Portability of using stddef.h's offsetof rather than rolling your own

Question

This is a nitpicky-details question with three parts. The context is that I wish to persuade some folks that it is safe to use <stddef.h>'s definition of offsetof unconditionally rather than (under some circumstances) rolling their own. The program in question is written entirely in plain old C, so please ignore C++ entirely when answering.

Part 1: When used in the same manner as the standard offsetof, does the expansion of this macro provoke undefined behavior per C89, why or why not, and is it different in C99?

#define offset_of(tp, member) (((char*) &((tp*)0)->member) - (char*)0)

Note: All implementations of interest to the people whose program this is supersede the standard's rule that pointers may only be subtracted from each other when they point into the same array, by defining all pointers, regardless of type or value, to point into a single global address space. Therefore, please do not rely on that rule when arguing that this macro's expansion provokes undefined behavior.

Part 2: To the best of your knowledge, has there ever been a released, production C implementation that, when fed the expansion of the above macro, would (under some circumstances) behave differently than it would have if its offsetof macro had been used instead?

Part 3: To the best of your knowledge, what is the most recently released production C implementation that either did not provide stddef.h or did not provide a working definition of offsetof in that header? Did that implementation claim conformance with any version of the C standard?

For parts 2 and 3, please answer only if you can name a specific implementation and give the date it was released. Answers that state general characteristics of implementations that may qualify are not useful to me.

Answer 1

To answer #2: yes, gcc-4* (I'm currently looking at v4.3.4, released 4 Aug 2009, but it should hold true for all gcc-4 releases to date). The following definition is used in their stddef.h:

#define offsetof(TYPE, MEMBER) __builtin_offsetof (TYPE, MEMBER)

where __builtin_offsetof is a compiler builtin like sizeof (that is, it's not implemented as a macro or run-time function). Compiling the code:

#include <stddef.h>

struct testcase {
    char array[256];
};

int main (void) {
    char buffer[offsetof(struct testcase, array[0])];
    return 0;
}

would result in an error using the expansion of the macro that you provided ("size of array ‘buffer’ is not an integral constant-expression") but would work when using the macro provided in stddef.h. Builds using gcc-3 used a macro similar to yours. I suppose that the gcc developers had many of the same concerns regarding undefined behavior, etc that have been expressed here, and created the compiler builtin as a safer alternative to attempting to generate the equivalent operation in C code.

Additional information:

• A mailing list thread from the Linux kernel developer's list
• GCC's documentation on offsetof
• A sort-of-related question on this site

Regarding your other questions: I think R's answer and his subsequent comments do a good job of outlining the relevant sections of the standard as far as question #1 is concerned. As for your third question, I have not heard of a modern C compiler that does not have stddef.h. I certainly wouldn't consider any compiler lacking such a basic standard header as "production". Likewise, if their offsetof implementation didn't work, then the compiler still has work to do before it could be considered "production", just like if other things in stddef.h (like NULL) didn't work. A C compiler released prior to C's standardization might not have these things, but the ANSI C standard is over 20 years old so it's extremely unlikely that you'll encounter one of these.

The whole premise to this problems begs a question: If these people are convinced that they can't trust the version of offsetof that the compiler provides, then what can they trust? Do they trust that NULL is defined correctly? Do they trust that long int is no smaller than a regular int? Do they trust that memcpy works like it's supposed to? Do they roll their own versions of the rest of the C standard library functionality? One of the big reasons for having language standards is so that you can trust the compiler to do these things correctly. It seems silly to trust the compiler for everything else except offsetof.

Update: (in response to your comments)

I think my co-workers behave like yours do :-) Some of our older code still has custom macros defining NULL, VOID, and other things like that since "different compilers may implement them differently" (sigh). Some of this code was written back before C was standardized, and many older developers are still in that mindset even though the C standard clearly says otherwise.

Here's one thing you can do to both prove them wrong and make everyone happy at the same time:

#include <stddef.h>

#ifndef offsetof
  #define offsetof(tp, member) (((char*) &((tp*)0)->member) - (char*)0)
#endif

In reality, they'll be using the version provided in stddef.h. The custom version will always be there, however, in case you run into a hypothetical compiler that doesn't define it.

Based on similar conversations that I've had over the years, I think the belief that offsetof isn't part of standard C comes from two places. First, it's a rarely used feature. Developers don't see it very often, so they forget that it even exists. Second, offsetof is not mentioned at all in Kernighan and Ritchie's seminal book "The C Programming Language" (even the most recent edition). The first edition of the book was the unofficial standard before C was standardized, and I often hear people mistakenly referring to that book as THE standard for the language. It's much easier to read than the official standard, so I don't know if I blame them for making it their first point of reference. Regardless of what they believe, however, the standard is clear that offsetof is part of ANSI C (see R's answer for a link).

---

Here's another way of looking at question #1. The ANSI C standard gives the following definition in section 4.1.5:

     offsetof( type,  member-designator)

which expands to an integral 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 ).

Using the offsetof macro does not invoke undefined behavior. In fact, the behavior is all that the standard actually defines. It's up to the compiler writer to define the offsetof macro such that its behavior follows the standard. Whether it's implemented using a macro, a compiler builtin, or something else, ensuring that it behaves as expected requires the implementor to deeply understand the inner workings of the compiler and how it will interpret the code. The compiler may implement it using a macro like the idiomatic version you provided, but only because they know how the compiler will handle the non-standard code.

On the other hand, the macro expansion you provided indeed invokes undefined behavior. Since you don't know enough about the compiler to predict how it will process the code, you can't guarantee that particular implementation of offsetof will always work. Many people define their own version like that and don't run into problems, but that doesn't mean that the code is correct. Even if that's the way that a particular compiler happens to define offsetof, writing that code yourself invokes UB while using the provided offsetof macro does not.

Rolling your own macro for offsetof can't be done without invoking undefined behavior (ANSI C section A.6.2 "Undefined behavior", 27th bullet point). Using stddef.h's version of offsetof will always produce the behavior defined in the standard (assuming a standards-compliant compiler). I would advise against defining a custom version since it can cause portability problems, but if others can't be persuaded then the #ifndef offsetof snippet provided above may be an acceptable compromise.