C语言中位域(bit struct)和#pragma pack(n)指令对其影响
2016-02-24 15:25
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刚才看INTERNETWORKING with TCP/IP Volume 3的时候看到下面代码
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
struct rtp{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned int rtp_cc:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
....
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
有点奇怪,不知道这个unsigned int rtp_cc:4是什么意思,照例google了下得到如下信息:
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:
struct 位域结构名
{ 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
例如:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:8;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:8;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}data;
说明data为bs变量,共占两个字节。其中位域a占8位,位域b占2位,位域c占6位。
(上面的说法好像不太对,我们假设sizeof(int) == 4,那么 sizeof(data) 的值应该是4,也就是说结构体 data 占用了一个 int 所占用的空间。
如图所示:
xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx
---- ---- (data.a 所占的空间 8 bits)
-- (data.b 所占的空间 2bits)
---- -- (data.c 所占的空间 6bits)
可以看到 a, b, c一共占用了低16位的空间。)
对于位域的定义尚有以下几点说明:
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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned a:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
unsigned :0 /*空域*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
unsigned b:4 /*从下一单元开始存放*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned c:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
在这个位域定义中,a占第一字节的4位,后4位填0表示不使用,b从第二字节开始,占用
4位,c占用4位。
(我觉得这里不应该是以字节为单位,应该是以定义的类型为单位,如上例中,就应该以 unsigned 为一个单位,b 从第二个unsigned 开始。
个人觉得,对于空域,不用去管里头到底是 0 还是1,意义不大,因为你访问不到他。
)
2. 由于位域不允许跨两个字节,因此位域的长度不能大于一个字节的长度,也
就是说不能超过8位二进位。
(按照以上的理解,就说明位域的长度不能够超过所定义类型的长度,例如 定义: int a:36就是不允许的)
3. 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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct k
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:1
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
int :2 /*该2位不能使用*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:3
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:2
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
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。
二、位域的使用位域的使用和结构成员的使用相同,其一般形式为:
位域变量名·位域名
位域允许用各种格式输出。
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
main(){
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned a:1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned b:3;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned c:4;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/717446ca04a6125dc5b6b54e0fa14ab4.gif)
} bit,*pbit;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.a=1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.b=7;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.c=15;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
printf("%d,%d,%d ",bit.a,bit.b,bit.c);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit=&bit;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->a=0;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->b&=3;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->c|=1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
printf("%d,%d,%d ",pbit->a,pbit->b,pbit->c);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
上例程序中定义了位域结构bs,三个位域为a,b,c。说明了bs类型的变量bit和指向bs类型
的指针变量pbit。这表示位域也是可以使用指针的。
程序的9、10、11三行分别给三个位域赋值。( 应注意赋值不能超过该位域的允许范围)程
序第12行以整型量格式输出三个域的内容。第13行把位域变量bit的地址送给指针变量
pbit。第14行用指针方式给位域a重新赋值,赋为0。第15行使用了复合的位运算符"&=",
该行相当于:pbit->b=pbit->b&3位域b中原有值为7,与3作按位与运算的结果为3
(111&011=011,十进制值为3)。同样,程序第16行中使用了复合位运算"|=", 相当于:pbit->c=pbit->c|1其结果为15。程序第17行用指针方式输出了这三个域的值。
使用位域的主要目的是压缩存储,其大致规则为:
1) 如果相邻位域字段的类型相同,且其位宽之和小于类型的sizeof大小,则后面的字段将紧邻前一个字段存储,直到不能容纳为止;
2) 如果相邻位域字段的类型相同,但其位宽之和大于类型的sizeof大小,则后面的字段将从新的存储单元开始,其偏移量为其类型大小的整数倍;
3) 如果相邻的位域字段的类型不同,则各编译器的具体实现有差异,VC6采取不压缩方式,Dev-C++采取压缩方式;
4) 如果位域字段之间穿插着非位域字段,则不进行压缩;
5) 整个结构体的总大小为最宽基本类型成员大小的整数倍。
上面的信息来自http://blog.csdn.net/jiyucn/archive/2006/07/01/862085.aspx。
对我来说位域对那些对关系每位是什么的程序提供了便利。
还有就是#pragma pack(n)对位域的影响和其他类型相同。
看下面程序:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
#include <iostream>
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
//#pragma pack(1)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:5;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:4;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int d:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char e:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char f:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct s
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int d;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char e;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char f;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
int main()
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
std::cout<<sizeof(bs)<<sizeof(s);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bs _bs;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
//std::cout<<sizeof(_bs.e); //error C2070: 'char': illegal sizeof operand
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
return 0;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
在这里bs总共占有 4+3+1 byte
4: sizeof(int)
3: for the alignment
1: sizeof(char)
位域是将C一个类型的变量分开(提供一个分开操作某种数据类型的机制,使操作粒度到达bit级别)管理
如果把上面程序中第二行的注释去掉
则bs总共占用 4+1 byte
4: sizeof(int)
//3: alignment every 1 byte accoeding to the #pragma pack(1), so this blank is removed.
1: sizeof(char)
这里给出struct s最为对比
注释掉pack指令
sizeof(s) = 4+4+4+4+2+2 = 20 byte
加上pack指令
sizeof(s) = 4+4+4+4+1+1 = 18 byte
又遇到一个C中不懂的东西。以后绝对不能在简历中写:精通C/C++.真的还有很长路要走。
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
struct rtp{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned int rtp_cc:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
....
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
有点奇怪,不知道这个unsigned int rtp_cc:4是什么意思,照例google了下得到如下信息:
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:
struct 位域结构名
{ 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
例如:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:8;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:8;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}data;
说明data为bs变量,共占两个字节。其中位域a占8位,位域b占2位,位域c占6位。
(上面的说法好像不太对,我们假设sizeof(int) == 4,那么 sizeof(data) 的值应该是4,也就是说结构体 data 占用了一个 int 所占用的空间。
如图所示:
xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx
---- ---- (data.a 所占的空间 8 bits)
-- (data.b 所占的空间 2bits)
---- -- (data.c 所占的空间 6bits)
可以看到 a, b, c一共占用了低16位的空间。)
对于位域的定义尚有以下几点说明:
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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned a:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
unsigned :0 /*空域*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
unsigned b:4 /*从下一单元开始存放*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned c:4
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
在这个位域定义中,a占第一字节的4位,后4位填0表示不使用,b从第二字节开始,占用
4位,c占用4位。
(我觉得这里不应该是以字节为单位,应该是以定义的类型为单位,如上例中,就应该以 unsigned 为一个单位,b 从第二个unsigned 开始。
个人觉得,对于空域,不用去管里头到底是 0 还是1,意义不大,因为你访问不到他。
)
2. 由于位域不允许跨两个字节,因此位域的长度不能大于一个字节的长度,也
就是说不能超过8位二进位。
(按照以上的理解,就说明位域的长度不能够超过所定义类型的长度,例如 定义: int a:36就是不允许的)
3. 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:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct k
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:1
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
int :2 /*该2位不能使用*/
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:3
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:2
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
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。
二、位域的使用位域的使用和结构成员的使用相同,其一般形式为:
位域变量名·位域名
位域允许用各种格式输出。
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
main(){
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/37c8bf68cdc3cc81759c34160776bc53.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned a:1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned b:3;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
unsigned c:4;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201104/717446ca04a6125dc5b6b54e0fa14ab4.gif)
} bit,*pbit;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.a=1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.b=7;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bit.c=15;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
printf("%d,%d,%d ",bit.a,bit.b,bit.c);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit=&bit;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->a=0;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->b&=3;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
pbit->c|=1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
printf("%d,%d,%d ",pbit->a,pbit->b,pbit->c);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
上例程序中定义了位域结构bs,三个位域为a,b,c。说明了bs类型的变量bit和指向bs类型
的指针变量pbit。这表示位域也是可以使用指针的。
程序的9、10、11三行分别给三个位域赋值。( 应注意赋值不能超过该位域的允许范围)程
序第12行以整型量格式输出三个域的内容。第13行把位域变量bit的地址送给指针变量
pbit。第14行用指针方式给位域a重新赋值,赋为0。第15行使用了复合的位运算符"&=",
该行相当于:pbit->b=pbit->b&3位域b中原有值为7,与3作按位与运算的结果为3
(111&011=011,十进制值为3)。同样,程序第16行中使用了复合位运算"|=", 相当于:pbit->c=pbit->c|1其结果为15。程序第17行用指针方式输出了这三个域的值。
使用位域的主要目的是压缩存储,其大致规则为:
1) 如果相邻位域字段的类型相同,且其位宽之和小于类型的sizeof大小,则后面的字段将紧邻前一个字段存储,直到不能容纳为止;
2) 如果相邻位域字段的类型相同,但其位宽之和大于类型的sizeof大小,则后面的字段将从新的存储单元开始,其偏移量为其类型大小的整数倍;
3) 如果相邻的位域字段的类型不同,则各编译器的具体实现有差异,VC6采取不压缩方式,Dev-C++采取压缩方式;
4) 如果位域字段之间穿插着非位域字段,则不进行压缩;
5) 整个结构体的总大小为最宽基本类型成员大小的整数倍。
上面的信息来自http://blog.csdn.net/jiyucn/archive/2006/07/01/862085.aspx。
对我来说位域对那些对关系每位是什么的程序提供了便利。
还有就是#pragma pack(n)对位域的影响和其他类型相同。
看下面程序:
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
#include <iostream>
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
//#pragma pack(1)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct bs
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a:1;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b:5;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c:4;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int d:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char e:2;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char f:6;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
struct s
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int a;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int b;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int c;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
int d;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char e;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
char f;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
};
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
![](https://oscdn.geek-share.com/Uploads/Images/Content/2015/2015030196/6810355c2f78c12e91b7997a8e8c583a.gif)
int main()
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/a41954a27d6ad96fa2c2cf816e677448.gif)
{
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
std::cout<<sizeof(bs)<<sizeof(s);
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
bs _bs;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
//std::cout<<sizeof(_bs.e); //error C2070: 'char': illegal sizeof operand
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/6a9c071a08f1dae2d3e1c512000eef41.gif)
return 0;
![](https://oscdn.geek-share.com/Uploads/Images/Content/201109/0196c3df5ea9e936f21e9932cca91014.gif)
}
在这里bs总共占有 4+3+1 byte
4: sizeof(int)
3: for the alignment
1: sizeof(char)
位域是将C一个类型的变量分开(提供一个分开操作某种数据类型的机制,使操作粒度到达bit级别)管理
如果把上面程序中第二行的注释去掉
则bs总共占用 4+1 byte
4: sizeof(int)
//3: alignment every 1 byte accoeding to the #pragma pack(1), so this blank is removed.
1: sizeof(char)
这里给出struct s最为对比
注释掉pack指令
sizeof(s) = 4+4+4+4+2+2 = 20 byte
加上pack指令
sizeof(s) = 4+4+4+4+1+1 = 18 byte
又遇到一个C中不懂的东西。以后绝对不能在简历中写:精通C/C++.真的还有很长路要走。
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