va_list used in <stdio.h>

[Doug Gwyn]

In article <18684 at princeton.Princeton.EDU> nfs at notecnirp.UUCP (Norbert Schlenker) writes:
>What is <stdio.h> allowed to define?  It has to pick up <stddef.h>, because
>the prototypes in <stdio.h> use size_t.  What makes <stddef.h> allowable
>and <stdarg.h> forbidden?

<stdio.h> must not #include <stddef.h> either.

Each standard header is allowed to declare/define only those identifiers
specified in the Standard, plus others specifically reserved for
implementation use (e.g. "_Iobuf").

For POSIX purposes, #defining _POSIX_SOURCE before including a standard
header enables the declaration/definition of the additional identifiers
specified in IEEE Std. 1003.1.  Other such "feature macros" could be
used to further extend the names supplied by the standard headers, but
I'd recommend that separate headers be used instead.