Writing a UPnP Service: GUPnP Reference Manual

## Writing a UPnP Service

## Introduction

This chapter explains how to implement a UPnP service using GUPnP. For this example we will create a virtual UPnP-enabled light bulb.

Before any code can be written, the device and services that it implement need to be described in XML. Although this can be frustrating, if you are implementing a standardised service (see http://upnp.org/sdcps-and-certification/standards/sdcps/ for the list of standard devices and services) then the service description is already written for you and the device description is trivial. UPnP has standardised Lighting Controls, so we'll be using the device and service types defined there.

## Defining the Device

The first step is to write the device description file. This is a short XML document which describes the device and what services it provides (for more details see the UPnP Device Architecture specification, section 2.1). We'll be using the `BinaryLight1` device type, but if none of the existing device types are suitable a custom device type can be created.

    <?xml version="1.0" encoding="utf-8"?>
    <root xmlns="urn:schemas-upnp-org:device-1-0">
      <specVersion>
        <major>1</major>
        <minor>0</minor>
      </specVersion>

      <device>
        <deviceType>urn:schemas-upnp-org:device:BinaryLight:1</deviceType>
        <friendlyName>Kitchen Lights</friendlyName>
        <manufacturer>OpenedHand</manufacturer>
        <modelName>Virtual Light</modelName>
        <UDN>uuid:cc93d8e6-6b8b-4f60-87ca-228c36b5b0e8</UDN>

        <serviceList>
          <service>
            <serviceType>urn:schemas-upnp-org:service:SwitchPower:1</serviceType>
            <serviceId>urn:upnp-org:serviceId:SwitchPower:1</serviceId>
            <SCPDURL>/SwitchPower1.xml</SCPDURL>
            <controlURL>/SwitchPower/Control</controlURL>
            <eventSubURL>/SwitchPower/Event</eventSubURL>
          </service>
        </serviceList>
      </device>
    </root>

The `specVersion` tag defines what version of the UPnP Device Architecture the document conforms to. At the time of writing the only version is 1.0.

Next there is the root `device` tag. This contains metadata about the device, lists the services it provides and any sub-devices present (there are none in this example). The `deviceType` tag specifies the type of the device.

Next we have `friendlyName`, `manufacturer` and `modelName`. The friendly name is a human-readable name for the device, the manufacturer and model name are self-explanatory.

Next there is the UDN, or Unique Device Name. This is an identifier which is unique for each device but persistent for each particular device. Although it has to start with `uuid:` note that it doesn't have to be an UUID. There are several alternatives here: for example it could be computed at built-time if the software will only be used on a single machine, or it could be calculated using the device's serial number or MAC address.

Finally we have the `serviceList` which describes the services this device provides. Each service has a service type (again there are types defined for standardised services or you can create your own), service identifier, and three URLs. As a service type we're using the standard `SwitchPower1` service. The `SCPDURL` field specifies where the Service Control Protocol Document can be found, this describes the service in more detail and will be covered next. Finally there are the control and event URLs, which need to be unique on the device and will be managed by GUPnP.

## Defining Services

Because we are using a standard service we can use the service description from the specification. This is the `SwitchPower1` service description file:

    <?xml version="1.0" encoding="utf-8"?>
    <scpd xmlns="urn:schemas-upnp-org:service-1-0">
      <specVersion>
        <major>1</major>
        <minor>0</minor>
      </specVersion>
      <actionList>
        <action>
          <name>SetTarget</name>
          <argumentList>
            <argument>
              <name>newTargetValue</name>
              <relatedStateVariable>Target</relatedStateVariable>
              <direction>in</direction>
            </argument>
          </argumentList>
        </action>
        <action>
          <name>GetTarget</name>
          <argumentList>
            <argument>
              <name>RetTargetValue</name>
              <relatedStateVariable>Target</relatedStateVariable>
              <direction>out</direction>
            </argument>
          </argumentList>
        </action>
        <action>
          <name>GetStatus</name>
          <argumentList>
            <argument>
              <name>ResultStatus</name>
              <relatedStateVariable>Status</relatedStateVariable>
              <direction>out</direction>
            </argument>
          </argumentList>
        </action>
      </actionList>
      <serviceStateTable>
        <stateVariable sendEvents="no">
          <name>Target</name>
          <dataType>boolean</dataType>
          <defaultValue>0</defaultValue>
        </stateVariable>
        <stateVariable sendEvents="yes">
          <name>Status</name>
          <dataType>boolean</dataType>
          <defaultValue>0</defaultValue>
        </stateVariable>
      </serviceStateTable>
    </scpd>

Again, the `specVersion` tag defines the UPnP version that is being used. The rest of the document consists of an `actionList` defining the actions available and a `serviceStateTable` defining the state variables.

Every `action` has a `name` and a list of `argument`s. Arguments also have a name, a direction (`in` or `out` for input or output arguments) and a related state variable. The state variable is used to determine the type of the argument, and as such is a required element. This can lead to the creation of otherwise unused state variables to define the type for an argument (the `WANIPConnection` service is a good example of this), thanks to the legacy behind UPnP.

`stateVariable`s need to specify their `name` and `dataType`. State variables by default send notifications when they change, to specify that a variable doesn't do this set the `sendEvents` attribute to `no`. Finally there are optional `defaultValue`, `allowedValueList` and `allowedValueRange` elements which specify what the default and valid values for the variable.

For the full specification of the service definition file, including a complete list of valid `dataType`s, see section 2.3 of the UPnP Device Architecture.

## Implementing the Device

Before starting to implement the device, some boilerplate code is needed to initialise GUPnP. GLib types and threading needs to be initialised, and then a GUPnP context can be created using `gupnp_context_new()`.

    GUPnPContext *context;
    /* Initialize required subsystems */
    #if !GLIB_CHECK_VERSION(2,35,0)
      g_type_init ();
    #endif
    /* Create the GUPnP context with default host and port */
    context = gupnp_context_new (NULL, NULL, 0, NULL);

Next the root device can be created. The name of the device description file can be passed as an absolute file path or a relative path to the second parameter of `gupnp_root_device_new()`. The service description files referenced in the device description are expected to be at the path given there as well.

    GUPnPRootDevice *dev;
    /* Create the root device object */
    dev = gupnp_root_device_new (context, "BinaryLight1.xml", ".");
    /* Activate the root device, so that it announces itself */
    gupnp_root_device_set_available (dev, TRUE);

GUPnP scans the device description and any service description files it refers to, so if the main loop was entered now the device and service would be available on the network, albeit with no functionality. The remaining task is to implement the services.

## Implementing a Service

To implement a service we first fetch the GUPnPService from the root device using `gupnp_device_info_get_service()` (GUPnPRootDevice is a subclass of GUPnPDevice, which implements GUPnPDeviceInfo). This returns a GUPnPServiceInfo which again is an interface, implemented by GUPnPService (on the server) and GUPnPServiceProxy (on the client).

    GUPnPServiceInfo *service;
    service = gupnp_device_info_get_service
      (GUPNP_DEVICE_INFO (dev), "urn:schemas-upnp-org:service:SwitchPower:1");

GUPnPService handles interacting with the network itself, leaving the implementation of the service itself to signal handlers that we need to connect. There are two signals: “action-invoked” and “query-variable”. “action-invoked” is emitted when a client invokes an action: the handler is passed a GUPnPServiceAction object that identifies which action was invoked, and is used to return values using `gupnp_service_action_set()`. “query-variable” is emitted for evented variables when a control point subscribes to the service (to announce the initial value), or whenever a client queries the value of a state variable (note that this is now deprecated behaviour for UPnP control points): the handler is passed the variable name and a GValue which should be set to the current value of the variable.

Handlers should be targetted at specific actions or variables by using the signal detail when connecting. For example, this causes `on_get_status_action` to be called when the `GetStatus` action is invoked:

    static void on_get_status_action (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data);
    …
    g_signal_connect (service, "action-invoked::GetStatus", G_CALLBACK (on_get_status_action), NULL);

The implementation of action handlers is quite simple. The handler is passed a GUPnPServiceAction object which represents the in-progress action. If required it can be queried using `gupnp_service_action_get_name()` to identify the action (this isn't required if detailed signals were connected). Any in arguments can be retrieving using `gupnp_service_action_get()`, and then return values can be set using `gupnp_service_action_set()`. Once the action has been performed, either `gupnp_service_action_return()` or `gupnp_service_action_return_error()` should be called to either return successfully or return an error code. If any evented state variables were modified during the action then a notification should be emitted using `gupnp_service_notify()`. This is an example implementation of `GetStatus` and `SetTarget`:

    static gboolean status;

    static void
    get_status_cb (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data)
    {
      gupnp_service_action_set (action,
                                "ResultStatus", G_TYPE_BOOLEAN, status,
                                NULL);
      gupnp_service_action_return (action);
    }

    void
    set_target_cb (GUPnPService *service, GUPnPServiceAction *action, gpointer user_data)
    {
      gupnp_service_action_get (action,
                                "NewTargetValue", G_TYPE_BOOLEAN, &status,
                                NULL);
      gupnp_service_action_return (action);
      gupnp_service_notify (service, "Status", G_TYPE_STRING, status, NULL);
    }
    …
    g_signal_connect (service, "action-invoked::GetStatus", G_CALLBACK (get_status_cb), NULL);
    g_signal_connect (service, "action-invoked::SetTarget", G_CALLBACK (set_target_cb), NULL);

State variable query handlers are called with the name of the variable and a GValue. This value should be initialized with the relevant type and then set to the current value. Again signal detail can be used to connect handlers to specific state variable callbacks.

    static gboolean status;

    static void
    query_status_cb (GUPnPService *service, char *variable, GValue *value, gpointer user_data)
    {
      g_value_init (value, G_TYPE_BOOLEAN);
      g_value_set_boolean (value, status);
    }
    …
    g_signal_connect (service, "query-variable::Status", G_CALLBACK (query_status_cb), NULL);

The service is now fully implemented. To complete it, enter a GLib main loop and wait for a client to connect. The complete source code for this example is available as `examples/light-server.c` in the GUPnP sources.

For services which have many actions and variables there is a convenience method `gupnp_service_signals_autoconnect()` which will automatically connect specially named handlers to signals. See the documentation for full details on how it works.

## Generating Service-specific Wrappers

Using service-specific wrappers can simplify the implementation of a service. Wrappers can be generated with gupnp-binding-tool(1) using the option `--mode server`.

In the following examples the wrapper has been created with `--mode server --prefix switch`. Please note that the callback handlers (`get_status_cb` and `set_target_cb`) are not automatically generated by gupnp-binding-tool(1) for you.

    static gboolean status;

    static void
    get_status_cb (GUPnPService *service,
                   GUPnPServiceAction *action,
                   gpointer user_data)
    {
      switch_get_status_action_set (action, status);

      gupnp_service_action_return (action);
    }

    static void
    set_target_cb (GUPnPService *service,
                   GUPnPServiceAction *action,
                   gpointer user_data)
    {
      switch_set_target_action_get (action, &status);
      switch_status_variable_notify (service, status);

      gupnp_service_action_return (action);
    }

    …

    switch_get_status_action_connect (service, G_CALLBACK(get_status_cb), NULL);
    switch_set_target_action_connect (service, G_CALLBACK(set_target_cb), NULL);

Note how many possible problem situations that were run-time errors are actually compile-time errors when wrappers are used: Action names, argument names and argument types are easier to get correct (and available in editor autocompletion).

State variable query handlers are implemented in a similar manner, but they are even simpler as the return value of the handler is the state variable value.

    static gboolean
    query_status_cb (GUPnPService *service,
                     gpointer user_data)
    {
      return status;
    }

    …

    switch_status_query_connect (service, query_status_cb, NULL);