The second phase of regular shutdown, as implemented by systemd-shutdown(8).

The third phase of regular shutdown, i.e. the return to the initrd context.

The hardware reboot operation.

The firmware initialization.

The boot loader initialization.

The kernel initialization.

The initrd initialization.

The OS update remains incomplete, as the kernel is not reset and continues running.

Kernel settings (such as /proc/sys/ settings, a.k.a. "sysctl", or /sys/ settings) are not reset.

File descriptors placed in the file descriptor store of services that remain active until the very end are passed to the next boot, where they are placed in the file descriptor store of the same unit. For this to work, units must declare DefaultDependencies=no (and avoid a manual Conflicts=shutdown.target or similar) to ensure they are not terminated as usual during the system shutdown operation. Alternatively, use FileDescriptorStorePreserve= to allow the file descriptor store to remain pinned even when the unit is down. See systemd.service(5) for details about the file descriptor store.

Similar to this, file descriptors associated with .socket units remain open (and connectible) if the units are not stopped during the transition. (Achieved by DefaultDependencies=no.)

The /run/ file system remains mounted and populated and may be used to pass state information between such userspace reboot cycles.

Service processes may continue to run over the transition, past soft-reboot and into the next session, if they are placed in services that remain active until the very end of shutdown (which again is achieved via DefaultDependencies=no). They must also be set up to avoid being killed by the aforementioned SIGTERM and SIGKILL via SurviveFinalKillSignal=yes, and also be configured to avoid being stopped on isolate via IgnoreOnIsolate=yes. They also have to be configured to be stopped on normal shutdown, reboot and maintenance mode. Finally, they have to be ordered after basic.target to ensure correct ordering on boot. Note that in case any new or custom units are used to isolate to, or that implement an equivalent shutdown functionality, they will also have to be configured manually for correct ordering and conflicting. For example:

[Unit]
Description=My Surviving Service
SurviveFinalKillSignal=yes
IgnoreOnIsolate=yes
DefaultDependencies=no
After=basic.target
Conflicts=reboot.target kexec.target poweroff.target halt.target rescue.target emergency.target
Before=shutdown.target rescue.target emergency.target

[Service]
Type=oneshot
ExecStart=sleep infinity

On top of the above, templated units also need a configuration file for their slice, as they by default use a slice named after the non-templated part of the unit. For example, for a foo@test.service instance, a system-foo.slice unit may be added with the following content:

[Unit]
SurviveFinalKillSignal=yes
IgnoreOnIsolate=yes
DefaultDependencies=no

File system mounts may remain mounted during the transition, and complex storage attached, if configured to remain until the very end of the shutdown process. (Also achieved via DefaultDependencies=no, and by avoiding Conflicts=umount.target)

If the unit publishes a service over D-Bus, the connection needs to be re-established after soft-reboot as the D-Bus broker will be stopped and then started again. When using the sd-bus(3) library this can be achieved by adapting the following example.

/* SPDX-License-Identifier: MIT-0 */

/* A D-Bus service that automatically reconnects when the system bus is
 * restarted.
 *
 * Compile with 'cc sd_bus_service_reconnect.c $(pkg-config --libs --cflags libsystemd)'
 *
 * To allow the program to take ownership of the name 'org.freedesktop.ReconnectExample',
 * add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf
 * and then reload the broker with 'systemctl reload dbus':

<?xml version="1.0"?> <!--*-nxml-*-->
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
  "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
  <policy user="root">
    <allow own="org.freedesktop.ReconnectExample"/>
    <allow send_destination="org.freedesktop.ReconnectExample"/>
    <allow receive_sender="org.freedesktop.ReconnectExample"/>
  </policy>

  <policy context="default">
    <allow send_destination="org.freedesktop.ReconnectExample"/>
    <allow receive_sender="org.freedesktop.ReconnectExample"/>
  </policy>
</busconfig>

 *
 * To get the property via busctl:
 *
 * $ busctl --user get-property org.freedesktop.ReconnectExample \
 *                              /org/freedesktop/ReconnectExample \
 *                              org.freedesktop.ReconnectExample \
 *                              Example
 * s "example"
 */

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-bus.h>

#define _cleanup_(f) __attribute__((cleanup(f)))

static int log_error(int r, const char *str) {
  fprintf(stderr, "%s failed: %s\n", str, strerror(-r));
  return r;
}

typedef struct object {
  const char *example;
  sd_bus **bus;
  sd_event **event;
} object;

static int property_get(
                sd_bus *bus,
                const char *path,
                const char *interface,
                const char *property,
                sd_bus_message *reply,
                void *userdata,
                sd_bus_error *error) {

  object *o = userdata;

  if (strcmp(property, "Example") == 0)
    return sd_bus_message_append(reply, "s", o->example);

  return sd_bus_error_setf(error,
                           SD_BUS_ERROR_UNKNOWN_PROPERTY,
                           "Unknown property '%s'",
                           property);
}

/* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html */
static const sd_bus_vtable vtable[] = {
  SD_BUS_VTABLE_START(0),
  SD_BUS_PROPERTY(
    "Example", "s",
    property_get,
    0,
    SD_BUS_VTABLE_PROPERTY_CONST),
  SD_BUS_VTABLE_END
};

static int setup(object *o);

static int on_disconnect(sd_bus_message *message, void *userdata, sd_bus_error *ret_error) {
  int r;

  r = setup((object *)userdata);
  if (r < 0) {
    object *o = userdata;
    r = sd_event_exit(*o->event, r);
    if (r < 0)
      return log_error(r, "sd_event_exit()");
  }

  return 1;
}

/* Ensure the event loop exits with a clear error if acquiring the well-known
 * service name fails */
static int request_name_callback(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
  int r;

  if (!sd_bus_message_is_method_error(m, NULL))
    return 1;

  const sd_bus_error *error = sd_bus_message_get_error(m);

  if (sd_bus_error_has_names(error, SD_BUS_ERROR_TIMEOUT, SD_BUS_ERROR_NO_REPLY))
    return 1; /* The bus is not available, try again later */

  fprintf(stderr, "Failed to request name: %s\n", error->message);
  object *o = userdata;
  r = sd_event_exit(*o->event, -sd_bus_error_get_errno(error));
  if (r < 0)
    return log_error(r, "sd_event_exit()");

  return 1;
}

static int setup(object *o) {
  int r;

  /* If we are reconnecting, then the bus object needs to be closed, detached
   * from the event loop and recreated.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_detach_event.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_close_unref.html
   */
  if (*o->bus) {
    r = sd_bus_detach_event(*o->bus);
    if (r < 0)
      return log_error(r, "sd_bus_detach_event()");
    *o->bus = sd_bus_close_unref(*o->bus);
  }

  /* Set up a new bus object for the system bus, configure it to wait for D-Bus
   * to be available instead of failing if it is not, and start it. All the
   * following operations are asynchronous and will not block waiting for D-Bus
   * to be available.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_new.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_address.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_bus_client.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_negotiate_creds.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_watch_bind.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_set_connected_signal.html
   * https://www.freedesktop.org/software/systemd/man/sd_bus_start.html
   */
  r = sd_bus_new(o->bus);
  if (r < 0)
    return log_error(r, "sd_bus_new()");
  r = sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket");
  if (r < 0)
    return log_error(r, "sd_bus_set_address()");
  r = sd_bus_set_bus_client(*o->bus, 1);
  if (r < 0)
    return log_error(r, "sd_bus_set_bus_client()");
  r = sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS);
  if (r < 0)
    return log_error(r, "sd_bus_negotiate_creds()");
  r = sd_bus_set_watch_bind(*o->bus, 1);
  if (r < 0)
    return log_error(r, "sd_bus_set_watch_bind()");
  r = sd_bus_start(*o->bus);
  if (r < 0)
    return log_error(r, "sd_bus_start()");

  /* Publish an interface on the bus, specifying our well-known object access
   * path and public interface name.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
   * https://dbus.freedesktop.org/doc/dbus-tutorial.html
   */
  r = sd_bus_add_object_vtable(*o->bus,
                               NULL,
                               "/org/freedesktop/ReconnectExample",
                               "org.freedesktop.ReconnectExample",
                               vtable,
                               o);
  if (r < 0)
    return log_error(r, "sd_bus_add_object_vtable()");
  /* By default the service is only assigned an ephemeral name. Also add a
   * well-known one, so that clients know whom to call. This needs to be
   * asynchronous, as D-Bus might not be yet available. The callback will check
   * whether the error is expected or not, in case it fails.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
   */
  r = sd_bus_request_name_async(*o->bus,
                                NULL,
                                "org.freedesktop.ReconnectExample",
                                0,
                                request_name_callback,
                                o);
  if (r < 0)
    return log_error(r, "sd_bus_request_name_async()");
  /* When D-Bus is disconnected this callback will be invoked, which will set up
   * the connection again. This needs to be asynchronous, as D-Bus might not yet
   * be available.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_match_signal_async.html
   */
  r = sd_bus_match_signal_async(*o->bus,
                                NULL,
                                "org.freedesktop.DBus.Local",
                                NULL,
                                "org.freedesktop.DBus.Local",
                                "Disconnected",
                                on_disconnect,
                                NULL,
                                o);
  if (r < 0)
    return log_error(r, "sd_bus_match_signal_async()");
  /* Attach the bus object to the event loop so that calls and signals are
   * processed.
   * https://www.freedesktop.org/software/systemd/man/sd_bus_attach_event.html
   */
  r = sd_bus_attach_event(*o->bus, *o->event, 0);
  if (r < 0)
    return log_error(r, "sd_bus_attach_event()");

  return 0;
}

int main(int argc, char **argv) {
  /* The bus should be relinquished before the program terminates. The cleanup
   * attribute allows us to do it nicely and cleanly whenever we exit the block.
   */
  _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
  _cleanup_(sd_event_unrefp) sd_event *event = NULL;
  object o = {
    .example = "example",
    .bus = &bus,
    .event = &event,
  };
  int r;

  /* Create an event loop data structure, with default parameters.
   * https://www.freedesktop.org/software/systemd/man/sd_event_default.html
   */
  r = sd_event_default(&event);
  if (r < 0)
    return log_error(r, "sd_event_default()");

  /* By default the event loop will terminate when all sources have disappeared,
   * so we have to keep it 'occupied'. Register signal handling to do so.
   * https://www.freedesktop.org/software/systemd/man/sd_event_add_signal.html
   */
  r = sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
  if (r < 0)
    return log_error(r, "sd_event_add_signal(SIGINT)");

  r = sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
  if (r < 0)
    return log_error(r, "sd_event_add_signal(SIGTERM)");

  r = setup(&o);
  if (r < 0)
    return EXIT_FAILURE;

  /* Enter the main loop, it will exit only on sigint/sigterm.
   * https://www.freedesktop.org/software/systemd/man/sd_event_loop.html
   */
  r = sd_event_loop(event);
  if (r < 0)
    return log_error(r, "sd_event_loop()");

  /* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */
  r = sd_bus_release_name(bus, "org.freedesktop.ReconnectExample");
  if (r < 0)
    return log_error(r, "sd_bus_release_name()");

  return 0;
}