Question

I’m writing a malloc function for a college assignment. Here’s a basic layout of my idea:

1)Define a node struct with pointers to previous node, next node, as well as a char for size and vacancy. Each region in the heap will contain a hidden node with this information.

2)Malloc function. Starting with first node loop through each node checking for vacancy. If a node is vacant and is large enough return a ptr to the beginning of the region not including the node. If no space is available use sbrk to allocate requested space PLUS space for a node.

3)Free function. Go to pointer passed as parameter-sizeof(struct node) and set the vacancy to vacant. Then starting with the beginning of the list, traverse the list merging adjacent free spaces.

How does this approach sound? My main concern is with actually starting the linked list. For instance, should I create a node with sbrk before I start to do any allocations and store a ptr to it as a global variable? If so how do I initialize a first node before I allow the malloc function to be called by a driver program?

Thanks in advance. I’m not asking for someone to write my code, only to provide some insight and suggestions regarding my ideas.

Answer 1

1. I would avoid keeping all the bookkeeping information on nodes while they’re allocated. I’d have the bare minimum of information (usually just the block size) at the beginning of the block, but nothing more.
2. I’d track free blocks and allocated blocks separately, so when you’re searching for a free block, you don’t waste time on blocks that are already in use.
3. I’d separate the free list into two pieces, and coalesce blocks lazily. In other words, have one free list you’re allocating from, and a second that’s just a holding area. When the user calls free, just link the block into the holding area, nothing more. When the list you’re using for allocations starts to run low, sort the blocks in the holding area by address, then merge with the allocation free list. Then walk the list and merge adjacent blocks.
4. When you do need to call sbrk (or whatever) to allocate more space from the system, do not just allocate enough space to satisfy the current allocation request. Instead, allocate a fairly large block (e.g., a megabyte) and then split that to get satisfy the request, and add the rest as block to the free list. If you’re running low enough on memory that you have to go to sbrk once, chances are the next few calls will do the same, so you might as well be greedy, and grab enough memory immediately to stand a decent chance of satisfying more requests as well.

The basic idea of the third is to avoid doing coalescing as long as possible to increase the chances of finding adjacent blocks, so when you do coalesce you’ll probably do some real good, and avoid wasting time trying to coalesce when there are only a few adjacent blocks free.