include/random-bits.h

   1 /* Fast pseudo-random bits based on clock_gettime.
   2    Copyright (C) 2019-2020 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _RANDOM_BITS_H
  20 # define _RANDOM_BITS_H
  21
  22 #include <time.h>
  23 #include <stdint.h>
  24
  25 /* Provides fast pseudo-random bits through clock_gettime.  It has unspecified
  26    starting time, nano-second accuracy, its randomness is significantly better
  27    than gettimeofday, and for mostly architectures it is implemented through
  28    vDSO instead of a syscall.  Since the source is a system clock, the upper
  29    bits will have less entropy. */
  30 static inline uint32_t
  31 random_bits (void)
  32 {
  33   struct timespec tv;
  34   __clock_gettime (CLOCK_MONOTONIC, &tv);
  35   /* Shuffle the lower bits to minimize the clock bias.  */
  36   uint32_t ret = tv.tv_nsec ^ tv.tv_sec;
  37   ret ^= (ret << 24) | (ret >> 8);
  38   return ret;
  39 }
  40
  41 #endif