1<?xml version='1.0'?> 2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" 3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> 4<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> 5 6<refentry id="sd_listen_fds" 7 xmlns:xi="http://www.w3.org/2001/XInclude"> 8 9 <refentryinfo> 10 <title>sd_listen_fds</title> 11 <productname>systemd</productname> 12 </refentryinfo> 13 14 <refmeta> 15 <refentrytitle>sd_listen_fds</refentrytitle> 16 <manvolnum>3</manvolnum> 17 </refmeta> 18 19 <refnamediv> 20 <refname>sd_listen_fds</refname> 21 <refname>sd_listen_fds_with_names</refname> 22 <refname>SD_LISTEN_FDS_START</refname> 23 <refpurpose>Check for file descriptors passed by the system manager</refpurpose> 24 </refnamediv> 25 26 <refsynopsisdiv> 27 <funcsynopsis> 28 <funcsynopsisinfo>#include <systemd/sd-daemon.h></funcsynopsisinfo> 29 30 <funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo> 31 32 <funcprototype> 33 <funcdef>int <function>sd_listen_fds</function></funcdef> 34 <paramdef>int <parameter>unset_environment</parameter></paramdef> 35 </funcprototype> 36 37 <funcprototype> 38 <funcdef>int <function>sd_listen_fds_with_names</function></funcdef> 39 <paramdef>int <parameter>unset_environment</parameter></paramdef> 40 <paramdef>char*** <parameter>names</parameter></paramdef> 41 </funcprototype> 42 </funcsynopsis> 43 </refsynopsisdiv> 44 45 <refsect1> 46 <title>Description</title> 47 48 <para><function>sd_listen_fds()</function> may be invoked by a daemon to check for file descriptors 49 passed by the service manager as part of the socket-based activation logic. It returns the number of 50 received file descriptors. If no file descriptors have been received, zero is returned. The first file 51 descriptor may be found at file descriptor number 3 (i.e. <constant>SD_LISTEN_FDS_START</constant>), the 52 remaining descriptors follow at 4, 5, 6, …, if any.</para> 53 54 <para>The file descriptors passed this way may be closed at will by the processes receiving them: it's up 55 to the processes themselves to close them after use or whether to leave them open until the process exits 56 (in which case the kernel closes them automatically). Note that the file descriptors received by daemons 57 are duplicates of the file descriptors the service manager originally allocated and bound and of which it 58 continuously keeps a copy (except if <varname>Accept=yes</varname> is used). This means any socket option 59 changes and other changes made to the sockets will be visible to the service manager too. Most 60 importantly this means it's generally not a good idea to invoke <citerefentry 61 project='man-pages'><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry> on 62 such sockets, since it will shut down communication on the file descriptor the service manager holds for 63 the same socket too. Also note that if a daemon is restarted (and its associated sockets are not) it will 64 receive file descriptors to the very same sockets as the earlier invocations, thus all socket options 65 applied then will still apply.</para> 66 67 <para>If a daemon receives more than one file descriptor, they will be passed in the same order as 68 configured in the systemd socket unit file (see 69 <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> for 70 details) — if there's only one such file (see below). Nonetheless, it is recommended to verify the 71 correct socket types before using them. To simplify this checking, the functions 72 <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 73 <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 74 <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 75 <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry> are 76 provided. In order to maximize flexibility, it is recommended to make these checks as loose as possible 77 without allowing incorrect setups. i.e. often, the actual port number a socket is bound to matters little 78 for the service to work, hence it should not be verified. On the other hand, whether a socket is a 79 datagram or stream socket matters a lot for the most common program logics and should be checked.</para> 80 81 <para>This function call will set the FD_CLOEXEC flag for all 82 passed file descriptors to avoid further inheritance to children 83 of the calling process.</para> 84 85 <para>If multiple socket units activate the same service, the order 86 of the file descriptors passed to its main process is undefined. 87 If additional file descriptors have been passed to the service 88 manager using 89 <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s 90 <literal>FDSTORE=1</literal> messages, these file descriptors are 91 passed last, in arbitrary order, and with duplicates 92 removed.</para> 93 94 <para>If the <parameter>unset_environment</parameter> parameter is 95 non-zero, <function>sd_listen_fds()</function> will unset the 96 <varname>$LISTEN_FDS</varname>, <varname>$LISTEN_PID</varname> and 97 <varname>$LISTEN_FDNAMES</varname> environment variables before 98 returning (regardless of whether the function call itself 99 succeeded or not). Further calls to 100 <function>sd_listen_fds()</function> will then return zero, but the 101 variables are no longer inherited by child processes.</para> 102 103 <para><function>sd_listen_fds_with_names()</function> is like 104 <function>sd_listen_fds()</function>, but optionally also returns 105 an array of strings with identification names for the passed file 106 descriptors, if that is available and the 107 <parameter>names</parameter> parameter is non-<constant>NULL</constant>. This 108 information is read from the <varname>$LISTEN_FDNAMES</varname> 109 variable, which may contain a colon-separated list of names. For 110 socket-activated services, these names may be configured with the 111 <varname>FileDescriptorName=</varname> setting in socket unit 112 files, see 113 <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> 114 for details. For file descriptors pushed into the file descriptor 115 store (see above), the name is set via the 116 <varname>FDNAME=</varname> field transmitted via 117 <function>sd_pid_notify_with_fds()</function>. The primary usecase 118 for these names are services which accept a variety of file 119 descriptors which are not recognizable with functions like 120 <function>sd_is_socket()</function> alone, and thus require 121 identification via a name. It is recommended to rely on named file 122 descriptors only if identification via 123 <function>sd_is_socket()</function> and related calls is not 124 sufficient. Note that the names used are not unique in any 125 way. The returned array of strings has as many entries as file 126 descriptors have been received, plus a final <constant>NULL</constant> pointer 127 terminating the array. The caller needs to free the array itself 128 and each of its elements with libc's <function>free()</function> 129 call after use. If the <parameter>names</parameter> parameter is 130 <constant>NULL</constant>, the call is entirely equivalent to 131 <function>sd_listen_fds()</function>.</para> 132 133 <para>Under specific conditions, the following automatic file 134 descriptor names are returned: 135 136 <table> 137 <title> 138 <command>Special names</command> 139 </title> 140 141 <tgroup cols='2'> 142 <thead> 143 <row> 144 <entry>Name</entry> 145 <entry>Description</entry> 146 </row> 147 </thead> 148 <tbody> 149 <row> 150 <entry><literal>unknown</literal></entry> 151 <entry>The process received no name for the specific file descriptor from the service manager.</entry> 152 </row> 153 154 <row> 155 <entry><literal>stored</literal></entry> 156 <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> 157 </row> 158 159 <row> 160 <entry><literal>connection</literal></entry> 161 <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> 162 </row> 163 </tbody> 164 </tgroup> 165 </table> 166 </para> 167 </refsect1> 168 169 <refsect1> 170 <title>Return Value</title> 171 172 <para>On failure, these calls returns a negative errno-style error 173 code. If 174 <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname> was 175 not set or was not correctly set for this daemon and hence no file 176 descriptors were received, 0 is returned. Otherwise, the number of 177 file descriptors passed is returned. The application may find them 178 starting with file descriptor SD_LISTEN_FDS_START, i.e. file 179 descriptor 3.</para> 180 </refsect1> 181 182 <refsect1> 183 <title>Notes</title> 184 185 <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/> 186 187 <para>Internally, <function>sd_listen_fds()</function> checks 188 whether the <varname>$LISTEN_PID</varname> environment variable 189 equals the daemon PID. If not, it returns immediately. Otherwise, 190 it parses the number passed in the <varname>$LISTEN_FDS</varname> 191 environment variable, then sets the FD_CLOEXEC flag for the parsed 192 number of file descriptors starting from SD_LISTEN_FDS_START. 193 Finally, it returns the parsed 194 number. <function>sd_listen_fds_with_names()</function> does the 195 same but also parses <varname>$LISTEN_FDNAMES</varname> if 196 set.</para> 197 </refsect1> 198 199 <refsect1> 200 <title>Environment</title> 201 202 <variablelist class='environment-variables'> 203 <varlistentry> 204 <term><varname>$LISTEN_PID</varname></term> 205 <term><varname>$LISTEN_FDS</varname></term> 206 <term><varname>$LISTEN_FDNAMES</varname></term> 207 208 <listitem><para>Set by the service manager for supervised 209 processes that use socket-based activation. This environment 210 variable specifies the data 211 <function>sd_listen_fds()</function> and 212 <function>sd_listen_fds_with_names()</function> parses. See 213 above for details.</para></listitem> 214 </varlistentry> 215 </variablelist> 216 </refsect1> 217 218 <refsect1> 219 <title>See Also</title> 220 221 <para> 222 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, 223 <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 224 <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 225 <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 226 <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 227 <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 228 <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>, 229 <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>, 230 <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>, 231 <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> 232 </para> 233 </refsect1> 234 235</refentry> 236