Assignment 7:  Dynamic Storage Allocator

DUE DATES:

For the “trivia” assignment:  11:59 pm, Thursday, December 3rd. 

For the main assignment:  11:59pm, Wednesday, December 9th. 

If you want to use slip days you have to tell us before the deadline by emailing Shuang Zhai.

Introduction

In this lab you will be writing a dynamic storage allocator for C programs, i.e., your own version of the malloc, free, and realloc routines.  You are encouraged to explore the design space creatively and implement an allocator that is correct, efficient, and fast. 

Logistics

You may work in a group of up to two people.  Any clarifications and revisions to the assignment will be posted on the blackboard discussion forum. 

Hand Out Instructions

Start by copying /u/cs252/labf20/malloclab/malloclab-handout.tar to a protected directory in which you plan to do your work.  Then give the command: 

    tar xvf malloclab-handout.tar
This will cause a number of files to be unpacked into the directory.  The only file you will be modifying and handing in is mm.c

The mdriver.c program is a driver program that allows you to evaluate the performance of your solution.  Use the command make to generate the driver code and run it with the command ./mdriver –V.  (The –V flag displays helpful summary information.)

Looking at the file mm.c you’ll notice a C structure team into which you should insert the requested identifying information about the one or two individuals comprising your programming team.  Do this right away so you don’t forget. 

When you have completed the lab, you will hand in only one file (mm.c), which contains your solution. 

How to Work on the Lab

Your dynamic storage allocator will consist of the following four functions, which are declared in mm.h and defined in mm.c

The mm.c file we have given you implements the simplest but still functionally correct malloc package that we could think of.  Using this as a starting place, modify these functions (and possibly define other private static functions), so that they obey the following semantics: 

These semantics match the the semantics of the corresponding libc malloc, realloc, and free routines.  Type man malloc to the shell for complete documentation. 

Heap Consistency Checker

Dynamic memory allocators are notoriously tricky beasts to program correctly and efficiently.  They are difficult to program correctly because they involve a lot of untyped pointer manipulation.  You will find it very helpful to write a heap checker that scans the heap and checks it for consistency. 

Some examples of what a heap checker might check are: 

Your heap checker will consist of the function int mm_check(void) in mm.c.  It will check any invariants or consistency conditions you consider prudent.  It returns a nonzero value if and only if your heap is consistent.  You are not limited to the listed suggestions nor are you required to check all of them.  You are encouraged to print out error messages when mm_check fails. 

This consistency checker is for your own debugging during development.  When you submit mm.c, make sure to remove any calls to mm_check as they will slow down your throughput.  Style points will be given for your mm_check function.  Make sure to put in comments and document what you are checking. 

Support Routines

The memlib.c package simulates the OS portion of the memory system for your dynamic memory allocator.  You can invoke the following functions in memlib.c

The Trace-driven Driver Program

The driver program mdriver.c in the malloc-handout.tar distribution tests your mm.c package for correctness, space utilization, and throughput.  The driver program is controlled by a set of trace files.  Some small traces are included in the malloc-handout.tar distribution.  The larger traces that we will test your file with are located at /u/cs252/labf20/malloclab/malloctraces/.  These larger traces are automatically run if you execute mdriver without a -f argument.  Each trace file contains a sequence of allocate, reallocate, and free directions that instruct the driver to call your mm_malloc, mm_realloc, and mm_free routines in some sequence. 

The driver mdriver.c accepts the following command line arguments: 

Programming Rules

This is a reminder of the honor code policy. You may use your text, course notes, and any other reference material (such as C reference books, C reference web pages, or Unix man pages). However, you may not obtain any code from anyone, regardless of whether or not they are in the course (aside from code provided as a part of the course, in reference books, or in standard code libraries). You may not consult solutions from prior semesters of this course or from similar courses at other universities or on the web. You may discuss problems, general strategies, or algorithms with other students in the course. However, you may not collaborate on the implementation of those strategies or algorithms with anyone outside your group and each group must write all of their own code. Specific programming rules for this assignment:

Evaluation

You will receive zero points if you break any of the rules or your code is buggy and crashes the driver.  Otherwise, your grade will be calculated as follows: 

Hints

“Trivia” Assignment

Before midnight on Thursday December 3rd, answer the following questions on blackboard. 

  1. Are you working alone or in a team of two?  If the latter, who is your partner? 
  2. After you have input your team information, what output do you get when you run the following command? 
        mdriver -V -f short2-bal.rep
    
  3. What is the value of ALIGNMENT defined in mm.c
  4. What does the mem_sbrk function do? 

Turn In Instructions

The “trivia” assignment will be submitted via blackboard.  The main assignment will be submitted using the script /u/cs252/bin/TURN_IN. 

    /u/cs252/bin/TURN_IN .
You only need to turn in your completed mm.c.  Watch the blackboard for details, and for any clarifications or revisions to the assignment. 

Before running the TURNIN script, be sure that you have