Issue 0419: What the heck is a "generic function"?

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

Authors: WG14, Douglas Walls
Date: 2012-09-16
Reference document: N1635
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

What the heck is a "generic function", and what are the sections of the standard covering how a user (or implementor) can write a stardard conforming program defining a "type generic function"?

I was trying to reconcile the rules in 7.1.4 Use of library functions allowing an implementation to define a function as a macro, and the user suppressing the macro by enclosing the name of the function in parentheses. But, I don't see how to make a function declaration, where a parameter can be any atomic type.

I've convinced myself, generic functions will take compiler magic. There is no way to declare them using C standard conforming code. Just like the type generic macros of <tgmath.h> in C99.

Somehow I missed this. I remember all the discussion of adding atomic operation to operators like += but somehow I missed the fact we were again adding in function specifications that cannot be implemented using standard C. I thought we were adding type generic macros. Sigh ...

I've been told that the discussion included talk about a proposal to recast them as generic macros, but that never happened so we ended up with generic functions through the back door without much explication.

Suggested Technical Corrigendum

Redefine the atomic type generic functions as type generic macros. Define the underlying functions to which the type generic macros expand.

Comment from WG14 on 2017-11-03:

Oct 2012 meeting

Proposed Technical Corrigendum

7.17.1 add a new paragraph after paragraph 5:

It is unspecified whether any generic function declared in stdatomic.h is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name of a generic function, the behavior is undefined.

J.2 add:

The macro definition of a generic function is suppressed in order to access an actual function (7.17.1)