2 <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
4 <chapter id="server-tutorial" xmlns:xi="http://www.w3.org/2001/XInclude">
5 <title>Writing a UPnP Service</title>
8 <title>Introduction</title>
10 This chapter explains how to implement a UPnP service using GUPnP. For
11 this example we will create a virtual UPnP-enabled light bulb.
14 Before any code can be written, the device and services that it implement
15 need to be described in XML. Although this can be frustrating, if you are
16 implementing a standardised service (see <ulink
17 url="http://upnp.org/sdcps-and-certification/standards/sdcps/"/> for the
18 list of standard devices and services) then the service description is
19 already written for you and the device description is trivial. UPnP has
20 standardised <ulink url="http://upnp.org/specs/ha/lighting/">Lighting
21 Controls</ulink>, so we'll be using the device and service types defined
27 <title>Defining the Device</title>
29 The first step is to write the <firstterm>device description</firstterm>
30 file. This is a short XML document which describes the device and what
31 services it provides (for more details see the <ulink
32 url="http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.0.pdf">UPnP
33 Device Architecture</ulink> specification, section 2.1). We'll be using
34 the <literal>BinaryLight1</literal> device type, but if none of the
35 existing device types are suitable a custom device type can be created.
37 <programlisting><xi:include href="../../examples/BinaryLight1.xml" parse="text"/></programlisting>
39 The <sgmltag>specVersion</sgmltag> tag defines what version of the UPnP
40 Device Architecture the document conforms to. At the time of writing the
44 Next there is the root <sgmltag>device</sgmltag> tag. This contains
45 metadata about the device, lists the services it provides and any
46 sub-devices present (there are none in this example). The
47 <sgmltag>deviceType</sgmltag> tag specifies the type of the device.
50 Next we have <sgmltag>friendlyName</sgmltag>,
51 <sgmltag>manufacturer</sgmltag> and <sgmltag>modelName</sgmltag>. The
52 friendly name is a human-readable name for the device, the manufacturer
53 and model name are self-explanatory.
56 Next there is the UDN, or <firstterm>Unique Device Name</firstterm>. This
57 is an identifier which is unique for each device but persistent for each
58 particular device. Although it has to start with <literal>uuid:</literal>
59 note that it doesn't have to be an UUID. There are several alternatives
60 here: for example it could be computed at built-time if the software will
61 only be used on a single machine, or it could be calculated using the
62 device's serial number or MAC address.
65 Finally we have the <sgmltag>serviceList</sgmltag> which describes the
66 services this device provides. Each service has a service type (again
67 there are types defined for standardised services or you can create your
68 own), service identifier, and three URLs. As a service type we're using
69 the standard <literal>SwitchPower1</literal> service. The
70 <sgmltag>SCPDURL</sgmltag> field specifies where the <firstterm>Service
71 Control Protocol Document</firstterm> can be found, this describes the
72 service in more detail and will be covered next. Finally there are the
73 control and event URLs, which need to be unique on the device and will be
79 <title>Defining Services</title>
81 Because we are using a standard service we can use the service description
82 from the specification. This is the <literal>SwitchPower1</literal>
83 service description file:
85 <programlisting><xi:include href="../../examples/SwitchPower1.xml" parse="text"/></programlisting>
87 Again, the <sgmltag>specVersion</sgmltag> tag defines the UPnP version
88 that is being used. The rest of the document consists of an
89 <sgmltag>actionList</sgmltag> defining the <glossterm
90 linkend="action">actions</glossterm> available and a
91 <sgmltag>serviceStateTable</sgmltag> defining the <glossterm
92 linkend="state-variable">state variables</glossterm>.
95 Every <sgmltag>action</sgmltag> has a <sgmltag>name</sgmltag> and a list
96 of <sgmltag>argument</sgmltag>s. Arguments also have a name, a direction
97 (<literal>in</literal> or <literal>out</literal> for input or output
98 arguments) and a related state variable. The state variable is used to
99 determine the type of the argument, and as such is a required element.
100 This can lead to the creation of otherwise unused state variables to
101 define the type for an argument (the <literal>WANIPConnection</literal>
102 service is a good example of this), thanks to the legacy behind UPnP.
105 <sgmltag>stateVariable</sgmltag>s need to specify their
106 <sgmltag>name</sgmltag> and <sgmltag>dataType</sgmltag>. State variables
107 by default send notifications when they change, to specify that a variable
108 doesn't do this set the <sgmltag>sendEvents</sgmltag> attribute to
109 <literal>no</literal>. Finally there are optional
110 <sgmltag>defaultValue</sgmltag>, <sgmltag>allowedValueList</sgmltag> and
111 <sgmltag>allowedValueRange</sgmltag> elements which specify what the
112 default and valid values for the variable.
115 For the full specification of the service definition file, including a
116 complete list of valid <sgmltag>dataType</sgmltag>s, see section 2.3 of
118 url="http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.0.pdf">UPnP
119 Device Architecture</ulink>.
124 <title>Implementing the Device</title>
126 Before starting to implement the device, some boilerplate code is needed
127 to initialise GUPnP. GLib types and threading needs to be initialised,
128 and then a GUPnP context can be created using gupnp_context_new().
130 <programlisting>GUPnPContext *context;
131 /* Initialize required subsystems */
133 /* Create the GUPnP context with default host and port */
134 context = gupnp_context_new (NULL, NULL, 0, NULL);</programlisting>
136 UPnP uses HTTP to provide the device and service description files, so
137 next we tell GUPnP to publish them. This is done with
138 gupnp_context_host_path() which takes a local filename to send when a
139 certain server path is requested.
141 <programlisting>gupnp_context_host_path (context, "BinaryLight1.xml", "/BinaryLight1.xml");
142 gupnp_context_host_path (context, "SwitchPower1.xml", "/SwitchPower1.xml");</programlisting>
144 Next the root device can be created.
146 <programlisting>GUPnPRootDevice *dev;
147 /* Create the root device object */
148 dev = gupnp_root_device_new (context, "/BinaryLight1.xml");
149 /* Activate the root device, so that it announces itself */
150 gupnp_root_device_set_available (dev, TRUE);</programlisting>
152 GUPnP scans the device description and any service description files it
153 refers to, so if the main loop was entered now the device and service
154 would be available on the network, albeit with no functionality. The
155 remaining task is to implement the services.
160 <title>Implementing a Service</title>
162 To implement a service we first fetch the #GUPnPService from the root
163 device using gupnp_device_info_get_service() (#GUPnPRootDevice is a
164 subclass of #GUPnPDevice, which implements #GUPnPDeviceInfo). This
165 returns a #GUPnPServiceInfo which again is an interface, implemented by
166 #GUPnPService (on the server) and #GUPnPServiceProxy (on the client).
168 <programlisting>GUPnPServiceInfo *service;
169 service = gupnp_device_info_get_service
170 (GUPNP_DEVICE_INFO (dev), "urn:schemas-upnp-org:service:SwitchPower:1");</programlisting>
172 #GUPnPService handles interacting with the network itself, leaving the
173 implementation of the service itself to signal handlers that we need to
174 connect. There are two signals: #GUPnPService::action-invoked and
175 #GUPnPService::query-variable. #GUPnPService::action-invoked is emitted
176 when a client invokes an action: the handler is passed a
177 #GUPnPServiceAction object that identifies which action was invoked, and
178 is used to return values using gupnp_service_action_set().
179 #GUPnPService::query-variable is emitted for evented variables when a
180 control point subscribes to the service (to announce the initial value),
181 or whenever a client queries the value of a state variable (note that this
182 is now deprecated behaviour for UPnP control points): the handler is
183 passed the variable name and a #GValue which should be set to the current
184 value of the variable.
187 There are two approaches that clients can take to handle these signals.
188 They can either connect a single handler to #GUPnPService::action-invoked
189 or #GUPnPService::query-variable and examine the arguments to decide what
190 action to take. Alternatively, handlers can be targetted at specific
191 actions or variables by using the <firstterm>signal detail</firstterm>
192 when connecting. For example, this causes
193 <function>on_get_status_action</function> to be called when the
194 <function>GetStatus</function> action is invoked:
196 <programlisting>static void on_get_status_action (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data);
198 g_signal_connect (service, "action-invoked::GetStatus", G_CALLBACK (on_get_status_action), NULL);</programlisting>
200 The implementation of action handlers is quite simple. The handler is
201 passed a #GUPnPServiceAction object which represents the in-progress
202 action. If required it can be queried using
203 gupnp_service_action_get_name() to identify the action (this isn't
204 required if detailed signals were connected). Any
205 <firstterm>in</firstterm> arguments can be retrieving using
206 gupnp_service_action_get(), and then return values can be set using
207 gupnp_service_action_set(). Once the action has been performed, either
208 gupnp_service_action_return() or gupnp_service_action_return_error()
209 should be called to either return successfully or return an error code.
210 If any evented state variables were modified during the action then a
211 notification should be emitted using gupnp_service_notify(). This is an
212 example implementation of <function>GetStatus</function> and
213 <function>SetTarget</function>:
215 <programlisting>static gboolean status;
218 get_status_cb (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data)
220 gupnp_service_action_set (action,
221 "ResultStatus", G_TYPE_BOOLEAN, status,
223 gupnp_service_action_return (action);
227 set_target_cb (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data)
229 gupnp_service_action_get (action,
230 "NewTargetValue", G_TYPE_BOOLEAN, &status,
232 gupnp_service_action_return (action);
233 gupnp_service_notify (service, "Status", G_TYPE_STRING, status, NULL);
236 g_signal_connect (service, "action-invoked::GetStatus", G_CALLBACK (get_status_cb), NULL);
237 g_signal_connect (service, "action-invoked::SetTarget", G_CALLBACK (set_target_cb), NULL);</programlisting>
239 State variable query handlers are called with the name of the variable and
240 a #GValue. This value should be initialized with the relevant type and
241 then set to the current value. Again signal detail can be used to connect
242 handlers to specific state variable callbacks.
244 <programlisting>static gboolean status;
247 query_status_cb (GUPnPService *service, char *variable, GValue *value, gpointer user_data)
249 g_value_init (value, G_TYPE_BOOLEAN);
250 g_value_set_boolean (value, status);
253 g_signal_connect (service, "query-variable::Status", G_CALLBACK (query_status_cb), NULL);</programlisting>
255 The service is now fully implemented. To complete it, enter a GLib main
256 loop and wait for a client to connect. The complete source code for this
257 example is available as <filename>examples/light-server.c</filename> in
261 For services which have many actions and variables there is a convenience
262 method gupnp_service_signals_autoconnect() which will automatically
263 connect specially named handlers to signals. See the documentation for
264 full details on how it works.
268 <title>Generating Service-specific Wrappers</title>
270 Using service-specific wrappers can simplify the implementation of a service.
271 Wrappers can be generated with <xref linkend="gupnp-binding-tool"/>
272 using the option <literal>--mode server</literal>.
275 In the following examples the wrapper has been created with
276 <literal>--mode server --prefix switch</literal>. Please note that the callback handlers
277 (<literal>get_status_cb</literal> and <literal>set_target_cb</literal>) are not automatically
278 generated by <xref linkend="gupnp-binding-tool"/> for you.
280 <programlisting>static gboolean status;
283 get_status_cb (GUPnPService *service,
284 GUPnPServiceAction *action,
287 switch_get_status_action_set (action, status);
289 gupnp_service_action_return (action);
293 set_target_cb (GUPnPService *service,
294 GUPnPServiceAction *action,
297 switch_set_target_action_get (action, &status);
298 switch_status_variable_notify (service, status);
300 gupnp_service_action_return (action);
305 switch_get_status_action_connect (service, G_CALLBACK(get_status_cb), NULL);
306 switch_set_target_action_connect (service, G_CALLBACK(set_target_cb), NULL);</programlisting>
308 Note how many possible problem situations that were run-time errors are
309 actually compile-time errors when wrappers are used: Action names,
310 argument names and argument types are easier to get correct (and available
311 in editor autocompletion).
314 State variable query handlers are implemented in a similar manner, but
315 they are even simpler as the return value of the handler is the state
318 <programlisting>static gboolean
319 query_status_cb (GUPnPService *service,
327 switch_status_query_connect (service, query_status_cb, NULL);</programlisting>