problems when accessing C union field

Question

I'd like to access the field of C union in Go. below is my source code, but i got an error when compile it:

package main

// #include <stdio.h>
// #include <stdlib.h>
// union bar {
//        char   c;
//        int    i;
//        double d;
// };
import "C"

import "fmt"

func main() {
    var b *C.union_bar = new(C.union_bar)
    b.c = 4
    fmt.Println(b)
}

when i build, i got errors like below:

b.c undefined (type *[8]byte has no field or method c)

Who could tell me the correct approach to access a union field?

Answer 1

Seems like unions are treated, for type safety, as [N]byte, N == size of the biggest union item. Thus it is necessary to handle the "Go visible" type as [8]byte in this case. Then it appears to work:

package main

/*

#include <stdio.h>
#include <stdlib.h>
union bar {
       char   c;
       int    i;
       double d;
} bar;

void foo(union bar *b) {
    printf("%i\n", b->i);
};

*/
import "C"

import "fmt"

func main() {
    b := new(C.union_bar)
    b[0] = 1
    b[1] = 2
    C.foo(b)
    fmt.Println(b)
}

---

(11:28) jnml@celsius:~/src/tmp/union$ go build && ./union
513
&[1 2 0 0 0 0 0 0]
(11:28) jnml@celsius:~/src/tmp/union$ 

---

Note: The same code would print a different number at a machine with other endianness.