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