User:Johannbg/QA/Systemd/Sd listen fds

Name
sd_listen_fds — Check for file descriptors passed by the init system.

Synopsis
int sd_listen_fds(	int unset_environment);
 * 1) include "sd-daemon.h"
 * 2) define SD_LISTEN_FDS_START 3

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.

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.