Why subtract null pointer in offsetof()?

Posted by Bruce Christensen on Stack Overflow See other posts from Stack Overflow or by Bruce Christensen
Published on 2010-04-02T19:18:39Z Indexed on 2010/04/02 19:23 UTC
Read the original article Hit count: 331

Filed under:

c

|

pointer-arithmetic

|

offsetof

Linux's stddef.h defines offsetof() as:

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

whereas the Wikipedia article on offsetof() (http://en.wikipedia.org/wiki/Offsetof) defines it as:

#define offsetof(st, m) \
    ((size_t) ( (char *)&((st *)(0))->m - (char *)0 ))

Why subtract (char *)0 in the Wikipedia version? Is there any case where that would actually make a difference?

Related posts about pointer-arithmetic

Pointer arithmetic and arrays: what's really legal?

as seen on Stack Overflow - Search for 'Stack Overflow'
Consider the following statements: int *pFarr, *pVarr; int farr[3] = {11,22,33}; int varr[3] = {7,8,9}; pFarr = &(farr[0]); pVarr = varr; At this stage, both pointers are pointing at the start of each respective array address. For *pFarr, we are presently looking at 11 and for *pVarr… >>> More
Pointer Arithmetic & Signed / Unsigned Conversions!

as seen on Stack Overflow - Search for 'Stack Overflow'
Incase of pointer arithmetic, are the integers automatically converted to their signed variants? If yes, why? Suppose I do pointer + uiVal where pointer is a pointer to int and uiVal is initialized to -1, then I find that the address in pointers get decremented by 4. Why is the unsigned value… >>> More
Question on Pointer Arithmetic

as seen on Stack Overflow - Search for 'Stack Overflow'
Heyy Everybody! I am trying to create a memory management system, so that a user can call myMalloc, a method I created. I have a linked list keeping track of my free memory. My problem is when I am attempting to find the end of a free bit in my linked list. I am attempting to add the size of the… >>> More
Array pointer arithmetic question

as seen on Stack Overflow - Search for 'Stack Overflow'
Is there a way to figure out where in an array a pointer is? Lets say we have done this: int nNums[10] = {'11','51','23', ... }; // Some random sequence int* pInt = nNums[4]; // Some index in the sequence. ... pInt++; // Assuming we have lost track of the index by this… >>> More
Pointer arithmetic.

as seen on Stack Overflow - Search for 'Stack Overflow'
Having code: int** a = new int*[2]; a[0] = new int(1); a[1] = new int(2); cout << "a[0] " << a[0] << '\n'; cout << "a[1] " << a[1] << '\n'; cout << "a[2] " << a[2] << '\n'; cout << "a[0] + 1 " << a[0] + 1 << '\n';//WHY THIS… >>> More

Developer IT

Why subtract null pointer in offsetof()? - Developer IT

Why subtract null pointer in offsetof()?

c

pointer-arithmetic

offsetof

Related posts about c

Related posts about pointer-arithmetic

Pointer arithmetic and arrays: what's really legal?

Pointer Arithmetic & Signed / Unsigned Conversions!

Question on Pointer Arithmetic

Array pointer arithmetic question

Pointer arithmetic.

Categories cloud