从Linux内核中获取真随机数

82199505

[37972.468580] We get random number: -276237802

[37972.468586] We get random number: 411869317

[37972.468590] We get random number: 1779353222

[37972.468594] We get random number: 823507551

[37972.468598] We get random number: 1061461415

[37972.468602] We get random number: 1372137935

[37972.468606] We get random number: 1460835009

[37972.468610] We get random number: 2002191729

[37972.468614] We get random number: -272204344

[38059.349589] quit get_random_num.

[38070.575433] Get some real random number.

[38070.575462] We get random number: 1111808207

[38070.575476] We get random number: -13789055

[38070.575481] We get random number: 240443446

[38070.575485] We get random number: -606998911

[38070.575489] We get random number: 538794850

[38070.575493] We get random number: -500786675

[38070.575497] We get random number: -1240394927

[38070.575501] We get random number: 1233931345

[38070.575504] We get random number: 1488497117

[38070.575508] We get random number: -177688514

/dev/random /dev/urandom

这两个特殊设备都是字符型设备。我们可以在用户空间通过read系统调用读这两个设备文件以此获取随机数。这两个设备文件的区别在于：如果内核熵池的估计值为0时，

/dev/random将被阻塞，而/dev/urandom不会有这个限制。

#include

#include

#include

#include

#include

/* 从min和max中返回一个随机值 */

int random_number(int min, int max)

{

static int dev_random_fd = -1;

char *next_random_byte;

int bytes_to_read;

unsigned random_value;

assert(max > min);

if (dev_random_fd == -1)

{

dev_random_fd = open(/dev/random, O_RDONLY);

assert(dev_random_fd != -1);

}

next_random_byte = (char *)random_value;

bytes_to_read = sizeof(random_value);

/* 因为是从/dev/random中读取，read可能会被阻塞，一次读取可能只能得到一个字节，

* 循环是为了让我们读取足够的字节数来填充random_value.

*/

do

{

int bytes_read;

bytes_read = read(dev_random_fd, next_random_byte, bytes_to_read);

bytes_to_read -= bytes_read;

next_random_byte += bytes_read;

}while(bytes_to_read > 0);

return min + (random_value % (max - min + 1));

}

同样，还可以用dd命令从/dev/urandom中获取指定字节数的随机值并写入文件中保存--如果你需要以文件的形式提供随机数的话。

dd if=/dev/urandom of = file count = 1 bs = bytes