Provided by: libsystemd-dev_256.5-2ubuntu3.1_amd64 
NAME
sd_notify, sd_notifyf, sd_pid_notify, sd_pid_notifyf, sd_pid_notify_with_fds,
sd_pid_notifyf_with_fds, sd_notify_barrier, sd_pid_notify_barrier - Notify service manager
about start-up completion and other service status changes
SYNOPSIS
#include <systemd/sd-daemon.h>
int sd_notify(int unset_environment, const char *state);
int sd_notifyf(int unset_environment, const char *format, ...);
int sd_pid_notify(pid_t pid, int unset_environment, const char *state);
int sd_pid_notifyf(pid_t pid, int unset_environment, const char *format, ...);
int sd_pid_notify_with_fds(pid_t pid, int unset_environment, const char *state,
const int *fds, unsigned n_fds);
int sd_pid_notifyf_with_fds(pid_t pid, int unset_environment, const int *fds,
size_t n_fds, const char *format, ...);
int sd_notify_barrier(int unset_environment, uint64_t timeout);
int sd_pid_notify_barrier(pid_t pid, int unset_environment, uint64_t timeout);
DESCRIPTION
sd_notify() may be called by a service to notify the service manager about state changes.
It can be used to send arbitrary information, encoded in an environment-block-like string.
Most importantly, it can be used for start-up or reload completion notifications.
If the unset_environment parameter is non-zero, sd_notify() will unset the $NOTIFY_SOCKET
environment variable before returning (regardless of whether the function call itself
succeeded or not). Further calls to sd_notify() will then silently do nothing, and the
variable is no longer inherited by child processes.
The state parameter should contain a newline-separated list of variable assignments,
similar in style to an environment block. A trailing newline is implied if none is
specified. The string may contain any kind of variable assignments, but see the next
section for a list of assignments understood by the service manager.
Note that systemd will accept status data sent from a service only if the NotifyAccess=
option is correctly set in the service definition file. See systemd.service(5) for
details.
Note that sd_notify() notifications may be attributed to units correctly only if either
the sending process is still around at the time PID 1 processes the message, or if the
sending process is explicitly runtime-tracked by the service manager. The latter is the
case if the service manager originally forked off the process, i.e. on all processes that
match NotifyAccess=main or NotifyAccess=exec. Conversely, if an auxiliary process of the
unit sends an sd_notify() message and immediately exits, the service manager might not be
able to properly attribute the message to the unit, and thus will ignore it, even if
NotifyAccess=all is set for it.
Hence, to eliminate all race conditions involving lookup of the client's unit and
attribution of notifications to units correctly, sd_notify_barrier() may be used. This
call acts as a synchronization point and ensures all notifications sent before this call
have been picked up by the service manager when it returns successfully. Use of
sd_notify_barrier() is needed for clients which are not invoked by the service manager,
otherwise this synchronization mechanism is unnecessary for attribution of notifications
to the unit.
sd_notifyf() is similar to sd_notify() but takes a printf()-like format string plus
arguments.
sd_pid_notify() and sd_pid_notifyf() are similar to sd_notify() and sd_notifyf() but take
a process ID (PID) to use as originating PID for the message as first argument. This is
useful to send notification messages on behalf of other processes, provided the
appropriate privileges are available. If the PID argument is specified as 0, the process
ID of the calling process is used, in which case the calls are fully equivalent to
sd_notify() and sd_notifyf().
sd_pid_notify_with_fds() is similar to sd_pid_notify() but takes an additional array of
file descriptors. These file descriptors are sent along the notification message to the
service manager. This is particularly useful for sending "FDSTORE=1" messages, as
described above. The additional arguments are a pointer to the file descriptor array plus
the number of file descriptors in the array. If the number of file descriptors is passed
as 0, the call is fully equivalent to sd_pid_notify(), i.e. no file descriptors are
passed. Note that file descriptors sent to the service manager on a message without
"FDSTORE=1" are immediately closed on reception.
sd_pid_notifyf_with_fds() is a combination of sd_pid_notify_with_fds() and sd_notifyf(),
i.e. it accepts both a PID and a set of file descriptors as input, and processes a format
string to generate the state string.
sd_notify_barrier() allows the caller to synchronize against reception of previously sent
notification messages and uses the BARRIER=1 command. It takes a relative timeout value in
microseconds which is passed to ppoll(2). A value of UINT64_MAX is interpreted as infinite
timeout.
sd_pid_notify_barrier() is just like sd_notify_barrier(), but allows specifying the
originating PID for the notification message.
WELL-KNOWN ASSIGNMENTS
The following assignments have a defined meaning:
READY=1
Tells the service manager that service startup is finished, or the service finished
re-loading its configuration. This is only used by systemd if the service definition
file has Type=notify or Type=notify-reload set. Since there is little value in
signaling non-readiness, the only value services should send is "READY=1" (i.e.
"READY=0" is not defined).
RELOADING=1
Tells the service manager that the service is beginning to reload its configuration.
This is useful to allow the service manager to track the service's internal state, and
present it to the user. Note that a service that sends this notification must also
send a "READY=1" notification when it completed reloading its configuration. Reloads
the service manager is notified about with this mechanisms are propagated in the same
way as they are when originally initiated through the service manager. This message is
particularly relevant for Type=notify-reload services, to inform the service manager
that the request to reload the service has been received and is now being processed.
Added in version 217.
STOPPING=1
Tells the service manager that the service is beginning its shutdown. This is useful
to allow the service manager to track the service's internal state, and present it to
the user.
Added in version 217.
MONOTONIC_USEC=...
A field carrying the monotonic timestamp (as per CLOCK_MONOTONIC) formatted in decimal
in μs, when the notification message was generated by the client. This is typically
used in combination with "RELOADING=1", to allow the service manager to properly
synchronize reload cycles. See systemd.service(5) for details, specifically
"Type=notify-reload".
Added in version 253.
STATUS=...
Passes a single-line UTF-8 status string back to the service manager that describes
the service state. This is free-form and can be used for various purposes: general
state feedback, fsck-like programs could pass completion percentages and failing
programs could pass a human-readable error message. Example: "STATUS=Completed 66% of
file system check..."
Added in version 233.
NOTIFYACCESS=...
Reset the access to the service status notification socket during runtime, overriding
NotifyAccess= setting in the service unit file. See systemd.service(5) for details,
specifically "NotifyAccess=" for a list of accepted values.
Added in version 254.
ERRNO=...
If a service fails, the errno-style error code, formatted as string. Example:
"ERRNO=2" for ENOENT.
Added in version 233.
BUSERROR=...
If a service fails, the D-Bus error-style error code. Example:
"BUSERROR=org.freedesktop.DBus.Error.TimedOut". Note that this assignment is currently
not used by systemd.
Added in version 233.
EXIT_STATUS=...
The exit status of a service or the manager itself. Note that systemd currently does
not consume this value when sent by services, so this assignment is only
informational. The manager will send this notification to its notification socket,
which may be used to collect an exit status from the system (a container or VM) as it
shuts down. For example, mkosi(1) makes use of this. The value to return may be set
via the systemctl(1) exit verb.
Added in version 254.
MAINPID=...
The main process ID (PID) of the service, in case the service manager did not fork off
the process itself. Example: "MAINPID=4711".
Added in version 233.
WATCHDOG=1
Tells the service manager to update the watchdog timestamp. This is the keep-alive
ping that services need to issue in regular intervals if WatchdogSec= is enabled for
it. See systemd.service(5) for information how to enable this functionality and
sd_watchdog_enabled(3) for the details of how the service can check whether the
watchdog is enabled.
WATCHDOG=trigger
Tells the service manager that the service detected an internal error that should be
handled by the configured watchdog options. This will trigger the same behaviour as if
WatchdogSec= is enabled and the service did not send "WATCHDOG=1" in time. Note that
WatchdogSec= does not need to be enabled for "WATCHDOG=trigger" to trigger the
watchdog action. See systemd.service(5) for information about the watchdog behavior.
Added in version 243.
WATCHDOG_USEC=...
Reset watchdog_usec value during runtime. Notice that this is not available when using
sd_event_set_watchdog() or sd_watchdog_enabled(). Example : "WATCHDOG_USEC=20000000"
Added in version 233.
EXTEND_TIMEOUT_USEC=...
Tells the service manager to extend the startup, runtime or shutdown service timeout
corresponding the current state. The value specified is a time in microseconds during
which the service must send a new message. A service timeout will occur if the message
isn't received, but only if the runtime of the current state is beyond the original
maximum times of TimeoutStartSec=, RuntimeMaxSec=, and TimeoutStopSec=. See
systemd.service(5) for effects on the service timeouts.
Added in version 236.
FDSTORE=1
Store file descriptors in the service manager. File descriptors sent this way will be
held for the service by the service manager and will later be handed back using the
usual file descriptor passing logic at the next start or restart of the service, see
sd_listen_fds(3). Any open sockets and other file descriptors which should not be
closed during a restart may be stored this way. When a service is stopped, its file
descriptor store is discarded and all file descriptors in it are closed, except when
overridden with FileDescriptorStorePreserve=, see systemd.service(5).
The service manager will accept messages for a service only if its
FileDescriptorStoreMax= setting is non-zero (defaults to zero, see
systemd.service(5)). The service manager will set the $FDSTORE environment variable
for services that have the file descriptor store enabled, see systemd.exec(5).
If FDPOLL=0 is not set and the file descriptors are pollable (see epoll_ctl(2)), then
any EPOLLHUP or EPOLLERR event seen on them will result in their automatic removal
from the store.
Multiple sets of file descriptors may be sent in separate messages, in which case the
sets are combined. The service manager removes duplicate file descriptors (those
pointing to the same object) before passing them to the service.
This functionality should be used to implement services that can restart after an
explicit request or a crash without losing state. Application state can either be
serialized to a file in /run/, or better, stored in a memfd_create(2) memory file
descriptor. Use sd_pid_notify_with_fds() to send messages with "FDSTORE=1". It is
recommended to combine FDSTORE= with FDNAME= to make it easier to manage the stored
file descriptors.
For further information on the file descriptor store see the File Descriptor Store[1]
overview.
Added in version 219.
FDSTOREREMOVE=1
Removes file descriptors from the file descriptor store. This field needs to be
combined with FDNAME= to specify the name of the file descriptors to remove.
Added in version 236.
FDNAME=...
When used in combination with FDSTORE=1, specifies a name for the submitted file
descriptors. When used with FDSTOREREMOVE=1, specifies the name for the file
descriptors to remove. This name is passed to the service during activation, and may
be queried using sd_listen_fds_with_names(3). File descriptors submitted without this
field will be called "stored".
The name may consist of arbitrary ASCII characters except control characters or ":".
It may not be longer than 255 characters. If a submitted name does not follow these
restrictions, it is ignored.
Note that if multiple file descriptors are submitted in a single message, the
specified name will be used for all of them. In order to assign different names to
submitted file descriptors, submit them in separate messages.
Added in version 233.
FDPOLL=0
When used in combination with FDSTORE=1, disables polling of the submitted file
descriptors regardless of whether or not they are pollable. As this option disables
automatic cleanup of the submitted file descriptors on EPOLLERR and EPOLLHUP, care
must be taken to ensure proper manual cleanup. Use of this option is not generally
recommended except for when automatic cleanup has unwanted behavior such as
prematurely discarding file descriptors from the store.
Added in version 246.
BARRIER=1
Tells the service manager that the client is explicitly requesting synchronization by
means of closing the file descriptor sent with this command. The service manager
guarantees that the processing of a BARRIER=1 command will only happen after all
previous notification messages sent before this command have been processed. Hence,
this command accompanied with a single file descriptor can be used to synchronize
against reception of all previous status messages. Note that this command cannot be
mixed with other notifications, and has to be sent in a separate message to the
service manager, otherwise all assignments will be ignored. Note that sending 0 or
more than 1 file descriptor with this command is a violation of the protocol.
Added in version 246.
The notification messages sent by services are interpreted by the service manager. Unknown
assignments are ignored. Thus, it is safe (but often without effect) to send assignments
which are not in this list. The protocol is extensible, but care should be taken to ensure
private extensions are recognizable as such. Specifically, it is recommend to prefix them
with "X_" followed by some namespace identifier. The service manager also sends some
messages to its notification socket, which may then consumed by a supervising machine or
container manager further up the stack. The service manager sends a number of extension
fields, for example X_SYSTEMD_UNIT_ACTIVE=, for details see systemd(1).
RETURN VALUE
On failure, these calls return a negative errno-style error code. If $NOTIFY_SOCKET was
not set and hence no status message could be sent, 0 is returned. If the status was sent,
these functions return a positive value. In order to support both service managers that
implement this scheme and those which do not, it is generally recommended to ignore the
return value of this call. Note that the return value simply indicates whether the
notification message was enqueued properly, it does not reflect whether the message could
be processed successfully. Specifically, no error is returned when a file descriptor is
attempted to be stored using FDSTORE=1 but the service is not actually configured to
permit storing of file descriptors (see above).
NOTES
Functions described here are available as a shared library, which can be compiled against
and linked to with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe.
This means that the code calling the functions described here must not call setenv(3) from
a parallel thread. It is recommended to only do calls to setenv() from an early phase of
the program when no other threads have been started.
These functions send a single datagram with the state string as payload to the socket
referenced in the $NOTIFY_SOCKET environment variable. If the first character of
$NOTIFY_SOCKET is "/" or "@", the string is understood as an AF_UNIX or Linux abstract
namespace socket (respectively), and in both cases the datagram is accompanied by the
process credentials of the sending service, using SCM_CREDENTIALS. If the string starts
with "vsock:" then the string is understood as an AF_VSOCK address, which is useful for
hypervisors/VMMs or other processes on the host to receive a notification when a virtual
machine has finished booting. Note that in case the hypervisor does not support SOCK_DGRAM
over AF_VSOCK, SOCK_SEQPACKET will be used instead. "vsock-stream", "vsock-dgram" and
"vsock-seqpacket" can be used instead of "vsock" to force usage of the corresponding
socket type. The address should be in the form: "vsock:CID:PORT". Note that unlike other
uses of vsock, the CID is mandatory and cannot be "VMADDR_CID_ANY". Note that PID1 will
send the VSOCK packets from a privileged port (i.e.: lower than 1024), as an attempt to
address concerns that unprivileged processes in the guest might try to send malicious
notifications to the host, driving it to make destructive decisions based on them.
Standalone Implementations
Note that, while using this library should be preferred in order to avoid code
duplication, it is also possible to reimplement the simple readiness notification protocol
without external dependencies, as demonstrated in the following self-contained examples
from several languages:
C
/* SPDX-License-Identifier: MIT-0 */
/* Implement the systemd notify protocol without external dependencies.
* Supports both readiness notification on startup and on reloading,
* according to the protocol defined at:
* https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
* This protocol is guaranteed to be stable as per:
* https://systemd.io/PORTABILITY_AND_STABILITY/ */
#define _GNU_SOURCE 1
#include <errno.h>
#include <inttypes.h>
#include <signal.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <time.h>
#include <unistd.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
static void closep(int *fd) {
if (!fd || *fd < 0)
return;
close(*fd);
*fd = -1;
}
static int notify(const char *message) {
union sockaddr_union {
struct sockaddr sa;
struct sockaddr_un sun;
} socket_addr = {
.sun.sun_family = AF_UNIX,
};
size_t path_length, message_length;
_cleanup_(closep) int fd = -1;
const char *socket_path;
/* Verify the argument first */
if (!message)
return -EINVAL;
message_length = strlen(message);
if (message_length == 0)
return -EINVAL;
/* If the variable is not set, the protocol is a noop */
socket_path = getenv("NOTIFY_SOCKET");
if (!socket_path)
return 0; /* Not set? Nothing to do */
/* Only AF_UNIX is supported, with path or abstract sockets */
if (socket_path[0] != '/' && socket_path[0] != '@')
return -EAFNOSUPPORT;
path_length = strlen(socket_path);
/* Ensure there is room for NUL byte */
if (path_length >= sizeof(socket_addr.sun.sun_path))
return -E2BIG;
memcpy(socket_addr.sun.sun_path, socket_path, path_length);
/* Support for abstract socket */
if (socket_addr.sun.sun_path[0] == '@')
socket_addr.sun.sun_path[0] = 0;
fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0);
if (fd < 0)
return -errno;
if (connect(fd, &socket_addr.sa, offsetof(struct sockaddr_un, sun_path) + path_length) != 0)
return -errno;
ssize_t written = write(fd, message, message_length);
if (written != (ssize_t) message_length)
return written < 0 ? -errno : -EPROTO;
return 1; /* Notified! */
}
static int notify_ready(void) {
return notify("READY=1");
}
static int notify_reloading(void) {
/* A buffer with length sufficient to format the maximum UINT64 value. */
char reload_message[sizeof("RELOADING=1\nMONOTONIC_USEC=18446744073709551615")];
struct timespec ts;
uint64_t now;
/* Notify systemd that we are reloading, including a CLOCK_MONOTONIC timestamp in usec
* so that the program is compatible with a Type=notify-reload service. */
if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0)
return -errno;
if (ts.tv_sec < 0 || ts.tv_nsec < 0 ||
(uint64_t) ts.tv_sec > (UINT64_MAX - (ts.tv_nsec / 1000ULL)) / 1000000ULL)
return -EINVAL;
now = (uint64_t) ts.tv_sec * 1000000ULL + (uint64_t) ts.tv_nsec / 1000ULL;
if (snprintf(reload_message, sizeof(reload_message), "RELOADING=1\nMONOTONIC_USEC=%" PRIu64, now) < 0)
return -EINVAL;
return notify(reload_message);
}
static int notify_stopping(void) {
return notify("STOPPING=1");
}
static volatile sig_atomic_t reloading = 0;
static volatile sig_atomic_t terminating = 0;
static void signal_handler(int sig) {
if (sig == SIGHUP)
reloading = 1;
else if (sig == SIGINT || sig == SIGTERM)
terminating = 1;
}
int main(int argc, char **argv) {
struct sigaction sa = {
.sa_handler = signal_handler,
.sa_flags = SA_RESTART,
};
int r;
/* Setup signal handlers */
sigemptyset(&sa.sa_mask);
sigaction(SIGHUP, &sa, NULL);
sigaction(SIGINT, &sa, NULL);
sigaction(SIGTERM, &sa, NULL);
/* Do more service initialization work here ... */
/* Now that all the preparations steps are done, signal readiness */
r = notify_ready();
if (r < 0) {
fprintf(stderr, "Failed to notify readiness to $NOTIFY_SOCKET: %s\n", strerror(-r));
return EXIT_FAILURE;
}
while (!terminating) {
if (reloading) {
reloading = false;
/* As a separate but related feature, we can also notify the manager
* when reloading configuration. This allows accurate state-tracking,
* and also automated hook-in of 'systemctl reload' without having to
* specify manually an ExecReload= line in the unit file. */
r = notify_reloading();
if (r < 0) {
fprintf(stderr, "Failed to notify reloading to $NOTIFY_SOCKET: %s\n", strerror(-r));
return EXIT_FAILURE;
}
/* Do some reconfiguration work here ... */
r = notify_ready();
if (r < 0) {
fprintf(stderr, "Failed to notify readiness to $NOTIFY_SOCKET: %s\n", strerror(-r));
return EXIT_FAILURE;
}
}
/* Do some daemon work here ... */
sleep(5);
}
r = notify_stopping();
if (r < 0) {
fprintf(stderr, "Failed to report termination to $NOTIFY_SOCKET: %s\n", strerror(-r));
return EXIT_FAILURE;
}
/* Do some shutdown work here ... */
return EXIT_SUCCESS;
}
Python
#!/usr/bin/env python3
# SPDX-License-Identifier: MIT-0
#
# Implement the systemd notify protocol without external dependencies.
# Supports both readiness notification on startup and on reloading,
# according to the protocol defined at:
# https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
# This protocol is guaranteed to be stable as per:
# https://systemd.io/PORTABILITY_AND_STABILITY/
import errno
import os
import signal
import socket
import sys
import time
reloading = False
terminating = False
def notify(message):
if not message:
raise ValueError("notify() requires a message")
socket_path = os.environ.get("NOTIFY_SOCKET")
if not socket_path:
return
if socket_path[0] not in ("/", "@"):
raise OSError(errno.EAFNOSUPPORT, "Unsupported socket type")
# Handle abstract socket.
if socket_path[0] == "@":
socket_path = "\0" + socket_path[1:]
with socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM | socket.SOCK_CLOEXEC) as sock:
sock.connect(socket_path)
sock.sendall(message)
def notify_ready():
notify(b"READY=1")
def notify_reloading():
microsecs = time.clock_gettime_ns(time.CLOCK_MONOTONIC) // 1000
notify(f"RELOADING=1\nMONOTONIC_USEC={microsecs}".encode())
def notify_stopping():
notify(b"STOPPING=1")
def reload(signum, frame):
global reloading
reloading = True
def terminate(signum, frame):
global terminating
terminating = True
def main():
print("Doing initial setup")
global reloading, terminating
# Set up signal handlers.
print("Setting up signal handlers")
signal.signal(signal.SIGHUP, reload)
signal.signal(signal.SIGINT, terminate)
signal.signal(signal.SIGTERM, terminate)
# Do any other setup work here.
# Once all setup is done, signal readiness.
print("Done setting up")
notify_ready()
print("Starting loop")
while not terminating:
if reloading:
print("Reloading")
reloading = False
# Support notifying the manager when reloading configuration.
# This allows accurate state tracking as well as automatically
# enabling 'systemctl reload' without needing to manually
# specify an ExecReload= line in the unit file.
notify_reloading()
# Do some reconfiguration work here.
print("Done reloading")
notify_ready()
# Do the real work here ...
print("Sleeping for five seconds")
time.sleep(5)
print("Terminating")
notify_stopping()
if __name__ == "__main__":
sys.stdout.reconfigure(line_buffering=True)
print("Starting app")
main()
print("Stopped app")
ENVIRONMENT
$NOTIFY_SOCKET
Set by the service manager for supervised processes for status and start-up completion
notification. This environment variable specifies the socket sd_notify() talks to. See
above for details.
EXAMPLES
Example 1. Start-up Notification
When a service finished starting up, it might issue the following call to notify the
service manager:
sd_notify(0, "READY=1");
Example 2. Extended Start-up Notification
A service could send the following after completing initialization:
sd_notifyf(0, "READY=1\n"
"STATUS=Processing requests...\n"
"MAINPID=%lu",
(unsigned long) getpid());
Example 3. Error Cause Notification
A service could send the following shortly before exiting, on failure:
sd_notifyf(0, "STATUS=Failed to start up: %s\n"
"ERRNO=%i",
strerror_r(errnum, (char[1024]){}, 1024),
errnum);
Example 4. Store a File Descriptor in the Service Manager
To store an open file descriptor in the service manager, in order to continue operation
after a service restart without losing state, use "FDSTORE=1":
sd_pid_notify_with_fds(0, 0, "FDSTORE=1\nFDNAME=foobar", &fd, 1);
Example 5. Eliminating race conditions
When the client sending the notifications is not spawned by the service manager, it may
exit too quickly and the service manager may fail to attribute them correctly to the unit.
To prevent such races, use sd_notify_barrier() to synchronize against reception of all
notifications sent before this call is made.
sd_notify(0, "READY=1");
/* set timeout to 5 seconds */
sd_notify_barrier(0, 5 * 1000000);
HISTORY
sd_pid_notify(), sd_pid_notifyf(), and sd_pid_notify_with_fds() were added in version 219.
sd_notify_barrier() was added in version 246.
sd_pid_notifyf_with_fds() and sd_pid_notify_barrier() were added in version 254.
SEE ALSO
systemd(1), sd-daemon(3), sd_listen_fds(3), sd_listen_fds_with_names(3),
sd_watchdog_enabled(3), daemon(7), systemd.service(5)
NOTES
1. File Descriptor Store
https://systemd.io/FILE_DESCRIPTOR_STORE