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The SIGIO signal allows a process to be notified via a signal when a socket (or more generally, a file descriptor) has data waiting to be read. Use of the SIGIO facility requires three steps: First, the process must set up a SIGIO signal handler by use of the signal, sigset, sigaction, or sigvec calls. Second, it must set the process id or process group id that is to receive notification of pending input to its own process id, or the process group id of its process group (note that the default process group of a socket is group zero). This can be done by using a fcntl(2) call. Third, it must enable asynchronous notification of pending I/O requests with another fcntl call. Sample code to allow a given process to receive information on pending I/O requests as they occur for a socket s is given below.
With the addition of a handler for SIGURG, this code can also be used to prepare for receipt of SIGURG signals.
#include <fcntl.h>
#include <sys/file.h>
...
int io_handler();
...
signal(SIGIO, io_handler);
/* Set the process receiving SIGIO/SIGURG signals to us. */
if (fcntl(s, F_SETOWN, getpid()) < 0) {
perror("fcntl F_SETOWN");
exit(1);
}
/* Allow receipt of asynchronous I/O signals. */
if (fcntl(s, F_SETFL, FASYNC) < 0) {
perror("fcntl F_SETFL, FASYNC");
exit(1);
}