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# ft_malloc
Custom dynamic memory allocator implementing `malloc`, `free`, and `realloc` as a shared library (`libft_malloc_$HOSTTYPE.so`).
42 School project.
## Build
```bash
make # build lib/libft_malloc_$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:
```bash
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.
|
