1  /* Fast pseudo-random bits based on clock_gettime.
       2     Copyright (C) 2019-2023 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 __timespec64 tv;
      34    __clock_gettime64 (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