# 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:

    ./configure
    make

Tip

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 'project.so' or
    # 'project.so.1', 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).

Tip

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.

Note

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).

Note

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 `output.wasm.map`. 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 `output.data`.

- `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.

Note

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 to 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

Note

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).

Note

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 in the ports directory, and have integration with emcc. When you request that a port be used, emcc will download, build and install it into the emscripten sysroot. For example, to use the SDL2 port in your project you would simply add `--use-port=sdl2` to your compiler and linker flags. For example:

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

If this if your first time using a given port you may see some notifications about it being downloaded and installed as your program is being compiled.

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

You can also use the standalone `embuilder` tools to explicitly build ports prior to running the compiler. For example, you can build SDL2 using `./embuilder build sdl2`. See `embuilder --help` for more information, including a list of all available targets.

Some ports take extra options. For example, when using the `sdl2_image` ports you can specify a list of image formats. e.g. `--use-port=sdl2_image:formats=bmp,png,xpm,jpg`. See Port-specific Notes below for more information on this.

Emscripten also has support for the older SDL 1.3, which is built-in. Use can use this via `-sUSE_SDL=1`. SDL 1.3 has support for sdl-config. Using the host version sdl-config may result in compilation errors. You may need to modify the build system to look for sdl-config* in the emscripten sysroot (`<sysroot>/bin/sdl-config`).

Note

When a port is built and installed into the sysroot it will then be available to all following `emcc` commands. For example, once you have run `emcc` with `--use-port=sdl2` or run `./embuilder build sdl2`, future `emcc` commands will be able to see the SDL2 headers and libraries.

Note

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.

### Port-specific Notes

The `sdl2_image` port generally requires the specification of set of supported imagine formats. For example `--use-port=sdl2_image:formats=bmp,png,xpm,jpg`. This ensures 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).

### 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 `README.md` 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/my_port.py`

Note

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.

### 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, and the recommendation is to avoiding dynamic linking in most cases.

See experimental support for more information if your project requires dynamic linking.

### 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).

Tip

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.

Note

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

## pkg-config

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/runner.py) 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.

## Troubleshooting

- 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.

  Note

  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.