einstein.c

/*
 * SPDX-FileCopyrightText: 2018 Bruce Cowan <bruce@bcowan.me.uk>
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 */

#include <stdio.h>
#include <stdlib.h>

#define C 299792458
#define C2 (C)*(C)

int
main (int argc, char **argv)
{
    double m;

    if (argc != 2)
    {
        printf ("Input the mass in kg: ");
        scanf ("%lf", &m);
    }
    else
    {
        if (sscanf (argv[1], "%lf", &m) != 1)
        {
            fprintf (stderr, "Couldn't parse number\n");
            return EXIT_FAILURE;
        }
    }

    double e = m * C2;
    printf ("E = %#.3G J\n", e);

    return 0;
}