# sd_notify

## NAME



           sd_notify, sd_notifyf, sd_pid_notify, sd_pid_notifyf,
           sd_pid_notify_with_fds, sd_pid_notifyf_with_fds,
           sd_notify_barrier, sd_pid_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_pid_notifyf_with_fds(pid_t** _pid_**, int** _unset_environment_**,**
                                       **const int ***_fds_**, size_t** _n_fds_**,**
                                       **const char ***_format_**, ...);**
    
           **int sd_notify_barrier(int** _unset_environment_**, uint64_t** _timeout_**);**
    
           **int sd_pid_notify_barrier(pid_t** _pid_**, 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 or reload completion
           notifications.
    
           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 silently do nothing, and
           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 see the next section
           for a list of assignments understood by the service manager.
    
           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. Effectively, this means that
           a privileged invocation of **sd_pid_notify()** may circumvent
           _NotifyAccess=main_ or _NotifyAccess=exec_ restrictions enforced for a
           service. 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 file descriptors sent to the service manager
           on a message without "FDSTORE=1" are immediately closed on
           reception.
    
           **sd_pid_notifyf_with_fds()** is a combination of
           **sd_pid_notify_with_fds()** and **sd_notifyf()** , i.e. it accepts both a
           PID and a set of file descriptors as input, and processes a format
           string to generate the state string.
    
           **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.
    
           **sd_pid_notify_barrier()** is just like **sd_notify_barrier()** , but
           allows specifying the originating PID for the notification
           message.
    


## WELL-KNOWN ASSIGNMENTS



           The following assignments have a defined meaning:
    
           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.
    
               Added in version 217.
    
           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.
    
               Added in version 217.
    
           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".
    
               Added in version 253.
    
           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..."
    
               Added in version 233.
    
           NOTIFYACCESS=...
               Reset the access to the service status notification socket
               during runtime, overriding _NotifyAccess=_ setting in the
               service unit file. See systemd.service(5) for details,
               specifically "NotifyAccess=" for a list of accepted values.
    
               Added in version 254.
    
           ERRNO=...
               If a service fails, the errno-style error code, formatted as
               string. Example: "ERRNO=2" for ENOENT.
    
               Added in version 233.
    
           BUSERROR=...
               If a service fails, the D-Bus error-style error code. Example:
               "BUSERROR=org.freedesktop.DBus.Error.TimedOut".
    
               Added in version 233.
    
           VARLINKERROR=...
               If a service fails, the Varlink error-style error code.
               Example: "VARLINKERROR=org.varlink.service.InvalidParameter".
    
               Added in version 257.
    
           EXIT_STATUS=...
               The exit status of a service or the manager itself. Note that
               **systemd** currently does not consume this value when sent by
               services, so this assignment is only informational. The
               manager will send this notification to _its_ notification
               socket, which may be used to collect an exit status from the
               system (a container or VM) as it shuts down. For example,
               **mkosi**(1) makes use of this. The value to return may be set via
               the systemctl(1) **exit** verb.
    
               Added in version 254.
    
           MAINPID=...
               Change the main process ID (PID) of the service. This is
               especially useful in the case where the real main process is
               not directly forked off by the service manager. Example:
               "MAINPID=4711".
    
               Added in version 233.
    
           MAINPIDFDID=...
               The pidfd inode number of the new main process (specified
               through _MAINPID=_). This information can be acquired through
               sd_pidfd_get_inode_id(3) on the pidfd and is used to identify
               the process in a race-free fashion. Alternatively, a pidfd can
               be sent directly to the service manager (see _MAINPIDFD=1_
               below).
    
               Added in version 257.
    
           MAINPIDFD=1
               Similar to _MAINPID=_ with _MAINPIDFDID=_ , but the process is
               referenced directly by the pidfd passed to the service
               manager. This is useful if pidfd id is not supported on the
               system. Exactly one fd is expected for this notification.
    
               Added in version 257.
    
           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.
    
               Added in version 243.
    
           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"
    
               Added in version 233.
    
           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 is not 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.
    
               Added in version 236.
    
           RESTART_RESET=1
               Reset the restart counter of the service, which has the effect
               of restoring the restart duration to _RestartSec=_ if
               _RestartSteps=_ and _RestartMaxDelaySec=_ are in use. For more
               information, refer to systemd.service(5).
    
               Added in version 258.
    
           FDSTORE=1
               Store file descriptors in the service manager. File
               descriptors sent this way will be held for the service by the
               service manager and will later be handed back using the usual
               file descriptor passing logic at the next start or restart of
               the service, see sd_listen_fds(3). Any open sockets and other
               file descriptors which should not be closed during a restart
               may be stored this way. When a service is stopped, its file
               descriptor store is discarded and all file descriptors in it
               are closed, except when overridden with
               _FileDescriptorStorePreserve=_ , see systemd.service(5).
    
               The service manager will accept messages for a service only if
               its _FileDescriptorStoreMax=_ setting is non-zero (defaults to
               zero, see systemd.service(5)). The service manager will set
               the _$FDSTORE_ environment variable for services that have the
               file descriptor store enabled, see systemd.exec(5).
    
               If _FDPOLL=0_ is not set and the file descriptors 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 sets of file descriptors may be sent in separate
               messages, in which case the sets are combined. The service
               manager removes duplicate file descriptors (those pointing to
               the same object) before passing them to the service.
    
               This functionality should be used to implement services that
               can restart after an explicit request or a crash without
               losing state. Application state can either be serialized to a
               file in /run/, or better, stored in a memfd_create(2) memory
               file descriptor. Use **sd_pid_notify_with_fds()** to send messages
               with "FDSTORE=1". It is recommended to combine _FDSTORE=_ with
               _FDNAME=_ to make it easier to manage the stored file
               descriptors.
    
               For further information on the file descriptor store see the
               **File Descriptor Store******[1] overview.
    
               Added in version 219.
    
           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.
    
               Added in version 236.
    
           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 will be called "stored".
    
               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.
    
               Note that if multiple file descriptors are submitted in a
               single message, the specified name will be used for all of
               them. In order to assign different names to submitted file
               descriptors, submit them in separate messages.
    
               Added in version 233.
    
           FDPOLL=0
               When used in combination with _FDSTORE=1_ , disables polling of
               the submitted file descriptors regardless of whether or not
               they are pollable. As this option disables automatic cleanup
               of the submitted 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.
    
               Added in version 246.
    
           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.
    
               Added in version 246.
    
           The notification messages sent by services are interpreted by the
           service manager. Unknown assignments are ignored. Thus, it is safe
           (but often without effect) to send assignments which are not in
           this list. The protocol is extensible, but care should be taken to
           ensure private extensions are recognizable as such. Specifically,
           it is recommend to prefix them with "X_" followed by some
           namespace identifier. The service manager also sends some messages
           to _its_ notification socket, which may then consumed by a
           supervising machine or container manager further up the stack. The
           service manager sends a number of extension fields, for example
           _X_SYSTEMD_UNIT_ACTIVE=_ , for details see systemd(1).
    


## 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).
    
       **Errors**
           Returned errors may indicate the following problems:
    
           **-E2BIG**
               More file descriptors passed at once than the system allows.
               On Linux the number of file descriptors that may be passed
               across **AF_UNIX** sockets at once is 253, see unix(7) for
               details.
    
               Added in version 257.
    


## NOTES



           Functions described here are available as a shared library, which
           can be compiled against and linked to with the
           **libsystemd pkg-config**(1) file.
    
           The code described here uses getenv(3), which is declared to be
           not multi-thread-safe. This means that the code calling the
           functions described here must not call setenv(3) from a parallel
           thread. It is recommended to only do calls to **setenv()** from an
           early phase of the program when no other threads have been
           started.
    
           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.  "vsock-stream",
           "vsock-dgram" and "vsock-seqpacket" can be used instead of "vsock"
           to force usage of the corresponding socket type. 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.
    
       **Standalone Implementations**
           Note that, while using this library should be preferred in order
           to avoid code duplication, it is also possible to reimplement the
           simple readiness notification protocol without external
           dependencies, as demonstrated in the following self-contained
           examples from several languages:
    
           **C**
    
                   /* SPDX-License-Identifier: MIT-0 */
    
                   /* Implement the systemd notify protocol without external dependencies.
                    * Supports both readiness notification on startup and on reloading,
                    * according to the protocol defined at:
                    * https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
                    * This protocol is guaranteed to be stable as per:
                    * https://systemd.io/PORTABILITY_AND_STABILITY/ */
    
                   #define _GNU_SOURCE 1
                   #include <errno.h>
                   #include <inttypes.h>
                   #include <signal.h>
                   #include <stdbool.h>
                   #include <stddef.h>
                   #include <stdlib.h>
                   #include <stdio.h>
                   #include <string.h>
                   #include <sys/socket.h>
                   #include <sys/un.h>
                   #include <time.h>
                   #include <unistd.h>
    
                   #define _cleanup_(f) __attribute__((cleanup(f)))
    
                   static void closep(int *fd) {
                     if (!fd || *fd < 0)
                       return;
    
                     close(*fd);
                     *fd = -1;
                   }
    
                   static int notify(const char *message) {
                     union sockaddr_union {
                       struct sockaddr sa;
                       struct sockaddr_un sun;
                     } socket_addr = {
                       .sun.sun_family = AF_UNIX,
                     };
                     size_t path_length, message_length;
                     _cleanup_(closep) int fd = -1;
                     const char *socket_path;
    
                     /* Verify the argument first */
                     if (!message)
                       return -EINVAL;
    
                     message_length = strlen(message);
                     if (message_length == 0)
                       return -EINVAL;
    
                     /* If the variable is not set, the protocol is a noop */
                     socket_path = getenv("NOTIFY_SOCKET");
                     if (!socket_path)
                       return 0; /* Not set? Nothing to do */
    
                     /* Only AF_UNIX is supported, with path or abstract sockets */
                     if (socket_path[0] != '/' && socket_path[0] != '@')
                       return -EAFNOSUPPORT;
    
                     path_length = strlen(socket_path);
                     /* Ensure there is room for NUL byte */
                     if (path_length >= sizeof(socket_addr.sun.sun_path))
                       return -E2BIG;
    
                     memcpy(socket_addr.sun.sun_path, socket_path, path_length);
    
                     /* Support for abstract socket */
                     if (socket_addr.sun.sun_path[0] == '@')
                       socket_addr.sun.sun_path[0] = 0;
    
                     fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0);
                     if (fd < 0)
                       return -errno;
    
                     if (connect(fd, &socket_addr.sa, offsetof(struct sockaddr_un, sun_path) + path_length) != 0)
                       return -errno;
    
                     ssize_t written = write(fd, message, message_length);
                     if (written != (ssize_t) message_length)
                       return written < 0 ? -errno : -EPROTO;
    
                     return 1; /* Notified! */
                   }
    
                   static int notify_ready(void) {
                     return notify("READY=1");
                   }
    
                   static int notify_reloading(void) {
                     /* A buffer with length sufficient to format the maximum UINT64 value. */
                     char reload_message[sizeof("RELOADING=1\nMONOTONIC_USEC=18446744073709551615")];
                     struct timespec ts;
                     uint64_t now;
    
                     /* Notify systemd that we are reloading, including a CLOCK_MONOTONIC timestamp in usec
                      * so that the program is compatible with a Type=notify-reload service. */
    
                     if (clock_gettime(CLOCK_MONOTONIC, &ts) < 0)
                       return -errno;
    
                     if (ts.tv_sec < 0 || ts.tv_nsec < 0 ||
                         (uint64_t) ts.tv_sec > (UINT64_MAX - (ts.tv_nsec / 1000ULL)) / 1000000ULL)
                       return -EINVAL;
    
                     now = (uint64_t) ts.tv_sec * 1000000ULL + (uint64_t) ts.tv_nsec / 1000ULL;
    
                     if (snprintf(reload_message, sizeof(reload_message), "RELOADING=1\nMONOTONIC_USEC=%" PRIu64, now) < 0)
                       return -EINVAL;
    
                     return notify(reload_message);
                   }
    
                   static int notify_stopping(void) {
                     return notify("STOPPING=1");
                   }
    
                   static volatile sig_atomic_t reloading = 0;
                   static volatile sig_atomic_t terminating = 0;
    
                   static void signal_handler(int sig) {
                     if (sig == SIGHUP)
                       reloading = 1;
                     else if (sig == SIGINT || sig == SIGTERM)
                       terminating = 1;
                   }
    
                   int main(int argc, char **argv) {
                     struct sigaction sa = {
                       .sa_handler = signal_handler,
                       .sa_flags = SA_RESTART,
                     };
                     int r;
    
                     /* Setup signal handlers */
                     sigemptyset(&sa.sa_mask);
                     sigaction(SIGHUP, &sa, NULL);
                     sigaction(SIGINT, &sa, NULL);
                     sigaction(SIGTERM, &sa, NULL);
    
                     /* Do more service initialization work here ... */
    
                     /* Now that all the preparations steps are done, signal readiness */
    
                     r = notify_ready();
                     if (r < 0) {
                       fprintf(stderr, "Failed to notify readiness to $NOTIFY_SOCKET: %s\n", strerror(-r));
                       return EXIT_FAILURE;
                     }
    
                     while (!terminating) {
                       if (reloading) {
                         reloading = false;
    
                         /* As a separate but related feature, we can also notify the manager
                          * when reloading configuration. This allows accurate state-tracking,
                          * and also automated hook-in of 'systemctl reload' without having to
                          * specify manually an ExecReload= line in the unit file. */
    
                         r = notify_reloading();
                         if (r < 0) {
                           fprintf(stderr, "Failed to notify reloading to $NOTIFY_SOCKET: %s\n", strerror(-r));
                           return EXIT_FAILURE;
                         }
    
                         /* Do some reconfiguration work here ... */
    
                         r = notify_ready();
                         if (r < 0) {
                           fprintf(stderr, "Failed to notify readiness to $NOTIFY_SOCKET: %s\n", strerror(-r));
                           return EXIT_FAILURE;
                         }
                       }
    
                       /* Do some daemon work here ... */
                       sleep(5);
                     }
    
                     r = notify_stopping();
                     if (r < 0) {
                       fprintf(stderr, "Failed to report termination to $NOTIFY_SOCKET: %s\n", strerror(-r));
                       return EXIT_FAILURE;
                     }
    
                     /* Do some shutdown work here ... */
    
                     return EXIT_SUCCESS;
                   }
    
           **Python**
    
                   #!/usr/bin/python
                   # SPDX-License-Identifier: MIT-0
                   #
                   # Implement the systemd notify protocol without external dependencies.
                   # Supports both readiness notification on startup and on reloading,
                   # according to the protocol defined at:
                   # https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
                   # This protocol is guaranteed to be stable as per:
                   # https://systemd.io/PORTABILITY_AND_STABILITY/
    
                   import errno
                   import os
                   import signal
                   import socket
                   import sys
                   import time
    
                   reloading = False
                   terminating = False
    
                   def notify(message):
                       if not message:
                           raise ValueError("notify() requires a message")
    
                       socket_path = os.environ.get("NOTIFY_SOCKET")
                       if not socket_path:
                           return
    
                       if socket_path[0] not in ("/", "@"):
                           raise OSError(errno.EAFNOSUPPORT, "Unsupported socket type")
    
                       # Handle abstract socket.
                       if socket_path[0] == "@":
                           socket_path = "\0" + socket_path[1:]
    
                       with socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM | socket.SOCK_CLOEXEC) as sock:
                           sock.connect(socket_path)
                           sock.sendall(message)
    
                   def notify_ready():
                       notify(b"READY=1")
    
                   def notify_reloading():
                       microsecs = time.clock_gettime_ns(time.CLOCK_MONOTONIC) // 1000
                       notify(f"RELOADING=1\nMONOTONIC_USEC={microsecs}".encode())
    
                   def notify_stopping():
                       notify(b"STOPPING=1")
    
                   def reload(signum, frame):
                       global reloading
                       reloading = True
    
                   def terminate(signum, frame):
                       global terminating
                       terminating = True
    
                   def main():
                       print("Doing initial setup")
                       global reloading, terminating
    
                       # Set up signal handlers.
                       print("Setting up signal handlers")
                       signal.signal(signal.SIGHUP, reload)
                       signal.signal(signal.SIGINT, terminate)
                       signal.signal(signal.SIGTERM, terminate)
    
                       # Do any other setup work here.
    
                       # Once all setup is done, signal readiness.
                       print("Done setting up")
                       notify_ready()
    
                       print("Starting loop")
                       while not terminating:
                           if reloading:
                               print("Reloading")
                               reloading = False
    
                               # Support notifying the manager when reloading configuration.
                               # This allows accurate state tracking as well as automatically
                               # enabling 'systemctl reload' without needing to manually
                               # specify an ExecReload= line in the unit file.
    
                               notify_reloading()
    
                               # Do some reconfiguration work here.
    
                               print("Done reloading")
                               notify_ready()
    
                           # Do the real work here ...
    
                           print("Sleeping for five seconds")
                           time.sleep(5)
    
                       print("Terminating")
                       notify_stopping()
    
                   if __name__ == "__main__":
                       sys.stdout.reconfigure(line_buffering=True)
                       print("Starting app")
                       main()
                       print("Stopped app")
    


## 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);
    


## NOTES



            1. File Descriptor Store
               https://systemd.io/FILE_DESCRIPTOR_STORE
    


* * *