C Programming - Const - Discussion

Discussion Forum : Const - Find Output of Program (Q.No. 4)
What will be the output of the program?

int main()
    const int x=5;
    const int *ptrx;
    ptrx = &x;
    *ptrx = 10;
    printf("%d\n", x);
    return 0;
Garbage value
Answer: Option

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;

18 comments Page 1 of 2.

Prudhvi said:   1 decade ago
Any one give me a clear explanation to step 4.

Hari said:   1 decade ago
I also don't think step 4 willl work how can it will try to change the value of x answer is 5.

Martyf said:   1 decade ago
It is very difficult paper.

Prakash said:   1 decade ago
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:   1 decade ago
In gcc compiler it will work o/p:10.

Voski said:   1 decade ago
A small correction in the Explanation Step 2 is needed: ptrx is not a constant, but a pointer to an integer constant.

Logeswari said:   1 decade ago
1)const x is declared as integer and assign the value 5.

2)const ptrx value declared as integer point.

3)ptrx=&x mean ptrx contain address of x which mean *ptrx contain x value 5.

4)then *ptrx=10 mean indirectly change the value of 5 know.

So its wrong.

If any wrong in above statement means please any one clear me.

Charan said:   10 years ago
How we can change const variable by using *(int *)&x = 10; (mentioned in explanation). However it is not allowing to change from 5 to 10. But no error also raising.

Hotcpu said:   9 years ago
This is an undefined behavior by casting away the const definition. However, it will not work in g++ 4.1+.

Shiva kumar said:   9 years ago
Using pointers we can change any kind it of data.

Even if it is constant variable also. That is why there is no security in C.

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