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<h4 class="subsection">24.1.1 Some Kinds of Signals</h4>

<p>A signal reports the occurrence of an exceptional event.  These are some
of the events that can cause (or <dfn>generate</dfn>, or <dfn>raise</dfn>) a
signal:

     <ul>
<li>A program error such as dividing by zero or issuing an address outside
the valid range.

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

     <li>The termination of a child process.

     <li>Expiration of a timer or alarm.

     <li>A call to <code>kill</code> or <code>raise</code> by the same process.

     <li>A call to <code>kill</code> from another process.  Signals are a limited but
useful form of interprocess communication.

     <li>An attempt to perform an I/O operation that cannot be done.  Examples
are reading from a pipe that has no writer (see <a href="Pipes-and-FIFOs.html#Pipes-and-FIFOs">Pipes and FIFOs</a>),
and reading or writing to a terminal in certain situations (see <a href="Job-Control.html#Job-Control">Job Control</a>). 
</ul>

   <p>Each of these kinds of events (excepting explicit calls to <code>kill</code>
and <code>raise</code>) generates its own particular kind of signal.  The
various kinds of signals are listed and described in detail in
<a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>.

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