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
#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
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 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.
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_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 process 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.
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).
Example 1. Create a simple listener on the bus
/* SPDX-License-Identifier: MIT-0 */ #define _GNU_SOURCE 1 #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) { int r; 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; r = sd_bus_message_read(m, "s", &string); if (r < 0) { fprintf(stderr, "sd_bus_message_read() failed: %s\n", strerror(-r)); return 0; } r = sd_bus_reply_method_return(m, "s", string); if (r < 0) { fprintf(stderr, "sd_bus_reply_method_return() failed: %s\n", strerror(-r)); return 0; } 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; int r; sd_bus_default(&bus); object object = { .number = 666 }; object.name = strdup("name"); if (!object.name) { fprintf(stderr, "OOM\n"); return EXIT_FAILURE; } r = sd_bus_add_object_vtable(bus, NULL, "/org/freedesktop/systemd/VtableExample", "org.freedesktop.systemd.VtableExample", vtable, &object); if (r < 0) { fprintf(stderr, "sd_bus_add_object_vtable() failed: %s\n", strerror(-r)); return EXIT_FAILURE; } r = sd_bus_request_name(bus, "org.freedesktop.systemd.VtableExample", 0); if (r < 0) { fprintf(stderr, "sd_bus_request_name() failed: %s\n", strerror(-r)); return EXIT_FAILURE; } for (;;) { r = sd_bus_wait(bus, UINT64_MAX); if (r < 0) { fprintf(stderr, "sd_bus_wait() failed: %s\n", strerror(-r)); return EXIT_FAILURE; } r = sd_bus_process(bus, NULL); if (r < 0) { fprintf(stderr, "sd_bus_process() failed: %s\n", strerror(-r)); return EXIT_FAILURE; } } r = sd_bus_release_name(bus, "org.freedesktop.systemd.VtableExample"); if (r < 0) { fprintf(stderr, "sd_bus_release_name() failed: %s\n", strerror(-r)); return EXIT_FAILURE; } 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>
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.
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, library or module instance.
-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
.
Functions described here are available as a shared
library, which can be compiled against and linked to with the
libsystemd
pkg-config(1)
file.
The code described here uses
getenv(3),
which is declared to be not multi-thread-safe. This means that the code calling the functions described
here must not call
setenv(3)
from a parallel thread. It is recommended to only do calls to setenv()
from an early phase of the program when no other threads have been started.
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 221.