dreamtworld / cmem-wasm

Single-page C memory management in WebAssembly

cmem-wasm

This library implements libc memory allocation functions in WebAssembly. The implementation assumes a single page of memory, that is, a 64 KiB buffer.

Available libc functions

• malloc
• calloc
• free
• realloc
• memset
• memcpy
• memmove

Usage

Initialize memory with cmem_init(start), where start is the 16 bit address of the first byte of the memory region to use. Proceed to use memory allocation functions as usual. Exceeding available memory will trap.

Additional functionality

cmem_end() returns a past-the-end pointer to the last buffer in memory, effectively retrieving the current length of memory. Intended use is backing up memory states at minimal size.

Build

build.sh runs the source code through the C language preprocessor, then translates it with wat2wasm from the wabt software package.

The result is an object file that can be linked using wasm-ld, as well as a WebAssembly module.

The script also produces an optimized module using wasm-opt -O3 from the binaryen package.

Build options

• EXPORT_ALL exports the memory and functions other than cmem_init and cmem_end, including memory and list management functions.
• PREFIX_LIBC prepends cmem_ to libc function names.
• FIRST_FIT employs a first-fit instead of a best-fit strategy for allocation.
• BULK_MEMORY enables use of bulk memory instructions, decreasing embedder compatibility.

Build dependencies

• cpp
• wabt
• binaryen

Implementation

Best-fit allocation. Doubly linked list to navigate buffers. 16-bit addresses constrict memory length to 64 KiB.