pset4 recover: read bytes does not match to the signature and seg fault

Question

I've spent hours and hours on this code but I can't get any picture out of raw file. I tracked the problem with gdb and figured that when the it reaches to place to test for signature of jpeg it does not match with the condition. So I printed the temp[0] value many times in almost 5 iterations but non was ff. I don't get where the problem is, but something should be wrong with the placeholder of the infile. Can anyone help me please? It's getting really frustrating.

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

// prototypes
int fsize(FILE *fp);

int main(int argc, char* argv[])
{
    if (argc != 1)
    {
        printf("Usage: recover currupted-file");
        return 1;
    }

    // opening the corrupted file
    FILE* infile = fopen("card.raw", "r");
    if (infile == NULL)
    {
        return 1;
    }

    // declaring and initializing output filename
    int filenumber = 0;
    char* filename = malloc(8 * sizeof(char));
    FILE* outfile = NULL;

    // allocating memory to temp read variable
    uint8_t temp[512];

    //declaring a variable for file size
    long filesize = fsize(infile);

    for (int i = 0; i < filesize; i += 512)
    {
        // reading every 512 Bytes to a temp
        fread(&temp, sizeof(temp), 1, infile);

        if (temp[0] == 0xff && temp[1] == 0xd8 && temp[2] == 0xff && (temp[3] == 0xe0 || temp[3]  == 0xe1))
        {
            if (filenumber > 0)
            {
                fclose(outfile);
            }
            sprintf(filename, "%3i.jpg", filenumber);
            FILE* outfile = fopen(filename, "w");
            if (outfile == NULL)
            {
                return 1;
            }
            filenumber++;
            fwrite(temp, 512, 1, outfile);
        }
        else if (filenumber > 0)
        {
            fwrite(temp, 512, 1, outfile);
        }
    }
    free(filename);
    fclose(outfile);
    fclose(infile);
}

// file size finder code
int fsize(FILE *fp)
{
    //find current place holder to return to later
    int last=ftell(fp);
    fseek(fp, 0, SEEK_END);
    int size=ftell(fp);
    //go back to where we were
    fseek(fp,last,SEEK_SET);
    return size;
}

Answer 1

You aren't seeing a match in the first several iterations because you have to run the loop many thousands of times before the first signature is detected.

However, the problem is elsewhere. You are hitting a seg fault because once you do detect the signature and create the file, your file pointer is disappearing before you can write the next block of data to the file.

The real problem is with these two lines:

FILE* outfile = NULL;
...
FILE* outfile = fopen(filename, "w");

You have declared the file pointer outfile twice. Since the second one is contained within a pair of curly braces, the second one supercedes the first one. It has scope within those curly braces. So, the second one points at the file when it is created, but the pointer disappears when you exit that if statement code block. Now, the original pointer, which points to NULL, becomes active again. When you try to write the second block of data, it seg faults.

The fix is simple. Remove 'FILE*' from the second declaration so that you are using the original one.

You have other issues, but that wasn't your question. Have fun! ;-)

If this answers your question, please click the check mark and accept. Let's keep up on forum maintenance. ;-)