Thread: Recovering jpeg images from a .raw file

Hi again,

I am working on a HOMEWORK assignment to recover jpg images from a .raw file. As this is something that I will be graded on, I am NOT asking you do "do it all for me" I am asking for hints and help so that I may continue to learn (and get a good grade at the same timehttp://im.cprogramming.com/images/smilies/wink.png).

The assignment I am currently working on involves recovering a jpg image by looking for the start of a jpeg, and then copying 512 byte chunks to an image file until the start of the next image is seen. I believe I am close to having a fully working program, The part I am stuck on is when I run the program I get a Segmentation fault.

If you could look over my program (below) and give me your opinion on what is wrong; that would be GREAT!!!

Code:

#include <stdio.h>
#include <stdlib.h>


#define false 0
#define true 1


typedef int bool;
typedef unsigned char BYTE;


int main(void)  {


FILE*outptr;


// open input file 
FILE* inptr = fopen("card.raw", "r");
if (inptr == NULL)
{
    printf("Sorry, me noes able to open the file!!!\n");
    return 2;
}


bool jpegfoundbefore = false;


int number = 0;


int block_size = 512;


BYTE buffer [512];


char image_name[8];


// untill eof is reached
while (! feof(inptr) == 0)
{
     //copy 512 bytes into buffer
     fread(buffer, block_size, 1, inptr);
     


     {
        // if a previous jpeg was found close file 
            if (jpegfoundbefore == true)
            {
                fclose(outptr);
            }


            jpegfoundbefore = true;
 
            // name the image
            sprintf(image_name, "%.3d.jpg", number);
            number++; 
            
            // create the new jpg image file
            outptr = fopen (image_name, "w");
            if (outptr == NULL)
            {
                printf("could not create jpeg file\n");
                return 2;
            }     
            
            // write 512 byte block to current jpeg file 
            fwrite(buffer, block_size, 1, outptr);        
      }


      // continue writing parts of a jpg to file on disk 
      if (jpegfoundbefore == true)
      {
          fwrite(buffer, block_size, 1, outptr);        
      } 
 
 
}


// close card.raw 
fclose(inptr);


// close last jpg
fclose(outptr);


}

How would I check the return value of fread() and would it fix my problem??? Also, I see why it might be bad to use feof, but wont it work???

Ok, so I ran it through gdb and this is what I got:

Program received signal SIGSEGV, Segmentation fault.
0xb7e5700e in _IO_new_fclose (fp=0xb7fa0ff4) at iofclose.c:66
66 _IO_FINISH (fp);




So the problem is (or at-least starts on) line 66 which is : outptr = fopen (image_name, "w");

I am not sure what is wrong, although I at-least know where the problem is. If you could take a look at it and give me your opinion that would be great!

Also, a few people gave me advice about checking the return value of fread... Is this something that I need to do? If so, how would I do it, I seem to have gotten conflicting answers from other people, so I am asking again.

Originally Posted by Dude22

Ok, so I ran it through gdb and this is what I got:

Program received signal SIGSEGV, Segmentation fault.
0xb7e5700e in _IO_new_fclose (fp=0xb7fa0ff4) at iofclose.c:66
66 _IO_FINISH (fp);

Is your file named "iofclose.c"? That's where your program finally crashes but I'm pretty sure that's a library file and not yours.

Salem told you already about the bt command (for backtracing). This will show you all frames which are active at the time of the crash. You have to find the frame where your program called the library function.

Here are some links:
GDB, The GNU Debugger, By Example - Cprogramming.com
Tutorials - An Introduction to GDB - Cprogramming.com
RMS's gdb Tutorial: Segmentation Fault Example

Also, a few people gave me advice about checking the return value of fread... Is this something that I need to do? If so, how would I do it, I seem to have gotten conflicting answers from other people, so I am asking again.

Yes it is very important to check the return value of fread()
Recovering jpeg images from a .raw file

Bye, Andreas

Ok, so I am working on rewriting the program with what I have learned, and my question is, what should I do with the return value of fread.... I know that It returns the number of successfully read elements, but how and where would that help???

Also, if you have any other recommendations for improvement on my old version, let me know!

Josh

The 2nd and 3rd parameters of both the fwrite and fread should be swapped about.

Originally Posted by stahta01

You are supposed to only fwrite the number of bytes fread!

Tim S.

In the course instructions they say that:

"The implication of all these details is that you, the investigator, can probably write a program that iterates over a copy of my CF 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 CF card, closing that file only once you encounter another signature. Moreover, rather than read my CF 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 CF 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."

If I am not mistaken, the only advantage to using the return value of fread to only fwrite the number of bytes read would be that if the extra area at the end of the 512 chunk contained something other then 0's it wouldn't cause any issue. But as they state that the extra space will be all 0's and they say that it wont cause any problems if that is in the recovered images. Is there still a need to : "only fwrite the number of bytes fread"