CS50 Recover - fread into buffer

Question

I am working on CS50 Recovery and I am unable to get the program to check the JPEG starting header.

I used debug50 and added a couple print statements to try and find the problem. Every time I run the program, it prints out "No Output File 2" meaning no outfile is created, and the IsJPEG boolean check is skipped.

Upon further debugging it seems that the buffer is 0'\000\ all along, so I suspect nothing is written into the buffer, causing the program to skip the boolean check (which trigger fopen) and hence causing it to break.

However, I struggle to find the reason why nothing is written into buffer.

Please help and thank you!

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

typedef uint8_t BYTE;
bool isJPEG(BYTE buffer[]);
int main(int argc, char *argv[])
{
    BYTE buffer[512];
    int fileCounter = 0;
    char filename[8];
    if (argc != 2)
    {
        printf("%s\n", "Usage: ./recovery file");
        return 1;
    }
    FILE* infile = fopen(argv[1], "r");
    if (infile == NULL)
    {
        printf("%s\n", "Error: File Not Found");
        return 1;
    }
    FILE* outfile;
    while (fread(buffer, 512, 1, infile))
    {
        if (isJPEG(buffer))
        {
            if (fileCounter == 0)
            {
                sprintf(filename, "%03i.jpg", fileCounter++);
                outfile = fopen(filename, "w");
                printf("%s\n", "Bool 1");
            }
            else
            {
                fclose(outfile);
                sprintf(filename, "%03i.jpg", fileCounter++);
                outfile = fopen(filename, "w");
                printf("%s\n", "Bool 2");
            }
            if (outfile == NULL)
            {
                printf("%s\n", "No Output File 1");
                return 1;
            }
            fwrite(buffer, 512, 1, outfile);
        }
        else
        {
            if (outfile == NULL)
            {
                printf("%s\n", "No Output File 2");
                return 1;
            }
            fwrite(buffer, 512, 1, outfile);
        }
    }
    fclose(outfile);
    fclose(infile);
}

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

score 1 · Answer 1 · answered Jun 06 '21 at 18:23

It prints "No output file 2" because it is told to so. When the script is running for the first time, execution is falling into else statement of if (isJPEG(buffer)) where it checks if outfile is NULL. It is indeed NULL because it is pointing to nothing and there No output file 2 is printed. Instead, it should be checked if the outfile is pointing to a file, only then we should write on it. I corrected the script as following;

 while (fread(buffer, 512, 1, infile))
    {
        if (isJPEG(buffer))
        {
            if (fileCounter == 0)
            {
                sprintf(filename, "%03i.jpg", fileCounter++);
                outfile = fopen(filename, "w");
                printf("%s\n", "Bool 1");
            }
            else
            {
                fclose(outfile);
                sprintf(filename, "%03i.jpg", fileCounter++);
                outfile = fopen(filename, "w");
                printf("%s\n", "Bool 2");
            }
            if (outfile != NULL)
            {
                fwrite(buffer, 512, 1, outfile);
            }
        }
        else
        {
            if (outfile != NULL)
            {
                fwrite(buffer, 512, 1, outfile);
            }
        }
    }

CS50 Recover - fread into buffer

1 Answers1