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(), 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 D-Bus
annotations
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.
