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3.6 Common Errors

This section contains a list of common errors made by programmers - things to watch out for in your programming.

1. The wrong value is tested for EOF instead of the returned value of scanf():

   flag = scanf("%d", &n);
        while (n != EOF)         /* should be: while (flag != EOF) */
             ...

   The value read is stored at the address given by &n, i.e. it is stored in n. The statement scanf("%d", &n) evaluates to a returned value which is either the number of data items read or EOF. In the above case, if an integer data item is read, the value returned will be 1. If no data item is read, scanf() returns EOF. The value returned by scanf() is stored in the variable flag, NOT in n. Test flag for EOF, NOT n.

2. An attempt is made by a called function to access a variable defined in the calling function.

   #include <stdio.h>
        #define TRUE 1
        main()
        {    int x, square(int x);
   
             x = 3;
             square(x);               /* x cannot be unchanged by square() */
             printf("x = %d\n", x);        /* prints: x = 3 */
        }
   
        int square(int x)        /* x is a new object, with initial value */
                                 /* passed by an argument in the function call */
        {
             x = x * x;          /* new x is changed */
             return TRUE;   /* a value is returned as the value of square() */
        }

   The variable x in main() is a different object from x in square(). The value of the local cell, x, is changed in square(), but that does not affect the cell x in main(). The cell, x, in main() will still have the value 3 after the function call to square(). If main() needs the squared value of x, then square() should return the squared value of x, NOT TRUE. This returned value should be saved in a local variable in main(). For example, if the return statement in square() is:

   return x;

   then the returned value can be saved in main():

   x = square(x);

3. A function is not declared with a prototype statement. Without a prototype, the compiler will not be able to check for consistency in usage of the function. When a function is declared, the compiler checks for a correct number of arguments in function calls and checks for correct types.

4. A default declaration of a function assumes an integer type function value. If the actual definition of that function returns a non-integer type, then the compiler will consider it an attempt to redeclare a function. The compiler will flag it as an error.

5. An erroneous keystroke is entered when an end of file is to be entered. For example, an attempt is made to enter 0 or -1 for an end of file. These values are not the end of file keystrokes; they represent the possible values returned by scanf() when an end of file keystroke ('136D or '136Z) is encountered.