看驱动的时候，时常会有如下代码：
.remove = __devexit_p(XX_exit),
这里的__devexit_p有什么作用呢？
我在include/linux/init.h中找到了它的定义：
/* Functions marked as __devexit may be discarded at kernel link time, depending
on config options. Newer versions of binutils detect references from
retained sections to discarded sections and flag an error. Pointers to
__devexit functions must use __devexit_p(function_name), the wrapper will
insert either the function_name or NULL, depending on the config options.
*/
#if defined(MODULE) || defined(CONFIG_HOTPLUG)
#define __devexit_p(x) x
#else
#define __devexit_p(x) NULL
#endif
编译成模块或有热拔插功能时， 就需要exit函数。 否则可以为NULL， 节约空间