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
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
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 sources:
resp.
sd_listen_fds() is
implemented in the reference implementation's
sd-daemon.c and
sd-daemon.h files. These
interfaces are available as shared library, which can
be compiled and linked to with the
libsystemd-daemon
pkg-config1
file. Alternatively, applications consuming these APIs
may copy the implementation into their source
tree. For more details about the reference
implementation see
sd-daemon3.
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.
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