How to access or modify the const variable in c ?

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Question Posted / leossk

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How to access or modify the const variable in c ?..

Answer / gaurav bhandarkar

Author : Gaurav M. Bhandarkar
Yes u can modify constants...

Theory:
const int z = 420; // z is constant
&z (address of constant)
z (value of constant)

imp:
*(int*)((char*)&(*((char*)&z+1))-1) is
a unity/identity pointer operation resulting in z
That is:-
printf("%d | %d",*(int*)((char*)&(*((char*)&z+1))-1),z);
OUTPUT: 420 | 420

code:

const int z = 420;

printf("%d | %d\n",*(int*)((char*)&(*((char*)&z+1))-1),z);
//o-p 420 | 420

*((char *)&z+1) = 21; //corrupting the constant

printf("%d | %d",*(int*)((char*)&(*((char*)&z+1))-1),z);
//o-p 5540 | 420

___
The 2 similar printf's(check they are same)
o/p different values for same "z"
which is a constant!
thus the constant was corrupted!

Is This Answer Correct ?

53 Yes

3 No

How to access or modify the const variable in c ?..

Answer / saneesh a t

Actually the keyword const is not for the programmer who work with the normal
general purpose computer. By declaring a variable as const, the compailar shows
warning/error to the modification to the variable. At runtime you can modify the memory location
using any technique, or an external hacking program can change the value of the variable.
Suppose your program is compiled to run with a microcontroller with flash or EEPROM or such
memory. Now your variable with const qualifier will be stored in the FLASH of EEPROM memory,
which cann't be modified, and the technique is used to save the RAM space which is too small in
size for a micro controller. In this case too, a FLASH/EEPROM write can chage the value of the
const varriable.

Is This Answer Correct ?

17 Yes

1 No

How to access or modify the const variable in c ?..

Answer / vignesh1988i

const. variable cannot be modified........ if you declare an
variable as:
const char i=90;
throught the program 'i' cant be modified
but #define macro can replace const.. and that macro can be
modified.... but its disadvantage is it will blindly
substitute the data which is #defined.......

Is This Answer Correct ?

21 Yes

9 No

How to access or modify the const variable in c ?..

Answer / vijay kumar (mainpuri)

Answer of #6 , output is wrong

correct out is given below :

Value is: 20
Value is: 20

(above output based on turbo c++ IDE)
Hence, const value can't be change.

Is This Answer Correct ?

15 Yes

5 No

How to access or modify the const variable in c ?..

Answer / sri harsha s

Its compiler dependant.
Using pointer you can modify the content of a 'const'
variable. It works in gcc.. you will get the following
warning when you compile that program.

warning: initialization discards qualifiers from pointer
target type.

I am not sure about turbo c++.

Is This Answer Correct ?

9 Yes

1 No

How to access or modify the const variable in c ?..

Answer / guest

use pointers and u can do anything...

Is This Answer Correct ?

23 Yes

17 No

How to access or modify the const variable in c ?..

Answer / mr.x

int main()
{
const volatile int no=10;
int *ptr;
ptr=(int *)&no;
*ptr=30;
printf("%d %d",no,*ptr);

return 0;
}

Is This Answer Correct ?

2 Yes

0 No

How to access or modify the const variable in c ?..

Answer / varunreddy

try this it will work:

#include<stdio.h>
int main()
{
const int i=10;

*(int *)&i=i++;
printf("%d\n",i);

return 0;
}

Is This Answer Correct ?

3 Yes

3 No

How to access or modify the const variable in c ?..

Answer / varunreddy

a small modification to answer #13

#include<stdio.h>
int main()
{
const int a=10;

*(int *)&a=25
printf("%d\n",a);
return 0;
}

Is This Answer Correct ?

3 Yes

3 No

How to access or modify the const variable in c ?..

Answer / raj

Answer 6 would be true if you change the variable to const
volatile.

Is This Answer Correct ?

0 Yes

1 No

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