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The wasm-ld linker. This the linker from the LLVM project. The external assembler llvm-mc is optional, since commit bc76487b878e59133ce3c2cc19ebb13f9a9826ea, in which, the internal assembler and object writer was enabled.

LLVM 11.0 is known to work. Development was done, using the Fedora packages llvm-11.0.0-1.fc33.x86_64 and lld-11.0.0-1.fc33.x86_64. Initial tests on MacOS used llvm-11 installed with MacPorts (https://www.macports.org) and according symlinks in /opt/local/bin (wasm32-wasi-llvm-mc -> llvm-mc-mp-11 and wasm32-embedded-llvm-mc -> llvm-mc-mp-11).

LLVM 12.0.1 also works.

Obtaining the compiler sources

The WebAssembly target was merged into FPC SVN trunk in r48955 on 2021-03-14. Since then, Free Pascal has moved to GitLab, so now it's now included in the git 'main' branch.

Get the sources:

git clone https://gitlab.com/freepascal.org/fpc/source.git fpc-wasm

enter the directory:

cd fpc-wasm

Supported targets

WASI - The WebAssembly System Interface

WASI is a modular system interface for WebAssembly. It allows creating portable and secure programs that can run in a sandboxed command-line environment. See the WASI website for more information.

To build the compiler for the WASI target:

make all OS_TARGET=wasi CPU_TARGET=wasm32 BINUTILSPREFIX= OPT="-O-" PP=fpc

The following units have been ported for the WASI target:

  • system
  • objpas
  • iso7185
  • ctypes
  • strings
  • wasiapi - interface for the WASI API

Additionally, these units compile, but are not yet fully functional and/or tested very well:

  • dos
  • sysutils
  • classes
  • math
  • fgl

See WebAssembly/Roadmap for details.

Compiled WASI binaries work with wasmtime version 0.23.0 or in the browser by opening:


and drag and dropping the .wasm binary directly into the browser window.

Embedded target

The Embedded target is primarily used for embedded systems, without an operating system. However, it also happens to be a perfect fit for creating WebAssembly modules that don't use any particular operating system-like API.

make all OS_TARGET=embedded CPU_TARGET=wasm32 BINUTILSPREFIX= OPT="-O-" PP=fpc

Using the compiler

Hint: If your fpc.cfg has


Enclose it in #IFNDEF CPUWASM32 :

#ifndef cpuwasm32

Otherwise, you have to create symlinks wasm32-wasi-llvm-mc to llvm-mc, and wasm32-wasi-wasm-ld to wasm-ld.

outdated: wasm-demo

Note: this section is outdated.

Wasm-demo is a Pascal WebAssembly demo project, ported Google's C WebAssembly example (https://codelabs.developers.google.com/codelabs/web-assembly-intro)

You follow these steps in order to get the demo project working:

1. Get the project sources: https://github.com/skalogryz/wasm-demo

Using in webassembly.studio

2. Run the wasm compiler against lyff.pas, but request the assembler file to be kept

 pp -a lyff.pas

Webassembly.studio accepts the assembler file and would compile it on the server side.

3. open up https://webassembly.studio and select "Create empty Wat Project". The project will consist of:


All files of the project can be edited through the web interface.

4. Copy and paste the contents of the following files:

(compiled) lyff.wat to main.wat.
wasm-demo\webassembly.studio\main.html to main.html
wasm-demo\webassembly.studio\main.js to main.js

Note: every time you copy and paste, don't forget to press "SAVE" button (on the right side of the web interface)

5. once all files are update, hit "Build and Run"

See Also