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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | ENVIRONMENT | EXAMPLES | SEE ALSO | COLOPHON |
SD_NOTIFY(3) sd_notify SD_NOTIFY(3)
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
#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);
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, 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. 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.
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).
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.
$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(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
"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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
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systemd 246 SD_NOTIFY(3)
Pages that refer to this page: 30-systemd-environment-d-generator(7) , systemd.index(7)