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(libc.info)Asynchronous I/O Signals

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Asynchronous I/O Signals
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   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 (Note: 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.  Note: 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.