why is my file not reading from a jpeg file

salterd · 02-09-2013, 02:54 AM

Hello all! This is my first time posting to this forum.I am having some serious difficulty reading from a jpeg file.I was told that the file has this format, at the beginning of each signature:
- 0xff 0xd8 0xff 0xe0
- 0xff 0xd8 0xff 0xe1

Now, knowing this, I have put in place this code:

Code:

 typedef unsigned char BYTE ;
    char image_name[8]; 
    int counter=1;
    FILE* fp = fopen("card.raw", "r");
    FILE *outfile=NULL;
    int size=512;
    BYTE buffer[size];
    while (feof(fp) == false) 
    {
        fread(buffer,size,sizeof(unsigned char),fp);
        if (fp== NULL)
        {
            printf("Could not open file \n");
            return 1;
        }
        if(buffer[0]==0xff && buffer[1]==0xd8 &&  buffer[2]==0xff && (buffer [3]==0xff || buffer[3]==0xe1))
        {  
            for(int i=0; i<51; ++i){        
                sprintf(image_name, "%.3d.jpg", counter);        
                outfile= fopen(image_name, "w");
                fwrite(buffer,sizeof(buffer),1,outfile); 
                counter=counter+1;
                if (outfile == NULL)              
                {                   
                    printf("could not create jpeg file\n");                   
                    return 2;              
                }      
            } 
        }
        fclose(fp);
        fclose(&outfile);
        return 0;
    }

NOw, my problem is I cant get the first if statement to return true.I was told the file is indeed legit and is not corrupt(and with certainty it has that signature within its structure). If anyone has any ideas on what could be my problem, I would love to hear them. Thank you all!

millgates · 02-09-2013, 03:44 AM

Hi,
1)

Code:

if (fp== NULL)
{
    printf("Could not open file \n");
    return 1;
}

This should go before the loop.

2)

Code:

while (feof(fp) == false)

Please note that feof() will only return true after you have attempted to read past the end of the file. So your loop will run one more iteration then you would expect.

3)

Code:

if(buffer[0]==0xff && buffer[1]==0xd8 &&  buffer[2]==0xff && (buffer [3]==0xff || buffer[3]==0xe1))

Notice the bold part. Shouldn't it be 0xe0? Anyway, maybe it would be more readable if you just defined the expected header sequence as an array/string and use something like strncmp() for the comparison.

4) why is the comparison in every iteration? Does the file have a header each 512 bytes?

5)

Code:

                if (outfile == NULL)              
                {                   
                    printf("could not create jpeg file\n");                   
                    return 2;              
                }

Again, you should check this before you attempt to write to the file.

6)

Code:

fclose(&outfile);

why the '&' ?

7) You open a new file in each of the 51 iterations of the inner loop. But you only fclose() once.

8) What is this program supposed to do in the first place?

salterd · 02-09-2013, 02:34 PM

Quote:

Originally Posted by millgates

Hi,
1)

Code:

if (fp== NULL)
{
    printf("Could not open file \n");
    return 1;
}

This should go before the loop.

2)

Code:

while (feof(fp) == false)

Please note that feof() will only return true after you have attempted to read past the end of the file. So your loop will run one more iteration then you would expect.

3)

Code:

if(buffer[0]==0xff && buffer[1]==0xd8 &&  buffer[2]==0xff && (buffer [3]==0xff || buffer[3]==0xe1))

Notice the bold part. Shouldn't it be 0xe0? Anyway, maybe it would be more readable if you just defined the expected header sequence as an array/string and use something like strncmp() for the comparison.

4) why is the comparison in every iteration? Does the file have a header each 512 bytes?

5)

Code:

                if (outfile == NULL)              
                {                   
                    printf("could not create jpeg file\n");                   
                    return 2;              
                }

Again, you should check this before you attempt to write to the file.

6)

Code:

fclose(&outfile);

why the '&' ?

7) You open a new file in each of the 51 iterations of the inner loop. But you only fclose() once.

8) What is this program supposed to do in the first place?

Yes I do open a new file! The premise of the program is to read in 512 bytes at a time.For each 512 bytes, there will be a new image and there are 51 images within this file. I'm not worried about that part so much as I'm worried on why I cant get the first if statement to return true...