Lethe — User-Space Memory Allocator

User-space memory allocator in C with free-list management, boundary-tag coalescing, and thread-local caching.

A drop-in replacement for malloc/free/realloc in C, implementing free-list management, boundary-tag coalescing, and thread-local caching. Injectable into any existing program via LD_PRELOAD without recompilation.

Benchmark Results

Here is a comparison between Lethe and glibc (default) allocators using bench/bench.c:

Single-threaded malloc/free (ns/op)

Size	Lethe	glibc
16	53.6	19.4
32	53.5	18.3
64	53.8	17.6
128	54.2	18.0
256	52.8	16.8
512	106.3	16.3
1024	98.6	19.9

Multi-threaded scaling (ops/sec)

Threads	Lethe	glibc
1	14,237,787	50,421,031
2	2,699,567	69,411,727
4	3,683,984	124,506,960
8	1,858,439	242,895,025
16	1,394,220	287,615,289

Note: Lethe shows contention at higher thread counts since allocations >= 512 bypass the thread cache and hit the global mutex, whereas glibc uses per-thread arenas.

Design Notes

• Boundary Tags: O(1) coalescing by storing block size and is_free status in both headers and footers.
• Thread-Local Caching: Uses __thread variables and pthread_key_create destructors for lock-free fast paths on small allocations.
• In-place Realloc: realloc aggressively tries to absorb free adjacent blocks (forward coalescing) to avoid memcpy.

All checklist features completed!