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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | ENVIRONMENT | EXAMPLES | SEE ALSO | COLOPHON |
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SD_NOTIFY(3) sd_notify SD_NOTIFY(3)
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
#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);
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 completion notification.
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 fail, but 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 the following shall
be considered well-known:
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.
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".
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.
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..."
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.
ERRNO=...
If a service fails, the errno-style error code, formatted as
string. Example: "ERRNO=2" for ENOENT.
BUSERROR=...
If a service fails, the D-Bus error-style error code.
Example: "BUSERROR=org.freedesktop.DBus.Error.TimedOut"
EXIT_STATUS=...
If a service exits, the return value of its main() function.
MAINPID=...
The main process ID (PID) of the service, in case the service
manager did not fork off the process itself. Example:
"MAINPID=4711"
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.
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"
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.
FDSTORE=1
Stores additional file descriptors in the service manager.
File descriptors sent this way will be maintained per-service
by the service manager and will later be handed back using
the usual file descriptor passing logic at the next
invocation of the service (e.g. when it is restarted), see
sd_listen_fds(3). This is useful for implementing services
that can restart after an explicit request or a crash without
losing state. Any open sockets and other file descriptors
which should not be closed during the restart may be stored
this way. Application state can either be serialized to a
file in /run/, or better, stored in a memfd_create(2) memory
file descriptor. Note that the service manager will accept
messages for a service only if its FileDescriptorStoreMax=
setting is non-zero (defaults to zero, see
systemd.service(5)). If FDPOLL=0 is not set and the file
descriptors sent 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 arrays of file
descriptors may be sent in separate messages, in which case
the arrays are combined. Note that the service manager
removes duplicate (pointing to the same object) file
descriptors before passing them to the service. When a
service is stopped, its file descriptor store is discarded
and all file descriptors in it are closed. Use
sd_pid_notify_with_fds() to send messages with "FDSTORE=1",
see below. The service manager will set the $FDSTORE
environment variable for services that have the file
descriptor store enabled.
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.
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 set, will implicitly get the name "stored"
assigned. Note that, if multiple file descriptors are
submitted at once, the specified name will be assigned to all
of them. In order to assign different names to submitted file
descriptors, submit them in separate invocations of
sd_pid_notify_with_fds(). 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.
FDPOLL=0
When used in combination with FDSTORE=1, disables polling of
the stored file descriptors regardless of whether or not they
are pollable. As this option disables automatic cleanup of
the stored 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.
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.
It is recommended to prefix variable names that are not listed
above with X_ to avoid namespace clashes.
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.
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).
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. 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.
$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.
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);
systemd(1), sd-daemon(3), sd_listen_fds(3),
sd_listen_fds_with_names(3), sd_watchdog_enabled(3), daemon(7),
systemd.service(5)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2023-06-23. (At that
time, the date of the most recent commit that was found in the
repository was 2023-06-23.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 253 SD_NOTIFY(3)
Pages that refer to this page: systemd(1), systemd-notify(1), systemd-nspawn(1), __pmServerNotifyServiceManagerReady(3), sd-daemon(3), sd_event_set_watchdog(3), sd_listen_fds(3), sd_watchdog_enabled(3), org.freedesktop.systemd1(5), systemd.exec(5), systemd.service(5), daemon(7), systemd.directives(7), systemd.index(7), systemd.system-credentials(7), start-stop-daemon(8)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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