Adding upstream version 255.4.upstream/255.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 589 insertions, 0 deletions

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+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/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 fail, 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. 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>. Note that this assignment is
+        currently not used by <command>systemd</command>.</para>
+
+        <xi:include href="version-info.xml" xpointer="v233"/></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 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>The main process ID (PID) of the service, in case the service manager did not fork
+        off the process itself. Example: <literal>MAINPID=4711</literal>.</para>
+
+        <xi:include href="version-info.xml" xpointer="v233"/></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 isn't 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>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>
+
+        <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
+        stored file descriptors regardless of whether or not they are pollable. As this option disables
+        automatic cleanup of the stored 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 may be logged, but are otherwise ignored. Thus, it is not useful to send assignments which
+    are not in this list. The service manager also sends some messages to <emphasis>its</emphasis>
+    notification socket, which are then consumed by the machine or container manager.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>On failure, these calls return a negative errno-style error code. If
+    <varname>$NOTIFY_SOCKET</varname> was not set and hence no status message could be sent, 0 is
+    returned. If the status was sent, these functions return a positive value. In order to support both
+    service managers that implement this scheme and those which do not, it is generally recommended to ignore
+    the return value of this call. Note that the return value simply indicates whether the notification
+    message was enqueued properly, it does not reflect whether the message could be processed
+    successfully. Specifically, no error is returned when a file descriptor is attempted to be stored using
+    <varname>FDSTORE=1</varname> but the service is not actually configured to permit storing of file
+    descriptors (see above).</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes</title>
+
+    <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
+    <xi:include href="threads-aware.xml" xpointer="getenv"/>
+
+    <para>These functions send a single datagram with the state string as payload to the socket referenced in
+    the <varname>$NOTIFY_SOCKET</varname> environment variable. If the first character of
+    <varname>$NOTIFY_SOCKET</varname> is <literal>/</literal> or <literal>@</literal>, the string is
+    understood as an <constant>AF_UNIX</constant> or Linux abstract namespace socket (respectively), and in
+    both cases the datagram is accompanied by the process credentials of the sending service, using
+    SCM_CREDENTIALS. If the string starts with <literal>vsock:</literal> then the string is understood as an
+    <constant>AF_VSOCK</constant> address, which is useful for hypervisors/VMMs or other processes on the
+    host to receive a notification when a virtual machine has finished booting. Note that in case the
+    hypervisor does not support <constant>SOCK_DGRAM</constant> over <constant>AF_VSOCK</constant>,
+    <constant>SOCK_SEQPACKET</constant> will be used instead. The address should be in the form:
+    <literal>vsock:CID:PORT</literal>. Note that unlike other uses of vsock, the CID is mandatory and cannot
+    be <literal>VMADDR_CID_ANY</literal>.  Note that PID1 will send the VSOCK packets from a privileged port
+    (i.e.: lower than 1024), as an attempt to address concerns that unprivileged processes in the guest might
+    try to send malicious notifications to the host, driving it to make destructive decisions based on
+    them.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Environment</title>
+
+    <variablelist class='environment-variables'>
+      <varlistentry>
+        <term><varname>$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>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_listen_fds_with_names</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>