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

RELOADING=1

STOPPING=1

MONOTONIC_USEC=...

STATUS=...

ERRNO=...

BUSERROR=...

EXIT_STATUS=...

MAINPID=...

WATCHDOG=1

FDSTORE=1

FDSTOREREMOVE=1

FDNAME=...

FDPOLL=0

BARRIER=1

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

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