Program in C to recover JPEGs from a memory card. CS50 Problem Set 4.
In anticipation of this problem, we spent the past several days taking photos around campus, all of which were saved on a digital camera as JPEGs on a memory card. Unfortunately, we somehow deleted them all! Thankfully, in the computer world, “deleted” tends not to mean “deleted” so much as “forgotten.” Even though the camera insists that the card is now blank, we’re pretty sure that’s not quite true. Indeed, we’re hoping (er, expecting!) you can write a program that recovers the photos for us!
Write a program to recover JPEGs from a memory card.
Even though JPEGs are more complicated than BMPs, JPEGs have “signatures,” patterns of bytes that can distinguish them from other file formats. Specifically, the first three bytes of JPEGs are
0xff 0xd8 0xff
from first byte to third byte, left to right. The fourth byte, meanwhile, is either 0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, or 0xef. Put another way, the fourth byte’s first four bits are 1110.
Odds are, if you find this pattern of four bytes on media known to store photos (e.g., my memory card), they demarcate the start of a JPEG. To be fair, you might encounter these patterns on some disk purely by chance, so data recovery isn’t an exact science.
Fortunately, digital cameras tend to store photographs contiguously on memory cards, whereby each photo is stored immediately after the previously taken photo. Accordingly, the start of a JPEG usually demarks the end of another. However, digital cameras often initialize cards with a FAT file system whose “block size” is 512 bytes (B). The implication is that these cameras only write to those cards in units of 512 B. A photo that’s 1 MB (i.e., 1,048,576 B) thus takes up 1048576 ÷ 512 = 2048 “blocks” on a memory card. But so does a photo that’s, say, one byte smaller (i.e., 1,048,575 B)! The wasted space on disk is called “slack space.” Forensic investigators often look at slack space for remnants of suspicious data.
The implication of all these details is that you, the investigator, can probably write a program that iterates over a copy of my memory card, looking for JPEGs’ signatures. Each time you find a signature, you can open a new file for writing and start filling that file with bytes from my memory card, closing that file only once you encounter another signature. Moreover, rather than read my memory card’s bytes one at a time, you can read 512 of them at a time into a buffer for efficiency’s sake. Thanks to FAT, you can trust that JPEGs’ signatures will be “block-aligned.” That is, you need only look for those signatures in a block’s first four bytes.
Realize, of course, that JPEGs can span contiguous blocks. Otherwise, no JPEG could be larger than 512 B. But the last byte of a JPEG might not fall at the very end of a block. Recall the possibility of slack space. But not to worry. Because this memory card was brand-new when I started snapping photos, odds are it’d been “zeroed” (i.e., filled with 0s) by the manufacturer, in which case any slack space will be filled with 0s. It’s okay if those trailing 0s end up in the JPEGs you recover; they should still be viewable.
Now, I only have one memory card, but there are a lot of you! And so I’ve gone ahead and created a “forensic image” of the card, storing its contents, byte after byte, in a file called card.raw. So that you don’t waste time iterating over millions of 0s unnecessarily, I’ve only imaged the first few megabytes of the memory card. But you should ultimately find that the image contains 50 JPEGs.
Implement a program called recover that recovers JPEGs from a forensic image.
- Implement your program in a file called recover.c in a directory called recover.
- Your program should accept exactly one command-line argument, the name of a forensic image from which to recover JPEGs.
- If your program is not executed with exactly one command-line argument, it should remind the user of correct usage, and main should return 1.
- If the forensic image cannot be opened for reading, your program should inform the user as much, and main should return 1.
- The files you generate should each be named ###.jpg, where ### is a three-digit decimal number, starting with 000 for the first image and counting up.
- Your program, if it uses malloc, must not leak any memory.
You can run the program by running:
./recover card.raw
Let's start to use this project.
A compiler for C must be installed
To execute the project open the terminal and go to the project folder. Then compile the code with a C compiler and execute the file generated:
make recover
./recover card.raw