sd_notify(3) — Linux manual page


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

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:

           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).

           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.

           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

           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..."

           If a service fails, the errno-style error code, formatted as
           string. Example: "ERRNO=2" for ENOENT.

           If a service fails, the D-Bus error-style error code.
           Example: "BUSERROR=org.freedesktop.DBus.Error.TimedOut"

           The main process ID (PID) of the service, in case the service
           manager did not fork off the process itself. Example:

           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.

           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.

           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"

           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.

           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.

           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
           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.

           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 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

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

           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

           sd_notifyf(0, "READY=1\n"
                   "STATUS=Processing requests...\n"
                   (unsigned long) getpid());

       Example 3. Error Cause Notification

       A service could send the following shortly before exiting, on

           sd_notifyf(0, "STATUS=Failed to start up: %s\n"
                   strerror_r(errnum, (char[1024]){}, 1024),

       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),

COLOPHON         top

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       manager) project.  Information about the project can be found at
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       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2022-12-17.  (At that
       time, the date of the most recent commit that was found in the
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systemd 252                                                 SD_NOTIFY(3)

Pages that refer to this page: systemd(1)systemd-notify(1)systemd-nspawn(1)__pmServerNotifyServiceManagerReady(3)sd-daemon(3)sd_event_set_watchdog(3)sd_listen_fds(3)sd_watchdog_enabled(3)org.freedesktop.systemd1(5)systemd.exec(5)systemd.service(5)daemon(7)systemd.directives(7)systemd.index(7)start-stop-daemon(8)