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<H1><A NAME="SECTION009100000000000000000">
7.1 <tt class="module">signal</tt> --
         Set handlers for asynchronous events</A>
</H1>

<P>
<A NAME="module-signal"></A>

<P>
This module provides mechanisms to use signal handlers in Python.
Some general rules for working with signals and their handlers:

<P>

<UL>
<LI>A handler for a particular signal, once set, remains installed until
it is explicitly reset (Python emulates the BSD style interface
regardless of the underlying implementation), with the exception of
the handler for <tt class="constant">SIGCHLD</tt>, which follows the underlying
implementation.

<P>
</LI>
<LI>There is no way to ``block'' signals temporarily from critical
sections (since this is not supported by all <span class="Unix">Unix</span> flavors).

<P>
</LI>
<LI>Although Python signal handlers are called asynchronously as far as
the Python user is concerned, they can only occur between the
``atomic'' instructions of the Python interpreter.  This means that
signals arriving during long calculations implemented purely in C
(such as regular expression matches on large bodies of text) may be
delayed for an arbitrary amount of time.

<P>
</LI>
<LI>When a signal arrives during an I/O operation, it is possible that the
I/O operation raises an exception after the signal handler returns.
This is dependent on the underlying <span class="Unix">Unix</span> system's semantics regarding
interrupted system calls.

<P>
</LI>
<LI>Because the C signal handler always returns, it makes little sense to
catch synchronous errors like <tt class="constant">SIGFPE</tt> or <tt class="constant">SIGSEGV</tt>.

<P>
</LI>
<LI>Python installs a small number of signal handlers by default:
<tt class="constant">SIGPIPE</tt> is ignored (so write errors on pipes and sockets can be
reported as ordinary Python exceptions) and <tt class="constant">SIGINT</tt> is translated
into a <tt class="exception">KeyboardInterrupt</tt> exception.  All of these can be
overridden.

<P>
</LI>
<LI>Some care must be taken if both signals and threads are used in the
same program.  The fundamental thing to remember in using signals and
threads simultaneously is: always perform <tt class="function">signal()</tt> operations
in the main thread of execution.  Any thread can perform an
<tt class="function">alarm()</tt>, <tt class="function">getsignal()</tt>, or <tt class="function">pause()</tt>;
only the main thread can set a new signal handler, and the main thread
will be the only one to receive signals (this is enforced by the
Python <tt class="module">signal</tt> module, even if the underlying thread
implementation supports sending signals to individual threads).  This
means that signals can't be used as a means of inter-thread
communication.  Use locks instead.

<P>
</LI>
</UL>

<P>
The variables defined in the <tt class="module">signal</tt> module are:

<P>
<dl><dt><b><tt id='l2h-2588' xml:id='l2h-2588'>SIG_DFL</tt></b></dt>
<dd>
  This is one of two standard signal handling options; it will simply
  perform the default function for the signal.  For example, on most
  systems the default action for <tt class="constant">SIGQUIT</tt> is to dump core
  and exit, while the default action for <tt class="constant">SIGCLD</tt> is to
  simply ignore it.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-2589' xml:id='l2h-2589'>SIG_IGN</tt></b></dt>
<dd>
  This is another standard signal handler, which will simply ignore
  the given signal.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-2590' xml:id='l2h-2590'>SIG*</tt></b></dt>
<dd>
  All the signal numbers are defined symbolically.  For example, the
  hangup signal is defined as <tt class="constant">signal.SIGHUP</tt>; the variable names
  are identical to the names used in C programs, as found in
  <code>&lt;signal.h&gt;</code>.
  The <span class="Unix">Unix</span> man page for `<tt class="cfunction">signal()</tt>' lists the existing
  signals (on some systems this is <span class="manpage"><i>signal</i>(2)</span>, on others the
  list is in <span class="manpage"><i>signal</i>(7)</span>).
  Note that not all systems define the same set of signal names; only
  those names defined by the system are defined by this module.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-2591' xml:id='l2h-2591'>NSIG</tt></b></dt>
<dd>
  One more than the number of the highest signal number.
</dd></dl>

<P>
The <tt class="module">signal</tt> module defines the following functions:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2592' xml:id='l2h-2592' class="function">alarm</tt></b>(</nobr></td>
  <td><var>time</var>)</td></tr></table></dt>
<dd>
  If <var>time</var> is non-zero, this function requests that a
  <tt class="constant">SIGALRM</tt> signal be sent to the process in <var>time</var> seconds.
  Any previously scheduled alarm is canceled (only one alarm can
  be scheduled at any time).  The returned value is then the number of
  seconds before any previously set alarm was to have been delivered.
  If <var>time</var> is zero, no alarm id scheduled, and any scheduled
  alarm is canceled.  The return value is the number of seconds
  remaining before a previously scheduled alarm.  If the return value
  is zero, no alarm is currently scheduled.  (See the <span class="Unix">Unix</span> man page
  <span class="manpage"><i>alarm</i>(2)</span>.)
  Availability: <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2593' xml:id='l2h-2593' class="function">getsignal</tt></b>(</nobr></td>
  <td><var>signalnum</var>)</td></tr></table></dt>
<dd>
  Return the current signal handler for the signal <var>signalnum</var>.
  The returned value may be a callable Python object, or one of the
  special values <tt class="constant">signal.SIG_IGN</tt>, <tt class="constant">signal.SIG_DFL</tt> or
  <tt class="constant">None</tt>.  Here, <tt class="constant">signal.SIG_IGN</tt> means that the
  signal was previously ignored, <tt class="constant">signal.SIG_DFL</tt> means that the
  default way of handling the signal was previously in use, and
  <code>None</code> means that the previous signal handler was not installed
  from Python.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2594' xml:id='l2h-2594' class="function">pause</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Cause the process to sleep until a signal is received; the
  appropriate handler will then be called.  Returns nothing.  Not on
  Windows. (See the <span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.)
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2595' xml:id='l2h-2595' class="function">signal</tt></b>(</nobr></td>
  <td><var>signalnum, handler</var>)</td></tr></table></dt>
<dd>
  Set the handler for signal <var>signalnum</var> to the function
  <var>handler</var>.  <var>handler</var> can be a callable Python object
  taking two arguments (see below), or
  one of the special values <tt class="constant">signal.SIG_IGN</tt> or
  <tt class="constant">signal.SIG_DFL</tt>.  The previous signal handler will be returned
  (see the description of <tt class="function">getsignal()</tt> above).  (See the
  <span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.)

<P>
When threads are enabled, this function can only be called from the
  main thread; attempting to call it from other threads will cause a
  <tt class="exception">ValueError</tt> exception to be raised.

<P>
The <var>handler</var> is called with two arguments: the signal number
  and the current stack frame (<code>None</code> or a frame object;
  for a description of frame objects, see the reference manual section
  on the standard type hierarchy or see the attribute descriptions in
  the <tt class="module"><a href="module-inspect.html">inspect</a></tt> module).
</dl>

<P>

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	54
	55	<H1><A NAME="SECTION009100000000000000000">
	56	7.1 <tt class="module">signal</tt> --
	57	Set handlers for asynchronous events</A>
	58	</H1>
	59
	60	<P>
	61	<A NAME="module-signal"></A>
	62
	63	<P>
	64	This module provides mechanisms to use signal handlers in Python.
65	Some general rules for working with signals and their handlers:
66
67	<P>
68
69	<UL>
70	<LI>A handler for a particular signal, once set, remains installed until
71	it is explicitly reset (Python emulates the BSD style interface
72	regardless of the underlying implementation), with the exception of
73	the handler for <tt class="constant">SIGCHLD</tt>, which follows the underlying
74	implementation.
75
76	<P>
77	</LI>
78	<LI>There is no way to ``block'' signals temporarily from critical
79	sections (since this is not supported by all <span class="Unix">Unix</span> flavors).
80
81	<P>
82	</LI>
83	<LI>Although Python signal handlers are called asynchronously as far as
84	the Python user is concerned, they can only occur between the
85	``atomic'' instructions of the Python interpreter. This means that
86	signals arriving during long calculations implemented purely in C
87	(such as regular expression matches on large bodies of text) may be
88	delayed for an arbitrary amount of time.
89
90	<P>
91	</LI>
92	<LI>When a signal arrives during an I/O operation, it is possible that the
93	I/O operation raises an exception after the signal handler returns.
94	This is dependent on the underlying <span class="Unix">Unix</span> system's semantics regarding
95	interrupted system calls.
96
97	<P>
98	</LI>
99	<LI>Because the C signal handler always returns, it makes little sense to
100	catch synchronous errors like <tt class="constant">SIGFPE</tt> or <tt class="constant">SIGSEGV</tt>.
101
102	<P>
103	</LI>
104	<LI>Python installs a small number of signal handlers by default:
105	<tt class="constant">SIGPIPE</tt> is ignored (so write errors on pipes and sockets can be
106	reported as ordinary Python exceptions) and <tt class="constant">SIGINT</tt> is translated
107	into a <tt class="exception">KeyboardInterrupt</tt> exception. All of these can be
108	overridden.
109
110	<P>
111	</LI>
112	<LI>Some care must be taken if both signals and threads are used in the
113	same program. The fundamental thing to remember in using signals and
114	threads simultaneously is: always perform <tt class="function">signal()</tt> operations
115	in the main thread of execution. Any thread can perform an
116	<tt class="function">alarm()</tt>, <tt class="function">getsignal()</tt>, or <tt class="function">pause()</tt>;
117	only the main thread can set a new signal handler, and the main thread
118	will be the only one to receive signals (this is enforced by the
119	Python <tt class="module">signal</tt> module, even if the underlying thread
120	implementation supports sending signals to individual threads). This
121	means that signals can't be used as a means of inter-thread
122	communication. Use locks instead.
123
124	<P>
125	</LI>
126	</UL>
127
128	<P>
129	The variables defined in the <tt class="module">signal</tt> module are:
130
131	<P>
132	<dl><dt><b><tt id='l2h-2588' xml:id='l2h-2588'>SIG_DFL</tt></b></dt>
133	<dd>
134	This is one of two standard signal handling options; it will simply
135	perform the default function for the signal. For example, on most
136	systems the default action for <tt class="constant">SIGQUIT</tt> is to dump core
137	and exit, while the default action for <tt class="constant">SIGCLD</tt> is to
138	simply ignore it.
139	</dd></dl>
140
141	<P>
142	<dl><dt><b><tt id='l2h-2589' xml:id='l2h-2589'>SIG_IGN</tt></b></dt>
143	<dd>
144	This is another standard signal handler, which will simply ignore
145	the given signal.
146	</dd></dl>
147
148	<P>
149	<dl><dt><b><tt id='l2h-2590' xml:id='l2h-2590'>SIG*</tt></b></dt>
150	<dd>
151	All the signal numbers are defined symbolically. For example, the
152	hangup signal is defined as <tt class="constant">signal.SIGHUP</tt>; the variable names
153	are identical to the names used in C programs, as found in
154	<code><signal.h></code>.
155	The <span class="Unix">Unix</span> man page for `<tt class="cfunction">signal()</tt>' lists the existing
156	signals (on some systems this is <span class="manpage"><i>signal</i>(2)</span>, on others the
157	list is in <span class="manpage"><i>signal</i>(7)</span>).
158	Note that not all systems define the same set of signal names; only
159	those names defined by the system are defined by this module.
160	</dd></dl>
161
162	<P>
163	<dl><dt><b><tt id='l2h-2591' xml:id='l2h-2591'>NSIG</tt></b></dt>
164	<dd>
165	One more than the number of the highest signal number.
166	</dd></dl>
167
168	<P>
169	The <tt class="module">signal</tt> module defines the following functions:
170
171	<P>
172	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
173	<td><nobr><b><tt id='l2h-2592' xml:id='l2h-2592' class="function">alarm</tt></b>(</nobr></td>
174	<td><var>time</var>)</td></tr></table></dt>
175	<dd>
176	If <var>time</var> is non-zero, this function requests that a
177	<tt class="constant">SIGALRM</tt> signal be sent to the process in <var>time</var> seconds.
178	Any previously scheduled alarm is canceled (only one alarm can
179	be scheduled at any time). The returned value is then the number of
180	seconds before any previously set alarm was to have been delivered.
181	If <var>time</var> is zero, no alarm id scheduled, and any scheduled
182	alarm is canceled. The return value is the number of seconds
183	remaining before a previously scheduled alarm. If the return value
184	is zero, no alarm is currently scheduled. (See the <span class="Unix">Unix</span> man page
185	<span class="manpage"><i>alarm</i>(2)</span>.)
186	Availability: <span class="Unix">Unix</span>.
187	</dl>
188
189	<P>
190	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
191	<td><nobr><b><tt id='l2h-2593' xml:id='l2h-2593' class="function">getsignal</tt></b>(</nobr></td>
192	<td><var>signalnum</var>)</td></tr></table></dt>
193	<dd>
194	Return the current signal handler for the signal <var>signalnum</var>.
195	The returned value may be a callable Python object, or one of the
196	special values <tt class="constant">signal.SIG_IGN</tt>, <tt class="constant">signal.SIG_DFL</tt> or
197	<tt class="constant">None</tt>. Here, <tt class="constant">signal.SIG_IGN</tt> means that the
198	signal was previously ignored, <tt class="constant">signal.SIG_DFL</tt> means that the
199	default way of handling the signal was previously in use, and
200	<code>None</code> means that the previous signal handler was not installed
201	from Python.
202	</dl>
203
204	<P>
205	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
206	<td><nobr><b><tt id='l2h-2594' xml:id='l2h-2594' class="function">pause</tt></b>(</nobr></td>
207	<td><var></var>)</td></tr></table></dt>
208	<dd>
209	Cause the process to sleep until a signal is received; the
210	appropriate handler will then be called. Returns nothing. Not on
211	Windows. (See the <span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.)
212	</dl>
213
214	<P>
215	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
216	<td><nobr><b><tt id='l2h-2595' xml:id='l2h-2595' class="function">signal</tt></b>(</nobr></td>
217	<td><var>signalnum, handler</var>)</td></tr></table></dt>
218	<dd>
219	Set the handler for signal <var>signalnum</var> to the function
220	<var>handler</var>. <var>handler</var> can be a callable Python object
221	taking two arguments (see below), or
222	one of the special values <tt class="constant">signal.SIG_IGN</tt> or
223	<tt class="constant">signal.SIG_DFL</tt>. The previous signal handler will be returned
224	(see the description of <tt class="function">getsignal()</tt> above). (See the
225	<span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.)
226
227	<P>
228	When threads are enabled, this function can only be called from the
229	main thread; attempting to call it from other threads will cause a
230	<tt class="exception">ValueError</tt> exception to be raised.
231
232	<P>
233	The <var>handler</var> is called with two arguments: the signal number
234	and the current stack frame (<code>None</code> or a frame object;
235	for a description of frame objects, see the reference manual section
236	on the standard type hierarchy or see the attribute descriptions in
237	the <tt class="module"><a href="module-inspect.html">inspect</a></tt> module).
238	</dl>
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