Cannot access c variables in cgo

Question

I\'m trying to access a c struct in cgo, but go this

could not determine kind of name for C.utmpx

utmpx is a c struct

here

Answer 1

#define's are problematic with CGo. I could get it to work with Go 1.8.1 on Linux amd64 like this:

package main

import "os"

/*
#define _GNU_SOURCE 1
#include <stdio.h>
#include <stdlib.h>
#include <utmpx.h>
#include <fcntl.h>
#include <unistd.h>

char *path_utmpx = UTMPX_FILE;

typedef struct utmpx utmpx;
*/
import "C"

type record C.utmpx

func main() {
    path := C.GoString(C.path_utmpx)
    fd, err := os.Open(path)
    if err != nil {
        panic("bad")
    }
    fd.Close()
}

1. I had to define _GNU_SOURCE to get the UTMPX_FILE definition.
2. I had to create the path_utmpx variable to get around the #define problems with CGo.
3. I had to do the typedef to get type record C.utmpx compile.
4. With Go, you can't use C strings directly. You must convert them to Go strings. Similarly, if you want to call C functions with Go strings, you must convert them to C strings (and free the space allocated in the heap).

A few pointers:

• https://blog.golang.org/c-go-cgo
• https://golang.org/cmd/cgo/

Good luck!