optimize.c

/********************************************************************
 * This program is wrong, I know, I just want to share. That's it.  *
 * I mean the way to cheat the compiler                             *
 ********************************************************************/
/* optimize.c */
#include <stdio.h>

int main()
{
    const int n = 1;    /* n is declared as a non-volatile constant */
    int *p = (int *)&n; /* p is a pointer to the constant n */

    printf("old n: %d\n", n);
    *p = 2;             /* the constant n should not be changed when optimized */
    printf("new n: %d\n", n);

    return 0;
}
/*****************************************************
 * Comments in the code are the comments by context. *
 * In fact, const is not constant. It is a keyword   *
 * preventing unintentional modification.            *
 *****************************************************/
/**********
 * OUTPUT *
 **********/
(I optimized with -O3.)
% gcc -W -Wall -O3 -ansi -pedantic -s optimize.c
% ./a.out
old n: 1
new n: 2
%
/* In the output, new n is still 2,
 * even if the compiler has already optimized.
 * Compiler optimization doesn't solve for this case.
 * And lastly, forget syntax highlighting for the output. */