C/C++ changing the value of a const

you need to cast away the constness:

linux ~ $ cat constTest.c
#include <stdio.h>


void modA( int *x )
{
        *x = 7;
}


int main( void )
{

        const int a = 3; // I promisse i won't change a
        int *ptr;
        ptr = (int*)( &a );

        printf( "A=%d\n", a );
        *ptr = 5; // I'm a liar, a is now 5
        printf( "A=%d\n", a );

        *((int*)(&a)) = 6;
        printf( "A=%d\n", a );

        modA( (int*)( &a ));
        printf( "A=%d\n", a );

        return 0;
}
linux ~ $ gcc constTest.c -o constTest
linux ~ $ ./constTest
A=3
A=5
A=6
A=7
linux ~ $ g++ constTest.c -o constTest
linux ~ $ ./constTest
A=3
A=3
A=3
A=3

also the common answer doesn’t work in g++ 4.1.2

linux ~ $ cat constTest2.cpp
#include <iostream>
using namespace std;
int main( void )
{
        const int a = 3; // I promisse i won't change a
        int *ptr;
        ptr = const_cast<int*>( &a );

        cout << "A=" << a << endl;
        *ptr = 5; // I'm a liar, a is now 5
        cout << "A=" << a << endl;

        return 0;
}
linux ~ $ g++ constTest2.cpp -o constTest2
linux ~ $ ./constTest2
A=3
A=3
linux ~ $

btw.. this is never recommended… I found that g++ doesn’t allow this to happen.. so that may be the issue you are experiencing.