Malloc

Part of 42's post common core UNIX projects.

The goal of this project is to recreate a dynamic memory allocator implementing:

• malloc()
• realloc()
• free()
• show_alloc_mem() to print allocations and their size

It also meets the following requirements:

• All pointers are aligned properly according to max_align_t
• Block headers and payload sizes are also aligned accordingly
• Minimizes the amount of calls to mmap() or munmap()

Architecture

Allocations are split into three categories/sizes:

• TINY
• SMALL
• LARGE

TINY and SMALL allocations are served from preallocated mmap() zones when possible.

LARGE allocations use their own dedicated mmap() region and are directly munmap()'d when freed.

Each memory block contains a header used to track:

• physical neighbours
• freelist membership
• returnable zone membership
• block size
• zone type
• mapping boundaries

Free blocks are stored in size-based freelists and reused by future allocations.

Adjacent free blocks belonging to the same mapped zone are merged to reduce fragmentation.

Bonus part

As part of 42's subject, all bonuses were implemented, features such as :

• Handling malloc related env variables
• Freed memory defragmentation
• Thread-safe multithreading capability
• Extended debug print tools

Building and usage

Build the shared library:

git clone https://github.com/0yech/malloc.git
cd malloc
make

This produces a platform-specific library such as libft_malloc_x86_64_Linux.so and symlink libft_malloc.so

Replace other implementations of malloc() in a program through LD_PRELOAD:

LD_PRELOAD=./libft_malloc.so ./program
    example
LD_PRELOAD=./libft_malloc.so /bin/ls

Debug functions

The project provides:

void show_alloc_mem(void);
void show_alloc_mem_ex(void);

show_alloc_mem() displays the currently allocated blocks.

show_alloc_mem_ex() provides a more detailed view of the allocator state, including additional block information and memory contents useful during debugging.

Debug output is implemented using write() calls rather than printf()to avoid recursive calls to malloc().

Env variables

MALLOC_DEBUG and MALLOC_SCRIBBLE are available as env variables.

MALLOC_DEBUG=1

Prints out a log whenever the three functions are called. malloc(55) - 0x...

MALLOC_SCRIBBLE=1

Fills allocated and freed memory with recognizable patterns. Fresh allocs are filled with 0xAA and freed allocs with 0xDD

Extras

Returnable zones

Fully free TINY and SMALL zones can be kept temporarily in a returnable list for reuse.

Keeping a limited number of free zones allows future allocations to reuse already mapped memory instead of immediately calling mmap() again.

When too many fully free zones accumulate, the excess zones are returned to the operating system with munmap().

LARGE allocations are always unmapped directly when freed.

This implementation of malloc was somewhat heavily slowed down by the subject requirements. Returnable lists should've been a major upgrade if users had to trim their zones manually. But since the project must not leak only using free(), its usefulness and overall performance was hurt.