/ dev / random总是返回相同的序列

我打算使用/ dev / random输出作为openssl密钥生成的种子,然后我编写这个小程序只是为了检查我要做什么：

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

#define LEN 128

void uc2hex(char* hex,unsigned char* uc,unsigned short uc_len)
{
    FILE* bp=fmemopen(hex,2*uc_len+1,"w");
    unsigned short i;
    for(i=0;i<uc_len;i++)
    {
        fprintf(bp,"%02x",uc[i]);
        //printf("%02x\n",uc[i]);
        //fprintf(bp,"%d-",i);
    }
    fprintf(bp,"%c",'\0');
    fclose(bp);
}

int main()
{
    unsigned char buf[LEN];
    char str[2*LEN+1];
    int fd=open("/dev/random",O_RDONLY);
    read(fd,buf,LEN);
    uc2hex(str,LEN);
    printf("%s\n",str);
    close(fd);
    return 0;
}

我运行程序一两次,一切似乎工作正常,但后来我再次按顺序运行了四次,这是输出：

[walter@eM350 ~]$./random 
0ee08c942ddf901af1278ba8f335b5df8db7cf18e5de2a67ac200f320a7a20e84866f533667a7e66a4572b3bf83d458e6f71f325783f2e3f921868328051f8f296800352cabeaf00000000000000000001000000000000005d08400000000000c080300e00000000000000000000000010084000000000000006400000000000
[walter@eM350 ~]$./random 
1f69a0b931c16f796bbb1345b3f58f17f74e3df600000000bb03400000000000ffffffff00000000880e648aff7f0000a88103b4d67f000000305cb4d67f000030415fb4d67f0000000000000000000001000000000000005d08400000000000c080300e00000000000000000000000010084000000000000006400000000000
[walter@eM350 ~]$./random 
4e8a1715238644a840eb66d9ff7f00002e4e3df600000000bb03400000000000ffffffff00000000a8ec66d9ff7f0000a871a37ad97f00000020fc7ad97f00003031ff7ad97f0000000000000000000001000000000000005d08400000000000c080300e00000000000000000000000010084000000000000006400000000000
[walter@eM350 ~]$./random 
598c57563e8951e6f0173f0cff7f00002e4e3df600000000bb03400000000000ffffffff0000000058193f0cff7f0000a8e1cbda257f0000009024db257f000030a127db257f0000000000000000000001000000000000005d08400000000000c080300e00000000000000000000000010084000000000000006400000000000

除了128字节的随机字符串之外,Theese对我来说似乎很重要,因为它们大多是相同的.然后,排除NSA篡改linux内核随机数生成器的可能性,我只能猜测这与我的机器中的可用熵有关,当我在序列中询问太多字节时,它会耗尽.我的问题是：
1)这个猜测是否正确？
2)假设1)是正确的,我怎么知道是否有足够的熵来产生真正的随机字节序列？

解决方法

从手册页中读取：

成功完成后,read(),readv()和pread()返回实际读取并放入缓冲区的字节数.如果描述符引用了在文件结束之前剩余那么多字节的普通文件,系统保证读取所请求的字节数,但不是其他情况.

底线：检查读取的返回值并查看实际读取的字节数 – 可能没有足够的熵来生成您请求的字节数.

int len = read(fd,LEN);
printf("read() returned %d bytes: ",len);
if (len > 0)
{
    uc2hex(str,len);
    printf("%s\n",str);
}

测试：

$./a.out 
read() returned 16 bytes: c3d5f6a8ee11ddc16f00a0dea4ef237a
$./a.out
read() returned 8 bytes: 24e23c57852a36bb
$./a.out 
read() returned 16 bytes: 4ead04d1eedb54ee99ab1b25a41e735b
$