L-32 MCS 260 Fri 2 Nov 2001

In this lecture we explained the difference between defining a constant with a preprocessor directive and using the const. The program below is an illustration of using const in combination with pointers.

/* L-32 MCS 260 Fri 2 Nov 2001 : pointers to const must be const 

When we compile the program below on a Sun workstation with gcc,
we get the following warning :

constptr.c: In function `main':
constptr.c:5: warning: initialization discards `const' from pointer target type

If we ignore the warning and execute the program, then we see that
the variable five, declared as a constant integer, has taken the value 6. 

To prevent such a mishap, the user must declare also the pointer, 
pointing to a const, as const.  Unfortunately, also with this correction,
the *p = 6 only caused a warning, and not an error...                     */

#include<stdio.h>

int main ( void )
{
   const int five = 5;
   int *p = &five;             /* wrong */

/* const int *p = &five; */    /* correct */

   printf("%d\n", five);
   *p = 6;                     /* should be prohibited ... */
   printf("%d\n", five);

   return 0;
}

For the remainder of the lecture we reviewed the answer to quiz 10 for the Thursday lab session.