The third last line of code is the one causing a segmentation fault. When removed, nothing happens and all I get is a $ on a new line in the terminal

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <cs50.h>

int main(int argc, char *argv[])
{
    #define BUFFER_SIZE 512
    FILE *img = NULL;

    if(argc == 1)
    {
        printf("Usage: ./recover image");
        return 1;
    }

    else if(argc > 2)
    {
        return 1;
    }

    else if(argc == 2)
    {
        FILE *file = fopen(argv[1], "r");

        if (file == NULL)
        {
            printf("Cannot open card.raw");
            return 1;
        }

        int file_counter = 0;

        unsigned char buffer[BUFFER_SIZE];

        while (fread(buffer, BUFFER_SIZE, 1, file) == 1)
        {
        fread(buffer, BUFFER_SIZE, 1, file);

            if(buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xe0) == 0xe0)
            {
                file_counter ++;

                //Creating the filename
                char filename[8];
                sprintf(filename, "%03i.jpg", file_counter);

                if(file_counter == 1)
                {
                    img = fopen("card.raw", "w");
                    fwrite(&buffer, BUFFER_SIZE, 1, img);
                }

                else
                {
                    fclose(img);
                    img = fopen("card.raw", "w");
                    fwrite(&buffer, BUFFER_SIZE, 1, img);
                }
            }

            else if(buffer[0] != 0xff || buffer[1] != 0xd8 || buffer[2] != 0xff || (buffer[3] & 0xe0) != 0xe0)
            {
                if (file_counter > 0)
                {
                    fwrite(&buffer, BUFFER_SIZE, 1, img);
                }

            }
        }
    }
    //Close any remaining files
    fclose(img); //This line of code is causing segmentation fault. But when I remove it, and run the program, nothing happens. All I get is $ on a new line in the command terminal.
    return 0;
}

Answer 1

The while executes an fread followed immediately by an fread. What is likely happening is: program never finds a jpeg signature because it is only processing every other block. Therefore, no img file is opened, therefore the close gives a seg fault.