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Old 12-29-2011, 03:11 AM   #1
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Registered: Dec 2011
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C typecasting offset


Hello everyone!!
I am new to Linux forum and have a question related to typecasting in C related to offset of structures.I have a piece of code and having a problem with this:
void fun(struct c *p)
{
int offset;
struct b *address;
offset=(char *)&((struct c *)(&((struct a*)0)->y)->j)-(char *)((struct a *)0);
address=(struct b *)((char*)&(p->j)-offset);
address->i=400;
address->f=3.14;
address->ch='c';
printf("%u",offset);
}
As you can see that this code is accessible in turbo C but not in Dev C++ and please anybody tell me why is this so and any method to easily typecast structures for finding their offset.
 
Old 12-29-2011, 06:06 AM   #2
Doc CPU
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Registered: Jun 2011
Location: Stuttgart, Germany
Distribution: Mint, Debian, Gentoo, Win 2k/XP
Posts: 1,099

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Hi there,

I haven't yet understood what you really want to do. Compute the byte offset of a structure member relative to the base address of that structure? So, for this arbitrary sample structure
Code:
typedef struct
 { WORD  w;
   BYTE  b;
   DWORD d;
 } FOO;
you want to get 3 for d?
Then all you need is
Code:
FOO *foo = NULL;
int  offset = (int) &(foo->d);
But your sophisticated sample, that I didn't dig into yet, looks like you want something else.

Code:
offset=(char *)&((struct c *)(&((struct a*)0)->y)->j)-(char *)((struct a *)0);
address=(struct b *)((char*)&(p->j)-offset);
address->i=400;
address->f=3.14;
address->ch='c';
Isn't there a more complicated way to write that? ;-)
Besides, for a better understanding you should have shown us the definition of struct a, struct b and struct c.

[X] Doc CPU
 
  


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