<!-- 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>

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

<p>
Building <code>gccgo</code> is just like building <code>gcc</code>
with one additional option.  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).
</p>

<p>
A number of prerequisites are required to build <code>gcc</code>, as
described on the <code>gcc</code> web site.  If those are all
available, then a typical build and install sequence would look like
this:

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

<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="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 with a fixed stack size.
    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.