sd_notifysystemdsd_notify3sd_notifysd_notifyfsd_pid_notifysd_pid_notifyfsd_pid_notify_with_fdssd_notify_barrierNotify service manager about start-up completion and other service status changes#include <systemd/sd-daemon.h>int sd_notifyint unset_environmentconst char *stateint sd_notifyfint unset_environmentconst char *format…int sd_pid_notifypid_t pidint unset_environmentconst char *stateint sd_pid_notifyfpid_t pidint unset_environmentconst char *format…int sd_pid_notify_with_fdspid_t pidint unset_environmentconst char *stateconst int *fdsunsigned n_fdsint sd_notify_barrierint unset_environmentuint64_t timeoutDescriptionsd_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=1Tells 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=1Tells 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.service5
for details, specifically Type=notify-reload.STOPPING=1Tells 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…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.TimedOutMAINPID=…The main process ID (PID) of the service, in
case the service manager did not fork off the process itself.
Example: MAINPID=4711WATCHDOG=1Tells 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.service5
for information how to enable this functionality and
sd_watchdog_enabled3
for the details of how the service can check whether the
watchdog is enabled. WATCHDOG=triggerTells 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.service5 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=20000000EXTEND_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.service5
for effects on the service timeouts.FDSTORE=1Stores 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_fds3. 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_create2 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.service5). If
FDPOLL=0 is not set and the file descriptors sent are pollable (see
epoll_ctl2), 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.FDSTOREREMOVE=1Removes 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_names3. 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=0When 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=1Tells 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.service5
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= or
NotifyAccess=. 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= 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 sending file descriptors
to the service manager on messages that do not expect them (i.e.
without FDSTORE=1) they are immediately closed
on reception.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
ppoll2. A value of UINT64_MAX is interpreted as infinite timeout.
Return ValueOn 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).NotesThese 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 $NOTIFY_SOCKET is
/ or @, the string is understood
as an AF_UNIX or Linux abstract namespace socket
(respectively), and in both cases the datagram is accompanied by the
process credentials of the sending service, using SCM_CREDENTIALS. If
the string starts with vsock: then the string is
understood as an AF_VSOCK address, which is useful
for hypervisors/VMMs or other processes on the host to receive a
notification when a virtual machine has finished booting. Note that in
case the hypervisor does not support SOCK_DGRAM
over AF_VSOCK, SOCK_SEQPACKET
will be used instead. The address should be in the form:
vsock:CID:PORT. Note that unlike other uses of vsock,
the CID is mandatory and cannot be VMADDR_CID_ANY.
Note that PID1 will send the VSOCK packets from a privileged port
(i.e.: lower than 1024), as an attempt to address concerns that unprivileged
processes in the guest might try to send malicious notifications to the
host, driving it to make destructive decisions based on them.Environment$NOTIFY_SOCKETSet 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.ExamplesStart-up NotificationWhen a service finished starting up, it might issue the
following call to notify the service manager:sd_notify(0, "READY=1");Extended Start-up NotificationA service could send the following after completing
initialization:
sd_notifyf(0, "READY=1\n"
"STATUS=Processing requests…\n"
"MAINPID=%lu",
(unsigned long) getpid());Error Cause NotificationA 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);Store a File Descriptor in the Service ManagerTo 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);Eliminating race conditionsWhen 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);
See Alsosystemd1,
sd-daemon3,
sd_listen_fds3,
sd_listen_fds_with_names3,
sd_watchdog_enabled3,
daemon7,
systemd.service5