SD_BUS_ADD_OBJECT_VTABLE(3)

NAME
sd_bus_add_object_vtable, sd_bus_add_fallback_vtable, SD_BUS_VTABLE_START, SD_BUS_VTABLE_END, SD_BUS_METHOD_WITH_NAMES_OFFSET, SD_BUS_METHOD_WITH_NAMES, SD_BUS_METHOD_WITH_OFFSET, SD_BUS_METHOD, SD_BUS_SIGNAL_WITH_NAMES, SD_BUS_SIGNAL, SD_BUS_WRITABLE_PROPERTY, SD_BUS_PROPERTY, SD_BUS_PARAM - Declare properties and methods for a D-Bus path

SYNOPSIS

#include <systemd/sd-bus-vtable.h>

typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata, sd_bus_error *ret_error);

typedef int (*sd_bus_property_get_t)(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);

typedef int (*sd_bus_property_set_t)(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *value, void *userdata, sd_bus_error *ret_error);

typedef int (*sd_bus_object_find_t)(const char *path, const char *interface, void *userdata, void **ret_found, sd_bus_error *ret_error);

int sd_bus_add_object_vtable(sd_bus *bus, sd_bus_slot **slot, const char *path, const char *interface, const sd_bus_vtable *vtable, void *userdata);

int sd_bus_add_fallback_vtable(sd_bus *bus, sd_bus_slot **slot, const char *prefix, const char *interface, const sd_bus_vtable *vtable, sd_bus_object_find_t find, void *userdata);

SD_BUS_VTABLE_START(flags)

SD_BUS_VTABLE_END

SD_BUS_METHOD_WITH_NAMES_OFFSET( member, signature, in_names, result, out_names, handler, offset, flags)

SD_BUS_METHOD_WITH_NAMES( member, signature, in_names, result, out_names, handler, flags)

SD_BUS_METHOD_WITH_OFFSET( member, signature, result, handler, offset, flags)

SD_BUS_METHOD( member, signature, result, handler, flags)

SD_BUS_SIGNAL_WITH_NAMES( member, signature, names, flags)

SD_BUS_SIGNAL( member, signature, flags)

SD_BUS_WRITABLE_PROPERTY( member, signature, get, set, offset, flags)

SD_BUS_PROPERTY( member, signature, get, offset, flags)

SD_BUS_PARAM(name)

DESCRIPTION

sd_bus_add_object_vtable() is used to declare attributes for the path object path path connected to the bus connection bus under the interface interface. The table vtable may contain property declarations using SD_BUS_PROPERTY() or SD_BUS_WRITABLE_PROPERTY(), method declarations using SD_BUS_METHOD(), SD_BUS_METHOD_WITH_NAMES(), SD_BUS_METHOD_WITH_OFFSET(), or SD_BUS_METHOD_WITH_NAMES_OFFSET(), and signal declarations using SD_BUS_SIGNAL_WITH_NAMES() or SD_BUS_SIGNAL(), see below. The userdata parameter contains a pointer that will be passed to various callback functions. It may be specified as NULL if no value is necessary.

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).

For both 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_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. 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. in_names and out_names should be created using the SD_BUS_PARAM() macro, see below. Parameter flags is a combination of flags, see below.

    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.

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.

    Equivalent to SD_BUS_SIGNAL_WITH_NAMES() with the names parameter unset (i.e. no parameter 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 has 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 valid variable of the corresponding type. A default setter and getters 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 D-Bus annotations[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.

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_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 calls return 0 or a positive 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)

NOTES

1.  D-Bus annotations
    https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format