SD_LISTEN_FDS(3) sd_listen_fds SD_LISTEN_FDS(3)
NAME
sd_listen_fds, sd_listen_fds_with_names, SD_LISTEN_FDS_START - Check
for file descriptors passed by the system manager
SYNOPSIS
#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);
DESCRIPTION
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.
The file descriptors passed this way may be closed at will by the
processes receiving them: it's up to the processes themselves to close
them after use or whether to leave them open until the process exits
(in which case the kernel closes them automatically). Note that the
file descriptors received by daemons are duplicates of the file
descriptors the service manager originally allocated and bound and of
which it continuously keeps a copy (except if Accept=yes is used). This
means any socket option changes and other changes made to the sockets
will be visible to the service manager too. Most importantly this means
it's generally not a good idea to invoke shutdown(2) on such sockets,
since it will shut down communication on the file descriptor the
service manager holds for the same socket too. Also note that if a
daemon is restarted (and its associated sockets are not) it will
receive file descriptors to the very same sockets as the earlier
invocations, thus all socket options applied then will still apply.
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. |
+-------------+----------------------------+
RETURN VALUE
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.
NOTES
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.
These functions are not designed for services that specify
StandardInput=socket as the $LISTEN_FDS variable is not set in their
environment.
ENVIRONMENT
$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
Set by the service manager for supervised processes that use
socket-based activation. This environment variable specifies the
data sd_listen_fds() and sd_listen_fds_with_names() parses. See
above for details.
SEE ALSO
systemd(1), sd-daemon(3), sd_is_fifo(3), sd_is_socket(3),
sd_is_socket_inet(3), sd_is_socket_unix(3), sd_pid_notify_with_fds(3),
daemon(7), systemd.service(5), systemd.socket(5)
systemd 252 SD_LISTEN_FDS(3)
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