|
|
西邮陈莉君老师【http://www.lupaworld.com/26540/viewspace-119901.html】 list_entry()宏: -------------------------------------------------------------------------------------------------- /** * list_entry - get the struct for this entry * @ptr: the &struct list_head pointer. * @type: the type of the struct this is embedded in. * @member: the name of the list_struct within the struct. */ #define list_entry(ptr, type, member) \ ((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member))) -------------------------------------------------------------------------------------------------- 指针ptr指向结构体type中的成员member;通过指针ptr,返回结构体type的起始地址,如图2。
type |----------| | | | | |----------| ptr--> | member --| |----------| | | | | |----------| 图2 list_entry()宏的示意图 为了便于理解,在此给予进一步说明。 例如my_list结构: struct my_list{ void *mydata; struct list_head list; }; struct list_head *pos; 则list_entry(pos, mylist, list)宏,就可以根据pos的值,获取mylist的地址,也就是指向mylist的指针,这样,我们就可以存取mylist->mydata字段了。 可为什么能够达到这样的效果? list_entry(pos, mylist, list) 展开以后为: ((struct my_list *)((char *)(pos) - (unsigned long)(&((struct my_list *)0)->list))) 这看起来会使大多数人眩晕,但仔细分析一下,实际很简单。 ((size_t) &(type *)0)->member)把0地址转化为type结构的指针,然后获取该结构中member成员的指针,并将其强制转换为size_t类型。于是,由于结构从0地址开始定义,因此,这样求出member的成员地址,实际上就是它在结构中的偏移量。为了更好的理解这些,我们可以写一段程序来验证: --------------------------------------------------------------------------------------- #include #include struct foobar{ unsigned int foo; char bar; char boo; }; int main(int argc, char** argv){ struct foobar tmp; printf("address of &tmp is= %p\n\n", &tmp); printf("address of tmp->foo= %p \t offset of tmp->foo= %lu\n", &tmp.foo, (unsigned long) &((struct foobar *)0)->foo); printf("address of tmp->bar= %p \t offset of tmp->bar= %lu\n", &tmp.bar, (unsigned long) &((struct foobar *)0)->bar); printf("address of tmp->boo= %p \t offset of tmp->boo= %lu\n\n", &tmp.boo, (unsigned long) &((struct foobar *)0)->boo); printf("computed address of &tmp using:\n"); printf("\taddress and offset of tmp->foo= %p\n", (struct foobar *) (((char *) &tmp.foo) - ((unsigned long) &((struct foobar *)0)->foo))); printf("\taddress and offset of tmp->bar= %p\n", (struct foobar *) (((char *) &tmp.bar) - ((unsigned long) &((struct foobar *)0)->bar))); printf("\taddress and offset of tmp->boo= %p\n", (struct foobar *) (((char *) &tmp.boo) - ((unsigned long) &((struct foobar *)0)->boo))); return 0; } Output from this code is: address of &tmp is= 0xbfffed00 address of tmp->foo= 0xbfffed00 offset of tmp->foo= 0 address of tmp->bar= 0xbfffed04 offset of tmp->bar= 4 address of tmp->boo= 0xbfffed05 offset of tmp->boo= 5 computed address of &tmp using: address and offset of tmp->foo= 0xbfffed00 address and offset of tmp->bar= 0xbfffed00 address and offset of tmp->boo= 0xbfffed00 ---------------------------------------------------------------------------------------- 到此,我们对链表的实现机制有所了解,但在此止步的话,我们依然无法领略这风景背后的韵味。 尽管list.h是内核代码中的头文件,但我们可以把它移植到用户空间使用。且看下一讲,链表接口之应用。 |