Issue 0044.01: What does it mean to say that the type of offsetof is size_t?

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
Cross-references: 0072
Converted from: dr.htm, dr_044.html

Subclause 7.1.6, page 98, lines 24-30 describe the macro

offsetof(type, member_designator)

“which expands to an integral constant expression that has type size_t, ...”

How is this statement to be interpreted? The expansion of the macro offsetof is

a. an expression which can be evaluated during translation, the value of which is in the range representable by a size_t type.

Or

b. an expression as (a) above, but further constrained to be an “integral constant expression” as defined in subclause 6.4, page 55, lines 17-21.

Comment from WG14 on 1997-09-23:

Response

Neither alternative (a) nor (b) in Question 1 fully captures the intent. What is intended is exactly what is specified in the C Standard. A strictly conforming program shall not produce output that varies depending upon details of implementation of facilities defined by the standard headers. Hence, use of the offsetof macro, in a context requiring an integer constant expression, per se does not render a program not strictly conforming.

Further clarification provided by David Prosser:

Although the replacement for the offsetof macro must be an integral constant expression, and must follow all the constraints appropriate to expressions, an implementation is permitted to make use of its extensions to constant expressions that behave like integral constant expressions. This is why the sample replacement expressions for the offsetof macro in the Rationale are valid candidates (for many implementations) but do not come under the strict definition of integral constant expression that strictly conforming code must follow. In particular, this is why the offsetof macro exists: there was otherwise no portable means to compute such translation-time constants. Therefore, of the two choices, (b) is the closest, but it is not the whole story.