NAME

       sd_listen_fds, SD_LISTEN_FDS_START - Check for file descriptors passed by the system manager

SYNOPSIS

       #include <systemd/sd-daemon.h>

       #define SD_LISTEN_FDS_START 3

       int sd_listen_fds(int unset_environment);

DESCRIPTION

       sd_listen_fds() shall be called by a daemon to check for file descriptors passed by the init system as
       part of the socket-based activation logic.

       If the unset_environment parameter is non-zero sd_listen_fds() will unset the $LISTEN_FDS/$LISTEN_PID
       environment variables before returning (regardless whether the function call itself succeeded or not).
       Further calls to sd_listen_fds() will then fail, but the variables are no longer inherited by child
       processes.

       If a daemon receives more than one file descriptor, they will be passed in the same order as configured
       in the systemd socket definition file. Nonetheless it is recommended to verify the correct socket types
       before using them. To simplify this checking the functions sd_is_fifo(3), sd_is_socket(3),
       sd_is_socket_inet(3), sd_is_socket_unix(3) 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.

       This function call will set the FD_CLOEXEC flag for all passed file descriptors to avoid further
       inheritance to children of the calling process.

RETURN VALUE

       On failure, this call returns a negative errno-style error code. If $LISTEN_FDS/$LISTEN_PID 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.

NOTES

       This function is provided by the reference implementation of APIs for new-style daemons and distributed
       with the systemd package. The algorithm it implements is simple, and can easily be reimplemented in
       daemons if it is important to support this interface without using the reference implementation.

       Internally, this function checks whether the $LISTEN_PID environment variable equals the daemon PID. If
       not, it returns immediately. Otherwise it parses the number passed in the $LISTEN_FDS 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.

       For details about the algorithm check the liberally licensed reference implementation sources:
       http://cgit.freedesktop.org/systemd/systemd/plain/src/libsystemd-daemon/sd-daemon.c and
       http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h

       sd_listen_fds() is implemented in the reference implementation's sd-daemon.c and sd-daemon.h files. These
       interfaces are available as shared library, which can be compiled and linked to with the
       libsystemd-daemonpkg-config(1) file. Alternatively, applications consuming these APIs may copy the
       implementation into their source tree. For more details about the reference implementation see sd-
       daemon(3).

       If the reference implementation is used as drop-in files and -DDISABLE_SYSTEMD is set during compilation
       this function will always return 0 and otherwise become a NOP.

ENVIRONMENT

       $LISTEN_PID, $LISTEN_FDS
           Set by the init system for supervised processes that use socket-based activation. This environment
           variable specifies the data sd_listen_fds() parses. See above for details.

SEE ALSO

       systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3), sd_is_socket_unix(3),
       daemon(7), systemd.service(5), systemd.socket(5)