Accessing Go compiled applications through FFI

I finally started to create some useful Go code and, just like some previous posts(in pt-BR, ) where I integrated python, lua and C, I would love to use them with old python code instead of C/C++. For that, I had to generate a shared library. The process is easy as expected, requiring only the import of library “C” and a comment before function definition, exporting the function name, as following examples.

package main

import "C"

//export ModXY
func ModXY(x int) int {
        return x * 2
}

func main() {}

Now, let’s compile it to generate a shared lib:

$ go build -o libmod.so -buildmode=c-shared

This results on two files, libmod.so and libmod.h where the first one is a shared library itself and the second one is the headers to include Go types into a C application. More on this soon. First let’s check libmod.so

$ file libmod.so
libmod.so: Mach-O 64-bit dynamically linked shared library x86_64

Nice, as expected on OSX. Now let’s try to import it with python ctypes!

Python 2.7.12 (default, Sep 28 2016, 18:41:32)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> lib = ctypes.CDLL('libmod.so')
>>> lib.ModXY(13,2)
1

It worked like a charm! It is, actually, pretty easy to deal with. Now let’s find out the raison d’etre of this .h file. If, for some crazy reason, you need to embbed Go code inside a C file, simply include the .h and use the functions normally, as the following example:

#include <stdio.h>
#include "libmod.h"

int main(void){
  printf("the rest of division of 13 per 2 is %d", (int)ModXY(13,2));
  return 0;
}

then, compile it with:

$ gcc -o purexecutable testlib.c libmod.so 

and

$ file purexecutable
purexecutable: Mach-O 64-bit executable x86_64
$ ./purexecutable
the rest of division of 13 per 2 is 1

Yey! Again, simple and efficient. The design documentation of this feature may be found here( http://golang.org/s/execmodes ). That’s all

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