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(libc.info)Alarm Signals

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Alarm Signals
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   These signals are used to indicate the expiration of timers.  Note:
Setting an Alarm, for information about functions that cause these
signals to be sent.

   The default behavior for these signals is to cause program
termination.  This default is rarely useful, but no other default would
be useful; most of the ways of using these signals would require
handler functions in any case.

 - Macro: int SIGALRM
     This signal typically indicates expiration of a timer that
     measures real or clock time.  It is used by the `alarm' function,
     for example.

 - Macro: int SIGVTALRM
     This signal typically indicates expiration of a timer that
     measures CPU time used by the current process.  The name is an
     abbreviation for "virtual time alarm".

 - Macro: int SIGPROF
     This signal typically indicates expiration of a timer that measures
     both CPU time used by the current process, and CPU time expended on
     behalf of the process by the system.  Such a timer is used to
     implement code profiling facilities, hence the name of this signal.