Solved

Strut pointer

Posted on 2007-11-27
7
415 Views
Last Modified: 2010-04-21
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
Comment
Question by:qiang_lin
7 Comments
 
LVL 55

Expert Comment

by:Jaime Olivares
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.
0
 
LVL 22

Accepted Solution

by:
grg99 earned 125 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.

0
 
LVL 10

Expert Comment

by:peetm
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!
0
Master Your Team's Linux and Cloud Stack

Come see why top tech companies like Mailchimp and Media Temple use Linux Academy to build their employee training programs.

 
LVL 40

Expert Comment

by:evilrix
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!
0
 
LVL 39

Expert Comment

by:itsmeandnobodyelse
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 *)( \
                                                  (PCHAR)(address) - \
                                                  (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
0
 
LVL 39

Expert Comment

by:itsmeandnobodyelse
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

by:qiang_lin
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.
0

Featured Post

NAS Cloud Backup Strategies

This article explains backup scenarios when using network storage. We review the so-called “3-2-1 strategy” and summarize the methods you can use to send NAS data to the cloud

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

IntroductionThis article is the second in a three part article series on the Visual Studio 2008 Debugger.  It provides tips in setting and using breakpoints. If not familiar with this debugger, you can find a basic introduction in the EE article loc…
This is a short and sweet, but (hopefully) to the point article. There seems to be some fundamental misunderstanding about the function prototype for the "main" function in C and C++, more specifically what type this function should return. I see so…
The goal of this video is to provide viewers with basic examples to understand how to create, access, and change arrays in the C programming language.
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…

823 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question