Ubuntu Manpage: sd_notify, sd_notifyf, sd_pid_notify, sd_pid_notifyf, sd_pid_notify_with

Provided by: libsystemd-dev_253.5-1ubuntu6.1_amd64

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
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..."

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 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_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);