This document explains how to use `gccgo`, a compiler for the Go language. The `gccgo` compiler is a new frontend for `gcc`, the widely used GNU compiler. Although the frontend itself is under a BSD-style license, `gccgo` is normally used as part of `gcc` and is then covered by the GNU General Public License.

Note that `gccgo` is not the `6g` compiler; see the Installing Go instructions for that compiler.

## Source code

The `gccgo` source code is accessible via Subversion. The `gcc` web site has instructions for getting the `gcc` source code. The `gccgo` source code is a branch of the main `gcc` code repository: `svn://gcc.gnu.org/svn/gcc/branches/gccgo`.

Note that although `gcc.gnu.org` 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 Contributing to gccgo.

## Building

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

On x86 GNU/Linux systems the `gccgo` 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 `gold` linker. The easiest way to do this is to build the GNU binutils, using `--enable-gold=default` when you run the `configure` script, and to use `--with-ld=GOLD_BINARY` when you configure `gccgo`. A typical sequence would look like this (you can replace `/opt/gold` with any directory to which you have write access):

    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

A number of prerequisites are required to build `gcc`, as described on the gcc web site. If those are all available, then a typical build and install sequence would look like this (only use the `--with-ld` option if you built and installed the gold linker as described above):

    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

## Using gccgo

The `gccgo` compiler works like other gcc frontends.

To compile a file:

    gccgo -c file.go

That produces `file.o`. To link files together to form an executable:

    gccgo -o file file.o

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 `LD_LIBRARY_PATH` in your environment:

    LD_LIBRARY_PATH=/usr/lib/gcc/MACHINE/VERSION

or by passing a `-Wl,-R` option when you link:

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

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

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

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

## Options

The `gccgo` compiler supports all `gcc` options that are language independent, notably the `-O` and `-g` options.

The `-fgo-prefix=PREFIX` 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 `PREFIX` may be any string; a good choice for the string is the directory where the package will be installed.

The `-fno-require-return-statement` 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 `6g`.

The `-I` and `-L` options, which are synonyms for the compiler, may be used to set the search path for finding imports.

## Imports

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 `gccgo` how to find the file.

When you import the package FILE with `gccgo`, it will look for the import data in the following files, and use the first one that it finds.

- `FILE.gox`
- `FILE.o`
- `libFILE.so`
- `libFILE.a`

`FILE.gox`, when used, will typically contain nothing but export data. This can be generated from `FILE.o` via

    objcopy -j .go_export FILE.o FILE.gox

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

    gccgo -c mypackage.go              # Exports mypackage
    gccgo -c main.go                   # Imports mypackage
    gccgo -o main main.o mypackage.o   # Explicitly links with mypackage.o

## Unimplemented

Some Go features are not yet implemented in `gccgo`. As of 2010-08-23, the following are not implemented:

- 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.

## Debugging

If you use the `-g` option when you compile, you can run `gdb` 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 `print *stringvar`. In general Go strings, maps, channels and interfaces are always represented as C pointers.

## C Interoperability

When using `gccgo` there is limited interoperability with C, or with C++ code compiled using `extern "C"`.

### Types

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

The Go `string` type is currently defined as a two-element structure (this is subject to change):

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

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 `*[10]int` is equivalent to C `int*`, assuming that the C pointer does point to 10 elements.

A slice in Go is a structure. The current definition is (this is subject to change):

    struct __go_slice {
      void *__values;
      int __count;
      int __capacity;
    };

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:

    func GoFunction(int) (int, float64)
    struct { int i; float64 f; } CFunction(int)

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

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

### Function names

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

    func c_open(name *byte, mode int, perm int) int __asm__ ("open");

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

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

(this serves as an example only, to open a file in Go please use Go's `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 `prefix.package.Functionname`. The prefix is set by the `-fgo-prefix` option used when the package is compiled; if the option is not used, the default is `go`. To call the function from C you must set the name using a `gcc` extension similar to the `gccgo` extension.

    extern int go_function(int) __asm__ ("myprefix.mypackage.Function");

### Automatic generation of Go declarations from C source code

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

Compile your C code as usual, but replace `-c` with `-S -ggo`. The result will be an assembler file with a `.s` 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 `unknowndefine`, and uses of `#undef` will be recorded as `undef`. So it is very approximately possible to get Go code by running

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

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.

## RTEMS Port

The `gccgo` compiler has been ported to `RTEMS`. `RTEMS` is a real-time executive that provides a high performance environment for embedded applications on a range of processors and embedded hardware. The current `gccgo` port is for x86. The goal is to extend the port to most of the architectures supported by `RTEMS`. For more information on the port, as well as instructions on how to install it, please see this `RTEMS` Wiki page.