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Problem set 3: Caching and File I/O

In this problem set, you’ll gain experience with caching by writing your own buffered I/O library.

Get the code

Our code is released in the cs61-psets repository. To update it, run

git remote show handout

If this reports an error, run

git remote add handout git://

Then run

git pull handout master

This will merge our handout code with your previous work. If you have any “conflicts” from problem set 1 or 2, resolve them before continuing further. Run git push to save this merge back to your private repository.

Use an explicit merge. If you copy code by hand, our automated scripts will have trouble analyzing your code, and it’ll be harder for you to incorporate our updates.

You may also create a new cs61-psets repository for this problem set. However, you will need our help: some issues with our use of GitHub Classroom make it difficult for you to create a new repository yourself. Write for help. Tell us (1) your GitHub username and (2) the name of the repo you want to create. Once the repo is ready, make sure you update the grading server.


We’ve released a simple library, IO61, to perform I/O to/from files. You will find the code in the pest directory in the file io61.c. Our version of IO61 is pretty stupid—it uses character-at-a-time system call I/O and is thus quite slow. Your goal is simple: speed it up using caching.

We also distribute several programs that use IO61. (The grading server has some secret extra programs too!)

Introduce buffering (that is, caching) to the io61_file abstraction and use them to speed up IO61 operations.

We’re giving you tons of freedom to implement the file caches as you like. You may even use memory-mapped I/O, prefetching system calls like madvise or posix_fadvise, or multiple threads or processes (although none of these are required). But you may not use another buffered I/O library or caching I/O library.

Your library should:

All your code should fit in io61.c.


We will evaluate you based on your code’s performance relative to a version of IO61 that uses stdio. That version is provided for you for testing in stdio-io61.c (and the makefile builds stdio-cat61, stdio-blockcat61, and so forth).

Run make check to see how your current implementation is doing on a battery of tests. This will also print summary statistics at the end. Run make check-TESTNUMBERS (e.g., make check-9 or make check-5-10 or make check-1,2,3) to run selected tests.

We may update the tests during the pset release period. We may also run additional tests during grading. In all instances, correctness counts—that is, a program that runs quickly but incorrectly is worse than a program that runs slowly!


It’s easy in this problem set to design something too complex and get yourself stuck. Avoid that problem by tackling the simple cases first. Here’s a possible roadmap.


You are allowed to make a couple assumptions.


If you want to compile without optimization (which might help you debug), run make O=0 or edit the GNUmakefile to set O = 0 by default. Your code should work safely with sanitizers turned on; try make SANITIZE=1 check.

Extra credit: Deadlock

We also release a program, pipeexchange61, that demonstrates a problem with conventional blocking I/O. The pipeexchange61 program forks two copies of itself. One copy, the requester, sends requests to the other copy, the responder. The requester can send many requests back-to-back and only then wait for responses (a form of batching). When you run pipeexchange61, the program appears to get stuck! Why? Can you construct an I/O library that unsticks pipeexchange61, without modifying pipeexchange61.c?


You will turn in your code by pushing your git repository to and updating the grading server with your repository.

Don’t forget to fill out and

This pset was originally created for CS61.