man/sd_listen_fds.xml

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  23
  24 <refentry id="sd_listen_fds">
  25
  26         <refentryinfo>
  27                 <title>sd_listen_fds</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>sd_listen_fds</refentrytitle>
  42                 <manvolnum>3</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>sd_listen_fds</refname>
  47                 <refname>SD_LISTEN_FDS_START</refname>
  48                 <refpurpose>Check for file descriptors passed by the system manager</refpurpose>
  49         </refnamediv>
  50
  51         <refsynopsisdiv>
  52                 <funcsynopsis>
  53                         <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>
  54
  55                         <funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
  56
  57                         <funcprototype>
  58                                 <funcdef>int <function>sd_listen_fds</function></funcdef>
  59                                 <paramdef>int <parameter>unset_environment</parameter></paramdef>
  60                         </funcprototype>
  61                 </funcsynopsis>
  62         </refsynopsisdiv>
  63
  64         <refsect1>
  65                 <title>Description</title>
  66
  67                 <para><function>sd_listen_fds()</function> shall be
  68                 called by a daemon to check for file descriptors
  69                 passed by the init system as part of the socket-based
  70                 activation logic.</para>
  71
  72                 <para>If the <parameter>unset_environment</parameter>
  73                 parameter is non-zero,
  74                 <function>sd_listen_fds()</function> will unset the
  75                 <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
  76                 environment variables before returning (regardless of
  77                 whether the function call itself succeeded or
  78                 not). Further calls to
  79                 <function>sd_listen_fds()</function> will then fail,
  80                 but the variables are no longer inherited by child
  81                 processes.</para>
  82
  83                 <para>If a daemon receives more than one file
  84                 descriptor, they will be passed in the same order as
  85                 configured in the systemd socket definition
  86                 file. Nonetheless, it is recommended to verify the
  87                 correct socket types before using them. To simplify
  88                 this checking, the functions
  89                 <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  90                 <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  91                 <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  92                 <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  93                 are provided. In order to maximize flexibility, it is
  94                 recommended to make these checks as loose as possible
  95                 without allowing incorrect setups. i.e. often, the
  96                 actual port number a socket is bound to matters little
  97                 for the service to work, hence it should not be
  98                 verified. On the other hand, whether a socket is a
  99                 datagram or stream socket matters a lot for the most
 100                 common program logics and should be checked.</para>
 101
 102                 <para>This function call will set the FD_CLOEXEC flag
 103                 for all passed file descriptors to avoid further
 104                 inheritance to children of the calling process.</para>
 105         </refsect1>
 106
 107         <refsect1>
 108                 <title>Return Value</title>
 109
 110                 <para>On failure, this call returns a negative
 111                 errno-style error code. If
 112                 <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
 113                 was not set or was not correctly set for this daemon and
 114                 hence no file descriptors were received, 0 is
 115                 returned. Otherwise, the number of file descriptors
 116                 passed is returned. The application may find them
 117                 starting with file descriptor SD_LISTEN_FDS_START,
 118                 i.e. file descriptor 3.</para>
 119         </refsect1>
 120
 121         <refsect1>
 122                 <title>Notes</title>
 123
 124                 <para>This function is provided by the reference
 125                 implementation of APIs for new-style daemons and
 126                 distributed with the systemd package. The algorithm it
 127                 implements is simple, and can easily be reimplemented
 128                 in daemons if it is important to support this
 129                 interface without using the reference
 130                 implementation.</para>
 131
 132                 <para>Internally, this function checks whether the
 133                 <varname>$LISTEN_PID</varname> environment variable
 134                 equals the daemon PID. If not, it returns
 135                 immediately. Otherwise, it parses the number passed in
 136                 the <varname>$LISTEN_FDS</varname> environment
 137                 variable, then sets the FD_CLOEXEC flag for the parsed
 138                 number of file descriptors starting from
 139                 SD_LISTEN_FDS_START. Finally, it returns the parsed
 140                 number.</para>
 141
 142                 <para>For details about the algorithm check the
 143                 liberally licensed reference implementation sources:
 144                 <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/libsystemd-daemon/sd-daemon.c"/>
 145                 and <ulink
 146                 url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h"/></para>
 147
 148                 <para><function>sd_listen_fds()</function> is
 149                 implemented in the reference implementation's
 150                 <filename>sd-daemon.c</filename> and
 151                 <filename>sd-daemon.h</filename> files. These
 152                 interfaces are available as a shared library, which can
 153                 be compiled and linked to with the
 154                 <constant>libsystemd-daemon</constant> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 155                 file. Alternatively, applications consuming these APIs
 156                 may copy the implementation into their source
 157                 tree. For more details about the reference
 158                 implementation, see
 159                 <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 160
 161                 <para>If the reference implementation is used as
 162                 drop-in files and -DDISABLE_SYSTEMD is set during
 163                 compilation, this function will always return 0 and
 164                 otherwise become a NOP.</para>
 165         </refsect1>
 166
 167         <refsect1>
 168                 <title>Environment</title>
 169
 170                 <variablelist class='environment-variables'>
 171                         <varlistentry>
 172                                 <term><varname>$LISTEN_PID</varname></term>
 173                                 <term><varname>$LISTEN_FDS</varname></term>
 174
 175                                 <listitem><para>Set by the init system
 176                                 for supervised processes that use
 177                                 socket-based activation. This
 178                                 environment variable specifies the
 179                                 data
 180                                 <function>sd_listen_fds()</function>
 181                                 parses. See above for
 182                                 details.</para></listitem>
 183                         </varlistentry>
 184                 </variablelist>
 185         </refsect1>
 186
 187         <refsect1>
 188                 <title>See Also</title>
 189
 190                 <para>
 191                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 192                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 193                         <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 194                         <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 195                         <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 196                         <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 197                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 198                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 199                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 200                 </para>
 201         </refsect1>
 202
 203 </refentry>