This example demonstrates how to use package:wasm to run a wasm build of the Brotli compression library.
dart brotli.dart lipsum.txt
This will compress lipsum.txt, report the compression ratio, then decompress it and verify that the result matches the input.
libbrotli.wasm was built by cloning the Brotli repo, and compiling it using wasienv.
There are several ways of building wasm code. The most important difference between the tool sets is how the wasm code they generate interacts with the OS. For very simple code this difference doesn't matter. But if the library does any sort of OS interaction, such as file IO, or even using malloc, it will need to use either Emscripten or WASI for that interaction. package:wasm only supports WASI at the moment.
To target WASI, one option is to use wasi-libc and a recent version of clang. Set the target to
--target=wasm32-unknown-wasi and the
--sysroot to wasi-libc.
Another option is to build using wasienv, which is a set of tools that are essentially just an ergonomic wrapper around the clang + wasi-libc approach. This is how libbrotli.wasm was built:
wasicc -c foo.c -o out/foo.o -I c/include
wasild --no-entry --export=bar out/foo.o $wasienv_sysroot/lib/wasm32-wasi/libc.aThe
--no-entryflag tells the linker to ignore the fact that there's no
main()function, which is important for libraries.
--export=barwill export the
bar()function from the library, so that it can be found by package:wasm. For libbrotli.wasm, every function in c/include/brotli/encode.h and decode.h was exported. Brotli used functions from libc, so the wasm version of libc that comes with wasienv was also linked in. If there are still undefined symbols after linking in the wasi libraries, the
--allow-undefinedflag tells the linker to treat undefined symbols as function imports. These functions can then be supplied from Dart code.