Thread: problem with file I/O, whole code

Hey, so like the descriptions says, this program is supposed to read a file called card.raw which has 30 jpgs on it and the rest is 0s. And then write those jpgs in a folder.

It's my first time using file I/O so I'm sure I made a bunch of mistakes. The file does compile but it doesn't even create any jpgs and it runs indefinitely. I'm using gdb to debug and I can't find a way to use it that makes sense with this problem.

My questions is, can you help me fix it? And also for the future, what is a good way to debug a program like this?

I'm using gcc to compile and OS is fedora.

Code:

/* 
 * ====================================
 *  Description: Recovers pictures from card.raw which
 *  contains 30 jpgs in a row, then is blanked out with 0s.
 * ====================================
 */
 
#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int
main(void)
{
    // open card.raw
    FILE *card = fopen("card.raw" , "r");
    if (card == NULL)
    {
        printf("Could not open card.raw");
        return 1;
    }
    
    // jpg number for output name
    int jpg_number = 1;
    
    // look at every byte of card.raw
    BYTE jpg_chunk[512];
    
    while (fread(&jpg_chunk, sizeof(BYTE), 512, card) != 0) // read current 512 byte        
    {    
        // test if that current 512 bytes is start of jpg
        if (jpg_chunk[0] == 0xff && jpg_chunk[1] == 0xd8 && jpg_chunk[2] == 0xff &&
           (jpg_chunk[3] == 0xe0 || jpg_chunk[3] == 0xe01))
        {
            // determine filename
            char *jpg_name = "";
            sprintf(jpg_name, "%d.jpg" , jpg_number);
            
            // create jpg   
            FILE *jpg_write = fopen(jpg_name, "w"); 
            if (jpg_write == NULL)
            {
                fclose(jpg_write);
                fprintf(stderr, "Could not create %s.\n", jpg_name);
                return 2;
            }
            
            do 
                // write to jpg
                fwrite(&jpg_chunk, sizeof(BYTE), 512, jpg_write);
            while (jpg_chunk[0] != 0xff && jpg_chunk[1] != 0xd8 && jpg_chunk[2] != 0xff &&
                  (jpg_chunk[3] != 0xe0 || jpg_chunk[3] != 0xe01));
            
            // close finished jpg
            fclose(jpg_write);
            
            // this jpg is done, next jpg
            jpg_number++;
        }
    }
    
    // close card.raw
    fclose(card);
    
    // end
    return 0;
}

> char *jpg_name = "";
> sprintf(jpg_name, "%d.jpg" , jpg_number);
You need to allocate some actual space for your string.

Say
char jpg_name[100];

Consider this to start with

Code:

    while (fread(&jpg_chunk, sizeof(BYTE), 512, card) != 0) // read current 512 byte        
    {    
        // test if that current 512 bytes is start of jpg
        if (jpg_chunk[0] == 0xff && jpg_chunk[1] == 0xd8 && jpg_chunk[2] == 0xff &&
           (jpg_chunk[3] == 0xe0 || jpg_chunk[3] == 0xe01))
        {
            printf("Found the start of a JPG file\n");
        }
    }

If this doesn't print 30 lines, then you need to think about this some more.

thanks for the replies, i have incorporated both suggestions, the result i get is now 37 jpgs (file does contain 37 even if i said 30 in the description in 1st post so that's correct) that are unreadable i also get the code below printed.

my question: im still not quite sure how to go about debugging the reason why the jpgs aren't being shown properly

Code:

found the start of a JPG file
jpg chunk: ff d8 ff e0
wrote name: 1.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0
wrote name: 2.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0
wrote name: 3.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0
wrote name: 4.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0
wrote name: 5.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0
...
...
wrote name: 36.jpgfound the start of a JPG file
jpg chunk: ff d8 ff e0

this is the updated code

Code:

/*
 * ========================================================
 *  Description: Recovers pictures from card.raw which
 *  contains **37** jpgs in a row, then is blanked out with 0s.
 * ========================================================
 */
 
#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int
main(void)
{
    // open card.raw
    FILE *card = fopen("card.raw" , "r");
    if (card == NULL)
    {
        printf("Could not open card.raw");
        return 1;
    }
    
    // jpg number for output name
    int jpg_number = 1;
    
    // look at every byte of card.raw
    BYTE jpg_chunk[512];
    
    while (fread(&jpg_chunk, sizeof(BYTE), 512, card) != 0) // read current 512 byte        
    {    
        // test if that current 512 bytes is start of jpg
        if (jpg_chunk[0] == 0xff && jpg_chunk[1] == 0xd8 && jpg_chunk[2] == 0xff && 
           (jpg_chunk[3] == 0xe0 || jpg_chunk[3] == 0xe1))
           
        {
            // determine filename
            char jpg_name[10];
            sprintf(jpg_name, "%d.jpg" , jpg_number);
            
            // create jpg   
            FILE *jpg_write = fopen(jpg_name, "w"); 
            if (jpg_write == NULL)
            {
                fclose(jpg_write);
                fprintf(stderr, "Could not create %s.\n", jpg_name);
                return 2;
            }
            
            // TESTING
            printf("found the start of a JPG file\n");
            printf("jpg chunk: %x %x %x %x\n" , jpg_chunk[0], jpg_chunk[1], jpg_chunk[2], jpg_chunk[3]);
            printf("wrote name: %s" , jpg_name);

            
            do 
                // write to jpg
                fwrite(&jpg_chunk, sizeof(BYTE), 512, jpg_write);
            while (jpg_chunk[0] != 0xff && jpg_chunk[1] != 0xd8 && jpg_chunk[2] != 0xff &&
                  (jpg_chunk[3] != 0xe0 || jpg_chunk[3] != 0xe1));
            
            // close finished jpg
            fclose(jpg_write);
            
            // this jpg is done, next jpg
            jpg_number++;
        }
    }
    
    // close card.raw
    fclose(card);
    
    // end
    return 0;
}

> fwrite(&jpg_chunk, sizeof(BYTE), 512, jpg_write);
You only have this inside the "new chunk" code.

You need to write all chunks to the file.

In other words, your "new chunk" detection code should do the following
- close the old file (if appropriate)
- generate a new filename
- open a new file

You're not writing everything to a file.

Code:

    while (fread(&jpg_chunk, sizeof(BYTE), 512, card) != 0) // read current 512 byte        
    {    
        // test if that current 512 bytes is start of jpg
        if (jpg_chunk[0] == 0xff && jpg_chunk[1] == 0xd8 && jpg_chunk[2] == 0xff && 
           (jpg_chunk[3] == 0xe0 || jpg_chunk[3] == 0xe1))
           
        {
            // determine filename
            char jpg_name[10];
            sprintf(jpg_name, "%d.jpg" , jpg_number);
            
            // create jpg   
            FILE *jpg_write = fopen(jpg_name, "w"); 
            if (jpg_write == NULL)
            {
                fclose(jpg_write);
                fprintf(stderr, "Could not create %s.\n", jpg_name);
                return 2;
            }
            
            // TESTING
            printf("found the start of a JPG file\n");
            printf("jpg chunk: %x %x %x %x\n" , jpg_chunk[0], jpg_chunk[1], jpg_chunk[2], jpg_chunk[3]);
            printf("wrote name: %s" , jpg_name);

            
            do 
                // write to jpg
                fwrite(&jpg_chunk, sizeof(BYTE), 512, jpg_write);
            while (jpg_chunk[0] != 0xff && jpg_chunk[1] != 0xd8 && jpg_chunk[2] != 0xff &&
                  (jpg_chunk[3] != 0xe0 || jpg_chunk[3] != 0xe1));
            
            // close finished jpg
            fclose(jpg_write);
            
            // this jpg is done, next jpg
            jpg_number++;
        }
        else
        {
            printf("Non header chunk lost to the sands of time\n");
        }
    }

Notice I've placed a final else statement.
Tell us how many times it gets printed, which tells you how many chunks are NOT being written to a file.