Configuring Emscripten Settings when Manually Building from Source

The compiler settings used by Emscripten are defined in the compiler configuration file (.emscripten). These settings include paths to the tools (LLVM, Clang, Java, etc.) and the compiler’s temporary directory for intermediate build files.

This article explains how to create and update the file when you are building Emscripten manually from source.

Creating the compiler configuration file

A settings file is required to run emcc (or any of the other Emscripten tools). When you first run emcc it will direct you to the --generate-config command line option which can be used to generate config file in the default location.

  1. Navigate to the directory where you cloned the Emscripten repository.

  2. Enter the command:

./emcc --generate-config

You should get a An Emscripten settings file has been generated at: message, along with the contents of the config file.

Emscripten makes a “best guess” at the correct locations for tools and updates the file appropriately. Where possible it will look for “system” apps (like Python and Java).

In most cases it is necessary to edit the generated file and modify at least the LLVM_ROOT and BINARYEN_ROOT settings to point to the correct location of your local LLVM and Binaryen installations respectively.

Locating the compiler configuration file (.emscripten)

The settings file (.emscripten) is created by default within the emscripten directory (alongsize emcc itself). In cases where the emscripten directory is read-only the users home directory will be used:

  • On Linux and macOS this file is named ~/.emscripten, where ~ is the user’s home directory.

    Note

    Files with the “.” prefix are hidden by default. You may need to change your view settings to find the file.

  • On Windows the file can be found at a path like: C:/Users/yourusername_000/.emscripten.

Compiler configuration file-format

Note

While the syntax is identical, the appearance of the default .emscripten file created by emcc is quite different than that created by emsdk. This is because emsdk manages multiple target environments, and where possible hard codes the locations of those tools when a new environment is activated. The default file, by contrast, is managed by the user — and is designed to make that task as easy as possible.

The file simply assigns paths to a number of variables representing the main tools used by Emscripten. For example, if the user installed python to the C:\Python38\ directory, then the file might have the line:

PYTHON = 'C:\\Python38\\python.exe'

The default emcc configuration file often gets the paths from environment variables if defined. If no variable is defined the system will also attempt to find “system executables”. For example:

PYTHON = os.path.expanduser(os.getenv('PYTHON', 'C:\\Python38\\python.exe'))

You can find out the other variable names from the default .emscripten file or the example here.

Editing the compiler configuration file

The compiler configuration file can be edited with the text editor of your choice. As stated above, most default settings are likely to be correct. If you’re building manually from source, you are most likely to have to update the variable LLVM_ROOT

  1. Edit the variable LLVM_ROOT to point to the directory where you built the LLVM binaries, such as:

    LLVM_ROOT = os.path.expanduser(os.getenv('LLVM', '/home/ubuntu/a-path/llvm/build/bin'))
    

    Note

    Use forward slashes!

After setting those paths, run emcc again. It should again perform the sanity checks to test the specified paths. There are further validation tests available at Verifying the Emscripten Development Environment.