systemd/man/sd_notify.xml

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        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<refentry id="sd_notify">

        <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>
                <refpurpose>Notify service manager about start-up completion and other daemon 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>
                </funcsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>
                <para><function>sd_notify()</function> shall be called
                by a daemon to notify the init system about status
                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
                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 init system
                                that daemon startup is finished. This
                                is only used by systemd if the service
                                definition file has Type=notify
                                set. The passed argument is a boolean
                                "1" or "0". Since there is little
                                value in signaling non-readiness, the
                                only value daemons should send is
                                "READY=1".</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>STATUS=...</term>

                                <listitem><para>Passes a single-line
                                status string back to the init system
                                that describes the daemon 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:
                                "STATUS=Completed 66% of file system
                                check..."</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>ERRNO=...</term>

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

                        <varlistentry>
                                <term>BUSERROR=...</term>

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

                        <varlistentry>
                                <term>MAINPID=...</term>

                                <listitem><para>The main pid of the
                                daemon, in case the init system did
                                not fork off the process
                                itself. Example:
                                "MAINPID=4711"</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Tells systemd 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 details. It is recommended to send
                                this message if the
                                <varname>WATCHDOG_USEC=</varname>
                                environment variable has been set for
                                the service process, in every half the
                                time interval that is specified in the
                                variable.</para></listitem>
                        </varlistentry>
                </variablelist>

                <para>It is recommended to prefix variable names that
                are not shown in the list above with
                <varname>X_</varname> to avoid namespace
                clashes.</para>

                <para>Note that systemd will accept status data sent
                from a daemon 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>
        </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
                don't, it is generally recommended to ignore the return
                value of this call.</para>
        </refsect1>

        <refsect1>
                <title>Notes</title>

                <para>These functions are provided by the reference
                implementation of APIs for new-style daemons and
                distributed with the systemd package. The algorithms
                they implement are simple, and can easily be
                reimplemented in daemons if it is important to support
                this interface without using the reference
                implementation.</para>

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

                <para>For details about the algorithms check the
                liberally licensed reference implementation sources:
                <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/libsystemd-daemon/sd-daemon.c"/>
                and <ulink
                url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h"/></para>

                <para><function>sd_notify()</function> and
                <function>sd_notifyf()</function> are implemented in
                the reference implementation's
                <filename>sd-daemon.c</filename> and
                <filename>sd-daemon.h</filename> files. These
                interfaces are available as shared library, which can
                be compiled and linked to with the
                <literal>libsystemd-daemon</literal>
                <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                file. Alternatively, applications consuming these APIs
                may copy the implementation into their source tree. For
                more details about the reference implementation see
                <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

                <para>If the reference implementation is used as
                drop-in files and -DDISABLE_SYSTEMD is set during
                compilation these functions will always return 0 and
                otherwise become a NOP.</para>
        </refsect1>

        <refsect1>
                <title>Environment</title>

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

                                <listitem><para>Set by the init system
                                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 daemon finished starting up, it
                        might issue the following call to notify
                        the init system:</para>

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

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

                        <para>A daemon 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 daemon 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>
        </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>
                </para>
        </refsect1>

</refentry>