sd_listen_fds, SD_LISTEN_FDS_START — Check for file descriptors passed by the system manager
#include <systemd/sd-daemon.h>
#define SD_LISTEN_FDS_START 3
| int sd_listen_fds( | int unset_environment ); | 
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.
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.
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-daemon
                pkg-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.