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>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </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>
    <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>
    </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. This is only used by systemd if the service
        definition file has Type=notify 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
        reloading 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.</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>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>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 be passed again using the usual file descriptor passing logic on the next
        invocation of the service, see
        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
        This is useful for implementing service restart schemes where services serialize
        their state to <filename>/run</filename>, push their file descriptors to the
        system manager, and are then restarted, retrieving their state again via socket
        passing and <filename>/run</filename>. Note that the service manager will accept
        messages for a service only if <varname>FileDescriptorStoreMax=</varname> is set
        to non-zero for it (defaults to zero, see
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
        File descriptors must be pollable, see
        <citerefentry><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
        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
        file descriptors before passing them to the service. Use
        <function>sd_pid_notify_with_fds()</function> to send messages with
        <literal>FDSTORE=1</literal>, see below.</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. 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 any ASCII character, but must not contain 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>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>

    </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><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 sending file descriptors
    to the service manager on messages that do not expect them (i.e.
    without <literal>FDSTORE=1</literal>) they are immediately closed
    on reception.</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 data could be sent, 0 is returned. If the status was
    sent, these functions return with a positive return value. In
    order to support both, init systems that implement this scheme and
    those which do not, it is generally recommended to ignore the
    return value of this call.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

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

    <para>These functions send a single datagram with the
    state string as payload to the <constant>AF_UNIX</constant> socket
    referenced in the <varname>$NOTIFY_SOCKET</varname> environment
    variable. If the first character of
    <varname>$NOTIFY_SOCKET</varname> is <literal>@</literal>, the
    string is understood as Linux abstract namespace socket. The
    datagram is accompanied by the process credentials of the sending
    service, using SCM_CREDENTIALS.</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(errno),
        errno);</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>
  </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>