Hey there – I’m Jean-Baptiste, just another developer doing weird things with code. All my projects live on jterrazz.com – complete with backstories and lessons learned. Feel free to poke around – you might just find something useful!

42 Malloc

Custom C implementation of the malloc library functions. It creates the shared library libft_malloc.so which can be used to replace malloc in any system commands.

Access the repository story here

It exposes the following methods:

void *malloc(size_t size);
void free(void *ptr);
void *realloc(void *ptr, size_t size);
void *calloc(size_t count, size_t size);
void *reallocf(void *ptr, size_t size);

// Debug calls

void show_alloc_mem(); // Print informations about allocated zones
void show_alloc_mem_ex(); // Print a hex dump of the heaps

Commands

Build

make
make clean # Clean temporary built files
make fclean # Clean all built files

gcc uses -fPIC to generate position independent code (PIC) for shared libraries.

Testing

# Use malloc with a ${CMD}
sh insert_lib.sh ${CMD} # ex: sh insert_lib.sh ls

# Run unit test
cd test
make && sh ./run_test.sh

Implementation

To understand in detail this implementation, please refer to the article of this project.

Bonus:

• calloc(), reallocf()
• Defragmentation of freed space
• Multi-thread safe with pthread
• Show hex dump of allocated zones with show_alloc_mem_ex()
• Debug environment variables: MyMallocStackLogging, MyMallocScribble, MyMallocFullLogging

Structure

The heap stores data about a mmap zone

typedef struct s_heap {
    struct s_heap	*prev;
    struct s_heap	*next;
    t_heap_group	group;
    size_t		    total_size;
    size_t		free_size;
    size_t		block_count;
} t_heap;

A block stores data about a malloc call.

typedef struct s_block {
    struct s_block	*prev;
    struct s_block	*next;
    size_t		data_size;
    t_bool		freed;
} t_block;

Constants

For better performance, we preallocate heaps for small malloc calls. We define 3 heap types: TINY, SMALL, LARGE.

#define TINY_HEAP_ALLOCATION_SIZE (4 * getpagesize())
#define TINY_BLOCK_SIZE (TINY_HEAP_ALLOCATION_SIZE / 128)
#define SMALL_HEAP_ALLOCATION_SIZE (32 * getpagesize())
#define SMALL_BLOCK_SIZE (SMALL_HEAP_ALLOCATION_SIZE / 128)