C Programming - Const - Discussion

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"I never think of the future. It comes soon enough."

- Albert Einstein

What will be the output of the program?

#include<stdio.h>

int main()
{
    const int x=5;
    const int *ptrx;
    ptrx = &x;
    *ptrx = 10;
    printf("%d\n", x);
    return 0;
}

[A].	5	[B].	10
[C].	Error	[D].	Garbage value

Answer: Option E

Explanation:

Step 1: const int x=5; The constant variable x is declared as an integer data type and initialized with value '5'.

Step 2: const int *ptrx; The constant variable ptrx is declared as an integer pointer.

Step 3: ptrx = &x; The address of the constant variable x is assigned to integer pointer variable ptrx.

Step 4: *ptrx = 10; Here we are indirectly trying to change the value of the constant vaiable x. This will result in an error.

To change the value of const variable x we have to use *(int *)&x = 10;

Prudhvi said: (Thu, Jul 14, 2011 12:23:18 PM)

Any one give me a clear explanation to step 4.

Hari said: (Thu, Aug 25, 2011 10:43:46 AM)

I also don't think step 4 willl work how can it will try to change the value of x answer is 5.

Prakash said: (Sun, Oct 30, 2011 12:16:54 PM)

int main() { int x=5; int ptrx; ptrx = &x; ptrx = 10; printf("%d\n", x); return 0; } To change the value of const variable x we have to use (int )&x = 10;

Bijay said: (Thu, Dec 1, 2011 01:09:52 PM)

In gcc compiler it will work o/p:10.

Voski said: (Mon, Jan 16, 2012 11:23:57 AM)

A small correction in the Explanation Step 2 is needed: ptrx is not a constant, but a pointer to an integer constant.