How to implement "malloc" in Wasm

Question

I am learning WebAssembly now and I find that the external function (libc or third party function) can't be accessed by Wasm world easily (using import is a general solution).

I'm trying to use emcc to compile my source code into wasm but malloc and free are widely used in my source code. I don't think that importing malloc and free from real world is practicable.

So, I'm pretty curious about how to implement malloc in wasm world. I have already known how the malloc works in glibc:using brk() or sbrk() to extend heap and some memory management for the liner address. But in wasm world I think it impossible to call brk() or sbrk() to get the liner address.

Is it reasonable to use global var to implement malloc like this ?

u_char mem[10240];


void *wasm_malloc(size_t num)
{
    /*get the free mem idx*/
    return &mem[idx];
}

Answer 1

Wait, you don't need to do that.

I don't think that importing malloc and free from real world is practicable.

Incorrect. That's exactly the point of using Emscripten. Emscripten is not just a C/C++ to Wasm compiler, but a complete toolchain that includes a web runtime and its own libc specifically designed for running C/C++ program for web browsers with minimal source code modifications.

Emscripten libc is a heavily modified fork of musl. It implements/emulates wide range of standard C libraries (including malloc, sbrk) and POSIX APIs (like pthread and BSD socket), except some APIs that doesn't make sense in a Wasm environment like exec and fork. By using emcc command, you will link those libc ports out of the box. So feel free just using malloc - you don't need to do anything!

If you are still wondering how to implement malloc for Emscripten, Emscripten has two options of malloc implementations - dlmalloc and emmalloc.

dlmalloc is a famous malloc implementation. glibc also uses a forked version of it. You can see Emscripten's version of dlmalloc here.

emmalloc might be more interesting to you. It's a simple and compact malloc implementation for web enviroment, designed by the Emscripten team. You can see the source code here.