Path: utzoo!attcan!uunet!husc6!uwvax!rutgers!columbia!garfield!aronoff
From: aronoff@garfield (Avram Aronoff)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MS-C 5.0 var_args bug?
Message-ID: <5696@columbia.edu>
Date: 13 Jun 88 07:51:26 GMT
References: <2664@bigtex.uucp>
Sender: nobody@columbia.edu
Reply-To: aronoff@garfield.UUCP (Avram Aronoff)
Organization: Columbia University CS Department
Lines: 25

In article <2664@bigtex.uucp> james@bigtex.uucp (James Van Artsdalen) writes:
>I am attempting to use the ANSI var_args support in MS-C 5.0...
>
>The problem is that Microsoft used sizeof()...
>...In particular, sizeof(char) is 1
>...but...
>it occupies two bytes on the stack.  Hence, if the fixed parameter in your
>function is a char, or any but the last variable argument is char, the
>Microsoft macros fail.
>
>/* ... */ char c; /* ... */ c = va_arg(ap, char); /* ... */

The problem lies not in the Microsoft macros, but in your code (WS :-)
Since the char parameter is part of a va_list, there is no prototype that says
it's being passed as a char. Therefore, it is widened to int by the standard
conversions before it is passed as a parameter.

In general, chars, shorts, and floats can not be used in va_arg. Instead, use
int or double, and if necessary, truncate by cast or assignment.

As far as structures, this would be a problem only if sizeof(struct a) was
different from its size on the stack. Hmmm...
(Mad dash to try struct { char a; } and struct { char a[3]; } :-)

							Hymie