CS50 Problem Set 4 - Recover, fclose undeclared identifier issue

Question

My first time posting on the forum so hope someone can help. I've been scratching my head over this code for a while now and can't understand why I get the undeclared identifier! My code seems to follow logically what I expect but here's my walk through with the actual code beneath:

While not end of file
Load 512 bytes into array
Check first four array elements for JPG header
IF JPG and it is for the first time
create filename 000.jpg
open file and write data
increment jpg counter
ELSE NOT FIRST JPG
close previous file
create new filename 001.jpg
open file and write data
increment jpg counter

**** CODE EXTRACT ****

BYTE buffer[BLOCK_SIZE];    // create an array BLOCK_SIZE big

// read BLOCK_SIZE worth of bytes into buffer
while (fread(buffer, sizeof(BYTE), BLOCK_SIZE, infile) == BLOCK_SIZE)
{
    // check if first four bytes of blockare JPG header
    if (buffer[0]==0xFF && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
    {
        // check if this is the first time a header has been found
        if (jpgcount == 0)
        {
            // first time JPG has been found
            // create filename
            sprintf(filename, "%03i.jpg", jpgcount);

            // open file
            FILE *img = fopen(filename, "w");
            if (img == NULL)
            {
                printf("Could not open file.\n");
                return 3;
            }

            printf("first header! jpgcount: %d\n", jpgcount);   //debug

            // write data to file
            fwrite (buffer, sizeof(BYTE), BLOCK_SIZE, img);

            // increment jpgcount
            jpgcount++;
        }

        else
        {
            // not first JPG
            // close previous file
            fclose (img);

            // create new filename
            sprintf(filename, "%03i.jpg", jpgcount);

            // open file
            FILE *img = fopen(filename, "w");
            if (img == NULL)
            {
                printf("Could not open file.\n");
                return 3;
            }

            printf("new header! jpgcount: %d\n", jpgcount);     //debug

            // write data to file
            fwrite (buffer, sizeof(BYTE), BLOCK_SIZE, img);

            // increment jpgcount
            jpgcount++;

        }
    }

I haven't explained rest of my code as the issue seems to be with the fclose in the else associated with the header check. The compiler is saying fclose(img) 'img' is undeclared, however this part of the loop won't be reached unless JPG counter is greater than 1 AND a valid header has been found. In which case the first IF will have already been executed and the file open?

The hints video suggests fclose in the way I've done it should be okay, and I've seen other examples but can't figure out what this bug is. Been stuck on this now for a couple of weeks.

Any help most appreciated, thankyou!!

Jon

Answer 1

It's a subtle issue. The problem is the scope of the file pointer. The code declares the file pointer img deep inside several nested curly brace pairs. Once the code steps out of the innermost set of curly braces surrounding the declaration, the pointer is destroyed. That means that on the next pass through the while loop, the pointer doesn't exist, resulting in an undeclared pointer variable error.

The fix is simple. Declare the pointer before the start of the while loop:

FILE *img = NULL;

This will give the pointer the correct scope and it will persist through every pass of the while loop.

If you don't follow what I'm saying, google "variable scope in c".

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