Native Client

Native Client Modules

This document describes the classes and functions that you need to implement in a Native Client module in order for Chrome to load, initialize, and run a Native Client module. The requirements depend on whether the module is written in C or C++.

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 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_InputEvent.h"
#include "ppapi/c/ppp_InputEvent.h"

// Create pointers for each PPB interface that your module uses.
static PPB_Instance* ppb_instance_interface = NULL;
static PPB_Instance* 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 struct 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 struct 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_Instance*)(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 interactive "Hello, World" example (examples/hello_world_interactive), these three components are specified in the file hello_world.cc. Here is the factory function:

Module* CreateModule() {
  return new hello_world::HelloWorldModule();
}

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, World" example:

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

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

The Module class must include a CreateInstance() function. The browser calls the CreateInstance() function of the Module object returned by CreateModule() 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, World" example:

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

  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() funtion. 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.

The module in the "Hello, World" example is created from two files: hello_world.cc and helper_functions.cc. The first file, hello_world.cc, contains the CreateModule() factory function and the Module and Instance classes described above. The second file, helper_functions.cc, contains plain C++ functions that do not use the Pepper API. This is a typical design pattern in Native Client, where plain C++ non-Pepper functions (functions that use standard types like string) are specified in a separate file from Pepper functions (functions that use Var, for example). This design pattern allows the plain C++ functions to be unit-tested with a command-line test (e.g., test_helper_functions.cc); this is easier than running tests inside Chrome.

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.

Threading

Currently, calls from the browser to a Native Client module always execute on the main thread of the module. Similarly, all Pepper API calls, both C and C++, must be made on the main thread of the module, with the exception of pp::Core::CallOnMainThread() and PPB_Core::CallOnMainThread().

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