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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.unix-wizards
Subject: Re: Function with variable number of args
Message-ID: <4011@utzoo.UUCP>
Date: Sat, 30-Jun-84 01:47:37 EDT
Article-I.D.: utzoo.4011
Posted: Sat Jun 30 01:47:37 1984
Date-Received: Sat, 30-Jun-84 01:47:37 EDT
References: <1273@sri-arpa.UUCP>
Organization: U of Toronto Zoology
Lines: 39

>I was amazed to read this statement in a recent post:
>
>	There is *no* fully machine-independent way to write a
>	function which takes a variable number of arguments, even
>	if all the function is trying to do is to pass its argument
>	list on to another function.
>
>The language BCPL has precisely this feature, and as far as I know
>there has been no major problem with it in 20 years.  The way to
>write the function is to declare it with the maximum number of
>arguments, and you can scan the argument list sequentially by
>simply building a pointer to the first argument and stepping it:
>
>	ptr := LV arg1
>	...
>	ptr := ptr + (LV arg2 - LV arg1)
>	
>(LV in BCPL is & in C).  The language semantics requires the args to be
>stored in consecutive cells, though they may run upwards or downwards in
>memory, hence the above code.  I've never heard of any implementation
>difficulties, and have never encountered any myself in the BCPL ports
>I've done.

For one thing, C is not BCPL, and C does *not* require consecutive cell
allocation of arguments.  So you can't (in principle) depend on that.
There are C compilers that put some of the arguments in registers, for
example.  (Please don't tell me that these compilers run into trouble
with various things; I'm quite aware of this.  But they do exist.)

For another thing, while I respect your BCPL porting experience, I would
say that you've never tried to implement BCPL on really difficult machines.
There exist machines -- with Unix implementations running on them -- which
absolutely require that the size of the argument list of a given function
be a compile-time constant, as a fundamental assumption of their only call
primitive.  Such machines break *all* the simplistic variable-arguments
techniques; really heavy kludging is required to make things work.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry