sd_pid_notify(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | ENVIRONMENT | EXAMPLES | SEE ALSO | COLOPHON

SD_NOTIFY(3)                      sd_notify                     SD_NOTIFY(3)

NAME         top

       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         top

       #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         top

       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.

RETURN VALUE         top

       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         top

       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         top

       $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         top

       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);

SEE ALSO         top

       systemd(1), sd-daemon(3), sd_listen_fds(3),
       sd_listen_fds_with_names(3), sd_watchdog_enabled(3), daemon(7),
       systemd.service(5)

COLOPHON         top

       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 2020-07-14.  (At that
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systemd 246                                                     SD_NOTIFY(3)

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