#ifdef Macro_Value 等价于 #if defined (Macro_Value)

在Linux和u-boot源码里面，经常看到 #ifdef 和 #if defined ( ) 的用法：

#include <stdio.h>#define hello_worldint main( void ){#if defined ( hello_world )    printf ( "hello world is defined.\n" );#endif #ifdef hello_world    printf ( "hello world is defined.\n" );#endif    return 0;}

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执行结果：
hello world is defined.
hello world is defined.

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总结：

define Macro_Value 是选择开关;

define Macro_Value 10 定义宏变量;