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看到内核代码中有很多地方都使用了#if defined(),可是却找不到这个东东的定义,主要是define()。它和我们常用的#ifdef有什么不同吗?

4.2.3 Defined The special operator defined is used in #if and #elif expressions to test whether a certain name is defined as a macro. defined name and defined (name) are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise. Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO.

defined is useful when you wish to test more than one macro for existence at once. For example,

#if defined (__vax__) || defined (__ns16000__)

would succeed if either of the names __vax__ or __ns16000__ is defined as a macro.

Conditionals written like this:

#if defined BUFSIZE && BUFSIZE >= 1024

can generally be simplified to just #if BUFSIZE >= 1024, since if BUFSIZE is not defined, it will be interpreted as having the value zero.

If the defined operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine defined operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option -pedantic, since other compilers may handle it differently.
posted on 2010-07-27 14:22 何克勤 阅读(2796) 评论(1)  编辑  收藏 所属分类: C/C++

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# re: #if defined()的含义(ZZ)[未登录]
2011-11-04 16:43 | jack
it's usefull   回复  更多评论
  

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