> >   Do you mean that we do:
> > 
> > #define stdin	(*__sFstdin)
> > #define stdout	(*__sFstdout)
> > #define stderr  (*__sfStderr)

> Right... that would be one way to do it.  

Recent GNU libc (glibc2? I forgot exact version) does this.
And some programs which are doing
	static FILE *debug_output = stderr;		-- (A)
cannot be compiled any more with the glibc.

> > or does POSIX screw this possibility?  
> 
> can any of the standards experts quote chapter and verse?

At least, ANSI/ISO-C permits that stdin/stdout/stderr are
variable (not constant), thus, programs which are doing (A)
are not portable, from the ANSI/ISO-C point of view.

But personally, I do not like changes like this which breaks
20 years working programs.
--
soda