Signals and process groups

Because of the existence of the SIGURG and SIGIO signals, each socket has an associated process number, just as is done for terminals. This value is initialized to zero, but may be redefined at a later time with the F_SETOWN fcntl such as was done in the code above for SIGIO.

To set the socket's process id for signals, positive arguments should be given to the fcntl call. To set the socket's process group for signals, negative arguments should be passed to fcntl.

The only acceptable arguments to these system calls are the caller's process id or a negative process group having the same absolute value as the caller's process id (the process must be the process group leader of its own process group). Therefore, the only allowed recipient of SIGURG and SIGIO signals is the calling process.

Note that the process number shows either the associated process id or the associated process group; it is impossible to specify both at the same time. A similar fcntl command, F_GETOWN, is available for determining the current process number of a socket.

Note that the receipt of SIGURG and SIGIO can also be enabled by using the ioctl(S) call to assign the socket to the user's process group:

   ...
   /* oobdata is the out-of-band data handling routine */
   signal(SIGURG, oobdata);
   . . .
   int pid = -getpid();
   

   if (ioctl(client, SIOCSPGRP, (char *)&pid) < 0) {
   	perror("ioctl: SIOCSPGRP");
   }
   ...

   int reaper();
    ...
   signal(SIGCHLD, reaper);
   listen(f, 5);
   for (;;) {
   	int g, len = sizeof(from);
   

   	g = accept(f, (struct sockaddr *)&from, &len,);
   	if (g < 0) {
   		if (errno != EINTR)
   			syslog(LOG_ERR, "rlogind: accept: %m");
   		continue;
   	}
   	...
   }
    ...
   #include <wait.h>
   reaper()
   {
   	int status;
   

   	while (wait(&status) > 0)
   		continue;
   }