Go: convert unsafe.Pointer to function pointer and vice versa

Question

In C you can put function pointers into an array of void pointers and convert them back to function pointers of any type:

extern int (*fn1)(void);
extern void (*fn2)(int);
void foo(void)
{
        void *array[2];
        int i;

        /* implicit cast from function pointer to void pointer */
        array[0] = fn1;
        array[1] = fn2;
        for (i = 0; i < 2; i++)
        {
                int (*fp)(int, int, int);

                /* implicit cast from void pointer to function pointer */
                fp = array[i];
                /* call function with a different signature */
                fp(1, 2, 3);
        }
}

I need to do the same in Go, using unsafe.Pointers. The questions are:

Can a Go function pointer be converted to an unsafe.Pointer?
Can an unsafe.Pointer be converted to a Go function pointer of a different (or the same) type as the original function pointer?

(The question is not why or whether I need to do that; in the given situation it is okay to call a function with the wrong set of parameters and to misinterpret the return value because the caller and the callees are able to deal with that.)

I answered: yes it works, though I have to admit I am incredibly curious about why you need to do this. — Linear, Apr 08 '14 at 09:57
Because I'm working on the gofrontend in gccgo and need to generate artificial code in tree representation that calls multiple functions from exactly the same address. I'm not really writing Go code, but if I can stick with the classes for parsing Go source code that are already in the frontend, it's much less work. — user3510167, Apr 08 '14 at 10:50
You can't do that in C. That's undefined behavior and a good compiler should warn you. function pointers are incompatible to data pointer for a series of very good reasons. — fuz, Apr 10 '14 at 14:40

score 3 · Answer 1 · edited Jun 12 '20 at 06:22

3

As LinearZoetrope's answer shows, you can do this. Beware that you can do bad things:

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    f1 := func(s string) {}
    f2 := func(i int) int { return i + 1 }
    pointers := []unsafe.Pointer{
        unsafe.Pointer(&f1),
        unsafe.Pointer(&f2),
    }
    f3 := (*func(int) bool)(pointers[1]) // note, not int
    fmt.Println((*f3)(1))
}

playground

edited Jun 12 '20 at 06:22

Linear

21,074
4
59
70

answered Apr 08 '14 at 09:59

mjibson

16,852
8
31
42

Wait... you can use unsafe on the playground? Is that new(-ish)? – Linear Apr 08 '14 at 10:03
Not sure how new it is, but you can open network sockets and files, too, due to the NaCl sandbox: http://blog.golang.org/playground – mjibson Apr 08 '14 at 10:20
--> move to a separate reply – user3510167 Apr 08 '14 at 10:36
Ah, I see; I tried to directly take the address of the functions, but you need to store a pointer in a variable and use & on that. Thanks. – user3510167 Apr 08 '14 at 10:44

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

2

It appears to work:

package main

import (
    "fmt"
    "unsafe"

    "math"
)

func main() {
    fn := print
    faked := *(*func(float64))(unsafe.Pointer(&fn))
    faked(1.0)

    // For comparison
    num := math.Float64bits(1.0)
    print(num)
}

func print(a uint64) {
    fmt.Println(a)
}

Will print

4607182418800017408

4607182418800017408

Of course, you're probably well aware of the potential problems with trying this.

edited Jun 20 '20 at 09:12

Community

1
1

answered Apr 08 '14 at 09:55

Linear

21,074
4
59
70

> Of course, you're probably well aware of the potential problems with trying this. -- Yes. – user3510167 Apr 08 '14 at 10:44

Go: convert unsafe.Pointer to function pointer and vice versa

2 Answers2

Linked