Memory

Why am I doing this lab?

  • Learn how to make a basic memory checker
  • Learn memory layout and how malloc/free works
  • How does free know how many bytes it has to free?

How does malloc() manage allocated memory?

  • As you’ll encounter in this lab, one way of keeping track of memory is via metadata tags
  • To accomplish this, we allocate more memory than the user requests
  • We’ll use pointer arithmetic to make sure that we return the right pointer to the user

What’s in the #tags?

Information about the current block of memory:

  • Pointer to next link, if using a linked list
  • Size of block
  • Whatever else you want…

typedef struct {
	struct metadata *next;
	size_t size;
} metadata;

int main(void)
{
	metadata* md = malloc(sizeof(metadata) + 64);
	
	printf("metadata address:\t%p\n", md);
	printf("start of allocation:\t%p\n", md + 1);

	free(md);
	return 0;
}


$ gcc malloc-main.c -o malloc-main
$ ./malloc-main
	metadata address:	0x563eaaa8f2a0
	start of allocation:	0x563eaaa8f2b0

The Big Picture

Map one

Map two

Hints

  • meta_data * head : head for linked-list
  • total_memory_requested: keep track of bytes used
  • total_memory_freed: keep track of bytes freed
  • Think about how to create a linked-list and insert/remove
  • Insert has to be constant time, but free doesn’t
  • Have to catch bad calls to free and increment invalid_addresses

Hey! Shouldn’t we not be able to do arithmetic with void pointers?

  • Technically yeah, but gcc and clang treats it as a char*
  • In short, don’t do it because the standard doesn’t guarantee it

Splitting and coalescing

  • Reduce memory wastage!
  • How can we use tags to split a block?
  • How can we use tags to merge adjacent blocks?

Understanding how to do this will be useful when you implement malloc!

Malloc split

Malloc merge

Questions?