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(libc.info)Kinds of Signals

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Some Kinds of Signals
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   A signal reports the occurrence of an exceptional event.  These are
some of the events that can cause (or "generate", or "raise") a signal:

   * A program error such as dividing by zero or issuing an address
     outside the valid range.

   * A user request to interrupt or terminate the program.  Most
     environments are set up to let a user suspend the program by
     typing `C-z', or terminate it with `C-c'.  Whatever key sequence
     is used, the operating system sends the proper signal to interrupt
     the process.

   * The termination of a child process.

   * Expiration of a timer or alarm.

   * A call to `kill' or `raise' by the same process.

   * A call to `kill' from another process.  Signals are a limited but
     useful form of interprocess communication.

   * An attempt to perform an I/O operation that cannot be done.
     Examples are reading from a pipe that has no writer (Note: Pipes
     and FIFOs), and reading or writing to a terminal in certain
     situations (Note: Job Control).

   Each of these kinds of events (excepting explicit calls to `kill'
and `raise') generates its own particular kind of signal.  The various
kinds of signals are listed and described in detail in Note: Standard
Signals.