One of the main advantages of debugging cross-platform Emscripten code is that the same cross-platform source code can be debugged on either the native platform or using the web browser’s increasingly powerful toolset — including debugger, profiler, and other tools.

Emscripten provides a lot of functionality and tools to aid debugging:

  • Compiler debug information flags that allow you to preserve debug information in compiled code and even create source maps so that you can step through the native C++ source when debugging in the browser.

  • Debug mode, which emits debug logs and stores intermediate build files for analysis.

  • Compiler settings to enable runtime checking of memory accesses and common allocation errors.

  • Manual print debugging of Emscripten-generated code is also supported, which is in some ways even better than on native platforms.

  • AutoDebugger, which automatically instruments LLVM bitcode to write out each store to memory.

This article describes the main tools and settings provided by Emscripten for debugging, along with a section explaining how to debug a number of Emscripten-specific issues.

Debugging in the browser

Emcc can output debug information in two formats, either as DWARF symbols or as source maps. Both allow you to view and debug the C/C++ source code in a browser’s debugger. DWARF offers the most precise and detailed debugging experience and is supported as an experiment in Chrome 88 with an extension <>. See here <> for a detailed usage guide. Source maps are more widely supported in Firefox, Chrome, and Safari, but unlike DWARF they cannot be used to inspect variables, for example.

Emcc strips out most of the debug information from optimized builds by default. DWARF can be produced with the emcc -g flag, and source maps can be emitted with the -gsource-map option. Be aware that optimisation levels -O1 and above increasingly remove LLVM debug information, and also disable runtime ASSERTIONS checks. Passing a -g flag also affects the generated JavaScript code and preserves white-space, function names, and variable names,


Even for medium-sized projects, DWARF debug information can be of substantial size and negatively impact the page performance, particularly compiling and loading of the module. Debug information can also be emitted in a file on the side instead with the -gseparate-dwarf option! The debug information size also affects the linking time, because the debug information in all object files needs to be linked as well. Passing the -gsplit-dwarf option can help here, which causes clang to leave debug information scattered across object files. That debug information needs to be linked into a DWARF package file (.dwp) using the emdwp tool then, but that could happen in parallel to the linking of the compiled output! When running it after linking, it’s as simple as emdwp -e foo.wasm -o foo.wasm.dwp, or emdwp -e foo.debug.wasm -o foo.debug.wasm.dwp when used together with -gseparate-dwarf (the dwp file should have the same file name as the main symbol file with an extra .dwp extension).

The -g flag can also be specified with an integer levels: -g0, -g1, -g2 (default with -gsource-map), and -g3 (default with -g). Each level builds on the last to provide progressively more debug information in the compiled output.


Because Binaryen optimization degrades the quality of DWARF info further, -O1 -g will skip running the Binaryen optimizer (wasm-opt) entirely unless required by other options. You can also throw in -sERROR_ON_WASM_CHANGES_AFTER_LINK option if you want to ensure the debug info is preserved. See Skipping Binaryen for more details.


Some optimizations may be disabled when used in conjunction with the debug flags both in the Binaryen optimizer (even if it runs) and JavaScript optimizer. For example, if you compile with -O3 -g, the Binaryen optimizer will skip some of the optimization passes that do not produce valid DWARF information, and also some of the normal JavaScript optimization will be disabled in order to better provide the requested debugging information.

Debug mode (EMCC_DEBUG)

The EMCC_DEBUG environment variable can be set to enable Emscripten’s debug mode:

# Linux or macOS
EMCC_DEBUG=1 emcc test/hello_world.cpp -o hello.html

# Windows
emcc test/hello_world.cpp -o hello.html

With EMCC_DEBUG=1 set, emcc emits debug output and generates intermediate files for the compiler’s various stages. EMCC_DEBUG=2 additionally generates intermediate files for each JavaScript optimizer pass.

The debug logs and intermediate files are output to TEMP_DIR/emscripten_temp, where TEMP_DIR is the OS default temporary directory (e.g. /tmp on UNIX).

The debug logs can be analysed to profile and review the changes that were made in each step.


The more limited amount of debug information can also be enabled by specifying the verbose output compiler flag (emcc -v).

Compiler settings

Emscripten has a number of compiler settings that can be useful for debugging. These are set using the emcc -s option, and will override any optimization flags. For example:

emcc -O1 -sASSERTIONS test/hello_world

Some important settings are:

  • ASSERTIONS=1 is used to enable runtime checks for common memory allocation errors (e.g. writing more memory than was allocated). It also defines how Emscripten should handle errors in program flow. The value can be set to ASSERTIONS=2 in order to run additional tests.

    ASSERTIONS=1 is enabled by default. Assertions are turned off for optimized code (-O1 and above).

  • SAFE_HEAP=1 adds additional memory access checks, and will give clear errors for problems like dereferencing 0 and memory alignment issues.

    You can also set SAFE_HEAP_LOG to log SAFE_HEAP operations.

  • Passing the STACK_OVERFLOW_CHECK=1 linker flag adds a runtime magic token value at the end of the stack, which is checked in certain locations to verify that the user code does not accidentally write past the end of the stack. While overrunning the Emscripten stack is not a security issue for JavaScript (which is unaffected), writing past the stack causes memory corruption in global data and dynamically allocated memory sections in the Emscripten HEAP, which makes the application fail in unexpected ways. The value STACK_OVERFLOW_CHECK=2 enables slightly more detailed stack guard checks, which can give a more precise callstack at the expense of some performance. Default value is 1 if ASSERTIONS=1 is set, and disabled otherwise.

A number of other useful debug settings are defined in src/settings.js. For more information, search that file for the keywords “check” and “debug”.


Emscripten also supports some of Clang’s sanitizers, such as Undefined Behaviour Sanitizer and Address Sanitizer.

emcc verbose output

Compiling with the emcc -v will cause Emscripten to output the sub-command that it runs as well as passes -v to Clang.

Manual print debugging

You can also manually instrument the source code with printf() statements, then compile and run the code to investigate issues. Note that printf() is line-buffered, make sure to add \n to see output in the console.

If you have a good idea of the problem line you can add print(new Error().stack) to the JavaScript to get a stack trace at that point.

Debug printouts can even execute arbitrary JavaScript. For example:

function _addAndPrint($left, $right) {
  $left = $left | 0;
  $right = $right | 0;
  if ($left < $right) console.log('l<r at ' + stackTrace());
  _printAnInteger($left + $right | 0);

Debugging with Chrome Devtools

Chrome devtools support source-level debugging on WebAssembly files with DWARF information. To use that, you need the Wasm debugging extension plugin here:

See Debugging WebAssembly with modern tools for the details.

Handling C++ Exceptions from JavaScript

See Handling C++ Exceptions from JavaScript.

Emscripten-specific issues

Memory Alignment Issues

The Emscripten memory representation is compatible with C and C++. However, when undefined behavior is involved you may see differences with native architectures, and also differences between Emscripten’s output for asm.js and WebAssembly:

  • In asm.js, loads and stores must be aligned, and performing a normal load or store on an unaligned address can fail silently (access the wrong address). If the compiler knows a load or store is unaligned, it can emulate it in a way that works but is slow.

  • In WebAssembly, unaligned loads and stores will work. Each one is annotated with its expected alignment. If the actual alignment does not match, it will still work, but may be slow on some CPU architectures.


SAFE_HEAP can be used to reveal memory alignment issues.

Generally it is best to avoid unaligned reads and writes — often they occur as the result of undefined behavior, as mentioned above. In some cases, however, they are unavoidable — for example if the code to be ported reads an int from a packed structure in some pre-existing data format. In that case, to make things work properly in asm.js, and be fast in WebAssembly, you must be sure that the compiler knows the load or store is unaligned. To do so you can:

  • Manually read individual bytes and reconstruct the full value

  • Use the emscripten_align* typedefs, which define unaligned versions of the basic types (short, int, float, double). All operations on those types are not fully aligned (use the 1 variants in most cases, which mean no alignment whatsoever).

Function Pointer Issues

If you get an abort() from a function pointer call to nullFunc or b0 or b1 (possibly with an error message saying “incorrect function pointer”), the problem is that the function pointer was not found in the expected function pointer table when called.


nullFunc is the function used to populate empty index entries in the function pointer tables (b0 and b1 are shorter names used for nullFunc in more optimized builds). A function pointer to an invalid index will call this function, which simply calls abort().

There are several possible causes:

  • Your code is calling a function pointer that has been cast from another type (this is undefined behavior but it does happen in real-world code). In optimized Emscripten output, each function pointer type is stored in a separate table based on its original signature, so you must call a function pointer with that same signature to get the right behavior (see Function Pointer Issues in the code portability section for more information).

  • Your code is calling a method on a NULL pointer or dereferencing 0. This sort of bug can be caused by any sort of coding error, but manifests as a function pointer error because the function can’t be found in the expected table at runtime.

In order to debug these sorts of issues:

  • Compile with -Werror. This turns warnings into errors, which can be useful as some cases of undefined behavior would otherwise show warnings.

  • Use -sASSERTIONS=2 to get some useful information about the function pointer being called, and its type.

  • Look at the browser stack trace to see where the error occurs and which function should have been called.

  • Enable clang warnings on dangerous function pointer casts using -Wcast-function-type.

  • Build with SAFE_HEAP=1.

  • Debugging with Sanitizers can help here, in particular UBSan.

Another function pointer issue is when the wrong function is called. SAFE_HEAP=1 can help with this as it detects some possible errors with function table accesses.

Infinite loops

Infinite loops cause your page to hang. After a period the browser will notify the user that the page is stuck and offer to halt or close it.

If your code hits an infinite loop, one easy way to find the problem code is to use a JavaScript profiler. In the Firefox profiler, if the code enters an infinite loop you will see a block of code doing the same thing repeatedly near the end of the profile.


The Browser main loop may need to be re-coded if your application uses an infinite main loop.



To profile your code for speed, build with profiling info, then run the code in the browser’s devtools profiler. You should then be able to see in which functions is most of the time spent.


The browser’s memory profiling tools generally only understand allocations at the JavaScript level. From that perspective, the entire linear memory that the emscripten-compiled application uses is a single big allocation (of a WebAssembly.Memory). The devtools will not show information about usage inside that object, so you need other tools for that, which we will now describe.

Emscripten supports mallinfo(), which lets you get information from dlmalloc about current allocations. For example usage, see the test.

Emscripten also has a --memoryprofiler option that displays memory usage in a visual manner, letting you see how fragmented it is and so forth. To use it, you can do something like

emcc test/hello_world.c --memoryprofiler -o page.html

Note that you need to emit HTML as in that example, as the memory profiler output is rendered onto the page. To view it, load page.html in your browser (remember to use a local webserver). The display auto-updates, so you can open the devtools console and run a command like _malloc(1024 * 1024). That will allocate 1MB of memory, which will then show up on the memory profiler display.


The AutoDebugger is the ‘nuclear option’ for debugging Emscripten code.


This option is primarily intended for Emscripten core developers.

The AutoDebugger will rewrite the output so it prints out each store to memory. This is useful because you can compare the output for different compiler settings in order to detect regressions.

The AutoDebugger can potentially find any problem in the generated code, so it is strictly more powerful than the CHECK_* settings and SAFE_HEAP. One use of the AutoDebugger is to quickly emit lots of logging output, which can then be reviewed for odd behavior. The AutoDebugger is also particularly useful for debugging regressions.

The AutoDebugger has some limitations:

  • It generates a lot of output. Using diff can be very helpful for identifying changes.

  • It prints out simple numerical values rather than pointer addresses (because pointer addresses change between runs, and hence can’t be compared). This is a limitation because sometimes inspection of addresses can show errors where the pointer address is 0 or impossibly large. It is possible to modify the tool to print out addresses as integers in tools/

To run the AutoDebugger, compile with the environment variable EMCC_AUTODEBUG=1 set. For example:

# Linux or macOS
EMCC_AUTODEBUG=1 emcc test/hello_world.cpp -o hello.html

# Windows
emcc test/hello_world.cpp -o hello.html

AutoDebugger Regression Workflow

Use the following workflow to find regressions with the AutoDebugger:

  • Compile the working code with EMCC_AUTODEBUG=1 set in the environment.

  • Compile the code using EMCC_AUTODEBUG=1 in the environment again, but this time with the settings that cause the regression. Following this step we have one build before the regression and one after.

  • Run both versions of the compiled code and save their output.

  • Compare the output using a diff tool.

Any difference between the outputs is likely to be caused by the bug.


You may want to use -sDETERMINISTIC which will ensure that timing and other issues don’t cause false positives.

Need help?

The Emscripten Test Suite contains good examples of almost all functionality offered by Emscripten. If you have a problem, it is a good idea to search the suite to determine whether test code with similar behavior is able to run.

If you’ve tried the ideas here and you need more help, please Get in touch.