sd_listen_fds, sd_listen_fds_with_names, 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); |
int sd_listen_fds_with_names( | int unset_environment, |
char*** names); |
sd_listen_fds() may be invoked by a
daemon to check for file descriptors passed by the service manager as
part of the socket-based activation logic. It returns the number
of received file descriptors. If no file descriptors have been
received, zero is returned. The first file descriptor may be found
at file descriptor number 3
(i.e. SD_LISTEN_FDS_START), the remaining
descriptors follow at 4, 5, 6, …, if any.
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) — if there's only one such file (see below). 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
manager using
sd_pid_notify_with_fds(3)'s
"FDSTORE=1" messages, these file descriptors are
passed last, in arbitrary order, and with duplicates
removed.
If the unset_environment parameter is
non-zero, sd_listen_fds() will unset the
$LISTEN_FDS, $LISTEN_PID and
$LISTEN_FDNAMES environment variables before
returning (regardless of whether the function call itself
succeeded or not). Further calls to
sd_listen_fds() will then return zero, but the
variables are no longer inherited by child processes.
sd_listen_fds_with_names() is like
sd_listen_fds(), but optionally also returns
an array of strings with identification names for the passed file
descriptors, if that is available and the
names parameter is non-NULL. This
information is read from the $LISTEN_FDNAMES
variable, which may contain a colon-separated list of names. For
socket-activated services, these names may be configured with the
FileDescriptorName= setting in socket unit
files, see
systemd.socket(5)
for details. For file descriptors pushed into the file descriptor
store (see above), the name is set via the
FDNAME= field transmitted via
sd_pid_notify_with_fds(). The primary usecase
for these names are services which accept a variety of file
descriptors which are not recognizable with functions like
sd_is_socket() alone, and thus require
identification via a name. It is recommended to rely on named file
descriptors only if identification via
sd_is_socket() and related calls is not
sufficient. Note that the names used are not unique in any
way. The returned array of strings has as many entries as file
descriptors have been received, plus a final NULL pointer
terminating the array. The caller needs to free the array itself
and each of its elements with libc's free()
call after use. If the names parameter is
NULL, the call is entirely equivalent to
sd_listen_fds().
Under specific conditions, the following automatic file descriptor names are returned:
Table 1. Special names
| Name | Description |
|---|---|
"unknown" | The process received no name for the specific file descriptor from the service manager. |
"stored" | The file descriptor originates in the service manager's per-service file descriptor store, and the FDNAME= field was absent when the file descriptor was submitted to the service manager. |
"connection" | The service was activated in per-connection style using Accept=yes in the socket unit file, and the file descriptor is the connection socket. |
On failure, these calls 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.
These APIs are implemented as a shared
library, which can be compiled and linked to with the
libsystemd pkg-config(1)
file.
Internally, sd_listen_fds() 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. sd_listen_fds_with_names() does the
same but also parses $LISTEN_FDNAMES if
set.