<!-- Setting up and using 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</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 --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 Go runtime library. 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
2009-11-06, the following are not implemented:

<ul>
<li>Garbage collection is not implemented. There is no way to free memory.
    Thus long running programs are not supported.

<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 a pointer to a structure.
The current definition is
(this is <b style="color: red;">expected to change</b>):

<pre>
struct __go_string {
  size_t __length;
  unsigned char __data[];
};
</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, float)
struct { int i; float 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 (they roughly correspond to pointers
to structs in C, but the structs are deliberately undocumented). C
<code>enum</code> types correspond to some Go 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>package.Functionname</code>. To call it 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__ ("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.