NAME¶

sd_bus_add_object, sd_bus_add_fallback, sd_bus_add_object_vtable, sd_bus_add_fallback_vtable, sd_bus_add_filter, SD_BUS_VTABLE_CAPABILITY, 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 <elogind/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(sd_bus *bus, sd_bus_slot **slot, const char *path, sd_bus_message_handler_t callback, void *userdata);

int sd_bus_add_fallback(sd_bus *bus, sd_bus_slot **slot, const char *path, sd_bus_message_handler_t callback, void *userdata);

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);

int sd_bus_add_filter(sd_bus *bus, sd_bus_slot **slot, sd_bus_message_handler_t callback, void *userdata);

SD_BUS_VTABLE_CAPABILITY(capability)

SD_BUS_VTABLE_START(flags)

SD_BUS_VTABLE_END

SD_BUS_METHOD_WITH_ARGS_OFFSET(member, args, result, handler, offset, flags)

SD_BUS_METHOD_WITH_ARGS(member, args, result, handler, flags)

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_ARGS(member, args, 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)

SD_BUS_ARGS(...)

SD_BUS_RESULT(...)

SD_BUS_NO_ARGS

SD_BUS_NO_RESULT

DESCRIPTION¶

sd_bus_add_object_vtable() is used to declare attributes for the 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. An interface can have any number of vtables attached to it.

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).

EXAMPLES¶

Example 1. Create a simple listener on the bus

/* SPDX-License-Identifier: MIT-0 */
#include <errno.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <elogind/sd-bus.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
#define check(x) ({                             \


  int r = (x);                                  \


  errno = r < 0 ? -r : 0;                       \


  printf(#x ": %m\n");                          \


  if (r < 0)                                    \


    return EXIT_FAILURE;                        \


  })
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);


  if (sd_bus_message_is_method_call(m,


                                    "org.freedesktop.systemd.VtableExample",


                                    "Method4"))


    return 1;


  const char *string;


  check(sd_bus_message_read(m, "s", &string));


  check(sd_bus_reply_method_return(m, "s", string));


  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
};
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,


                                 "/org/freedesktop/systemd/VtableExample",


                                 "org.freedesktop.systemd.VtableExample",


                                 vtable,


                                 &object));


  check(sd_bus_request_name(bus,


                            "org.freedesktop.systemd.VtableExample",


                            0));


  for (;;) {


    check(sd_bus_wait(bus, UINT64_MAX));


    check(sd_bus_process(bus, NULL));


  }


  check(sd_bus_release_name(bus, "org.freedesktop.systemd.VtableExample"));


  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"
"https://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<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="xml_data" type="s" direction="out"/>


  </method>


 </interface>


 <interface name="org.freedesktop.DBus.Properties">


  <method name="Get">


   <arg name="interface_name" direction="in" type="s"/>


   <arg name="property_name" direction="in" type="s"/>


   <arg name="value" direction="out" type="v"/>


  </method>


  <method name="GetAll">


   <arg name="interface_name" direction="in" type="s"/>


   <arg name="props" direction="out" type="a{sv}"/>


  </method>


  <method name="Set">


   <arg name="interface_name" direction="in" type="s"/>


   <arg name="property_name" direction="in" type="s"/>


   <arg name="value" direction="in" type="v"/>


  </method>


  <signal name="PropertiesChanged">


   <arg type="s" name="interface_name"/>


   <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.

NOTES¶

Functions described here are available as a shared library, which can be compiled against and linked to with the libelogind pkg-config(8) file.

HISTORY¶

sd_bus_property_get_t(), sd_bus_property_set_t(), sd_bus_object_find_t(), sd_bus_add_object(), sd_bus_add_fallback(), sd_bus_add_object_vtable(), sd_bus_add_fallback_vtable(), and sd_bus_add_filter() were added in version 246.

SEE ALSO¶

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

NOTES¶

1.

D-Bus annotations

2.

polkit