recover pset4 file handling

Question

I got this code to work, but I have a few questions for my own clarification.

1. On line 68 where I define the jpg title, why is it a single character? The actual title 001.jpg is much longer than a character. Any why does 'char pic[8];' have to show an array of 8?

2. In fwrite(), the code works correctly either with bookmark or &bookmark as the first parameter. Why is this? Is it because it only has to be set as the address one time, and was already set in fread()?

3. In sprintf(pic, "%03d.jpg", counter); are we using a 3 because the number format shows 3 digits? This ran correctly with either d or i. Is that important, whether it runs as an integer or double? Is there any difference here?

Thanks for any clarification.

/**************************************************************************** * recover.c * * Computer Science 50 * Problem Set 4 * * Recovers JPEGs from a forensic image. ***************************************************************************/

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

 int main(int argc, char *argv[])
 {
    // define and store the string for the cmd line argument to take card.raw
    char *cardraw = (argv[1]);
    // ensure proper usage
    if (argc != 2)
    {
        fprintf(stderr, "Usage: ./recover card.raw\n");
        return 1;
    }

    FILE *card = fopen(cardraw, "r");
    if(card == NULL)
    {
        printf("Error opening the file \"ecard.raw\"...");
        return 2;
    }


    //Alternate way
    //check that the file to open is not null
    // if ((f = fopen(card, "r")) == NULL)
    // {
    //     printf("Error opening the file \"ecard.raw\"...");
    //     return 1;
    // }

    //create an address (or pointer) for reading
    //uint8_t is a byte, an unsigned integer 8 bits long
    //the array contains 512 bytes
    //byteDatatype arrayName[# in array] - to create an array of bytes
    uint8_t bookmark[512];

    //create a variable for the size
    int block = 512;

    //initialize the while loop
    int counter = 0;

    //set the outfile construct pointer to null
    FILE *pix = NULL;

    // Iterate over file contents to read
    while(fread(&bookmark, block, 1, card))
    {
        //JPEG header starts with same 3 bytes - 0xff 0xd8 0xff 0xe Last byte start with 1110 or 0xe 1-9 or 0xe a-f
        // Check if the first four bytes are a JPEG signature
        if(bookmark[0] == 0xff && bookmark[1] == 0xd8 && bookmark[2] == 0xff &&(bookmark[3] & 0xf0) == 0xe0)
        {
            // Close the file, if it is opened
            if(pix != NULL)
            {
                fclose(pix);
            }
            // create an array for each individual jpg title
            //?why is this only a character? why is it an array of 8?
            char pic[8];

            //store the jpg title in sprintf
            // sprintf(outfile, "%03d.jpg", variable for the jpeg #);
            //? the 3 means 3 digits?
            sprintf(pic, "%03d.jpg", counter);

            // Open the new JPEG file for writing and store in the outfile
            pix = fopen(pic, "w");

            counter++;
        }
        //if outfilefile is open, then continue writing to it
        if(pix != NULL)
        {
            //? why does this also work as bookmark with no ampersand?
            fwrite(&bookmark, block, 1, pix);
        }
    }
    //after reading
    //if outfile is open, then close it
    if(pix != NULL)
    {
        fclose(pix);
    }

    //close infile
    fclose(card);

    //success
    return 0;
 }

Answer 1

1 It's not a single char. You've declared an array of 8 chars, which is what you need to hold "000.jpg"

{'0', '0', '0', '.', 'j', 'p', 'g', '\0'}

2 bookmark is an array. An array will "decay" to a pointer type when needed. So you can use either.

3 %03d the 03 means you want a 3-digit number which will use leading zeroes. Using d vs i makes no difference in this case. (d does not mean double, it means digit.)