C语言中<<、>>、&、|的实际用途
作为一个开发人员，在看别人项目或者看第三方库的源代码时，可能经常会看到譬如a>>4&0x0f这样的写法，对于一些初入行的童鞋来说可能就不是太明白这是什么意思。下面我们就来讲解这几个运算符的实际用途。
一、运算符的含义
| 运算符 | 含义 | 描述 |
|---|---|---|
| << | 左移 | 用来将一个数的各二进制位全部左移N位，高位舍弃，低位补0。 |
| >> | 右移 | 将一个数的各二进制位右移N位，移到右端的低位被舍弃，对于无符号数，高位补0。 |
| & | 按位与 | 如果两个相应的二进制位都为1，则该位的结果值为1，否则为0。 |
| l | 按位或 | 两个相应的二进制位中只要有一个为1，该位的结果值为1，否则为0。 |
二、用途
按位与的用途
1.清零
若想对一个存储单元清零，即使其全部二进制位为0，只要对这个存储单元当前的值取反(也就是原来的数中为1变为0，0变为1），然后使二者进行&运算，即可达到清零目的。
比如a=23,b=~a:
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">a<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00010111
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:11101000
a&<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00000000
<span aria-hidden="true" class="line-numbers-rows" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">2.取一个数中某些指定位
比如a=23，我想取a的二进制的后面4位数，那么可以找一个后4位是1其余位是0的数b，即b=0x0f(十六进制，转换为二进制为00001111)，a&b就得到了a的后四位。
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">a<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00010111
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00001111
a&<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00000111
<span aria-hidden="true" class="line-numbers-rows" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">3.保留指定位
比如a=23(用8bit表示)，我想保留其二进制的第4和第6位(最左边为第1位)，其余位置0。那么可以找一个第4和第6位是1其余位是0的数b与a进行按位与运算
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">a<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00010111
<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00010100
a&<span class="token property" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">b<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">:00010100
<span aria-hidden="true" class="line-numbers-rows" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">按位或的用途
常用来对一个数的某些位置1，其余位不变。比如a=23，我想将其第2和第3位置为1，可以找一个第2、3位是1其余位是0的数与a进行按位或运算。
a:00010111
b:01100000
a|b:01110111
<span aria-hidden="true" class="line-numbers-rows" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">三、项目实例
在实际项目中，很多时候&和|都是和>>、<<配合使用的。比如在蓝牙项目开发中，2个设备之间进行日期数据传输时，我们先定一个协议，用4个byte来表示一个日期，其中byte0表示年份的高位数，byte1表示年份的低位数，byte2表示月份，byte3表示日期。
设备端现在收到另外一台设备传过来的日期数据00010100 00010011 00000110 00011101（为了阅读方便我将每个byte之间用空格分开）。那么我要如何解析这个数据来得到实际日期呢？
<span class="token comment" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">/*
*第一步，获取日期。
*日期是最后一个byte，也就是最后8位，用一个后8位是1其余位是0的数(11111111,十六进制表示是0xff)与数据进行按位与计算即可
*/
date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010100 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010011 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00000110 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00011101<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">;
day <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">& <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">0xff<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">;<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00011101，十进制表示是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">29，也就是日期是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">29<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)。
<span class="token comment" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">/*
*第二步，获取月份。
*月份是倒数第2个byte，此时需要先将最后一个byte砍掉(也就是右移8位)，然后再和0xff进行按位与运算
*下面代码可以简写成date=date>>8&0xff;
*/
date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date<span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">>><span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">8<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">; <span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010100 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010011 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00000110<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)
month <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">& <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">0xff<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">;<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00000110，十进制表示是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">6，也就是月份是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">6月<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)。
<span class="token comment" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">/*
*第三步，获取年份低位。
*先将最后一个byte砍掉(也就是右移8位)，然后再和0xff进行按位与运算
*下面代码可以简写成date=date>>8&0xff;
*/
date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date<span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">>><span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">8<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">; <span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010100 <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010011<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)
year_low <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">& <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">0xff<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">;<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010011，十进制表示是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">19<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)。
<span class="token comment" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">/*
*第四步，获取年份高位。
*先将最后一个byte砍掉(也就是右移8位)，然后再和0xff进行按位与运算
*下面代码可以简写成date=date>>8&0xff;
*/
date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date<span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">>><span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">8<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">; <span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010100<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)
year_heigh <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">= date <span class="token operator" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">& <span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">0xff<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">;<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">(计算结果是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">00010011，十进制表示是<span class="token number" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">20<span class="token punctuation" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">)。
<span aria-hidden="true" class="line-numbers-rows" style="font-size: inherit; color: inherit; line-height: inherit; margin: 0px; padding: 0px; word-wrap: inherit !important; word-break: inherit !important;">最后将4个数拼起来就得到日期为2019年6月29日。
