man/sd_listen_fds.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2010 Lennart Poettering
  11 -->
  12
  13 <refentry id="sd_listen_fds"
  14   xmlns:xi="http://www.w3.org/2001/XInclude">
  15
  16   <refentryinfo>
  17     <title>sd_listen_fds</title>
  18     <productname>systemd</productname>
  19
  20     <authorgroup>
  21       <author>
  22         <contrib>Developer</contrib>
  23         <firstname>Lennart</firstname>
  24         <surname>Poettering</surname>
  25         <email>lennart@poettering.net</email>
  26       </author>
  27     </authorgroup>
  28   </refentryinfo>
  29
  30   <refmeta>
  31     <refentrytitle>sd_listen_fds</refentrytitle>
  32     <manvolnum>3</manvolnum>
  33   </refmeta>
  34
  35   <refnamediv>
  36     <refname>sd_listen_fds</refname>
  37     <refname>sd_listen_fds_with_names</refname>
  38     <refname>SD_LISTEN_FDS_START</refname>
  39     <refpurpose>Check for file descriptors passed by the system manager</refpurpose>
  40   </refnamediv>
  41
  42   <refsynopsisdiv>
  43     <funcsynopsis>
  44       <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>
  45
  46       <funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
  47
  48       <funcprototype>
  49         <funcdef>int <function>sd_listen_fds</function></funcdef>
  50         <paramdef>int <parameter>unset_environment</parameter></paramdef>
  51       </funcprototype>
  52
  53       <funcprototype>
  54         <funcdef>int <function>sd_listen_fds_with_names</function></funcdef>
  55         <paramdef>int <parameter>unset_environment</parameter></paramdef>
  56         <paramdef>char*** <parameter>names</parameter></paramdef>
  57       </funcprototype>
  58     </funcsynopsis>
  59   </refsynopsisdiv>
  60
  61   <refsect1>
  62     <title>Description</title>
  63
  64     <para><function>sd_listen_fds()</function> may be invoked by a
  65     daemon to check for file descriptors passed by the service manager as
  66     part of the socket-based activation logic. It returns the number
  67     of received file descriptors. If no file descriptors have been
  68     received, zero is returned. The first file descriptor may be found
  69     at file descriptor number 3
  70     (i.e. <constant>SD_LISTEN_FDS_START</constant>), the remaining
  71     descriptors follow at 4, 5, 6, …, if any.</para>
  72
  73     <para>If a daemon receives more than one file descriptor, they
  74     will be passed in the same order as configured in the systemd
  75     socket unit file (see
  76     <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  77     for details). Nonetheless, it is recommended to verify the correct
  78     socket types before using them. To simplify this checking, the
  79     functions
  80     <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  81     <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  82     <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  83     <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  84     are provided. In order to maximize flexibility, it is recommended
  85     to make these checks as loose as possible without allowing
  86     incorrect setups. i.e. often, the actual port number a socket is
  87     bound to matters little for the service to work, hence it should
  88     not be verified. On the other hand, whether a socket is a datagram
  89     or stream socket matters a lot for the most common program logics
  90     and should be checked.</para>
  91
  92     <para>This function call will set the FD_CLOEXEC flag for all
  93     passed file descriptors to avoid further inheritance to children
  94     of the calling process.</para>
  95
  96     <para>If multiple socket units activate the same service, the order
  97     of the file descriptors passed to its main process is undefined.
  98     If additional file descriptors have been passed to the service
  99     manager using
 100     <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
 101     <literal>FDSTORE=1</literal> messages, these file descriptors are
 102     passed last, in arbitrary order, and with duplicates
 103     removed.</para>
 104
 105     <para>If the <parameter>unset_environment</parameter> parameter is
 106     non-zero, <function>sd_listen_fds()</function> will unset the
 107     <varname>$LISTEN_FDS</varname>, <varname>$LISTEN_PID</varname> and
 108     <varname>$LISTEN_FDNAMES</varname> environment variables before
 109     returning (regardless of whether the function call itself
 110     succeeded or not). Further calls to
 111     <function>sd_listen_fds()</function> will then return zero, but the
 112     variables are no longer inherited by child processes.</para>
 113
 114     <para><function>sd_listen_fds_with_names()</function> is like
 115     <function>sd_listen_fds()</function>, but optionally also returns
 116     an array of strings with identification names for the passed file
 117     descriptors, if that is available and the
 118     <parameter>names</parameter> parameter is non-NULL. This
 119     information is read from the <varname>$LISTEN_FDNAMES</varname>
 120     variable, which may contain a colon-separated list of names. For
 121     socket-activated services, these names may be configured with the
 122     <varname>FileDescriptorName=</varname> setting in socket unit
 123     files, see
 124     <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 125     for details. For file descriptors pushed into the file descriptor
 126     store (see above), the name is set via the
 127     <varname>FDNAME=</varname> field transmitted via
 128     <function>sd_pid_notify_with_fds()</function>. The primary usecase
 129     for these names are services which accept a variety of file
 130     descriptors which are not recognizable with functions like
 131     <function>sd_is_socket()</function> alone, and thus require
 132     identification via a name. It is recommended to rely on named file
 133     descriptors only if identification via
 134     <function>sd_is_socket()</function> and related calls is not
 135     sufficient. Note that the names used are not unique in any
 136     way. The returned array of strings has as many entries as file
 137     descriptors have been received, plus a final NULL pointer
 138     terminating the array. The caller needs to free the array itself
 139     and each of its elements with libc's <function>free()</function>
 140     call after use. If the <parameter>names</parameter> parameter is
 141     NULL, the call is entirely equivalent to
 142     <function>sd_listen_fds()</function>.</para>
 143
 144     <para>Under specific conditions, the following automatic file
 145     descriptor names are returned:
 146
 147     <table>
 148       <title>
 149          <command>Special names</command>
 150       </title>
 151
 152       <tgroup cols='2'>
 153         <thead>
 154           <row>
 155             <entry>Name</entry>
 156             <entry>Description</entry>
 157           </row>
 158         </thead>
 159         <tbody>
 160           <row>
 161             <entry><literal>unknown</literal></entry>
 162             <entry>The process received no name for the specific file descriptor from the service manager.</entry>
 163           </row>
 164
 165           <row>
 166             <entry><literal>stored</literal></entry>
 167             <entry>The file descriptor originates in the service manager's per-service file descriptor store, and the <varname>FDNAME=</varname> field was absent when the file descriptor was submitted to the service manager.</entry>
 168           </row>
 169
 170           <row>
 171             <entry><literal>connection</literal></entry>
 172             <entry>The service was activated in per-connection style using <varname>Accept=yes</varname> in the socket unit file, and the file descriptor is the connection socket.</entry>
 173           </row>
 174         </tbody>
 175       </tgroup>
 176     </table>
 177     </para>
 178   </refsect1>
 179
 180   <refsect1>
 181     <title>Return Value</title>
 182
 183     <para>On failure, these calls returns a negative errno-style error
 184     code. If
 185     <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname> was
 186     not set or was not correctly set for this daemon and hence no file
 187     descriptors were received, 0 is returned. Otherwise, the number of
 188     file descriptors passed is returned. The application may find them
 189     starting with file descriptor SD_LISTEN_FDS_START, i.e. file
 190     descriptor 3.</para>
 191   </refsect1>
 192
 193   <refsect1>
 194     <title>Notes</title>
 195
 196     <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
 197
 198     <para>Internally, <function>sd_listen_fds()</function> checks
 199     whether the <varname>$LISTEN_PID</varname> environment variable
 200     equals the daemon PID. If not, it returns immediately. Otherwise,
 201     it parses the number passed in the <varname>$LISTEN_FDS</varname>
 202     environment variable, then sets the FD_CLOEXEC flag for the parsed
 203     number of file descriptors starting from SD_LISTEN_FDS_START.
 204     Finally, it returns the parsed
 205     number. <function>sd_listen_fds_with_names()</function> does the
 206     same but also parses <varname>$LISTEN_FDNAMES</varname> if
 207     set.</para>
 208   </refsect1>
 209
 210   <refsect1>
 211     <title>Environment</title>
 212
 213     <variablelist class='environment-variables'>
 214       <varlistentry>
 215         <term><varname>$LISTEN_PID</varname></term>
 216         <term><varname>$LISTEN_FDS</varname></term>
 217         <term><varname>$LISTEN_FDNAMES</varname></term>
 218
 219         <listitem><para>Set by the service manager for supervised
 220         processes that use socket-based activation. This environment
 221         variable specifies the data
 222         <function>sd_listen_fds()</function> and
 223         <function>sd_listen_fds_with_names()</function> parses. See
 224         above for details.</para></listitem>
 225       </varlistentry>
 226     </variablelist>
 227   </refsect1>
 228
 229   <refsect1>
 230     <title>See Also</title>
 231
 232     <para>
 233       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 234       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 235       <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 236       <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 237       <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 238       <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 239       <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 240       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 241       <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 242       <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 243     </para>
 244   </refsect1>
 245
 246 </refentry>