PSET 4 Problem Set Recover

Question

I've been working on Recover a few days now and managed to actually create a program that retrieves the images. The only problem is that they are not retrieved correctly and don't match what they are actually supposed to be. After running debugger, it continues to land on the line "while (fread(buffer, sizeof(buffer), 1, inputfile) == 1)". I have changed that line so many times, but everytime I change it (along with other lines of code), but everytime I do, I get a segmentation fault. I have no idea what to do. Any help would be greatly appreciated!

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <stdint.h>
#define BLOCKSIZE 512

typedef uint8_t BYTE;

bool found_jpeg(BYTE buffer[])
{
    return (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0);
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./recover image\n");
        return 1;
    }
    FILE *inputfile = fopen(argv[1], "r");
    if (inputfile == NULL)
    {
        printf("File not found\n");
        return 1;
    }
    else
    {
    BYTE buffer[BLOCKSIZE];
    FILE *outputfile = NULL;
    char filename[8];
    int counter = 0;
    bool first_jpeg = false;

        while (fread(buffer, sizeof(buffer), 1, inputfile) == 1)
        {
            if (found_jpeg(buffer) == true)
            {
                if (first_jpeg == false)
                {
                    sprintf(filename, "%03i.jpg", counter);
                    outputfile = fopen(filename, "w");
                    if (outputfile == NULL)
                    {
                        return 1;
                    }
                    fwrite(buffer, sizeof(buffer), 1, outputfile);
                    counter++;
                    first_jpeg = true;
                }
                else if (first_jpeg == true)
                {
                    fclose(outputfile);
                    sprintf(filename, "%03i.jpg", counter);
                    outputfile = fopen(filename, "w");
                    if (outputfile == NULL)
                    {
                        return 1;
                    }
                    fwrite(buffer, sizeof(buffer), 1, outputfile);
                    counter++;
                }
            }
            if (first_jpeg == true)
            {
                fwrite(buffer, sizeof(buffer), 1, outputfile);
            }
        }
    fclose(outputfile);
    fclose(inputfile);
    }
}

Answer 1

When this code opens a new file, it writes the first 512 byte block to the output file. But what happens next? The code drops down to the last if block:

        if (first_jpeg == true)
        {
            fwrite(buffer, sizeof(buffer), 1, outputfile);
        }

Since first_jpeg is now true, that first 512 byte signature data block gets written out a second time! That's what is corrupting your output files.

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

Answer 2

The command line argument for this programm is a file called card.raw, which containes raw data which programm needs to read and extracts pictures from - right?

There is a code line FILE *inputfile = fopen(argv[1], "r"); which opens files for reading and stores file pointer returned by fopen() into inputfile variable. That is a first step to make reading file content possible. Farther to read file content function fread() should be used. Examples can be found here -https://www.tutorialspoint.com/c_standard_library/c_function_fread.htm

In short - fread() doesn't read the whole card.raw file in one go. It should read not more than can be fit be fitted into the buffer BYTE array...

I think in this particular situation it will benefitial to first create a programm that 1) reads card.raw file and 2) writes its content into another file, for example copy_of_card.raw. Nothing more - nothing less.

This way you will get a bit confident with fopen(), fread(), fwrite() and fclose() functions. Before tackling the main objective.