c++ - “#define X X”是什么意思？

Question

在Linux头文件epoll.h中，我找到了以下代码:

enum EPOLL_EVENTS
{
    EPOLLIN = 0x001,
#define EPOLLIN EPOLLIN
...
}

#define EPOLLIN EPOLLIN是什么意思？

Answer 1

这将创建一个名为EPOLLIN的宏，其替换文本也是EPOLLIN。

这很可能是预处理器检查哪些事件代码可用并根据需要有条件地编译代码的一种方式。如果转到glibc的git repo并查看git blame的输出，我们将看到enum EPOLL_EVENTS的以下内容:

ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  34) enum EPOLL_EVENTS
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  35)   { 
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  36)     EPOLLIN = 0x001,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  37) #define EPOLLIN EPOLLIN
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  38)     EPOLLPRI = 0x002,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  39) #define EPOLLPRI EPOLLPRI
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  40)     EPOLLOUT = 0x004,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  41) #define EPOLLOUT EPOLLOUT
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  42)     EPOLLRDNORM = 0x040,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  43) #define EPOLLRDNORM EPOLLRDNORM
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  44)     EPOLLRDBAND = 0x080,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  45) #define EPOLLRDBAND EPOLLRDBAND
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  46)     EPOLLWRNORM = 0x100,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  47) #define EPOLLWRNORM EPOLLWRNORM
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  48)     EPOLLWRBAND = 0x200,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  49) #define EPOLLWRBAND EPOLLWRBAND
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  50)     EPOLLMSG = 0x400,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  51) #define EPOLLMSG EPOLLMSG
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  52)     EPOLLERR = 0x008,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  53) #define EPOLLERR EPOLLERR
5e826ab5 (Ulrich Drepper 2003-03-25 01:14:36 +0000  54)     EPOLLHUP = 0x010,
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  55) #define EPOLLHUP EPOLLHUP
94833f11 (Ulrich Drepper 2007-10-28 01:34:10 +0000  56)     EPOLLRDHUP = 0x2000,
94833f11 (Ulrich Drepper 2007-10-28 01:34:10 +0000  57) #define EPOLLRDHUP EPOLLRDHUP
981569c7 (Joseph Myers   2016-03-14 19:04:53 +0000  58)     EPOLLEXCLUSIVE = 1u << 28,
981569c7 (Joseph Myers   2016-03-14 19:04:53 +0000  59) #define EPOLLEXCLUSIVE EPOLLEXCLUSIVE
f8d44fdc (Andreas Jaeger 2012-07-26 13:11:33 +0200  60)     EPOLLWAKEUP = 1u << 29,
f8d44fdc (Andreas Jaeger 2012-07-26 13:11:33 +0200  61) #define EPOLLWAKEUP EPOLLWAKEUP
4920765e (Ulrich Drepper 2011-12-21 22:14:05 -0500  62)     EPOLLONESHOT = 1u << 30,
e11676dd (Ulrich Drepper 2004-01-21 06:23:26 +0000  63) #define EPOLLONESHOT EPOLLONESHOT
4920765e (Ulrich Drepper 2011-12-21 22:14:05 -0500  64)     EPOLLET = 1u << 31
5e826ab5 (Ulrich Drepper 2003-03-25 01:14:36 +0000  65) #define EPOLLET EPOLLET
ad3bf20c (Ulrich Drepper 2002-12-16 23:38:42 +0000  66)   };

从这里，您可以看到这些事件大多数是2002年创建的，但后来又添加了其他事件。因此，这些宏允许您检查特定标志是否可用，如下所示:

#ifdef EPOLLEXCLUSIVE 
/* code that can handle EPOLLEXCLUSIVE */
#else
/* code that uses an alternate event */
#endif

这样，代码可以在具有可用新事件的较新Linux版本上运行，也可以在不具有新事件的较旧版本上运行。