Writing a Custom Memory Allocator in C++

2025-06-23T00:00:00+00:00

Writing a Memory Allocator in C

Every C program leans on malloc and free without much thought. In this post we rip out the guts of a simple allocator and build one from scratch.

The Core Problem

The OS hands your process raw pages via sbrk() or mmap(). Your allocator’s job is to carve that slab into chunks and hand them out on request.

typedef struct block {
    size_t        size;
    int           free;
    struct block *next;
} block_t;

#define BLOCK_SIZE sizeof(block_t)

Free List Strategy

We keep a linked list of every allocated block. On malloc:

Walk the list for a free block that fits (first-fit / best-fit)
If none found — extend the heap with sbrk(size)
Mark the block as used and return a pointer past the header

void *my_malloc(size_t size) {
    block_t *cur = head;

    while (cur) {
        if (cur->free && cur->size >= size) {
            cur->free = 0;
            return (void *)(cur + 1);   /* skip header */
        }
        cur = cur->next;
    }

    /* grow heap */
    block_t *blk = sbrk(BLOCK_SIZE + size);
    blk->size = size;
    blk->free = 0;
    blk->next = NULL;
    /* ... append to list */
    return (void *)(blk + 1);
}

Why Alignment Matters

sbrk doesn’t guarantee alignment. Unaligned access on x86 is slow; on ARM it’s a bus fault. Always round up to sizeof(max_align_t):

#define ALIGN(n) (((n) + sizeof(max_align_t) - 1) & ~(sizeof(max_align_t) - 1))

Coalescing Free Blocks

Naive free causes fragmentation — lots of small gaps that can’t satisfy a large request even if total free memory is sufficient. Fix it by merging adjacent free blocks during free().

Next Steps

Implement best-fit vs first-fit and benchmark them
Add a slab layer for fixed-size objects
Use mmap instead of sbrk for large allocations