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(2795)
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