得到某一范围的随机数

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <stdlib.h>
int main()
{
    unsigned int seed;
    struct timeval now;
    int i=0;
    float tmp;
    float rand;
    gettimeofday(&now, NULL);
    seed = now.tv_sec ^ now.tv_usec;

    while (i<10)
    {
        printf("rand = %.1f\n", (float)rand_r(&seed)/RAND_MAX*0.4+0.1);
        i++;
    }
}

[zhanghua@melon TEST]$ gcc rand_r-test.c -o rand_r-test
[zhanghua@melon TEST]$ ./rand_r-test
rand = 0.4
rand = 0.1
rand = 0.4
rand = 0.3
rand = 0.3
rand = 0.3
rand = 0.2
rand = 0.4
rand = 0.2
rand = 0.4
[zhanghua@melon TEST]$ ./rand_r-test
rand = 0.2
rand = 0.3
rand = 0.1
rand = 0.2
rand = 0.3
rand = 0.5
rand = 0.2
rand = 0.1
rand = 0.2
rand = 0.2

上面例子生成0.1 - 0.5之间的随机数.
其中:
rand_r(&seed)/RAND_MAX将得到0 -1 之间的数.
乘以0.4则生成0 - 0.4之间的数, 再加上0.1得到0.1 - 0.5之间的随机数.