Message-ID: <3EA395F3.78C4A4D4@acm.org>
From: Eric Sosman <esosman AT acm DOT org>
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Subject: Re: __builtin_va_list bug?
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Date: Mon, 21 Apr 2003 11:55:21 GMT
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Gisle Vanem wrote:
> 
> "Eric Sosman" <esosman AT acm DOT org> said:
> 
> > > #include <stdarg.h>
> > > char x;
> > > void foo (va_list arg)
> >
> >     Hold on; stop right there.  You've included <stdarg.h>, but you're
> > using the argument list declaration from the pre-Standard <varargs.h>
> > (or <vararg.h> with some compilers).  The two are not the same, they
> > are not interchangeable, and they are *definitely* not miscible.
> 
> You mean using "va_list" is pre-standard? (it's used in <stadarg.h>)
> That's funny since djgpp uses it all the time. Have a look at e.g.
> src\libc\ansi\stdio\doprnt.c.

    My apologies; Richard Dawe pointed out my error.  <stdarg.h> uses
some of the same names as the pre-Standard <varags.h>, and I mis-read
the usage you quoted.

> > >   x = va_arg (arg, char);
> >
> >     This will be wrong with either <stdarg.h> or <varargs.h>, since
> > the actual type of the parameter is not `char' but whatever `char'
> > gets promoted to: `int' on most machines, but perhaps `unsigned int'
> > on some oddball architectures.  You must specify the promoted type,
> > not the type to which you will later "demote" it.
> 
> Yes, I know. I use "x = (char) va_arg(arg,int)" to stop gcc complaining.
> But my question is why the "int $5" is generated. [...]

    No idea.  The code as it stands (with `char' instead of `int') *is*
incorrect, and the implementation of <stdarg.h> is necessarily
"magical."
We've all heard stories of the unexpected and usually unwelcome things
that happen when sorcerers mis-pronounce their incantations; perhaps the
mysterious `int $5' is one such consequence.  If it goes away when the
va_arg() call is corrected, I think that's a practical resolution to the
problem.

-- 
Eric Sosman
esosman AT acm DOT org