scan_merge.c

/* scan_merge scans the free list in order to find
 * continuous free blocks that can be merged and also
 * checks if our last free block ends where the program
 * break is. If it does, and the free block is larger then
 * MIN_DEALLOC then the block is released to the OS, by
 * calling brk to set the program break to the begin of
 * the block */
void
scan_merge()
{
	block_t        *curr = head;
	unsigned long	header_curr, header_next;
	unsigned long	program_break = (unsigned long)sbrk(0);
	if (program_break == 0) {
		printf("failed to retrieve program break\n");
		return;
	}
	while (curr->next) {
		header_curr = (unsigned long)curr;
		header_next = (unsigned long)curr->next;
		if (header_curr + curr->size + sizeof(block_t) == header_next) {
			/* found two continuous addressed blocks, merge them
			 * and create a new block with the sum of their sizes */
			curr->size += curr->next->size + sizeof(block_t);
			curr->next = curr->next->next;
			if (curr->next) {
				curr->next->prev = curr;
			} else {
				break;
			}
		}
		curr = curr->next;
	}
	stats("after merge");
	header_curr = (unsigned long)curr;
	/* last check if our last free block ends on the program break and is
	 * big enough to be released to the OS (this check is to reduce the
	 * number of calls to sbrk/brk */
	if (header_curr + curr->size + sizeof(block_t) == program_break
	    && curr->size >= MIN_DEALLOC) {
		fl_remove(curr);
		if (brk(curr) != 0) {
			printf("error freeing memory\n");
		}
	}
}