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                 <refpurpose>Check for file descriptors passed by the init system.</refpurpose>
  48         </refnamediv>
  49
  50         <refsynopsisdiv>
  51                 <funcsynopsis>
  52                         <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>
  53
  54                         <funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
  55
  56                         <funcprototype>
  57                                 <funcdef>int <function>sd_listen_fds</function></funcdef>
  58                                 <paramdef>int <parameter>unset_environment</parameter></paramdef>
  59                         </funcprototype>
  60                 </funcsynopsis>
  61         </refsynopsisdiv>
  62
  63         <refsect1>
  64                 <title>Description</title>
  65
  66                 <para><function>sd_listen_fds()</function> shall be
  67                 called by a daemon to check for file descriptors
  68                 passed by the init system as part of the socket-based
  69                 activation logic.</para>
  70
  71                 <para>If the <parameter>unset_environment</parameter>
  72                 parameter is non-zero
  73                 <function>sd_listen_fds()</function> will unset the
  74                 <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
  75                 environment variables before returning (regardless
  76                 whether the function call itself succeeded or
  77                 not). Further calls to
  78                 <function>sd_listen_fds()</function> will then fail,
  79                 but the variables are no longer inherited by child
  80                 processes.</para>
  81
  82                 <para>If a daemon receives more than one file
  83                 descriptor, they will be passed in the same order as
  84                 configured in the systemd socket definition
  85                 file. Nonetheless it is recommended to verify the
  86                 correct socket types before using them. To simplify
  87                 this checking the functions
  88                 <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  89                 <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  90                 <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  91                 <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  92                 are provided. In order to maximize flexibility it is
  93                 recommended to make these checks as loose as possible
  94                 without allowing incorrect setups. i.e. often the
  95                 actual port number a socket is bound to matters little
  96                 for the service to work, hence it should not be
  97                 verified. On the other hand, whether a socket is a
  98                 datagram or stream socket matters a lot for the most
  99                 common program logics and should be checked.</para>
 100
 101                 <para>This function call will set the FD_CLOEXEC flag
 102                 for all passed file descriptors to avoid further
 103                 inheritance to children of the calling process.</para>
 104         </refsect1>
 105
 106         <refsect1>
 107                 <title>Return Value</title>
 108
 109                 <para>On failure, this call returns a negative
 110                 errno-style error code. If
 111                 <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
 112                 was not set or was not correctly set for this daemon and
 113                 hence no file descriptors were received, 0 is
 114                 returned. Otherwise the number of file descriptors
 115                 passed is returned. The application may find them
 116                 starting with file descriptor SD_LISTEN_FDS_START,
 117                 i.e. file descriptor 3.</para>
 118         </refsect1>
 119
 120         <refsect1>
 121                 <title>Notes</title>
 122
 123                 <para>This function is provided by the reference
 124                 implementation of APIs for new-style daemons and
 125                 distributed with the systemd package. The algorithm it
 126                 implements is simple, and can easily be reimplemented
 127                 in daemons if it is important to support this
 128                 interface without using the reference
 129                 implementation.</para>
 130
 131                 <para>Internally, this function checks whether the
 132                 <varname>$LISTEN_PID</varname> environment variable
 133                 equals the daemon PID. If not, it returns
 134                 immediately. Otherwise it parses the number passed in
 135                 the <varname>$LISTEN_FDS</varname> environment
 136                 variable, then sets the FD_CLOEXEC flag for the parsed
 137                 number of file descriptors starting from
 138                 SD_LISTEN_FDS_START. Finally it returns the parsed
 139                 number.</para>
 140
 141                 <para>For details about the algorithm check the
 142                 liberally licensed reference implementation sources:
 143                 <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/sd-daemon.c"/>
 144                 resp. <ulink
 145                 url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h"/></para>
 146
 147                 <para><function>sd_listen_fds()</function> is
 148                 implemented in the reference implementation's
 149                 <filename>sd-daemon.c</filename> and
 150                 <filename>sd-daemon.h</filename> files. These
 151                 interfaces are available as shared library, which can
 152                 be compiled and linked to with the
 153                 <literal>libsystemd-daemon</literal>
 154                 <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>7</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>
 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>7</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>