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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.

Ubuntu 20.04

The wasm-ld is part of package lld-12. fpc searches for the tool wasm32-wasi-wasm-ld or wasm-ld depending on the -XP flag you use (see /etc/fpc.cfg):

apt install lld-12
ln -sf /usr/lib/llvm-12/bin/wasm-ld ~/bin/wasm32-wasi-wasm-ld

Note: This assumes you have a $HOME/bin directory and it is in PATH.

If you need the external assembler, install the package llvm-12 and set a symlink:

apt install llvm-12
ln -sf /usr/lib/llvm-12/bin/llvm-mc ~/bin/wasm32-wasi-llvm-mc

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

enter the directory:

cd fpc

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 or in the browser. 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 (not a browser), 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

By default /etc/fpc.cfg contains a -XP$FPCTARGET- flag, so fpc searches for wasm32-wasi-wasm-ld. If you want to search for wasm-ld instead adapt your fpc.cfg :

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.

See Also