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malloc

A malloc implementation

This screenshots displays the memory managed by the library. Yellow represents block headers, green client data and blue free block headers and trailers. screenshot

API

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

void	show_alloc_mem(void); /* print a summary of allocated blocks */
void	hexdump_mem(void);    /* semantic hexdump of the allocated memory */

Usage & testing

To create the library:

make

To run a program, dynamically link it by running

scripts/inject.sh COMMAND

Or link with libft_malloc.so.

Strategy

  • Memory is managed via mmap(2) and mmunmap(2).

  • mmaped areas are managed via the area struct and area__* functions. Areas are a doubly linked list containing individual blocks, which are taken from an available area.

  • blocks have a 1 word header encoding size, free status, and free status of the previous block. An additional 3 word minimum size is required to store free list nodes.

  • free_blocks are doubly-linked list nodes. They have the same header as the underlying block, 2 pointers to adjacent free_blocks, an an additional footer encoding the block's size. This footer, in addition to block.prev_free allows for linear time coalescing of neighboring blocks.

  • Free blocks are stored in an adress-ordered linked list, such that free_block->prev < free_block < free_block->next is true for all free blocks for a free list. This ensures that the memory close to the begining of the mmaped areas gets reused sooner, reducing memory fragmentation and improving locality.

  • When freeing the last block of an area, the block is destroyed and area.cur_size is reduced, effectively destroying the block. This significantly reduces external fragmentation.

  • 3 free lists are used. Each list contains blocks of a specific size range (0-255, 256-1023, 1024+).

  • Free blocks are selected on a first fit basis.

  • Memory is defragmented at allocation size. When searching the free list for a block of the requested size, neighboring free blocks are included in the current blocks's size, and merged if the combined size is sufficient.

Further improvements

  • Block checksums for fast freeing