assert.c

#include <stdio.h> 
#include <assert.h> 

int main() 
{ 
    int x = 7; 

    /*  Some big code in between and let's say x  
     *         is accidentally changed to 9  */
    x = 9; 

    // Programmer assumes x to be 7 in rest of the code 
    assert(x==7); 

    /* Rest of the code */

    return 0; 
}