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 (the license covers gccgo itself as part of GCC; it does not cover code generated by gccgo).

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

## Releases

The simplest way to install gccgo is to install a GCC binary release built to include Go support. GCC binary releases are available from various websites and are typically included as part of GNU/Linux distributions. We expect that most people who build these binaries will include Go support.

The GCC 4.7.1 release and all later 4.7 releases include a complete Go 1 compiler and libraries.

Due to timing, the GCC 4.8.0 and 4.8.1 releases are close to but not identical to Go 1.1. The GCC 4.8.2 release includes a complete Go 1.1.2 implementation.

The GCC 4.9 releases include a complete Go 1.2 implementation.

The GCC 5 releases include a complete implementation of the Go 1.4 user libraries. The Go 1.4 runtime is not fully merged, but that should not be visible to Go programs.

The GCC 6 releases include a complete implementation of the Go 1.6.1 user libraries. The Go 1.6 runtime is not fully merged, but that should not be visible to Go programs.

The GCC 7 releases are expected to include a complete implementation of the Go 1.8 user libraries. As with earlier releases, the Go 1.8 runtime is not fully merged, but that should not be visible to Go programs.

## Source code

If you cannot use a release, or prefer to build gccgo for yourself, 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 included. As a convenience, a stable version of the Go support is available in a branch of the main GCC code repository: `svn://gcc.gnu.org/svn/gcc/branches/gccgo`. This branch is periodically updated with stable Go compiler sources.

Note that although `gcc.gnu.org` is the most convenient way to get the source code for the Go frontend, it is not where the master sources live. If you want to contribute changes to the Go frontend 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). If you are targeting a 64-bit x86, but sometimes want to use the `-m32` option, then use the `configure` option `--with-arch-32=i586`.

### Gold

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 the gold linker version 2.22 or later. You can either install GNU binutils 2.22 or later, or you can build gold yourself.

To build gold yourself, build the GNU binutils, using `--enable-gold=default` when you run the `configure` script. Before building, you must install the flex and bison packages. 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"]
    [The next command will create a directory named src, not binutils]
    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

However you install gold, when you configure gccgo, use the option `--with-ld=GOLD_BINARY`.

### Prerequisites

A number of prerequisites are required to build GCC, as described on the gcc web site. It is important to install all the prerequisites before running the gcc `configure` script. The prerequisite libraries can be conveniently downloaded using the script `contrib/download_prerequisites` in the GCC sources.

### Build commands

Once all the prerequisites are installed, then a typical build and install sequence would look like this (only use the `--with-ld` option if you are using the gold linker as described above):

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

## Using gccgo

The gccgo compiler works like other gcc frontends. As of GCC 5 the gccgo installation also includes a version of the `go` command, which may be used to build Go programs as described at https://golang.org/cmd/go.

To compile a file without using the `go` command:

    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. There are a few ways to do this:

- Set the `LD_LIBRARY_PATH` environment variable:

      LD_LIBRARY_PATH=${prefix}/lib/gcc/MACHINE/VERSION
      [or]
      LD_LIBRARY_PATH=${prefix}/lib64/gcc/MACHINE/VERSION
      export LD_LIBRARY_PATH

  Here `${prefix}` is the `--prefix` option used when building gccgo. For a binary install this is normally `/usr`. Whether to use `lib` or `lib64` depends on the target. Typically `lib64` is correct for x86_64 systems, and `lib` is correct for other systems. The idea is to name the directory where `libgo.so` is found.

- Passing a `-Wl,-R` option when you link (replace lib with lib64 if appropriate for your system):

      go build -gccgoflags -Wl,-R,${prefix}/lib/gcc/MACHINE/VERSION
      [or]
      gccgo -o file file.o -Wl,-R,${prefix}/lib/gcc/MACHINE/VERSION

- Use the `-static-libgo` option to link statically against the compiled packages.

- Use the `-static` option to do a fully static link (the default for the `gc` compiler).

## Options

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

The `-fgo-pkgpath=PKGPATH` option may be used to set a unique prefix for the package being compiled. This option is automatically used by the go command, but you may want to use it if you invoke gccgo directly. 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 `PKGPATH` may be any string; a good choice for the string is the path used to import the package.

The `-I` and `-L` options, which are synonyms for the compiler, may be used to set the search path for finding imports. These options are not needed if you build with the go command.

## Imports

When you compile a file that exports something, the export information will be stored directly in the object file. If you build with gccgo directly, rather than with the go command, then 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`
- `libFILE.so`
- `libFILE.a`
- `FILE.o`

`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 (2015-06-15) record the file name of imported packages in the object file. You must arrange for the imported data to be linked into the program. Again, this is not necessary when building with the go command.

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

## Debugging

If you use the `-g` option when you compile, you can run `gdb` on your executable. The debugger has only limited knowledge about Go. 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 and interfaces will show up as two-element structures. Go maps and channels are always represented as C pointers to run-time structures.

## 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 `int32` in Go is an `int32_t` in C, an `int64` is an `int64_t`, etc. The Go type `int` is an integer that is the same size as a pointer, and as such corresponds to the C type `intptr_t`. 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;
      intptr_t __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;
      intptr_t __count;
      intptr_t __capacity;
    };

The type of a Go function is a pointer to a struct (this is subject to change). The first field in the struct points to the code of the function, which will be equivalent to a pointer to a C function whose parameter types are equivalent, with an additional trailing parameter. The trailing parameter is the closure, and the argument to pass is a pointer to the Go function struct. When a Go function returns more than one value, the C function returns a struct. For example, these functions are roughly equivalent:

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

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 preceded by `//extern NAME`. For example, here is how the C function `open` can be declared in Go:

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

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 `syscall` package):

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

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

Note that if the C function can block, such as in a call to `read`, calling the C function may block the Go program. Unless you have a clear understanding of what you are doing, all calls between C and Go should be implemented through cgo or SWIG, as for the `gc` compiler.

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.

    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, and most users should use the cgo program with the `-gccgo` option instead.

Compile your C code as usual, and add the option `-fdump-go-spec=FILENAME`. This will create the file `FILENAME` as a side effect of the compilation. This file will contain Go declarations for the types, variables and functions declared in the C code. C types that can not be represented in Go will be recorded as comments in the Go code. The generated file will not have a `package` declaration, but can otherwise be compiled directly by gccgo.

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.