Issue 0005: May a conforming implementation define and recognize a pragma which would change the semantics of the language?

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

Authors: Walter J. Murray, WG14
Date: 1992-12-10
Reference document: X3J11/90-020
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_005.html

According to subclause 6.8.6, a pragma directive “causes the implementation to behave in an implementation-defined manner.” May a conforming implementation define and recognize a pragma which would change the semantics of the language? For example, might a conforming implementation recognize and honor the directive

#pragma UNSIGNED_PRESERVING

as a way for a program to request non-standard integral promotions?

I also pose the corollary question. May a strictly conforming program contain a pragma directive? According to subclause 4, a strictly conforming program “shall use only those features of the language ... specified in this standard. It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior...”

If there is no constraint on how a conforming implementation may behave when encountering a pragma directive, would it not follow that a strictly conforming program may not contain a pragma directive?

Comment from WG14 on 1997-09-23:

Response

The relevant citations are subclause 6.8.6:

A ... pragma ... causes the implementation to behave in an implementation-defined manner.

and clause 4:

A strictly conforming program ... shall not produce output dependent on any ... implementation-defined behavior ...

In response to each question:

1. Yes, a conforming implementation may define and recognize a pragma which would change the semantics of the language.
2. Yes, for example, it might honor UNSIGNED_PRESERVING.
3. No, a strictly conforming program may not contain a pragma directive.
4. We agree with your conclusion, reasserting answer number 3.