(building_from_sources)=
# Building from sources

Building is easiest on Linux and relatively straightforward on Mac. For
Windows, we currently recommend using the Docker image (described below) to
build Pyodide. Another option for building on Windows is to use [WSL2](https://docs.microsoft.com/en-us/windows/wsl/install-win10) to create a Linux build environment.

## Build using `make`

Make sure the prerequisites for [emsdk](https://github.com/emscripten-core/emsdk) are
installed. Pyodide will build a custom, patched version of emsdk, so there is no
need to build it yourself prior.

Additional build prerequisites are:

- A working native compiler toolchain, enough to build [CPython](https://devguide.python.org/setup/#linux).
- A native Python 3.8 to run the build scripts.
- CMake
- PyYAML
- FreeType 2 development libraries to compile Matplotlib.
- Cython to compile SciPy
- gfortran (GNU Fortran 95 compiler)
- [f2c](http://www.netlib.org/f2c/)
- [ccache](https://ccache.samba.org) (optional) *highly* recommended for much faster rebuilds.

On Mac, you will also need:

- [Homebrew](https://brew.sh/) for installing dependencies
- System libraries in the root directory (`sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /` should do it, see https://github.com/pyenv/pyenv/issues/1219#issuecomment-428305417)
- coreutils for md5sum and other essential Unix utilities (`brew install coreutils`)
- cmake (`brew install cmake`)
- Cython to compile SciPy (`brew install cython`)
- pkg-config (`brew install pkg-config`)
- openssl (`brew install openssl`)
- gfortran (`brew cask install gfortran`)
- f2c: Install wget (`brew install wget`), and then run the buildf2c script from the root directory (`sudo ./tools/buildf2c`)


After installing the build prerequisites, run from the command line:

```bash
make
```

## Using Docker

We provide a Debian-based Docker image on Docker Hub with the dependencies
already installed to make it easier to build Pyodide. On top of that we provide a
pre-built image which can be used for fast custom and partial builds of pyodide.
Note that building from the non pre-built the Docker image is *very* slow on Mac,
building on the host machine is preferred if at all possible.

1. Install Docker

2. From a git checkout of Pyodide, run `./run_docker` or `./run_docker --pre-built`

3. Run `make` to build.

Note: You can control the resources allocated to the build by setting the env vars
`EMSDK_NUM_CORE`, `EMCC_CORES` and `PYODIDE_JOBS` (the default for each is 4).


If running ``make`` deterministically stops at one point in each subsequent try, increasing
the maximum RAM usage available to the docker container might help [This is different
from the physical RAM capacity inside the system]. Ideally, at least 3 GB of RAM
should be available to the docker container to build `pyodide` smoothly. These settings can
be changed via Docker Preferences (See [here](https://stackoverflow.com/questions/44533319/how-to-assign-more-memory-to-docker-container)).

You can edit the files in your source checkout on your host machine, and then
repeatedly run `make` inside the Docker environment to test your changes.

(partial-builds)=
## Partial builds

To build a subset of available packages in pyodide, set the environment
variable `PYODIDE_PACKAGES` to a comma separated list of packages. For
instance,

```
PYODIDE_PACKAGES="toolz,attrs" make
```

Dependencies of the listed packages will be built automatically as well.
The package names must match the folder names in `packages/` exactly; in
particular they are case sensitive.

To build a minimal version of pyodide, set `PYODIDE_PACKAGES="micropip"`. The
packages micropip and distutils are always automatically included (but an empty
`PYODIDE_PACKAGES` is interpreted as unset).

## Environment variables

Following environment variables additionally impact the build,
 - `PYODIDE_JOBS`: the `-j` option passed to the `emmake make` command when applicable for parallel compilation. Default: 3.
 - `PYODIDE_BASE_URL`: Base URL where pyodide packages are deployed. It must
   end with a trailing `/`. Default: `./` to load pyodide packages from the
   same base URL path as where `pyodide.js` is located.  Example:
   `https://cdn.jsdelivr.net/pyodide/dev/full/`