位域3

1、

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char** argv)
{
    union
    {
        struct
        {
            unsigned char a:1;
            unsigned char b:2;
            unsigned char c:3;
        }d;
        unsigned char e;
    } f;
    f.e = 1;
    printf("%d\n",f.d.a);
    return 0;
}

（小端情况下）输出：1

为什么要加上小端情况下这个限制呢？我是这样想的，正确性有待证明，一个字节有8比特位，从左到右暂且称之为

b7 b6 b5 b4 b3 b2 b1 b0， 那么unsigned char a:1;到底对应哪一位呢，是b7还是b0。如果是小端的话就是b0，如果是大端的话，就是b7。

这个知识点还有待继续深挖，下面两个网址里的内容以后要研究一下

http://blog.163.com/niuxiangshan@126/blog/static/1705965952011529103742195/

http://en.wikipedia.org/wiki/Endianess

2、

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char** argv)
{
    union
    {
        struct
        {
            char a:1;
            char b:2;
            char c:3;
        }d;
        char e;
    } f;
    f.e = 1;
    printf("%d\n",f.d.a);
    return 0;
}

（小端情况下）输出：-1

 3、unsigned 与signed

    struct test 
    {
       short    a : 2;
       unsigned short  b : 3;
    };
//输出 2,not 4

 

4、地址不可取

最后说的一点就是位域是一个字节单元里面的一段，是没有地址的！

5、

While we're on the subject of structures, we might as well look at bitfields. They can only be declared inside a structure or a union (or a class in c++), and allow you to specify some very small objects of a given number of bits in length. Their usefulness is limited and they aren't seen in many programs, but we'll deal with them anyway.

6、

#include <stdio.h>

int main(void)
{

    struct {

          /* field 4 bits wide */
          unsigned field1 :4;

          /*
           * unnamed 3 bit field
           * unnamed fields allow for padding
           */
          unsigned        :3;

          /*
           * one-bit field
           * can only be 0 or -1 in two's complement!
           */
          signed field2   :1;

          /* align next field on a storage unit */
          unsigned        :0;
          unsigned field3 :6;
    }full_of_fields;
    printf("%d\n", sizeof(full_of_fields));

    return 0;
}
输出8

7、

#include <stdio.h>

int main(void)
{

    struct {

          /* field 4 bits wide */
          unsigned field1 :4;

          /*
           * unnamed 3 bit field
           * unnamed fields allow for padding
           */
          unsigned        :3;

          /*
           * one-bit field
           * can only be 0 or -1 in two's complement!
           */
          signed field2   :1;

          unsigned field3 :6;
    }full_of_fields;
    printf("%d\n", sizeof(full_of_fields));

    return 0;
}
//输出4