Recover: Segmentation fault

Question

Everytime I try to run this code, it gives me a segmentation fault error. What is wrong with my logic?

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

int main(int argc, char *argv[])
{
    // Ensures proper usage
    if (argc != 2)
    {
        fprintf(stderr, "Usage: ./recover card.raw\n");
        return 1;
    }

    // Remembers arguments
    char *raw_image = argv[1];

    // Opens image
    FILE *raw_file = fopen(raw_image, "r");
    if (raw_file == NULL)
    {
        fclose(raw_file);
        fprintf(stderr, "Could not open %s.\n", raw_image);
        return 2;
    }

    // Counts JPEGs
    int jpegCounter = 0;

    unsigned char buffer[512];

    // Stores JPEG names
    char jpegs [10];

    // Creates output file for JPEGs
    FILE *img = NULL;

    while (1)
    {
        // Reads card.raw
        fread(&buffer, 512, 1, raw_file);

        // Checks if JPEG
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && 
        (buffer[3] & 0xf0) == 0xe0)
        {
            // Names JPEGs by number
            sprintf(jpegs, "%03i.jpg", 2);

            // Opens found jpeg
            img = fopen(jpegs, "w");
            fwrite(&buffer, 512, 1, img);

            // Adds to counter
            jpegCounter++;
        }
        else
        {
            fwrite(&buffer, 512, 1, img);
        }

        // on end of file terminate program
        if (feof(raw_file))
        {
            fclose(raw_file);
            fclose(img);
            return 0;
        }
    }

    // close file
    fclose(raw_file);

    // Success!
    return 0;
}

Answer 1

First, you should always isolate which line of code is causing the seg fault.

In this code, the logic is to look for a signature. If a signature is not found, then execute the else clause.

Now, let's look at the else clause.

    else
    {
        fwrite(&buffer, 512, 1, img);
    }

This will attempt to write the buffer contents.

So consider what happens with the first block of data. There's no signature because it's garbage data. The code first checks for a signature and finds none. It then executes the fwrite, but no file has been opened yet. So, img is a null pointer. That throws a seg fault.

There's nothing in the code to handle discarding the garbage data that's read before the first signature.

There may be other issues, but this is your seg fault.

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