<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta charset="utf-8" /> <title>signal — Set handlers for asynchronous events — Python 3.7.4 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.7.4 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="mmap — Memory-mapped file support" href="mmap.html" /> <link rel="prev" title="asynchat — Asynchronous socket command/response handler" href="asynchat.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/signal.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> <script type="text/javascript" src="../_static/switchers.js"></script> <style> @media only screen { table.full-width-table { width: 100%; } } </style> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="mmap.html" title="mmap — Memory-mapped file support" accesskey="N">next</a> |</li> <li class="right" > <a href="asynchat.html" title="asynchat — Asynchronous socket command/response handler" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <span class="language_switcher_placeholder">en</span> <span class="version_switcher_placeholder">3.7.4</span> <a href="../index.html">Documentation </a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" accesskey="U">Networking and Interprocess Communication</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <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: <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code> is ignored (so write errors on pipes and sockets can be reported as ordinary Python exceptions) and <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code> 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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGCHLD</span></code>, 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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGFPE</span></code> or <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGSEGV</span></code> 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="descclassname">signal.</code><code class="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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGCHLD</span></code> is to simply ignore it.</p> </dd></dl> <dl class="data"> <dt id="signal.SIG_IGN"> <code class="descclassname">signal.</code><code class="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> <code class="descname">SIG*</code></dt> <dd><p>All the signal numbers are defined symbolically. For example, the hangup signal is defined as <code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGHUP</span></code>; 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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code> upon expiration.</p> </dd></dl> <dl class="data"> <dt id="signal.ITIMER_VIRTUAL"> <code class="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="descname">alarm</code><span class="sig-paren">(</span><em>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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code> 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. (See the Unix man page <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="function"> <dt id="signal.getsignal"> <code class="descclassname">signal.</code><code class="descname">getsignal</code><span class="sig-paren">(</span><em>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="descclassname">signal.</code><code class="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. Not on Windows. (See the Unix man page <em class="manpage">signal(2)</em>.)</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="descclassname">signal.</code><code class="descname">pthread_kill</code><span class="sig-paren">(</span><em>thread_id</em>, <em>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="descclassname">signal.</code><code class="descname">pthread_sigmask</code><span class="sig-paren">(</span><em>how</em>, <em>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. {<code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGINT</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">signal.SIGTERM</span></code>}). 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 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="descclassname">signal.</code><code class="descname">setitimer</code><span class="sig-paren">(</span><em>which</em>, <em>seconds</em>, <em>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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGALRM</span></code>, <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="descclassname">signal.</code><code class="descname">getitimer</code><span class="sig-paren">(</span><em>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="descclassname">signal.</code><code class="descname">set_wakeup_fd</code><span class="sig-paren">(</span><em>fd</em>, <em>*</em>, <em>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="descclassname">signal.</code><code class="descname">siginterrupt</code><span class="sig-paren">(</span><em>signalnum</em>, <em>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="descclassname">signal.</code><code class="descname">signal</code><span class="sig-paren">(</span><em>signalnum</em>, <em>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>.)</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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGABRT</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGFPE</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGILL</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGINT</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGSEGV</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGTERM</span></code>, or <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGBREAK</span></code>. 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="descclassname">signal.</code><code class="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="descclassname">signal.</code><code class="descname">sigwait</code><span class="sig-paren">(</span><em>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="descclassname">signal.</code><code class="descname">sigwaitinfo</code><span class="sig-paren">(</span><em>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="descclassname">signal.</code><code class="descname">sigtimedwait</code><span class="sig-paren">(</span><em>sigset</em>, <em>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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code> 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 <code class="xref py py-const docutils literal notranslate"><span class="pre">SIGPIPE</span></code>’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> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <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" title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">asynchat</span></code> — Asynchronous socket command/response handler</a></p> <h4>Next topic</h4> <p class="topless"><a href="mmap.html" title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">mmap</span></code> — Memory-mapped file support</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.7/Doc/library/signal.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="mmap.html" title="mmap — Memory-mapped file support" >next</a> |</li> <li class="right" > <a href="asynchat.html" title="asynchat — Asynchronous socket command/response handler" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <span class="language_switcher_placeholder">en</span> <span class="version_switcher_placeholder">3.7.4</span> <a href="../index.html">Documentation </a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" >Networking and Interprocess Communication</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 2001-2019, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Jul 13, 2019. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 2.0.1. </div> </body> </html>