offsetof的使用？ [英] Uses of offsetof?
问题描述
出于个人的好奇心:)
人们使用offsetof（）做什么？我的意思是，我可以理解为什么你会想要能够获取结构成员的地址，但是你只需&&b
就可以;（sa）或类似的。为什么你会关心
偏移量（这肯定取决于编译器如何选择在内存中放置
结构），我真的不知道。如果你确实这么做了，那么就不会抵消（s，a）只是&（sa） - & s？
Just out of personal curiosity :)
What do people use offsetof() for? I mean, I can understand why you''d
want to be able to take the address of a member of a struct, but you
can do that with just &(s.a) or similar. Why you''d care about the
offset (which surely depends on how the compiler chooses to lay the
struct out in memory), I don''t really know. And if you did really
care, won''t offset(s,a) just be &(s.a) - &s ?
推荐答案
3月24日上午12:06，Francine.Ne ... @ googlemail.com写道：
On Mar 24, 12:06 am, Francine.Ne...@googlemail.com wrote:
Just出于个人的好奇心:)
人们使用offsetof（）做什么？我的意思是，我可以理解为什么你会想要能够获取结构成员的地址，但是你只需&&b
就可以;（sa）或类似的。为什么你会关心
偏移量（这肯定取决于编译器如何选择在内存中放置
结构），我真的不知道。如果你确实这么做了，那么就不会错过（s，a）只是&（s.a） - & s？
Just out of personal curiosity :)
What do people use offsetof() for? I mean, I can understand why you''d
want to be able to take the address of a member of a struct, but you
can do that with just &(s.a) or similar. Why you''d care about the
offset (which surely depends on how the compiler chooses to lay the
struct out in memory), I don''t really know. And if you did really
care, won''t offset(s,a) just be &(s.a) - &s ?
嗯，我自己一直在做一些实验...我的最后一个
声明似乎是假的。例如，考虑
以下代码：
#include< stdio.h>
#include< stdlib。 h>
#include< stddef.h>
#include< string.h>
#define STRING" ;只是一个测试
struct s {int a; char * b; int c; };
struct t {char * b; int c; };
main（）
{
struct sa;
aa = 10，ab = malloc（strlen（STRING）+1），strcpy（ab，STRING），ac = 20;
struct t * b =& ab;
struct t * c =& a + offsetof（struct s，b）;
printf（"％s：％d \ n"，b-> b，b-> c）;
printf（"％s：％d \ n"，c-> b，c-> c）;
}
现在看看会发生什么：
Hmmm, I''ve been doing some experiments with this myself... The last
statement of mine seems to be false. For example, consider the
following code:
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#define STRING "just a test"
struct s { int a; char *b; int c; };
struct t { char *b; int c; };
main()
{
struct s a;
a.a=10, a.b=malloc(strlen(STRING)+1), strcpy(a.b,STRING), a.c=20;
struct t *b=&a.b;
struct t *c=&a+offsetof(struct s,b);
printf("%s : %d\n", b->b, b->c);
printf("%s : %d\n", c->b, c->c);
}
Now see what happens:
./ a.out
只是一个测试：20
（null）： - 1208827916
所以offsetof（）对于找到子结构的方式甚至没有用！
./a.out
just a test : 20
(null) : -1208827916
So offsetof() isn''t even useful for finding your way to substructures!
在文章< 11 ********************** @ b75g2000hsg.googlegroups .com> ;,
< Fr ************ @ googlemail.comwrote：
In article <11**********************@b75g2000hsg.googlegroups .com>,
<Fr************@googlemail.comwrote:
struct sa;
struct s a;
[...]
[...]
struct t * c =& a + offsetof（struct s ，b）;
struct t *c=&a+offsetof(struct s,b);
由于& a属于（struct s *）类型，因此添加的是
大小的单位。如果你这样做
struct t * c =（（char *）& a）+ offsetof（struct s，b）;
你会得到你期望的答案。
offsetof（）的一个用途是有效地传递一个
结构的成员，当* which *成员时它在编译时是不知道的。
而是传递所需成员的偏移量。当然，你还需要以某种方式知道你想要操作的
对象的类型 - 或者至少是大小 -
- Richard
-
考虑到需要多达32个字符
字母" - 1963年的X3.4。
Since &a is of type (struct s *), the addition is in units of the
size of s. If you do
struct t *c=((char *)&a)+offsetof(struct s,b);
you will get the answer you expect.
One use of offsetof() is to effectively pass around a member of a
structure, when *which* member it is is not known at compile time.
Instead you pass the offset of the desired member. Of course, you
also need to somehow know the type - or at least the size - of the
object you wish to manipulate.
-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.
这篇关于offsetof的使用？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
