$ busctl list --no-pager --system
NAME                                    PID PROCESS         USER             CONNECTION    UNIT                        SESSION DESCRIPTION
:1.10                                  1067 udisksd         root             :1.10         udisks2.service             -       -
:1.14                                  1122 ModemManager    root             :1.14         ModemManager.service        -       -
:1.16                                  1155 gdm3            root             :1.16         gdm.service                 -       -
:1.22                                  1064 systemd-logind  root             :1.22         systemd-logind.service      -       -
:1.25                                  1198 unattended-upgr root             :1.25         unattended-upgrades.service -       -
:1.2633                             1382069 networkd-dispat root             :1.2633       networkd-dispatcher.service -       -
org.bluez                           1283814 bluetoothd      root             :1.7917       bluetooth.service           -       -
org.debian.apt                            - -               -                (activatable) -                           -       -
org.freedesktop.Accounts               1023 accounts-daemon root             :1.7          accounts-daemon.service     -       -
org.freedesktop.Avahi                  1027 avahi-daemon    avahi            :1.3          avahi-daemon.service        -       -
org.freedesktop.DBus                      1 systemd         root             -             init.scope                  -       -
org.freedesktop.NetworkManager         1034 NetworkManager  root             :1.11         NetworkManager.service      -       -
org.freedesktop.UDisks2                1067 udisksd         root             :1.10         udisks2.service             -       -
org.freedesktop.thermald                  - -               -                (activatable) -                           -       -

$ busctl --user |grep 1.43
:1.43                                           5089 gnome-shell     yk   :1.43         user@1000.service -       -
org.gnome.Panel                                 5089 gnome-shell     yk   :1.43         user@1000.service -       -
org.gnome.ScreenSaver                           5089 gnome-shell     yk   :1.43         user@1000.service -       -
org.gnome.Shell                                 5089 gnome-shell     yk   :1.43         user@1000.service -       -
...省略N行...

$ cat /usr/lib/systemd/system/thermald.service
[Unit]
Description=Thermal Daemon Service
ConditionVirtualization=no

[Service]
Type=dbus
SuccessExitStatus=1
BusName=org.freedesktop.thermald
ExecStart=/usr/sbin/thermald --systemd --dbus-enable --adaptive

$ busctl introspect org.freedesktop.login1 /org/freedesktop/login1/session/auto
NAME                                TYPE      SIGNATURE RESULT/VALUE                             FLAGS
org.freedesktop.login1.Session      interface -         -                                        -
.Lock                               method    -         -                                        -
.Active                             property  b         true                                     emits-change
.Remote                             property  b         false                                    const
.RemoteHost                         property  s         ""                                       const
.RemoteUser                         property  s         ""                                       const
.Scope                              property  s         "session-2.scope"                        const
.Seat                               property  (so)      "seat0" "/org/freedesktop/login1/seat/s… const
.Lock                               signal    -         -                                        -

$ busctl --user monitor
Monitoring bus message stream.
‣ Type=method_call  Endian=l  Flags=0  Version=1  Priority=0 Cookie=2
  Sender=:1.21712  Destination=org.freedesktop.DBus  Path=/org/freedesktop/DBus  Interface=org.freedesktop.DBus  Member=ListNames
  UniqueName=:1.21712
  MESSAGE "" {
  };

‣ Type=method_return  Endian=l  Flags=1  Version=1  Priority=0 Cookie=3  ReplyCookie=2
  Sender=org.freedesktop.DBus  Destination=:1.21712
  MESSAGE "as" {
          ARRAY "s" {
                  STRING "org.freedesktop.DBus";
                  STRING ":1.10105";
                  STRING "org.freedesktop.Notifications";
                  此处省略N个结果
          };
  };

   ‣ Type=method_call  Endian=l  Flags=0  Version=1  Priority=0 Cookie=1
     Sender=:1.23493  Destination=org.freedesktop.DBus  Path=/org/freedesktop/DBus  Interface=org.freedesktop.DBus  Member=Hello
     UniqueName=:1.23493
     MESSAGE "" {
     };
     
     ‣ Type=method_return  Endian=l  Flags=1  Version=1  Priority=0 Cookie=1  ReplyCookie=1
     Sender=org.freedesktop.DBus  Destination=:1.23493
     MESSAGE "s" {
             STRING ":1.23493";
     };

   ‣ Type=method_call  Endian=l  Flags=0  Version=1  Priority=0 Cookie=2
     Sender=:1.23493  Destination=org.freedesktop.DBus  Path=/org/freedesktop/DBus  Interface=org.freedesktop.DBus  Member=RequestName
     UniqueName=:1.23493
     MESSAGE "su" {
             STRING "net.poettering.Calculator";
             UINT32 4;
     };
   
   ‣ Type=method_return  Endian=l  Flags=1  Version=1  Priority=0 Cookie=4  ReplyCookie=2
     Sender=org.freedesktop.DBus  Destination=:1.23493
     MESSAGE "u" {
             UINT32 1;
     };

   ‣ Type=method_call  Endian=l  Flags=0  Version=1  Priority=0 Cookie=2
     Sender=:1.28315  Destination=org.freedesktop.DBus  Path=/org/freedesktop/DBus  Interface=org.freedesktop.DBus  Member=AddMatch
     UniqueName=:1.28315
     MESSAGE "s" {
             STRING "type='signal',sender='org.sdbuscpp.examplemanager',interface='org.freedesktop.DBus.ObjectManager',member='InterfacesAdded',path='/org/sdbuscpp/examplemanager'";
     };
   
   ‣ Type=method_return  Endian=l  Flags=1  Version=1  Priority=0 Cookie=3  ReplyCookie=2
     Sender=org.freedesktop.DBus  Destination=:1.28315
     MESSAGE "" {
     };

   ‣ Type=method_call  Endian=l  Flags=0  Version=1  Priority=0 Cookie=2
     Sender=:1.26055  Destination=net.poettering.Calculator  Path=/net/poettering/Calculator  Interface=org.freedesktop.DBus.Introspectable  Member=Introspect
     UniqueName=:1.26055
     MESSAGE "" {
     };
   
   ‣ Type=method_return  Endian=l  Flags=1  Version=1  Priority=0 Cookie=52  ReplyCookie=2
     Sender=:1.25848  Destination=:1.26055
     UniqueName=:1.25848
     MESSAGE "s" {
             STRING "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
   "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
   <node>
    <interface name="org.freedesktop.DBus.Peer">
     <method name="Ping"/>
     <method name="GetMachineId">
      <arg type="s" name="machine_uuid" direction="out"/>
     </method>
    此处省略N字...
   </node>
   ";
     };

<!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.sdbuscpp.concatenator"/>
    <allow send_destination="org.sdbuscpp"/>
    <allow send_interface="org.sdbuscpp.concatenator"/>
  </policy>
</busconfig>

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

int main(int argc, char *argv[]) {
        sd_bus_error error = SD_BUS_ERROR_NULL;
        sd_bus_message *m = NULL;
        sd_bus *bus = NULL;
        const char *path;
        int r;

        /* Connect to the system bus */
        r = sd_bus_open_system(&bus);
        if (r < 0) {
                fprintf(stderr, "Failed to connect to system bus: %s\n", strerror(-r));
                goto finish;
        }

        /* Issue the method call and store the respons message in m */
        r = sd_bus_call_method(bus,
                               "org.freedesktop.systemd1",           /* service to contact */
                               "/org/freedesktop/systemd1",          /* object path */
                               "org.freedesktop.systemd1.Manager",   /* interface name */
                               "StartUnit",                          /* method name */
                               &error,                               /* object to return error in */
                               &m,                                   /* return message on success */
                               "ss",                                 /* input signature */
                               "cups.service",                       /* first argument */
                               "replace");                           /* second argument */
        if (r < 0) {
                fprintf(stderr, "Failed to issue method call: %s\n", error.message);
                goto finish;
        }

        /* Parse the response message */
        r = sd_bus_message_read(m, "o", &path);
        if (r < 0) {
                fprintf(stderr, "Failed to parse response message: %s\n", strerror(-r));
                goto finish;
        }

        printf("Queued service job as %s.\n", path);

finish:
        sd_bus_error_free(&error);
        sd_bus_message_unref(m);
        sd_bus_unref(bus);

        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}

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

static int method_multiply(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
        int64_t x, y;
        int r;

        /* Read the parameters */
        r = sd_bus_message_read(m, "xx", &x, &y);
        if (r < 0) {
                fprintf(stderr, "Failed to parse parameters: %s\n", strerror(-r));
                return r;
        }

        /* Reply with the response */
        return sd_bus_reply_method_return(m, "x", x * y);
}

static int method_divide(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
        int64_t x, y;
        int r;

        /* Read the parameters */
        r = sd_bus_message_read(m, "xx", &x, &y);
        if (r < 0) {
                fprintf(stderr, "Failed to parse parameters: %s\n", strerror(-r));
                return r;
        }

        /* Return an error on division by zero */
        if (y == 0) {
                sd_bus_error_set_const(ret_error, "net.poettering.DivisionByZero", "Sorry, can't allow division by zero.");
                return -EINVAL;
        }

        return sd_bus_reply_method_return(m, "x", x / y);
}

/* The vtable of our little object, implements the net.poettering.Calculator interface */
static const sd_bus_vtable calculator_vtable[] = {
        SD_BUS_VTABLE_START(0),
        SD_BUS_METHOD("Multiply", "xx", "x", method_multiply, SD_BUS_VTABLE_UNPRIVILEGED),
        SD_BUS_METHOD("Divide",   "xx", "x", method_divide,   SD_BUS_VTABLE_UNPRIVILEGED),
        SD_BUS_VTABLE_END
};

int main(int argc, char *argv[]) {
        sd_bus_slot *slot = NULL;
        sd_bus *bus = NULL;
        int r;

        /* Connect to the user bus this time */
        r = sd_bus_open_user(&bus);
        if (r < 0) {
                fprintf(stderr, "Failed to connect to system bus: %s\n", strerror(-r));
                goto finish;
        }

        /* Install the object */
        r = sd_bus_add_object_vtable(bus,
                                     &slot,
                                     "/net/poettering/Calculator",  /* object path */
                                     "net.poettering.Calculator",   /* interface name */
                                     calculator_vtable,
                                     NULL);
        if (r < 0) {
                fprintf(stderr, "Failed to issue method call: %s\n", strerror(-r));
                goto finish;
        }

        /* Take a well-known service name so that clients can find us */
        r = sd_bus_request_name(bus, "net.poettering.Calculator", 0);
        if (r < 0) {
                fprintf(stderr, "Failed to acquire service name: %s\n", strerror(-r));
                goto finish;
        }

        for (;;) {
                /* Process requests */
                r = sd_bus_process(bus, NULL);
                if (r < 0) {
                        fprintf(stderr, "Failed to process bus: %s\n", strerror(-r));
                        goto finish;
                }
                if (r > 0) /* we processed a request, try to process another one, right-away */
                        continue;

                /* Wait for the next request to process */
                r = sd_bus_wait(bus, (uint64_t) -1);
                if (r < 0) {
                        fprintf(stderr, "Failed to wait on bus: %s\n", strerror(-r));
                        goto finish;
                }
        }

finish:
        sd_bus_slot_unref(slot);
        sd_bus_unref(bus);

        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}