This is not the current version of the class.

Problem set 4: Stdio

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

Get the code

Get our code with

$ git pull; git pull handout main

or, alternately

$ git pull; git pull main

This will merge our Problem Set 4 code with your previous work. If you have any “conflicts” from prior problem sets, resolve them before continuing further. Run git push to save your work back to your personal repository.

You may also create a new cs61-psets repository for this assignment. Don’t forget to enter your repository URL on the grading server.


Our simple IO61 library performs I/O on files. You will find the code in the pset directory in the file Our version of IO61 is pretty stupid—it uses byte-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, and it has tests that use our handout programs in different ways.)

And there are others.

You will introduce caching to the io61_file abstraction and use your cache to speed up IO61 operations. We’re giving you tons of freedom to implement the cache 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:

Code will be graded based on performance on the grading server.

All your code for the main pset should fit in


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 in (You can build a stdio test with make stdio-cat61 and so forth.)

Run make check to check your current implementation on a battery of tests and print summary statistics at the end. The tests are divided into groups:

Run make check-TESTS (e.g., make check-seqperf or make check-seqperf1-3) 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 matters most. A program that runs quickly but incorrectly is worse than a program that runs slowly but correctly.


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.

Phase 1

You can improve the way io61_read and io61_write work without introducing a cache. Use strace to investigate the operation of blockcat61. Look at the blockcat61 code; what system calls do you think it should make? Then run strace -o strace.out ./blockcat61 (That command line tells strace to write its output to strace.out, so examine strace.out with a command like less strace.out.) What system calls does blockcat61 actually make?

This will improve your performance on the block I/O tests (e.g., make check-mseq2). Also check that your code still passes the correctness tests (make check-c).

As you work on improving the performance of your code on sequential access patterns, you may want to ignore non-sequential access patterns. This will mean your code will produce incorrect results for non-sequential tests like C12, C13, and C14. That’s OK as long as you fix it later!

Phase 2

Implement a single-slot cache buffer for sequential reads. This will hold bytes [N, N+B) of the file, where B is some largish number (try different numbers). Any read request that lies within that range of bytes can be satisfied without making a system call. Check your work by examining strace output.

This will improve your performance on byte I/O tests (e.g., make check-mseq1).

After you've implemented your cache buffer for reading, consider a special case for reading one character at a time from your cache in io61_readc. Can we avoid calling io61_read if we only want to read a single byte?

Phase 3

Extend your single-slot cache buffer to also support sequential writes.

This will improve your performance on byte I/O tests. If you do this well, you should now match or beat stdio’s performance on all sequential I/O tests!

Phase 4

Fix your code to handle seeks correctly (but not necessarily in a high-performance way).

This should make your code produce correct results for all tests.

Phase 5

Change your code to continue handling seeks correctly, but with good performance for reverse61. For ideas, try running strace on the stdio-reverse61 variant. What does it look like stdio is doing?

This should make your code both roughly equal stdio’s performance and produce correct results on all tests.

Phase 6

Finally, it’s time to outperform stdio on some tests! Perhaps a bigger cache buffer will outperform stdio; perhaps memory-mapped files will help. If you have worked through the previous phases, you will have ideas of your own.

Hints and troubleshooting

Read our file descriptors notes if you are confused about file descriptor system calls.

You will likely need to change most of the functions in

Stdio is a well-written package! It is OK if you can’t always beat it, especially for sequential I/O.

Write your own tests! This will help you shake out bugs in your code, particularly correctness bugs. What diabolical things can you think of to try? You can add new tests pretty easily; just edit GNUmakefile and

If a program using your library produces incorrect results, it can be hard to figure out whether the problem is with reads, writes, or both. For instance, perhaps your read cache is wrong, so the program is reading incorrect data; or perhaps the write cache is wrong, so the the program is trying to write correct data, but the data passed through to the kernel via system calls is wrong. To distinguish these scenarios, you can use strace to check things like file positions. You can also use the read61, write61, blockread61, and blockwrite61 programs. These behave like cat61 and blockcat61, but they use stdio for half their tasks. The read variants use IO61 for reading and stdio for writing, whereas the write variants use IO61 for writing and stdio for reading. If cat61 and write61 produce bad results but read61 on the same workload produces correct results, you can be pretty sure the problem is with writes.

You may make a couple assumptions:

Add assertions in your code, since they can help you check correctness. However, assertions do take time to check, which could leave your code at a disadvantage relative to stdio. We will check the performance of your code with assertions disabled by running make NDEBUG=1 check-TEST.

If your code is slow, consider (1) the system calls it makes and (2) its CPU performance. For system calls, use strace to check: Is a mistake in your code accidentally causing expensive system calls, such as many read calls that read one byte each? For CPU performance, consider whether there are functions that can avoid some extra work. It might pay off to have specialized io61_readc and io61_writec calls, for example.

Make targets

Your code must work correctly with sanitizers turned on (though it will not be very fast). Run make SAN=1 check to check for errors.

To get an strace log for your code on a specific test, use make STRACE=1 check-TEST. The strace output will be stored in strace.out.

To run without running the stdio tests, use make NOSTDIO=1 check.

To run with one trial rather than multiple trials, use make TRIALS=1 check.

To compile without optimization (which might help you debug), run make O=0.

To disable assertions, use make NDEBUG=1 check.

Going beyond

If you feel ambitious, you might try changing your code to use memory-mapped I/O when this is possible. Memory-mapped I/O is not always possible, though, so your old block cache should be used for pipe files and other non-mappable files. This should make your code beat stdio on some non-sequential I/O tests!

If you have additional time and feel even more ambitious, you can implement an associative cache that holds multiple blocks and works for (a) non-mappable files or (b) very large memory-mapped files. Something like this will get the highest performance.

It is difficult and rewarding to implement an associative cache. You’ll learn a lot! But design carefully so you don't make your other code more difficult to read and maintain. In fact, you might try working on a separate branch initially.


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.