How to pass command line arguments to C code with WebAssembly and JS?

Question

I'm trying to run some small C demos on the web with WebAssembly and pure JS, and I'm compiling my code using WASI-SDK/WASI-libc:

clang --target=wasm32-unknown-wasi --sysroot=<sysroot> -nostartfiles -O3 -flto -Wl,--no-entry -Wl,--export=malloc -Wl,--export-all -Wl,--lto-O3 src.c -o src.wasm

I'm then using this small JS library to implement the WASI functions. This works fine for printing with stdout, and I've even tested passing strings and other types into different functions. But I can't figure out how to pass an array of strings into main as an argument.

I don't want to use Node or Emscripten, which is why I went with a minimal JS implementation.

Edit:

To add command line arguments, I removed both -nostartfiles and -Wl,--no-entry from my compiler call and implemented args_get and args_sizes_get from the WASI standard. From there, it was as simple as calling _start from the wasm's exported functions.

sbc100 · Accepted Answer · 2021-05-12 07:11:28Z

2

If you want to use WASI then the way to pass args to main is to implement the wasi_snapshot_preview1.args_get and wasi_snapshot_preview1.args_sizes_get syscalls which are used by WASI programs to access the argv values. In that case you would want to call _start rather than main.

If you want to bypass that and call main directly you would need to somehow allocate and array of char * pointers in the linear memory which you could then pass as your argv value. The problem that you face if you try to take this approach is that allocating memory (e.g. via malloc) before calling main is hard. My advise would be to go with the first method which is to call _start and implement that wasi syscall needed to access argv. (You can call also then remove the -Wl,--no-entry from your link command).

answered May 12, 2021 at 7:11

sbc100

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2 Comments

Tommy Over a year ago

I assumed it would be start but I wasn't sure. How would I specify the entry point? Removing -Wl,--no-entry gives wasm-ld: error: entry symbol not defined (pass --no-entry to suppress): _start

Tommy Over a year ago

Nevermind, I solved it by removing both -nostartfiles and -Wl,--no-entry

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