sd_listen_fds
systemd
Developer
Lennart
Poettering
lennart@poettering.net
sd_listen_fds
3
sd_listen_fds
SD_LISTEN_FDS_START
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
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 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_fifo3,
sd_is_socket3,
sd_is_socket_inet3,
sd_is_socket_unix3
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.
Notes
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.
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.
See Also
systemd1,
sd-daemon3,
sd_is_fifo3,
sd_is_socket3,
sd_is_socket_inet3,
sd_is_socket_unix3,
daemon7,
systemd.service5,
systemd.socket5