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<h4 class="subsection">24.1.3 How Signals Are Delivered</h4>

<p><a name="index-delivery-of-signals-2812"></a><a name="index-pending-signals-2813"></a><a name="index-blocked-signals-2814"></a>
When a signal is generated, it becomes <dfn>pending</dfn>.  Normally it
remains pending for just a short period of time and then is
<dfn>delivered</dfn> to the process that was signaled.  However, if that kind
of signal is currently <dfn>blocked</dfn>, it may remain pending
indefinitely&mdash;until signals of that kind are <dfn>unblocked</dfn>.  Once
unblocked, it will be delivered immediately.  See <a href="Blocking-Signals.html#Blocking-Signals">Blocking Signals</a>.

   <p><a name="index-specified-action-_0028for-a-signal_0029-2815"></a><a name="index-default-action-_0028for-a-signal_0029-2816"></a><a name="index-signal-action-2817"></a><a name="index-catching-signals-2818"></a>When the signal is delivered, whether right away or after a long delay,
the <dfn>specified action</dfn> for that signal is taken.  For certain
signals, such as <code>SIGKILL</code> and <code>SIGSTOP</code>, the action is fixed,
but for most signals, the program has a choice: ignore the signal,
specify a <dfn>handler function</dfn>, or accept the <dfn>default action</dfn> for
that kind of signal.  The program specifies its choice using functions
such as <code>signal</code> or <code>sigaction</code> (see <a href="Signal-Actions.html#Signal-Actions">Signal Actions</a>).  We
sometimes say that a handler <dfn>catches</dfn> the signal.  While the
handler is running, that particular signal is normally blocked.

   <p>If the specified action for a kind of signal is to ignore it, then any
such signal which is generated is discarded immediately.  This happens
even if the signal is also blocked at the time.  A signal discarded in
this way will never be delivered, not even if the program subsequently
specifies a different action for that kind of signal and then unblocks
it.

   <p>If a signal arrives which the program has neither handled nor ignored,
its <dfn>default action</dfn> takes place.  Each kind of signal has its own
default action, documented below (see <a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>).  For most kinds
of signals, the default action is to terminate the process.  For certain
kinds of signals that represent &ldquo;harmless&rdquo; events, the default action
is to do nothing.

   <p>When a signal terminates a process, its parent process can determine the
cause of termination by examining the termination status code reported
by the <code>wait</code> or <code>waitpid</code> functions.  (This is discussed in
more detail in <a href="Process-Completion.html#Process-Completion">Process Completion</a>.)  The information it can get
includes the fact that termination was due to a signal and the kind of
signal involved.  If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.

   <p>The signals that normally represent program errors have a special
property: when one of these signals terminates the process, it also
writes a <dfn>core dump file</dfn> which records the state of the process at
the time of termination.  You can examine the core dump with a debugger
to investigate what caused the error.

   <p>If you raise a &ldquo;program error&rdquo; signal by explicit request, and this
terminates the process, it makes a core dump file just as if the signal
had been due directly to an error.

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