CS50 Problem Set 4: Recover

Question

I am getting a 'Segmentation fault (core dumped) error'

I know that means I am accessing memory I shouldn't. I feel like it has something to do with my fread or fwrite functions.

'''

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

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./recover IMAGE\n");
        return 1;
    }

    if (argv[1] == NULL)
    {
        printf("File entered does not exist\n");
        return 1;
    }

    FILE *file = fopen(argv[1], "r");
    typedef uint8_t BYTE;
    BYTE buffer[512];
    char filename[8];
    int i = 0;
    FILE *img;
    while (fread(buffer, 1, 512, file) == 512)
    {
        // check if first three bytes are 0xff 0xd8 0xff
        // check if 4th byte is 0xe0, 0xe1, 0xe2, ..., 0xef
        // if those 4 bytes are found then start writing these bytes to a file ###.jpg
        // once you see the 4 bytes again, stop writing to the current file and make a new file ###.jpg to write to
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
        {
            if (i == 0)
            {
                sprintf(filename, "%03i.jpg", i);
                img = fopen(filename, "w");
                i += 1;
            }
            else
            {
                fclose(img);
                sprintf(filename, "%03i.jpg", i);
                img = fopen(filename, "w");
                i += 1;
            }
            fwrite(buffer, 1, 512, img);
        }
        else
        {
            img = fopen(filename, "w");
            fwrite(buffer, 1, 512, img);
        }
    }
    fclose(img);

    return 0;
}

'''

Any ideas?

Answer 1

I think there's an issue with your second else statement. Since this else statement has you writing when you are NOT at the start of a new jpeg file, you should make sure you've already opened and started writing to at least one image.