Building Projects

Emscripten provides two scripts that configure your makefiles to use emcc as a drop-in replacement for gcc — in most cases the rest of your project’s current build system remains unchanged.

Integrating with a build system

To build using Emscripten you need to replace gcc with emcc in your makefiles. This is done using emconfigure, which sets the appropriate environment variables like CXX (C++ compiler) and CC (the compiler).

Consider the case where you normally build with the following commands:



If you’re not familiar with these build commands, the article The magic behind configure, make, make install is a good primer.

To build with Emscripten, you would instead use the following commands:

# Run emconfigure with the normal configure command as an argument.
emconfigure ./configure

# Run emmake with the normal make to generate Wasm object files.
emmake make

# Compile the linked code generated by make to JavaScript + WebAssembly.
# 'project.o' should be replaced with the make output for your project, and
# you may need to rename it if it isn't something emcc recognizes
# (for example, it might have a different suffix like '' or
# '', or no suffix like just 'project' for an executable).
# If the project output is a library, you may need to add your 'main.c' file
# here as well.
# [-Ox] represents build optimisations (discussed in the next section).
emcc [-Ox] project.o -o project.js

emconfigure is called with the normal configure as an argument (in configure-based build systems), and emmake with make as an argument. If your build system uses CMake, replace ./configure with cmake . etc. in the above example. If your build system doesn’t use configure or CMake, then you can omit the first step and just run make (although then you may need to edit the Makefile manually).


We recommend you call both emconfigure and emmake scripts in configure- and CMake-based build systems. Whether you actually need to call both tools depends on the build system (some systems will store the environment variables in the configure step, and others will not).

Make generates Wasm object files. It may also link the object files into libraries and/or Wasm executables. Unless such a build system has been modified to also emit JavaScript output, you will need to run an additional emcc command as shown above, that will emit the final runnable JavaScript and WebAssembly.


The file output from make might have a different suffix: .a for a static library archive, .so for a shared library, .o for object files (these file extensions are the same as gcc would use for the different types). Irrespective of the file extension, these files contain something that emcc can compile into the final JavaScript + WebAssembly (typically the contents will be Wasm object files, but if you build with LTO then they will contain LLVM bitcode).


Some build systems may not properly emit Wasm object files using the above procedure, and you may see is not a valid input file warnings. You can run file to check what a file contains (also you can manually check if the contents start with \0asm to see if they are Wasm object files, or BC if they are LLVM bitcode). It is also worth running emmake make VERBOSE=1 which will print out the commands it runs - you should see emcc being used, and not the native system compiler. If emcc is not used, you may need to modify the configure or cmake scripts.

Emscripten linker output files

Unless run with certain specific flags (such as -c, -S, -r, or -shared) emcc will run the link phase which can produce more than just one file. The set of produced files changes depending on the final flags passed to emcc and the name of the specified output file. Here is a cheat sheet of which files are produced under which conditions:

  • emcc ... -o output.html builds a output.html file as an output, as well as an accompanying output.js launcher file, and a output.wasm WebAssembly file.

  • emcc ... -o output.js omits generating a HTML launcher file (expecting you to provide it yourself if you plan to run in browser), and produces two files, output.js and output.wasm. (that can be run in e.g. node.js shell)

  • emcc ... -o output.wasm omits generating either JavaScript or HTML launcher file, and produces a single Wasm file built in standalone mode as if the -sSTANDALONE_WASM setting had been used. The resulting file expects to be run with the WASI ABI - in particular, as soon as you initialize the module you must manually invoke either the _start export or (in the case of --no-entry) the _initialize export before doing anything else with it.

  • emcc ... -o output.{html,js} -sWASM=0 causes the compiler to target JavaScript, and therefore a .wasm file is not produced.

  • emcc ... -o output.{html,js} --emit-symbol-map produces a file output.{html,js}.symbols if WebAssembly is being targeted (-sWASM=0 not specified), or if JavaScript is being targeted and -Os, -Oz or -O2 or higher is specified, but debug level setting is -g1 or lower (i.e. if symbols minification did occur).

  • emcc ... -o output.{html,js} -gsource-map generates a source map file If targeting JavaScript with -sWASM=0, the filename is output.{html,js}.map.

  • emcc ... -o output.{html,js} --preload-file xxx directive generates a preloaded MEMFS filesystem file

  • emcc ... -o output.{html,js} -sWASM={0,1} -sSINGLE_FILE merges JavaScript and WebAssembly code in the single output file output.{html,js} (in base64) to produce only one file for deployment. (If paired with --preload-file, the preloaded .data file still exists as a separate file)

This list is not exhaustive, but illustrates most commonly used combinations.


Regardless of the name of the output file emcc will always perform linking and produce a final executable, unless a specific flags (e.g. -c) direct it do something else. This differs to previous behaviour where emcc would default to combining object files (essentially assuming -r) unless given a specific executable extension (e.g. .js or .html).

Building projects with optimizations

Emscripten performs compiler optimization at two levels: each source file is optimized by LLVM as it is compiled into an object file, and then JavaScript/WebAssembly-specific optimizations are applied when converting object files into the final JavaScript/WebAssembly.

In order to properly optimize code, it is usually best to use the same optimization flags and other compiler options when compiling source to object code, and object code to JavaScript (or HTML).

Consider the examples below:

# Sub-optimal - JavaScript/WebAssembly optimizations are omitted
emcc -O2 a.cpp -c -o a.o
emcc -O2 b.cpp -c -o b.o
emcc a.o b.o -o project.js

# Sub-optimal - LLVM optimizations omitted
emcc a.cpp -c -o a.o
emcc b.cpp -c -o b.o
emcc -O2 a.o b.o -o project.js

# Usually the right thing: The same options are provided at compile and link.
emcc -O2 a.cpp -c -o a.o
emcc -O2 b.cpp -c -o b.o
emcc -O2 a.o b.o -o project.js

However, sometimes you may want slightly different optimizations on certain files:

# Optimize the first file for size, and the rest using `-O2`.
emcc -Oz a.cpp -c -o a.o
emcc -O2 b.cpp -c -o b.o
emcc -O2 a.o b.o -o project.js


Unfortunately each build-system defines its own mechanisms for setting compiler and optimization methods. You will need to work out the correct approach to set the LLVM optimization flags for your system.

  • Some build systems have a flag like ./configure --enable-optimize.

JavaScript/WebAssembly optimizations are specified in the final step (sometimes called “link”, as that step typically also links together a bunch of files that are all compiled together into one JavaScript/WebAssembly output). For example, to compile with -O1:

# Compile the object file to JavaScript with -O1 optimizations.
emcc -O1 project.o -o project.js

Building projects with debug information

Building a project containing debug information requires that debug flags are specified for both the LLVM and JavaScript compilation phases.

To make Clang and LLVM emit debug information in object files you need to compile the sources with -g (exactly the same as with clang or gcc normally).


Each build-system defines its own mechanisms for setting debug flags. To get Clang to emit LLVM debug information, you will need to work out the correct approach for your system.

  • Some build systems have a flag like ./configure --enable-debug. In CMake-based build systems, set the CMAKE_BUILD_TYPE to "Debug".

To get emcc to include the debug information present in object files when generating the final JavaScript and WebAssembly, your final emcc command must specify -g or one of the -gN debug level options.

# Compile the Wasm object file to JavaScript+WebAssembly, with debug info
# -g or -gN can be used to set the debug level (N)
emcc -g project.o -o project.js

For more general information, see the topic Debugging.

Using libraries

Built-in support is available for a number of standard libraries: libc, libc++ and SDL. These will automatically be linked when you compile code that uses them (you do not even need to add -lSDL, but see below for more SDL-specific details).

If your project uses other libraries, for example zlib or glib, you will need to build and link them. The normal approach is to build the libraries (to object files, or .a archives of them) and then link those with your main program to emit JavaScript+WebAssembly.

For example, consider the case where a project “project” uses a library “libstuff”:

# Compile libstuff to libstuff.a
emconfigure ./configure
emmake make

# Compile project to project.o
emconfigure ./configure
emmake make

# Link the library and code together.
emcc project.o libstuff.a -o final.html

Emscripten Ports

Emscripten Ports is a collection of useful libraries, ported to Emscripten. They reside on GitHub, and have integration support in emcc. When you request that a port be used, emcc will fetch it from the remote server, set it up and build it locally, then link it with your project, add necessary include to your build commands, etc. For example, SDL2 is in ports, and you can request that it be used with --use-port=sdl2. For example,

emcc test/browser/test_sdl2_glshader.c --use-port=sdl2 -sLEGACY_GL_EMULATION -o sdl2.html

You should see some notifications about SDL2 being used, and built if it wasn’t previously. You can then view sdl2.html in your browser.

To see a list of all available ports, run emcc --show-ports.


SDL_image has also been added to ports, use it with --use-port=sdl2_image. For sdl2_image to be useful, you generally need to specify the image formats you are planning on using with e.g. --use-port=sdl2_image:formats=bmp,png,xpm,jpg. This will also ensure that IMG_Init works properly when you specify those formats. Alternatively, you can use emcc --use-preload-plugins and --preload-file your images, so the browser codecs decode them (see Preloading files). A code path in the sdl2_image port will load through emscripten_get_preloaded_image_data(), but then your calls to IMG_Init with those image formats will fail (as while the images will work through preloading, IMG_Init reports no support for those formats, as it doesn’t have support compiled in - in other words, IMG_Init does not report support for formats that only work through preloading).


SDL_net has also been added to ports, use it with --use-port=sdl2_net.


Emscripten also has support for older SDL1, which is built-in. If you do not specify SDL2 as in the command above, then SDL1 is linked in and the SDL1 include paths are used. SDL1 has support for sdl-config, which is present in system/bin. Using the native sdl-config may result in compilation or missing-symbol errors. You will need to modify the build system to look for files in emscripten/system or emscripten/system/bin in order to use the Emscripten sdl-config.


You can also build a library from ports in a manual way if you prefer that, but then you will need to also apply the python logic that ports does. That code (under tools/ports/) may do things like ensure necessary JS functions are included in the build, add exports, and so forth. In general, it’s better to use the ports version as it is what is tested and known to work.


Since emscripten 3.1.54, --use-port is the preferred syntax to use a port in your project. The legacy syntax (for example -sUSE_SDL2, -sUSE_SDL_IMAGE=2) remains available.

Contrib ports

Contrib ports are contributed by the wider community and supported on a “best effort” basis. Since they are not run as part of emscripten CI they are not always guaranteed to build or function. See Contrib Ports for more information.

Adding more ports

The simplest way to add a new port is to put it under the contrib directory. Basically, the steps are:

  • Make sure the port is open source and has a suitable license.

  • Read the file under tools/ports/contrib which contains more information.

External ports

Emscripten also supports external ports (ports that are not part of the distribution). In order to use such a port, you simply provide its path: --use-port=/path/to/


Be aware that if you are working on the code of a port, the port API used by emscripten is not 100% stable and could change between versions.

Build system issues

Build system self-execution

Some large projects generate executables and run them in order to generate input for later parts of the build process (for example, a parser may be built and then run on a grammar, which then generates C/C++ code that implements that grammar). This sort of build process causes problems when using Emscripten because you cannot directly run the code you are generating.

The simplest solution is usually to build the project twice: once natively, and once to JavaScript. When the JavaScript build procedure fails because a generated executable is not present, you can then copy that executable from the native build, and continue to build normally. For example, this approach has been successfully used for compiling Python (which needs to run its pgen executable during the build).

In some cases it makes sense to modify the build scripts so that they build the generated executable natively. For example, this can be done by specifying two compilers in the build scripts, emcc and gcc, and using gcc just for generated executables. However, this can be more complicated than the previous solution because you need to modify the project build scripts, and you may have to work around cases where code is compiled and used both for the final result and for a generated executable.

Faux Dynamic Linking

Emscripten’s goal is to generate the fastest and smallest possible code. For that reason it focuses on compiling an entire project into a single Wasm file, avoiding dynamic linking when possible.

By default, when the -shared flag is used to build a shared library, Emscripten will produce an .so library that is actually just a regular .o object file (Under the hood it uses ld -r to combine objects into a single larger object). When these faux “shared libraries” are linked into your application they are effectively linked as static libraries. When building these shared libraries Emcc will ignore other shared libraries on the command line. This is to ensure that the same dynamic library is not linked multiple times in intermediate build stages, which would result in duplicate symbol errors.

See experimental support for how to build true dynamic libraries, which can be linked together either at load time, or at runtime (via dlopen).

Configure may run checks that appear to fail

Projects that use configure, cmake, or some other portable configuration method may run checks during the configure phase to verify that the toolchain and paths are set up properly. Emcc tries to get checks to pass where possible, but you may need to disable tests that fail due to a “false negative” (for example, tests that would pass in the final execution environment, but not in the shell during configure).


Ensure that if a check is disabled, the tested functionality does work. This might involve manually adding commands to the make files using a build system-specific method.


In general configure is not a good match for a cross-compiler like Emscripten. configure is designed to build natively for the local setup, and works hard to find the native build system and the local system headers. With a cross-compiler, you are targeting a different system, and ignoring these headers etc.

Archive (.a) files

Emscripten supports .a archive files, which are bundles of object files. This is a simple format for libraries, that has special semantics - for example, the order of linking matters with .a files, but not with plain object files. For the most part those special semantics should work the same in Emscripten as elsewhere.

Manually using emcc

The Emscripten Tutorial showed how emcc can be used to compile single files into JavaScript. Emcc can also be used in all the other ways you would expect of gcc:

# Generate a.out.js from C++. Can also take .ll (LLVM assembly) or .bc (LLVM bitcode) as input
emcc src.cpp

# Generate an object file called src.o.
emcc src.cpp -c

# Generate result.js containing JavaScript.
emcc src.cpp -o result.js

# Generate an object file called result.o
emcc src.cpp -c -o result.o

# Generate a.out.js from two C++ sources.
emcc src1.cpp src2.cpp

# Generate object files src1.o and src2.o
emcc src1.cpp src2.cpp -c

# Combine two object files into a.out.js
emcc src1.o src2.o

# Combine two object files into another object file (not normally needed)
emcc src1.o src2.o -r -o combined.o

# Combine two object files into library file
emar rcs libfoo.a src1.o src2.o

In addition to the capabilities it shares with gcc, emcc supports options to optimize code, control what debug information is emitted, generate HTML and other output formats, etc. These options are documented in the emcc tool reference (emcc --help on the command line).

Detecting Emscripten in Preprocessor

Emscripten provides the following preprocessor macros that can be used to identify the compiler version and platform:

  • The preprocessor define __EMSCRIPTEN__ is always defined when compiling programs with Emscripten.

  • The preprocessor variables __EMSCRIPTEN_major__, __EMSCRIPTEN_minor__ and __EMSCRIPTEN_tiny__ are defined in emscripten/version.h and specify, as integers, the currently used Emscripten compiler version.

  • Emscripten behaves like a variant of Unix, so the preprocessor defines unix, __unix and __unix__ are always present when compiling code with Emscripten.

  • Emscripten uses Clang/LLVM as its underlying codegen compiler, so the preprocessor defines __llvm__ and __clang__ are defined, and the preprocessor defines __clang_major__, __clang_minor__ and __clang_patchlevel__ indicate the version of Clang that is used.

  • Clang/LLVM is GCC-compatible, so the preprocessor defines __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__ are also defined to represent the level of GCC compatibility that Clang/LLVM provides.

  • The preprocessor string __VERSION__ indicates the GCC compatible version, which is expanded to also show Emscripten version information.

  • Likewise, __clang_version__ is present and indicates both Emscripten and LLVM version information.

  • Emscripten is a 32-bit platform, so size_t is a 32-bit unsigned integer, __POINTER_WIDTH__=32, __SIZEOF_LONG__=4 and __LONG_MAX__ equals 2147483647L.

  • When targeting SSEx SIMD APIs using one of the command line compiler flags -msse, -msse2, -msse3, -mssse3, or -msse4.1, one or more of the preprocessor flags __SSE__, __SSE2__, __SSE3__, __SSSE3__, __SSE4_1__ will be present to indicate available support for these instruction sets.

  • If targeting the pthreads multithreading support with the compiler & linker flag -pthread, the preprocessor define __EMSCRIPTEN_PTHREADS__ will be present.

Using a compiler wrapper

Sometimes it can be useful to use a compiler wrapper in order to do things like ccache, distcc or gomacc. For ccache the normal method of simply wrapping the entire compiler should work, e.g. ccache emcc. For distributed builds it can be beneficial to run the emscripten driver locally and distribute only the underlying clang commands. If this is desirable, the COMPILER_WRAPPER setting in the config file can be used to add a wrapper around the internal calls to clang. Like other config settings this can also be set via an environment variable. e.g:

EM_COMPILER_WRAPPER=gomacc emcc -c hello.c


emconfigure and emmake configure pkg-config for cross compiling and set the environment variable PKG_CONFIG_LIBDIR and PKG_CONFIG_PATH. To provide custom pkg-config paths, set the environment variable EM_PKG_CONFIG_PATH.

Examples / test code

The Emscripten test suite (test/ contains a number of good examples — large C/C++ projects that are built using their normal build systems as described above: freetype, openjpeg, zlib, bullet and poppler.

It is also worth looking at the build scripts in the ammo.js project.


  • Make sure to use emar (which calls llvm-ar), as the system ar may not support our object files. emmake and emconfigure set the AR environment variable correctly, but a build system might incorrectly hardcode ar.

  • Similarly, using the system ranlib instead of emranlib (which calls llvm-ranlib) may lead to problems, like not supporting our object files and removing the index, leading to archive has no index; run ranlib to add one from wasm-ld. Again, using emmake/emconfigure should avoid this by setting the env var RANLIB, but a build system might have it hardcoded, or require you to pass an option.

  • The compilation error multiply defined symbol indicates that the project has linked a particular static library multiple times. The project will need to be changed so that the problem library is linked only once.


    You can use llvm-nm to see which symbols are defined in each object file.

    One solution is to use dynamic-linking. This ensures that libraries are linked only once, in the final build stage.

  • When generating standalone Wasm, make sure to invoke the _start or (for --no-entry) _initialize export before attempting to use the module.