sd_notify, sd_notifyf, sd_pid_notify, sd_pid_notifyf, sd_pid_notify_with_fds, sd_notify_barrier — Notify service manager about start-up completion and other service status changes
|const char *state
|const char *format,
|const char *state
|const char *format,
|const char *state,
|const int *fds,
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.
unset_environment parameter is
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.
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
Tells the service manager that service startup is finished, or the service finished loading its
configuration. This is only used by systemd if the service definition file has
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).
Tells the service manager that the service is
reloading 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 "
notification when it completed reloading its
configuration. Reloads are propagated in the same way as they
are when initiated by the user.
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.
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
If a service fails, the errno-style error
code, formatted as string. Example: "
If a service fails, the D-Bus error-style
error code. Example:
The main process ID (PID) of the service, in
case the service manager did not fork off the process itself.
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
for information how to enable this functionality and
for the details of how the service can check whether the
watchdog is enabled.
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
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
information about the watchdog behavior.
watchdog_usec value during runtime.
Notice that this is not available when using
Example : "
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
for effects on the service timeouts.
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, 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
FDPOLL=0 is not set and the file descriptors sent are pollable (see
epoll_ctl(2)), then any
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. Use
to send messages with "
FDSTORE=1", see below.
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.
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
descriptors submitted without this field set, will implicitly get the name "
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
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.
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.
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
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
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
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
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
sd_notifyf() is similar to
sd_notify() but takes a
printf()-like format string plus
sd_pid_notifyf() are similar to
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_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
no file descriptors are passed. Note that sending file descriptors
to the service manager on messages that do not expect them (i.e.
FDSTORE=1") they are immediately closed
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.
On failure, these calls return a negative errno-style error code. If
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
These APIs are implemented as a shared
library, which can be compiled and linked to with the
These functions send a single datagram with the
state string as payload to the
referenced in the
variable. If the first character of
$NOTIFY_SOCKET is "
string is understood as Linux abstract namespace socket. The
datagram is accompanied by the process credentials of the sending
service, using SCM_CREDENTIALS.
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
Example 1. Start-up Notification
When a service finished starting up, it might issue the following call to notify the service manager:
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(errno), errno);
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 "
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
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);