Native Client Modules

Introduction

Native Client modules do not have a main() function. When a module loads, the Native Client runtime calls the code in the module to create an instance and initialize the interfaces for the APIs the module uses. This initialization sequence depends on whether the module is written in C or C++ and requires that you implement specific functions in each case.

Writing modules in C

The C API uses a prefix convention to show whether an interface is implemented in the browser or in a module. Interfaces starting with PPB_ (which can be read as “Pepper browser”) are implemented in the browser and they are called from your module. Interfaces starting with PPP_ (“Pepper plugin”) are implemented in the module; they are called from the browser and will execute on the main thread of the module instance.

When you implement a Native Client module in C you must include these components:

• The functions PPP_InitializeModule and PPP_GetInterface
• Code that implements the interface PPP_Instance and any other C interfaces that your module uses

For each PPP interface, you must implement all of its functions, create the struct through which the browser calls the interface, and insure that the function PPP_GetInterface returns the appropriate struct for the interface.

For each PPB interface, you must declare a pointer to the interface and initialize the pointer with a call to get_browser inside PPP_InitializeModule.

These steps are illustrated in the code excerpt below, which shows the implementation and initialization of the required PPP_Instance interface. The code excerpt also shows the initialization of three additional interfaces which are not required: PPB_Instance (through which the Native Client module calls back to the browser) and PPB_InputEvent and PPP_InputEvent.

#include <stdlib.h>
#include <string.h>
#include "ppapi/c/pp_errors.h"
#include "ppapi/c/ppp.h"
// Include the interface headers.
// PPB APIs describe calls from the module to the browser.
// PPP APIs describe calls from the browser to the functions defined in your module.
#include "ppapi/c/ppb_instance.h"
#include "ppapi/c/ppp_instance.h"
#include "ppapi/c/ppb_input_event.h"
#include "ppapi/c/ppp_input_event.h"

// Create pointers for each PPB interface that your module uses.
static PPB_Instance* ppb_instance_interface = NULL;
static PPB_InputEvent* ppb_input_event_interface = NULL;

// Define all the functions for each PPP interface that your module uses.
// Here is a stub for the first function in PPP_Instance.
static PP_Bool Instance_DidCreate(PP_Instance instance,
                                  uint32_t argc,
                                  const char* argn[],
                                  const char* argv[]) {
        return PP_TRUE;
}
// ... more API functions ...

// Define PPP_GetInterface.
// This function should return a non-NULL value for every interface you are using.
// The string for the name of the interface is defined in the interface's header file.
// The browser calls this function to get pointers to the interfaces that your module implements.
PP_EXPORT const void* PPP_GetInterface(const char* interface_name) {
        // Create structs for each PPP interface.
        // Assign the interface functions to the data fields.
         if (strcmp(interface_name, PPP_INSTANCE_INTERFACE) == 0) {
                static PPP_Instance instance_interface = {
                        &Instance_DidCreate,
                        // The definitions of these functions are not shown
                        &Instance_DidDestroy,
                        &Instance_DidChangeView,
                        &Instance_DidChangeFocus,
                        &Instance_HandleDocumentLoad
                };
                return &instance_interface;
         }

         if (strcmp(interface_name, PPP_INPUT_EVENT_INTERFACE) == 0) {
                static PPP_InputEvent input_interface = {
                        // The definition of this function is not shown.
                        &Instance_HandleInput,
                };
                return &input_interface;
         }
         // Return NULL for interfaces that you do not implement.
         return NULL;
}

// Define PPP_InitializeModule, the entry point of your module.
// Retrieve the API for the browser-side (PPB) interfaces you will use.
PP_EXPORT int32_t PPP_InitializeModule(PP_Module a_module_id, PPB_GetInterface get_browser) {
        ppb_instance_interface = (PPB_Instance*)(get_browser(PPB_INSTANCE_INTERFACE));
        ppb_input_event_interface = (PPB_InputEvent*)(get_browser(PPB_INPUT_EVENT_INTERFACE));
        return PP_OK;
}

Writing modules in C++

When you implement a Native Client module in C++ you must include these components:

• The factory function called CreateModule()
• Code that defines your own Module class (derived from the pp::Module class)
• Code that defines your own Instance class (derived from the pp:Instance class)

In the “Hello tutorial” example (in the getting_started/part1 directory of the NaCl SDK), these three components are specified in the file hello_tutorial.cc. Here is the factory function:

namespace pp {
Module* CreateModule() {
  return new HelloTutorialModule();
}
}

The CreateModule() factory function is the main binding point between a module and the browser, and serves as the entry point into the module. The browser calls CreateModule() when a module is first loaded; this function returns a Module object derived from the pp::Module class. The browser keeps a singleton of the Module object.

Below is the Module class from the “Hello tutorial” example:

class HelloTutorialModule : public pp::Module {
 public:
  HelloTutorialModule() : pp::Module() {}
  virtual ~HelloTutorialModule() {}

  virtual pp::Instance* CreateInstance(PP_Instance instance) {
    return new HelloTutorialInstance(instance);
  }
};

The Module class must include a CreateInstance() method. The browser calls the CreateInstance() method every time it encounters an <embed> element on a web page that references the same module. The CreateInstance() function creates and returns an Instance object derived from the pp::Instance class.

Below is the Instance class from the “Hello tutorial” example:

class HelloTutorialInstance : public pp::Instance {
 public:
  explicit HelloTutorialInstance(PP_Instance instance) : pp::Instance(instance) {}
  virtual ~HelloTutorialInstance() {}

  virtual void HandleMessage(const pp::Var& var_message) {}
};

As in the example above, the Instance class for your module will likely include an implementation of the HandleMessage() function. The browser calls an instance’s HandleMessage() function every time the JavaScript code in an application calls postMessage() to send a message to the instance. See the Native Client messaging system for more information about how to send messages between JavaScript code and Native Client modules.

While the CreateModule() factory function, the Module class, and the Instance class are required for a Native Client application, the code samples shown above don’t actually do anything. Subsequent documents in the Developer’s Guide build on these code samples and add more interesting functionality.