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 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.
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.
$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.
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)