could not determine kind of name for C.free

main.go

package main

// #include <stdlib.h>
// #include <unistd.h>
// #include <sys/types.h>
// #include <pwd.h>
// #include <grp.h>

import "C"
import "fmt"

func main() {
	fmt.Println("Hello, playground")
}

func lookupGroup(groupname string, f func(*C.struct_group) *Group) (*Group, os.Error) {
	var grp C.struct_group
	var result *C.struct_group

	buf, bufSize, err := allocBuffer(groupBuffer)
	if err != nil {
		return nil, err
	}
	defer C.free(buf)

	nameC := C.CString(groupname)
	defer C.free(unsafe.Pointer(nameC))
	rv := C.getgrnam_r(nameC,
		&grp,
		(*C.char)(buf),
		C.size_t(bufSize),
		&result)
	if rv != 0 {
		return nil, fmt.Errorf("group: lookup groupname %s: %s", groupname, os.Errno(rv))
	}
	if result == nil {
		return nil, UnknownGroupError(groupname)
	}
	g := f(&grp)
	return g, nil
}