doc/gccgo_install.html - go - Git at Google

 <!--{
 	"Title": "Setting up and using gccgo",
 	"Path": "/install/gccgo/"
 }-->

 <p>
 This document explains how to use <code>gccgo</code>, a compiler for
 the Go language.  The <code>gccgo</code> compiler is a new frontend
 for <code>gcc</code>, the widely used GNU compiler.  Although the
 frontend itself is under a BSD-style license, <code>gccgo</code> is
 normally used as part of <code>gcc</code> and is then covered by
 the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public
 License</a>.
 </p>

 <p>
 Note that <code>gccgo</code> is not the <code>6g</code> compiler; see
 the <a href="install.html">Installing Go</a> instructions for that
 compiler.
 </p>

 <h2 id="Source_code">Source code</h2>

 <p>
 The <code>gccgo</code> source code is accessible via Subversion.  The
 <code>gcc</code> web site
 has <a href="http://gcc.gnu.org/svn.html">instructions for getting the
 <code>gcc</code> source code</a>.  The <code>gccgo</code> source code
 is a branch of the main <code>gcc</code> code
 repository: <code>svn://gcc.gnu.org/svn/gcc/branches/gccgo</code>.
 </p>

 <p>
 Note that although <code>gcc.gnu.org</code> is the most convenient way
 to get the source code for the compiler, that is not where the master
 sources live.  If you want to contribute changes to the gccgo
 compiler, see <a href="gccgo_contribute.html">Contributing to
 gccgo</a>.
 </p>


 <h2 id="Building">Building</h2>

 <p>
 Building <code>gccgo</code> is just like building <code>gcc</code>
 with one or two additional options.  See
 the <a href="http://gcc.gnu.org/install/">instructions on the gcc web
 site</a>.  When you run <code>configure</code>, add the
 option <code>--enable-languages=c,c++,go</code> (along with other
 languages you may want to build).  If you are targeting a 32-bit x86,
 then you will want to build <code>gccgo</code> to default to
 supporting locked compare and exchange instructions; do this by also
 using the <code>configure</code> option <code>--with-arch=i586</code>
 (or a newer architecture, depending on where you need your programs to
 run).
 </p>

 <p>
 On x86 GNU/Linux systems the <code>gccgo</code> compiler is able to
 use a small discontiguous stack for goroutines.  This permits programs
 to run many more goroutines, since each goroutine can use a relatively
 small stack.  Doing this requires using a development version of
 the <code>gold</code> linker.  The easiest way to do this is to build
 the GNU binutils, using <code>--enable-gold=default</code> when you run
 the <code>configure</code> script, and to
 use <code>--with-ld=GOLD_BINARY</code> when you
 configure <code>gccgo</code>.  A typical sequence would look like
 this (you can replace <code>/opt/gold</code> with any directory to
 which you have write access):
 </p>

 <pre>
 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
 [password is "anoncvs"]
 cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
 mkdir binutils-objdir
 cd binutils-objdir
 ../src/configure --enable-gold=default --prefix=/opt/gold
 make
 make install
 </pre>

 <p>
 A number of prerequisites are required to build <code>gcc</code>, as
 described on the <a href="http://gcc.gnu.org/">gcc web site</a>.  If
 those are all available, then a typical build and install sequence
 would look like this (only use the <code>--with-ld</code> option if
 you built and installed the gold linker as described above):
 </p>

 <pre>
 svn checkout svn://gcc.gnu.org/svn/gcc/branches/gccgo gccgo
 mkdir objdir
 cd objdir
 ../gccgo/configure --enable-languages=c,c++,go --with-ld=/opt/gold/bin/ld
 make
 make install
 </pre>

 <h2 id="Using_gccgo">Using gccgo</h2>

 <p>
 The <code>gccgo</code> compiler works like other gcc frontends.

 <p>
 To compile a file:

 <pre>
 gccgo -c file.go
 </pre>

 <p>
 That produces <code>file.o</code>. To link files together to form an
 executable:

 <pre>
 gccgo -o file file.o
 </pre>

 <p>
 To run the resulting file, you will need to tell the program where to
 find the compiled Go packages. This can be done either by setting
 <code>LD_LIBRARY_PATH</code> in your environment:

 <pre>
 LD_LIBRARY_PATH=/usr/lib/gcc/MACHINE/VERSION
 </pre>

 <p>
 or by passing a <code>-Wl,-R</code> option when you link:

 <pre>
 gccgo -o file file.o -Wl,-R,/usr/lib/gcc/MACHINE/VERSION
 </pre>

 <p>
 or you can use the <code>-static-libgo</code> link-time option to link
 statically against libgo, or you can do a fully static link (static
 linking is the default for the <code>6l</code> Go linker).  On most
 systems, a static link will look something like:

 <pre>
 gccgo -o file file.o -static -L /usr/lib/nptl -lgobegin -lgo -lpthread
 </pre>

 <p>
 You may get a warning about not creating an <code>.eh_frame_hdr</code>
 section; this has nothing to do with Go, and may be ignored. In the
 future the requirement of explicitly specifying
 <code>-L /usr/lib/nptl -lgobegin -lgo -lpthread</code>
 may be removed.


 <h2 id="Options">Options</h2>

 <p>
 The <code>gccgo</code> compiler supports all <code>gcc</code> options
 that are language independent, notably the <code>-O</code>
 and <code>-g</code> options.

 <p>
 The <code>-fgo-prefix=PREFIX</code> option may be used to set a unique
 prefix for the package being compiled.  This option is intended for
 use with large programs that contain many packages, in order to allow
 multiple packages to use the same identifier as the package name.
 The <code>PREFIX</code> may be any string; a good choice for the
 string is the directory where the package will be installed.

 <p>
 The <code>-fno-require-return-statement</code> option may be used to
 disable the compiler error about functions missing return statements.
 Note that there is no way to disable this error in <code>6g</code>.

 <p>
 The <code>-I</code> and <code>-L</code> options, which are synonyms
 for the compiler, may be used to set the search path for finding
 imports.


 <h2 id="Imports">Imports</h2>

 <p>
 When you compile a file which exports something, the export
 information will be stored directly in the object file. When
 you import a package, you must tell <code>gccgo</code> how to
 find the file.

 <p>
 When you import the package <var>FILE</var> with <code>gccgo</code>,
 it will look for the import data in the following files, and use the
 first one that it finds.

 <ul>
 <li><code><var>FILE</var>.gox</code>
 <li><code><var>FILE</var>.o</code>
 <li><code>lib<var>FILE</var>.so</code>
 <li><code>lib<var>FILE</var>.a</code>
 </ul>

 <p>
 <code><var>FILE</var>.gox</code>, when used, will typically contain
 nothing but export data. This can be generated from
 <code><var>FILE</var>.o</code> via

 <pre>
 objcopy -j .go_export FILE.o FILE.gox
 </pre>

 <p>
 The <code>gccgo</code> compiler will look in the current
 directory for import files. In more complex scenarios you
 may pass the <code>-I</code> or <code>-L</code> option to
 <code>gccgo</code>. Both options take directories to search. The
 <code>-L</code> option is also passed to the linker.

 The <code>gccgo</code> compiler does not currently (2009-11-06) record
 the file name of imported packages in the object file. You must
 arrange for the imported data to be linked into the program.

 <pre>
 gccgo -c mypackage.go              # Exports mypackage
 gccgo -c main.go                   # Imports mypackage
 gccgo -o main main.o mypackage.o   # Explicitly links with mypackage.o
 </pre>

 <h2 id="Unimplemented">Unimplemented</h2>

 <p>
 Some Go features are not yet implemented in <code>gccgo</code>.  As of
 2010-08-23, the following are not implemented:

 <ul>
 <li>goroutines are implemented as NPTL threads.  If you can not use
     the gold linker as described above, they are created with a fixed
     stack size, and the number of goroutines that may be created at
     one time is limited.
 </ul>

 <h2 id="Debugging">Debugging</h2>

 <p>
 If you use the <code>-g</code> option when you compile, you can run
 <code>gdb</code> on your executable.  The debugger doesn't (yet)
 know anything about Go. However, you can set breakpoints, single-step,
 etc.  You can print variables, but they will be printed as though they
 had C/C++ types. For numeric types this doesn't matter. Go strings
 will show up as pointers to structures; to see the value
 <code>print *stringvar</code>. In general Go strings, maps, channels
 and interfaces are always represented as C pointers.

 <h2 id="C_Interoperability">C Interoperability</h2>

 <p>
 When using <code>gccgo</code> there is limited interoperability with C,
 or with C++ code compiled using <code>extern "C"</code>.

 <h3 id="Types">Types</h3>

 <p>
 Basic types map directly: an <code>int</code> in Go is an <code>int</code>
 in C, etc. Go <code>byte</code> is equivalent to C <code>unsigned char</code>.
 Pointers in Go are pointers in C. A Go <code>struct</code> is the same as C
 <code>struct</code> with the same fields and types.

 <p>
 The Go <code>string</code> type is currently defined as a two-element
 structure (this is <b style="color: red;">subject to change</b>):

 <pre>
 struct __go_string {
   const unsigned char *__data;
   int __length;
 };
 </pre>

 <p>
 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</code> is equivalent to C <code>int*</code>,
 assuming that the C pointer does point to 10 elements.

 <p>
 A slice in Go is a structure.  The current definition is
 (this is <b style="color: red;">subject to change</b>):

 <pre>
 struct __go_slice {
   void *__values;
   int __count;
   int __capacity;
 };
 </pre>

 <p>
 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:

 <pre>
 func GoFunction(int) (int, float64)
 struct { int i; float64 f; } CFunction(int)
 </pre>

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

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

 <p>
 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.

 <h3 id="Function_names">Function names</h3>

 <p>
 Go code can call C functions directly using a Go extension implemented
 in <code>gccgo</code>: a function declaration may be followed by
 <code>__asm__("NAME")</code>. For example, here is how the C function
 <code>open</code> can be declared in Go:

 <pre>
 func c_open(name *byte, mode int, perm int) int __asm__ ("open");
 </pre>

 <p>
 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</code> with a terminating zero byte. So a sample call
 from Go would look like (after importing the <code>os</code> package):

 <pre>
 var name = [4]byte{'f', 'o', 'o', 0};
 i := c_open(&amp;name[0], os.O_RDONLY, 0);
 </pre>

 <p>
 (this serves as an example only, to open a file in Go please use Go's
 <code>os.Open</code> function instead).

 <p>
 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>prefix.package.Functionname</code>. The prefix is set by
 the <code>-fgo-prefix</code> option used when the package is compiled;
 if the option is not used, the default is <code>go</code>.
 To call the function from C you must set the name using
 a <code>gcc</code> extension similar to the <code>gccgo</code>
 extension.

 <pre>
 extern int go_function(int) __asm__ ("myprefix.mypackage.Function");
 </pre>

 <h3 id="Automatic_generation_of_Go_declarations_from_C_source_code">
 Automatic generation of Go declarations from C source code</h3>

 <p>
 The Go version of <code>gcc</code> supports automatically generating
 Go declarations from C code. The facility is rather awkward at present,
 and a better mechanism is under development.

 <p>
 Compile your C code as usual, but replace <code>-c</code> with
 <code>-S&nbsp;-ggo</code>. The result will be an assembler file
 with a <code>.s</code> extension. This assembler file will contain
 comments beginning with #GO. Those comments are declarations in the Go
 language for the C types, variables and functions declared in the C code.
 C types which can not be represented in Go will contain the string INVALID.
 Unsupported macro definitions will be recorded as <code>unknowndefine</code>,
 and uses of <code>#undef</code> will be recorded as <code>undef</code>.
 So it is very approximately possible to get Go code by running

 <pre>
 gcc -S -ggo foo.c
 grep '#GO' foo.s | grep -v INVALID | grep -v unknowndefine | grep -v undef > foo.go
 </pre>

 <p>
 This procedure is full of unstated caveats and restrictions and we make no
 guarantee that it will not change in the future. It is more useful as a
 starting point for real Go code than as a regular procedure.

 <h2 id="RTEMS_Port">RTEMS Port</h2>
 <p>
 The <code>gccgo</code> compiler has been ported to <a href="http://www.rtems.com/">
 <code>RTEMS</code></a>. <code>RTEMS</code> is a real-time executive
 that provides a high performance environment for embedded applications
 on a range of processors and embedded hardware. The current <code>gccgo</code>
 port is for x86. The goal is to extend the port to most of the
 <a href="http://www.rtems.org/wiki/index.php/SupportedCPUs">
 architectures supported by <code>RTEMS</code></a>. For more information on the port,
 as well as instructions on how to install it, please see this
 <a href="http://www.rtems.com/wiki/index.php/GCCGoRTEMS"><code>RTEMS</code> Wiki page</a>.
	<!--{
	"Title": "Setting up and using gccgo",
	"Path": "/install/gccgo/"
	}-->

	<p>
	This document explains how to use <code>gccgo</code>, a compiler for
	the Go language. The <code>gccgo</code> compiler is a new frontend
	for <code>gcc</code>, the widely used GNU compiler. Although the
	frontend itself is under a BSD-style license, <code>gccgo</code> is
	normally used as part of <code>gcc</code> and is then covered by
	the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public
	License</a>.
	</p>

	<p>
	Note that <code>gccgo</code> is not the <code>6g</code> compiler; see
	the <a href="install.html">Installing Go</a> instructions for that
	compiler.
	</p>

	<h2 id="Source_code">Source code</h2>

	<p>
	The <code>gccgo</code> source code is accessible via Subversion. The
	<code>gcc</code> web site
	has <a href="http://gcc.gnu.org/svn.html">instructions for getting the
	<code>gcc</code> source code</a>. The <code>gccgo</code> source code
	is a branch of the main <code>gcc</code> code
	repository: <code>svn://gcc.gnu.org/svn/gcc/branches/gccgo</code>.
	</p>

	<p>
	Note that although <code>gcc.gnu.org</code> is the most convenient way
	to get the source code for the compiler, that is not where the master
	sources live. If you want to contribute changes to the gccgo
	compiler, see <a href="gccgo_contribute.html">Contributing to
	gccgo</a>.
	</p>


	<h2 id="Building">Building</h2>

	<p>
	Building <code>gccgo</code> is just like building <code>gcc</code>
	with one or two additional options. See
	the <a href="http://gcc.gnu.org/install/">instructions on the gcc web
	site</a>. When you run <code>configure</code>, add the
	option <code>--enable-languages=c,c++,go</code> (along with other
	languages you may want to build). If you are targeting a 32-bit x86,
	then you will want to build <code>gccgo</code> to default to
	supporting locked compare and exchange instructions; do this by also
	using the <code>configure</code> option <code>--with-arch=i586</code>
	(or a newer architecture, depending on where you need your programs to
	run).
	</p>

	<p>
	On x86 GNU/Linux systems the <code>gccgo</code> compiler is able to
	use a small discontiguous stack for goroutines. This permits programs
	to run many more goroutines, since each goroutine can use a relatively
	small stack. Doing this requires using a development version of
	the <code>gold</code> linker. The easiest way to do this is to build
	the GNU binutils, using <code>--enable-gold=default</code> when you run
	the <code>configure</code> script, and to
	use <code>--with-ld=GOLD_BINARY</code> when you
	configure <code>gccgo</code>. A typical sequence would look like
	this (you can replace <code>/opt/gold</code> with any directory to
	which you have write access):
	</p>

	<pre>
	cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
	[password is "anoncvs"]
	cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
	mkdir binutils-objdir
	cd binutils-objdir
	../src/configure --enable-gold=default --prefix=/opt/gold
	make
	make install
	</pre>

	<p>
	A number of prerequisites are required to build <code>gcc</code>, as
	described on the <a href="http://gcc.gnu.org/">gcc web site</a>. If
	those are all available, then a typical build and install sequence
	would look like this (only use the <code>--with-ld</code> option if
	you built and installed the gold linker as described above):
	</p>

	<pre>
	svn checkout svn://gcc.gnu.org/svn/gcc/branches/gccgo gccgo
	mkdir objdir
	cd objdir
	../gccgo/configure --enable-languages=c,c++,go --with-ld=/opt/gold/bin/ld
	make
	make install
	</pre>

	<h2 id="Using_gccgo">Using gccgo</h2>

	<p>
	The <code>gccgo</code> compiler works like other gcc frontends.

	<p>
	To compile a file:

	<pre>
	gccgo -c file.go
	</pre>

	<p>
	That produces <code>file.o</code>. To link files together to form an
	executable:

	<pre>
	gccgo -o file file.o
	</pre>

	<p>
	To run the resulting file, you will need to tell the program where to
	find the compiled Go packages. This can be done either by setting
	<code>LD_LIBRARY_PATH</code> in your environment:

	<pre>
	LD_LIBRARY_PATH=/usr/lib/gcc/MACHINE/VERSION
	</pre>

	<p>
	or by passing a <code>-Wl,-R</code> option when you link:

	<pre>
	gccgo -o file file.o -Wl,-R,/usr/lib/gcc/MACHINE/VERSION
	</pre>

	<p>
	or you can use the <code>-static-libgo</code> link-time option to link
	statically against libgo, or you can do a fully static link (static
	linking is the default for the <code>6l</code> Go linker). On most
	systems, a static link will look something like:

	<pre>
	gccgo -o file file.o -static -L /usr/lib/nptl -lgobegin -lgo -lpthread
	</pre>

	<p>
	You may get a warning about not creating an <code>.eh_frame_hdr</code>
	section; this has nothing to do with Go, and may be ignored. In the
	future the requirement of explicitly specifying
	<code>-L /usr/lib/nptl -lgobegin -lgo -lpthread</code>
	may be removed.


	<h2 id="Options">Options</h2>

	<p>
	The <code>gccgo</code> compiler supports all <code>gcc</code> options
	that are language independent, notably the <code>-O</code>
	and <code>-g</code> options.

	<p>
	The <code>-fgo-prefix=PREFIX</code> option may be used to set a unique
	prefix for the package being compiled. This option is intended for
	use with large programs that contain many packages, in order to allow
	multiple packages to use the same identifier as the package name.
	The <code>PREFIX</code> may be any string; a good choice for the
	string is the directory where the package will be installed.

	<p>
	The <code>-fno-require-return-statement</code> option may be used to
	disable the compiler error about functions missing return statements.
	Note that there is no way to disable this error in <code>6g</code>.

	<p>
	The <code>-I</code> and <code>-L</code> options, which are synonyms
	for the compiler, may be used to set the search path for finding
	imports.


	<h2 id="Imports">Imports</h2>

	<p>
	When you compile a file which exports something, the export
	information will be stored directly in the object file. When
	you import a package, you must tell <code>gccgo</code> how to
	find the file.

	<p>
	When you import the package <var>FILE</var> with <code>gccgo</code>,
	it will look for the import data in the following files, and use the
	first one that it finds.

	<ul>
	<li><code><var>FILE</var>.gox</code>
	<li><code><var>FILE</var>.o</code>
	<li><code>lib<var>FILE</var>.so</code>
	<li><code>lib<var>FILE</var>.a</code>
	</ul>

	<p>
	<code><var>FILE</var>.gox</code>, when used, will typically contain
	nothing but export data. This can be generated from
	<code><var>FILE</var>.o</code> via

	<pre>
	objcopy -j .go_export FILE.o FILE.gox
	</pre>

	<p>
	The <code>gccgo</code> compiler will look in the current
	directory for import files. In more complex scenarios you
	may pass the <code>-I</code> or <code>-L</code> option to
	<code>gccgo</code>. Both options take directories to search. The
	<code>-L</code> option is also passed to the linker.

	The <code>gccgo</code> compiler does not currently (2009-11-06) record
	the file name of imported packages in the object file. You must
	arrange for the imported data to be linked into the program.

	<pre>
	gccgo -c mypackage.go # Exports mypackage
	gccgo -c main.go # Imports mypackage
	gccgo -o main main.o mypackage.o # Explicitly links with mypackage.o
	</pre>

	<h2 id="Unimplemented">Unimplemented</h2>

	<p>
	Some Go features are not yet implemented in <code>gccgo</code>. As of
	2010-08-23, the following are not implemented:

	<ul>
	<li>goroutines are implemented as NPTL threads. If you can not use
	the gold linker as described above, they are created with a fixed
	stack size, and the number of goroutines that may be created at
	one time is limited.
	</ul>

	<h2 id="Debugging">Debugging</h2>

	<p>
	If you use the <code>-g</code> option when you compile, you can run
	<code>gdb</code> on your executable. The debugger doesn't (yet)
	know anything about Go. However, you can set breakpoints, single-step,
	etc. You can print variables, but they will be printed as though they
	had C/C++ types. For numeric types this doesn't matter. Go strings
	will show up as pointers to structures; to see the value
	<code>print *stringvar</code>. In general Go strings, maps, channels
	and interfaces are always represented as C pointers.

	<h2 id="C_Interoperability">C Interoperability</h2>

	<p>
	When using <code>gccgo</code> there is limited interoperability with C,
	or with C++ code compiled using <code>extern "C"</code>.

	<h3 id="Types">Types</h3>

	<p>
	Basic types map directly: an <code>int</code> in Go is an <code>int</code>
	in C, etc. Go <code>byte</code> is equivalent to C <code>unsigned char</code>.
	Pointers in Go are pointers in C. A Go <code>struct</code> is the same as C
	<code>struct</code> with the same fields and types.

	<p>
	The Go <code>string</code> type is currently defined as a two-element
	structure (this is <b style="color: red;">subject to change</b>):

	<pre>
	struct __go_string {
	const unsigned char *__data;
	int __length;
	};
	</pre>

	<p>
	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</code> is equivalent to C <code>int</code>,
	assuming that the C pointer does point to 10 elements.

	<p>
	A slice in Go is a structure. The current definition is
	(this is <b style="color: red;">subject to change</b>):

	<pre>
	struct __go_slice {
	void *__values;
	int __count;
	int __capacity;
	};
	</pre>

	<p>
	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:

	<pre>
	func GoFunction(int) (int, float64)
	struct { int i; float64 f; } CFunction(int)
	</pre>

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

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

	<p>
	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.

	<h3 id="Function_names">Function names</h3>

	<p>
	Go code can call C functions directly using a Go extension implemented
	in <code>gccgo</code>: a function declaration may be followed by
	<code>__asm__("NAME")</code>. For example, here is how the C function
	<code>open</code> can be declared in Go:

	<pre>
	func c_open(name *byte, mode int, perm int) int __asm__ ("open");
	</pre>

	<p>
	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</code> with a terminating zero byte. So a sample call
	from Go would look like (after importing the <code>os</code> package):

	<pre>
	var name = [4]byte{'f', 'o', 'o', 0};
	i := c_open(&name[0], os.O_RDONLY, 0);
	</pre>

	<p>
	(this serves as an example only, to open a file in Go please use Go's
	<code>os.Open</code> function instead).

	<p>
	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>prefix.package.Functionname</code>. The prefix is set by
	the <code>-fgo-prefix</code> option used when the package is compiled;
	if the option is not used, the default is <code>go</code>.
	To call the function from C you must set the name using
	a <code>gcc</code> extension similar to the <code>gccgo</code>
	extension.

	<pre>
	extern int go_function(int) __asm__ ("myprefix.mypackage.Function");
	</pre>

	<h3 id="Automatic_generation_of_Go_declarations_from_C_source_code">
	Automatic generation of Go declarations from C source code</h3>

	<p>
	The Go version of <code>gcc</code> supports automatically generating
	Go declarations from C code. The facility is rather awkward at present,
	and a better mechanism is under development.

	<p>
	Compile your C code as usual, but replace <code>-c</code> with
	<code>-S -ggo</code>. The result will be an assembler file
	with a <code>.s</code> extension. This assembler file will contain
	comments beginning with #GO. Those comments are declarations in the Go
	language for the C types, variables and functions declared in the C code.
	C types which can not be represented in Go will contain the string INVALID.
	Unsupported macro definitions will be recorded as <code>unknowndefine</code>,
	and uses of <code>#undef</code> will be recorded as <code>undef</code>.
	So it is very approximately possible to get Go code by running

	<pre>
	gcc -S -ggo foo.c
	grep '#GO' foo.s \| grep -v INVALID \| grep -v unknowndefine \| grep -v undef > foo.go
	</pre>

	<p>
	This procedure is full of unstated caveats and restrictions and we make no
	guarantee that it will not change in the future. It is more useful as a
	starting point for real Go code than as a regular procedure.

	<h2 id="RTEMS_Port">RTEMS Port</h2>
	<p>
	The <code>gccgo</code> compiler has been ported to <a href="http://www.rtems.com/">
	<code>RTEMS</code></a>. <code>RTEMS</code> is a real-time executive
	that provides a high performance environment for embedded applications
	on a range of processors and embedded hardware. The current <code>gccgo</code>
	port is for x86. The goal is to extend the port to most of the
	<a href="http://www.rtems.org/wiki/index.php/SupportedCPUs">
	architectures supported by <code>RTEMS</code></a>. For more information on the port,
	as well as instructions on how to install it, please see this
	<a href="http://www.rtems.com/wiki/index.php/GCCGoRTEMS"><code>RTEMS</code> Wiki page</a>.