Issue 0044.05: Is offsetof the only standard macro that renders a program not strictly conforming?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Steve M. Hoxey, WG14
Date: 1992-12-10
Reference document: X3J11/92-010
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_044.html

Revisiting (b) as the correct interpretation of Question 1, it seems the only possibility for a definition of the macro offsetof constitutes use of an identifier from the reserved name space to define a builtin which interrogates the translator's symbol table in a fashion analogous to the sizeof operator. Further, this builtin must appear syntactically as a keyword rather than an identifier to avoid the constraint violation of subclause 6.4, page 55, line 9, which invalidates the use of what appears to be a function call within that which is otherwise required to be a constant expression.

Further, implementing an expansion for offsetof as described in the previous paragraph would violate the implementation constraint outlined in Question 2 above, since the expansion would inject preprocessing tokens requiring recognition of a keyword outside the scope of a strictly conforming program.

In any case, the implication is that the fragment:

#include <stddef.h>
 static struct x {int field1, field2; } s;
 enum fields {F0, F1, F2 = offsetof(struct x,field2), F3 };

is either rendered not strictly conforming or the implementation is rendered a nonconforming implementation.

Alternatively, if the answer to Question 2 above is no, then the following questions are raised:

Since translation phases 1 through 4 may introduce into the translation unit token sequences which are not strictly conforming, what mechanism exists, if any, to determine whether such sequences originated from the program source?

How is one to interpret the meaning of “strictly conforming program” from clause 4, page 3, lines 38-40, given that subclause 5.1.1.1, page 5, lines 12-15 define the translation unit to be “a source file together with all the headers and source files included via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion preprocessing directives?”

It seems that any program which makes use of the macro offsetof in the context of a constraint requirement mandating an “integer constant expression” will require use of unspecified, undefined, or implementation-defined behavior.

As near as I can tell, offsetof is the only macro defined by the C Standard which can alter the behavior of a strictly conforming program as a consequence of its own definition.

Comment from WG14 on 1997-09-23:

Response

The response to Question 1 addresses this issue.