systemd/man/sd_notify.xml

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<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_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_notify_barrier</function></funcdef>
        <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 completion notification.</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, but 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 the following shall be considered
    well-known:</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></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></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></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></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></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></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></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></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></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></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></listitem>
      </varlistentry>

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

        <listitem><para>Stores additional file descriptors in the service manager. File descriptors sent this
        way will be maintained per-service by the service manager and will later be handed back using the
        usual file descriptor passing logic at the next invocation of the service (e.g. when it is
        restarted), see
        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        This is useful for implementing services that can restart after an explicit request or a crash
        without losing state. Any open sockets and other file descriptors which should not be closed during
        the restart may be stored this way. 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. Note that 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>). If
        <varname>FDPOLL=0</varname> is not set and the file descriptors sent 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. Multiple arrays of file descriptors may be sent in separate
        messages, in which case the arrays are combined. Note that the service manager removes duplicate
        (pointing to the same object) file descriptors before passing them to the service. When a service is
        stopped, its file descriptor store is discarded and all file descriptors in it are closed. Use
        <function>sd_pid_notify_with_fds()</function> to send messages with <literal>FDSTORE=1</literal>, see
        below. The service manager will set the <varname>$FDSTORE</varname> environment variable for services
        that have the file descriptor store enabled.</para></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></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 set, will implicitly get the name <literal>stored</literal>
        assigned. Note that, if multiple file descriptors are submitted at once, the specified name will be assigned to
        all of them. In order to assign different names to submitted file descriptors, submit them in separate
        invocations of <function>sd_pid_notify_with_fds()</function>. 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></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></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></listitem>
      </varlistentry>
    </variablelist>

    <para>It is recommended to prefix variable names that are not
    listed above with <varname>X_</varname> to avoid namespace
    clashes.</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_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>
  </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>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>