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# Strut pointer

Posted on 2007-11-27
Medium Priority
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Given a poniter to a member "m" within a struct, write a function that returns the pointer to the struct.
struct s {
.....
int m;
.....
};

struct s * get_struct_pointer(int * ptr);
0
Question by:qiang_lin
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LVL 55

Expert Comment

ID: 20359496
Homework?
Some steps, you have to know the "distance" between some struct member and the FIRST member in the same structure
Once you know the distance (some few bytes, usually), sustract this distance from the pointer.
to do calculation, all your pointers should be casted to (char *) to operate in bytes, if not you will have strange results.
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LVL 22

Accepted Solution

grg99 earned 375 total points
ID: 20359580
Hint:   The  & operator can be used to take the address of structs or fields in a struct.

And if you store addresses in a char *,  then subtract those, you can get the distance between.

Similarly you can take the address of a field and subtract from it to get ....

BTW this is a very very dirty thing to do.  You should never work backwards from fields to the (supposedly) encompassing structure.

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LVL 10

Expert Comment

ID: 20359844
You presumably either have the member 'm' as the first member of the struct, OR, you know the types of the members before 'm'?

My gut feel is that this is a pretty horrible thing to ever do or attempt - lock down the struct definition for sure!
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LVL 40

Expert Comment

ID: 20360292
In theory, if m is the first element in s then m and s should have the same address, if not you have no way to calculate this without knowing that comes before it. Even then you have no guarantees of this as it all depends upon the packing and alignment used by the compiler!
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LVL 39

Expert Comment

ID: 20360444
Here is a macro defined in winnt.h that does the job for an arbitrary struct:

#define CONTAINING_RECORD(address, type, field) ((type *)( \
(UINT_PTR)(&((type *)0)->field)))

You would call it like

struct s *  ps = CONTAINING_RECORD(pm, struct s, m);

It was casting a NULL pointer to struct s pointer and retrieves the 'address' of member m of that NULL pointer, so getting the offset in bytes. Then it makes a cast to a  unsigned int* (what I don't understand) and subtracts that address (what actually is the offset of m within s) from the given pointer casted to a PCHAR what is a char pointer.

Regards, Alex
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LVL 39

Expert Comment

ID: 20360494
Note, the above was a macro. So it can use the fieldname 'm' to access member m within struct s and can use the typename 'struct s' passed as argument  to make casts. If your task is to make it a C function you have to 'know' that it is 'struct s' and that the address passed is that of member 'm'.
0

Author Closing Comment

ID: 31411234
The hint is very good. Actually we can creat a "struct s" object and find out the distance between this object and its member m. The distance is consistant among all "struct s" objects.
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