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.

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).

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: 

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

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 Go runtime library. 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.

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, 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 2009-11-06, the following are not implemented:

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 a pointer to a structure. The current definition is (this is expected to change): 

struct __go_string {
  size_t __length;
  unsigned char __data[];
};

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, float)
struct { int i; float 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 (they roughly correspond to pointers to structs in C, but the structs are deliberately undocumented). C enum types correspond to some Go 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 package.Functionname. To call it from C you must set the name using a gcc extension similar to the gccgo extension. 

extern int go_function(int) __asm__ ("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.