Example: Worker program used for sendmsg() and recvmsg()

/**************************************************************************/
/* Worker job that uses the recvmsg to process client requests            */
/**************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>

main (int argc, char *argv[])
{
   int    rc, len;
   int    worker_sd, pass_sd;
   char   buffer[80];
   struct iovec   iov[1];
   struct msghdr  msg;

   /*************************************************/
   /* One of the socket descriptors that was        */
   /* returned by socketpair(), is passed to this   */
   /* worker job as descriptor 0.                   */
   /*************************************************/
   worker_sd = 0;

   /*************************************************/
   /* Initialize message header structure           */
   /*************************************************/
   memset(&msg,   0, sizeof(msg));
   memset(iov,    0, sizeof(iov));

   /*************************************************/
   /* The recvmsg() call will NOT block unless a    */
   /* non-zero length data buffer is specified      */
   /*************************************************/
   iov[0].iov_base = buffer;
   iov[0].iov_len  = sizeof(buffer);
   msg.msg_iov     = iov;
   msg.msg_iovlen  = 1;

   /*************************************************/
   /* Fill in the msg_accrights fields so that we   */
   /* can receive the descriptor                    */
   /*************************************************/
   msg.msg_accrights    = (char *)&pass_sd;
   msg.msg_accrightslen = sizeof(pass_sd);

   /*************************************************/
   /* Wait for the descriptor to arrive             */
   /*************************************************/
   printf("Waiting on recvmsg\n");
   rc = recvmsg(worker_sd, &msg, 0);
   if (rc < 0)
   {
      perror("recvmsg() failed");
      close(worker_sd);
      exit(-1);
   }
   else if (msg.msg_accrightslen <= 0)
   {
      printf("Descriptor was not received\n");
      close(worker_sd);
      exit(-1);
   }
   else
   {
      printf("Received descriptor = %d\n", pass_sd);
   }

   /*************************************************/
   /* Receive a message from the client             */
   /*************************************************/
   printf("Wait for client to send us a message\n");
   rc = recv(pass_sd, buffer, sizeof(buffer), 0);
   if (rc <= 0)
   {
      perror("recv() failed");
      close(worker_sd);
      close(pass_sd);
      exit(-1);
   }
   printf("<%s>\n", buffer);

   /*************************************************/
   /* Echo the data back to the client              */
   /*************************************************/
   printf("Echo it back\n");
   len = rc;
   rc = send(pass_sd, buffer, len, 0);
   if (rc <= 0)
   {
      perror("send() failed");
      close(worker_sd);
      close(pass_sd);
      exit(-1);
   }

   /*************************************************/
   /* Close down the descriptors                    */
   /*************************************************/
   close(worker_sd);
   close(pass_sd);
}