FAQ

This FAQ contains answers to many questions asked on IRC and the mailing list.

How do I compile code?

See the Emscripten Tutorial and emcc.

Why do I get errors building basic code and the tests?

All the tests in the Emscripten test suite are known to build and pass on our test infrastructure, so if you see failures locally it is likely that there is some problem with your environment. (Rarely, there may be temporary breakage, but never on a tagged release version.)

First call emcc --check, which runs basic sanity checks and prints out useful environment information. If that doesn’t help, follow the instructions in Verifying the Emscripten Development Environment.

You might also want to go through the Emscripten Tutorial again, as it is updated as Emscripten changes.

Also make sure that you have the necessary requirements for running Emscripten as specified in the SDK section, including new-enough versions of the dependencies.

I tried something: why doesn’t it work?

Some general steps that might help figure things out:

  • See if the problem happens without optimizations (-O0, or not specifying any optimization level). Without optimizations, emscripten enables many assertions at compile and runtime, which may catch a problem and display an error message with a suggestion for how to fix it.

  • Search the documentation on this site.

  • Check if there is a test for the failing functionality in the Emscripten test suite (run grep -r in test/). They should all pass (with only rare exceptions), so they provide concrete “known-good” examples of how various options and code are used.

Do I need to change my build system to use Emscripten?

In most cases you will be able to use your project’s current build system with Emscripten. See Building Projects.

Why is code compilation slow?

Emscripten makes some trade-offs that make the generated code faster and smaller, at the cost of longer link times. For example, we build parts of the standard library with -flto (Link Time Optimization), which enables some additional optimizations, but can take longer to build. We also (in optimized builds) run the Binaryen optimizer on the entire output even without LTO.

Note

You can determine what compilation steps take longest by compiling with EMCC_DEBUG=1 in the environment and then reviewing the debug logs (by default in /tmp/emscripten_temp). Note that compiling in debug mode takes longer than normal, because we print out a lot of intermediate steps to disk, so it’s useful for debugging but not for actual compiling.

The main tips for improving build time are:

  • Use -O0 for fast iteration builds. You can still compile with higher optimization levels, but specifying -O0 during link will make the link step much faster.

  • Compile on a machine with more cores:

    • For compiling your source files, use a parallel build system (for example, in make you can do something like make -j8 to run using 8 cores).

    • For the link step, Emscripten can run some optimizations in parallel (specifically, Binaryen optimizations for Wasm, and our JavaScript optimizations). Increasing the number of cores results in an almost linear improvement. Emscripten will automatically use more cores if they are available, but you can control that with EMCC_CORES=N in the environment (which is useful if you have many cores but relatively less memory).

Why does my code run slowly?

Make sure you optimize code by building with -O2 (even more aggressive optimization is available, at the cost of significantly increased compilation time).

Why is my compiled code big?

Make sure you build with -O3 or -Os so code is fully optimized and minified. You should use the closure compiler, gzip compression on your webserver, etc., see the section on code size in Optimizing code.

Why does compiling code that works on another machine gives me errors?

Make sure you are using the Emscripten bundled system headers. Using emcc will do so by default, but problems may occur if you use your local system headers with emcc.

How can I reduce startup time?

Make sure that you are running an optimized build (smaller builds are faster to start up).

Network latency is also a possible factor in startup time. Consider putting the file loading code in a separate script element from the generated code so that the browser can start the network download in parallel to starting up the codebase (run the file packager and put file loading code in one script element, and the generated codebase in a later script element).

How do I run a local webserver for testing / why does my program stall in “Downloading…” or “Preparing…”?

That error can happen when loading the page using a file:// URL, which works in some browsers but not in others. Instead, it’s best to use a local webserver. For example, Python has one built in, python -m http.server in Python 3 or python -m SimpleHTTPServer in Python 2. After doing that, you can visit http://localhost:8000/. You can also use emrun FILENAME.html (which will run a python webserver for you).

When doing quick local testing, another option than a local webserver is to bundle everything into a single file, using -sSINGLE_FILE (as then no XHRs will be made to file:// URLs).

Otherwise, to debug this, look for an error reported on the page itself, or in the browser devtools (web console and network tab), or in your webserver’s logging.

Why do I get machine type must be wasm32 or unknown file type during linking?

This means that one or more of this linker input files were not build by Emscripten (or, more-specifically, not built for the correct target architecture).

Most often the file in question will be an ELF file or Mach-O file built for the host machine. You can run the file command-line utility to see what they actually contain.

Common issues are:

  • Attempting to link against libraries built for the host system. For example, if you have something like -L/usr/lib in your link command that is almost always going to cause these errors since the libraries that exist in those system directories are almost certainly not built with/for Emscripten. This solution is to use Emscripten to build all the libraries that you depend on, and never use host libraries.

  • Some libraries or object files in your project were built using the host compiler rather then the emscripten compiler. If you are using autoconf or cmake make sure you use the emconfigure/emmake wrapper, see Building Projects.

  • LLVM IR from the old backend, if you built the project with a version before 1.39.0 (which used the old backend by default), and are doing an incremental rebuild now. To fix that, do a complete rebuild from scratch of all your project’s files, including libraries (this error often happens if you have prebuilt libraries from a third party; those must be recompiled too with the new backend).

Why does my code fail to compile with an error message about inline assembly (or {"text":"asm"})?

Emscripten cannot compile inline assembly code (unless that assembly code is specifically written to target WebAssembly).

You will need to find where inline assembly is used, and disable it or replace it with platform-independent code.

Why does my HTML app hang?

The browser event model uses co-operative multitasking — each event has a “turn” to run, and must then return control to the browser event loop so that other events can be processed. A common cause of HTML pages hanging is JavaScript that does not complete and return control to the browser.

Graphical C++ apps typically have an infinite main loop in which event handling, processing and rendering is done, followed by a delay to keep the frame-rate right (SDL_DELAY in SDL apps). As the main loop does not complete (is infinite) it cannot return control to the browser, and the app will hang.

Apps that use an infinite main loop should be re-coded to put the actions for a single iteration of the loop into a single “finite” function. In the native build this function can be run in an infinite loop as before. In the Emscripten build it is set as the main loop function and will be called by the browser at a specified frequency.

There is more information on this topic in Emscripten Runtime Environment.

How do I run an event loop?

To run a C function repeatedly, use emscripten_set_main_loop() (this is discussed in Emscripten Runtime Environment). The related functions in emscripten.h are also useful, allowing you to add events that block the main loop, etc.

To respond to browser events use the SDL API in the normal way. There are examples in the SDL tests (search for SDL in test/runner.py).

See also: Why does my HTML app hang?

Why doesn’t my SDL app work?

See the SDL automatic tests for working examples: test/runner.py browser.

What are my options for audio playback?

Emscripten has partial support for SDL1 and 2 audio, and OpenAL.

To use SDL1 audio, include it as #include <SDL/SDL_mixer.h>. You can use it that way alongside SDL1, SDL2, or another library for platform integration.

To use SDL2 audio, include it as #include <SDL2/SDL_mixer.h> and use -sUSE_SDL_MIXER=2. Format support is currently limited to OGG, WAV, MID, and MOD.

How can my compiled program access files?

Emscripten uses a virtual file system that may be preloaded with data or linked to URLs for lazy loading. See the File System Overview for more details.

Why can’t my code access a file in the same directory?

Emscripten-generated code running in the browser cannot access files in the local file system. Instead you can use preloading and embedding to work around the lack of synchronous file IO. See File System Overview for more information.

It is possible to allow access to local file system for code running in node.js, use the NODEFS filesystem option.

How can I tell when the page is fully loaded and it is safe to call compiled functions?

(You may need this answer if you see an error saying something like native function `x` called before runtime initialization, which is a check enabled in ASSERTIONS builds.)

Calling a compiled function before a page has fully loaded can result in an error, if the function relies on files that may not be present (for example preloaded files are loaded asynchronously, and therefore if you just place some JS that calls compiled code in a --post-js, that code will be called synchronously at the end of the combined JS file, potentially before the asynchronous event happens, which is bad).

The easiest way to find out when loading is complete is to add a main() function, and within it call a JavaScript function to notify your code that loading is complete.

Note

The main() function is called after startup is complete as a signal that it is safe to call any compiled method.

For example, if allReady() is a JavaScript function you want called when everything is ready, you can do:

#include <emscripten.h>

int main() {
  EM_ASM( allReady() );
}

Another option is to define an onRuntimeInitialized function,

Module['onRuntimeInitialized'] = function() { ... };

That method will be called when the runtime is ready and it is ok for you to call compiled code. In practice, that is exactly the same time at which main() would be called, so onRuntimeInitialized doesn’t let you do anything new, but you can set it from JavaScript at runtime in a flexible way.

Here is an example of how to use it:

<script type="text/javascript">
  var Module = {
    onRuntimeInitialized: function() {
      Module._foobar(); // foobar was exported
    }
  };
</script>
<script type="text/javascript" src="my_project.js"></script>

The crucial thing is that Module exists, and has the property onRuntimeInitialized, before the script containing emscripten output (my_project.js in this example) is loaded.

Another option is to use the MODULARIZE option, using -sMODULARIZE. That puts all of the generated JavaScript into a factory function, which you can call to create an instance of your module. The factory function returns a Promise that resolves with the module instance. The promise is resolved once it’s safe to call the compiled code, i.e. after the compiled code has been downloaded and instantiated. For example, if you build with -sMODULARIZE -s 'EXPORT_NAME="createMyModule"', then you can do this:

createMyModule(/* optional default settings */).then(function(Module) {
  // this is reached when everything is ready, and you can call methods on Module
});

Note that in MODULARIZE mode we do not look for a global Module object for default values. Default values must be passed as a parameter to the factory function. (see details in settings.js)

What does “exiting the runtime” mean? Why don’t atexit()s run?

(You may need this answer if you see an error saying something like atexit() called, but EXIT_RUNTIME is not set or stdio streams had content in them that was not flushed. you should set EXIT_RUNTIME to 1.)

By default Emscripten sets EXIT_RUNTIME=0, which means that we don’t include code to shut down the runtime. That means that when main() exits, we don’t flush the stdio streams, or call the destructors of global C++ objects, or call atexit callbacks. This lets us emit smaller code by default, and is normally what you want on the web: even though main() exited, you may have something asynchronous happening later that you want to execute.

In some cases, though, you may want a more “commandline” experience, where we do shut down the runtime when main() exits. You can build with -sEXIT_RUNTIME, and then we will call atexits and so forth. When you build with ASSERTIONS, you should get a warning when you need this. For example, if your program prints something without a newline,

#include <stdio.h>

int main() {
  printf("hello"); // note no newline
}

If we don’t shut down the runtime and flush the stdio streams, “hello” won’t be printed. In an ASSERTIONS build you’ll get a notification saying stdio streams had content in them that was not flushed. you should set EXIT_RUNTIME to 1.

Why do functions in my C/C++ source code vanish when I compile to WebAssembly?

Emscripten does dead code elimination of functions that are not called from the compiled code. While this does minimize code size, it can remove functions that you plan to call yourself (outside of the compiled code).

To make sure a C function remains available to be called from normal JavaScript, it must be added to the EXPORTED_FUNCTIONS using the emcc command line. For example, to prevent functions my_func() and main() from being removed/renamed, run emcc with:

emcc -sEXPORTED_FUNCTIONS=_main,_my_func  ...

Note

_main should be in the export list, as in that example, if you have a main() function. Otherwise, it will be removed as dead code; there is no special logic to keep main() alive by default.

Note

EXPORTED_FUNCTIONS affects compilation to JavaScript. If you first compile to an object file, then compile the object to JavaScript, you need that option on the second command.

If your function is used in other functions, LLVM may inline it and it will not appear as a unique function in the JavaScript. Prevent inlining by defining the function with EMSCRIPTEN_KEEPALIVE:

void EMSCRIPTEN_KEEPALIVE yourCfunc() {..}

EMSCRIPTEN_KEEPALIVE also exports the function, as if it were on EXPORTED_FUNCTIONS.

Note

  • All functions not kept alive through EXPORTED_FUNCTIONS or EMSCRIPTEN_KEEPALIVE will potentially be removed. Make sure you keep the things you need alive using one or both of those methods.

  • Exported functions need to be C functions (to avoid C++ name mangling).

  • Decorating your code with EMSCRIPTEN_KEEPALIVE can be useful if you don’t want to have to keep track of functions to export explicitly, and when these exports do not change. It is not necessarily suitable for exporting functions from other libraries — for example it is not a good idea to decorate and recompile the source code of the C standard library. If you build the same source in multiple ways and change what is exported, then managing exports on the command line is easier.

  • Running emcc with -sLINKABLE will also disable link-time optimizations and dead code elimination. This is not recommended as it makes the code larger and less optimized.

Another possible cause of missing code is improper linking of .a files. The .a files link only the internal object files needed by previous files on the command line, so the order of files matters, and this can be surprising. If you are linking .a files, make sure they are at the end of the list of files, and in the right order amongst themselves. Alternatively, just use .so files instead in your project.

Tip

It can be useful to compile with EMCC_DEBUG=1 set for the environment (EMCC_DEBUG=1 emcc ... on Linux, set EMCC_DEBUG=1 on Windows). This splits up the compilation steps and saves them in /tmp/emscripten_temp. You can then see at what stage the code vanishes (you will need to do llvm-dis on the bitcode stages to read them, or llvm-nm, etc.).

Why is the File System API is not available when I build with closure?

The Closure Compiler will minify the File Server API code. Code that uses the file system must be optimized with the File System API, using emcc’s --pre-js option.

Why does my code break and gives odd errors when using -O2 --closure 1?

The Closure Compiler minifies variable names, which results in very short variable names like i, j, xa, etc. If other code declares variables with the same names in global scope, this can cause serious problems.

This is likely to be the cause if you can successfully run code compiled with -O2 set and --closure unset.

One solution is to stop using small variable names in the global scope (often this is a mistake — forgetting to use var when assigning to a variable).

Another alternative is to wrap the generated code (or your other code) in a closure, as shown:

var CompiledModule = (function() {
  .. GENERATED CODE ..
  return Module;
  })();

Why do I get TypeError: Module.someThing is not a function?

The Module object will contain exported methods. For something to appear there, you should add it to EXPORTED_FUNCTIONS for compiled code, or EXPORTED_RUNTIME_METHODS for a runtime method (like getValue). For example,

emcc -sEXPORTED_FUNCTIONS=_main,_my_func ...

would export a C method my_func (in addition to main, in this example). And

emcc -sEXPORTED_RUNTIME_METHODS=ccall ...

will export ccall. In both cases you can then access the exported function on the Module object.

Note

You can use runtime methods directly, without exporting them, if the compiler can see them used. For example, you can use getValue in EM_ASM code, or a --pre-js, by calling it directly. The optimizer will not remove that JS runtime method because it sees it is used. You only need to use Module.getValue if you want to call that method from outside the JS code the compiler can see, and then you need to export it.

Note

Emscripten used to export many runtime methods by default. This increased code size, and for that reason we’ve changed that default. If you depend on something that used to be exported, you should see a warning pointing you to the solution, in an unoptimized build, or a build with ASSERTIONS enabled, which we hope will minimize any annoyance. See ChangeLog.md for details.

Why does Runtime no longer exist? Why do I get an error trying to access Runtime.someThing?

1.37.27 includes a refactoring to remove the Runtime object. This makes the generated code more efficient and compact, but requires minor changes if you used Runtime.* APIs. You just need to remove the Runtime. prefix, as those functions are now simple functions in the top scope (an error message in -O0 or builds with assertions enabled with suggest this). In other words, replace

x = Runtime.stackAlloc(10);

with

x = stackAlloc(10);

Note

The above will work for code in a --pre-js or JS library, that is, code that is compiled together with the emscripten output. If you try to access Runtime.* methods from outside the compiled code, then you must export that function (using EXPORTED_RUNTIME_METHODS), and use it on the Module object, see that FAQ entry.

Why do I get a NameError or a problem occurred in evaluating content after a "-s" when I use a -s option?

This can occur if you have non-trivial strings in -s argument and are having trouble getting the shell quoting / escaping correct.

Using the simpler list form (without quotes, spaces or square brackets) can sometimes help:

emcc a.c -sEXPORTED_RUNTIME_METHODS=foo,bar

It is also possible to use a response file, that is,

emcc a.c -sEXPORTED_RUNTIME_METHODS=@extra.txt

with extra.txt being a plain text file that contains foo and bar on separate lines.

How do I specify -s options in a CMake project?

Simple things like this should just work in a CMakeLists.txt file:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -sUSE_SDL=2")

However, some -s options may require quoting, or the space between -s and the next argument may confuse CMake, when using things like target_link_options. To avoid those problems, you can use -sX=Y notation, that is, without spaces and without square brackets or quotes:

# same as before but no space after -s
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -sUSE_SDL=2")
# example of target_link_options with a list of names
target_link_options(example PRIVATE "-sEXPORTED_FUNCTIONS=_main")

Note also that _main does not need to be quoted, even though it’s a string name (emcc knows that the argument to EXPORTED_FUNCTIONS is a list of strings, so it accepts a or a,b etc.).

Why do I get a Python SyntaxError: invalid syntax on file=.. or on a string starting with f'..'?

Emscripten requires a recent-enough version of Python. An older Python version, like 2.*, will not support the print statement by default, so it will error on syntax like print('..', file=..). And an older 3.* Python may not support f-strings, which look like f'..'.

Make sure that you have a new enough version of Python installed, as specified in the SDK instructions, and that it is used by emcc (for example by running emcc.py using that Python).

In a CI environment you may need to specify the Python version to use, if the default is not new enough. For example, on Netlify you can use PYTHON_VERSION.

Why do I get a stack size error when optimizing: RangeError: Maximum call stack size exceeded or similar?

You may need to increase the stack size for node.js.

On Linux and Mac macOS, you can just do NODE_JS = ['/path/to/node', '--stack_size=8192'] in the Emscripten Compiler Configuration File (.emscripten). On Windows (for node versions older than v19), you will also need --max-stack-size=8192, and also run editbin /stack:33554432 node.exe.

How do I pass int64_t and uint64_t values from js into Wasm functions?

If you build using the -sWASM_BIGINT flag, then int64_t and uint64_t will be represented as bigint values in JS. Without the -sWASM_BIGINT flag, the values will be represented as number in JS which can’t represent int64s, so what happens is that in exported functions (that you can call from JS) we “legalize” the types, by turning an i64 argument into two i32s (low and high bits), and an i64 return value becomes an i32, and you can access the high bits by calling a helper function called getTempRet0.

Can I use multiple Emscripten-compiled programs on one Web page?

Emscripten output by default is just some code. When put in a script tag, that means the code is in the global scope. So multiple such modules on the same page can’t work.

But by putting each module in a function scope, that problem is avoided. Emscripten even has a compile flag for this, MODULARIZE, useful in conjunction with EXPORT_NAME (details in settings.js).

However, there are still some issues if the same Module object (that defines the canvas, text output area, etc.) is used among separate modules. By default Emscripten output even looks for Module in the global scope, but when using MODULARIZE, you get a function you must call with the Module as a param, so that problem is avoided. But note that each module will probably want its own canvas, text output area, etc.; just passing in the same Module object (e.g. from the default HTML shell) may not work.

So by using MODULARIZE and creating a proper Module object for each module, and passing those in, multiple modules can work fine.

Another option is to use an iframe, in which case the default HTML shell will just work, as each will have its own canvas, etc. But this is overkill for small programs, which can run modularly as described above.

Can I build JavaScript that only runs on the Web?

Yes, you can use the ENVIRONMENT option in settings.js. For example, building with emcc -sENVIRONMENT=web will emit code that only runs on the Web, and does not include support code for Node.js and other environments.

This can be useful to reduce code size, and also works around issues like the Node.js support code using require(), which Webpack will process and include unnecessary code for.

Why the weird name for the project?

I don’t know why; it’s a perfectly cromulent word!