man/sd_notify.xml

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  23
  24 <refentry id="sd_notify">
  25
  26         <refentryinfo>
  27                 <title>sd_notify</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>sd_notify</refentrytitle>
  42                 <manvolnum>3</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>sd_notify</refname>
  47                 <refname>sd_notifyf</refname>
  48                 <refpurpose>Notify init system about start-up completion and other daemon status changes</refpurpose>
  49         </refnamediv>
  50
  51         <refsynopsisdiv>
  52                 <funcsynopsis>
  53                         <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>
  54
  55                         <funcprototype>
  56                                 <funcdef>int <function>sd_notify</function></funcdef>
  57                                 <paramdef>int <parameter>unset_environment</parameter></paramdef>
  58                                 <paramdef>const char *<parameter>state</parameter></paramdef>
  59                         </funcprototype>
  60
  61                         <funcprototype>
  62                                 <funcdef>int <function>sd_notifyf</function></funcdef>
  63                                 <paramdef>int <parameter>unset_environment</parameter></paramdef>
  64                                 <paramdef>const char *<parameter>format</parameter></paramdef>
  65                                 <paramdef>...</paramdef>
  66                         </funcprototype>
  67                 </funcsynopsis>
  68         </refsynopsisdiv>
  69
  70         <refsect1>
  71                 <title>Description</title>
  72                 <para><function>sd_notify()</function> shall be called
  73                 by a daemon to notify the init system about status
  74                 changes. It can be used to send arbitrary information,
  75                 encoded in an environment-block-like string. Most
  76                 importantly it can be used for start-up completion
  77                 notification.</para>
  78
  79                 <para>If the <parameter>unset_environment</parameter>
  80                 parameter is non-zero <function>sd_notify()</function>
  81                 will unset the <varname>$NOTIFY_SOCKET</varname>
  82                 environment variable before returning (regardless
  83                 whether the function call itself succeeded or
  84                 not). Further calls to
  85                 <function>sd_notify()</function> will then fail, but
  86                 the variable is no longer inherited by child
  87                 processes.</para>
  88
  89                 <para>The <parameter>state</parameter> parameter
  90                 should contain an newline-separated list of variable
  91                 assignments, similar in style to an environment
  92                 block. A trailing newline is implied if none is
  93                 specified. The string may contain any kind of variable
  94                 assignments, but the following shall be considered
  95                 well-known:</para>
  96
  97                 <variablelist>
  98                         <varlistentry>
  99                                 <term>READY=1</term>
 100
 101                                 <listitem><para>Tells the init system
 102                                 that daemon startup is finished. This
 103                                 is only used by systemd if the service
 104                                 definition file has Type=notify
 105                                 set. The passed argument is a boolean
 106                                 "1" or "0". Since there is little
 107                                 value in signalling non-readiness, the
 108                                 only value daemons should send is
 109                                 "READY=1".</para></listitem>
 110                         </varlistentry>
 111
 112                         <varlistentry>
 113                                 <term>STATUS=...</term>
 114
 115                                 <listitem><para>Passes a single-line
 116                                 status string back to the init system
 117                                 that describes the daemon state. This
 118                                 is free-form and can be used for
 119                                 various purposes: general state
 120                                 feedback, fsck-like programs could
 121                                 pass completion percentages and
 122                                 failing programs could pass a human
 123                                 readable error message. Example:
 124                                 "STATUS=Completed 66% of file system
 125                                 check..."</para></listitem>
 126                         </varlistentry>
 127
 128                         <varlistentry>
 129                                 <term>ERRNO=...</term>
 130
 131                                 <listitem><para>If a daemon fails, the
 132                                 errno-style error code, formatted as
 133                                 string. Example: "ERRNO=2" for
 134                                 ENOENT.</para></listitem>
 135                         </varlistentry>
 136
 137                         <varlistentry>
 138                                 <term>BUSERROR=...</term>
 139
 140                                 <listitem><para>If a daemon fails, the
 141                                 D-Bus error-style error code. Example:
 142                                 "BUSERROR=org.freedesktop.DBus.Error.TimedOut"</para></listitem>
 143                         </varlistentry>
 144
 145                         <varlistentry>
 146                                 <term>MAINPID=...</term>
 147
 148                                 <listitem><para>The main pid of the
 149                                 daemon, in case the init system did
 150                                 not fork off the process
 151                                 itself. Example:
 152                                 "MAINPID=4711"</para></listitem>
 153                         </varlistentry>
 154
 155                         <varlistentry>
 156                                 <term>WATCHDOG=1</term>
 157
 158                                 <listitem><para>Tells systemd to
 159                                 update the watchdog timestamp.
 160                                 Services using this feature should do
 161                                 this in regular intervals. A watchdog
 162                                 framework can use the timestamps to
 163                                 detect failed
 164                                 services.</para></listitem>
 165                         </varlistentry>
 166                 </variablelist>
 167
 168                 <para>It is recommended to prefix variable names that
 169                 are not shown in the list above with
 170                 <varname>X_</varname> to avoid namespace
 171                 clashes.</para>
 172
 173                 <para>Note that systemd will accept status data sent
 174                 from a daemon only if the
 175                 <varname>NotifyAccess=</varname> option is correctly
 176                 set in the service definition file. See
 177                 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 178                 for details.</para>
 179
 180                 <para><function>sd_notifyf()</function> is similar to
 181                 <function>sd_notify()</function> but takes a
 182                 <function>printf()</function>-like format string plus
 183                 arguments.</para>
 184         </refsect1>
 185
 186         <refsect1>
 187                 <title>Return Value</title>
 188
 189                 <para>On failure, these calls return a negative
 190                 errno-style error code. If
 191                 <varname>$NOTIFY_SOCKET</varname> was not set and
 192                 hence no status data could be sent, 0 is returned. If
 193                 the status was sent these functions return with a
 194                 positive return value. In order to support both, init
 195                 systems that implement this scheme and those which
 196                 don't, it is generally recommended to ignore the return
 197                 value of this call.</para>
 198         </refsect1>
 199
 200         <refsect1>
 201                 <title>Notes</title>
 202
 203                 <para>These functions are provided by the reference
 204                 implementation of APIs for new-style daemons and
 205                 distributed with the systemd package. The algorithms
 206                 they implement are simple, and can easily be
 207                 reimplemented in daemons if it is important to support
 208                 this interface without using the reference
 209                 implementation.</para>
 210
 211                 <para>Internally, these functions send a single
 212                 datagram with the state string as payload to the
 213                 AF_UNIX socket referenced in the
 214                 <varname>$NOTIFY_SOCKET</varname> environment
 215                 variable. If the first character of
 216                 <varname>$NOTIFY_SOCKET</varname> is @ the string is
 217                 understood as Linux abstract namespace socket. The
 218                 datagram is accompanied by the process credentials of
 219                 the sending daemon, using SCM_CREDENTIALS.</para>
 220
 221                 <para>For details about the algorithms check the
 222                 liberally licensed reference implementation sources:
 223                 <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/sd-daemon.c"/>
 224                 resp. <ulink
 225                 url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h"/></para>
 226
 227                 <para><function>sd_notify()</function> and
 228                 <function>sd_notifyf()</function> are implemented in
 229                 the reference implementation's
 230                 <filename>sd-daemon.c</filename> and
 231                 <filename>sd-daemon.h</filename> files. These
 232                 interfaces are available as shared library, which can
 233                 be compiled and linked to with the
 234                 <literal>libsystemd-daemon</literal>
 235                 <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 236                 file. Alternatively, applications consuming these APIs
 237                 may copy the implementation into their source tree. For
 238                 more details about the reference implementation see
 239                 <citerefentry><refentrytitle>sd_daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 240
 241                 <para>If the reference implementation is used as
 242                 drop-in files and -DDISABLE_SYSTEMD is set during
 243                 compilation these functions will always return 0 and
 244                 otherwise become a NOP.</para>
 245         </refsect1>
 246
 247         <refsect1>
 248                 <title>Environment</title>
 249
 250                 <variablelist>
 251                         <varlistentry>
 252                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 253
 254                                 <listitem><para>Set by the init system
 255                                 for supervised processes for status
 256                                 and start-up completion
 257                                 notification. This environment variable
 258                                 specifies the socket
 259                                 <function>sd_notify()</function> talks
 260                                 to. See above for details.</para></listitem>
 261                         </varlistentry>
 262                 </variablelist>
 263         </refsect1>
 264
 265         <refsect1>
 266                 <title>Examples</title>
 267
 268                 <example>
 269                         <title>Start-up Notification</title>
 270
 271                         <para>When a daemon finished starting up, it
 272                         might issue the following call to notify
 273                         the init system:</para>
 274
 275                         <programlisting>sd_notify(0, "READY=1");</programlisting>
 276                 </example>
 277
 278                 <example>
 279                         <title>Extended Start-up Notification</title>
 280
 281                         <para>A daemon could send the following after
 282                         completing initialization:</para>
 283
 284                         <programlisting>sd_notifyf(0, "READY=1\n"
 285               "STATUS=Processing requests...\n"
 286               "MAINPID=%lu",
 287               (unsigned long) getpid());</programlisting>
 288                 </example>
 289
 290                 <example>
 291                         <title>Error Cause Notification</title>
 292
 293                         <para>A daemon could send the following shortly before exiting, on failure</para>
 294
 295                         <programlisting>sd_notifyf(0, "STATUS=Failed to start up: %s\n"
 296               "ERRNO=%i",
 297               strerror(errno),
 298               errno);</programlisting>
 299                 </example>
 300         </refsect1>
 301
 302         <refsect1>
 303                 <title>See Also</title>
 304                 <para>
 305                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 306                         <citerefentry><refentrytitle>sd_daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 307                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 308                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 309                 </para>
 310         </refsect1>
 311
 312 </refentry>