malloc from Scratch

Atlas I explained the heap and a custom bump/arena/pool allocator conceptually in pointers & memory. This branch builds the general-purpose case those simpler allocators avoid: a malloc/free/realloc that has to handle arbitrary alloc/free order without leaking or corrupting itself.

Planned notes

• The contract: what malloc/free/realloc/calloc actually promise
• Requesting memory from the OS: sbrk vs mmap, and where the line is
• A first-fit free list — the simplest allocator that works
• Block headers, footers, and the bookkeeping tax of every allocation
• Alignment requirements and why they're not optional
• Coalescing adjacent free blocks on free()
• Splitting blocks on allocation to fight fragmentation
• Best-fit vs first-fit vs segregated free lists — a measured comparison
• Implementing realloc without always copying
• Thread-safety: a global lock, then why that doesn't scale
• Benchmarking against glibc's malloc and mimalloc under real workloads
• Where this allocator breaks (and what production allocators do instead)

Core sources

• Computer Systems: A Programmer's Perspective (Bryant & O'Hallaron) — the dynamic memory allocation chapter, already cited in Atlas I.
• Doug Lea's malloc design notes — the design that shaped glibc's allocator.
• "Understanding and Using C Pointers" (Reese) — pointer arithmetic discipline this build leans on hard.

Connects to: KV Store & Durability · Craftsmanship Low-Level II