How to make a callback into C from Go

Example: Count() method with a callback from Go

C Code

Here's the main.c:

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

void onCount(int n) {
    printf("Count: %d\n", n);
}

int main(void) {
    int rc = Count(3, onCount); // implemented in Go
    printf("Done!\n");
    return rc;
}

The Count() method is implemented in Go, which is compiled as a shared library. The method takes a number and a callback function. It calls the callback function, specified number of times, with interval of 1-second, then exits.

Go Code

Here's the implemention of Count() method in Go:

counter_api.go:

package main

// typedef void (*count_cb)(int);
// void makeCallback(int sum, count_cb cb);
import "C"

//export Count
func Count(n C.int, cb C.count_cb) int {
	goCb := func(n int) {
		C.makeCallback(C.int(n), cb)
	}
	return count(int(n), goCb)
}

counter.go:

package main

// typedef void (*count_cb)(int);
// extern void bridge(int sum, count_cb cb) {
//     cb(sum);
// }
import "C"
import (
	"time"
)

type countCb func(n int)

func count(n int, goCb countCb) int {
	for i := 0; i < n; i++ {
		time.Sleep(time.Second * 1)
		goCb(i + 1)
	}

	return 0
}

func main() {}

How to Build

First build a shared library from Go code.

go build -v -buildmode=c-shared -o libcounter.so *.go

Then build a C executable using the library.

gcc -o main main.c libcounter.so

Makefile is provided. For a quick build, type make.

To run:

$ ./main
Count: 1
Count: 2
Count: 3
Done!

Also try:

$ go run counter.go

It demonstrate that by putting C-typed (binding) code as a separate file (seen as counter_api.go), the rest of pure Go code become testable and runnable within Go land.

Comments / Tips

• No blank-line between C-code comment lines and import "C"
• Go code can call C-function in the comment lines. But;
• Go code can NOT directly call function pointer.
• Go method meant to be called by C must have //export Xxx right above the function.
• The *.go file that contains //export Xxx cannot have definition in the comment line, declaration only. (see counter.go)
• For that reason, use a sperate go file to place the definition. (see counter.go)
• No space between // and expose. (// export Xxx is tempting, but wound't work)
• For more details, see The Go Programming Language, Command cgo.