GNU C Library (libc) Programming Guide - Asynchronous I/O Signals

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24.2.4 Asynchronous I/O Signals

The signals listed in this section are used in conjunction with asynchronous I/O facilities. You have to take explicit action by calling fcntl to enable a particular file descriptor to generate these signals (see Interrupt Input). The default action for these signals is to ignore them.

— Macro: int SIGIO

This signal is sent when a file descriptor is ready to perform input or output.
On most operating systems, terminals and sockets are the only kinds of files that can generate SIGIO; other kinds, including ordinary files, never generate SIGIO even if you ask them to.
In the GNU system SIGIO will always be generated properly if you successfully set asynchronous mode with fcntl.

— Macro: int SIGURG

This signal is sent when “urgent” or out-of-band data arrives on a socket. See Out-of-Band Data.

— Macro: int SIGPOLL

This is a System V signal name, more or less similar to SIGIO. It is defined only for compatibility.