How to modify a const variable in C?

Whenever we use const qualifier with variable name, it becomes a read-only variable and get stored in .rodata segment. Any attempt to modify this read-only variable will then result in a compilation error: "assignment of read-only variable".

In the below program, a read-only variable declared using the const qualifier is tried to modify:

#include <stdio.h>
int main()
{
    const int var = 10;
    var = 15;
    printf("var = %d\n", var);
    return 0;
}

Output

prog.c: In function 'main':
prog.c:5:9: error: assignment of read-only variable 'var'

Changing Value of a const Variable Through Pointer

The variables declared using the const keyword get stored in the .rodata segment. However, it was previously possible to access and change the value of such variables through pointers by casting the address of the const variable to a non-constant pointer. This method relied on the compiler's behavior to discard the const qualifier, allowing modification of the supposedly read-only variable.

However, modern C++ compilers enforce stricter type checking, and this approach will result in a compilation error. In C, while it may only generate a warning, it is not recommended due to potential undefined behavior and the violation of const correctness.

Below program illustrates this:

// C program to demonstrate changing Value of a const
// variable through pointer
#include <stdio.h>
#include <stdlib.h>
int main()
{
    const int var = 10;

    int* ptr = &var;
    *ptr = 12;

    printf("var = %d\n", var);

    return 0;
}

Output

error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]

Conclusion

In conclusion, while there were ways to modify const variables using pointers in older compilers, modern compilers enforce stricter rules to prevent such practices. It is best to avoid such hacks and adhere to const correctness to write safe and reliable code.