I wrote a UDP server on Linux systemand used signal-driven
I/O to receive datagram. That is, when a new UDP datagram
arrives, the server program will receive a SIGIO signal from
the kernel.
I wrote such code to prepare for this:
==============================================
    /* set the signal handler */
    sigset(SIGIO, sigio_handler);

    /* set the owner of UDP socket */
    pid = getpid();
    if(ioctl(usock, FIOSETOWN, &pid) < 0){
        perror("ioctl FIOSETOWN");
        exit(1);
    }

    /* set socket as async mode to enable
     * signal driven I/O   */
    if(ioctl(usock, FIOASYNC, &on) < 0){
        perror("ioctl FIOASYNC");
        exit(1);
    }
==============================================
When I ran UDP client and sent UDP datagrams,
the UDP server failed to work. I used tcpdump
and saw that datagrams had been transferred
successfully to the server machine. I sent signal
to UDP server using 'kill -SIGIO' command and
proved that the signal handler worked well.
I copied the source code to the Sun Solaris system
and compiled it. It worked well.
Then I returned back to Linux and changed the
FIOSETOWN of ioctl to SIOCSPGRP of ioctl. Two
options are the same. It failed to work as well.
At last, I changed
   ioctl(usock, FIOSETOWN, &pid)
to
   fcntl(usock, F_SETOWN, pid)
then succeeded.

Therefore, I think there is some bug in implementation
of setting FIOSETOWN option of ioctl() function.

My Linux version: Red Hat Linux 6.0,
kernel is 2.2.5-15 and the CPU is Pentium.