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. */
30static inline uint32_t
31random_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
42