SD_NOTIFY(3) sd_notify SD_NOTIFY(3)
NAME
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
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_notify_barrier(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 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 loading its configuration. This is only used by
systemd if the service definition file has Type=notify 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 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 "READY=1"
notification when it completed reloading its configuration. Reloads
are propagated in the same way as they are when initiated by the
user.
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..."
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"
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.
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 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 ppoll(2). A value of UINT64_MAX is interpreted as infinite
timeout.
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
These APIs are implemented as a shared library, which can be compiled
and linked to with the libsystemd pkg-config(1) file.
These functions send a single datagram with the state string as payload
to the AF_UNIX socket referenced in the $NOTIFY_SOCKET environment
variable. If the first character of $NOTIFY_SOCKET is "@", the string
is understood as Linux abstract namespace socket. The datagram is
accompanied by the process credentials of the sending service, using
SCM_CREDENTIALS.
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);
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)
systemd 252 SD_NOTIFY(3)
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