SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
sd_listen_fds - Check for file descriptors passed by the init 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
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_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.
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:
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-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(7).
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(7), sd_is_fifo(3), sd_is_socket(3), sd_is_socket_inet(3), sd_is_socket_unix(3), daemon(7), systemd.service(5),
Lennart Poettering <firstname.lastname@example.org>
systemd 10/07/2013 SD_LISTEN_FDS(3)