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
#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.
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
sd_listen_fds() is implemented in the reference implementation's sd-daemon.c and
sd-daemon.h files. These interfaces are available as a 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.
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 208 SD_LISTEN_FDS(3)