1 {{+bindTo:partials.standard_nacl_article}}
3 <section id=
"application-structure">
4 <span id=
"devcycle-application-structure"></span><h1 id=
"application-structure"><span id=
"devcycle-application-structure"></span>Application Structure
</h1>
5 <div class=
"contents local" id=
"contents" style=
"display: none">
7 <li><a class=
"reference internal" href=
"#application-components" id=
"id1">Application components
</a></li>
8 <li><a class=
"reference internal" href=
"#html-file-and-the-embed-element" id=
"id2">HTML file and the
<embed
> element
</a></li>
9 <li><a class=
"reference internal" href=
"#manifest-files" id=
"id3">Manifest Files
</a></li>
10 <li><a class=
"reference internal" href=
"#modules-and-instances" id=
"id4">Modules and instances
</a></li>
11 <li><a class=
"reference internal" href=
"#native-client-modules-a-closer-look" id=
"id5">Native Client modules: A closer look
</a></li>
14 </div><p>This section of the Developer
’s Guide describes the general structure of a
15 Native Client application. The section assumes you are familiar with the
16 material presented in the
<a class=
"reference internal" href=
"/native-client/overview.html"><em>Technical Overview
</em></a>.
</p>
18 The
“Hello, World
” example is used here to illustrate basic
19 Native Client programming techniques. You can find this code in the
20 <em>/getting_started/part1
</em> directory in the Native Client SDK download.
22 <h2 id=
"application-components">Application components
</h2>
23 <p>A Native Client application typically contains the following components:
</p>
24 <ul class=
"small-gap">
25 <li>an HTML file;
</li>
26 <li>JavaScript code, which can be included in the HTML file or contained in one or
27 more separate .js files;
</li>
28 <li>CSS styles, which can be included in the HTML file or contained in one or more
29 separate .css files;
</li>
30 <li>a Native Client manifest file (with a .nmf extension) that specifies how to
31 load a Native Client module for different processors; and
</li>
32 <li>a Native Client module, written in C or C++, and compiled into a portable
33 executable file (with a .pexe extension) or (if using the Chrome Web Store),
34 architecture-specific executable files (with .nexe extensions).
</li>
36 <p>Applications that are published in the
<a class=
"reference external" href=
"https://chrome.google.com/webstore/search?q=%22Native+Client%22+OR+NativeClient+OR+NaCl">Chrome Web Store
</a>
38 Web Store manifest file
<code>(manifest.json)
</code> and one or more icon files.
</p>
39 <h2 id=
"html-file-and-the-embed-element"><span id=
"html-file"></span>HTML file and the
<embed
> element
</h2>
40 <p>The
<code><embed
></code> element in an HTML file triggers the loading of a Native Client
41 module and specifies the rectangle on the web page that is managed by the
42 module. Here is the
<embed
> element from the
“Hello, World
” application:
</p>
43 <pre class=
"prettyprint">
44 <embed id=
"hello_tutorial
"
46 src=
"hello_tutorial.nmf
"
47 type=
"application/x-pnacl
" /
>
49 <p>In the
<code><embed
></code> element:
</p>
52 <dd>is the DOM name attribute for the Native Client module
53 (
“nacl_module
” is often used as a convention)
</dd>
55 <dd>specifies the DOM ID for the Native Client module
</dd>
56 <dt>width, height
</dt>
57 <dd>specify the size in pixels of the rectangle on the web page that is
58 managed by the Native Client module (if the module does not have a
59 visible area, these values can be
0)
</dd>
61 <dd>refers to the Native Client manifest file that is used to determine
62 which version of a module to load based on the architecture of the
63 user
’s computer (see the following section for more information)
</dd>
65 <dd>specifies the MIME type of the embedded content; for Portable Native Client
66 modules the type must be
“application/x-pnacl
”. For architecture-specific
67 Native Client modules the type must be
“application/x-nacl
”</dd>
69 <h2 id=
"manifest-files"><span id=
"manifest-file"></span>Manifest Files
</h2>
70 <p>Native Client applications have two types of manifest files: a Chrome Web Store
71 manifest file and a Native Client manifest file.
</p>
72 <p>A
<strong>Chrome Web Store manifest file
</strong> is a file with information about a web
73 application that is published in the Chrome Web Store. This file, named
74 <code>manifest.json
</code>, is required for applications that are published in the
75 Chrome Web Store. For more information about this file see
<a class=
"reference internal" href=
"/native-client/devguide/distributing.html"><em>Distributing
76 Your Application
</em></a>. and the
<a class=
"reference external" href=
"/extensions/manifest">Chrome Web Store manifest file
78 <p>A
<strong>Native Client manifest file
</strong> is a file that specifies which Native Client
79 module (executable) to load. For PNaCl it specifies a single portable
80 executable; otherwise it specifies one for each of the supported end-user
81 computer architectures (for example x86-
32, x86-
64, or ARM). This file is
82 required for all Native Client applications. The extension for this file is
84 <p>Manifest files for applications that use PNaCl are simple. Here is the manifest
85 for the hello world example:
</p>
86 <pre class=
"prettyprint">
88 "program
": {
89 "portable
": {
90 "pnacl-translate
": {
91 "url
":
"hello_tutorial.pexe
"
97 <p>For Chrome Web Store applications that do not use PNaCl, a typical manifest file
98 contains a
<a class=
"reference external" href=
"http://www.json.org/">JSON
</a> dictionary with a single top-level
99 key/value pair: the
“program
” key and a value consisting of a nested
100 dictionary. The nested dictionary contains keys corresponding to the names of
101 the supported computer architectures, and values referencing the file to load
102 for a given architecture—specifically, the URL of the .nexe file, given by the
103 <code>"url
"</code> key. URLs are specified relative to the location of the manifest file.
104 Here is an example:
</p>
105 <pre class=
"prettyprint">
107 "program
": {
108 "x86-
32": {
109 "url
":
"hello_tutorial_x86_32.nexe
"
111 "x86-
64": {
112 "url
":
"hello_tutorial_x86_64.nexe
"
115 "url
":
"hello_tutorial_arm.nexe
"
120 <p>For applications that use the
<a class=
"reference internal" href=
"/native-client/devguide/devcycle/dynamic-loading.html#c-libraries"><em>glibc
</em></a>
121 library, the manifest file must also contain a
“files
” key that specifies the
122 shared libraries that the applications use. This is discussed in detail in
123 <a class=
"reference internal" href=
"/native-client/devguide/devcycle/dynamic-loading.html"><em>Dynamic Linking and Loading with glibc
</em></a>. To
124 see some example manifest files, build some of the example applications in the
125 SDK (run
<code>make
</code> in the example subdirectories) and look at the generated
127 <p>In most cases, you can simply use the Python script provided with the SDK,
128 <code>create_nmf.py
</code>, to create a manifest file for your application as part of the
129 compilation step (see the Makefile in any of the SDK examples for an
130 illustration of how to do so). The manifest file format is also
131 <a class=
"reference internal" href=
"/native-client/reference/nacl-manifest-format.html"><em>documented
</em></a>.
</p>
132 <h2 id=
"modules-and-instances">Modules and instances
</h2>
133 <p>A Native Client
<strong>module
</strong> is C or C++ code compiled into a PNaCl .pexe file or
134 a NaCl .nexe file.
</p>
135 <p>An
<strong>instance
</strong> is a rectangle on a web page that is managed by a module. An
136 instance may have a dimension of width=
0 and height=
0, meaning that the instance
137 does not have any visible component on the web page. An instance is created by
138 including an
<code><embed
></code> element in a web page. The
<code><embed
></code> element
139 references a Native Client manifest file that loads the appropriate version of
140 the module (either portable, or specific to the end-user
’s architecture). A
141 module may be included in a web page multiple times by using multiple
142 <code><embed
></code> elements that refer to the module; in this case the Native Client
143 runtime system loads the module once and creates multiple instances that are
144 managed by the module.
</p>
145 <h2 id=
"native-client-modules-a-closer-look">Native Client modules: A closer look
</h2>
146 <p>A Native Client module must include three components:
</p>
147 <ul class=
"small-gap">
148 <li>a factory function called
<code>CreateModule()
</code></li>
149 <li>a Module class (derived from the
<code>pp::Module
</code> class)
</li>
150 <li>an Instance class (derived from the
<code>pp:Instance
</code> class)
</li>
152 <p>In the
“Hello tutorial
” example (in the
<code>getting_started/part1
</code> directory of
153 the NaCl SDK), these three components are specified in the file
154 <code>hello_tutorial.cc
</code>. Here is the factory function:
</p>
155 <pre class=
"prettyprint">
156 Module* CreateModule() {
157 return new HelloTutorialModule();
160 <p>Native Client modules do not have a
<code>main()
</code> function. The
<code>CreateModule()
</code>
161 factory function is the main binding point between a module and the browser, and
162 serves as the entry point into the module. The browser calls
<code>CreateModule()
</code>
163 when a module is first loaded; this function returns a Module object derived
164 from the
<code>pp::Module
</code> class. The browser keeps a singleton of the Module
166 <p>Below is the Module class from the
“Hello tutorial
” example:
</p>
167 <pre class=
"prettyprint">
168 class HelloTutorialModule : public pp::Module {
170 HelloTutorialModule() : pp::Module() {}
171 virtual ~HelloTutorialModule() {}
173 virtual pp::Instance* CreateInstance(PP_Instance instance) {
174 return new HelloTutorialInstance(instance);
178 <p>The Module class must include a
<code>CreateInstance()
</code> method. The browser calls
179 the
<code>CreateInstance()
</code> method every time it encounters an
<code><embed
></code> element
180 on a web page that references the same module. The
<code>CreateInstance()
</code> function
181 creates and returns an Instance object derived from the
<code>pp::Instance
</code> class.
</p>
182 <p>Below is the Instance class from the
“Hello tutorial
” example:
</p>
183 <pre class=
"prettyprint">
184 class HelloTutorialInstance : public pp::Instance {
186 explicit HelloTutorialInstance(PP_Instance instance) : pp::Instance(instance) {}
187 virtual ~HelloTutorialInstance() {}
189 virtual void HandleMessage(const pp::Var
& var_message) {}
192 <p>As in the example above, the Instance class for your module will likely include
193 an implementation of the
<code>HandleMessage()
</code> function. The browser calls an
194 instance
’s
<code>HandleMessage()
</code> function every time the JavaScript code in an
195 application calls
<code>postMessage()
</code> to send a message to the instance. See the
196 <a class=
"reference internal" href=
"/native-client/devguide/coding/message-system.html"><em>Native Client messaging system
</em></a> for more information about
197 how to send messages between JavaScript code and Native Client modules.
</p>
198 <p>The NaCl code is only invoked to handle various browser-issued
199 events and callbacks. There is no need to shut down the NaCl instance by
200 calling the
<code>exit()
</code> function. NaCl modules will be shut down when the user
201 leaves the web page, or the NaCl module
’s
<code><embed
></code> is otherwise destroyed.
202 If the NaCl module does call the
<code>exit()
</code> function, the instance will
203 issue a
<code>crash
</code> event
204 <a class=
"reference internal" href=
"/native-client/devguide/coding/progress-events.html"><em>which can be handled in Javascript
</em></a>.
</p>
205 <p>While the
<code>CreateModule()
</code> factory function, the
<code>Module
</code> class, and the
206 <code>Instance
</code> class are required for a Native Client application, the code
207 samples shown above don
’t actually do anything. Subsequent sections in the
208 Developer
’s Guide build on these code samples and add more interesting
212 {{/partials.standard_nacl_article}}