Mutual call between GO and C (2)-C language callback function of go dynamic library

In order to call the Go program in the C/C++ program, the Go library is written, so how to call the callback function of the C language in the library function? Let's try it.

#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef void (*callback)(void *,int);
extern void c_callback (void *,int);
extern callback _cb;
*/
import "C"
import (
//	"encoding/json"
	"unsafe"
)

func funccallback(gostring string,goint int) {
	var cmsg *C.char = C.CString(gostring)
	var ivalue C.int = C.int(len(gostring))
	defer C.free(unsafe.Pointer(cmsg))
	C.c_callback(unsafe.Pointer(cmsg),ivalue)
}

//export register_callback
func register_callback(p C.callback) {
	C._cb = p;
	return
}

//export test_callback
func test_callback() {
	funccallback("hello,this is func callback.",123)
}

func main() {
}

Two functions register_callback and test_callback are opened to C

Go library functions call back functions in C programs through a bridge function

package main

/*
#include <stdio.h>

typedef void (*callback)(void *,int);

callback _cb;

void c_callback(void* p,int i)
{
	_cb(p,i);
}
*/
import "C"

Compile the go library

go build -o libcallback.so  -buildmode=c-shared libcallback.go bridge.go 

One thing I didn’t understand, why can’t I put the bridge function of bridge.go in libcallback.go, but need to separate a file bridge.go?

 C main program

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

void gocallback(void* s,int len) {
     printf("raw data: %s\n",(char *)s);
}

int main() {
    // 注册回调
    register_callback(gocallback);
    
    // 测试回调
    test_callback();
}

Compile

 gcc -o testcallback main.c ./libcallback.so 

operation result

$ ./testcallback
raw data: hello,this is func callback.

Program call relationship

Gray is Go function, red is C language function, Call _cb stores the pointer of callback function.