SD_NOTIFY(3)

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 <elogind/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 fail, 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 elogind will accept status data sent from a service only if the NotifyAccess= option is correctly set in the service definition file. See elogind.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 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
            configuration. Since there is little value in signaling non-readiness, the only value services should send is
            Tells the service manager that service startup is finished, or the service finished re-loading its configuration. This is only used by elogind 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.

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.

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". Note that this assignment is currently not used by elogind.

    Added in version 233.

EXIT_STATUS=...
    The exit status of a service or the manager itself. Note that elogind 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 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=...
    The main process ID (PID) of the service, in case the service manager did not fork off the process itself. Example: "MAINPID=4711".

    Added in version 233.

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 a watchdog is enabled for them. See sd_watchdog_enabled(3) for the details of how the service can check whether the watchdog is enabled.

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). memory file descriptor. Note that it depends on the service manager if it will accept messages for a service. If file descriptors sent are pollable (see epoll_ctl(2)), then any EPOLLHUP or EPOLLERR event seen on them will result in messages, in which case the arrays are combined. Note that the service manager should remove duplicate 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 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.

    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 may be logged, but are otherwise ignored. Thus, it is not useful to send assignments which are not in this list. The service manager also sends some messages to its notification socket, which are then consumed by the machine or container manager.

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

Please note that this is a description of the notification technique as used by elogind. If the service manager you use does not set the needed environment variables, none of the functions described here do anything but return 0.

Functions described here are available as a shared library, which can be compiled against and linked to with the libelogind pkg-config(8) 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.

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 example:

    /* SPDX-License-Identifier: MIT-0 */
    /* Implement the elogind 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/elogind/man/latest/sd_notify.html
     * This protocol is guaranteed to be stable as per:
     * https://systemd.io/PORTABILITY_AND_STABILITY/ */
    #include <errno.h>
    #include <inttypes.h>
    #include <signal.h>
    #include <stdbool.h>
    #include <stddef.h>
    #include <stdlib.h>
    #include <stdio.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;
      socket_path = getenv("NOTIFY_SOCKET");
      if (!socket_path)
        return 0; /* Not running under elogind? Nothing to do */
      if (!message)
        return -EINVAL;
      message_length = strlen(message);
      if (message_length == 0)
        return -EINVAL;
      /* 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 elogind 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 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);
      }
      return EXIT_SUCCESS;
    }

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);

HISTORY

sd_pid_notify(), sd_pid_notifyf(), and sd_pid_notify_with_fds() were added in version 219.

sd_notify_barrier() was added in version 246.

sd_pid_notifyf_with_fds() and sd_pid_notify_barrier() were added in version 254.

SEE ALSO

elogind(8), sd-daemon(3), sd_listen_fds(3), sd_listen_fds_with_names(3), sd_watchdog_enabled(3),

NOTES

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