How to reuse a Go callback in several packages from C?

Question

Is there a way to build a Go + C application that:

• From main package X, import packages Y and Z.
• Package M exports a go callback F.
• Packages X and Y are both built with accompanying C files, both want to call F from C source code.

Generally speaking I'm trying to figure out how to call a callback from accompanying C files in other modules which are used to build a final application. I coudn't figure out how to achieve this or something similar. I'm also interested in convoluted solutions.

Answer 1

I don't see a way to call a Go function across packages, but all cgo packages are linked into the same binary and can call each other. This means that you can export M.F to a C function in package M and call that C function from packages Y and Z.

m/m.go:

package m

// void F();
import "C"
import "fmt"

//export F
func F() {
    fmt.Println("m.f")
}

m/m.h:

void m_f();

m/m.c:

#include <stdio.h>
#include "_cgo_export.h"
#include "m.h"

void m_f() {
    printf("m_f\n")
    F();
}

y/y.go:

package y

// The LDFLAGS lines below are needed to prevent linker errors
// since not all packages are present while building intermediate
// packages. The darwin build tag is used as a proxy for clang
// versus gcc because there doesn't seem to be a better way
// to detect this.

// #cgo darwin LDFLAGS: -Wl,-undefined -Wl,dynamic_lookup
// #cgo !darwin LDFLAGS: -Wl,-unresolved-symbols=ignore-all
// #include "y.h"
import "C"

import (
    "fmt"
    _ "m"
)

func Y() {
    fmt.Println("y.Y")
    C.y()
}

y/y.h:

void y();

y/y.c:

#include <stdio.h>
#include "../m/m.h"

void y() {
    printf("y.C.y\n");
    m_f();
}