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 if the 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 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>
                                for details. Multiple arrays of file
                                descriptors may be sent in seperate
                                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>

                </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>Internally, 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", &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>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd_watchdog_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>