Issue 0147: Is there a requirement for a sequence point to occur within a library function?

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

Authors: Clive D.W. Feather, BSI
Date: 1995-02-23
Submitted against: C90
Status: Fixed
Fixed in: C99
Converted from: dr.htm, dr_147.html

Submitted to BSI by Clive D.W. Feather clive@sco.com.

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 031: Sequence points in library functions

There is no requirement for a sequence point to occur within a library function, since it might not be written in C. Consider the following code:

#include <string.h>
 char s[10];
 /* ... */
 (strcpy)(s, "Testing") [0] = 'X';

Any function written in C must have a sequence point after the last full expression evaluated (which will be the returned value if there is one), so if strcpy were a C function, the assigning of 'T' to s[0] would be completed before the call returned.

However, since library functions might not be written in C, they might not have such a sequence point. If not, then the above statement is in breach of the requirements of the second paragraph of subclause 6.3.

Suggested Technical Corrigendum:

Add to the end of subclause 7.1.7:

There is a sequence point immediately before a library function returns.

Add to the end of annex C:

Immediately before a library function returns (7.1.7).

Add a reference to 7.1.7 in the Forward References of 5.1.2.3, and in the relevant Index entry.

Comment from WG14 on 1997-09-23:

Response

We agree that the current wording does not make this clear. The next revision of the C Standard will clarify that every function return is a sequence point. The suggested changes will be used.