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<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code> — Set handlers for asynchronous events</a><ul>
<li><a class="reference internal" href="#general-rules">General rules</a><ul>
<li><a class="reference internal" href="#execution-of-python-signal-handlers">Execution of Python signal handlers</a></li>
<li><a class="reference internal" href="#signals-and-threads">Signals and threads</a></li>
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<li><a class="reference internal" href="#note-on-sigpipe">Note on SIGPIPE</a></li>
<li><a class="reference internal" href="#note-on-signal-handlers-and-exceptions">Note on Signal Handlers and Exceptions</a></li>
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<section id="module-signal">
<span id="signal-set-handlers-for-asynchronous-events"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code> — Set handlers for asynchronous events<a class="headerlink" href="#module-signal" title="Link to this heading"></a></h1>
<p><strong>Source code:</strong> <a class="extlink-source reference external" href="https://github.com/python/cpython/tree/3.13/Lib/signal.py">Lib/signal.py</a></p>
<hr class="docutils" />
<p>This module provides mechanisms to use signal handlers in Python.</p>
<section id="general-rules">
<h2>General rules<a class="headerlink" href="#general-rules" title="Link to this heading"></a></h2>
<p>The <a class="reference internal" href="#signal.signal" title="signal.signal"><code class="xref py py-func docutils literal notranslate"><span class="pre">signal.signal()</span></code></a> function allows defining custom handlers to be
executed when a signal is received. A small number of default handlers are
installed: <a class="reference internal" href="#signal.SIGPIPE" title="signal.SIGPIPE"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code></a> is ignored (so write errors on pipes and sockets
can be reported as ordinary Python exceptions) and <a class="reference internal" href="#signal.SIGINT" title="signal.SIGINT"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code></a> is
translated into a <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> exception if the parent process
has not changed it.</p>
<p>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
<a class="reference internal" href="#signal.SIGCHLD" title="signal.SIGCHLD"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGCHLD</span></code></a>, which follows the underlying implementation.</p>
<p>On WebAssembly platforms, signals are emulated and therefore behave
differently. Several functions and signals are not available on these
platforms.</p>
<section id="execution-of-python-signal-handlers">
<h3>Execution of Python signal handlers<a class="headerlink" href="#execution-of-python-signal-handlers" title="Link to this heading"></a></h3>
<p>A Python signal handler does not get executed inside the low-level (C) signal
handler. Instead, the low-level signal handler sets a flag which tells the
<a class="reference internal" href="../glossary.html#term-virtual-machine"><span class="xref std std-term">virtual machine</span></a> to execute the corresponding Python signal handler
at a later point(for example at the next <a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a> instruction).
This has consequences:</p>
<ul class="simple">
<li><p>It makes little sense to catch synchronous errors like <a class="reference internal" href="#signal.SIGFPE" title="signal.SIGFPE"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGFPE</span></code></a> or
<a class="reference internal" href="#signal.SIGSEGV" title="signal.SIGSEGV"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGSEGV</span></code></a> that are caused by an invalid operation in C code. Python
will return from the signal handler to the C code, which is likely to raise
the same signal again, causing Python to apparently hang. From Python 3.3
onwards, you can use the <a class="reference internal" href="faulthandler.html#module-faulthandler" title="faulthandler: Dump the Python traceback."><code class="xref py py-mod docutils literal notranslate"><span class="pre">faulthandler</span></code></a> module to report on synchronous
errors.</p></li>
<li><p>A long-running calculation implemented purely in C (such as regular
expression matching on a large body of text) may run uninterrupted for an
arbitrary amount of time, regardless of any signals received. The Python
signal handlers will be called when the calculation finishes.</p></li>
<li><p>If the handler raises an exception, it will be raised “out of thin air” in
the main thread. See the <a class="reference internal" href="#handlers-and-exceptions"><span class="std std-ref">note below</span></a> for a
discussion.</p></li>
</ul>
</section>
<section id="signals-and-threads">
<span id="id1"></span><h3>Signals and threads<a class="headerlink" href="#signals-and-threads" title="Link to this heading"></a></h3>
<p>Python signal handlers are always executed in the main Python thread of the main interpreter,
even if the signal was received in another thread. This means that signals
can’t be used as a means of inter-thread communication. You can use
the synchronization primitives from the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module instead.</p>
<p>Besides, only the main thread of the main interpreter is allowed to set a new signal handler.</p>
</section>
</section>
<section id="module-contents">
<h2>Module contents<a class="headerlink" href="#module-contents" title="Link to this heading"></a></h2>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>signal (SIG*), handler (<a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_DFL</span></code></a>, <a class="reference internal" href="#signal.SIG_IGN" title="signal.SIG_IGN"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_IGN</span></code></a>) and sigmask
(<a class="reference internal" href="#signal.SIG_BLOCK" title="signal.SIG_BLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_BLOCK</span></code></a>, <a class="reference internal" href="#signal.SIG_UNBLOCK" title="signal.SIG_UNBLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_UNBLOCK</span></code></a>, <a class="reference internal" href="#signal.SIG_SETMASK" title="signal.SIG_SETMASK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_SETMASK</span></code></a>)
related constants listed below were turned into
<a class="reference internal" href="enum.html#enum.IntEnum" title="enum.IntEnum"><code class="xref py py-class docutils literal notranslate"><span class="pre">enums</span></code></a> (<a class="reference internal" href="#signal.Signals" title="signal.Signals"><code class="xref py py-class docutils literal notranslate"><span class="pre">Signals</span></code></a>, <a class="reference internal" href="#signal.Handlers" title="signal.Handlers"><code class="xref py py-class docutils literal notranslate"><span class="pre">Handlers</span></code></a> and <a class="reference internal" href="#signal.Sigmasks" title="signal.Sigmasks"><code class="xref py py-class docutils literal notranslate"><span class="pre">Sigmasks</span></code></a> respectively).
<a class="reference internal" href="#signal.getsignal" title="signal.getsignal"><code class="xref py py-func docutils literal notranslate"><span class="pre">getsignal()</span></code></a>, <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a>, <a class="reference internal" href="#signal.sigpending" title="signal.sigpending"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigpending()</span></code></a> and
<a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a> functions return human-readable
<a class="reference internal" href="enum.html#enum.IntEnum" title="enum.IntEnum"><code class="xref py py-class docutils literal notranslate"><span class="pre">enums</span></code></a> as <a class="reference internal" href="#signal.Signals" title="signal.Signals"><code class="xref py py-class docutils literal notranslate"><span class="pre">Signals</span></code></a> objects.</p>
</div>
<p>The signal module defines three enums:</p>
<dl class="py class">
<dt class="sig sig-object py" id="signal.Signals">
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">Signals</span></span><a class="headerlink" href="#signal.Signals" title="Link to this definition"></a></dt>
<dd><p><a class="reference internal" href="enum.html#enum.IntEnum" title="enum.IntEnum"><code class="xref py py-class docutils literal notranslate"><span class="pre">enum.IntEnum</span></code></a> collection of SIG* constants and the CTRL_* constants.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="py class">
<dt class="sig sig-object py" id="signal.Handlers">
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">Handlers</span></span><a class="headerlink" href="#signal.Handlers" title="Link to this definition"></a></dt>
<dd><p><a class="reference internal" href="enum.html#enum.IntEnum" title="enum.IntEnum"><code class="xref py py-class docutils literal notranslate"><span class="pre">enum.IntEnum</span></code></a> collection the constants <a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_DFL</span></code></a> and <a class="reference internal" href="#signal.SIG_IGN" title="signal.SIG_IGN"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_IGN</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="py class">
<dt class="sig sig-object py" id="signal.Sigmasks">
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">Sigmasks</span></span><a class="headerlink" href="#signal.Sigmasks" title="Link to this definition"></a></dt>
<dd><p><a class="reference internal" href="enum.html#enum.IntEnum" title="enum.IntEnum"><code class="xref py py-class docutils literal notranslate"><span class="pre">enum.IntEnum</span></code></a> collection the constants <a class="reference internal" href="#signal.SIG_BLOCK" title="signal.SIG_BLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_BLOCK</span></code></a>, <a class="reference internal" href="#signal.SIG_UNBLOCK" title="signal.SIG_UNBLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_UNBLOCK</span></code></a> and <a class="reference internal" href="#signal.SIG_SETMASK" title="signal.SIG_SETMASK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_SETMASK</span></code></a>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigprocmask(2)">sigprocmask(2)</a></em> and
<em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/pthread_sigmask(3)">pthread_sigmask(3)</a></em> for further information.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.5.</span></p>
</div>
</dd></dl>
<p>The variables defined in the <a class="reference internal" href="#module-signal" title="signal: Set handlers for asynchronous events."><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code></a> module are:</p>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIG_DFL">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIG_DFL</span></span><a class="headerlink" href="#signal.SIG_DFL" title="Link to this definition"></a></dt>
<dd><p>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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGQUIT</span></code> is to dump core and exit, while the
default action for <a class="reference internal" href="#signal.SIGCHLD" title="signal.SIGCHLD"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGCHLD</span></code></a> is to simply ignore it.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIG_IGN">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIG_IGN</span></span><a class="headerlink" href="#signal.SIG_IGN" title="Link to this definition"></a></dt>
<dd><p>This is another standard signal handler, which will simply ignore the given
signal.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGABRT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGABRT</span></span><a class="headerlink" href="#signal.SIGABRT" title="Link to this definition"></a></dt>
<dd><p>Abort signal from <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/abort(3)">abort(3)</a></em>.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGALRM">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGALRM</span></span><a class="headerlink" href="#signal.SIGALRM" title="Link to this definition"></a></dt>
<dd><p>Timer signal from <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/alarm(2)">alarm(2)</a></em>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGBREAK">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGBREAK</span></span><a class="headerlink" href="#signal.SIGBREAK" title="Link to this definition"></a></dt>
<dd><p>Interrupt from keyboard (CTRL + BREAK).</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGBUS">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGBUS</span></span><a class="headerlink" href="#signal.SIGBUS" title="Link to this definition"></a></dt>
<dd><p>Bus error (bad memory access).</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGCHLD">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGCHLD</span></span><a class="headerlink" href="#signal.SIGCHLD" title="Link to this definition"></a></dt>
<dd><p>Child process stopped or terminated.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGCLD">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGCLD</span></span><a class="headerlink" href="#signal.SIGCLD" title="Link to this definition"></a></dt>
<dd><p>Alias to <a class="reference internal" href="#signal.SIGCHLD" title="signal.SIGCHLD"><code class="xref py py-data docutils literal notranslate"><span class="pre">SIGCHLD</span></code></a>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: not macOS.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGCONT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGCONT</span></span><a class="headerlink" href="#signal.SIGCONT" title="Link to this definition"></a></dt>
<dd><p>Continue the process if it is currently stopped</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGFPE">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGFPE</span></span><a class="headerlink" href="#signal.SIGFPE" title="Link to this definition"></a></dt>
<dd><p>Floating-point exception. For example, division by zero.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a> is raised when the second argument of a division
or modulo operation is zero.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGHUP">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGHUP</span></span><a class="headerlink" href="#signal.SIGHUP" title="Link to this definition"></a></dt>
<dd><p>Hangup detected on controlling terminal or death of controlling process.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGILL">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGILL</span></span><a class="headerlink" href="#signal.SIGILL" title="Link to this definition"></a></dt>
<dd><p>Illegal instruction.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGINT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGINT</span></span><a class="headerlink" href="#signal.SIGINT" title="Link to this definition"></a></dt>
<dd><p>Interrupt from keyboard (CTRL + C).</p>
<p>Default action is to raise <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a>.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGKILL">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGKILL</span></span><a class="headerlink" href="#signal.SIGKILL" title="Link to this definition"></a></dt>
<dd><p>Kill signal.</p>
<p>It cannot be caught, blocked, or ignored.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGPIPE">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGPIPE</span></span><a class="headerlink" href="#signal.SIGPIPE" title="Link to this definition"></a></dt>
<dd><p>Broken pipe: write to pipe with no readers.</p>
<p>Default action is to ignore the signal.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGSEGV">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGSEGV</span></span><a class="headerlink" href="#signal.SIGSEGV" title="Link to this definition"></a></dt>
<dd><p>Segmentation fault: invalid memory reference.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGSTKFLT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGSTKFLT</span></span><a class="headerlink" href="#signal.SIGSTKFLT" title="Link to this definition"></a></dt>
<dd><blockquote>
<div><p>Stack fault on coprocessor. The Linux kernel does not raise this signal: it
can only be raised in user space.</p>
</div></blockquote>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Linux.</p>
<p>On architectures where the signal is available. See
the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/signal(7)">signal(7)</a></em> for further information.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.11.</span></p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGTERM">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGTERM</span></span><a class="headerlink" href="#signal.SIGTERM" title="Link to this definition"></a></dt>
<dd><p>Termination signal.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGUSR1">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGUSR1</span></span><a class="headerlink" href="#signal.SIGUSR1" title="Link to this definition"></a></dt>
<dd><p>User-defined signal 1.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGUSR2">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGUSR2</span></span><a class="headerlink" href="#signal.SIGUSR2" title="Link to this definition"></a></dt>
<dd><p>User-defined signal 2.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIGWINCH">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIGWINCH</span></span><a class="headerlink" href="#signal.SIGWINCH" title="Link to this definition"></a></dt>
<dd><p>Window resize signal.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py">
<span class="sig-name descname"><span class="pre">SIG*</span></span></dt>
<dd><p>All the signal numbers are defined symbolically. For example, the hangup signal
is defined as <a class="reference internal" href="#signal.SIGHUP" title="signal.SIGHUP"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGHUP</span></code></a>; the variable names are identical to the
names used in C programs, as found in <code class="docutils literal notranslate"><span class="pre">&lt;signal.h&gt;</span></code>. The Unix man page for
‘<code class="xref c c-func docutils literal notranslate"><span class="pre">signal()</span></code>’ lists the existing signals (on some systems this is
<em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/signal(2)">signal(2)</a></em>, on others the list is in <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/signal(7)">signal(7)</a></em>). Note that
not all systems define the same set of signal names; only those names defined by
the system are defined by this module.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.CTRL_C_EVENT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">CTRL_C_EVENT</span></span><a class="headerlink" href="#signal.CTRL_C_EVENT" title="Link to this definition"></a></dt>
<dd><p>The signal corresponding to the <kbd class="kbd docutils literal notranslate">Ctrl</kbd>+<kbd class="kbd docutils literal notranslate">C</kbd> keystroke event. This signal can
only be used with <a class="reference internal" href="os.html#os.kill" title="os.kill"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.kill()</span></code></a>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.CTRL_BREAK_EVENT">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">CTRL_BREAK_EVENT</span></span><a class="headerlink" href="#signal.CTRL_BREAK_EVENT" title="Link to this definition"></a></dt>
<dd><p>The signal corresponding to the <kbd class="kbd docutils literal notranslate">Ctrl</kbd>+<kbd class="kbd docutils literal notranslate">Break</kbd> keystroke event. This signal can
only be used with <a class="reference internal" href="os.html#os.kill" title="os.kill"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.kill()</span></code></a>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.NSIG">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">NSIG</span></span><a class="headerlink" href="#signal.NSIG" title="Link to this definition"></a></dt>
<dd><p>One more than the number of the highest signal number.
Use <a class="reference internal" href="#signal.valid_signals" title="signal.valid_signals"><code class="xref py py-func docutils literal notranslate"><span class="pre">valid_signals()</span></code></a> to get valid signal numbers.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.ITIMER_REAL">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">ITIMER_REAL</span></span><a class="headerlink" href="#signal.ITIMER_REAL" title="Link to this definition"></a></dt>
<dd><p>Decrements interval timer in real time, and delivers <a class="reference internal" href="#signal.SIGALRM" title="signal.SIGALRM"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code></a> upon
expiration.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.ITIMER_VIRTUAL">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">ITIMER_VIRTUAL</span></span><a class="headerlink" href="#signal.ITIMER_VIRTUAL" title="Link to this definition"></a></dt>
<dd><p>Decrements interval timer only when the process is executing, and delivers
SIGVTALRM upon expiration.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.ITIMER_PROF">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">ITIMER_PROF</span></span><a class="headerlink" href="#signal.ITIMER_PROF" title="Link to this definition"></a></dt>
<dd><p>Decrements interval timer both when the process executes and when the
system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL,
this timer is usually used to profile the time spent by the application
in user and kernel space. SIGPROF is delivered upon expiration.</p>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIG_BLOCK">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIG_BLOCK</span></span><a class="headerlink" href="#signal.SIG_BLOCK" title="Link to this definition"></a></dt>
<dd><p>A possible value for the <em>how</em> parameter to <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a>
indicating that signals are to be blocked.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIG_UNBLOCK">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIG_UNBLOCK</span></span><a class="headerlink" href="#signal.SIG_UNBLOCK" title="Link to this definition"></a></dt>
<dd><p>A possible value for the <em>how</em> parameter to <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a>
indicating that signals are to be unblocked.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py data">
<dt class="sig sig-object py" id="signal.SIG_SETMASK">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">SIG_SETMASK</span></span><a class="headerlink" href="#signal.SIG_SETMASK" title="Link to this definition"></a></dt>
<dd><p>A possible value for the <em>how</em> parameter to <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a>
indicating that the signal mask is to be replaced.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<p>The <a class="reference internal" href="#module-signal" title="signal: Set handlers for asynchronous events."><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code></a> module defines one exception:</p>
<dl class="py exception">
<dt class="sig sig-object py" id="signal.ItimerError">
<em class="property"><span class="k"><span class="pre">exception</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">ItimerError</span></span><a class="headerlink" href="#signal.ItimerError" title="Link to this definition"></a></dt>
<dd><p>Raised to signal an error from the underlying <a class="reference internal" href="#signal.setitimer" title="signal.setitimer"><code class="xref py py-func docutils literal notranslate"><span class="pre">setitimer()</span></code></a> or
<a class="reference internal" href="#signal.getitimer" title="signal.getitimer"><code class="xref py py-func docutils literal notranslate"><span class="pre">getitimer()</span></code></a> implementation. Expect this error if an invalid
interval timer or a negative time is passed to <a class="reference internal" href="#signal.setitimer" title="signal.setitimer"><code class="xref py py-func docutils literal notranslate"><span class="pre">setitimer()</span></code></a>.
This error is a subtype of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3: </span>This error used to be a subtype of <a class="reference internal" href="exceptions.html#IOError" title="IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</span></code></a>, which is now an
alias of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p>
</div>
</dd></dl>
<p>The <a class="reference internal" href="#module-signal" title="signal: Set handlers for asynchronous events."><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code></a> module defines the following functions:</p>
<dl class="py function">
<dt class="sig sig-object py" id="signal.alarm">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">alarm</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">time</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.alarm" title="Link to this definition"></a></dt>
<dd><p>If <em>time</em> is non-zero, this function requests that a <a class="reference internal" href="#signal.SIGALRM" title="signal.SIGALRM"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code></a> signal be
sent to the process in <em>time</em> 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 <em>time</em> is zero, no alarm is scheduled, and any scheduled alarm is
canceled. If the return value is zero, no alarm is currently scheduled.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/alarm(2)">alarm(2)</a></em> for further information.</p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.getsignal">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">getsignal</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">signalnum</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.getsignal" title="Link to this definition"></a></dt>
<dd><p>Return the current signal handler for the signal <em>signalnum</em>. The returned value
may be a callable Python object, or one of the special values
<a class="reference internal" href="#signal.SIG_IGN" title="signal.SIG_IGN"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_IGN</span></code></a>, <a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_DFL</span></code></a> or <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>. Here,
<a class="reference internal" href="#signal.SIG_IGN" title="signal.SIG_IGN"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_IGN</span></code></a> means that the signal was previously ignored,
<a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_DFL</span></code></a> means that the default way of handling the signal was
previously in use, and <code class="docutils literal notranslate"><span class="pre">None</span></code> means that the previous signal handler was not
installed from Python.</p>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.strsignal">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">strsignal</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">signalnum</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.strsignal" title="Link to this definition"></a></dt>
<dd><p>Returns the description of signal <em>signalnum</em>, such as “Interrupt”
for <a class="reference internal" href="#signal.SIGINT" title="signal.SIGINT"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code></a>. Returns <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> if <em>signalnum</em> has no
description. Raises <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if <em>signalnum</em> is invalid.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.8.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.valid_signals">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">valid_signals</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#signal.valid_signals" title="Link to this definition"></a></dt>
<dd><p>Return the set of valid signal numbers on this platform. This can be
less than <code class="docutils literal notranslate"><span class="pre">range(1,</span> <span class="pre">NSIG)</span></code> if some signals are reserved by the system
for internal use.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.8.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.pause">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">pause</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#signal.pause" title="Link to this definition"></a></dt>
<dd><p>Cause the process to sleep until a signal is received; the appropriate handler
will then be called. Returns nothing.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/signal(2)">signal(2)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a>, <a class="reference internal" href="#signal.sigwaitinfo" title="signal.sigwaitinfo"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwaitinfo()</span></code></a>, <a class="reference internal" href="#signal.sigtimedwait" title="signal.sigtimedwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigtimedwait()</span></code></a> and
<a class="reference internal" href="#signal.sigpending" title="signal.sigpending"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigpending()</span></code></a>.</p>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.raise_signal">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">raise_signal</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">signum</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.raise_signal" title="Link to this definition"></a></dt>
<dd><p>Sends a signal to the calling process. Returns nothing.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.8.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.pidfd_send_signal">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">pidfd_send_signal</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">pidfd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">sig</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">siginfo</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">flags</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.pidfd_send_signal" title="Link to this definition"></a></dt>
<dd><p>Send signal <em>sig</em> to the process referred to by file descriptor <em>pidfd</em>.
Python does not currently support the <em>siginfo</em> parameter; it must be
<code class="docutils literal notranslate"><span class="pre">None</span></code>. The <em>flags</em> argument is provided for future extensions; no flag
values are currently defined.</p>
<p>See the <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/pidfd_send_signal(2)">pidfd_send_signal(2)</a></em> man page for more information.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Linux &gt;= 5.1, Android &gt;= <a class="reference internal" href="sys.html#sys.getandroidapilevel" title="sys.getandroidapilevel"><code class="xref py py-func docutils literal notranslate"><span class="pre">build-time</span></code></a> API level 31</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.9.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.pthread_kill">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">pthread_kill</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">thread_id</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">signalnum</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.pthread_kill" title="Link to this definition"></a></dt>
<dd><p>Send the signal <em>signalnum</em> to the thread <em>thread_id</em>, another thread in the
same process as the caller. The target thread can be executing any code
(Python or not). However, if the target thread is executing the Python
interpreter, the Python signal handlers will be <a class="reference internal" href="#signals-and-threads"><span class="std std-ref">executed by the main
thread of the main interpreter</span></a>. Therefore, the only point of sending a
signal to a particular Python thread would be to force a running system call
to fail with <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p>
<p>Use <a class="reference internal" href="threading.html#threading.get_ident" title="threading.get_ident"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.get_ident()</span></code></a> or the <a class="reference internal" href="threading.html#threading.Thread.ident" title="threading.Thread.ident"><code class="xref py py-attr docutils literal notranslate"><span class="pre">ident</span></code></a>
attribute of <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a> objects to get a suitable value
for <em>thread_id</em>.</p>
<p>If <em>signalnum</em> is 0, then no signal is sent, but error checking is still
performed; this can be used to check if the target thread is still running.</p>
<p class="audit-hook">Raises an <a class="reference internal" href="sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">signal.pthread_kill</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">thread_id</span></code>, <code class="docutils literal notranslate"><span class="pre">signalnum</span></code>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/pthread_kill(3)">pthread_kill(3)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="os.html#os.kill" title="os.kill"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.kill()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.pthread_sigmask">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">pthread_sigmask</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">how</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">mask</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.pthread_sigmask" title="Link to this definition"></a></dt>
<dd><p>Fetch and/or change the signal mask of the calling thread. The signal mask
is the set of signals whose delivery is currently blocked for the caller.
Return the old signal mask as a set of signals.</p>
<p>The behavior of the call is dependent on the value of <em>how</em>, as follows.</p>
<ul class="simple">
<li><p><a class="reference internal" href="#signal.SIG_BLOCK" title="signal.SIG_BLOCK"><code class="xref py py-data docutils literal notranslate"><span class="pre">SIG_BLOCK</span></code></a>: The set of blocked signals is the union of the current
set and the <em>mask</em> argument.</p></li>
<li><p><a class="reference internal" href="#signal.SIG_UNBLOCK" title="signal.SIG_UNBLOCK"><code class="xref py py-data docutils literal notranslate"><span class="pre">SIG_UNBLOCK</span></code></a>: The signals in <em>mask</em> are removed from the current
set of blocked signals. It is permissible to attempt to unblock a
signal which is not blocked.</p></li>
<li><p><a class="reference internal" href="#signal.SIG_SETMASK" title="signal.SIG_SETMASK"><code class="xref py py-data docutils literal notranslate"><span class="pre">SIG_SETMASK</span></code></a>: The set of blocked signals is set to the <em>mask</em>
argument.</p></li>
</ul>
<p><em>mask</em> is a set of signal numbers (e.g. {<a class="reference internal" href="#signal.SIGINT" title="signal.SIGINT"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGINT</span></code></a>,
<a class="reference internal" href="#signal.SIGTERM" title="signal.SIGTERM"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGTERM</span></code></a>}). Use <a class="reference internal" href="#signal.valid_signals" title="signal.valid_signals"><code class="xref py py-func docutils literal notranslate"><span class="pre">valid_signals()</span></code></a> for a full
mask including all signals.</p>
<p>For example, <code class="docutils literal notranslate"><span class="pre">signal.pthread_sigmask(signal.SIG_BLOCK,</span> <span class="pre">[])</span></code> reads the
signal mask of the calling thread.</p>
<p><a class="reference internal" href="#signal.SIGKILL" title="signal.SIGKILL"><code class="xref py py-data docutils literal notranslate"><span class="pre">SIGKILL</span></code></a> and <code class="xref py py-data docutils literal notranslate"><span class="pre">SIGSTOP</span></code> cannot be blocked.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigprocmask(2)">sigprocmask(2)</a></em> and
<em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/pthread_sigmask(3)">pthread_sigmask(3)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.pause" title="signal.pause"><code class="xref py py-func docutils literal notranslate"><span class="pre">pause()</span></code></a>, <a class="reference internal" href="#signal.sigpending" title="signal.sigpending"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigpending()</span></code></a> and <a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.setitimer">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">setitimer</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">which</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">seconds</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">interval</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0.0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.setitimer" title="Link to this definition"></a></dt>
<dd><p>Sets given interval timer (one of <a class="reference internal" href="#signal.ITIMER_REAL" title="signal.ITIMER_REAL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_REAL</span></code></a>,
<a class="reference internal" href="#signal.ITIMER_VIRTUAL" title="signal.ITIMER_VIRTUAL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_VIRTUAL</span></code></a> or <a class="reference internal" href="#signal.ITIMER_PROF" title="signal.ITIMER_PROF"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_PROF</span></code></a>) specified
by <em>which</em> to fire after <em>seconds</em> (float is accepted, different from
<a class="reference internal" href="#signal.alarm" title="signal.alarm"><code class="xref py py-func docutils literal notranslate"><span class="pre">alarm()</span></code></a>) and after that every <em>interval</em> seconds (if <em>interval</em>
is non-zero). The interval timer specified by <em>which</em> can be cleared by
setting <em>seconds</em> to zero.</p>
<p>When an interval timer fires, a signal is sent to the process.
The signal sent is dependent on the timer being used;
<a class="reference internal" href="#signal.ITIMER_REAL" title="signal.ITIMER_REAL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_REAL</span></code></a> will deliver <a class="reference internal" href="#signal.SIGALRM" title="signal.SIGALRM"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code></a>,
<a class="reference internal" href="#signal.ITIMER_VIRTUAL" title="signal.ITIMER_VIRTUAL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_VIRTUAL</span></code></a> sends <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGVTALRM</span></code>,
and <a class="reference internal" href="#signal.ITIMER_PROF" title="signal.ITIMER_PROF"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.ITIMER_PROF</span></code></a> will deliver <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPROF</span></code>.</p>
<p>The old values are returned as a tuple: (delay, interval).</p>
<p>Attempting to pass an invalid interval timer will cause an
<a class="reference internal" href="#signal.ItimerError" title="signal.ItimerError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ItimerError</span></code></a>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.getitimer">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">getitimer</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">which</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.getitimer" title="Link to this definition"></a></dt>
<dd><p>Returns current value of a given interval timer specified by <em>which</em>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.set_wakeup_fd">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">set_wakeup_fd</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fd</span></span></em>, <em class="sig-param"><span class="keyword-only-separator o"><abbr title="Keyword-only parameters separator (PEP 3102)"><span class="pre">*</span></abbr></span></em>, <em class="sig-param"><span class="n"><span class="pre">warn_on_full_buffer</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.set_wakeup_fd" title="Link to this definition"></a></dt>
<dd><p>Set the wakeup file descriptor to <em>fd</em>. When a signal is received, the
signal number is written as a single byte into the fd. This can be used by
a library to wakeup a poll or select call, allowing the signal to be fully
processed.</p>
<p>The old wakeup fd is returned (or -1 if file descriptor wakeup was not
enabled). If <em>fd</em> is -1, file descriptor wakeup is disabled.
If not -1, <em>fd</em> must be non-blocking. It is up to the library to remove
any bytes from <em>fd</em> before calling poll or select again.</p>
<p>When threads are enabled, this function can only be called
from <a class="reference internal" href="#signals-and-threads"><span class="std std-ref">the main thread of the main interpreter</span></a>;
attempting to call it from other threads will cause a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>
exception to be raised.</p>
<p>There are two common ways to use this function. In both approaches,
you use the fd to wake up when a signal arrives, but then they
differ in how they determine <em>which</em> signal or signals have
arrived.</p>
<p>In the first approach, we read the data out of the fd’s buffer, and
the byte values give you the signal numbers. This is simple, but in
rare cases it can run into a problem: generally the fd will have a
limited amount of buffer space, and if too many signals arrive too
quickly, then the buffer may become full, and some signals may be
lost. If you use this approach, then you should set
<code class="docutils literal notranslate"><span class="pre">warn_on_full_buffer=True</span></code>, which will at least cause a warning
to be printed to stderr when signals are lost.</p>
<p>In the second approach, we use the wakeup fd <em>only</em> for wakeups,
and ignore the actual byte values. In this case, all we care about
is whether the fd’s buffer is empty or non-empty; a full buffer
doesn’t indicate a problem at all. If you use this approach, then
you should set <code class="docutils literal notranslate"><span class="pre">warn_on_full_buffer=False</span></code>, so that your users
are not confused by spurious warning messages.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>On Windows, the function now also supports socket handles.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>Added <code class="docutils literal notranslate"><span class="pre">warn_on_full_buffer</span></code> parameter.</p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.siginterrupt">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">siginterrupt</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">signalnum</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">flag</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.siginterrupt" title="Link to this definition"></a></dt>
<dd><p>Change system call restart behaviour: if <em>flag</em> is <a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a>, system
calls will be restarted when interrupted by signal <em>signalnum</em>, otherwise
system calls will be interrupted. Returns nothing.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/siginterrupt(3)">siginterrupt(3)</a></em> for further information.</p>
</div>
<p>Note that installing a signal handler with <a class="reference internal" href="#module-signal" title="signal: Set handlers for asynchronous events."><code class="xref py py-func docutils literal notranslate"><span class="pre">signal()</span></code></a> will reset the
restart behaviour to interruptible by implicitly calling
<code class="xref c c-func docutils literal notranslate"><span class="pre">siginterrupt()</span></code> with a true <em>flag</em> value for the given signal.</p>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.signal">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">signal</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">signalnum</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">handler</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.signal" title="Link to this definition"></a></dt>
<dd><p>Set the handler for signal <em>signalnum</em> to the function <em>handler</em>. <em>handler</em> can
be a callable Python object taking two arguments (see below), or one of the
special values <a class="reference internal" href="#signal.SIG_IGN" title="signal.SIG_IGN"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_IGN</span></code></a> or <a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIG_DFL</span></code></a>. The previous
signal handler will be returned (see the description of <a class="reference internal" href="#signal.getsignal" title="signal.getsignal"><code class="xref py py-func docutils literal notranslate"><span class="pre">getsignal()</span></code></a>
above). (See the Unix man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/signal(2)">signal(2)</a></em> for further information.)</p>
<p>When threads are enabled, this function can only be called
from <a class="reference internal" href="#signals-and-threads"><span class="std std-ref">the main thread of the main interpreter</span></a>;
attempting to call it from other threads will cause a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>
exception to be raised.</p>
<p>The <em>handler</em> is called with two arguments: the signal number and the current
stack frame (<code class="docutils literal notranslate"><span class="pre">None</span></code> or a frame object; for a description of frame objects,
see the <a class="reference internal" href="../reference/datamodel.html#frame-objects"><span class="std std-ref">description in the type hierarchy</span></a> or see the
attribute descriptions in the <a class="reference internal" href="inspect.html#module-inspect" title="inspect: Extract information and source code from live objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">inspect</span></code></a> module).</p>
<p>On Windows, <a class="reference internal" href="#module-signal" title="signal: Set handlers for asynchronous events."><code class="xref py py-func docutils literal notranslate"><span class="pre">signal()</span></code></a> can only be called with <a class="reference internal" href="#signal.SIGABRT" title="signal.SIGABRT"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGABRT</span></code></a>,
<a class="reference internal" href="#signal.SIGFPE" title="signal.SIGFPE"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGFPE</span></code></a>, <a class="reference internal" href="#signal.SIGILL" title="signal.SIGILL"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGILL</span></code></a>, <a class="reference internal" href="#signal.SIGINT" title="signal.SIGINT"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code></a>, <a class="reference internal" href="#signal.SIGSEGV" title="signal.SIGSEGV"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGSEGV</span></code></a>,
<a class="reference internal" href="#signal.SIGTERM" title="signal.SIGTERM"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGTERM</span></code></a>, or <a class="reference internal" href="#signal.SIGBREAK" title="signal.SIGBREAK"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGBREAK</span></code></a>.
A <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> will be raised in any other case.
Note that not all systems define the same set of signal names; an
<a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> will be raised if a signal name is not defined as
<code class="docutils literal notranslate"><span class="pre">SIG*</span></code> module level constant.</p>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.sigpending">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">sigpending</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigpending" title="Link to this definition"></a></dt>
<dd><p>Examine the set of signals that are pending for delivery to the calling
thread (i.e., the signals which have been raised while blocked). Return the
set of the pending signals.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigpending(2)">sigpending(2)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.pause" title="signal.pause"><code class="xref py py-func docutils literal notranslate"><span class="pre">pause()</span></code></a>, <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a> and <a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.sigwait">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">sigwait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">sigset</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigwait" title="Link to this definition"></a></dt>
<dd><p>Suspend execution of the calling thread until the delivery of one of the
signals specified in the signal set <em>sigset</em>. The function accepts the signal
(removes it from the pending list of signals), and returns the signal number.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigwait(3)">sigwait(3)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.pause" title="signal.pause"><code class="xref py py-func docutils literal notranslate"><span class="pre">pause()</span></code></a>, <a class="reference internal" href="#signal.pthread_sigmask" title="signal.pthread_sigmask"><code class="xref py py-func docutils literal notranslate"><span class="pre">pthread_sigmask()</span></code></a>, <a class="reference internal" href="#signal.sigpending" title="signal.sigpending"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigpending()</span></code></a>,
<a class="reference internal" href="#signal.sigwaitinfo" title="signal.sigwaitinfo"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwaitinfo()</span></code></a> and <a class="reference internal" href="#signal.sigtimedwait" title="signal.sigtimedwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigtimedwait()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.sigwaitinfo">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">sigwaitinfo</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">sigset</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigwaitinfo" title="Link to this definition"></a></dt>
<dd><p>Suspend execution of the calling thread until the delivery of one of the
signals specified in the signal set <em>sigset</em>. The function accepts the
signal and removes it from the pending list of signals. If one of the
signals in <em>sigset</em> is already pending for the calling thread, the function
will return immediately with information about that signal. The signal
handler is not called for the delivered signal. The function raises an
<a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a> if it is interrupted by a signal that is not in
<em>sigset</em>.</p>
<p>The return value is an object representing the data contained in the
<code class="xref c c-type docutils literal notranslate"><span class="pre">siginfo_t</span></code> structure, namely: <code class="xref py py-attr docutils literal notranslate"><span class="pre">si_signo</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">si_code</span></code>,
<code class="xref py py-attr docutils literal notranslate"><span class="pre">si_errno</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">si_pid</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">si_uid</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">si_status</span></code>,
<code class="xref py py-attr docutils literal notranslate"><span class="pre">si_band</span></code>.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigwaitinfo(2)">sigwaitinfo(2)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.pause" title="signal.pause"><code class="xref py py-func docutils literal notranslate"><span class="pre">pause()</span></code></a>, <a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a> and <a class="reference internal" href="#signal.sigtimedwait" title="signal.sigtimedwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigtimedwait()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried if interrupted by a signal not in <em>sigset</em>
and the signal handler does not raise an exception (see <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for
the rationale).</p>
</div>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="signal.sigtimedwait">
<span class="sig-prename descclassname"><span class="pre">signal.</span></span><span class="sig-name descname"><span class="pre">sigtimedwait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">sigset</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigtimedwait" title="Link to this definition"></a></dt>
<dd><p>Like <a class="reference internal" href="#signal.sigwaitinfo" title="signal.sigwaitinfo"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwaitinfo()</span></code></a>, but takes an additional <em>timeout</em> argument
specifying a timeout. If <em>timeout</em> is specified as <code class="docutils literal notranslate"><span class="pre">0</span></code>, a poll is
performed. Returns <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> if a timeout occurs.</p>
<div class="availability docutils container">
<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
<p>See the man page <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/sigtimedwait(2)">sigtimedwait(2)</a></em> for further information.</p>
</div>
<p>See also <a class="reference internal" href="#signal.pause" title="signal.pause"><code class="xref py py-func docutils literal notranslate"><span class="pre">pause()</span></code></a>, <a class="reference internal" href="#signal.sigwait" title="signal.sigwait"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwait()</span></code></a> and <a class="reference internal" href="#signal.sigwaitinfo" title="signal.sigwaitinfo"><code class="xref py py-func docutils literal notranslate"><span class="pre">sigwaitinfo()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.3.</span></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with the recomputed <em>timeout</em> if interrupted
by a signal not in <em>sigset</em> and the signal handler does not raise an
exception (see <span class="target" id="index-1"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale).</p>
</div>
</dd></dl>
</section>
<section id="examples">
<span id="signal-example"></span><h2>Examples<a class="headerlink" href="#examples" title="Link to this heading"></a></h2>
<p>Here is a minimal example program. It uses the <a class="reference internal" href="#signal.alarm" title="signal.alarm"><code class="xref py py-func docutils literal notranslate"><span class="pre">alarm()</span></code></a> function to limit
the time spent waiting to open a file; this is useful if the file is for a
serial device that may not be turned on, which would normally cause the
<a class="reference internal" href="os.html#os.open" title="os.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.open()</span></code></a> to hang indefinitely. The solution is to set a 5-second alarm
before opening the file; if the operation takes too long, the alarm signal will
be sent, and the handler raises an exception.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">signal</span><span class="o">,</span><span class="w"> </span><span class="nn">os</span>
<span class="k">def</span><span class="w"> </span><span class="nf">handler</span><span class="p">(</span><span class="n">signum</span><span class="p">,</span> <span class="n">frame</span><span class="p">):</span>
<span class="n">signame</span> <span class="o">=</span> <span class="n">signal</span><span class="o">.</span><span class="n">Signals</span><span class="p">(</span><span class="n">signum</span><span class="p">)</span><span class="o">.</span><span class="n">name</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;Signal handler called with signal </span><span class="si">{</span><span class="n">signame</span><span class="si">}</span><span class="s1"> (</span><span class="si">{</span><span class="n">signum</span><span class="si">}</span><span class="s1">)&#39;</span><span class="p">)</span>
<span class="k">raise</span> <span class="ne">OSError</span><span class="p">(</span><span class="s2">&quot;Couldn&#39;t open device!&quot;</span><span class="p">)</span>
<span class="c1"># Set the signal handler and a 5-second alarm</span>
<span class="n">signal</span><span class="o">.</span><span class="n">signal</span><span class="p">(</span><span class="n">signal</span><span class="o">.</span><span class="n">SIGALRM</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span>
<span class="n">signal</span><span class="o">.</span><span class="n">alarm</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="c1"># This open() may hang indefinitely</span>
<span class="n">fd</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s1">&#39;/dev/ttyS0&#39;</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">O_RDWR</span><span class="p">)</span>
<span class="n">signal</span><span class="o">.</span><span class="n">alarm</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="c1"># Disable the alarm</span>
</pre></div>
</div>
</section>
<section id="note-on-sigpipe">
<h2>Note on SIGPIPE<a class="headerlink" href="#note-on-sigpipe" title="Link to this heading"></a></h2>
<p>Piping output of your program to tools like <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/head(1)">head(1)</a></em> will
cause a <a class="reference internal" href="#signal.SIGPIPE" title="signal.SIGPIPE"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code></a> signal to be sent to your process when the receiver
of its standard output closes early. This results in an exception
like <code class="code docutils literal notranslate"><span class="pre">BrokenPipeError:</span> <span class="pre">[Errno</span> <span class="pre">32]</span> <span class="pre">Broken</span> <span class="pre">pipe</span></code>. To handle this
case, wrap your entry point to catch this exception as follows:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">os</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">sys</span>
<span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
<span class="k">try</span><span class="p">:</span>
<span class="c1"># simulate large output (your code replaces this loop)</span>
<span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10000</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;y&quot;</span><span class="p">)</span>
<span class="c1"># flush output here to force SIGPIPE to be triggered</span>
<span class="c1"># while inside this try block.</span>
<span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">BrokenPipeError</span><span class="p">:</span>
<span class="c1"># Python flushes standard streams on exit; redirect remaining output</span>
<span class="c1"># to devnull to avoid another BrokenPipeError at shutdown</span>
<span class="n">devnull</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">devnull</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">O_WRONLY</span><span class="p">)</span>
<span class="n">os</span><span class="o">.</span><span class="n">dup2</span><span class="p">(</span><span class="n">devnull</span><span class="p">,</span> <span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">fileno</span><span class="p">())</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="c1"># Python exits with error code 1 on EPIPE</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
<p>Do not set <a class="reference internal" href="#signal.SIGPIPE" title="signal.SIGPIPE"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code></a>’s disposition to <a class="reference internal" href="#signal.SIG_DFL" title="signal.SIG_DFL"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIG_DFL</span></code></a> in
order to avoid <a class="reference internal" href="exceptions.html#BrokenPipeError" title="BrokenPipeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenPipeError</span></code></a>. Doing that would cause
your program to exit unexpectedly whenever any socket
connection is interrupted while your program is still writing to
it.</p>
</section>
<section id="note-on-signal-handlers-and-exceptions">
<span id="handlers-and-exceptions"></span><h2>Note on Signal Handlers and Exceptions<a class="headerlink" href="#note-on-signal-handlers-and-exceptions" title="Link to this heading"></a></h2>
<p>If a signal handler raises an exception, the exception will be propagated to
the main thread and may be raised after any <a class="reference internal" href="../glossary.html#term-bytecode"><span class="xref std std-term">bytecode</span></a> instruction. Most
notably, a <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> may appear at any point during execution.
Most Python code, including the standard library, cannot be made robust against
this, and so a <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> (or any other exception resulting from
a signal handler) may on rare occasions put the program in an unexpected state.</p>
<p>To illustrate this issue, consider the following code:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">SpamContext</span><span class="p">:</span>
<span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="k">def</span><span class="w"> </span><span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># If KeyboardInterrupt occurs here, everything is fine</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">acquire</span><span class="p">()</span>
<span class="c1"># If KeyboardInterrupt occurs here, __exit__ will not be called</span>
<span class="o">...</span>
<span class="c1"># KeyboardInterrupt could occur just before the function returns</span>
<span class="k">def</span><span class="w"> </span><span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_val</span><span class="p">,</span> <span class="n">exc_tb</span><span class="p">):</span>
<span class="o">...</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="o">.</span><span class="n">release</span><span class="p">()</span>
</pre></div>
</div>
<p>For many programs, especially those that merely want to exit on
<a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a>, this is not a problem, but applications that are
complex or require high reliability should avoid raising exceptions from signal
handlers. They should also avoid catching <a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> as a means
of gracefully shutting down. Instead, they should install their own
<a class="reference internal" href="#signal.SIGINT" title="signal.SIGINT"><code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code></a> handler. Below is an example of an HTTP server that avoids
<a class="reference internal" href="exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">signal</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">socket</span>
<span class="kn">from</span><span class="w"> </span><span class="nn">selectors</span><span class="w"> </span><span class="kn">import</span> <span class="n">DefaultSelector</span><span class="p">,</span> <span class="n">EVENT_READ</span>
<span class="kn">from</span><span class="w"> </span><span class="nn">http.server</span><span class="w"> </span><span class="kn">import</span> <span class="n">HTTPServer</span><span class="p">,</span> <span class="n">SimpleHTTPRequestHandler</span>
<span class="n">interrupt_read</span><span class="p">,</span> <span class="n">interrupt_write</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socketpair</span><span class="p">()</span>
<span class="k">def</span><span class="w"> </span><span class="nf">handler</span><span class="p">(</span><span class="n">signum</span><span class="p">,</span> <span class="n">frame</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Signal handler called with signal&#39;</span><span class="p">,</span> <span class="n">signum</span><span class="p">)</span>
<span class="n">interrupt_write</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="sa">b</span><span class="s1">&#39;</span><span class="se">\0</span><span class="s1">&#39;</span><span class="p">)</span>
<span class="n">signal</span><span class="o">.</span><span class="n">signal</span><span class="p">(</span><span class="n">signal</span><span class="o">.</span><span class="n">SIGINT</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span>
<span class="k">def</span><span class="w"> </span><span class="nf">serve_forever</span><span class="p">(</span><span class="n">httpd</span><span class="p">):</span>
<span class="n">sel</span> <span class="o">=</span> <span class="n">DefaultSelector</span><span class="p">()</span>
<span class="n">sel</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">interrupt_read</span><span class="p">,</span> <span class="n">EVENT_READ</span><span class="p">)</span>
<span class="n">sel</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">httpd</span><span class="p">,</span> <span class="n">EVENT_READ</span><span class="p">)</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">_</span> <span class="ow">in</span> <span class="n">sel</span><span class="o">.</span><span class="n">select</span><span class="p">():</span>
<span class="k">if</span> <span class="n">key</span><span class="o">.</span><span class="n">fileobj</span> <span class="o">==</span> <span class="n">interrupt_read</span><span class="p">:</span>
<span class="n">interrupt_read</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">return</span>
<span class="k">if</span> <span class="n">key</span><span class="o">.</span><span class="n">fileobj</span> <span class="o">==</span> <span class="n">httpd</span><span class="p">:</span>
<span class="n">httpd</span><span class="o">.</span><span class="n">handle_request</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Serving on port 8000&quot;</span><span class="p">)</span>
<span class="n">httpd</span> <span class="o">=</span> <span class="n">HTTPServer</span><span class="p">((</span><span class="s1">&#39;&#39;</span><span class="p">,</span> <span class="mi">8000</span><span class="p">),</span> <span class="n">SimpleHTTPRequestHandler</span><span class="p">)</span>
<span class="n">serve_forever</span><span class="p">(</span><span class="n">httpd</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Shutdown...&quot;</span><span class="p">)</span>
</pre></div>
</div>
</section>
</section>
<div class="clearer"></div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="Main">
<div class="sphinxsidebarwrapper">
<div>
<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">signal</span></code> — Set handlers for asynchronous events</a><ul>
<li><a class="reference internal" href="#general-rules">General rules</a><ul>
<li><a class="reference internal" href="#execution-of-python-signal-handlers">Execution of Python signal handlers</a></li>
<li><a class="reference internal" href="#signals-and-threads">Signals and threads</a></li>
</ul>
</li>
<li><a class="reference internal" href="#module-contents">Module contents</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
<li><a class="reference internal" href="#note-on-sigpipe">Note on SIGPIPE</a></li>
<li><a class="reference internal" href="#note-on-signal-handlers-and-exceptions">Note on Signal Handlers and Exceptions</a></li>
</ul>
</li>
</ul>
</div>
<div>
<h4>Previous topic</h4>
<p class="topless"><a href="selectors.html"
title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">selectors</span></code> — High-level I/O multiplexing</a></p>
</div>
<div>
<h4>Next topic</h4>
<p class="topless"><a href="mmap.html"
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