Defect Report #144

Submission Date: 23 Feb 95
Submittor: BSI
Source: Clive D.W. Feather
Question
Submitted to BSI by Clive D.W. Feather [email protected].
In this Defect Report, identifiers lexically identical to those declared in standard headers refer to the identifiers declared in those standard headers, whether or not the header is explicitly mentioned.
This Defect Report has been prepared with considerable help from Mark Brader, Jutta Degener, Ronald Guilmette, and a person whose employment conditions require anonymity. However, except where stated, opinions expressed or implied should not be assumed to be those of any person other than myself.
Defect Report UK 028: Preprocessing of preprocessing directives
Can the white space preceeding the initial # of a preprocessing directive be derived from macro expansion? Consider the following code extract:
#define EMPTY # EMPTY include <file.h> /* Line A */ EMPTY # include <file.h> /* Line B */ Line A is clearly forbidden by subclause 6.8:

The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

However, this text does not appear to forbid line B. Nor does subclause 6.8.3.4:

The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one. If that subclause applies only to the expansion of EMPTY, it is not relevant. If it applies to both the expansion and the following preprocessing token sequence, then no subsequent preprocessing directive could ever be processed.

Is line B strictly conforming, or does it violate a constraint (and if so, which), or does it cause undefined behavior?
Suggested Technical Corrigendum:
In subclause 6.8 Description, change:

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that is either ...

to:

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that (at the start of translation phase 4, before any preprocessing takes place) is either ...

Suggested Future Change
The current C Standard has correct meaning, but the wording could be clearer. We suggest the following change for the revised C Standard:

A preprocessing directive consists of a directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that (at the start of translation phase 4) is either ...

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