man/sd_notify.xml - main - niceSOFT/systemd

man/sd_notify.xml
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Luca Boccassi
docs: document LUO support
15 май 2026, 15:46
15 май 2026, 15:460854ac4
<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="sd_notify"
  xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_notify</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_notify</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_notify</refname>
    <refname>sd_notifyf</refname>
    <refname>sd_pid_notify</refname>
    <refname>sd_pid_notifyf</refname>
    <refname>sd_pid_notify_with_fds</refname>
    <refname>sd_pid_notifyf_with_fds</refname>
    <refname>sd_notify_barrier</refname>
    <refname>sd_pid_notify_barrier</refname>
    <refpurpose>Notify service manager about start-up completion and other service status changes</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_notify</function></funcdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const char *<parameter>state</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_notifyf</function></funcdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const char *<parameter>format</parameter></paramdef>
        <paramdef>…</paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_pid_notify</function></funcdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const char *<parameter>state</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_pid_notifyf</function></funcdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const char *<parameter>format</parameter></paramdef>
        <paramdef>…</paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_pid_notify_with_fds</function></funcdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const char *<parameter>state</parameter></paramdef>
        <paramdef>const int *<parameter>fds</parameter></paramdef>
        <paramdef>unsigned <parameter>n_fds</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_pid_notifyf_with_fds</function></funcdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>const int *<parameter>fds</parameter></paramdef>
        <paramdef>size_t <parameter>n_fds</parameter></paramdef>
        <paramdef>const char *<parameter>format</parameter></paramdef>
        <paramdef>…</paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_notify_barrier</function></funcdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>uint64_t <parameter>timeout</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_pid_notify_barrier</function></funcdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>uint64_t <parameter>timeout</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_notify()</function> may be called by a service to notify the service manager about
    state changes. It can be used to send arbitrary information, encoded in an environment-block-like string.
    Most importantly, it can be used for start-up or reload completion notifications.</para>

    <para>If the <parameter>unset_environment</parameter> parameter is non-zero,
    <function>sd_notify()</function> will unset the <varname>$NOTIFY_SOCKET</varname> environment variable
    before returning (regardless of whether the function call itself succeeded or not). Further calls to
    <function>sd_notify()</function> will then silently do nothing, and the variable is no longer inherited
    by child processes.</para>

    <para>The <parameter>state</parameter> parameter should contain a newline-separated list of variable
    assignments, similar in style to an environment block. A trailing newline is implied if none is
    specified. The string may contain any kind of variable assignments, but see the next section
    for a list of assignments understood by the service manager.</para>

    <para>Note that systemd will accept status data sent from a service only if the
    <varname>NotifyAccess=</varname> option is correctly set in the service definition file. See
    <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
    details.</para>

    <para>Note that <function>sd_notify()</function> notifications may be attributed to units correctly only
    if either the sending process is still around at the time PID 1 processes the message, or if the sending
    process is explicitly runtime-tracked by the service manager. The latter is the case if the service
    manager originally forked off the process, i.e. on all processes that match
    <varname>NotifyAccess=</varname><option>main</option> or
    <varname>NotifyAccess=</varname><option>exec</option>. Conversely, if an auxiliary process of the unit
    sends an <function>sd_notify()</function> message and immediately exits, the service manager might not be
    able to properly attribute the message to the unit, and thus will ignore it, even if
    <varname>NotifyAccess=</varname><option>all</option> is set for it.</para>

    <para>Hence, to eliminate all race conditions involving lookup of the client's unit and attribution of
    notifications to units correctly, <function>sd_notify_barrier()</function> may be used. This call acts as
    a synchronization point and ensures all notifications sent before this call have been picked up by the
    service manager when it returns successfully. Use of <function>sd_notify_barrier()</function> is needed
    for clients which are not invoked by the service manager, otherwise this synchronization mechanism is
    unnecessary for attribution of notifications to the unit.</para>

    <para><function>sd_notifyf()</function> is similar to <function>sd_notify()</function> but takes a
    <function>printf()</function>-like format string plus arguments.</para>

    <para><function>sd_pid_notify()</function> and <function>sd_pid_notifyf()</function> are similar to
    <function>sd_notify()</function> and <function>sd_notifyf()</function> but take a process ID (PID) to use
    as originating PID for the message as first argument. This is useful to send notification messages on
    behalf of other processes, provided the appropriate privileges are available. Effectively, this means
    that a privileged invocation of <command>sd_pid_notify()</command> may circumvent
    <varname>NotifyAccess=main</varname> or <varname>NotifyAccess=exec</varname> restrictions enforced for a
    service. If the PID argument is specified as 0, the process ID of the calling process is used, in which
    case the calls are fully equivalent to <function>sd_notify()</function> and
    <function>sd_notifyf()</function>.</para>

    <para><function>sd_pid_notify_with_fds()</function> is similar to <function>sd_pid_notify()</function>
    but takes an additional array of file descriptors. These file descriptors are sent along the notification
    message to the service manager. This is particularly useful for sending <literal>FDSTORE=1</literal>
    messages, as described above. The additional arguments are a pointer to the file descriptor array plus
    the number of file descriptors in the array. If the number of file descriptors is passed as 0, the call
    is fully equivalent to <function>sd_pid_notify()</function>, i.e. no file descriptors are passed. Note
    that file descriptors sent to the service manager on a message without <literal>FDSTORE=1</literal> are
    immediately closed on reception.</para>

    <para><function>sd_pid_notifyf_with_fds()</function> is a combination of
    <function>sd_pid_notify_with_fds()</function> and <function>sd_notifyf()</function>, i.e. it accepts both
    a PID and a set of file descriptors as input, and processes a format string to generate the state
    string.</para>

    <para><function>sd_notify_barrier()</function> allows the caller to synchronize against reception of
    previously sent notification messages and uses the <varname>BARRIER=1</varname> command. It takes a
    relative <varname>timeout</varname> value in microseconds which is passed to
    <citerefentry><refentrytitle>ppoll</refentrytitle><manvolnum>2</manvolnum>
    </citerefentry>. A value of UINT64_MAX is interpreted as infinite timeout.</para>

    <para><function>sd_pid_notify_barrier()</function> is just like <function>sd_notify_barrier()</function>,
    but allows specifying the originating PID for the notification message.</para>
  </refsect1>

  <refsect1>
    <title>Well-known assignments</title>

    <para>The following assignments have a defined meaning:</para>

    <variablelist>
      <varlistentry>
        <term>READY=1</term>

        <listitem><para>Tells the service manager that service startup is finished, or the service finished
        re-loading its configuration. This is only used by systemd if the service definition file has
        <varname>Type=notify</varname> or <varname>Type=notify-reload</varname> set. Since there is little
        value in signaling non-readiness, the only value services should send is <literal>READY=1</literal>
        (i.e.  <literal>READY=0</literal> is not defined).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>RELOADING=1</term>

        <listitem><para>Tells the service manager that the service is beginning to reload its configuration.
        This is useful to allow the service manager to track the service's internal state, and present it to
        the user. Note that a service that sends this notification must also send a
        <literal>READY=1</literal> notification when it completed reloading its configuration. Reloads the
        service manager is notified about with this mechanisms are propagated in the same way as they are
        when originally initiated through the service manager. This message is particularly relevant for
        <varname>Type=notify-reload</varname> services, to inform the service manager that the request to
        reload the service has been received and is now being processed.</para>

        <xi:include href="version-info.xml" xpointer="v217"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>STOPPING=1</term>

        <listitem><para>Tells the service manager that the service is beginning its shutdown. This is useful
        to allow the service manager to track the service's internal state, and present it to the user.
        </para>

        <xi:include href="version-info.xml" xpointer="v217"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>MONOTONIC_USEC=…</term>

        <listitem><para>A field carrying the monotonic timestamp (as per
        <constant>CLOCK_MONOTONIC</constant>) formatted in decimal in μs, when the notification message was
        generated by the client. This is typically used in combination with <literal>RELOADING=1</literal>,
        to allow the service manager to properly synchronize reload cycles. See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details, specifically <literal>Type=notify-reload</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>STATUS=…</term>

        <listitem><para>Passes a single-line UTF-8 status string back to the service manager that describes
        the service state. This is free-form and can be used for various purposes: general state feedback,
        fsck-like programs could pass completion percentages and failing programs could pass a human-readable
        error message. Example: <literal>STATUS=Completed 66% of file system check…</literal></para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>NOTIFYACCESS=…</term>

        <listitem><para>Reset the access to the service status notification socket during runtime, overriding
        <varname>NotifyAccess=</varname> setting in the service unit file. See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details, specifically <literal>NotifyAccess=</literal> for a list of accepted values.
        </para>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>ERRNO=…</term>

        <listitem><para>If a service fails, the errno-style error code, formatted as string. Example:
        <literal>ERRNO=2</literal> for ENOENT.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>BUSERROR=…</term>

        <listitem><para>If a service fails, the D-Bus error-style error code. Example:
        <literal>BUSERROR=org.freedesktop.DBus.Error.TimedOut</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>VARLINKERROR=…</term>

        <listitem><para>If a service fails, the Varlink error-style error code. Example:
        <literal>VARLINKERROR=org.varlink.service.InvalidParameter</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>EXIT_STATUS=…</term>

        <listitem><para>The exit status of a service or the manager itself. Note that
        <command>systemd</command> currently does not consume this value when sent by services, so this
        assignment is only informational. The manager will send this notification to <emphasis>its</emphasis>
        notification socket, which may be used to collect an exit status from the system (a container or
        VM) as it shuts down. For example,
        <citerefentry project='debian'><refentrytitle>mkosi</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        makes use of this. The value to return may be set via the
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        <command>exit</command> verb.</para>

        <xi:include href="version-info.xml" xpointer="v254"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>MAINPID=…</term>

        <listitem><para>Change the main process ID (PID) of the service. This is especially useful in the case
        where the real main process is not directly forked off by the service manager.
        Example: <literal>MAINPID=4711</literal>.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>MAINPIDFDID=…</term>

        <listitem><para>The pidfd inode number of the new main process (specified through <varname>MAINPID=</varname>).
        This information can be acquired through
        <citerefentry><refentrytitle>sd_pidfd_get_inode_id</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        on the pidfd and is used to identify the process in a race-free fashion. Alternatively,
        a pidfd can be sent directly to the service manager (see <varname>MAINPIDFD=1</varname> below).</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>MAINPIDFD=1</term>

        <listitem><para>Similar to <varname>MAINPID=</varname> with <varname>MAINPIDFDID=</varname>, but
        the process is referenced directly by the pidfd passed to the service manager. This is useful
        if pidfd id is not supported on the system. Exactly one fd is expected for this notification.</para>

        <xi:include href="version-info.xml" xpointer="v257"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>WATCHDOG=1</term>

        <listitem><para>Tells the service manager to update the watchdog timestamp. This is the keep-alive
        ping that services need to issue in regular intervals if <varname>WatchdogSec=</varname> is enabled
        for it. See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for information how to enable this functionality and
        <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for the details of how the service can check whether the watchdog is enabled.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>WATCHDOG=trigger</term>

        <listitem><para>Tells the service manager that the service detected an internal error that should be
        handled by the configured watchdog options. This will trigger the same behaviour as if
        <varname>WatchdogSec=</varname> is enabled and the service did not send <literal>WATCHDOG=1</literal>
        in time. Note that <varname>WatchdogSec=</varname> does not need to be enabled for
        <literal>WATCHDOG=trigger</literal> to trigger the watchdog action. See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for information about the watchdog behavior.</para>

        <xi:include href="version-info.xml" xpointer="v243"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>WATCHDOG_USEC=…</term>

        <listitem><para>Reset <varname>watchdog_usec</varname> value during runtime.  Notice that this is not
        available when using <function>sd_event_set_watchdog()</function> or
        <function>sd_watchdog_enabled()</function>.  Example :
        <literal>WATCHDOG_USEC=20000000</literal></para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>EXTEND_TIMEOUT_USEC=…</term>

        <listitem><para>Tells the service manager to extend the startup, runtime or shutdown service timeout
        corresponding the current state. The value specified is a time in microseconds during which the
        service must send a new message. A service timeout will occur if the message is not received, but only
        if the runtime of the current state is beyond the original maximum times of
        <varname>TimeoutStartSec=</varname>, <varname>RuntimeMaxSec=</varname>, and
        <varname>TimeoutStopSec=</varname>.  See
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for effects on the service timeouts.</para>

        <xi:include href="version-info.xml" xpointer="v236"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>RESTART_RESET=1</term>

        <listitem><para>Reset the restart counter of the service, which has the effect of restoring
        the restart duration to <varname>RestartSec=</varname> if <varname>RestartSteps=</varname> and
        <varname>RestartMaxDelaySec=</varname> are in use. For more information, refer to
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para>

        <xi:include href="version-info.xml" xpointer="v258"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>FDSTORE=1</term>

        <listitem><para>Store file descriptors in the service manager. File descriptors sent this way will be
        held for the service by the service manager and will later be handed back using the usual file
        descriptor passing logic at the next start or restart of the service, see
        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        Any open sockets and other file descriptors which should not be closed during a restart may be stored
        this way. When a service is stopped, its file descriptor store is discarded and all file descriptors
        in it are closed, except when overridden with <varname>FileDescriptorStorePreserve=</varname>, see
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para>

        <para>The service manager will accept messages for a service only if its
        <varname>FileDescriptorStoreMax=</varname> setting is non-zero (defaults to zero, see
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        The service manager will set the <varname>$FDSTORE</varname> environment variable for services that
        have the file descriptor store enabled, see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para>

        <para>If <varname>FDPOLL=0</varname> is not set and the file descriptors are pollable (see
        <citerefentry><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>), then
        any <constant>EPOLLHUP</constant> or <constant>EPOLLERR</constant> event seen on them will result in
        their automatic removal from the store.</para>

        <para>Multiple sets of file descriptors may be sent in separate messages, in which case the sets are
        combined. The service manager removes duplicate file descriptors (those pointing to the same object)
        before passing them to the service.</para>

        <para>This functionality should be used to implement services that can restart after an explicit
        request or a crash without losing state. Application state can either be serialized to a file in
        <filename>/run/</filename>, or better, stored in a
        <citerefentry><refentrytitle>memfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry>
        memory file descriptor. Use <function>sd_pid_notify_with_fds()</function> to send messages with
        <literal>FDSTORE=1</literal>. It is recommended to combine <varname>FDSTORE=</varname> with
        <varname>FDNAME=</varname> to make it easier to manage the stored file descriptors.</para>

        <para>For further information on the file descriptor store see the <ulink
        url="https://systemd.io/FILE_DESCRIPTOR_STORE">File Descriptor Store</ulink> overview.</para>

        <para>On kernels that support the <ulink
        url="https://docs.kernel.org/userspace-api/liveupdate.html">Live Update Orchestrator</ulink>,
        compatible file descriptors stored this way (such as <citerefentry
        project='man-pages'><refentrytitle>memfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry>)
        are additionally preserved across <literal>kexec</literal>-based reboots and handed back to the
        service on the other side, provided <varname>FileDescriptorStorePreserve=yes</varname> is set on
        the service (see
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        See the <ulink url="https://systemd.io/FILE_DESCRIPTOR_STORE">File Descriptor Store</ulink>
        overview for details.</para>

        <xi:include href="version-info.xml" xpointer="v219"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>FDSTOREREMOVE=1</term>

        <listitem><para>Removes file descriptors from the file descriptor store. This field needs to be
        combined with <varname>FDNAME=</varname> to specify the name of the file descriptors to
        remove.</para>

        <xi:include href="version-info.xml" xpointer="v236"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>FDNAME=…</term>

        <listitem><para>When used in combination with <varname>FDSTORE=1</varname>, specifies a name for the
        submitted file descriptors. When used with <varname>FDSTOREREMOVE=1</varname>, specifies the name for
        the file descriptors to remove. This name is passed to the service during activation, and may be
        queried using
        <citerefentry><refentrytitle>sd_listen_fds_with_names</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        File descriptors submitted without this field will be called <literal>stored</literal>.</para>

        <para>The name may consist of arbitrary ASCII characters except control characters or
        <literal>:</literal>. It may not be longer than 255 characters. If a submitted name does not follow
        these restrictions, it is ignored.</para>

        <para>Note that if multiple file descriptors are submitted in a single message, the specified name
        will be used for all of them. In order to assign different names to submitted file descriptors,
        submit them in separate messages.</para>

        <xi:include href="version-info.xml" xpointer="v233"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>FDPOLL=0</term>

        <listitem><para>When used in combination with <varname>FDSTORE=1</varname>, disables polling of the
        submitted file descriptors regardless of whether or not they are pollable. As this option disables
        automatic cleanup of the submitted file descriptors on EPOLLERR and EPOLLHUP, care must be taken to
        ensure proper manual cleanup.  Use of this option is not generally recommended except for when
        automatic cleanup has unwanted behavior such as prematurely discarding file descriptors from the
        store.</para>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>

      <varlistentry>
        <term>BARRIER=1</term>

        <listitem><para>Tells the service manager that the client is explicitly requesting synchronization by
        means of closing the file descriptor sent with this command. The service manager guarantees that the
        processing of a <varname>BARRIER=1</varname> command will only happen after all previous notification
        messages sent before this command have been processed. Hence, this command accompanied with a single
        file descriptor can be used to synchronize against reception of all previous status messages. Note
        that this command cannot be mixed with other notifications, and has to be sent in a separate message
        to the service manager, otherwise all assignments will be ignored. Note that sending 0 or more than 1
        file descriptor with this command is a violation of the protocol.</para>

        <xi:include href="version-info.xml" xpointer="v246"/></listitem>
      </varlistentry>
    </variablelist>

    <para>The notification messages sent by services are interpreted by the service manager. Unknown
    assignments are ignored. Thus, it is safe (but often without effect) to send assignments which are not
    in this list. The protocol is extensible, but care should be taken to ensure private extensions are
    recognizable as such. Specifically, it is recommend to prefix them with <literal>X_</literal> followed by
    some namespace identifier. The service manager also sends some messages to <emphasis>its</emphasis>
    notification socket, which may then consumed by a supervising machine or container manager further up the
    stack. The service manager sends a number of extension fields, for example
    <varname>X_SYSTEMD_UNIT_ACTIVE=</varname>, for details see
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On failure, these calls return a negative errno-style error code. If
    <varname>$NOTIFY_SOCKET</varname> was not set and hence no status message could be sent, 0 is
    returned. If the status was sent, these functions return a positive value. In order to support both
    service managers that implement this scheme and those which do not, it is generally recommended to ignore
    the return value of this call. Note that the return value simply indicates whether the notification
    message was enqueued properly, it does not reflect whether the message could be processed
    successfully. Specifically, no error is returned when a file descriptor is attempted to be stored using
    <varname>FDSTORE=1</varname> but the service is not actually configured to permit storing of file
    descriptors (see above).</para>

    <refsect2 id='errors'>
      <title>Errors</title>

      <para>Returned errors may indicate the following problems:</para>

      <variablelist>
        <varlistentry>
          <term><constant>-E2BIG</constant></term>

          <listitem><para>More file descriptors passed at once than the system allows. On Linux the number of
          file descriptors that may be passed across <constant>AF_UNIX</constant> sockets at once is 253, see
          <citerefentry
          project='man-pages'><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
          details.</para>

          <xi:include href="version-info.xml" xpointer="v257"/></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
    <xi:include href="threads-aware.xml" xpointer="getenv"/>

    <para>These functions send a single datagram with the state string as payload to the socket referenced in
    the <varname>$NOTIFY_SOCKET</varname> environment variable. If the first character of
    <varname>$NOTIFY_SOCKET</varname> is <literal>/</literal> or <literal>@</literal>, the string is
    understood as an <constant>AF_UNIX</constant> or Linux abstract namespace socket (respectively), and in
    both cases the datagram is accompanied by the process credentials of the sending service, using
    SCM_CREDENTIALS. If the string starts with <literal>vsock:</literal> then the string is understood as an
    <constant>AF_VSOCK</constant> address, which is useful for hypervisors/VMMs or other processes on the
    host to receive a notification when a virtual machine has finished booting. Note that in case the
    hypervisor does not support <constant>SOCK_DGRAM</constant> over <constant>AF_VSOCK</constant>,
    <constant>SOCK_SEQPACKET</constant> will be used instead. <literal>vsock-stream</literal>,
    <literal>vsock-dgram</literal> and <literal>vsock-seqpacket</literal> can be used instead of
    <literal>vsock</literal> to force usage of the corresponding socket type. The address should be in the
    form: <literal>vsock:CID:PORT</literal>. Note that unlike other uses of vsock, the CID is mandatory and
    cannot be <literal>VMADDR_CID_ANY</literal>.  Note that PID1 will send the VSOCK packets from a
    privileged port (i.e.: lower than 1024), as an attempt to address concerns that unprivileged processes in
    the guest might try to send malicious notifications to the host, driving it to make destructive decisions
    based on them.</para>

    <refsect2>
      <title>Standalone Implementations</title>
      <para>Note that, while using this library should be preferred in order to avoid code duplication, it is
      also possible to reimplement the simple readiness notification protocol without external dependencies,
      as demonstrated in the following self-contained examples from several languages:</para>
      <refsect3>
        <title>C</title>
        <programlisting><xi:include href="notify-selfcontained-example.c" parse="text"/></programlisting>
      </refsect3>
      <refsect3>
        <title>Python</title>
        <programlisting><xi:include href="notify-selfcontained-example.py" parse="text"/></programlisting>
      </refsect3>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <variablelist class='environment-variables'>
      <varlistentry>
        <term><varname>$NOTIFY_SOCKET</varname></term>

        <listitem><para>Set by the service manager for supervised processes for status and start-up
        completion notification.  This environment variable specifies the socket
        <function>sd_notify()</function> talks to. See above for details.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Start-up Notification</title>

      <para>When a service finished starting up, it might issue the following call to notify the service
      manager:</para>

      <programlisting>sd_notify(0, "READY=1");</programlisting>
    </example>

    <example>
      <title>Extended Start-up Notification</title>

      <para>A service could send the following after completing initialization:</para>

      <programlisting>
sd_notifyf(0, "READY=1\n"
              "STATUS=Processing requests…\n"
              "MAINPID=%lu",
           (unsigned long) getpid());</programlisting>
    </example>

    <example>
      <title>Error Cause Notification</title>

      <para>A service could send the following shortly before exiting, on failure:</para>

      <programlisting>
sd_notifyf(0, "STATUS=Failed to start up: %s\n"
              "ERRNO=%i",
           strerror_r(errnum, (char[1024]){}, 1024),
           errnum);</programlisting>
    </example>

    <example>
      <title>Store a File Descriptor in the Service Manager</title>

      <para>To store an open file descriptor in the service manager, in order to continue operation after a
      service restart without losing state, use <literal>FDSTORE=1</literal>:</para>

      <programlisting>sd_pid_notify_with_fds(0, 0, "FDSTORE=1\nFDNAME=foobar", &amp;fd, 1);</programlisting>
    </example>

    <example>
      <title>Eliminating race conditions</title>

      <para>When the client sending the notifications is not spawned by the service manager, it may exit too
      quickly and the service manager may fail to attribute them correctly to the unit. To prevent such
      races, use <function>sd_notify_barrier()</function> to synchronize against reception of all
      notifications sent before this call is made.</para>

      <programlisting>
sd_notify(0, "READY=1");
/* set timeout to 5 seconds */
sd_notify_barrier(0, 5 * 1000000);
</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>History</title>
    <para><function>sd_pid_notify()</function>,
    <function>sd_pid_notifyf()</function>, and
    <function>sd_pid_notify_with_fds()</function> were added in version 219.</para>
    <para><function>sd_notify_barrier()</function> was added in version 246.</para>
    <para><function>sd_pid_notifyf_with_fds()</function> and
    <function>sd_pid_notify_barrier()</function> were added in version 254.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_listen_fds_with_names</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>