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<div class="section" id="module-signal">
<span id="signal-set-handlers-for-asynchronous-events"></span><h1><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> — Set handlers for asynchronous events<a class="headerlink" href="#module-signal" title="Permalink to this headline">¶</a></h1>
<hr class="docutils" />
<p>This module provides mechanisms to use signal handlers in Python.</p>
<div class="section" id="general-rules">
<h2>General rules<a class="headerlink" href="#general-rules" title="Permalink to this headline">¶</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>
<div class="section" id="execution-of-python-signal-handlers">
<h3>Execution of Python signal handlers<a class="headerlink" href="#execution-of-python-signal-handlers" title="Permalink to this headline">¶</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>
</ul>
</div>
<div class="section" id="signals-and-threads">
<span id="id1"></span><h3>Signals and threads<a class="headerlink" href="#signals-and-threads" title="Permalink to this headline">¶</a></h3>
<p>Python signal handlers are always executed in the main Python thread,
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 is allowed to set a new signal handler.</p>
</div>
</div>
<div class="section" id="module-contents">
<h2>Module contents<a class="headerlink" href="#module-contents" title="Permalink to this headline">¶</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.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>.</p>
</div>
<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="data">
<dt id="signal.SIG_DFL">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIG_DFL</code><a class="headerlink" href="#signal.SIG_DFL" title="Permalink 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="data">
<dt id="signal.SIG_IGN">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIG_IGN</code><a class="headerlink" href="#signal.SIG_IGN" title="Permalink 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="data">
<dt id="signal.SIGABRT">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGABRT</code><a class="headerlink" href="#signal.SIGABRT" title="Permalink to this definition">¶</a></dt>
<dd><p>Abort signal from <em class="manpage">abort(3)</em>.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGALRM">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGALRM</code><a class="headerlink" href="#signal.SIGALRM" title="Permalink to this definition">¶</a></dt>
<dd><p>Timer signal from <em class="manpage">alarm(2)</em>.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGBREAK">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGBREAK</code><a class="headerlink" href="#signal.SIGBREAK" title="Permalink to this definition">¶</a></dt>
<dd><p>Interrupt from keyboard (CTRL + BREAK).</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGBUS">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGBUS</code><a class="headerlink" href="#signal.SIGBUS" title="Permalink to this definition">¶</a></dt>
<dd><p>Bus error (bad memory access).</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGCHLD">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGCHLD</code><a class="headerlink" href="#signal.SIGCHLD" title="Permalink to this definition">¶</a></dt>
<dd><p>Child process stopped or terminated.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGCLD">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGCLD</code><a class="headerlink" href="#signal.SIGCLD" title="Permalink 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>
</dd></dl>
<dl class="data">
<dt id="signal.SIGCONT">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGCONT</code><a class="headerlink" href="#signal.SIGCONT" title="Permalink to this definition">¶</a></dt>
<dd><p>Continue the process if it is currently stopped</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGFPE">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGFPE</code><a class="headerlink" href="#signal.SIGFPE" title="Permalink 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="data">
<dt id="signal.SIGHUP">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGHUP</code><a class="headerlink" href="#signal.SIGHUP" title="Permalink to this definition">¶</a></dt>
<dd><p>Hangup detected on controlling terminal or death of controlling process.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGILL">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGILL</code><a class="headerlink" href="#signal.SIGILL" title="Permalink to this definition">¶</a></dt>
<dd><p>Illegal instruction.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGINT">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGINT</code><a class="headerlink" href="#signal.SIGINT" title="Permalink 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="data">
<dt id="signal.SIGKILL">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGKILL</code><a class="headerlink" href="#signal.SIGKILL" title="Permalink to this definition">¶</a></dt>
<dd><p>Kill signal.</p>
<p>It cannot be caught, blocked, or ignored.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGPIPE">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGPIPE</code><a class="headerlink" href="#signal.SIGPIPE" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGSEGV">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGSEGV</code><a class="headerlink" href="#signal.SIGSEGV" title="Permalink to this definition">¶</a></dt>
<dd><p>Segmentation fault: invalid memory reference.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGTERM">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGTERM</code><a class="headerlink" href="#signal.SIGTERM" title="Permalink to this definition">¶</a></dt>
<dd><p>Termination signal.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGUSR1">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGUSR1</code><a class="headerlink" href="#signal.SIGUSR1" title="Permalink to this definition">¶</a></dt>
<dd><p>User-defined signal 1.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGUSR2">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGUSR2</code><a class="headerlink" href="#signal.SIGUSR2" title="Permalink to this definition">¶</a></dt>
<dd><p>User-defined signal 2.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt id="signal.SIGWINCH">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIGWINCH</code><a class="headerlink" href="#signal.SIGWINCH" title="Permalink to this definition">¶</a></dt>
<dd><p>Window resize signal.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="data">
<dt>
<code class="sig-name descname">SIG*</code></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"><signal.h></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">signal(2)</em>, on others the list is in <em class="manpage">signal(7)</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="data">
<dt id="signal.CTRL_C_EVENT">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">CTRL_C_EVENT</code><a class="headerlink" href="#signal.CTRL_C_EVENT" title="Permalink to this definition">¶</a></dt>
<dd><p>The signal corresponding to the <kbd class="kbd docutils literal notranslate">Ctrl+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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="signal.CTRL_BREAK_EVENT">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">CTRL_BREAK_EVENT</code><a class="headerlink" href="#signal.CTRL_BREAK_EVENT" title="Permalink to this definition">¶</a></dt>
<dd><p>The signal corresponding to the <kbd class="kbd docutils literal notranslate">Ctrl+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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Windows.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="signal.NSIG">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">NSIG</code><a class="headerlink" href="#signal.NSIG" title="Permalink to this definition">¶</a></dt>
<dd><p>One more than the number of the highest signal number.</p>
</dd></dl>
<dl class="data">
<dt id="signal.ITIMER_REAL">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">ITIMER_REAL</code><a class="headerlink" href="#signal.ITIMER_REAL" title="Permalink 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="data">
<dt id="signal.ITIMER_VIRTUAL">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">ITIMER_VIRTUAL</code><a class="headerlink" href="#signal.ITIMER_VIRTUAL" title="Permalink 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="data">
<dt id="signal.ITIMER_PROF">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">ITIMER_PROF</code><a class="headerlink" href="#signal.ITIMER_PROF" title="Permalink 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="data">
<dt id="signal.SIG_BLOCK">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIG_BLOCK</code><a class="headerlink" href="#signal.SIG_BLOCK" title="Permalink 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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="signal.SIG_UNBLOCK">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIG_UNBLOCK</code><a class="headerlink" href="#signal.SIG_UNBLOCK" title="Permalink 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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="signal.SIG_SETMASK">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">SIG_SETMASK</code><a class="headerlink" href="#signal.SIG_SETMASK" title="Permalink 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">New 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="exception">
<dt id="signal.ItimerError">
<em class="property">exception </em><code class="sig-prename descclassname">signal.</code><code class="sig-name descname">ItimerError</code><a class="headerlink" href="#signal.ItimerError" title="Permalink 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">New 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="function">
<dt id="signal.alarm">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">alarm</code><span class="sig-paren">(</span><em class="sig-param">time</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.alarm" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">alarm(2)</em> for further
information.</p>
</dd></dl>
<dl class="function">
<dt id="signal.getsignal">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">getsignal</code><span class="sig-paren">(</span><em class="sig-param">signalnum</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.getsignal" title="Permalink 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="function">
<dt id="signal.pause">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">pause</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#signal.pause" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">signal(2)</em> for further
information.</p>
<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="function">
<dt id="signal.pthread_kill">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">pthread_kill</code><span class="sig-paren">(</span><em class="sig-param">thread_id</em>, <em class="sig-param">signalnum</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.pthread_kill" title="Permalink 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</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="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">pthread_kill(3)</em> for further
information.</p>
<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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="signal.pthread_sigmask">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">pthread_sigmask</code><span class="sig-paren">(</span><em class="sig-param">how</em>, <em class="sig-param">mask</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.pthread_sigmask" title="Permalink 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 <code class="docutils literal notranslate"><span class="pre">range(1,</span> <span class="pre">signal.NSIG)</span></code> 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">sigprocmask(3)</em> and
<em class="manpage">pthread_sigmask(3)</em> for further information.</p>
<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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="signal.setitimer">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">setitimer</code><span class="sig-paren">(</span><em class="sig-param">which</em>, <em class="sig-param">seconds</em>, <em class="sig-param">interval=0.0</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.setitimer" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="function">
<dt id="signal.getitimer">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">getitimer</code><span class="sig-paren">(</span><em class="sig-param">which</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.getitimer" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns current value of a given interval timer specified by <em>which</em>.</p>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix.</p>
</dd></dl>
<dl class="function">
<dt id="signal.set_wakeup_fd">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">set_wakeup_fd</code><span class="sig-paren">(</span><em class="sig-param">fd</em>, <em class="sig-param">*</em>, <em class="sig-param">warn_on_full_buffer=True</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.set_wakeup_fd" title="Permalink 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 the main thread;
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="function">
<dt id="signal.siginterrupt">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">siginterrupt</code><span class="sig-paren">(</span><em class="sig-param">signalnum</em>, <em class="sig-param">flag</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.siginterrupt" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">siginterrupt(3)</em>
for further information.</p>
<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="function">
<dt id="signal.signal">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">signal</code><span class="sig-paren">(</span><em class="sig-param">signalnum</em>, <em class="sig-param">handler</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.signal" title="Permalink 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">signal(2)</em> for further information.)</p>
<p>When threads are enabled, this function can only be called from the main thread;
attempting to call it from other threads will cause a <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="function">
<dt id="signal.sigpending">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">sigpending</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigpending" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">sigpending(2)</em> for further
information.</p>
<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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="signal.sigwait">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">sigwait</code><span class="sig-paren">(</span><em class="sig-param">sigset</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigwait" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">sigwait(3)</em> for further
information.</p>
<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">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="signal.sigwaitinfo">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">sigwaitinfo</code><span class="sig-paren">(</span><em class="sig-param">sigset</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigwaitinfo" title="Permalink 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">sigwaitinfo(2)</em> for further
information.</p>
<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">New 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://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for
the rationale).</p>
</div>
</dd></dl>
<dl class="function">
<dt id="signal.sigtimedwait">
<code class="sig-prename descclassname">signal.</code><code class="sig-name descname">sigtimedwait</code><span class="sig-paren">(</span><em class="sig-param">sigset</em>, <em class="sig-param">timeout</em><span class="sig-paren">)</span><a class="headerlink" href="#signal.sigtimedwait" title="Permalink 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="xref py py-const 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>
<p class="availability"><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix. See the man page <em class="manpage">sigtimedwait(2)</em> for further
information.</p>
<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">New 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://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale).</p>
</div>
</dd></dl>
</div>
<div class="section" id="example">
<span id="signal-example"></span><h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</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="nn">signal</span><span class="o">,</span> <span class="nn">os</span>
<span class="k">def</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">'Signal handler called with signal'</span><span class="p">,</span> <span class="n">signum</span><span class="p">)</span>
<span class="k">raise</span> <span class="ne">OSError</span><span class="p">(</span><span class="s2">"Couldn't open device!"</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">'/dev/ttyS0'</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>
</div>
<div class="section" id="note-on-sigpipe">
<h2>Note on SIGPIPE<a class="headerlink" href="#note-on-sigpipe" title="Permalink to this headline">¶</a></h2>
<p>Piping output of your program to tools like <em class="manpage">head(1)</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="nn">os</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">def</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">"y"</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">'__main__'</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 also whenever any socket connection
is interrupted while your program is still writing to it.</p>
</div>
</div>
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<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="#example">Example</a></li>
<li><a class="reference internal" href="#note-on-sigpipe">Note on SIGPIPE</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="asynchat.html"
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<p class="topless"><a href="mmap.html"
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