SD_BUS_ADD_OBJECT - Online Linux Manual PageSection : 3
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Source : systemd 250
Note : sd_bus_add_object

NAMEsd_bus_add_object, sd_bus_add_fallback, sd_bus_add_object_vtable, sd_bus_add_fallback_vtable, sd_bus_add_filter, 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(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_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

DESCRIPTIONsd_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)​.

The sd_bus_vtable arrayThe 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​.

FlagsThe 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 this 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​.

EXAMPLESExample ​1. ​Create a simple listener on the bus /* SPDX−License−Identifier: CC0−1​.0 */ #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"> <!−− 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 VALUEOn 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​.

ErrorsReturned 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​.

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

SEE ALSOsd-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
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