sd_listen_fds, SD_LISTEN_FDS_START - Check for file descriptors passed
by the system manager
#define SD_LISTEN_FDS_START 3
int sd_listen_fds(int unset_environment);
DESCRIPTIONsd_listen_fds() shall be called by a daemon to check for file
descriptors passed by the init system as part of the socket-based
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
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 ALSOsystemd(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 212SD_LISTEN_FDS(3)