Assignment 6. Low-level refactoring and performance

Introduction

This assignment is designed to give you some skills with low-level programming, which is used in later courses like the operating system class, as well as in real-world applications like the Internet of things (IoT). You’ll start with a working program; you’ll add a few features, and tune and refactor the program to make it better.

Note: Use a private local Git repository (not a repository host like GitHub) to keep track of your work in this assignment when you’re modifying code, data, or notes.txt. Don’t put big output files into your repository; use it only for sources that you maintain by hand.

Useful pointers

• Jeremy W. Sheaffer et al, C tutorial for C++ programmers (2022)
• Debugging with gdb (2022)
• Valgrind Quick Start Guide (2022)
• Valgrind User Manual (2022)

Homework: Tuning and refactoring a C program

Keep a log in the file notes.txt of what you do in the homework so that you can reproduce the results later. This should not merely be a transcript of what you typed: it should be more like a true lab notebook, in which you briefly note down what you did and what happened.

You’re trying to generate large quantities of random numbers for use in a machine-learning experiment. You have a program randall that can generate random byte streams, but it has problems. You want it to be (a) faster and (b) better-organized.

You can find a copy of a repository for the randall source code in the tarball randall-git.tgz. Unpack that tarball, clone the resulting repository, and look at the resulting source code. It should contain:

• A .gitignore file listing built files that Git should ignore (and that you should not commit). Feel free to update this file if it’s incomplete for your work.
• A COPYING file giving license permissions for this program.
• A Makefile that can build the program randall and can build the tarball.
• A file randall.c that is a single main program, which you would like to modularize.

Add notes.txt to your clone of the repository, and commit changes to it as needed while you work on this assignment.

Read and understand the code in randall.c and in Makefile.

Modify the Makefile so that the command 'make check' tests your program. You can supply just a simple test, e.g., that the output is the correct length. You’re doing this step first because you believe in test-driven development (TDD).

Next, split the randall implementation by copying its source code into the following modules, which you will need to likely need to modify to get everything to work:

• options.c with header options.h, which does command-line options processing. Initially there isn’t very much of this.
• output.c with header output.h, which does the output.
• rand64-hw.c with header rand64-hw.h, which does the hardware-based random-number generation.
• rand64-sw.c with header rand64-sw.h, which does the software-based random-number generation.
• randall.c should contain the main program that glues together everything else. It should include the .h files mentioned above, and use their APIs to do its work.

You may add other modules if you like. Each module should #include only the files that it needs; for example, since rand64-hw.c doesn't need to do I/O, it shouldn't #include <stdio.h>. Also, each module should keep as many symbols private as it can.

Next, modify the Makefile to compile and link your better-organized program.

Next, add some options to your program to help you try to improve its performance. Redo the program so that it has an option '-i input', where input is one of the following:

• rdrand – the hardware random-number generation supported by x86-64 processors if available; randall should issue an error message and fail if it is not available. This option is the default.
• lrand48_r – the lrand48_r function of the GNU C library.
• /F (i.e., any argument beginning with /) – use the file /F as a source of random data, instead of using /dev/random.

Also, redo the program so that it has an option -o output, where output is one of the following:

• stdio – Use stdio output, as before. This is the default.
• N (a positive decimal integer) – Output N bytes at a time, using the write system call. If the write call reports a successful write of fewer than N bytes, do not consider this to be an error; just add the successfully-written number of bytes to your count of bytes written. The last output block might be smaller than usual, if needed to get the total size to be correct. You can use the malloc function to allocate your output buffer.

You can use getopt to implement your option processing.

Add some 'make check' tests to check your additions to randall.

When debugging, you may find the valgrind program useful. Also, the AddressSanitizer (asan) and the Undefined Behavior Sanitizer (ubsan) may be useful; these can be enabled with the GCC options -fsanitize=address and -fsanitize=undefined, respectively.

If the program encounters an error of any kind (including option, output and memory allocation failures), it should report the error to stderr and exit with status 1; otherwise, the program should succeed and exit with status 0. The program need not report stderr output errors.

Finally, time your implementation as follows ...

    # This is a sanity check to test whether you’re in the right ballpark.
    time dd if=/dev/urandom ibs=8192 obs=8192 count=16384 >/dev/null

    time ./randall 133562368 >/dev/null
    time ./randall 133562368 | cat >/dev/null
    time ./randall 133562368 >rand.data

... except that you may need different numbers if your implementation is faster or slower. Also you should try various combinations of the above options to see which gives you random data the fastest. One option that you should try is '-i /dev/urandom'.

Record your results (including your slow results) in notes.txt.

Submit

Submit two files:

1. The file randall-submission.tgz, which you can build by running the command "make submission-tarball". Test your tarball before submitting it, by extracting from it into a fresh directory and by running 'make check' there.
2. The file randall-git.tgz, which is a gzipped tarball of your private local Git repository and configuration, created by the command "make repository-tarball".

Neither submitted file should be all that large, since it should contain only information about source files maintained by hand, as opposed to generated files.

All source files should be ASCII text files, with no carriage returns, and with no more than 100 columns per line. The shell command

expand Makefile notes.txt *.c *.h |
  awk '/\r/ || 100 < length'

should output nothing.