Use golang function inside C-program

Question

I have created a golang program to pass some values to c program. I used this example to do so

My simple golang code :

package main

import "C"

func Add() int {
        var a = 23
        return a 
 }
func main() {}

Then I compiled this using go build -o test.so -buildmode=c-shared test.go

My C code :

#include "test.h"

int *http_200 = Add(); 

When I try to compile it using gcc -o test test.c ./test.so

I get

int *http_200 = Add(); ^ http_server.c:75:17: error: initializer element is not constant

Why I am getting this error? How to initialize that variable properly in my C code.

PS : edited after first comment.

Answer 1

There are a couple of issues here. First is the incompatibility of the types. Go will return a GoInt. Second issues is that the Add() function has to be exported to get the desired header file. If you don't want to change your Go code, then in C you have to use the GoInt which is a long long.

A complete example is:

test.go

package main

import "C"

//export Add
func Add() C.int {
    var a = 23
    return C.int(a)
}

func main() {}

test.c

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

int main() {
    int number = Add();
    printf("%d\n", number);
}

Then compile and run:

go build -o test.so -buildmode=c-shared test.go
gcc -o test test.c ./test.so &&
./test

23

---

A second example using the GoInt: test.go

package main

import "C"

//export Add
func Add() int { // returns a GoInt (typedef long long GoInt)
    var a = 23
    return a
}

func main() {}

test.c

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

int main() {
    long long number = Add();
    printf("%lld\n", number);
}

Then compile and run:

go build -o test.so -buildmode=c-shared test.go
gcc -o test test.c ./test.so &&
./test

23