This document describes tools and techniques you can use to debug, monitor, and measure your application’s performance.

Diagnostic information

Viewing process statistics with the task manager

You can use Chrome’s Task Manager to display information about a Native Client application:

  1. Open the Task Manager by clicking the menu icon menu-icon and choosing Tools > Task manager.
  2. When the Task Manager window appears, verify that the columns displaying memory information are visible. If they are not, right click in the header row and select the memory items from the popup menu that appears.

A browser window running a Native Client application has at least two processes associated with it: a process for the app’s top level (the render process managing the page including its HTML and JavaScript) and one or more processes for each instance of a Native Client module embedded in the page (each process running native code from one nexe or pexe file). The top-level process appears with the application’s icon and begins with the text “Tab:”. A Native Client process appears with a Chrome extension icon (a jigsaw puzzle piece puzzle) and begins with the text “Native Client module:” followed by the URL of its manifest file.

From the Task Manager you can view the changing memory allocations of all the processes associated with a Native Client application. Each process has its own memory footprint. You can also see the rendering rate displayed as frames per second (FPS). Note that the computation of render frames can be performed in any process, but the rendering itself is always done in the top level application process, so look for the rendering rate there.

Controlling the level of Native Client error and warning messages

Native Client prints warning and error messages to stdout and stderr. You can increase the amount of Native Client’s diagnostic output by setting the following environment variables:

  • NACL_SRPC_DEBUG=[1-255] (use a higher number for more verbose debug output)

Basic debugging

Writing messages to the JavaScript console

You can send messages from your C/C++ code to JavaScript using the PostMessage() call in the Pepper messaging system. When the JavaScript code receives a message, its message event handler can call console.log() to write the message to the JavaScript console in Chrome’s Developer Tools.

Debugging with printf

Your C/C++ code can perform inline printf debugging to stdout and stderr by calling fprintf() directly, or by using cover functions like these:

#include <stdio.h>
void logmsg(const char* pMsg){
  fprintf(stdout,"logmsg: %s\n",pMsg);
void errormsg(const char* pMsg){
  fprintf(stderr,"logerr: %s\n",pMsg);

Using Chrome’s stdout and stderr Streams

By default stdout and stderr will appear in Chrome’s stdout and stderr stream but they can also be redirected to log files. (See the next section.) On Mac and Linux, launching Chrome from a terminal makes stderr and stdout appear in that terminal. If you launch Chrome this way, be sure it doesn’t attach to an existing instance. One simple way to do this is to pass a new directory to chrome as your user data directory (chrome --user-data-dir=<newdir>).

Redirecting output to log files

You can redirect stdout and stderr to output files by setting these environment variables:

  • NACL_EXE_STDOUT=c:\nacl_stdout.log
  • NACL_EXE_STDERR=c:\nacl_stderr.log

There is another variable, NACLLOG, that you can use to redirect Native Client’s internally-generated messages. This variable is set to stderr by default; you can redirect these messages to an output file by setting the variable as follows:

  • NACLLOG=c:\nacl.log

Logging calls to Pepper interfaces

You can log all Pepper calls your module makes by passing the following flags to Chrome on startup:

--vmodule=ppb*=4 --enable-logging=stderr

The vmodule flag tells Chrome to log all calls to C Pepper interfaces that begin with “ppb” (that is, the interfaces that are implemented by the browser and that your module calls). The enable-logging flag tells Chrome to log the calls to stderr.

Debugging with Visual Studio

If you develop on a Windows platform you can use the Native Client Visual Studio add-in to write and debug your code. The add-in defines new project platforms that let you run your module in two different modes: As a Pepper plugin and as a Native Client module. When running as a Pepper plugin you can use the built-in Visual Studio debugger. When running as a Native Client module Visual Studio will launch an instance of nacl-gdb for you and link it to the running code.

Debugging with nacl-gdb

The Native Client SDK includes a command-line debugger that you can use to debug Native Client modules. The debugger is based on the GNU debugger gdb, and is located at pepper_<version>/toolchain/<platform>_x86_newlib/bin/x86_64-nacl-gdb (where <platform> is the platform of your development machine: win, mac, or linux).

Note that this same copy of GDB can be used to debug any NaCl program, whether built using newlib or glibc for x86-32, x86-64 or ARM. In the SDK, i686-nacl-gdb is an alias for x86_64-nacl-gdb, and the newlib and glibc toolchains both contain the same version of GDB.

Debugging PNaCl pexes (Pepper 35 or later)

If you want to use GDB to debug a program that is compiled with the PNaCl toolchain, you must have a copy of the pexe from before running pnacl-finalize. The pnacl-finalize tool converts LLVM bitcode to the stable PNaCl bitcode format, but it also strips out debug metadata, which we need for debugging. In this section we’ll give the LLVM bitcode file a .bc file extension, and the PNaCl bitcode file a .pexe file extension. The actual extension should not matter, but it helps distinguish between the two types of files.

Note unlike the finalized copy of the pexe, the non-finalized debug copy is not considered stable. This means that a debug copy of the PNaCl application created by a Pepper N SDK is only guaranteed to run with a matching Chrome version N. If the version of the debug bitcode pexe does not match that of Chrome then the translation process may fail, and you will see an error message in the JavaScript console.

Also, make sure you are passing the -g compile option to pnacl-clang to enable generating debugging info. You might also want to omit -O2 from the compile-time and link-time options, otherwise GDB not might be able to print variables’ values when debugging (this is more of a problem with the PNaCl/LLVM toolchain than with GCC).

Once you have built a non-stable debug copy of the pexe, list the URL of that copy in your application’s manifest file:

  "program": {
    "portable": {
      "pnacl-translate": {
        "url": "release_version.pexe",
        "optlevel": 2
      "pnacl-debug": {
        "url": "debug_version.bc",
        "optlevel": 0

Copy the debug_version.bc and nmf files to the location that your local web server serves files from.

When you run Chrome with --enable-nacl-debug, Chrome will translate and run the debug_version.bc instead of release_version.pexe. Once the debug version is loaded, you are ready to run nacl-gdb

Whether you publish the NMF file containing the debug URL to the release web server, is up to you. One reason to avoid publishing the debug URL is that it is only guaranteed to work for the Chrome version that matches the SDK version. Developers who may have left the --enable-nacl-debug flag turned on may end up loading the debug copy of your application (which may or may not work, depending on their version of Chrome).

Debugging PNaCl pexes (with older Pepper toolchains)

If you want to use GDB to debug a program that is compiled with the PNaCl toolchain, you must convert the pexe file to a nexe. (You can skip this step if you are using the GCC toolchain, or if you are using pepper 35 or later.)

  • Firstly, make sure you are passing the -g compile option to pnacl-clang to enable generating debugging info. You might also want to omit -O2 from the compile-time and link-time options.
  • Secondly, use pnacl-translate to convert your pexe to one or more

    nexe files. For example:

    nacl_sdk/pepper_<version>/toolchain/win_pnacl/bin/pnacl-translate \
      --allow-llvm-bitcode-input hello_world.pexe -arch x86-32 \
      -o hello_world_x86_32.nexe
    nacl_sdk/pepper_<version>/toolchain/win_pnacl/bin/pnacl-translate \
      --allow-llvm-bitcode-input hello_world.pexe -arch x86-64 \
      -o hello_world_x86_64.nexe

    For this, use the non-finalized pexe file produced by pnacl-clang, not the pexe file produced by pnacl-finalize. The latter pexe has debugging info stripped out. The option --allow-llvm-bitcode-input tells pnacl-translate to accept a non-finalized pexe.

  • Replace the nmf manifest file that points to your pexe file with one that points to the nexe files. For the example nexe filenames above, the new nmf file would contain:

      "program": {
        "x86-32": {"url": "hello_world_x86_32.nexe"},
        "x86-64": {"url": "hello_world_x86_64.nexe"},
  • Change the <embed> HTML element to use type="application/x-nacl" rather than type="application/x-pnacl".
  • Copy the nexe and nmf files to the location that your local web server serves files from.

Running nacl-gdb

Before you start using nacl-gdb, make sure you can build your module and run your application normally. This will verify that you have created all the required application parts (.html, .nmf, and .nexe files, shared libraries, etc.), that your server can access those resources, and that you’ve configured Chrome correctly to run your application. The instructions below assume that you are using a local server to run your application; one benefit of doing it this way is that you can check the web server output to confirm that your application is loading the correct resources. However, some people prefer to run their application as an unpacked extension, as described in Running Native Client Applications.

Follow the instructions below to debug your module with nacl-gdb:

  1. Compile your module with the -g flag so that your .nexe retains symbols and other debugging information (see the recommended compile flags).

  2. Launch a local web server (e.g., the web server included in the SDK).

  3. Launch Chrome with these three required flags: --enable-nacl --enable-nacl-debug --no-sandbox.

    You may also want to use some of the optional flags listed below. A typical command looks like this:

    chrome --enable-nacl --enable-nacl-debug --no-sandbox --disable-hang-monitor localhost:5103

    Required flags:


    Enables Native Client for all applications, including those that are launched outside the Chrome Web Store.


    Turns on the Native Client debug stub, opens TCP port 4014, and pauses Chrome to let the debugger connect.


    Turns off the Chrome sandbox (not the Native Client sandbox). This enables the stdout and stderr streams, and lets the debugger connect.

    Optional flags:


    Prevents Chrome from displaying a warning when a tab is unresponsive.


    Specifies the user data directory from which Chrome should load its state. You can specify a different user data directory so that changes you make to Chrome in your debugging session do not affect your personal Chrome data (history, cookies, bookmarks, themes, and settings).


    Specifies a set of debug mask patterns. This allows you to selectively choose to debug certain applications and not debug others. For example, if you only want to debug the NMF files for your applications at, and no other NaCl applications found on the web, specify --nacl-debug-mask=*.nmf. This helps prevent accidentally debugging other NaCl applications if you like to leave the --enable-nacl-debug flag turned on. The pattern language for the mask follows chrome extension match patterns. The pattern set can be inverted by prefixing the pattern set with the ! character.


    Specifies the URL Chrome should open when it launches. The local server that comes with the SDK listens on port 5103 by default, so the URL when you’re debugging is typically localhost:5103 (assuming that your application’s page is called index.html and that you run the local server in the directory where that page is located).

  4. Navigate to your application’s page in Chrome. (You don’t need to do this if you specified a URL when you launched Chrome in the previous step.) Chrome will start loading the application, then pause and wait until you start nacl-gdb and run the continue command.

  5. Go to the directory with your source code, and run nacl-gdb from there. For example:

    cd nacl_sdk/pepper_<version>/examples/demo/drive

    The debugger will start and show you a gdb prompt:

  6. Run the debugging command lines.

    For PNaCl:

    (gdb) target remote localhost:4014
    (gdb) remote get nexe <path-to-save-translated-nexe-with-debug-info>
    (gdb) file <path-to-save-translated-nexe-with-debug-info>
    (gdb) remote get irt <path-to-save-NaCl-integrated-runtime>
    (gdb) nacl-irt <path-to-saved-NaCl-integrated-runtime>

    For NaCl:

    (gdb) target remote localhost:4014
    (gdb) nacl-manifest <path-to-your-.nmf-file>
    (gdb) remote get irt <path-to-save-NaCl-integrated-runtime>
    (gdb) nacl-irt <path-to-saved-NaCl-integrated-runtime>
  7. The command used for PNaCl and NaCl are described below:

    target remote localhost:4014

    Tells the debugger how to connect to the debug stub in the Native Client application loader. This connection occurs through TCP port 4014 (note that this port is distinct from the port which the local web server uses to listen for incoming requests, typically port 5103). If you are debugging multiple applications at the same time, the loader may choose a port that is different from the default 4014 port. See the Chrome task manager for the debug port.

    remote get nexe <path>

    This saves the application’s main executable (nexe) to <path>. For PNaCl, this provides a convenient way to access the nexe that is a result of translating your pexe. This can then be loaded with the file <path> command.

    nacl-manifest <path>

    For NaCl (not PNaCl), this tells the debugger where to find your application’s executable (.nexe) files. The application’s manifest (.nmf) file lists your application’s executable files, as well as any libraries that are linked with the application dynamically.

    remote get irt <path>

    This saves the Native Client Integrated Runtime (IRT). Normally, the IRT is located in the same directory as the Chrome executable, or in a subdirectory named after the Chrome version. For example, if you’re running Chrome canary on Windows, the path to the IRT typically looks something like C:/Users/<username>/AppData/Local/Google/Chrome SxS/Application/23.0.1247.1/nacl_irt_x86_64.nexe. The remote get irt <path> saves that to the current working directory so that you do not need to find where exactly the IRT is stored.

    nacl-irt <path>

    Tells the debugger where to find the Native Client Integrated Runtime (IRT). <path> can either be the location of the copy saved by remote get irt <path> or the copy that is installed alongside Chrome.

    A couple of notes on how to specify path names in the nacl-gdb commands above:

    • You can use a forward slash to separate directories on Linux, Mac, and Windows. If you use a backslash to separate directories on Windows, you must escape the backslash by using a double backslash “\” between directories.

    • If any directories in the path have spaces in their name, you must put quotation marks around the path.

    As an example, here is a what these nacl-gdb commands might look like on Windows:

    target remote localhost:4014
    nacl-manifest "C:/nacl_sdk/pepper_<version>/examples/hello_world_gles/newlib/Debug/hello_world_gles.nmf"
    nacl-irt "C:/Users/<username>/AppData/Local/Google/Chrome SxS/Application/23.0.1247.1/nacl_irt_x86_64.nexe"

    To save yourself some typing, you can put put these nacl-gdb commands in a script file, and execute the file when you run nacl-gdb, like so:

    nacl_sdk/pepper_<version>/toolchain/win_x86_newlib/bin/x86_64-nacl-gdb -x <nacl-script-file>

    If nacl-gdb connects successfully to Chrome, it displays a message such as the one below, followed by a gdb prompt:

    0x000000000fc00200 in _start ()

    If nacl-gdb can’t connect to Chrome, it displays a message such as “localhost:4014: A connection attempt failed” or “localhost:4014: Connection timed out.” If you see a message like that, make sure that you have launched a web server, launched Chrome, and navigated to your application’s page before starting nacl-gdb.

Once nacl-gdb connects to Chrome, you can run standard gdb commands to execute your module and inspect its state. Some commonly used commands are listed below.

break <location>

set a breakpoint at <location>, e.g.:

break hello_world::HelloWorldInstance::HandleMessage
break Render
resume normal execution of the program
execute the next source line, stepping over functions
execute the next source line, stepping into functions
print <expression>
print the value of <expression> (e.g., variables)
print a stack backtrace
info breakpoints
print a table of all breakpoints
delete <breakpoint>
delete the specified breakpoint (you can use the breakpoint number displayed by the info command)
help <command>
print documentation for the specified gdb <command>
quit gdb

See the gdb documentation for a comprehensive list of gdb commands. Note that you can abbreviate most commands to just their first letter (b for break, c for continue, and so on).

To interrupt execution of your module, press <Ctrl-c>. When you’re done debugging, close the Chrome window and type q to quit gdb.

Debugging with other tools

If you cannot use the Visual Studio add-in, or you want to use a debugger other than nacl-gdb, you must manually build your module as a Pepper plugin (sometimes referred to as a “trusted” or “in-process” plugin). Pepper plugins (.DLL files on Windows; .so files on Linux; .bundle files on Mac) are loaded directly in either the Chrome renderer process or a separate plugin process, rather than in Native Client. Building a module as a trusted Pepper plugin allows you to use standard debuggers and development tools on your system, but when you’re finished developing the plugin, you need to port it to Native Client (i.e., build the module with one of the toolchains in the NaCl SDK so that the module runs in Native Client). For details on this advanced development technique, see Debugging a Trusted Plugin. Note that starting with the pepper_22 bundle, the NaCl SDK for Windows includes pre-built libraries and library source code, making it much easier to build a module into a .DLL.