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NOTE:
Deprecation of the technologies described here has been announced
for platforms other than ChromeOS.
Please visit our
migration guide
for details.
This tutorial shows how to convert the finished PNaCl web application from Part 1 to use the Native Client SDK build system and common JavaScript files. It also demonstrates some techniques to make your web application Content Security Policy (CSP)-compliant, which is necessary for Chrome Apps.
Using the Native Client SDK build system makes it easy to build with all of the SDK toolchains, and switch between the Debug and Release configurations. It also simplifies the makefiles for your project, as we’ll see in the next section. Finally, it adds some useful commands for running and debugging your application.
The finished code for this example can be found in the
pepper_$(VERSION)/getting_started/part2
directory in the Native Client SDK
download.
This section describes how to use the SDK build system. To do so, we’ll make changes in the makefile. Because the makefile in part1 and part2 are so different, it is easier to start from scratch. Here is the contents of the new makefile. The following sections will describe it in more detail.
The makefile from part1 only supports one toolchain (PNaCl) and one configuration (Release). It also only supports one source file. It’s relatively simple, but if we want to add support for multiple toolchains, configurations, source files, or build steps, it would grow increasingly complex. The SDK build system uses a set of variables and macros to make this possible, without significantly increasing the complexity of the makefile.
Here is the new makefile, supporting three toolchains (PNaCl, Newlib NaCl, Glibc NaCl) and two configurations (Debug, Release).
VALID_TOOLCHAINS := pnacl clang-newlib glibc NACL_SDK_ROOT ?= $(abspath $(CURDIR)/../..) include $(NACL_SDK_ROOT)/tools/common.mk TARGET = part2 LIBS = ppapi_cpp ppapi CFLAGS = -Wall SOURCES = hello_tutorial.cc # Build rules generated by macros from common.mk: $(foreach src,$(SOURCES),$(eval $(call COMPILE_RULE,$(src),$(CFLAGS)))) # The PNaCl workflow uses both an unstripped and finalized/stripped binary. # On NaCl, only produce a stripped binary for Release configs (not Debug). ifneq (,$(or $(findstring pnacl,$(TOOLCHAIN)),$(findstring Release,$(CONFIG)))) $(eval $(call LINK_RULE,$(TARGET)_unstripped,$(SOURCES),$(LIBS),$(DEPS))) $(eval $(call STRIP_RULE,$(TARGET),$(TARGET)_unstripped)) else $(eval $(call LINK_RULE,$(TARGET),$(SOURCES),$(LIBS),$(DEPS))) endif $(eval $(call NMF_RULE,$(TARGET),))
The makefile begins by specifying the toolchains that are valid for this project. The Native Client SDK build system supports multi-toolchain projects for its examples and libraries, but generally you will choose one toolchain when you begin your project and never change it. Please see the Toolchains section of the Native Client overview for more information.
For this example, we support the pnacl
, clang-newlib
and glibc
toolchains.
VALID_TOOLCHAINS := pnacl clang-newlib glibc
Next, as a convenience, we specify where to find NACL_SDK_ROOT
. Because
this example is located in pepper_$(VERSION)/getting_started/part2
, the
root of the SDK is two directories up.
NACL_SDK_ROOT ?= $(abspath $(CURDIR)/../..)
Next, we include the file tools/common.mk
. This file provides the
functionality for the Native Client SDK build system, including new build rules
to compile and link a project, which we’ll use below.
include $(NACL_SDK_ROOT)/tools/common.mk
After including tools/common.mk
, we configure the project by specifying its
name, the sources and libraries it uses:
TARGET = part2 LIBS = ppapi_cpp ppapi CFLAGS = -Wall SOURCES = hello_tutorial.cc
These variable names are not required and not used by the SDK build system; they are only used in the rules described below. By convention, all SDK makefiles use the following variables:
part2
, which will generate an executable called
part2.pexe
for PNaCl. For NaCl toolchains, the executable’s file name
will be given a suffix for its architecture. For example, the ARM executable
is called part2_arm.nexe
.ppapi_cpp
and ppapi
. ppapi_cpp
is needed to use the Pepper C++ interface. ppapi
is needed for communicating
with the browser.-Wall
, which turns on all warnings.\
as a line-continuation character. Here’s an example:SOURCES = foo.cc \ bar.cc \ baz.cc \ quux.cc
For many projects, the following build macros do not need to be changed; they will use the variables we’ve defined above.
$(foreach src,$(SOURCES),$(eval $(call COMPILE_RULE,$(src),$(CFLAGS)))) ifneq (,$(or $(findstring pnacl,$(TOOLCHAIN)),$(findstring Release,$(CONFIG)))) $(eval $(call LINK_RULE,$(TARGET)_unstripped,$(SOURCES),$(LIBS),$(DEPS))) $(eval $(call STRIP_RULE,$(TARGET),$(TARGET)_unstripped)) else $(eval $(call LINK_RULE,$(TARGET),$(SOURCES),$(LIBS),$(DEPS))) endif $(eval $(call NMF_RULE,$(TARGET),))
The first line defines rules to compile each source in SOURCES
, using the
flags in CFLAGS
:
$(foreach src,$(SOURCES),$(eval $(call COMPILE_RULE,$(src),$(CFLAGS))))
The next six lines define rules to link the object files into one or more
executables. When TOOLCHAIN
is pnacl
, there is only one executable
generated: in the example above, part2.pexe
. When using a NaCl toolchain,
there will be three executables generated, one for each architecture: in the
example above, part2_arm.nexe
, part2_x86_32.nexe
and
part2_x86_64.nexe
.
When CONFIG
is Release
, each executable is also stripped to remove
debug information and reduce the file size. Otherwise, when the TOOLCHAIN
is pnacl
, the workflow involves creating an unstripped binary for debugging
and then finalizing it and stripping it for publishing.
ifneq (,$(or $(findstring pnacl,$(TOOLCHAIN)),$(findstring Release,$(CONFIG)))) $(eval $(call LINK_RULE,$(TARGET)_unstripped,$(SOURCES),$(LIBS),$(DEPS))) $(eval $(call STRIP_RULE,$(TARGET),$(TARGET)_unstripped)) else $(eval $(call LINK_RULE,$(TARGET),$(SOURCES),$(LIBS),$(DEPS))) endif
Finally, the NMF rule generates a NaCl manifest file (.nmf
) that references
each executable generated in the previous step:
$(eval $(call NMF_RULE,$(TARGET),))
This section describes the changes necessary to make the HTML and JavaScript in part1 CSP-compliant. This is required if you want to build a Chrome App, but is not necessary if you want to use PNaCl on the open web.
Chrome Apps CSP restricts you from doing the following:
<script>
blocks and event handlers (<button onclick="...">
).eval()
and new
Function()
.To make our application CSP-compliant, we have to remove inline scripting. As
described above, we can’t use inline <script>
blocks or event handlers. This
is easy to do—we’ll just reference some new files from our script tag, and
remove all of our inlined scripts:
<head> ... <script type="text/javascript" src="common.js"></script> <script type="text/javascript" src="example.js"></script> </head>
common.js
has shared code used by all SDK examples, and is described
later in this document. example.js
is a script that has code specific to
this example.
We also need to remove the inline event handler on the body tag:
<body onload="pageDidLoad()"> ...
This logic is now handled by common.js
.
Finally, there are a few changes to index.html
that are not necessary for
CSP-compliance, but help make the SDK examples more generic.
First, we add some data attributes
to the body element to specify the name, supported toolchains, supported
configurations, and path to the .nmf
file:
<body data-name="part2" data-tools="clang-newlib glibc pnacl" data-configs="Debug Release" data-path="{tc}/{config}"> ...
common.js
will read these data attributes to allow you to load the same
example with different toolchains by changing the URL’s query string. For example, you can load the
glibc Debug version of this example by navigating to
index.html?tc=glibc&config=Debug
. Path URI’s such as ../
, for example
do not work for either the data-path parameter or its corresponding query
string.
Next, we remove the embed
element that is described in HTML. This will be
automatically added for us by common.js
, based on the current
toolchain/configuration combination:
<!-- Just as in part1, the <embed> element will be wrapped inside the <div> element with the id "listener". In part1, the embed was specified in HTML, here the common.js module creates a new <embed> element and adds it to the <div> for us. --> <div id="listener"></div>
common.js
contains JavaScript code that each example uses to create a
NaCl module, handle messages from that module and other common tasks like
displaying the module load status and logging messages. Explaining all of
common.js
is outside the scope of this document, but please look at the
documentation in that file for more information.
Since we’ve added <script>
tags for common.js
and example.js
to the
head
element, they will be loaded and executed before the rest of the
document has been parsed. As a result, we have to wait for the page to finish
loading before we try to create the embed element and add it to the page.
We can do that by calling addEventListener
and listening for the
DOMContentLoaded
event:
// Listen for the DOM content to be loaded. This event is fired when parsing of // the page's document has finished. document.addEventListener('DOMContentLoaded', function() { ... });
Inside this function, we parse the URL query string, and compare that to the data attributes:
// From https://developer.mozilla.org/en-US/docs/DOM/window.location var searchVars = {}; if (window.location.search.length > 1) { var pairs = window.location.search.substr(1).split('&'); for (var key_ix = 0; key_ix < pairs.length; key_ix++) { var keyValue = pairs[key_ix].split('='); searchVars[unescape(keyValue[0])] = keyValue.length > 1 ? unescape(keyValue[1]) : ''; } } ... var toolchains = body.dataset.tools.split(' '); var configs = body.dataset.configs.split(' '); ... var tc = toolchains.indexOf(searchVars.tc) !== -1 ? searchVars.tc : toolchains[0]; // If the config value is included in the search vars, use that. // Otherwise default to Release if it is valid, or the first value if // Release is not valid. if (configs.indexOf(searchVars.config) !== -1) var config = searchVars.config; else if (configs.indexOf('Release') !== -1) var config = 'Release'; else var config = configs[0];
Then domContentLoaded
is called, which performs some checks to see if the
browser supports Native Client, then creates the NaCl module.
function domContentLoaded(name, tool, path, width, height, attrs) { updateStatus('Page loaded.'); if (!browserSupportsNaCl(tool)) { updateStatus( 'Browser does not support NaCl (' + tool + '), or NaCl is disabled'); } else if (common.naclModule == null) { updateStatus('Creating embed: ' + tool); // We use a non-zero sized embed to give Chrome space to place the bad // plug-in graphic, if there is a problem. width = typeof width !== 'undefined' ? width : 200; height = typeof height !== 'undefined' ? height : 200; attachDefaultListeners(); createNaClModule(name, tool, path, width, height, attrs); } else { // It's possible that the Native Client module onload event fired // before the page's onload event. In this case, the status message // will reflect 'SUCCESS', but won't be displayed. This call will // display the current message. updateStatus('Waiting.'); } }
attachDefaultListeners
is added before the creation of the module, to make
sure that no messages are lost. Note that window.attachListeners
is also
called; this is the way that common.js
allows each example to configure
itself differently. If an example defines the attachListeners
function, it
will be called by common.js
.
function attachDefaultListeners() { var listenerDiv = document.getElementById('listener'); listenerDiv.addEventListener('load', moduleDidLoad, true); listenerDiv.addEventListener('message', handleMessage, true); listenerDiv.addEventListener('crash', handleCrash, true); if (typeof window.attachListeners !== 'undefined') { window.attachListeners(); } }
Finally, createNaClModule
actually creates the embed
, and appends it as
a child of the element with id listener
:
function createNaClModule(name, tool, path, width, height, attrs) { var moduleEl = document.createElement('embed'); moduleEl.setAttribute('name', 'nacl_module'); moduleEl.setAttribute('id', 'nacl_module'); moduleEl.setAttribute('width', width); moduleEl.setAttribute('height', height); moduleEl.setAttribute('path', path); moduleEl.setAttribute('src', path + '/' + name + '.nmf'); ... var mimetype = mimeTypeForTool(tool); moduleEl.setAttribute('type', mimetype); var listenerDiv = document.getElementById('listener'); listenerDiv.appendChild(moduleEl); ... }
When the module finishes loading, it will dispatch a load
event, and the
event listener function that was registered above (moduleDidLoad
) will be
called. Note that common.js
allows each example to define a
window.moduleDidLoad
function, that will be called here as well.
function moduleDidLoad() { common.naclModule = document.getElementById('nacl_module'); updateStatus('RUNNING'); if (typeof window.moduleDidLoad !== 'undefined') { window.moduleDidLoad(); } }
As described in the previous section, common.js
will call certain functions
during the module loading process. This example only needs to respond to two:
moduleDidLoad
and handleMessage
.
// This function is called by common.js when the NaCl module is // loaded. function moduleDidLoad() { // Once we load, hide the plugin. In this example, we don't display anything // in the plugin, so it is fine to hide it. common.hideModule(); // After the NaCl module has loaded, common.naclModule is a reference to the // NaCl module's <embed> element. // // postMessage sends a message to it. common.naclModule.postMessage('hello'); } // This function is called by common.js when a message is received from the // NaCl module. function handleMessage(message) { var logEl = document.getElementById('log'); logEl.textContent += message.data; }
Compile the Native Client module by running the make
command again.
Start the SDK web server by running make server
.
Re-run the application by reloading http://localhost:5103/part2
in
Chrome.
After Chrome loads the Native Client module, you should see the message sent from the module.