sd_listen_fds man page on Archlinux

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SD_LISTEN_FDS(3)		 sd_listen_fds		      SD_LISTEN_FDS(3)

       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 of 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

       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.

       These APIs are implemented as a shared library, which can be compiled
       and linked to with the libsystemd pkg-config(1) file.

       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.

	   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.

       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)

systemd 212						      SD_LISTEN_FDS(3)

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