How to pass a C function pointer around in go

Question

I can pass a C function pointer to a C function, but passing it to a go function gives invalid operation.

I have 100 go functions wrapping C functions and most share the same setup and arguments so I wanted to implement one go function that can take a C function pointer to make this easier to maintain.

package main

/*
#include <stdio.h>
typedef void (*func_t)(int);
void this_works(int a, func_t f){
    f(a);
}
void tst(int a) {
    printf("YAY\n");
}
*/
import "C"

func this_does_not_work( fp C.func_t ) {
    fp(2) // invalid operation: cannot call non-function fp (variable of type _Ctype_func_t)
}

func main() {
    C.this_works(1, C.func_t(C.tst))
    this_does_not_work( C.func_t(C.tst) )
}

kostix · Accepted Answer · 2024-05-16 15:18:55Z

1

Impossible, but what's wrong with using a helper?

package main

/*
#include <stdio.h>
typedef void (*func_t)(int);
void this_works(int a, func_t f){
    f(a);
}
void tst(int a) {
    printf("YAY\n");
}

void call_my_c_func(func_t f, int a)
{
    f(a);
}
*/
import "C"

func this_does_not_work(fp C.func_t) {
    C.call_my_c_func(fp, 2)
}

func main() {
    C.this_works(1, C.func_t(C.tst))
    this_does_not_work(C.func_t(C.tst))
}

Basically, you will use C.call_my_c_func(fp, a) instead of fp(a) where applicable—not very beautiful but works.

answered May 16, 2024 at 15:18

kostix

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2 Comments

Craftables Over a year ago

Ah of course thanks! I'm concerned with performance, but this doesn't materially affect it.

kostix Over a year ago

@Craftables, even if Go would allows such "direct" ("transparent"?) calls to C you were thinking of, the mechanism it would have to exploit to do them would necessarily have the same performance you will have in the case with the helper; if you're interested, there I've explained to someone the basics of how Go calls to C. If you meant that the extra C call in this chanin affects performance, then yes, it does, but I'm sure in such simplistic case (two machine-word-sized arguments and no return value) the overhead will indeed be negligible.

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