#define _GNU_SOURCE 1

#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>

int main(void)
{
 int fds[2];
 struct msghdr msg;
 struct iovec iov[1];
 union
 { /* union for &snd_u.control alignment */
  struct cmsghdr cm;
  char control[CMSG_SPACE(sizeof(int))];
 } snd_u;
 union
 { /* union for &rcv_u.control alignment */
  struct cmsghdr cm;
  char control[CMSG_SPACE(sizeof(int)) + CMSG_SPACE(sizeof(struct ucred))];
 } rcv_u;
 char buf[1];
 struct cmsghdr *hdr = NULL;

 if (socketpair(PF_LOCAL, SOCK_STREAM, IPPROTO_IP, fds) == -1)
   exit (EXIT_FAILURE); 
 
 iov[0].iov_base = buf;
 iov[0].iov_len = 1;

 msg.msg_control = snd_u.control;
 msg.msg_controllen = sizeof(snd_u.control);
 msg.msg_name = NULL;
 msg.msg_namelen = 0;
 msg.msg_iov = iov;
 msg.msg_iovlen = 1;
 msg.msg_flags = 0;

 hdr = CMSG_FIRSTHDR(&msg);
 hdr->cmsg_len = CMSG_LEN(sizeof(int)); 
 hdr->cmsg_level = SOL_SOCKET;
 hdr->cmsg_type = SCM_RIGHTS;
 *((int *)CMSG_DATA(hdr)) = fds[0];

 if (sendmsg(fds[0], &msg, 0) == -1)
   exit (EXIT_FAILURE);

/* now recv is at the other end... */

 iov[0].iov_base = buf;
 iov[0].iov_len = 1;

 msg.msg_control = rcv_u.control;
 msg.msg_controllen = sizeof(rcv_u.control);
 msg.msg_name = NULL;
 msg.msg_namelen = 0;
 msg.msg_iov = iov;
 msg.msg_iovlen = 1;
 msg.msg_flags = 0;

 {
  socklen_t len = sizeof(int);
  int val = 1;

  if (setsockopt(fds[1], SOL_SOCKET, SO_PASSCRED, &val, len) == -1)
    exit (EXIT_FAILURE);
 }

 if (recvmsg(fds[1], &msg, 0) == -1)
   exit (EXIT_FAILURE);

 hdr = CMSG_FIRSTHDR(&msg);

 if (!hdr || (hdr->cmsg_level != SOL_SOCKET) ||
     ((hdr->cmsg_type != SCM_RIGHTS) && (hdr->cmsg_type != SCM_CREDENTIALS)))
   exit (EXIT_FAILURE);

 hdr = CMSG_NXTHDR(&msg, hdr);

 if (!hdr || (hdr->cmsg_level != SOL_SOCKET) ||
     ((hdr->cmsg_type != SCM_RIGHTS) && (hdr->cmsg_type != SCM_CREDENTIALS)))
   exit (EXIT_FAILURE);
 
 hdr = CMSG_NXTHDR(&msg, hdr);

 if (hdr)
   exit (EXIT_FAILURE);

 exit (EXIT_SUCCESS);
}