cmd/cgo: macros for C field names not recognized by cgo

[jeeger]

The linux manpage ipv6(7) states that in6_addr is a C struct containing an unsigned char array[16] with name s6_addr. When compiling a C program, this works:

#include <stdio.h>
#include <netinet/in.h>

int main() {
  struct in6_addr addr;
  printf("%s\n", addr.s6_addr);
}

However, the following Go program does not compile with the error addr.s6_addr undefined (type C.struct_in6_addr has no field or method s6_addr):

package main

// #include <netinet/in.h>
// #include <sys/socket.h>
import "C"

import "fmt"

func main() {
    // See man 7 ipv6
    var addr C.struct_in6_addr
    fmt.Printf("%v", addr.s6_addr)
}

Testing with go tool cgo reveals that the struct used is defined in in.h, and thus using addr.__in6_u works with go.

When the include file linux/in6.h is included, and go tool cgo -- -D__USE_KERNEL_IPV6_DEFS test.go is run, the correct (man page) definition of struct in6_addr shows up in _obj/_cgo_gotypes.go. I do not know how to get go to compile this correctly however.

Go version used is go version go1.3.3 linux/amd64. Contact me if you need any more information.

[jeeger]

Ah, Linux version is Gentoo with Kernel 3.18.1.

[ianlancetaylor]

As you probably know, this doesn't work because struct in6_addr has no field named s6_addr. Instead, the header file has

#define s6_addr __in6_u.__u6_addr8

So if we are going to fix this, the fix would be to somehow get cgo to recognize defined macro names when they are used as field names. I don't think this is likely to happen any time soon, but I will leave the issue open since it would be nice if it worked.