#if defined() 含义
来源:互联网 发布:mac html5开发工具 编辑:程序博客网 时间:2024/06/05 20:21
文章出处:http://www.blogjava.net/tinysun/archive/2010/07/27/327215.html
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 expressionsto 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.
- #if defined(...)的含义
- #if defined() 含义
- #if defined() 含义
- #if defined(...)
- #if defined
- #if defined和#if !defined
- #if,#ifdef,#if defined
- #if #ifdef #if defined
- #if/#if defined/#ifdef
- #if,#ifdef和#if defined
- #if #ifdef #if defined 区别
- #if,#ifdef和#if defined
- #if,#ifdef和#if defined
- #if #ifdef #if defined 区别
- #if #ifndef #if defined等
- #if defined()的用法
- ifdef 与if defined
- #if defined 的意思?
- Linq中Group和Sum的应用
- SQLite使用手记
- Objective-C中委托和协议详解
- 计数排序的C++实现
- 静态NAT配置
- #if defined() 含义
- 装系统时出现蓝屏
- RIP协议配置
- js 操作select和option(转载)
- 新学期的总结
- logback的使用(1)
- OSPF协议配置
- HTML5是什么东东,为什么要关注它
- peipei