imported>潜水飞机：更正链接

2020-12-25T17:57:50Z

更正链接

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{{lowercase}}
{{NoteTA
|G1 = IT
|1 = zh-cn:宏; zh-tw:巨集;
}}
[[C语言]]的'''<tt>offsetof()</tt>'''宏，是定义在<tt>[[stddef.h]]</tt>。用于求出一个[[结构体_(C语言)|struct]]或[[联合体|union]]数据类型的给定成员的<tt>[[size_t]]</tt>类型的字节偏移值。<tt>offsetof()</tt>宏有两个参数，分别是结构名与结构内的成员名。不能声明为C原型。<ref>{{cite web
| url = http://msdn.microsoft.com/en-us/library/dz4y9b9a.aspx
| publisher = [[MSDN]]
| title = offsetof reference
| accessdate = 2010-09-19
| archive-date = 2011-10-10
| archive-url = https://web.archive.org/web/20111010130120/http://msdn.microsoft.com/en-us/library/dz4y9b9a.aspx
| dead-url = no
}}</ref>

==实现==
传统实现依赖于编译器对指针不是很挑剔。它假定结构的地址为0，然后获得成员的偏移值：
<syntaxhighlight lang="c">
#define offsetof(st, m) ((size_t)&(((st *)0)->m))
</syntaxhighlight>

上述定义在[[C11]]语言标准下是[[未定义行为]]，<ref>{{cite web
|url=https://stackoverflow.com/questions/26906621/does-struct-name-null-b-cause-undefined-behaviour-in-c11
|title=Does &((struct name *)NULL -> b) cause undefined behaviour in C11?
|accessdate=2015-02-07
|archive-date=2015-02-07
|archive-url=https://web.archive.org/web/20150207105717/http://stackoverflow.com/questions/26906621/does-struct-name-null-b-cause-undefined-behaviour-in-c11
|dead-url=no
}}</ref> 因为它对空指针做了解引用（dereference）。[[GCC]]现在定义该宏为：<ref>{{cite web
| url = https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Offsetof.html
| title = GCC offsetof reference
| publisher = [[Free Software Foundation]]
| accessdate = 2010-09-19
| archive-date = 2010-07-24
| archive-url = https://web.archive.org/web/20100724235737/http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Offsetof.html
| dead-url = no
}}</ref>
<syntaxhighlight lang="c">
#define offsetof(st, m) __builtin_offsetof(st, m)
</syntaxhighlight>
这种内置对C++的<tt>class</tt>或<tt>struct</tt>也适用。<ref>{{cite web
| url = https://stackoverflow.com/questions/400116/what-is-the-purpose-and-return-type-of-the-builtin-offsetof-operator/406941#406941
| title = what is the purpose and return type of the __builtin_offsetof operator?
| accessdate = 2012-10-20
| archive-date = 2014-12-15
| archive-url = https://web.archive.org/web/20141215044302/http://stackoverflow.com/questions/400116/what-is-the-purpose-and-return-type-of-the-builtin-offsetof-operator/406941#406941
| dead-url = no
}}</ref>

==用途==
[[Linux内核]]使用<tt>offsetof()</tt>来实现<tt>container_of()</tt>，这允许类似于[[mixin]]类型以发现包含它的结构:<ref>{{cite web
| url = http://www.kroah.com/log/linux/container_of.html
| title = <tt>container_of()</tt>
| author = Greg Kroah-Hartman
| publisher = [[Linux Journal]]
| date = June 2003
| accessdate = 2010-09-19
| archive-date = 2010-02-13
| archive-url = https://web.archive.org/web/20100213091627/http://www.kroah.com/log/linux/container_of.html
| dead-url = no
}}</ref>
<syntaxhighlight lang="c">
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
</syntaxhighlight>

<tt>container_of()</tt>宏被用于从指向内嵌成员的指针获得外包的结构的指针。如链表<tt>my_struct</tt>:
<syntaxhighlight lang="c">
struct my_struct {
const char *name;
struct list_node list;
};

extern struct list_node * list_next(struct list_node *);

struct list_node *current = /* ... */
while(current != NULL){
struct my_struct *element = container_of(current, struct my_struct, list);
printf("%s\n", element->name);
current = list_next(&element->list);
}
</syntaxhighlight>

Linux内核实现<tt>container_of()</tt>时，使用了GNU C扩展''statement expressions''.<ref>{{cite web
| url = https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
| title = Statements and Declarations in Expressions
| publisher = [[Free Software Foundation]]
| accessdate = 2016-01-01
| archive-date = 2016-01-05
| archive-url = https://web.archive.org/web/20160105221853/http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
| dead-url = no
}}</ref>下述实现也能确保类型安全:

<syntaxhighlight lang="c">
#define container_of(ptr, type, member) ((type *)((char *)(1 ? (ptr) : &((type *)0)->member) - offsetof(type, member)))
</syntaxhighlight>

粗看起来，上述的不寻常的?:条件运算符是不适当的。可以写成更简单的形式:

<syntaxhighlight lang="c">
#define container_of(ptr, type, member) ((type *)((char *)(ptr) - offsetof(type, member)))
</syntaxhighlight>

这种写法忽略了检查ptr的类型是否是member的类型，而Linux内核的实现需要这种安全检查。而?:条件运算符要求如果操作数是一个类型的两个指针值，那么它们应当是兼容的类型。所以第三个操作数虽然不会被使用，但编译器要检查<code>(ptr)</code>与<code>&((type *)0)->member</code>是不是兼容的指针类型。

==局限性==
C++03要求<tt>offsetof</tt>限于[[POD_(程序设计)|POD]]类型。[[C++11]]要求<tt>offsetof</tt>限于[[标准布局类型]]，<ref>{{cite web
| url = http://www.cplusplus.com/reference/cstddef/offsetof
| title = <tt>offsetof reference</tt>
| publisher = [[cplusplus.com]]
| accessdate = 2016-04-01
| archive-date = 2016-03-30
| archive-url = https://web.archive.org/web/20160330195954/http://www.cplusplus.com/reference/cstddef/offsetof/
| dead-url = no
}}</ref> 但仍存在未定义行为。特别是[[虚继承]]情形。<ref>{{cite web
| url = https://stackoverflow.com/a/1130760
| title = <tt>Why can't you use offsetof on non-POD structures in C</tt>
| author = Steve Jessop
| publisher = [[Stack Overflow]]
| date = July 2009
| accessdate = 2016-04-01
| archive-date = 2019-10-19
| archive-url = https://web.archive.org/web/20191019011135/https://stackoverflow.com/questions/1129894/why-cant-you-use-offsetof-on-non-pod-structures-in-c/1130760
| dead-url = no
}}</ref>
下述代码用gcc 4.7.3 amd64编译器，产生的结果是有问题的:
<syntaxhighlight lang="c++">
#include <stddef.h>
#include <stdio.h>

struct A
{
int a;
virtual void dummy() {}
};

struct B: public virtual A
{
int b;
};

int main()
{
printf("offsetof(A,a) : %zu\n", offsetof(A, a));
printf("offsetof(B,b) : %zu\n", offsetof(B, b));
return 0;
}
</syntaxhighlight>
Output is:
<pre>
offsetof(A,a) : 8
offsetof(B,b) : 8
</pre>

==参考文献==
{{reflist}}

[[Category:C標準函式庫]]

Offsetof - 版本历史

imported>潜水飞机：​更正链接

imported>潜水飞机：更正链接