这仅仅是扩大，好了，没事的宏。然而，现在宏已经被定义，你可以用检查＃如果定义（或 #IFDEF ）有否被定义。

It is simply a macro that expands to, well, nothing. However, now that the macro has been defined you can check with #if defined (or #ifdef) whether it has been defined.

#define FOO

int main(){
    FOO FOO FOO
    printf("Hello world");
}

将扩大到

int main(){

    printf("Hello world");
}

有某些情况下这是非常方便，例如额外的调试信息，这些信息你不想在你的发布版本显示：

There are certain situations where this comes in very handy, for example additional debug information, which you don't want to show in your release version:

#ifndef NDEBUG
#define DEBUG_MSG(x) print(x)
#else
#define DEBUG_MSG(x) 
#endif

int main(){
    DEBUG_MSG("Entering main");
    /* ... */
}

如果 NDEBUG （没有调试）已定义的宏， DEBUG_MSG 将扩大到什么，否则你会获得进入主。需要注意的是杂散分号; 是没有问题的。这是一个有效的空语句。

If the macro NDEBUG (no debug) has been defined, DEBUG_MSG will expand to nothing, otherwise you will get Entering main. Note that the stray semicolon ; isn't a problem. It's a valid empty statement.