Using Go code in an existing C project

Question

Ever since Go 1.5 came out, I started taking another look at how I could integrate it into an existing project of mine.

The project's codebase is written entirely in C for low level access to to hardware and other fun stuff. However, some of the higher level things are tedious, and I would like to start writing them in a higher level language (Go)

Is there any way I can call Go code from a C program? I installed Go 1.5, which added -buildmode=c-archive (https://golang.org/s/execmodes) which I am trying to get working.

However, I can't seem to get Go to generate the appropriate header files to allow my project to actually compile. When I generate the archive, I see the function in the exported symbols (using objdump), but without the header files to include gcc complains about the function not existing (as expected)

I'm quite new to Go - however, I love the language and would like to make use of it. Is there any idiomatic way ("idiomatic" gets used a lot in the world of Go I see...) to get this to play nicely with each other?

The reason I asked this question and specifically mentioned Go 1.5 is that according to this document, https://docs.google.com/document/d/1nr-TQHw_er6GOQRsF6T43GGhFDelrAP0NqSS_00RgZQ/edit?pli=1#heading=h.1gw5ytjfcoke Go 1.5 added support for non-Go programs to call Go code. Specifically, mentioned under the section "Go code linked into, and called from, a non-Go program"

Answer 1

To build an archive callable from C, you will need to mark them as exported CGo symbols.
For example, if I create a file foo.go with the following contents:

package main

import (
    "C"
    "fmt"
)

//export PrintInt
func PrintInt(x int) {
    fmt.Println(x)
}

func main() {}

The important things to note are:

• The package needs to be called main
• You need to have a main function, although it can be empty.
• You need to import the package C
• You need special //export comments to mark the functions you want callable from C.

I can compile it as a C callable static library with the following command:

go build -buildmode=c-archive foo.go

The results will be an archive foo.a and a header foo.h. In the header, we get the following (eliding irrelevant parts):

...
typedef long long GoInt64;
...
typedef GoInt64 GoInt;
...
extern void PrintInt(GoInt p0);
...

So that's enough to call the exported function. We can write a simple C program that calls it like so:

#include "foo.h"

int main(int argc, char **argv) {
    PrintInt(42);
    return 0;
}

We can compile it with a command like:

gcc -pthread foo.c foo.a -o foo

The -pthread option is needed because the Go runtime makes use of threads. When I run the resulting executable it prints 42.

Answer 2

The code above work just fine, but gcc will complain about functions and headers.

The includes should be:

#define _GNU_SOURCE
#include <stdio.h>
#include "mygopkg.h"

If you forget the #define _GNU_SOURCE, the gcc will complain:

warning: implicit declaration of function 'asprintf'; did you mean 'vsprintf'? [-Wimplicit-function-declaration]

If you forget the #include "mygopkg.h", the gcc will complain:

warning: implicit declaration of function 'PrintString' [-Wimplicit-function-declaration]

The last but not less important. The build command line I recommend for production code is:

go build -ldflags "-s -w"  -buildmode c-archive -o mygopkg.a

It'll save you 53% size of final mygopkg.a.