深入理解Linux内核list_entry（）宏

list_entry（）宏：

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/**

* 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)))

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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:

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#include <stdio.h>

#include <stdlib.h>

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

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