SD_BUS_ADD_OBJECT

sd_bus_add_fallback_vtable() is similar to sd_bus_add_object_vtable(), but is used to register "fallback" attributes. When looking for an attribute declaration, bus object paths registered with sd_bus_add_object_vtable() are checked first. If no match is found, the fallback vtables are checked for each prefix of the bus object path, i.e. with the last slash-separated components successively removed. This allows the vtable to be used for an arbitrary number of dynamically created objects.

Parameter find is a function which is used to locate the target object based on the bus object path path. It must return 1 and set the ret_found output parameter if the object is found, return 0 if the object was not found, and return a negative errno-style error code or initialize the error structure ret_error on error. The pointer passed in ret_found will be used as the userdata parameter for the callback functions (offset by the offset offsets as specified in the vtable entries).

sd_bus_add_object() attaches a callback directly to the object path path. An object path can have any number of callbacks attached to it. Each callback is prepended to the list of callbacks which are always called in order. sd_bus_add_fallback() is similar to sd_bus_add_object() but applies to fallback paths instead.

sd_bus_add_filter() installs a callback that is invoked for each incoming D-Bus message. Filters can be used to handle logic common to all messages received by a service (e.g. authentication or authorization).

When a request is received, any associated callbacks are called sequentially until a callback returns a non-zero integer. Return zero from a callback to give other callbacks the chance to process the request. Callbacks are called in the following order: first, global callbacks installed with sd_bus_add_filter() are called. Second, callbacks attached directly to the request object path are called, followed by any D-Bus method callbacks attached to the request object path, interface and member. Finally, the property callbacks attached to the request object path, interface and member are called. If the final callback returns zero, an error reply is sent back to the caller indicating no matching object for the request was found.

Note that you can return a positive integer from a method callback without immediately sending a reply. This informs sd-bus this callback will take responsibility for replying to the request without forcing the callback to produce a reply immediately. This allows a callback to perform any number of asynchronous operations required to construct a reply. However, if producing a reply takes too long, the method call will time out at the caller. This is only available to methods and not properties.

If a callback was invoked to handle a request that expects a reply and the callback returns a negative value, the value is interpreted as a negative errno-style error code and sent back to the caller as a D-Bus error as if sd_bus_reply_method_errno(3) was called. Additionally, all callbacks take a sd_bus_error output parameter that can be used to provide more detailed error information. If ret_error is set when the callback finishes, the corresponding D-Bus error is sent back to the caller as if sd_bus_reply_method_error(3) was called. Any error stored in ret_error takes priority over any negative values returned by the same callback when determining which error to send back to the caller. Use sd_bus_error_set(3) or one of its variants to set ret_error and return a negative integer from a callback with a single function call. To send an error reply after a callback has already finished, use sd_bus_reply_method_errno(3) or one of its variants.

For all functions, a match slot is created internally. If the output parameter slot is NULL, a "floating" slot object is created, see sd_bus_slot_set_floating(3). Otherwise, a pointer to the slot object is returned. In that case, the reference to the slot object should be dropped when the vtable is not needed anymore, see sd_bus_slot_unref(3).  

The sd_bus_vtable array

The array consists of the structures of type sd_bus_vtable, but it should never be filled in manually, but through one of the following macros:

SD_BUS_VTABLE_START(), SD_BUS_VTABLE_END

Those must always be the first and last element.

SD_BUS_METHOD_WITH_ARGS_OFFSET(), SD_BUS_METHOD_WITH_ARGS()

Declare a D-Bus method with the name member, arguments args and result result. args expects a sequence of argument type/name pairs wrapped in the SD_BUS_ARGS() macro. The elements at even indices in this list describe the types of the method's arguments. The method's parameter signature is the concatenation of all the string literals at even indices in args. If a method has no parameters, pass SD_BUS_NO_ARGS to args. The elements at uneven indices describe the names of the method's arguments. result expects a sequence of type/name pairs wrapped in the SD_BUS_RESULT() macro in the same format as SD_BUS_ARGS(). The method's result signature is the concatenation of all the string literals at even indices in result. If a method has no result, pass SD_BUS_NO_RESULT to result. Note that argument types are expected to be quoted string literals and argument names are expected to be unquoted string literals. See below for a complete example.

The handler function handler must be of type sd_bus_message_handler_t. It will be called to handle the incoming messages that call this method. It receives a pointer that is the userdata parameter passed to the registration function offset by offset bytes. This may be used to pass pointers to different fields in the same data structure to different methods in the same vtable. To send a reply from handler, call sd_bus_reply_method_return(3) with the message the callback was invoked with. Parameter flags is a combination of flags, see below.

SD_BUS_METHOD_WITH_ARGS() is a shorthand for calling SD_BUS_METHOD_WITH_ARGS_OFFSET() with an offset of zero.

SD_BUS_METHOD_WITH_NAMES_OFFSET(), SD_BUS_METHOD_WITH_NAMES(), SD_BUS_METHOD_WITH_OFFSET(), SD_BUS_METHOD()

Declare a D-Bus method with the name member, parameter signature signature, result signature result. Parameters in_names and out_names specify the argument names of the input and output arguments in the function signature. in_names and out_names should be created using the SD_BUS_PARAM() macro, see below. In all other regards, this macro behaves exactly the same as SD_BUS_METHOD_WITH_ARGS_OFFSET().

SD_BUS_METHOD_WITH_NAMES(), SD_BUS_METHOD_WITH_OFFSET(), and SD_BUS_METHOD() are variants which specify zero offset (userdata parameter is passed with no change), leave the names unset (i.e. no parameter names), or both.

Prefer using SD_BUS_METHOD_WITH_ARGS_OFFSET() and SD_BUS_METHOD_WITH_ARGS() over these macros as they allow specifying argument types and names next to each other which is less error-prone than first specifying all argument types followed by specifying all argument names.

SD_BUS_SIGNAL_WITH_ARGS()

Declare a D-Bus signal with the name member and arguments args. args expects a sequence of argument type/name pairs wrapped in the SD_BUS_ARGS() macro. The elements at even indices in this list describe the types of the signal's arguments. The signal's parameter signature is the concatenation of all the string literals at even indices in args. If a signal has no parameters, pass SD_BUS_NO_ARGS to args. The elements at uneven indices describe the names of the signal's arguments. Parameter flags is a combination of flags. See below for a complete example.

SD_BUS_SIGNAL_WITH_NAMES(), SD_BUS_SIGNAL()

Declare a D-Bus signal with the name member, parameter signature signature, and argument names names. names should be created using the SD_BUS_PARAM() macro, see below. Parameter flags is a combination of flags, see below.

SD_BUS_SIGNAL() is equivalent to SD_BUS_SIGNAL_WITH_NAMES() with the names parameter unset (i.e. no parameter names).

Prefer using SD_BUS_SIGNAL_WITH_ARGS() over these macros as it allows specifying argument types and names next to each other which is less error-prone than first specifying all argument types followed by specifying all argument names.

SD_BUS_WRITABLE_PROPERTY(), SD_BUS_PROPERTY()

Declare a D-Bus property with the name member and value signature signature. Parameters get and set are the getter and setter methods. They are called with a pointer that is the userdata parameter passed to the registration function offset by offset bytes. This may be used pass pointers to different fields in the same data structure to different setters and getters in the same vtable. Parameter flags is a combination of flags, see below.

The setter and getter methods may be omitted (specified as NULL), if the property is one of the basic types or "as" in case of read-only properties. In those cases, the userdata and offset parameters must together point to a valid variable of the corresponding type. A default setter and getter will be provided, which simply copy the argument between this variable and the message.

SD_BUS_PROPERTY() is used to define a read-only property.

SD_BUS_PARAM()

Parameter names should be wrapped in this macro, see the example below.

Flags

The flags parameter is used to specify a combination of m[blue]D-Bus annotationsm[][1].

SD_BUS_VTABLE_DEPRECATED

Mark this vtable entry as deprecated using the org.freedesktop.DBus.Deprecated annotation in introspection data. If specified for SD_BUS_VTABLE_START(), the annotation is applied to the enclosing interface.

SD_BUS_VTABLE_HIDDEN

Make this vtable entry hidden. It will not be shown in introspection data. If specified for SD_BUS_VTABLE_START(), all entries in the array are hidden.

SD_BUS_VTABLE_UNPRIVILEGED

Mark this vtable entry as unprivileged. If not specified, the org.freedesktop.systemd1.Privileged annotation with value "true" will be shown in introspection data.

SD_BUS_VTABLE_METHOD_NO_REPLY

Mark his vtable entry as a method that will not return a reply using the org.freedesktop.DBus.Method.NoReply annotation in introspection data.

SD_BUS_VTABLE_PROPERTY_CONST, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE, SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION

Those three flags correspond to different values of the org.freedesktop.DBus.Property.EmitsChangedSignal annotation, which specifies whether the org.freedesktop.DBus.Properties.PropertiesChanged signal is emitted whenever the property changes. SD_BUS_VTABLE_PROPERTY_CONST corresponds to const and means that the property never changes during the lifetime of the object it belongs to, so no signal needs to be emitted. SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE corresponds to true and means that the signal is emitted. SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION corresponds to invalidates and means that the signal is emitted, but the value is not included in the signal.

SD_BUS_VTABLE_PROPERTY_EXPLICIT

Mark this vtable property entry as requiring explicit request to for the value to be shown (generally because the value is large or slow to calculate). This entry cannot be combined with SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE, and will not be shown in property listings by default (e.g. busctl introspect). This corresponds to the org.freedesktop.systemd1.Explicit annotation in introspection data.

SD_BUS_VTABLE_SENSITIVE

Mark this vtable method entry as processing sensitive data. When set, incoming method call messages and their outgoing reply messages are marked as sensitive using sd_bus_message_sensitive(3), so that they are erased from memory when freed.

SD_BUS_VTABLE_ABSOLUTE_OFFSET

Mark this vtable method or property entry so that the user data pointer passed to its associated handler functions is determined slightly differently: instead of adding the offset parameter of the entry to the user data pointer specified during vtable registration, the offset is passed directly, converted to a pointer, without taking the user data pointer specified during vtable registration into account.

EXAMPLES

Example 1. Create a simple listener on the bus

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

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

typedef struct object {
  char *name;
  uint32_t number;
} object;

static int method(sd_bus_message *m, void *userdata, sd_bus_error *error) {
  printf("Got called with userdata=%p\n", userdata);
  return 1;
}

static const sd_bus_vtable vtable[] = {
        SD_BUS_VTABLE_START(0),
        SD_BUS_METHOD(
            "Method1", "s", "s", method, 0),
        SD_BUS_METHOD_WITH_NAMES_OFFSET(
            "Method2",
            "so", SD_BUS_PARAM(string) SD_BUS_PARAM(path),
            "s", SD_BUS_PARAM(returnstring),
            method, offsetof(object, number),
            SD_BUS_VTABLE_DEPRECATED),
        SD_BUS_METHOD_WITH_ARGS_OFFSET(
            "Method3",
            SD_BUS_ARGS("s", string, "o", path),
            SD_BUS_RESULT("s", returnstring),
            method, offsetof(object, number),
            SD_BUS_VTABLE_UNPRIVILEGED),
        SD_BUS_METHOD_WITH_ARGS(
            "Method4",
            SD_BUS_NO_ARGS,
            SD_BUS_NO_RESULT,
            method,
            SD_BUS_VTABLE_UNPRIVILEGED),
        SD_BUS_SIGNAL(
            "Signal1",
            "so",
            0),
        SD_BUS_SIGNAL_WITH_NAMES(
            "Signal2",
            "so", SD_BUS_PARAM(string) SD_BUS_PARAM(path),
            0),
        SD_BUS_SIGNAL_WITH_ARGS(
            "Signal3",
            SD_BUS_ARGS("s", string, "o", path),
            0),
        SD_BUS_WRITABLE_PROPERTY(
            "AutomaticStringProperty", "s", NULL, NULL,
            offsetof(object, name),
            SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
        SD_BUS_WRITABLE_PROPERTY(
            "AutomaticIntegerProperty", "u", NULL, NULL,
            offsetof(object, number),
            SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION),
        SD_BUS_VTABLE_END
};

#define check(x) ({                             \
  int r = x;                                    \
  errno = r < 0 ? -r : 0;                       \
  printf(#x ": %m\n");                          \
  if (r < 0)                                    \
    return EXIT_FAILURE;                        \
  })

int main(int argc, char **argv) {
  _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;

  sd_bus_default(&bus);

  object object = { .number = 666 };
  check((object.name = strdup("name")) != NULL);

  check(sd_bus_add_object_vtable(bus, NULL, "/object",
                                 "org.freedesktop.systemd.VtableExample",
                                 vtable,
                                 &object));

  for (;;) {
    check(sd_bus_wait(bus, UINT64_MAX));
    check(sd_bus_process(bus, NULL));
  }

  free(object.name);

  return 0;
}

This creates a simple client on the bus (the user bus, when run as normal user). We may use the D-Bus org.freedesktop.DBus.Introspectable.Introspect call to acquire the XML description of the interface:

<!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>
 </interface>
 <interface name="org.freedesktop.DBus.Introspectable">
  <method name="Introspect">
   <arg name="data" type="s" direction="out"/>
  </method>
 </interface>
 <interface name="org.freedesktop.DBus.Properties">
  <method name="Get">
   <arg name="interface" direction="in" type="s"/>
   <arg name="property" direction="in" type="s"/>
   <arg name="value" direction="out" type="v"/>
  </method>
  <method name="GetAll">
   <arg name="interface" direction="in" type="s"/>
   <arg name="properties" direction="out" type="a{sv}"/>
  </method>
  <method name="Set">
   <arg name="interface" direction="in" type="s"/>
   <arg name="property" direction="in" type="s"/>
   <arg name="value" direction="in" type="v"/>
  </method>
  <signal name="PropertiesChanged">
   <arg type="s" name="interface"/>
   <arg type="a{sv}" name="changed_properties"/>
   <arg type="as" name="invalidated_properties"/>
  </signal>
 </interface>
 <interface name="org.freedesktop.systemd.VtableExample">
  <method name="Method1">
   <arg type="s" direction="in"/>
   <arg type="s" direction="out"/>
  </method>
  <method name="Method2">
   <arg type="s" name="string" direction="in"/>
   <arg type="o" name="path" direction="in"/>
   <arg type="s" name="returnstring" direction="out"/>
   <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
  </method>
  <property name="AutomaticStringProperty" type="s" access="readwrite">
  </property>
  <property name="AutomaticIntegerProperty" type="u" access="readwrite">
   <annotation name="org.freedesktop.DBus.Property.EmitsChangedSignal" value="invalidates"/>
  </property>
 </interface>
</node>

RETURN VALUE

On success, sd_bus_add_object_vtable() and sd_bus_add_fallback_vtable() return a non-negative integer. On failure, they return a negative errno-style error code.  

Errors

Returned errors may indicate the following problems:

-EINVAL

One of the required parameters is NULL or invalid. A reserved D-Bus interface was passed as the interface parameter.

-ENOPKG

The bus cannot be resolved.

-ECHILD

The bus was created in a different process.

-ENOMEM

Memory allocation failed.

-EPROTOTYPE

sd_bus_add_object_vtable() and sd_bus_add_fallback_vtable() have been both called for the same bus object path, which is not allowed.

-EEXIST

This vtable has already been registered for this interface and path.

NOTES

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.  

SEE ALSO

sd-bus(3), busctl(1), sd_bus_emit_properties_changed(3), sd_bus_emit_object_added(3)  

NOTES

1.

D-Bus annotations

https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format