CINXE.COM

<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 3143 – Standard daemon process library | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3143/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 3143 – Standard daemon process library | peps.python.org'> <meta property="og:description" content="Writing a program to become a well-behaved Unix daemon is somewhat complex and tricky to get right, yet the steps are largely similar for any daemon regardless of what else the program may need to do."> <meta property="og:type" content="website"> <meta property="og:url" content="https://peps.python.org/pep-3143/"> <meta property="og:site_name" content="Python Enhancement Proposals (PEPs)"> <meta property="og:image" content="https://peps.python.org/_static/og-image.png"> <meta property="og:image:alt" content="Python PEPs"> <meta property="og:image:width" content="200"> <meta property="og:image:height" content="200"> <meta name="description" content="Writing a program to become a well-behaved Unix daemon is somewhat complex and tricky to get right, yet the steps are largely similar for any daemon regardless of what else the program may need to do."> <meta name="theme-color" content="#3776ab"> </head> <body> <svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <symbol id="svg-sun-half" viewBox="0 0 24 24" pointer-events="all"> <title>Following system colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="9"></circle> <path d="M12 3v18m0-12l4.65-4.65M12 14.3l7.37-7.37M12 19.6l8.85-8.85"></path> </svg> </symbol> <symbol id="svg-moon" viewBox="0 0 24 24" pointer-events="all"> <title>Selected dark colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <path stroke="none" d="M0 0h24v24H0z" fill="none"></path> <path d="M12 3c.132 0 .263 0 .393 0a7.5 7.5 0 0 0 7.92 12.446a9 9 0 1 1 -8.313 -12.454z"></path> </svg> </symbol> <symbol id="svg-sun" viewBox="0 0 24 24" pointer-events="all"> <title>Selected light colour scheme</title> <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"> <circle cx="12" cy="12" r="5"></circle> <line x1="12" y1="1" x2="12" y2="3"></line> <line x1="12" y1="21" x2="12" y2="23"></line> <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line> <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line> <line x1="1" y1="12" x2="3" y2="12"></line> <line x1="21" y1="12" x2="23" y2="12"></line> <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line> <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line> </svg> </symbol> </svg> <script> document.documentElement.dataset.colour_scheme = localStorage.getItem("colour_scheme") || "auto" </script> <section id="pep-page-section"> <header> <h1>Python Enhancement Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 3143</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> Toggle light / dark / auto colour theme </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 3143 – Standard daemon process library</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author:</dt> <dd class="field-odd">Ben Finney <ben+python at benfinney.id.au></dd> <dt class="field-even">Status:</dt> <dd class="field-even"><abbr title="Inactive draft that may be taken up again at a later time">Deferred</abbr></dd> <dt class="field-odd">Type:</dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Created:</dt> <dd class="field-even">26-Jan-2009</dd> <dt class="field-odd">Python-Version:</dt> <dd class="field-odd">3.x</dd> <dt class="field-even">Post-History:</dt> <dd class="field-even"></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#pep-deferral">PEP Deferral</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#example-usage">Example usage</a></li> <li><a class="reference internal" href="#interface">Interface</a></li> <li><a class="reference internal" href="#daemoncontext-objects"><code class="docutils literal notranslate">DaemonContext</code> objects</a></li> </ul> </li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#correct-daemon-behaviour">Correct daemon behaviour</a></li> <li><a class="reference internal" href="#a-daemon-is-not-a-service">A daemon is not a service</a></li> </ul> </li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a><ul> <li><a class="reference internal" href="#other-daemon-implementations">Other daemon implementations</a></li> </ul> </li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> Writing a program to become a well-behaved Unix daemon is somewhat complex and tricky to get right, yet the steps are largely similar for any daemon regardless of what else the program may need to do. This PEP introduces a package to the Python standard library that provides a simple interface to the task of becoming a daemon process. </section> <section id="pep-deferral"> <h2><a class="toc-backref" href="#pep-deferral" role="doc-backlink">PEP Deferral</a></h2> Further exploration of the concepts covered in this PEP has been deferred for lack of a current champion interested in promoting the goals of the PEP and collecting and incorporating feedback, and with sufficient available time to do so effectively. </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <section id="example-usage"> <h3><a class="toc-backref" href="#example-usage" role="doc-backlink">Example usage</a></h3> Simple example of direct <code class="docutils literal notranslate">DaemonContext</code> usage: <div class="highlight-default notranslate"><div class="highlight"><pre>import daemon from spam import do_main_program with daemon.DaemonContext(): do_main_program() </pre></div> </div> More complex example usage: <div class="highlight-default notranslate"><div class="highlight"><pre>import os import grp import signal import daemon import lockfile from spam import ( initial_program_setup, do_main_program, program_cleanup, reload_program_config, ) context = daemon.DaemonContext( working_directory='/var/lib/foo', umask=0o002, pidfile=lockfile.FileLock('/var/run/spam.pid'), ) context.signal_map = { signal.SIGTERM: program_cleanup, signal.SIGHUP: 'terminate', signal.SIGUSR1: reload_program_config, } mail_gid = grp.getgrnam('mail').gr_gid context.gid = mail_gid important_file = open('spam.data', 'w') interesting_file = open('eggs.data', 'w') context.files_preserve = [important_file, interesting_file] initial_program_setup() with context: do_main_program() </pre></div> </div> </section> <section id="interface"> <h3><a class="toc-backref" href="#interface" role="doc-backlink">Interface</a></h3> A new package, <code class="docutils literal notranslate">daemon</code>, is added to the standard library. A class, <code class="docutils literal notranslate">DaemonContext</code>, is defined to represent the settings and process context for the program running as a daemon process. </section> <section id="daemoncontext-objects"> <h3><a class="toc-backref" href="#daemoncontext-objects" role="doc-backlink"><code class="docutils literal notranslate">DaemonContext</code> objects</a></h3> A <code class="docutils literal notranslate">DaemonContext</code> instance represents the behaviour settings and process context for the program when it becomes a daemon. The behaviour and environment is customised by setting options on the instance, before calling the <code class="docutils literal notranslate">open</code> method. Each option can be passed as a keyword argument to the <code class="docutils literal notranslate">DaemonContext</code> constructor, or subsequently altered by assigning to an attribute on the instance at any time prior to calling <code class="docutils literal notranslate">open</code>. That is, for options named <code class="docutils literal notranslate">wibble</code> and <code class="docutils literal notranslate">wubble</code>, the following invocation: <div class="highlight-default notranslate"><div class="highlight"><pre>foo = daemon.DaemonContext(wibble=bar, wubble=baz) foo.open() </pre></div> </div> is equivalent to: <div class="highlight-default notranslate"><div class="highlight"><pre>foo = daemon.DaemonContext() foo.wibble = bar foo.wubble = baz foo.open() </pre></div> </div> The following options are defined. <dl> <dt><code class="docutils literal notranslate">files_preserve</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> List of files that should not be closed when starting the daemon. If <code class="docutils literal notranslate">None</code>, all open file descriptors will be closed. Elements of the list are file descriptors (as returned by a file object’s <code class="docutils literal notranslate">fileno()</code> method) or Python <code class="docutils literal notranslate">file</code> objects. Each specifies a file that is not to be closed during daemon start. </dd> <dt><code class="docutils literal notranslate">chroot_directory</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Full path to a directory to set as the effective root directory of the process. If <code class="docutils literal notranslate">None</code>, specifies that the root directory is not to be changed. </dd> <dt><code class="docutils literal notranslate">working_directory</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">'/'</code></dd> </dl> Full path of the working directory to which the process should change on daemon start. Since a filesystem cannot be unmounted if a process has its current working directory on that filesystem, this should either be left at default or set to a directory that is a sensible “home directory” for the daemon while it is running. </dd> <dt><code class="docutils literal notranslate">umask</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">0</code></dd> </dl> File access creation mask (“umask”) to set for the process on daemon start. Since a process inherits its umask from its parent process, starting the daemon will reset the umask to this value so that files are created by the daemon with access modes as it expects. </dd> <dt><code class="docutils literal notranslate">pidfile</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Context manager for a PID lock file. When the daemon context opens and closes, it enters and exits the <code class="docutils literal notranslate">pidfile</code> context manager. </dd> <dt><code class="docutils literal notranslate">detach_process</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> If <code class="docutils literal notranslate">True</code>, detach the process context when opening the daemon context; if <code class="docutils literal notranslate">False</code>, do not detach. If unspecified (<code class="docutils literal notranslate">None</code>) during initialisation of the instance, this will be set to <code class="docutils literal notranslate">True</code> by default, and <code class="docutils literal notranslate">False</code> only if detaching the process is determined to be redundant; for example, in the case when the process was started by <code class="docutils literal notranslate">init</code>, by <code class="docutils literal notranslate">initd</code>, or by <code class="docutils literal notranslate">inetd</code>. </dd> <dt><code class="docutils literal notranslate">signal_map</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd">system-dependent</dd> </dl> Mapping from operating system signals to callback actions. The mapping is used when the daemon context opens, and determines the action for each signal’s signal handler: <ul class="simple"> <li>A value of <code class="docutils literal notranslate">None</code> will ignore the signal (by setting the signal action to <code class="docutils literal notranslate">signal.SIG_IGN</code>).</li> <li>A string value will be used as the name of an attribute on the <code class="docutils literal notranslate">DaemonContext</code> instance. The attribute’s value will be used as the action for the signal handler.</li> <li>Any other value will be used as the action for the signal handler.</li> </ul> The default value depends on which signals are defined on the running system. Each item from the list below whose signal is actually defined in the <code class="docutils literal notranslate">signal</code> module will appear in the default map: <ul class="simple"> <li><code class="docutils literal notranslate">signal.SIGTTIN</code>: <code class="docutils literal notranslate">None</code></li> <li><code class="docutils literal notranslate">signal.SIGTTOU</code>: <code class="docutils literal notranslate">None</code></li> <li><code class="docutils literal notranslate">signal.SIGTSTP</code>: <code class="docutils literal notranslate">None</code></li> <li><code class="docutils literal notranslate">signal.SIGTERM</code>: <code class="docutils literal notranslate">'terminate'</code></li> </ul> Depending on how the program will interact with its child processes, it may need to specify a signal map that includes the <code class="docutils literal notranslate">signal.SIGCHLD</code> signal (received when a child process exits). See the specific operating system’s documentation for more detail on how to determine what circumstances dictate the need for signal handlers. </dd> <dt><code class="docutils literal notranslate">uid</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">os.getuid()</code></dd> </dl> </dd> <dt><code class="docutils literal notranslate">gid</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">os.getgid()</code></dd> </dl> The user ID (“UID”) value and group ID (“GID”) value to switch the process to on daemon start. The default values, the real UID and GID of the process, will relinquish any effective privilege elevation inherited by the process. </dd> <dt><code class="docutils literal notranslate">prevent_core</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">True</code></dd> </dl> If true, prevents the generation of core files, in order to avoid leaking sensitive information from daemons run as <code class="docutils literal notranslate">root</code>. </dd> <dt><code class="docutils literal notranslate">stdin</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> </dd> <dt><code class="docutils literal notranslate">stdout</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> </dd> <dt><code class="docutils literal notranslate">stderr</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Default:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Each of <code class="docutils literal notranslate">stdin</code>, <code class="docutils literal notranslate">stdout</code>, and <code class="docutils literal notranslate">stderr</code> is a file-like object which will be used as the new file for the standard I/O stream <code class="docutils literal notranslate">sys.stdin</code>, <code class="docutils literal notranslate">sys.stdout</code>, and <code class="docutils literal notranslate">sys.stderr</code> respectively. The file should therefore be open, with a minimum of mode ‘r’ in the case of <code class="docutils literal notranslate">stdin</code>, and mode ‘w+’ in the case of <code class="docutils literal notranslate">stdout</code> and <code class="docutils literal notranslate">stderr</code>. If the object has a <code class="docutils literal notranslate">fileno()</code> method that returns a file descriptor, the corresponding file will be excluded from being closed during daemon start (that is, it will be treated as though it were listed in <code class="docutils literal notranslate">files_preserve</code>). If <code class="docutils literal notranslate">None</code>, the corresponding system stream is re-bound to the file named by <code class="docutils literal notranslate">os.devnull</code>. </dd> </dl> The following methods are defined. <dl> <dt><code class="docutils literal notranslate">open()</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Open the daemon context, turning the current program into a daemon process. This performs the following steps: <ul> <li>If this instance’s <code class="docutils literal notranslate">is_open</code> property is true, return immediately. This makes it safe to call <code class="docutils literal notranslate">open</code> multiple times on an instance.</li> <li>If the <code class="docutils literal notranslate">prevent_core</code> attribute is true, set the resource limits for the process to prevent any core dump from the process.</li> <li>If the <code class="docutils literal notranslate">chroot_directory</code> attribute is not <code class="docutils literal notranslate">None</code>, set the effective root directory of the process to that directory (via <code class="docutils literal notranslate">os.chroot</code>).This allows running the daemon process inside a “chroot gaol” as a means of limiting the system’s exposure to rogue behaviour by the process. Note that the specified directory needs to already be set up for this purpose. </li> <li>Set the process UID and GID to the <code class="docutils literal notranslate">uid</code> and <code class="docutils literal notranslate">gid</code> attribute values.</li> <li>Close all open file descriptors. This excludes those listed in the <code class="docutils literal notranslate">files_preserve</code> attribute, and those that correspond to the <code class="docutils literal notranslate">stdin</code>, <code class="docutils literal notranslate">stdout</code>, or <code class="docutils literal notranslate">stderr</code> attributes.</li> <li>Change current working directory to the path specified by the <code class="docutils literal notranslate">working_directory</code> attribute.</li> <li>Reset the file access creation mask to the value specified by the <code class="docutils literal notranslate">umask</code> attribute.</li> <li>If the <code class="docutils literal notranslate">detach_process</code> option is true, detach the current process into its own process group, and disassociate from any controlling terminal.</li> <li>Set signal handlers as specified by the <code class="docutils literal notranslate">signal_map</code> attribute.</li> <li>If any of the attributes <code class="docutils literal notranslate">stdin</code>, <code class="docutils literal notranslate">stdout</code>, <code class="docutils literal notranslate">stderr</code> are not <code class="docutils literal notranslate">None</code>, bind the system streams <code class="docutils literal notranslate">sys.stdin</code>, <code class="docutils literal notranslate">sys.stdout</code>, and/or <code class="docutils literal notranslate">sys.stderr</code> to the files represented by the corresponding attributes. Where the attribute has a file descriptor, the descriptor is duplicated (instead of re-binding the name).</li> <li>If the <code class="docutils literal notranslate">pidfile</code> attribute is not <code class="docutils literal notranslate">None</code>, enter its context manager.</li> <li>Mark this instance as open (for the purpose of future <code class="docutils literal notranslate">open</code> and <code class="docutils literal notranslate">close</code> calls).</li> <li>Register the <code class="docutils literal notranslate">close</code> method to be called during Python’s exit processing.</li> </ul> When the function returns, the running program is a daemon process. </dd> <dt><code class="docutils literal notranslate">close()</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Close the daemon context. This performs the following steps: <ul class="simple"> <li>If this instance’s <code class="docutils literal notranslate">is_open</code> property is false, return immediately. This makes it safe to call <code class="docutils literal notranslate">close</code> multiple times on an instance.</li> <li>If the <code class="docutils literal notranslate">pidfile</code> attribute is not <code class="docutils literal notranslate">None</code>, exit its context manager.</li> <li>Mark this instance as closed (for the purpose of future <code class="docutils literal notranslate">open</code> and <code class="docutils literal notranslate">close</code> calls).</li> </ul> </dd> <dt><code class="docutils literal notranslate">is_open</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd"><code class="docutils literal notranslate">True</code> if the instance is open, <code class="docutils literal notranslate">False</code> otherwise.</dd> </dl> This property exposes the state indicating whether the instance is currently open. It is <code class="docutils literal notranslate">True</code> if the instance’s <code class="docutils literal notranslate">open</code> method has been called and the <code class="docutils literal notranslate">close</code> method has not subsequently been called. </dd> <dt><code class="docutils literal notranslate">terminate(signal_number, stack_frame)</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd"><code class="docutils literal notranslate">None</code></dd> </dl> Signal handler for the <code class="docutils literal notranslate">signal.SIGTERM</code> signal. Performs the following step: <ul class="simple"> <li>Raise a <code class="docutils literal notranslate">SystemExit</code> exception explaining the signal.</li> </ul> </dd> </dl> The class also implements the context manager protocol via <code class="docutils literal notranslate">__enter__</code> and <code class="docutils literal notranslate">__exit__</code> methods. <dl> <dt><code class="docutils literal notranslate">__enter__()</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd">The <code class="docutils literal notranslate">DaemonContext</code> instance</dd> </dl> Call the instance’s <code class="docutils literal notranslate">open()</code> method, then return the instance. </dd> <dt><code class="docutils literal notranslate">__exit__(exc_type, exc_value, exc_traceback)</code></dt><dd><dl class="field-list simple"> <dt class="field-odd">Return:</dt> <dd class="field-odd"><code class="docutils literal notranslate">True</code> or <code class="docutils literal notranslate">False</code> as defined by the context manager protocol</dd> </dl> Call the instance’s <code class="docutils literal notranslate">close()</code> method, then return <code class="docutils literal notranslate">True</code> if the exception was handled or <code class="docutils literal notranslate">False</code> if it was not. </dd> </dl> </section> </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> The majority of programs written to be Unix daemons either implement behaviour very similar to that in the <a class="reference internal" href="#specification">specification</a>, or are poorly-behaved daemons by the <a class="reference internal" href="#correct-daemon-behaviour">correct daemon behaviour</a>. Since these steps should be much the same in most implementations but are very particular and easy to omit or implement incorrectly, they are a prime target for a standard well-tested implementation in the standard library. </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <section id="correct-daemon-behaviour"> <h3><a class="toc-backref" href="#correct-daemon-behaviour" role="doc-backlink">Correct daemon behaviour</a></h3> According to Stevens in <a class="reference internal" href="#stevens" id="id1">[stevens]</a> §2.6, a program should perform the following steps to become a Unix daemon process. <ul class="simple"> <li>Close all open file descriptors.</li> <li>Change current working directory.</li> <li>Reset the file access creation mask.</li> <li>Run in the background.</li> <li>Disassociate from process group.</li> <li>Ignore terminal I/O signals.</li> <li>Disassociate from control terminal.</li> <li>Don’t reacquire a control terminal.</li> <li>Correctly handle the following circumstances:<ul> <li>Started by System V <code class="docutils literal notranslate">init</code> process.</li> <li>Daemon termination by <code class="docutils literal notranslate">SIGTERM</code> signal.</li> <li>Children generate <code class="docutils literal notranslate">SIGCLD</code> signal.</li> </ul> </li> </ul> The <code class="docutils literal notranslate">daemon</code> tool <a class="reference internal" href="#slack-daemon" id="id2">[slack-daemon]</a> lists (in its summary of features) behaviour that should be performed when turning a program into a well-behaved Unix daemon process. It differs from this PEP’s intent in that it invokes a separate program as a daemon process. The following features are appropriate for a daemon that starts itself once the program is already running: <ul class="simple"> <li>Sets up the correct process context for a daemon.</li> <li>Behaves sensibly when started by <code class="docutils literal notranslate">initd(8)</code> or <code class="docutils literal notranslate">inetd(8)</code>.</li> <li>Revokes any suid or sgid privileges to reduce security risks in case daemon is incorrectly installed with special privileges.</li> <li>Prevents the generation of core files to prevent leaking sensitive information from daemons run as root (optional).</li> <li>Names the daemon by creating and locking a PID file to guarantee that only one daemon with the given name can execute at any given time (optional).</li> <li>Sets the user and group under which to run the daemon (optional, root only).</li> <li>Creates a chroot gaol (optional, root only).</li> <li>Captures the daemon’s stdout and stderr and directs them to syslog (optional).</li> </ul> </section> <section id="a-daemon-is-not-a-service"> <h3><a class="toc-backref" href="#a-daemon-is-not-a-service" role="doc-backlink">A daemon is not a service</a></h3> This PEP addresses only Unix-style daemons, for which the above correct behaviour is relevant, as opposed to comparable behaviours on other operating systems. There is a related concept in many systems, called a “service”. A service differs from the model in this PEP, in that rather than having the current program continue to run as a daemon process, a service starts an additional process to run in the background, and the current process communicates with that additional process via some defined channels. The Unix-style daemon model in this PEP can be used, among other things, to implement the background-process part of a service; but this PEP does not address the other aspects of setting up and managing a service. </section> </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> The <code class="docutils literal notranslate">python-daemon</code> package <a class="reference internal" href="#python-daemon" id="id3">[python-daemon]</a>. <section id="other-daemon-implementations"> <h3><a class="toc-backref" href="#other-daemon-implementations" role="doc-backlink">Other daemon implementations</a></h3> Prior to this PEP, several existing third-party Python libraries or tools implemented some of this PEP’s <a class="reference internal" href="#correct-daemon-behaviour">correct daemon behaviour</a>. The <a class="reference internal" href="#reference-implementation">reference implementation</a> is a fairly direct successor from the following implementations: <ul class="simple"> <li>Many good ideas were contributed by the community to Python cookbook recipes #66012 <a class="reference internal" href="#cookbook-66012" id="id4">[cookbook-66012]</a> and #278731 <a class="reference internal" href="#cookbook-278731" id="id5">[cookbook-278731]</a>.</li> <li>The <code class="docutils literal notranslate">bda.daemon</code> library <a class="reference internal" href="#bda-daemon" id="id6">[bda.daemon]</a> is an implementation of <a class="reference internal" href="#cookbook-66012" id="id7">[cookbook-66012]</a>. It is the predecessor of <a class="reference internal" href="#python-daemon" id="id8">[python-daemon]</a>.</li> </ul> Other Python daemon implementations that differ from this PEP: <ul class="simple"> <li>The <code class="docutils literal notranslate">zdaemon</code> tool <a class="reference internal" href="#zdaemon" id="id9">[zdaemon]</a> was written for the Zope project. Like <a class="reference internal" href="#slack-daemon" id="id10">[slack-daemon]</a>, it differs from this specification because it is used to run another program as a daemon process.</li> <li>The Python library <code class="docutils literal notranslate">daemon</code> <a class="reference internal" href="#clapper-daemon" id="id11">[clapper-daemon]</a> is (according to its homepage) no longer maintained. As of version 1.0.1, it implements the basic steps from <a class="reference internal" href="#stevens" id="id12">[stevens]</a>.</li> <li>The <code class="docutils literal notranslate">daemonize</code> library <a class="reference internal" href="#seutter-daemonize" id="id13">[seutter-daemonize]</a> also implements the basic steps from <a class="reference internal" href="#stevens" id="id14">[stevens]</a>.</li> <li>Ray Burr’s <code class="docutils literal notranslate">daemon.py</code> module <a class="reference internal" href="#burr-daemon" id="id15">[burr-daemon]</a> provides the <a class="reference internal" href="#stevens" id="id16">[stevens]</a> procedure as well as PID file handling and redirection of output to syslog.</li> <li>Twisted <a class="reference internal" href="#twisted" id="id17">[twisted]</a> includes, perhaps unsurprisingly, an implementation of a process daemonisation API that is integrated with the rest of the Twisted framework; it differs significantly from the API in this PEP.</li> <li>The Python <code class="docutils literal notranslate">initd</code> library <a class="reference internal" href="#dagitses-initd" id="id18">[dagitses-initd]</a>, which uses <a class="reference internal" href="#clapper-daemon" id="id19">[clapper-daemon]</a>, implements an equivalent of Unix <code class="docutils literal notranslate">initd(8)</code> for controlling a daemon process.</li> </ul> </section> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <div role="list" class="citation-list"> <div class="citation" id="stevens" role="doc-biblioentry"> <dt class="label" id="stevens">[stevens] (<a href='#id1'>1</a>, <a href='#id12'>2</a>, <a href='#id14'>3</a>, <a href='#id16'>4</a>) </dt> <dd><code class="docutils literal notranslate">Unix Network Programming</code>, W. Richard Stevens, 1994 Prentice Hall.</div> <div class="citation" id="slack-daemon" role="doc-biblioentry"> <dt class="label" id="slack-daemon">[slack-daemon] (<a href='#id2'>1</a>, <a href='#id10'>2</a>) </dt> <dd>The (non-Python) “libslack” implementation of a <code class="docutils literal notranslate">daemon</code> tool <a class="reference external" href="http://www.libslack.org/daemon/">http://www.libslack.org/daemon/</a> by “raf” <<a class="reference external" href="mailto:raf%40raf.org">raf@raf.org</a>>.</div> <div class="citation" id="python-daemon" role="doc-biblioentry"> <dt class="label" id="python-daemon">[python-daemon] (<a href='#id3'>1</a>, <a href='#id8'>2</a>) </dt> <dd>The <code class="docutils literal notranslate">python-daemon</code> library <a class="reference external" href="http://pypi.python.org/pypi/python-daemon/">http://pypi.python.org/pypi/python-daemon/</a> by Ben Finney et al.</div> <div class="citation" id="cookbook-66012" role="doc-biblioentry"> <dt class="label" id="cookbook-66012">[cookbook-66012] (<a href='#id4'>1</a>, <a href='#id7'>2</a>) </dt> <dd>Python Cookbook recipe 66012, “Fork a daemon process on Unix” <a class="reference external" href="http://code.activestate.com/recipes/66012/">http://code.activestate.com/recipes/66012/</a>.</div> <div class="citation" id="cookbook-278731" role="doc-biblioentry"> <dt class="label" id="cookbook-278731">[<a href="#id5">cookbook-278731</a>]</dt> <dd>Python Cookbook recipe 278731, “Creating a daemon the Python way” <a class="reference external" href="http://code.activestate.com/recipes/278731/">http://code.activestate.com/recipes/278731/</a>.</div> <div class="citation" id="bda-daemon" role="doc-biblioentry"> <dt class="label" id="bda-daemon">[<a href="#id6">bda.daemon</a>]</dt> <dd>The <code class="docutils literal notranslate">bda.daemon</code> library <a class="reference external" href="http://pypi.python.org/pypi/bda.daemon/">http://pypi.python.org/pypi/bda.daemon/</a> by Robert Niederreiter et al.</div> <div class="citation" id="zdaemon" role="doc-biblioentry"> <dt class="label" id="zdaemon">[<a href="#id9">zdaemon</a>]</dt> <dd>The <code class="docutils literal notranslate">zdaemon</code> tool <a class="reference external" href="http://pypi.python.org/pypi/zdaemon/">http://pypi.python.org/pypi/zdaemon/</a> by Guido van Rossum et al.</div> <div class="citation" id="clapper-daemon" role="doc-biblioentry"> <dt class="label" id="clapper-daemon">[clapper-daemon] (<a href='#id11'>1</a>, <a href='#id19'>2</a>) </dt> <dd>The <code class="docutils literal notranslate">daemon</code> library <a class="reference external" href="http://pypi.python.org/pypi/daemon/">http://pypi.python.org/pypi/daemon/</a> by Brian Clapper.</div> <div class="citation" id="seutter-daemonize" role="doc-biblioentry"> <dt class="label" id="seutter-daemonize">[<a href="#id13">seutter-daemonize</a>]</dt> <dd>The <code class="docutils literal notranslate">daemonize</code> library <a class="reference external" href="http://daemonize.sourceforge.net/">http://daemonize.sourceforge.net/</a> by Jerry Seutter.</div> <div class="citation" id="burr-daemon" role="doc-biblioentry"> <dt class="label" id="burr-daemon">[<a href="#id15">burr-daemon</a>]</dt> <dd>The <code class="docutils literal notranslate">daemon.py</code> module <a class="reference external" href="http://www.nightmare.com/~ryb/code/daemon.py">http://www.nightmare.com/~ryb/code/daemon.py</a> by Ray Burr.</div> <div class="citation" id="twisted" role="doc-biblioentry"> <dt class="label" id="twisted">[<a href="#id17">twisted</a>]</dt> <dd>The <code class="docutils literal notranslate">Twisted</code> application framework <a class="reference external" href="http://pypi.python.org/pypi/Twisted/">http://pypi.python.org/pypi/Twisted/</a> by Glyph Lefkowitz et al.</div> <div class="citation" id="dagitses-initd" role="doc-biblioentry"> <dt class="label" id="dagitses-initd">[<a href="#id18">dagitses-initd</a>]</dt> <dd>The Python <code class="docutils literal notranslate">initd</code> library <a class="reference external" href="http://pypi.python.org/pypi/initd/">http://pypi.python.org/pypi/initd/</a> by Michael Andreas Dagitses.</div> </div> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> This work is hereby placed in the public domain. To the extent that placing a work in the public domain is not legally possible, the copyright holder hereby grants to all recipients of this work all rights and freedoms that would otherwise be restricted by copyright. </section> </section> <hr class="docutils" /> Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-3143.rst">https://github.com/python/peps/blob/main/peps/pep-3143.rst</a> Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3143.rst">2025-02-01 08:59:27 GMT</a> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#pep-deferral">PEP Deferral</a></li> <li><a class="reference internal" href="#specification">Specification</a><ul> <li><a class="reference internal" href="#example-usage">Example usage</a></li> <li><a class="reference internal" href="#interface">Interface</a></li> <li><a class="reference internal" href="#daemoncontext-objects"><code class="docutils literal notranslate">DaemonContext</code> objects</a></li> </ul> </li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#rationale">Rationale</a><ul> <li><a class="reference internal" href="#correct-daemon-behaviour">Correct daemon behaviour</a></li> <li><a class="reference internal" href="#a-daemon-is-not-a-service">A daemon is not a service</a></li> </ul> </li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a><ul> <li><a class="reference internal" href="#other-daemon-implementations">Other daemon implementations</a></li> </ul> </li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-3143.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>