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)