Middle square method (PRNG)

/*
    middle_square_method.c computes pseudorandom numbers starting with a number
    given at the command line. (Outdated method, for educational purposes only.
    Try 9319 or 12314.) To compile run "gcc -ansi -o msm middle_square_method.c".
    Copyright (C) <February 29, 2016> Henning Polzer,
    send comments and error reports to: h underscore polzer at gmx dot de.

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
*/


#include <math.h>           /* fabs(), floor(), log10(), pow() */
#include <stdio.h>          /* printf(), scanf() */
#include <stdlib.h>         /* atol() */

#define OBERGRENZE 50       /* für sicheren Programmabruch, willkuerlich gesetzt */

typedef unsigned long ganzzahl;

ganzzahl stellen (wert)     /* Stellenzahl ermitteln */
{
    if (wert != 0) return floor (log10(fabs(wert)))+1; else return 1;
}

ganzzahl qmg (z)
{
    ganzzahl quadrat=z*z, diff=stellen(quadrat)-stellen(z), links=floor(diff/2), t;

    if (z < 4) return 0;
    quadrat=floor(quadrat/powl(10,diff-links));
    t=pow(10,stellen(quadrat)-links);
    return quadrat%t;
}

int main (int argc, char *argv[])
{
    ganzzahl erg, i=1, z, verlassen=0;

    if (((argc==2) && (z = fabs(atol(argv[1])))) != 0) {
      do {
        erg = qmg (z);
        if (erg == z) verlassen=1;
        z = erg;
        printf ("%ld ", erg);
            i += 1;
      } while ((verlassen==0) && (erg != 0) && (i < OBERGRENZE));
      printf ("\n");
    } else printf ("Aufruf: qmg eine_zahl_groesser_null\n");
}