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Building SWI-Prolog using Emscripten for WebAssembly (WASM)

As of version 8.1.2, building an executable for WASM is fully supported by the standard build process. The initial port is the work of Raivo Laanemets. Currently Jesse Wright is most active in maintaining the port. Jesse maintains the npm build process on github.

(Infrequent) status updates and various hints on using the WASM version are on this Discourse wiki page

The remainder of this page guides you through building the WASM version yourself.


You need to download the Emscripten compiler. Follow the instruction on its homepage.

After the successful installation load the Emscripten environment into the current terminal session (adjust path):

source ./emsdk_env.sh

We currently (May 3, 2023) use version 3.1.37 of Emscripten. The build process also requires Node.js. Recent versions of Emscripten require recent version of Node.js. We use v18.16. We build using the ninja build tool. This is not necessary, but speeds up the build considerably.

The build process assumes


We need several dependencies. zlib is required, the others are optional. We install the dependencies with prefix ~/wasm We assume running the commands from ~/wasm. We also assume you setup the environment from Emscripten as described above, so emcc is in our $PATH.

Installing zlib

This is mandatory

wget https://zlib.net/zlib-1.2.13.tar.gz
tar -xf "zlib-1.2.13.tar.gz"
cd zlib-1.2.13
emconfigure ./configure --static --prefix=$HOME/wasm
emmake make
emmake make install

Installing pcre

The pcre library adds Perl Compatible Regular Expressions to the WASM version. This is optional.

git clone https://github.com/PCRE2Project/pcre2
cd pcre2
git checkout pcre2-10.42
ninja install

Installing GMPlib

The GMP library brings unbounded integers, rational numbers and good random numbers to SWI-Prolog. The price is its size and for some users the LGPL license. As all code is linked statically this effectively makes the resulting system LGPL. Without GMP, the build falls back to the bundled LibBF by Fabrice Bellard. This provides all the same functionality thanks to a compatibility wrapper developed by Jan Wielemaker and Rick Workman. Notably performance on rational number arithmetic is considerably worse though. But, the LibBF verion is smaller and causes all code to be covered by permissive licenses (MIT, BSD, Apache). This is optional

wget https://gmplib.org/download/gmp/gmp-6.1.2.tar.lz
tar xf gmp-6.1.2.tar.lz
cd gmp-6.1.2
emconfigure ./configure --disable-assembly --prefix=${HOME}/wasm
make -j
make install


The Emscripten/WASM build is fully supported from the default SWI-Prolog sources. You find the WASM specific code in the directory src/wasm of the source tree. This contains the JavaScript wrapper, several examples and a Prolog web server for the examples.


Go to where you downloaded the swipl-devel source tree. Create a build directory:

mkdir build.wasm
cd build.wasm

Now, create a script configure with this content (adjust as required):

source $WASM_HOME/emsdk/emsdk_env.sh
[ -f $TOOLCHAIN ] || echo "Could not find emscripten toolchain"

      -DCMAKE_BUILD_TYPE=Release \
      -G Ninja ..

Now configure and build the system using these commands

bash configure

Optionally you may test the system by running the command below. The `-j 8` are the number of tests run in paralel. Choosing the number of cores of your system is a good start.

ctest -j 8

Run the system

You can run the commandline SWI-Prolog with full access to the filesystem using the command below (from the build.wasm directory).

node src/swipl.js

The build products for deployment are in src. You can run the demo with a native SWI-Prolog using

swipl ../src/wasm/server.pl [--port=8080]

Now open the URL that is printed in your browser. You should get the same as https://dev.swi-prolog.org/wasm/

See also
- https://github.com/SWI-Prolog/npm-swipl-wasm
- https://swi-prolog.discourse.group/t/swi-prolog-in-the-browser-using-wasm/5650/1