random_seed.c
Go to the documentation of this file.
1 /*
2  * Copyright (c) 2009 Baptiste Coudurier <baptiste.coudurier@gmail.com>
3  *
4  * This file is part of Libav.
5  *
6  * Libav is free software; you can redistribute it and/or
7  * modify it under the terms of the GNU Lesser General Public
8  * License as published by the Free Software Foundation; either
9  * version 2.1 of the License, or (at your option) any later version.
10  *
11  * Libav is distributed in the hope that it will be useful,
12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14  * Lesser General Public License for more details.
15  *
16  * You should have received a copy of the GNU Lesser General Public
17  * License along with Libav; if not, write to the Free Software
18  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
19  */
20 
21 #include <unistd.h>
22 #include <fcntl.h>
23 #include <math.h>
24 #include <time.h>
25 #include "timer.h"
26 #include "random_seed.h"
27 
28 static int read_random(uint32_t *dst, const char *file)
29 {
30  int fd = open(file, O_RDONLY);
31  int err = -1;
32 
33  if (fd == -1)
34  return -1;
35  err = read(fd, dst, sizeof(*dst));
36  close(fd);
37 
38  return err;
39 }
40 
41 static uint32_t get_generic_seed(void)
42 {
43  clock_t last_t = 0;
44  int bits = 0;
45  uint64_t random = 0;
46  unsigned i;
47  float s = 0.000000000001;
48 
49  for (i = 0; bits < 64; i++) {
50  clock_t t = clock();
51  if (last_t && fabs(t - last_t) > s || t == (clock_t) -1) {
52  if (i < 10000 && s < (1 << 24)) {
53  s += s;
54  i = t = 0;
55  } else {
56  random = 2 * random + (i & 1);
57  bits++;
58  }
59  }
60  last_t = t;
61  }
62 #ifdef AV_READ_TIME
63  random ^= AV_READ_TIME();
64 #else
65  random ^= clock();
66 #endif
67 
68  random += random >> 32;
69 
70  return random;
71 }
72 
73 uint32_t av_get_random_seed(void)
74 {
75  uint32_t seed;
76 
77  if (read_random(&seed, "/dev/urandom") == sizeof(seed))
78  return seed;
79  if (read_random(&seed, "/dev/random") == sizeof(seed))
80  return seed;
81  return get_generic_seed();
82 }