一、如何使用随机函数
查看man可以找到
#include <stdlib.h>
int rand(void);
int rand_r(unsigned int *seedp);
void srand(unsigned int seed);
rand返回0~RAND_MAX之间的整数，是“伪随机”
srand()用来设置rand函数的随机种子，随机种子相同产生的随机数序列也是相同的。
如果没调用srand那么rand函数自动使用1作为种子。
在函数初始化的时候srand()调用设置一次随机种子就可以里，不必每次调用rand都使用srand调用。
随机函数原理
RAND_MAX的值为32767
原理是使用Linear-Congruential-Random-Number-Generator算法产生伪随机数。（具体的我还没弄明白）弄明白再补一篇。
函数如下
static int32_t randtbl[DEG_3 + 1] =
{
TYPE_3,
-1726662223, 379960547, 1735697613, 1040273694, 1313901226,
1627687941, -179304937, -2073333483, 1780058412, -1989503057,
-615974602, 344556628, 939512070, -1249116260, 1507946756,
-812545463, 154635395, 1388815473, -1926676823, 525320961,
-1009028674, 968117788, -123449607, 1284210865, 435012392,
-2017506339, -911064859, -370259173, 1132637927, 1398500161,
-205601318,
};
/* Initialize the random number generator based on the given seed. If the
type is the trivial no-state-information type, just remember the seed.
Otherwise, initializes state[] based on the given "seed" via a linear
congruential generator. Then, the pointers are set to known locations
that are exactly rand_sep places apart. Lastly, it cycles the state
information a given number of times to get rid of any initial dependencies
introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
for default usage relies on values produced by this routine. */
int
__srandom_r (seed, buf)
unsigned int seed;
struct random_data *buf;
{
int type;
int32_t *state;
long int i;
int32_t word;
int32_t *dst;
int kc;
if (buf == NULL)
goto fail;
type = buf->rand_type;
if ((unsigned int) type >= MAX_TYPES)
goto fail;
state = buf->state;
/* We must make sure the seed is not 0. Take arbitrarily 1 in this case. */
if (seed == 0)
seed = 1;
state[0] = seed;
if (type == TYPE_0)
goto done;
dst = state;
word = seed;
kc = buf->rand_deg;
for (i = 1; i < kc; ++i)
{
/* This does:
state[i] = (16807 * state[i - 1]) % 2147483647;
but avoids overflowing 31 bits. */
long int hi = word / 127773;
long int lo = word % 127773;
word = 16807 * lo - 2836 * hi;
if (word < 0)
word += 2147483647;
*++dst = word;
}
buf->fptr = &state[buf->rand_sep];
buf->rptr = &state[0];
kc *= 10;
while (--kc >= 0)
{
int32_t discard;
(void) __random_r (buf, &discard);
}
done:
return 0;
fail:
return -1;
}
int
__random_r (buf, result)
struct random_data *buf;
int32_t *result;
{
int32_t *state;
if (buf == NULL || result == NULL)
goto fail;
state = buf->state;
if (buf->rand_type == TYPE_0)
{
int32_t val = state[0];
val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
state[0] = val;
*result = val;
}
else
{
int32_t *fptr = buf->fptr;
int32_t *rptr = buf->rptr;
int32_t *end_ptr = buf->end_ptr;
int32_t val;
val = *fptr += *rptr;
/* Chucking least random bit. */
*result = (val >> 1) & 0x7fffffff;
++fptr;
if (fptr >= end_ptr)
{
fptr = state;
++rptr;
}
else
{
++rptr;
if (rptr >= end_ptr)
rptr = state;
}
buf->fptr = fptr;
buf->rptr = rptr;
}
return 0;
fail:
__set_errno (EINVAL);
return -1;
}
可以看到，设置随机种子的时候操作很复杂，如果每次rand都调用srand是非常耗时，而且没有用途的。特别是有人总是用srand(time(NULL))，那样一秒钟产生的随机数都是相同的，根本没有作用。
二、使用场景
如果只是为了使用随机得到尽量乱平均的数字，可以使用rand函数，或者使用自己写的
static unsigned long next = 1;
/* RAND_MAX assumed to be 32767 */
int myrand(void) {
next = next * 1103515245 + 12345;
return((unsigned)(next/65536) % 32768);
}
void mysrand(unsigned seed) {
next = seed;
}
也能达到效果，例如在寻址函数中随机挑选一个服务器，为的就是平均，即使轮询影响也不大。
如果是为了加密，构造随机数，那么rand函数不适合你，还是用硬件的函数吧
参考硬件相关的安全随机数：/dev/random和/dev/urandom
参考：
http://blog.csdn.net/flyoxs/article/details/5331718
http://hi.baidu.com/tlogin/item/e619169c2c0dfdc9b6253149