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有些信息在存储时,并不需要占用一个完整的字节, 而只需占几个或一个二进制位。例如在存放一个开关量时,只有0和1 两种状态, 用一位二进位即可。为了节省存储空间,并使处理简便,C语言又提供了一种数据结构,称为“位域”或“位段”。所谓“位域”是把一个字节中的二进位划分为几个不同的区域,并说明每个区域的位数。每个域有一个域名,允许在程序中按域名进行操作。这样就可以把几个不同的对象用一个字节的二进制位域来表示。一、位域的定义和位域变量的说明位域定义与结构定义相仿,其形式为:


{ 位域列表 };

其中位域列表的形式为: 类型说明符 位域名:位域长度

struct bs 
    int a:8; 
    int b:2; 
    int c:6; 

位域变量的说明与结构变量说明的方式相同。 可采用先定义后说明,同时定义说明或者直接说明这三种方式。例如:

struct bs 
    int a:8; 
    int b:2; 
    int c:6; 

1. 一个位域必须存储在同一个字节中,不能跨两个字节。如一个字节所剩空间不够存放另一位域时,应从下一单元起存放该位域。也可以有意使某位域从下一单元开始。例如:

struct bs 
    unsigned a:4 
    unsigned :0 /*空域*/ 
    unsigned b:4 /*从下一单元开始存放*/ 
    unsigned c:4 

2. 位域可以无位域名,这时它只用来作填充或调整位置。无名的位域是不能使用的。例如:

struct k 
    int a:1 
    int :2 /*该2位不能使用*/ 
    int b:3 
    int c:2 

从以上分析可以看出,位域在本质上就是一种结构类型, 不过其成员是按二进位分配的。
这是位域操作的表示方法,也就是说后面加上“:1”的意思是这个成员的大小占所定义类型的1 bit,“:2”占2 bit,依次类推。当然大小不能超过所定义类型包含的总bit数。
一个bytes(字节)是8 bit(bit)。例如你的结构中定义的类型是u_char,一个字节,共8bit,最大就不能超过8。
因此你的结构的大小如果用sizeof(struct frame_control)计算,就是2bytes.

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