C语言中的位字段

C语言学习笔记---位字段    在C语言中通常操作只有真假两种状态的的数据时使用布尔bool变量比较多,如果需要同时观察多个状态,这时候选择位操作效率会更高,用一个字节的8位分别表示8种状态。比较节省内存,处理起来效率更高。但是这种方法有一个缺点,就是看起来不直观,比如0x5C要想知道每一位的状态还得换算一下,同时要操作某一个单独位时,还必须使用位操作运算,比如位与、位或、异或。    如果熟悉单片机的就会想到,能不能在C语言中像操作寄存器那样直接操作一个字节的单独一位呢?    比如这是一个单片机的端口方向配置寄存器,每一个端口 阅读详情

位字段(bit filed)是C语言中一种存储结构,不同于一般结构体的是它在定义成员的时候需要指定成员所占的位数。位字段是一个signed int或unsigned int类型变量中一组相邻的位(C99和C11新增了Bool类型的位字段)。位字段通过一个结构声明来建立,该结构声明为每个字段提供标签,并确定该字段的宽度。例如,下面的声明建立了4个1位的字段:

struct {

    unsigned autfd : 1;
    unsigned bldfc : 1;
    unsigned undln : 1;
    unsigned itals : 1;
} prnt;

根据该声明,prnt包含了4个1位的字段。现在,可以通过普通的结构成员运算符(.)单独给这些字段赋值:

    prnt.itals=1;
    prnt.undln=0;
    prnt.bldfc=1;
    prnt.autfd=0;

下面查看一下prnt的值:

    char str[33];
    int* value=reinterpret_cast<int*>(&prnt);
    itoa(*value,str,2);
    printf("%d %d %d %d\n", prnt.autfd,prnt.bldfc,prnt.undln,prnt.itals);
    printf("sizeof(prnt) = %d\n",sizeof(prnt));
    printf("十进制: %d\n",prnt);
    printf("二进制: %032s\n",str);

输出的结果

C语言基础知识:位与位字段 计算机中的CPU位数指的是CPU一次能处理的最大位数,例如32位计算机的CPU一次最多能处理32位数据,计算机中的CPU位数也成为机器字长,和数据总线(CPU与内部存储器之间连接的用于传输数据的线的根数)的概念是统一的。在有些机器上,存储的顺序是从左往右的,而在另一些机器上,是从右往左的。stuff的大小为8个字节。注:这个字节与比特的关系是规定的,记住就好,通用于任何场景,容易混淆的是字长和字节,字长指的是cpu一次性能够运算的数据的位数,不同的计算机可能不一样,但是字节这个概念是恒久不变的。 阅读详情

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