1  /* Return a NULL pointer, without letting the compiler know it.
       2     Copyright (C) 2017-2021 Free Software Foundation, Inc.
       3  
       4     This program is free software: you can redistribute it and/or modify
       5     it under the terms of the GNU General Public License as published by
       6     the Free Software Foundation; either version 3 of the License, or
       7     (at your option) any later version.
       8  
       9     This program is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12     GNU General Public License for more details.
      13  
      14     You should have received a copy of the GNU General Public License
      15     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      16  
      17  #include <stdlib.h>
      18  
      19  /* Return NULL.
      20     Usual compilers are not able to infer something about the return value.  */
      21  static void *
      22  null_ptr (void)
      23  {
      24    unsigned int x = rand ();
      25    unsigned int y = x * x;
      26    if (y & 2)
      27      return (void *) -1;
      28    else
      29      return (void *) 0;
      30  }
      31  
      32  /* If you want to know why this always returns NULL, read
      33     https://en.wikipedia.org/wiki/Quadratic_residue#Prime_power_modulus .  */