, @command{gccgo}
will look for the import data using the following filenames, using the
first one that it finds.

@table @file
@item @var{gopackage}.gox
@item lib@var{gopackage}.so
@item lib@var{gopackage}.a
@item @var{gopackage}.o
@end table

The compiler will search for these files in the directories named by
any @option{-I} options, in order in which the directories appear on
the command line.  The compiler will then search several standard
system directories.  Finally the compiler will search the current
directory (to search the current directory earlier, use @samp{-I.}).

The compiler will extract the export information directly from the
compiled object file.  The file @file{@var{gopackage}.gox} will
typically contain nothing but export data.  This can be generated from
@file{@var{gopackage}.o} via

@smallexample
objcopy -j .go_export @var{gopackage}.o @var{gopackage}.gox
@end smallexample

For example, it may be desirable to extract the export information
from several different packages into their independent
@file{@var{gopackage}.gox} files, and then to combine the different
package object files together into a single shared library or archive.

At link time you must explicitly tell @command{gccgo} which files to
link together into the executable, as is usual with @command{gcc}.
This is different from the behavior of other Go compilers.

@node Compiler Directives
@chapter Compiler Directives

The Go compiler supports a few compiler directives.  A compiler
directive uses a @code{//} comment at the start of a line.  There must
be no space between the @code{//} and the name of the directive.

@table @code
@item //line @var{file}:@var{line}
The @code{//line} directive specifies that the source line that
follows should be recorded as having come from the given file path and
line number.  Successive lines are recorded using increasing line
numbers, until the next directive.  This directive typically appears
in machine-generated code, so that compilers and debuggers will show
lines in the original input to the generator.

@item //extern @var{extern_name}
The @code{extern} directive sets the externally visible name of the
next function declaration.  See @ref{Function Names}.

@item //go:compile @var{go_name} @var{extern_name}
The @code{go:compile} directives sets the externally visible name of a
function definition or declaration.  See @ref{Function Names}.

@item //go:noescape
The @code{//go:noescape} directive specifies that the next declaration
in the file, which must be a func without a body (meaning that it has
an implementation not written in Go) does not allow any of the
pointers passed as arguments to escape into the heap or into the
values returned from the function. This information can be used during
the compiler's escape analysis of Go code calling the function.

@item //go:nosplit
The @code{//go:nosplit} directive specifies that the next function
declared in the file must not include a stack overflow check. This is
most commonly used by low-level runtime sources invoked at times when
it is unsafe for the calling goroutine to be preempted.

@item //go:noinline
The @code{//go:noinline} directive specifies that the next function
defined in the file may not be inlined.

@end table

@node C Interoperability
@chapter C Interoperability

When using @command{gccgo} there is limited interoperability with C,
or with C++ code compiled using @code{extern "C"}.

This information is provided largely for documentation purposes.  For
ordinary use it is best to build programs with the go tool and then
use @code{import "C"}, as described at
@url{http://golang.org/cmd/cgo}.

@menu
* C Type Interoperability::     How C and Go types match up.
* Function Names::              How Go functions are named.
@end menu

@node C Type Interoperability
@section C Type Interoperability

Basic types map directly: an @code{int} in Go is an @code{int} in C,
etc.  Go @code{byte} is equivalent to C @code{unsigned char}.
Pointers in Go are pointers in C.  A Go @code{struct} is the same as C
@code{struct} with the same field names and types.

@cindex @code{string} in C
The Go @code{string} type is currently defined as a two-element
structure:

@smallexample
struct __go_string @{
  const unsigned char *__data;
  int __length;
@};
@end smallexample

You can't pass arrays between C and Go.  However, a pointer to an
array in Go is equivalent to a C pointer to the equivalent of the
element type.  For example, Go @code{*[10]int} is equivalent to C
@code{int*}, assuming that the C pointer does point to 10 elements.

@cindex @code{slice} in C
A slice in Go is a structure.  The current definition is:

@smallexample
struct __go_slice @{
  void *__values;
  int __count;
  int __capacity;
@};
@end smallexample

The type of a Go function with no receiver is equivalent to a C
function whose parameter types are equivalent.  When a Go function
returns more than one value, the C function returns a struct.  For
example, these functions have equivalent types:

@smallexample
func GoFunction(int) (int, float)
struct @{ int i; float f; @} CFunction(int)
@end smallexample

A pointer to a Go function is equivalent to a pointer to a C function
when the functions have equivalent types.

Go @code{interface}, @code{channel}, and @code{map} types have no
corresponding C type (@code{interface} is a two-element struct and
@code{channel} and @code{map} are pointers to structs in C, but the
structs are deliberately undocumented).  C @code{enum} types
correspond to some integer type, but precisely which one is difficult
to predict in general; use a cast.  C @code{union} types have no
corresponding Go type.  C @code{struct} types containing bitfields
have no corresponding Go type.  C++ @code{class} types have no
corresponding Go type.

Memory allocation is completely different between C and Go, as Go uses
garbage collection.  The exact guidelines in this area are
undetermined, but it is likely that it will be permitted to pass a
pointer to allocated memory from C to Go.  The responsibility of
eventually freeing the pointer will remain with C side, and of course
if the C side frees the pointer while the Go side still has a copy the
program will fail.  When passing a pointer from Go to C, the Go
function must retain a visible copy of it in some Go variable.
Otherwise the Go garbage collector may delete the pointer while the C
function is still using it.

@node Function Names
@section Function Names

@cindex @code{extern}
@cindex external names
Go code can call C functions directly using the @code{//extern} or
@code{//go:linkname} compiler directives.  An @code{//extern}
directive must be at the beginning of the line and must start with
@code{//extern}.  This must be followed by a space and then the
external name of the function.  The function declaration must be on
the line immediately after the comment.  For example, here is how the
C function @code{open} can be declared in Go:

@smallexample
//extern open
func c_open(name *byte, mode int, perm int) int
@end smallexample

You can do the same thing using the @code{//go:linkname} compiler
directive.  The @code{//go:linkname} directive must be at the start of
the line.  It is followed by whitespace, the name of the Go function,
more whitespace, and the external name of the function.  Unlike
@code{//extern}, @code{//go:linkname} does not need to appear
immediately adjacent to the function definition or declaration.

@smallexample
//go:linkname c_open open
func c_open(name *byte, mode int, perm int) int
@end smallexample

The C function naturally expects a nul terminated string, which in Go
is equivalent to a pointer to an array (not a slice!) of @code{byte}
with a terminating zero byte.  So a sample call from Go would look
like (after importing the @code{os} package):

@smallexample
var name = [4]byte@{'f', 'o', 'o', 0@};
i := c_open(&name[0], os.O_RDONLY, 0);
@end smallexample

Note that this serves as an example only.  To open a file in Go please
use Go's @code{os.Open} function instead.

The name of Go functions accessed from C is subject to change.  At
present the name of a Go function that does not have a receiver is
@code{pkgpath.Functionname}.  The @var{pkgpath} is set by the
@option{-fgo-pkgpath} option used when the package is compiled; if the
option is not used, the default is @code{go.@var{packagename}}.  To
call the function from C you must set the name using the @command{gcc}
@code{__asm__} extension.

@smallexample
extern int go_function(int) __asm__ ("mypkgpath.Function");
@end smallexample

@node Index
@unnumbered Index

@printindex cp

@bye