jammy (3) SD_BUS_PARAM.3.gz

Provided by: libsystemd-dev_249.11-0ubuntu3.12_amd64 bug

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 <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_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).

   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(flags), SD_BUS_VTABLE_END
           Those must always be the first and last element. The flags parameter can be used to
           set attributes that apply to the whole array; see the "Flags" section below.

       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 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_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.

       SD_BUS_VTABLE_CAPABILITY(capability)
           Access to this vtable entry will be allowed if the calling proccess has the capability
           capability, as described in sd_bus_query_sender_privilege(3). If used for
           SD_BUS_VTABLE_START(), provides a default for all entries in the array. If not
           specified, either for an individual entry or the whole array, CAP_SYS_ADMIN is checked
           by default. See capabilities(7) for information about capabilities.

           Note that vtable entries may be marked as unprivileged and the whole bus may be marked
           as trusted, see the discussion of SD_BUS_VTABLE_UNPRIVILEGED below.

       SD_BUS_VTABLE_UNPRIVILEGED
           Mark this vtable entry as unprivileged. Access to privileged entries is limited to
           users with appropriate capabilities as described above. In practice many vtable
           entries are marked as unprivileged, and either are open to everyone, or the decision
           whether to allow access is taken later, e.g. by delegating to polkit[2].

           The whole bus may be marked as trusted, in which case annotations at the entry level
           are ignored, see sd_bus_set_trusted(3).

           When not specified, the org.freedesktop.systemd1.Privileged annotation with value
           "true" will be shown in introspection data.

           Note that this page describes checks implemented in the D-Bus client. The D-Bus server
           has an additional policy that may permit or deny connections, see "CONFIGURATION FILE"
           in dbus-daemon(1).

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="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.

   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

        2. polkit
           https://www.freedesktop.org/software/polkit/docs/latest/