ft_malloc

ft_malloc is a custom memory allocator written in C for Linux. It provides a shared library that can replace the standard malloc, free, and realloc functions at runtime through LD_PRELOAD.

The project is built around a zone-based allocator with three allocation classes:

• TINY for allocations from 1 to 128 bytes
• SMALL for allocations from 129 to 1024 bytes
• LARGE for allocations above 1024 bytes

The code also includes a custom ft_printf implementation used for allocator diagnostics and debug output.

• Custom implementations of malloc, free, and realloc
• Thread-safe allocation paths protected by a global mutex
• 8-byte aligned allocations
• Zone splitting and block merging to reduce fragmentation
• Optional memory perturbation for debugging with pseudo-random filling
• Optional error logging controlled through environment variables
• Memory visualisation helpers with detailed zone and block output
• Extended memory dump with fragmentation statistics
• Built-in test programs for single-threaded and multi-threaded validation

The shared library exposes the following functions:

void *ft_malloc(size_t size);
void ft_free(void *ptr);
void *ft_realloc(void *ptr, size_t size);
void show_alloc_mem(void);
void show_alloc_mem_ex(void);

The allocator manages memory using mmap and stores metadata for each zone and block.

• TINY and SMALL allocations are served from reusable pre-allocated zones.
• LARGE allocations are handled with a dedicated zone per request.
• Free blocks can be merged when they are adjacent in memory.
• Reallocations try to grow or shrink in place when possible before falling back to allocate-copy-free behavior.

The implementation is Linux-specific and relies on sysconf(_SC_PAGESIZE), mmap, and munmap.

The allocator reads the following environment variables when an allocation function is first called:

• MALLOC_LOG_
  • Enables colored error logging when set to a non-zero numeric value.
• MALLOC_PERTURB_
  • Fills allocated blocks with pseudo-random data and inverts the pattern on free.
• MALLOC_MMAP_THRESHOLD_
  • Forces allocations greater than or equal to the given size to use the LARGE path.
• MALLOC_MMAP_MAX_
  • Limits the number of large allocations handled through mmap.

Values must be valid positive integers. A value of 0 disables the option.

Example:

export MALLOC_LOG_=1
export MALLOC_PERTURB_=45
export MALLOC_MMAP_THRESHOLD_=700

make

This builds a shared library named libft_malloc_<HOSTTYPE>.so and creates a symlink called libft_malloc.so.

Additional targets:

make test # build the test binaries
make clean # remove object files
make fclean # remove objects, shared library, symlink, and test binaries
make re # rebuild everything from scratch

To preload the allocator into another program:

LD_PRELOAD=./libft_malloc.so ./your_program

You can also link against the shared library directly from a test or another program using the public header in include/malloc.h.

Running make test builds two executables:

• test_normal, a functional test suite that checks basic allocations, reallocations, edge cases, stress cases, and allocator dumps
• test_thread, a multi-threaded stress test that performs concurrent allocations, reallocations, and frees across several worker threads

Both tests are linked with an rpath so they can find libft_malloc.so in the local directory.

• This project is intended for Linux only.
• The allocator is designed as a learning and experimentation project.
• Debug output is intentionally verbose to help inspect internal allocation state.