Playing with wasm

Installing tools

# Compiler
brew install emscripten

# Runtime
brew install wasmer

# Tool
brew install wabt

Hello world

via JavaScript

Prepare a source code named helloworld.c.

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Hello world\n");
    return 0;
}

Compile it with emcc.

emcc -o helloworld.js helloworld.c

This generates helloworld.js and helloworld.wasm. You can run it with Node.js.

node helloworld.js

In this case, the system interface will be given by JavaScript through helloworld.js, and helloworld.js calls __main_argc_argv exported from helloworld.wasm to start running the code.

When you run wasm2wbt helloworld.wasm, you can find these imports and exports.

(import "wasi_snapshot_preview1" "fd_write" (func (;0;) (type 13)))
(import "env" "emscripten_memcpy_js" (func (;1;) (type 7)))

(export "memory" (memory 0))
(export "__wasm_call_ctors" (func 2))
(export "__main_argc_argv" (func 3))
(export "__errno_location" (func 19))
(export "fflush" (func 52))
(export "__indirect_function_table" (table 0))
(export "emscripten_stack_init" (func 48))
(export "emscripten_stack_get_free" (func 49))
(export "emscripten_stack_get_base" (func 50))
(export "emscripten_stack_get_end" (func 51))
(export "stackSave" (func 53))
(export "stackRestore" (func 54))
(export "stackAlloc" (func 55))
(export "emscripten_stack_get_current" (func 56))
(export "dynCall_jiji" (func 58))

Standalone

One the other hand, you can also compile it to a standalone wasm.

emcc -o helloworld.wasm helloworld.c

You can run it with wasmer.

wasmer helloworld.wasm

In this case, the system interface will be given by wasmer based on WASI. wasmer calls _start exported from helloworld.wasm to start running the code.

You can see these imports and exports with wasm2wbt.

(import "wasi_snapshot_preview1" "args_sizes_get" (func (;0;) (type 5)))
(import "wasi_snapshot_preview1" "args_get" (func (;1;) (type 5)))
(import "wasi_snapshot_preview1" "proc_exit" (func (;2;) (type 3)))
(import "wasi_snapshot_preview1" "fd_write" (func (;3;) (type 11)))

(export "memory" (memory 0))
(export "_start" (func 6))
(export "__indirect_function_table" (table 0))
(export "__errno_location" (func 19))
(export "emscripten_stack_init" (func 54))
(export "emscripten_stack_get_free" (func 55))
(export "emscripten_stack_get_base" (func 56))
(export "emscripten_stack_get_end" (func 57))
(export "stackSave" (func 58))
(export "stackRestore" (func 59))
(export "stackAlloc" (func 60))
(export "emscripten_stack_get_current" (func 61))