How to cast bytes to struct(c struct) in go? - c

Question

package main
/*
#define _GNU_SOURCE 1
#include <stdio.h>
#include <stdlib.h>
#include <utmpx.h>
#include <fcntl.h>
#include <unistd.h>
char *path_utmpx = _PATH_UTMPX;
typedef struct utmpx utmpx;
*/
import "C"
import (
"fmt"
"io/ioutil"
)
type Record C.utmpx
func main() {
path := C.GoString(C.path_utmpx)
content, err := ioutil.ReadFile(path)
handleError(err)
var records []Record
// now we have the bytes(content), the struct(Record/C.utmpx)
// how can I cast bytes to struct ?
}
func handleError(err error) {
if err != nil {
panic("bad")
}
}
I'm trying to read content into Record
I have asked a few related questions.
Cannot access c variables in cgo
Can not read utmpx file in go
I have read some articles and posts but still cannot figure out a way to do this.

Answer 1

I think you're going about this the wrong way. If you were wanting to use the C library, you would use the C library to read the file.
Don't use cgo purely to have struct definitions, you should create these in Go yourself. You could then write the appropriate marshal / unmarshal code to read from the raw bytes.
A quick Google shows that someone has already done the work required to convert a look of the relevant C library to Go. See the utmp repository.
A short example of how this could be used is:
package main
import (
"bytes"
"fmt"
"log"
"github.com/ericlagergren/go-gnulib/utmp"
)
func handleError(err error) {
if err != nil {
log.Fatal(err)
}
}
func byteToStr(b []byte) string {
i := bytes.IndexByte(b, 0)
if i == -1 {
i = len(b)
}
return string(b[:i])
}
func main() {
list, err := utmp.ReadUtmp(utmp.UtmpxFile, 0)
handleError(err)
for _, u := range list {
fmt.Println(byteToStr(u.User[:]))
}
}
You can view the GoDoc for the utmp package for more information.