Call a C function from go

Question

I'm quite new on learning go language, and I start to be a great lover of this language. I hope, I will be a good gopher soon. Currently I try to call a C function to read the shadow file, my code is:

// #cgo CFLAGS: -D_POSIX_SOURCE=1
// #include <stdlib.h>
// #include <shadow.h>
// size_t size_of_shadow() { return sizeof(struct spwd); }
import "C"
import "unsafe"
import "fmt"

type Shadow struct {
    Name   string
    Passwd string
}

func Getspnam(name string) (*Shadow, error) {
    cname := C.CString(name)
    cspwd := (*C.struct_passwd)(C.malloc(C.size_of_shadow()))
    buf := (*C.char)(C.malloc(1024))
    _, err := C.getspnam_r(cname, cspwd, 1024, &cpwd)

    if unsafe.Pointer(cspwd) == unsafe.Pointer(uintptr(0)) {
        C.free(unsafe.Pointer(cname))

        if err == nil {
            err = fmt.Errorf("User %s not found", name)
        }

        return nil, err
    }

    s := Shadow{
        Name: C.GoString(cspwd.sp_namp),
        Passwd: C.GoString(cspwd.sp_pwdp),
    }

    C.free(unsafe.Pointer(cname))
    C.free(unsafe.Pointer(cspwd))
    C.free(unsafe.Pointer(buf))

    return &s, nil
}

Inspired by this little project and the documentation of the function of course:

https://github.com/LTD-Beget/passwd http://linux.die.net/man/3/getspnam

I'm on debian stretch and go 1.6 version, installed with the package manager. I got an error when I try to compile my file:

could not determine kind of name for C.getspnam_r

But when I open the header file shadow.h, the function is however present on the file.

Answer 1

I fixed my mistake. The error was the used of the flag some was unnecessary and the typo on the name of the struct:

// #include <stdlib.h>
// #include <shadow.h>
// size_t size_of_shadow() { return sizeof(struct spwd); }
import "C"

import "C"
import "unsafe"
import "fmt"

type Shadow struct {
    Name   string
    Passwd string
}

func Getspnam(name string) (*Shadow, error) {
    cname := C.CString(name)
    defer C.free(unsafe.Pointer(cname))

    cspwd := (*C.struct_spwd)(C.malloc(C.size_of_shadow()))
    defer C.free(unsafe.Pointer(cspwd))

    buf := (*C.char)(C.malloc(1024))
    defer C.free(unsafe.Pointer(buf))

    _, err := C.getspnam_r(cname, cspwd, buf, 1024, &cspwd)

    if unsafe.Pointer(cspwd) == unsafe.Pointer(uintptr(0)) {
        if err == nil {
            err = fmt.Errorf("User %s not found", name)
        }

        return nil, err
    }

    s := Shadow{
        Name:   C.GoString(cspwd.sp_namp),
        Passwd: C.GoString(cspwd.sp_pwdp),
    }

    return &s, nil
}

The new version of the code.