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<refentry id="sd_listen_fds"
<refpurpose>Check for file descriptors passed by the system manager</refpurpose>
<funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>
<funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
<funcdef>int <function>sd_listen_fds</function></funcdef>
<paramdef>int <parameter>unset_environment</parameter></paramdef>
<funcdef>int <function>sd_listen_fds_with_names</function></funcdef>
<paramdef>int <parameter>unset_environment</parameter></paramdef>
<paramdef>char*** <parameter>names</parameter></paramdef>
<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>The file descriptors passed this way may be closed at will by the processes receiving them: it's up
to the processes themselves to close them after use or whether to leave them open until the process exits
(in which case the kernel closes them automatically). Note that the file descriptors received by daemons
are duplicates of the file descriptors the service manager originally allocated and bound and of which it
continously keeps a copy (except if <varname>Accept=yes</varname> is used). This means any socket option
changes and other changes made to the sockets will visible to the service manager too. Most importanly
this means it's generally not a good idea to invoke <citerefentry
project='man-pages'><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry> on
such sockets, since it will shut down communication on the file descriptor the service manager holds for
the same socket, too. Also note that if a daemon is restarted (and its associated sockets are not) it
will receive file descriptors to the very same sockets as the earlier invocations, thus all socket
options applied then will still apply.</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) — if there's only one such file (see below). Nonetheless, it is recommended to verify the
correct socket types before using them. To simplify this checking, the functions
<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
<literal>FDSTORE=1</literal> messages, these file descriptors are
passed last, in arbitrary order, and with duplicates
<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>$LISTEN_FDNAMES</varname> environment variables before
returning (regardless of whether the function call itself
succeeded or not). Further calls to
<function>sd_listen_fds()</function> will then return zero, but the
variables are no longer inherited by child processes.</para>
<para><function>sd_listen_fds_with_names()</function> is like
<function>sd_listen_fds()</function>, but optionally also returns
an array of strings with identification names for the passed file
descriptors, if that is available and the
<parameter>names</parameter> parameter is non-<constant>NULL</constant>. This
information is read from the <varname>$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
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 <constant>NULL</constant> 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
<constant>NULL</constant>, the call is entirely equivalent to
<para>Under specific conditions, the following automatic file
descriptor names are returned:
<command>Special names</command>
<tgroup cols='2'>
<entry>The process received no name for the specific file descriptor from the service manager.</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>
<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>
<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>
<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
<variablelist class='environment-variables'>
<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>
<title>See Also</title>