libavutil/random_seed.c
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00001 /*
00002  * Copyright (c) 2009 Baptiste Coudurier <baptiste.coudurier@gmail.com>
00003  *
00004  * This file is part of Libav.
00005  *
00006  * Libav is free software; you can redistribute it and/or
00007  * modify it under the terms of the GNU Lesser General Public
00008  * License as published by the Free Software Foundation; either
00009  * version 2.1 of the License, or (at your option) any later version.
00010  *
00011  * Libav is distributed in the hope that it will be useful,
00012  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00013  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00014  * Lesser General Public License for more details.
00015  *
00016  * You should have received a copy of the GNU Lesser General Public
00017  * License along with Libav; if not, write to the Free Software
00018  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
00019  */
00020 
00021 #include <unistd.h>
00022 #include <fcntl.h>
00023 #include <math.h>
00024 #include <time.h>
00025 #include "timer.h"
00026 #include "random_seed.h"
00027 
00028 static int read_random(uint32_t *dst, const char *file)
00029 {
00030     int fd = open(file, O_RDONLY);
00031     int err = -1;
00032 
00033     if (fd == -1)
00034         return -1;
00035     err = read(fd, dst, sizeof(*dst));
00036     close(fd);
00037 
00038     return err;
00039 }
00040 
00041 static uint32_t get_generic_seed(void)
00042 {
00043     clock_t last_t  = 0;
00044     int bits        = 0;
00045     uint64_t random = 0;
00046     unsigned i;
00047     float s = 0.000000000001;
00048 
00049     for (i = 0; bits < 64; i++) {
00050         clock_t t = clock();
00051         if (last_t && fabs(t - last_t) > s || t == (clock_t) -1) {
00052             if (i < 10000 && s < (1 << 24)) {
00053                 s += s;
00054                 i = t = 0;
00055             } else {
00056                 random = 2 * random + (i & 1);
00057                 bits++;
00058             }
00059         }
00060         last_t = t;
00061     }
00062 #ifdef AV_READ_TIME
00063     random ^= AV_READ_TIME();
00064 #else
00065     random ^= clock();
00066 #endif
00067 
00068     random += random >> 32;
00069 
00070     return random;
00071 }
00072 
00073 uint32_t av_get_random_seed(void)
00074 {
00075     uint32_t seed;
00076 
00077     if (read_random(&seed, "/dev/urandom") == sizeof(seed))
00078         return seed;
00079     if (read_random(&seed, "/dev/random")  == sizeof(seed))
00080         return seed;
00081     return get_generic_seed();
00082 }