#define / #undef /#progma 预处理器

╰半夏微凉° 2022-07-15 13:17 119阅读 0赞

预处理，在程序编译之前先进行处理，

\#define     reg     register

\#define     do\_forever     for( ; ; )//实现无限循环的语句

\#define     CASE     break;case   //会在每个case之前放一个break

\#define     malloc                       //将语句注释空，可以有效控制程序随意引用

\#undef     malloc                         //取消定义恢复原本状态

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当定义中的stuff 很长，可以分成几行，除了最后一行之外，其余每一行都要加入一个反斜杠  \\  ，如下面的例子

\#define     DEBUG\_PRINT     printf(  "File %s line %d:"  \\

"x = %d, y = %d, z = %d", \\

\_\_FILE\_\_，\_\_LINE\_\_,     \\

x ,y ,z )

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预定义的符号

<table style=""> 
 <tbody> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.29145728643216%"> 符号</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.29145728643216%"> 样例值</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.29145728643216%"> 含义</td> 
  </tr> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> __FILE__</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> "name.c"</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 进行编译的源文件名 &nbsp; //在确认调试输出的来源很有用处</td> 
  </tr> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> __LINE__</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 25</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 文件当前行的行号 &nbsp; &nbsp; //在确认调试输出的来源很有用处</td> 
  </tr> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> __DATE__</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> "Jan 31 1995 "</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 文件被编译的日期</td> 
  </tr> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> __TIME__</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> "18 : 04：30 "</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 文件被编译的时间</td> 
  </tr> 
  <tr> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> __STDC__</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 1</td> 
   <td style="border-style:solid; border-width:1px; border-color:rgb(211,211,211); padding:10px; margin:0px; width:33.33%"> 如果遵循ANSIC,其值就是1，否则为定义</td> 
  </tr> 
 </tbody> 
</table>

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在define的语句之后不能加“；” 系统在编译的时候会自动加上去

**假设你需要这样的宏定义**

**\#define     SQUARE(x)   ((X) \* (X))**

**这样就可有效的避免很多问题**

使用技巧

\#define     PRINT( FORMAT , VALUE)     \\

printf( "The value  of " **\#VALUE**   \\

" is " FORMAT "\\n" ,VALUE)

PRINT( "%d" ,x+3);

the value of x+3 is 25

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