sd_listen_fds, SD_LISTEN_FDS_START — Check for file descriptors passed by the system manager
#define SD_LISTEN_FDS_START 3
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.
unset_environment parameter is
sd_listen_fds() will unset the
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 unit file (see systemd.socket(5) for details). 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.
If multiple socket units activate the same service the order
of the file descriptors passed to its main process is undefined.
If additional file descriptors have been passed to the service
FDSTORE=1" messages, these file descriptors are
passed last, in arbitrary order, and with duplicates
On failure, this call returns a negative errno-style error
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
These APIs are implemented as a shared
library, which can be compiled and linked to with the
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
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
parses. See above for