In our discussions of dynamic memory management we discussed the operation of the standard C library call, malloc(). Malloc designates a region of a processís address space from the symbol _end (where the code and global data ends) to brk as the heap.
As part of dynamic memory management, we also discussed various algorithms for the management of the empty spaces that may be created after a malloc()-managed heap has had some of itís allocations freed. In this assignment, you are asked to create your own version of malloc, one that uses the next-fit algorithm.
We are programmatically able to grow or shrink the size of the heap by setting new values of brk. The function sbrk() handles scaling the brk value by its parameter:
A simple place to start then is to create a malloc which, with each request, simply increments brk by the amount requested and returns the old value of brk as the pointer. However, when you want to write a free() function, the parameter to free() is just a pointer to the start of the region, so you have no way to determine how much space to deallocate. In order to know what space is free or used, and how big each region is, we must use one of the techniques from class: Bitmaps or Linked lists.
From our discussion in class, linked lists seem like the better choice, but now we need some place to store this dynamic list of free and occupied memory regions inside of the heap. If we just allocated some fixed-size region, that space may not be adequate for how many nodes in the list weíd need to create. A better idea is illustrated in the figure below:
We can add some additional space to each update of brk in order to accommodate a structure that is a node in our linked list, and this structure can contain useful things like:
We then return back a pointer that is in the middle of the chunk we allocated, and thus the program calling malloc() will never notice the additional structure. However, when we get a pointer back to free, we can simply look at the memory before it for the structure that we wrote there with the information we need.
You are to create two functions for this project.
Your free function should coalesce adjacent free blocks as we described in class. If the block that touches brk is free, you should use sbrk() with a negative offset to reduce the size of the heap.
As you are developing, you will want to create a driver program that tests your calls to your mallocs and frees. A week before the due date, I will provide a sample driver program that must work. During grading, we will use a second driver program in addition to the first one in order to test that your code works.
For this project we will again be working on thot.cs.pitt.edu
This machine is a 64-bit machine, and to avoid pointer cast warnings, build using the -m32 option for gcc. This will build a 32-bit program instead of a 64-bit one.
To create a tar.gz file, if your code is in a folder named project3, execute the following commands:
Where USERNAME is your username.
Then copy your file to: