C. CS50 recover assignment, recovering Jpg images - c

Question

I have a question about how to recover jpg images (it's an assignment for CS50).
My code works for the most part (I believe), however I only get a bunch of thumbnails when I open the jpg's I found.
I've been trying to solve this exercise for quite some time now but I can't seem to figure out why it doesn't work. Could somebody give me a push into the right direction.
Here is my code (also available at http://pastebin.com/U2pwJd5e):
#include <stdio.h>
#include <stdlib.h>
#include "bmp.h"
int main(int argc, char* argv[])
{
//get input file
char* infile = "card.raw";
// open card file
FILE* inptr;
inptr = fopen("card.raw", "r");
// error checking (copied from copy.c)
if (inptr == NULL)
{
printf("Could not open %s.\n", infile);
return 2;
}
// initialize buffer
BYTE buffer[512];
//initialize jpg variables:
int increment = 0;
char outfilename[8];
// while the end of the file is not reached, continue process & write to buffer next block of 512 bytes
while (fread(buffer, 512, 1, inptr) != 0)
{
// if the inpointer is not empty
if(inptr != NULL)
{
// If the block of 512 bytes starts with markers
if(buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3]== 0xe1 || buffer[3]== 0xe0))
{
// increase file number by 1
sprintf(outfilename,"%.3d.jpg", ++increment);
// open new file
FILE* outptr;
outptr = fopen(outfilename, "a");
// write first block of 512 bytes, then read next block
fwrite(buffer, 512, 1, outptr);
if(fread(buffer, 512, 1, inptr) == 0)
break;
// copy all information from inpointer to buffer to jpg
while((buffer[0] != 0xff && buffer[1] != 0xd8 && buffer[2] != 0xff && (buffer[3]!= 0xe1 || buffer[3]!= 0xe0) ))
{
// if next byte is NULL break
if(fread(buffer, 512, 1, inptr) == 0)
break;
fread(buffer, 512, 1, inptr);
//copies jpg file 1 byte at a time
fwrite(buffer, 512, 1, outptr);
}
// close file
fclose(outptr);
}
}
}
return 0;
}

Answer 1

There's a lot of missing information in your problem but I can see a few potential problems:
You only check for the JPEG file at the start of each 512 byte block. Unless you are guaranteed that this is the case you should probably check for start of JPEG files in the entire memory block.
You only check for JPEG files with a FFD8FFE1 or FFD8FFE0 start. What if the second block in the JPEG is not FFE1/FFE0?
The following check in your second if block is not correct:
(buffer[3] != 0xe1 || buffer[3] != 0xe0)
This is always true as buffer[3] can't be both 0xE1 and 0xE0 at the same time. This should be:
(buffer[3] != 0xe1 && buffer[3] != 0xe0)
Your checking for the end of the JPEG image probably doesn't do what you wanted:
while ( buffer[0] != 0xff && buffer[1] != 0xd8 &&
buffer[2] != 0xff && buffer[3] != 0xe1 && buffer[3]!= 0xe0 )
This ends the JPEG when you find any of those values at the start of the 512 byte block. For example the bytes 01DB0203 would end the JPEG since buffer[1] != 0xd8 is false, even though this is not a JPEG block marker.
I would think that finding the end of a JPEG file would require you to search the entire memory block for the FFD9 byte marker which signifies the end of a JPEG file. If I understand the JPEG format correct a FFD9 byte combination can only occur at the end of a valid JPEG file.
If you still run into issues I would create a test file composed of several known JPEG files and other data. You can then directly compare what is being output to what you know should be output to narrow down where/what is causing the problem.