Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mstan!amull
From: amull@Morgan.COM (Andrew P. Mullhaupt)
Newsgroups: comp.lang.c
Subject: Re: Variable Parameters
Summary: Variable Arguments + Structure Assignment
Message-ID: <631@s5.Morgan.COM>
Date: 31 Dec 89 08:07:30 GMT
References: <1169@zip.eecs.umich.edu> <1989Dec31.013746.2349@utzoo.uucp>
Organization: Morgan Stanley & Co. NY, NY
Lines: 43


I need to make assignment of any number of variables of any type
to a temporary stack, and then reassign them back again. I'm at
present using two versions (one with varargs.h for System V and
one with stdarg.h for ANSI C). I pass the 'return addresses' and
sizes of the variables, a single punctuational NULL, and then the
list of values as function arguments. I then memcpy the appropriate
sizes for the values (indexed just after the NULL marker argument)
into the waiting addresses. This (for those who may be interested
in speed) is not too slow - on a CISC architecture like the 386 it's
only about 2.5 times as much time as the equivalent structure
assignment, (which can't be coded because the arguments are not always
of some given type), but on a RISC (like a Sun 4) it's about 20 times
as slow. (Gack!). 

Now this business is not without at least one fine point I can't
really get around; and that is the size of a variable is not always
the same as it's size as an argument passed to a function on a 
stack. The complicated part (aside from the instant Hell of memory
model complications) seems to come when using type char data which
gets widened to int. It seems that where your character ends up in the 
int cannot be known without knowledge of the dreaded byte-ordering
of the machine. Also; since there is no way to know at run-time 
what type the argument has, sizeof isn't so helpful on the way into
this function. Essentially, I'm trying to write a 'remember' function
which retains the values of any collection of assignable l-values
for future re-assignment. The function is of type void, but could
be assumed to be called 'recursively' in the sense that evaluation
of the r-values may call functions which call 'remember'. Has
anyone got a standard or portable way to do this? 

P.S. Is there some strange side effect of going from System V style
varargs to ANSI stdarg which changes the code generation between
caller and called? The profile timings of the two otherwise
identical versions of a function using 'remember' compiled on the
SCO / Microsoft UNIX System V/386 r3.2 C compiler differ in how
the time is split between the caller and the called routine. The
ANSI version has a faster called routine, but the System V UNIX-style
version makes up the difference (nearly exactly) by having a faster
caller. Is this due to the dPANS?

Thanks in Advance,
Andrew Mullhaupt