Issue 1003: Linkage between library functions

Authors: Joseph Myers
Date: 2025-03-07
Submitted against: C23
Status: Open
Cross-references: 0078

C23 footnote 20, in 6.2.2 (Linkages of identifiers), says:

There is no linkage between different identifiers.

Is this footnote (not normative) referring only to linkage between identifiers defined by the user's program? Or is it also intended to say that there cannot be linkage between different library functions?

In practice, it is common that pointers to library functions with compatible semantics may compare equal: that memcpy == memmove, or ldexp == scalbn, for example. The response to C90 issue 0078 said that this is permitted. However, the footnote in question was added in C99, which makes it less clear if that issue response reflects the current intent.

It seems appropriate to amend both normative and non-normative text to make clear that it is OK for pointers to different library functions to compare equal.

Suggested correction

In C23 7.1.2 (Standard headers), insert text as follows.

Any declaration of a library function shall have external linkage. Pointers to different library functions may compare equal where this is compatible with the semantics of those functions.^FN)

^FN)For example, it is possible that memcpy == memmove or that ldexp == scalbn.

In C23 6.2.2 (Linkages of identifiers), amend footnote 20 as follows.

There is no This document does not define any mechanism to establish linkage between different identifiers. As described in 7.1.2, it is possible that pointers to different library functions compare equal where that is compatible with the semantics of those functions.