send_fd.c

#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>

#ifndef NULL
#define NULL ((void *) 0)
#endif

// http://www.thomasstover.com/uds.html

 int send_fd(int socket, int fd_to_send)
 {
  if(fd_to_send==-1) {
    return send(socket, "X", 1, 0);
  }
  struct msghdr socket_message;
  struct iovec io_vector[1];
  struct cmsghdr *control_message = NULL;
  char message_buffer[1];
  /* storage space needed for an ancillary element with a paylod of length is CMSG_SPACE(sizeof(length)) */
  char ancillary_element_buffer[CMSG_SPACE(sizeof(int))];
  int available_ancillary_element_buffer_space;

  /* at least one vector of one byte must be sent */
  message_buffer[0] = 'F';
  io_vector[0].iov_base = message_buffer;
  io_vector[0].iov_len = 1;

  /* initialize socket message */
  memset(&socket_message, 0, sizeof(struct msghdr));
  socket_message.msg_iov = io_vector;
  socket_message.msg_iovlen = 1;

  /* provide space for the ancillary data */
  available_ancillary_element_buffer_space = CMSG_SPACE(sizeof(int));
  memset(ancillary_element_buffer, 0, available_ancillary_element_buffer_space);
  socket_message.msg_control = ancillary_element_buffer;
  socket_message.msg_controllen = available_ancillary_element_buffer_space;

  /* initialize a single ancillary data element for fd passing */
  control_message = CMSG_FIRSTHDR(&socket_message);
  control_message->cmsg_level = SOL_SOCKET;
  control_message->cmsg_type = SCM_RIGHTS;
  control_message->cmsg_len = CMSG_LEN(sizeof(int));
  *((int *) CMSG_DATA(control_message)) = fd_to_send;

  return sendmsg(socket, &socket_message, 0);
 }