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Answer 1

+1 A:

Your thinking is correct! And the name of the macro gives a good hint, too. ;)

Dmitry Brant 2010-08-10 20:20:45

Answer 2

+3 A:

It gets the offset of the member of a struct. It does so by casting address zero to a struct of that type then taking the address of the member.

Amardeep 2010-08-10 20:22:35

C/C++ compilers have lately been adding an intrinsic function for this, mainly because you can break this technique with overloaded operators in C++, but also because it *is* technically undefined behavior and you can imagine a sufficiently aggressive points-to optimization breaking it.

Zack 2010-08-10 20:26:11

@Zack: It's a library feature, so no, it's not undefined behavior. An implementation is free to implement it however they want.

GMan 2010-08-10 20:28:12

He's implying that this macro is defined inside an application program in which case it _is_ undefined behaviour (although extremely likely to work as expected of course). If the definition was in the standard library then it wouldn't be undefined of course.

John Burton 2010-08-10 20:30:47

@John: If `offsetof` is indeed (re)defined by some library, they've made the program ill-formed by using a reserved identifier. They should use the one defined in the standard library.

GMan 2010-08-10 20:34:46

I'm going to be incredibly hair-splitty and insist that "`((size_t) C++ is a little different).

Zack 2010-08-10 20:41:52

@Zack: I hope you see I didn't recognize the macro was being redefined outside the standard library. And fair, though I doubt anything could get very far without eventually including `stddef.h`.

GMan 2010-08-10 21:03:45

Yeah, I did see; I just felt like making microfibers ;-) FYI, in C, the standard headers are required to behave as-if they do not include each other, so you can actually get quite far without picking up stddef.h. (That's what I meant when I said C++ is a little different. I would have unpacked it but I ran out of characters.)

Zack 2010-08-10 21:32:30

Zack 2010-08-10 21:38:46

@Zack: Ah, good to know. I'm a C++ person, not a C person.

GMan 2010-08-11 04:54:09

Answer 3

A:

It's defined in §7.17/3:

offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given
static type t;
then the expression &(t.member-designator) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.)

Because the library doesn't have to necessarily follow language rules, an implementation is free to get the result however it pleases.

So the result of this particular implementation is not undefined behavior, because you aren't suppose to care how it's implemented. (In other words, your implementation makes the guarantee that taking the address of an indirection through a null pointer is well-defined. You of course can't assume this in your own programs.)

If that some library has (re)defined offsetof, they've made your program behavior undefined and should be using the standard library instead. (The dummies.)

GMan 2010-08-10 20:27:36

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