Description

Assignment 1: Reading Data from Portable Network Graphics Files
In a source file called pngq.c write a C program that reads and extracts information from a Portable Network Graphics file (PNG). Use the W3C documentation of the PNG format as a reference specification. The general usage of the pngq program is as follows: pngq [options] [–] file1 [[options] file2 …]
This means that the program takes, on the command line, zero or more options and one or more file names. An optional “–” argument (without quotes) indicates that all subsequent arguments are to be interpreted as file names. For each file name, the program must read and validate the file. That is, the program must reject an invalid PNG file with a specific error message printed on the standard error. A PNG is considered invalid if it does not conform with the specification, and also if any of the CRC check-sums does not check.
When the file is valid, the output is determined by three format strings as explained below. These are the PNG-file format or pformat, the chunk format or cformat, and the keywords format or kformat. They can be set with a command-line argument p=pformat, c=cformat, or k=kformat, respectively.

pformat “_f: _w x _h, _c, _d bits per sample, _N chunks _C”
cformat “ _n: _t (_l) ”
kformat “ _k: _t ”

Table 1: Default format strings
Output Format
For each file f, the program must output information following the currently set pformat string. In the format string, every character must be copied onto the output verbatim, except for the underscore character, which introduces place-holders for information on the file. The information must be derived from the chunks that make up the PNG file, including in particular the IHDR chunk. The following placeholders are defined for a pformat string:
• _f print the file name as passed on the command line
• _w print the width of the PNG image
• _h print the height of the PNG image
• _d print the bit depth of the PNG image
• _c print the color type class as a string. The values are “Greyscale”, “Truecolor”, “Indexed”, “Greyscale+alpha”, and “Truecolor+alpha”.
• _N print the number of chunk the image consists of.
• _C print the chunks that make up the PNG image file, each formatted according to the cformat
• _K print the keywords and corresponding text, as specified in tEXt chunks, formatted according to the kformat string.
The following placeholders are defined for a cformat string:
• _n chunk sequence number (starting from 1)
• _t chunk type, four-character type identifier (e.g., “IDAT”)
• _l length of the chunk (in bytes)
• _c CRC32 checksum for the chunk
• _D chunk data. Print each byte of the content of the chunk as an hexadecimal two-digit number. Byte values on the same line are separated by a single space. A line consists of at most 16 hexadecimal values (bytes).
The following placeholders are defined for a kformat string:
• _k keyword
• _t corresponding text
In all format strings, any character following an underscore other than those specified for the place-holders is printed verbatim on the output. So, for example, __ print a single underscore character (“_”).
Examples
With a single file 1.png as command-line argument, the output should be something like the following:
1.png: 3383 x 1949, Truecolor, 8 bits per sample, 36 chunks
1: IHDR (13)
2: pHYs (9)
3: IDAT (8192)
4: IDAT (8192)
5: IDAT (8192)
6: IDAT (8192)
7: IDAT (8192)
8: IDAT (8192)
9: IDAT (8192)
…
35: IDAT (2784)
36: IEND (0)
The following command processes the same single file 1.png with the given cformat option. Notice that new-line characters are given directly in the single-quoted argument.
$ ./pngq c=’_n: _t (_l)
_D
’ 1.png
In this case, the _D placeholder prints the data content of each chunk. The output is as follows:
1.png: 3383 x 1949, Truecolor, 8 bits per sample, 36 chunks
1: IHDR (13)
0 0 d 37 0 0 7 9d 8 2 0 0 0
2: pHYs (9)
0 0 e c4 0 0 e c4 1
3: IDAT (8192)
78 5e ec dd 7f 74 14 f7 7d ef ff 77 62 4b a3 20 6b 24 87 1d b9 91 56 58 f2 62 49 ec 1a 7d 59 62 b0 88 64 2f d0 cb 8a 24 16 86 7e 45 ec 46 f4 ab 46 71 68 94 13 72 39 df d2 e2 94 ef c5 d 3d f8 9a 5e dc c3 a9 72 4a 4b 1d f5 aa 45 8e 5d eb d6 d8 4a 1d bc 4e 8d 65 8b b0 b6 29 eb a 6b 91 88 d7 52 61 25 37 da 55 40 a3 8 6b 56 4e fb fd 63 57 d2 ee 68 f5 b 10 c6 f0 7c 1c 9f 63 f1 f9 cc cc ce 8c 4 e7 cc 5b af 79 7f 3e 73 f7 dd 85 69 69 8a 0 98 c9 c8 88 21 22 86 31 62 9e 0 f0 e9
…