Question: The correct way to compile without main function

[timonson]

Hi guys, I would like to create a WASM file from the following C file:

int total_count = 0;

// Modules can have state
int count() {
  return ++total_count;
}

// They can do work
int add(int a, int b) {
  return a + b;
}

// But they only work with numbers
char * string() {
  return "Hello, world!";
}

Besides other things, I tried to run $WASI_SDK_DIR/bin/clang --target=wasm32-unknown-wasi -Wl,--no-entry -nostartfiles --sysroot $WASI_SDK_DIR/share/wasi-sysroot -o module.wasm module.c. This command creates a module.wasm file without compiler error but the file is almost empty unfortunately.

My goal is to create an equal file with wasi-sdk to what the following command does: clang --target=wasm32 --no-standard-libraries -Wl,--export-all -Wl,--no-entry -o module.wasm module.c

I don't need a main function because I want to import these functions as ES Modules later with the help from deno.

What am I doing wrong?
Thank you!

[sbc100]

Currently WASI only supports programs with main functions. See WebAssembly/WASI#13 for the plan to extend this.

Having said that there are various ways to achieve what you want in a non-standards-conforming way today.

I would still recommend keeping an empty main function around if only to ensure that all the correct startup functions are called. For example the _start function that calls main is also in charge of calling things like C++ constructor (you can do this youself by calling __wasm_call_ctors if you prefer).

In terms of exporting functions probably the best way is to use the __attribute__((export_name)) clang attribute to tag the functions you want to export and the name you want to export them under.
I would also have though the --export command line flag should work too. Is it not working for you?

[timonson]

Hi @sbc100 thank you a lot. I could make it work with the --export-all flag thanks to your advice.

[mmathys]

For anyone looking at this in the future, I made it work like this:

__attribute__((export_name("entry")))
int entry(int argc, char **argv) {
...
}

compile using

clang -Wl,--no-entry -mexec-model=reactor -o example.wasm example.c

using clang from the WASI-SDK.

[sbc100]

You shouldn't need to define a dummy main like that. I think the link flag you want is -mexec-model=reactor

[mmathys]

Thanks! Updated the example.