Signal and process groups (Internet domain)

Each socket has an associated process group, just as a terminal does.
The socket's process group is initialized to the process group of its creator, but may be redefined at a later time with the

SIOCSPGRP ioctl:

ioctl(s, SIOCSPGRP, &pgrp);

A similar ioctl, SIOCGPGRP, is available for determining the current process group of a socket.

The presence of out-of-band data is signaled to the socket's process group through the use of the SIGUSR1 signal
(used where a BSD system would use the SIGURG signal).