🚀 go-pugleaf

> The implementation's <stdlib.h>
> has to declare abs but does not have to declare a prototype for it.

If you strain to read the standard so that it provides the least
possible information, you can interpret it as not requiring one.

> > 7.10.6.1/4.10.6.1 shows the prototype for abs() ...
> 
> 7.10.6.1/4.10.6.1 has the line
>   #include <stdlib.h>
> right above the line
>   int abs(int j);
> Does this mean that <stdlib.h> is required to #include itself before
> declaring abs with a prototype?  No ... the section is labelled "Synopsis."
> It describes what programs have to do or what alternative methods used
> by programs must be equivalent to.

Nothing in the standard gives it that meaning, so we must fall back on
a common sense reading of the synopsis.  Sections 7.1.2/4.1.2 and
7.1.3/4.1.2.1 state that the header declares the function.  In order for
the standard to be useful, the reader of the standard must know how the
function is declared.  The only information provided is the declaration
in the synopsis.  Ergo, the only sensible reading is that the standard
requires the header to provide that declaration (or an equivalent one).
Which means that the prototype is required.
-- 
Mark Brader, msb@sq.com   |  "Common sense isn't any more common on Usenet
SoftQuad Inc., Toronto    |   than it is anywhere else."     -- Henry Spencer

My text in this article is in the public domain.