ft_malloc

Custom dynamic memory allocator implementing malloc, free, and realloc as a shared library (libft_malloc_$HOSTTYPE.so).

42 School project.

Build

make        # build lib/libft_malloc_$HOSTTYPE.so + symlink
make test   # build & run tests
make clean  # remove object files
make fclean # full clean (library, Libft, tests)
make re     # fclean + all

Usage

The library is loaded at runtime via LD_PRELOAD, which instructs the Linux dynamic linker to load the specified shared library before any other, including libc. Any symbols it exports (such as malloc, free, realloc) override the default ones. This makes it a drop-in replacement with no recompilation needed:

LD_PRELOAD=./lib/libft_malloc.so ./my_program

Every call to malloc, free, or realloc inside my_program (and any library it uses) will go through ft_malloc instead of the system allocator.

API

Function	Description
`void *malloc(size_t size)`	Allocate `size` bytes
`void free(void *ptr)`	Release a previously allocated block
`void realloc(void ptr, size_t size)`	Resize a block to `size` bytes
`void show_alloc_mem(void)`	Print all zones and allocations to stdout

Architecture

Allocations are divided into three categories:

Category	Size range	Strategy
TINY	1 -- 128 bytes	Pre-allocated zone (fits 100+ allocations)
SMALL	129 -- 1024 bytes	Pre-allocated zone (fits 100+ allocations)
LARGE	1025+ bytes	Individual `mmap` per allocation

TINY and SMALL zones are page-aligned regions obtained via mmap. New zones are allocated on demand when the current one is full. LARGE allocations each get their own mmap region and are released individually via munmap.

All returned pointers are 16-byte aligned.

Testing

AddressSanitizer (ASan) cannot be used to test this allocator. ASan intercepts malloc/free itself and expects specific allocator metadata (redzones, quarantine zones) that a custom allocator does not provide. The two implementations would conflict. Since allocations go through mmap, they also bypass ASan's shadow memory tracking entirely.

Alternatives for catching bugs: - Valgrind — operates at the binary level without replacing malloc, so it can detect issues in the allocator's own internal logic. - Custom tests — targeted checks for write-after-free, double free, boundary writes, and stress scenarios. - show_alloc_mem() — inspect zone and chunk state visually after operations.