Grid step (machine epsilon e.t.c.)

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C++

C

C#

#include <stdio.h>
int main(void)
{
    long double x,eps=1.0,a; 
    scanf("%Lg",&x);
    do
    {
        a=eps;
        eps/=2.0;
    }
    while( x+eps>x);
    printf("Grid step: %Le",(long double)a);
    return 0;
}

What I have tried:

I wrote such a code to find the grid step(difference between two adjacent machine-representable numbers, machine epsilon the difference between the minimum machine-representable number greater than one and one), the teacher says: Give examples for which the program gives the wrong answer. How do you fix the program to make it work correctly? Help please