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my sample code, thanks for you attention.

mike20030405(麦客)

#include <stdio.h>

struct s{
char c;
int a;
long l;
};

struct s *get_s_ptr(int *a_ptr)
{
struct s r;
return (struct s *)((unsigned long)a_ptr - ((unsigned long)&r.a - (unsigned long)&r));

}

main()
{
struct s r, *r_ptr;

r.c = 'A';
r.a = 10;
r.l = 123456789;

printf("r.c=%c r.a=%d r.l=%ld\n", r.c, r.a, r.l);
r_ptr = get_s_ptr(&r.a);
printf("r_ptr->c=%c r_ptr->a=%d r_ptr->l=%ld\n", r_ptr->c, r_ptr->a, r_ptr->l);
}

(#3810885@0)
Last Updated: 2007-7-16
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工作学习 / 专业技术讨论 / 一个c语言的考题看不明白, 那位帮忙解释解释? 谢谢. (考题原文见内) -mike20030405(麦客); 2007-7-16 {199} (#3810771@0)
Given a pointer to member a within a struct, write a routine that returns a pointer to the struct.

struct s
{
...
int a;
…
};

struct s *get_s_ptr(int *a_ptr)
{

}

The key is the offset of a in s ==> s = a - offset -realengineer(engineer); 2007-7-16 (#3810801@0)

seems we have the same way to understand the question. now the problem is: how to know the offset? -mike20030405(麦客); 2007-7-16 (#3810816@0)

在函数里建一个s的实例r，用r.a的地址减r的地址就是offset.这样理解对吗？ -mike20030405(麦客); 2007-7-16 (#3810831@0)

应该有 macros 可以做这个的吧。不行再在 function 里建 instance，只是要设计好一点，重用结果，而不是an instance per call。不用C十几年了，不知道对不对。 -realengineer(engineer); 2007-7-16 (#3810866@0)

my sample code, thanks for you attention. -mike20030405(麦客); 2007-7-16 {546} (#3810885@0)
#include <stdio.h>

struct s{
char c;
int a;
long l;
};

struct s *get_s_ptr(int *a_ptr)
{
struct s r;
return (struct s *)((unsigned long)a_ptr - ((unsigned long)&r.a - (unsigned long)&r));

}

main()
{
struct s r, *r_ptr;

r.c = 'A';
r.a = 10;
r.l = 123456789;

printf("r.c=%c r.a=%d r.l=%ld\n", r.c, r.a, r.l);
r_ptr = get_s_ptr(&r.a);
printf("r_ptr->c=%c r_ptr->a=%d r_ptr->l=%ld\n", r_ptr->c, r_ptr->a, r_ptr->l);
}

加减instance的地址？还是用sizeof算offset? -ice(Endless Waiting); 2007-7-16 (#3810904@0)

result of sizeof() has no meaning to the offset -mike20030405(麦客); 2007-7-16 (#3810939@0)

structure should have a continuous memery chunk. You got a pointer pointing to somewhere in the middle of an instance in the momory. should you use the given pointer minus sizeof() whatever variables -ice(Endless Waiting); 2007-7-16 {121} (#3810976@0)
which are allocated in front of the given pointer?

just don't understand why you want to involve a whatever instance.

不一定。可能有对其的问题，页就是会有空穴。 -wangqingshui(忘情水); 2007-7-16 (#3811088@0)

有一个宏offsetof可以直接用，你这个也算可以但是到64位上就不行了。 -baalinca(scarb); 2007-7-16 (#3811200@0)
return (struct s *)( (char *)a_ptr - ((cha *)&r - (char *)&r.a); -useless(爱雨虹); 2007-7-18 (#3815337@0)

打漏个字符 return (struct s *)( (char *)a_ptr - ((cha *)&r - (char *)&r.a); -useless(爱雨虹); 2007-7-18 (#3815341@0)

反了：return (struct s *)( (char *)a_ptr - ((cha *)&r.a - (char *)&r); -useless(爱雨虹); 2007-7-18 (#3815351@0)

return (struct s *) ( (int)(a_ptr) - (int) ( &(( (struct s * )( 0 ) )->a ) ) ); -frankwoo(柳五随风); 2007-7-16 (#3811129@0)

这个64位也不行，最好是用（BYTE*）（a_ptr)代替（int)(a_ptr)。不过还是用offsetof宏好，让面试的人可以抖个包袱，问你不用这个宏怎么实现，呵呵。 -baalinca(scarb); 2007-7-16 (#3811205@0)

那也是long long，而不是byte *, 另外用MACRO的话就不这么用了。LISTOFFSET本质上也是把0转化成结构指针，再去值，得到offset. -frankwoo(柳五随风); 2007-7-16 (#3811219@0)

longlong也可以，不分32、64总是64位。byte*在32位os上是32反之是64位的。你得到offset的方法没问题，记得microsoft的宏offsetof就是这么定义的。用offsetof宏的话，大概就是(struct×)((byte*)(a_ptr) - offsetof(struct s, a)); -baalinca(scarb); 2007-7-16 (#3811234@0)

我说的意思是如果要用macro,就无需这个什么函数了，直接 #define BASE_ADDR(ADDR, TYPE, MEMB) ( (TYPE *) ( ADDR - ((size_t) &((TYPE *)0)->MEMB) ) 的了。 -frankwoo(柳五随风); 2007-7-16 (#3811262@0)

offsetof是已经定义好的标准宏,不是用户定义的宏. -baalinca(scarb); 2007-7-16 (#3811270@0)
fix bug: #define BASE_ADDR(ADDR, TYPE, MEMB) ( (TYPE *) ( ADDR - ((size_t) &((TYPE *)0)->MEMB) ) ==>> #define BASE_ADDR( ADDR, TYPE, MEMB) ( (TYPE *) ( (char *)ADDR - ((size_t) &((TYPE *)0)->MEMB) ) -frankwoo(柳五随风); 2007-7-17 (#3811356@0)

P.S offsetof属于C99范畴，不是所有compiler都支持。 -frankwoo(柳五随风); 2007-7-16 (#3811264@0)

有可能，我没查过。不过比较新的gcc，microsoft编译器都支持。 -baalinca(scarb); 2007-7-16 (#3811272@0)

I don't know how to do that, but my question is why having this function in design? If this is needed, why not create the struct as a class and simply implement a member function to return class pointer? -james_sep(James); 2007-7-17 (#3812729@0)

As the title says, it's C not C++. -stevensun2000(小胖子); 2007-7-17 (#3812803@0)

what about this -james_sep(James); 2007-7-17 {218} (#3812887@0)
struct s
{
...
int a;
…
};

struct s *get_s_ptr(int *a_ptr)
{
s m_s;
void *p_s = null, *p_a = null;
int offset;

p_s = &m_s;
p_a = &(m_s.a);

offset = p_a - p_s;
return (a_ptr - offset);
}

I like this one! -digitworm(digitworm); 2007-7-19 (#3817383@0)

This is the MACRO used in the well-known Apache web server. it works for me on a IA64 box. -whatthehell(cyberman); 2007-7-18 {109} (#3816523@0)
#define APR_OFFSET (p_type, field ) ((long) (((char *) (&(((p_type)NULL)->field))) - ((char *) NULL)))

@Florida

my sample code, thanks for you attention.

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