Re: va_list function portability doco

[Akim Demaille]

It seems like this guy never went in.  Would you install it please?

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I don't suppose there's anything in the below which isn't a straight
consequence of the relevant standards, but perhaps a paragraph on the
traps might keep people out of trouble.  The call-by-reference
behaviour on powerpc for instance seems to me a particular trap if one
hasn't encountered it before.

        * autoconf.texi (Function Portability): Notes on va_list.


@item @code{va_list}
@c @fuindex va_list
@prindex @code{va_list}
Often @code{va_list} is just a pointer, but this is not always so.
Sometimes it's a structure (eg. @command{gcc} on Alpha), which means for
instance @code{NULL} should not be used.  And sometimes it's an array
(eg. @command{gcc} on PowerPC, in some configurations), which means for
instance a @code{va_list} function parameter can be call-by-reference.
Applications may need to watch out for functions modifying the
@code{va_list} they're given (eg. @code{vsnprintf} in the GNU C Library
2.1 modifies it).

Any copying of a @code{va_list} should be done with @code{va_copy}, not
just @code{=}.  @code{va_copy} is standard in C99 and hopefully will be
available on older compilers if @code{=} is not enough, though perhaps
as @code{__va_copy} (eg. @command{gcc} in strict C89 mode).
@code{#ifdef}s should suffice to test for these.