llvm dev - Sep 2020 - WebAssembly - Import functions from runtime

Hi,

Is there a method for importing functions from the Web Assembly runtime
when writing C? I know Emscripten has support for calling JS functions from
C code, but I was wondering if there's an easy way to do this without
Emscripten (pure Clang)?

Best,
Ved
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Hi Ved,

Yes, the trick is to declare the functions you want to import in your C
source, then pass the `--allow-undefined` flag to wasm-ld when linking your
wasm module. Here's a complete example:

hello.c:

```
int print_int(int x);

int do_work(int x) {
    print_int(x);
    return x;
}
```

clang -target wasm32 -c hello.c -o hello.o
wasm-ld hello.o -o hello.wasm --no-entry --allow-undefined --export=do_work

This will give you a wasm module that imports function "env"
"print_int"
and exports function "do_work". Note that Wasm uses a two-level
namespace
for imports, but the first level will always be "env" unless you
annotate
the function declaration with
`__attribute__((import_module("my_other_namespace")))`.

Best,

Thomas

On Sun, Sep 13, 2020 at 8:04 PM Vedant Roy via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> Is there a method for importing functions from the Web Assembly runtime
> when writing C? I know Emscripten has support for calling JS functions from
> C code, but I was wondering if there's an easy way to do this without
> Emscripten (pure Clang)?
>
> Best,
> Ved
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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