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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="sd_listen_fds"
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_listen_fds</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>sd_listen_fds</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_listen_fds</refname>
<refname>sd_listen_fds_with_names</refname>
<refname>SD_LISTEN_FDS_START</refname>
<refpurpose>Check for file descriptors passed by the system manager</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-daemon.h></funcsynopsisinfo>
<funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_listen_fds</function></funcdef>
<paramdef>int <parameter>unset_environment</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_listen_fds_with_names</function></funcdef>
<paramdef>int <parameter>unset_environment</parameter></paramdef>
<paramdef>char*** <parameter>names</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_listen_fds()</function> may be invoked by a
daemon to check for file descriptors passed by the service manager as
part of the socket-based activation logic. It returns the number
of received file descriptors. If no file descriptors have been
received, zero is returned. The first file descriptor may be found
at file descriptor number 3
(i.e. <constant>SD_LISTEN_FDS_START</constant>), the remaining
descriptors follow at 4, 5, 6, …, if any.</para>
<para>If a daemon receives more than one file descriptor, they
will be passed in the same order as configured in the systemd
socket unit file (see
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details). Nonetheless, it is recommended to verify the correct
socket types before using them. To simplify this checking, the
functions
<citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
are provided. In order to maximize flexibility, it is recommended
to make these checks as loose as possible without allowing
incorrect setups. i.e. often, the actual port number a socket is
bound to matters little for the service to work, hence it should
not be verified. On the other hand, whether a socket is a datagram
or stream socket matters a lot for the most common program logics
and should be checked.</para>
<para>This function call will set the FD_CLOEXEC flag for all
passed file descriptors to avoid further inheritance to children
of the calling process.</para>
<para>If multiple socket units activate the same service, the order
of the file descriptors passed to its main process is undefined.
If additional file descriptors have been passed to the service
manager using
<citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
<literal>FDSTORE=1</literal> messages, these file descriptors are
passed last, in arbitrary order, and with duplicates
removed.</para>
<para>If the <parameter>unset_environment</parameter> parameter is
non-zero, <function>sd_listen_fds()</function> will unset the
<varname>$LISTEN_FDS</varname>, <varname>$LISTEN_PID</varname> and
<varname>$LISTEN_FDNAMES</varname> environment variables before
returning (regardless of whether the function call itself
succeeded or not). Further calls to
<function>sd_listen_fds()</function> will then return zero, but the
variables are no longer inherited by child processes.</para>
<para><function>sd_listen_fds_with_names()</function> is like
<function>sd_listen_fds()</function>, but optionally also returns
an array of strings with identification names for the passed file
descriptors, if that is available and the
<parameter>names</parameter> parameter is non-NULL. This
information is read from the <varname>$LISTEN_FDNAMES</varname>
variable, which may contain a colon-separated list of names. For
socket-activated services, these names may be configured with the
<varname>FileDescriptorName=</varname> setting in socket unit
files, see
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. For file descriptors pushed into the file descriptor
store (see above), the name is set via the
<varname>FDNAME=</varname> field transmitted via
<function>sd_pid_notify_with_fds()</function>. The primary usecase
for these names are services which accept a variety of file
descriptors which are not recognizable with functions like
<function>sd_is_socket()</function> alone, and thus require
identification via a name. It is recommended to rely on named file
descriptors only if identification via
<function>sd_is_socket()</function> and related calls is not
sufficient. Note that the names used are not unique in any
way. The returned array of strings has as many entries as file
descriptors have been received, plus a final NULL pointer
terminating the array. The caller needs to free the array itself
and each of its elements with libc's <function>free()</function>
call after use. If the <parameter>names</parameter> parameter is
NULL, the call is entirely equivalent to
<function>sd_listen_fds()</function>.</para>
<para>Under specific conditions, the following automatic file
descriptor names are returned:
<table>
<title>
<command>Special names</command>
</title>
<tgroup cols='2'>
<thead>
<row>
<entry>Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>unknown</literal></entry>
<entry>The process received no name for the specific file descriptor from the service manager.</entry>
</row>
<row>
<entry><literal>stored</literal></entry>
<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>
</row>
<row>
<entry><literal>connection</literal></entry>
<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>
</row>
</tbody>
</tgroup>
</table>
</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On failure, these calls returns a negative errno-style error
code. If
<varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname> was
not set or was not correctly set for this daemon and hence no file
descriptors were received, 0 is returned. Otherwise, the number of
file descriptors passed is returned. The application may find them
starting with file descriptor SD_LISTEN_FDS_START, i.e. file
descriptor 3.</para>
</refsect1>
<refsect1>
<title>Notes</title>
<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
<para>Internally, <function>sd_listen_fds()</function> checks
whether the <varname>$LISTEN_PID</varname> environment variable
equals the daemon PID. If not, it returns immediately. Otherwise,
it parses the number passed in the <varname>$LISTEN_FDS</varname>
environment variable, then sets the FD_CLOEXEC flag for the parsed
number of file descriptors starting from SD_LISTEN_FDS_START.
Finally, it returns the parsed
number. <function>sd_listen_fds_with_names()</function> does the
same but also parses <varname>$LISTEN_FDNAMES</varname> if
set.</para>
</refsect1>
<refsect1>
<title>Environment</title>
<variablelist class='environment-variables'>
<varlistentry>
<term><varname>$LISTEN_PID</varname></term>
<term><varname>$LISTEN_FDS</varname></term>
<term><varname>$LISTEN_FDNAMES</varname></term>
<listitem><para>Set by the service manager for supervised
processes that use socket-based activation. This environment
variable specifies the data
<function>sd_listen_fds()</function> and
<function>sd_listen_fds_with_names()</function> parses. See
above for details.</para></listitem>
</varlistentry>
</variablelist>
</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>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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