Go code can call C functions directly using the //extern or
//go:linkname compiler directives. An //extern
directive must be at the beginning of the line and must start with
//extern. This must be followed by a space and then the
external name of the function. The function declaration must be on
the line immediately after the comment. 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
You can do the same thing using the //go:linkname compiler
directive. The //go:linkname directive must be at the start of
the line. It is followed by whitespace, the name of the Go function,
more whitespace, and the external name of the function. Unlike
//extern, //go:linkname does not need to appear
immediately adjacent to the function definition or declaration.
//go:linkname c_open 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 os package):
var name = [4]byte{'f', 'o', 'o', 0};
i := c_open(&name[0], os.O_RDONLY, 0);
Note that 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
pkgpath.Functionname. The pkgpath is set by the
-fgo-pkgpath option used when the package is compiled; if the
option is not used, the default is go.packagename. To
call the function from C you must set the name using the gcc
__asm__ extension.
extern int go_function(int) __asm__ ("mypkgpath.Function");