MP 3: malloc

Due Date: Completed and turned in via git before March 1, 2021 at 11:59pm
Points: MP 3 is worth 50 points


Your third MP in CS 240 is writing your own memory allocator! You should write your implementations of calloc, malloc, realloc, and free in alloc.c.

Memory allocation seems like a mystery, but in actuality, we are making a wrapper around the system call sbrk. Here’s a really simple implementation of malloc:

void *malloc(size_t size) {
    return sbrk(size);

As you can see, when we request size bytes of memory, we call sbrk(size) to increase the heap by size bytes. Then, we return a pointer to this memory, and we’re done. Simple!

Here is our implementation of free:

void free(void *ptr) {

This is a “correct” way to implement free. However, the obvious drawback with our implementation is that we can’t reuse memory after we are done with it. Also, we have not checked for errors when we call sbrk, and we have not implemented realloc or calloc.

Despite all of this, this is still a “working” implementation of malloc. So, the job of malloc is not really to allocate memory, but to keep track of the memory we’ve allocated so that we can reuse it.

Initial Files

In your CS 240 directory, merge the initial starting files with the following commands:

git fetch release
git merge release/mp3 -m "Merging initial files"

Machine Problem

Writing malloc

Using sbrk, write your malloc implementation! Your malloc should allocate a block of size bytes of memory, returning a pointer to the beginning of the block. The contents of the newly allocated block of memory are not initialized.

Writing free

Write your implementation of free! This function will take a pointer to a block of previously allocated memory and make it available again for future allocations.

Writing calloc

Write your implementation of calloc! This function will take in two parameters, num and size. It will allocate a block of memory for an array of num elements, each of them being size bytes long, and initialize all of its bits to zero.

Writing realloc

Write your implementation of realloc! This function will take in two parameters: a pointer to a previously allocated block of memory, and the desired size of the returned memory. You must return a pointer to a block of allocated memory of at least size, with this block of memory containing the same contents as the passed in pointer.

Getting Started Guide

Not sure how to get started or test this program? We’ve made a “Building Malloc” guide to help you get started!

Testing Your Code

In order to run your solution on the testers, run ./mstats with the tester you want. You must do this, or your code will be run with the glibc implementation!


./mstats testers_exe/tester-1
Memory failed to allocate!
[mstats]: STATUS: FAILED=(256)
[mstats]: MAX: 0
[mstats]: AVG: 0.000000
[mstats]: TIME: 0.000000

Here is what some of the error codes you may encounter mean:

11: (SIGSEGV) Segmentation fault
15: (SIGTERM) Timed out
65, 66: Dynamic linking error
67: Failed to collect memory info
68: Exceeded memory limit
91: Data allocated outside of heap
92: Data allocated exceeds heap limit

Adding Print Statements

I highly encourage you to add a print_heap function to your code, if you have not done so already. Our “Building Malloc” has code to print your heap as part of malloc and you just need to make it into a function.

When you want to run your program without the print output, you can simple comment out the contents of your print_heap function. You will find for all non-trivial tests the print statements will slow down your code far, far too much.

Running Samples

Several sample programs are provided and can be built using make. Specifically, six samples are part of mp3. In each of these descriptions, the below values are the MAX values expected when running ./mstats (ex: ./mstats ./samples/sample1).

  • ./samples/sample1 is the sample from the “Building Malloc” guide. and is a great sample to test block splitting.

    • Without block splitting, sample1 should MAX around 1,600 bytes of memory.
    • With block splitting, sample1 should MAX around 1,100 bytes of memory.

  • ./samples/sample2 is a great sample to debug memory coalescing.

    • Without memory coalescing, sample2 should MAX around 9,000 bytes of memory.
    • With memory coalescing, sample2 should MAX around 6,500 bytes of memory.

  • ./samples/sample3 tests memory coalescing in reverse order (this one is trickier).

    • Without memory coalescing, sample3 should MAX around 9,000 bytes of memory.
    • With memory coalescing, sample3 should MAX around 6,500 bytes of memory.

  • ./samples/sample4 tests memory coalescing with extra free blocks around that cannot be coalesced.

    • Without memory coalescing, sample4 should MAX around 11,000 bytes of memory.
    • With memory coalescing, sample4 should MAX around 9,000 bytes of memory.

  • ./samples/sample5 tests a “free list” by having allocated blocks before/after the freed element. This is a simple version of sample6.

  • ./samples/sample6 tests your “free list” by having LOTS of allocated blocks before/after your free block.

    • If you are NOT reusing blocks, sample6 will run quickly but will use over 3,000,000 bytes of memory.
    • If you are re-using blocks, sample6 should run quickly when a linked list of free nodes are maintained (ex: TIME less than about 0.1s).
    • If you are re-using blocks but are NOT maintaining a linked list of free nodes, sample6 will run slower (TIME greater than 1s).
    • (The speed of your system will impact the speed, but there will be a significant speed difference.)

Real Programs

Both mstats and mreplace can be used to launch “real” programs (not just the testers). For example:

# ignore the warning about an invalid terminal, if you get it
./mreplace /usr/bin/less alloc.c


./mstats /bin/ls

You can even run your version of MP2 using your malloc! For example:

./mstats ../mp2/png-extractGIF PATH_TO_PNG


The MP is worth a total of 50 points, and the points are split in the following way:

  • Implement a basic memory allocator, 10 points
  • Implement block splitting, 15 points (see sample1)
  • Implement memory coalescing, 15 points (see sample{2,3,4})
  • Implement free lists, 10 points (see sample{5,6})

Extra Credit

You can earn extra credit in two different ways for this MP!

  1. Submitting Early: We will grade the files you submit by Monday, Feb. 22 at 11:59pm for extra credit:

    • If your program is a working basic memory allocator, earn +1 EC point
    • If your program implements memory coalescing, earn +2 EC points
    • If your program implements block splitting, earn +2 EC points
  2. Passing Advanced Tests: On your final submission (Mar. 1), we will run some advanced tests on your code:

    • The testers directory contains many advanced tests that do really mean things to your memory allocator in terms of the number of requests, the sizes of the requests, and more. Earn +1 EC for each of tester{1-5} that successfully run using ./mstats within 30 seconds.


When you have completed your program, double-check all four functions run without errors and gets the result your expect. When you are ready, submit the code via the following git commands:

git add -A
git commit -m "MP submission"
git push origin master

You can verify your code was successfully submitted by viewing your git repo via the web interface here: