1 Initial Files

mp3.zip contains header files, testers, and a Makefile. You will add to the following files:

• pngchunklist.c
• extractuiucchunk.c
• insertuiucchunk.c
• pnglib.c and pnglib.h (optional, provided to allow sharing code between the other three files)

The first three of these will be command-line application; we provide the basic command-line argument count checking for you and you write the rest. The pnglib files will not be directly tested, but can be used to keep your code organized.

These initial files provide very little structure. We have a general guide on coding from blank files that might be helpful to some of you.

A learning goal of this MP is working without us giving you a design or guide to getting started on this MP specifically. Please don’t ask course staff to bypass that goal by telling you how to design your code.

2 Create application pngchunklist

This application lists the type and size of each chunk in the PNG file in the order that they appear in the file. The format of this should be one printed-out line per chunk, each with the type, a space, and the length.

You can implement it in whatever way you wish, provided that you write all the code yourself without copying from any AI system or third party. We recommend using the following libraries:

• fopen
• fseek
• fread
• ntohl
• printf or an alternative display function. Remember that if you are printing a string it needs to be null-terminated; for example a 4-byte string is stored in 5 bytes, with the last one being 0.

Coding this application shouldn’t require malloc and should fseek past most of the bytes in the file.

3 Create application extractuiucchunk

This application looks for a special chunk with type uiuc; if found, it copies its contents into a separate file. If there is more than one such chunk, it copies the first and then exits with code 0. If there is no such chunk, prints an error message to stderr and exists with code 4.

You can implement it in whatever way you wish, provided that you write all the code yourself without copying from any AI system or third party. We recommend sharing significant code from pngchunklist (ideally by putting the chared code in library functions in pnglib.c and pnglib.h), and additionally using the following libraries:

• memcmp (or strncmp or the like)
• fwrite
• malloc and free – not strictly required, but several simple approaches to the task would need them.

4 Create application insertuiucchunk

Makes a copy of a PNG file with a new uiuc chunk. If the input PNG file already has a uiuc chunk, replaces it. If not, adds it between two existing chunks.

You can implement it in whatever way you wish, provided that you write all the code yourself without copying from any AI system or third party. We recommend sharing significant code with the other two applications (ideally by putting the shared code in library functions in pnglib.c and pnglib.h).

You’ll also need to generate a crc32 checksum; this is a computation based in coding theory which most CS programs either don’t teach at all or only teach it in an elective course. Perhaps that is why the code you need is given in the PNG specification in Appendix D. Note there are three functions in that appendix: the first to be run just once, the second to be given bytes of a chunk (either all at once or a bit at a time, it makes no difference), and the third a reference showing how you’d use the second if you have the entire chunk in one array of unsigned characters. Also note that the appendix is somewhat old and assumes that long means a 32-bit integer; currently int (or int32_t from #include <stdint.h>) is the type with that size.

5 Use insertuiucchunk

Put a secret payload inside a PNG of your choice and share it on the CampusWire post MP3 Image Share. This is an opportunity to share a bit about yourself and your interests; please keep it (both the image and its hidden contents) inoffensive.

6 Submission and Grading

Submit on the submission site. Submit pngchunklist.c, extractuiucchunk.c, and insertuiucchunk.c; if you changed them, also submit pnglib.c and pnglib.h. Do not change or submit the Makefile, tests, or example images.