Path: utzoo!attcan!uunet!cbmvax!snark!eric
From: eric@snark.UUCP (Eric S. Raymond)
Newsgroups: comp.lang.c
Subject: Re: why does yacc use array[-n]?
Summary: yypvt is a pointer, not an array
Message-ID: <e1qC1#1uFNy5=eric@snark.UUCP>
Date: 28 Sep 88 23:25:48 GMT
References: <1237@imagine.pawl.rpi.edu> <816@goofy.megatest.uucp> <227@olive.athertn.atherton.com>
Organization: Wild-Eyed Extremists for Freedom, Inc.
Lines: 19
Back-References: <2712@ima.ima.isc.com>

In article <227@olive.athertn.atherton.com>, joshua@atherton.com (Sleaze Hack) writes:
> >case 28:
> ># line 160 "gram.y"
> >{ Mpc_insert_with_searchdir(yypvt[-3],yypvt[-1]);
> >                          yyval = tree(N_INSERT_DECL3);} break;
> 
> My question is: why does YACC do this? [incorrect hypothesis follows].

The answer: YACC isn't doing what you thought it was doing. It turns out (as
you'd know if you'd looked at /usr/lib/yaccpar) that yypvt is a *pointer*, 
a register pointer in fact, into the parser stack.

In fact, it seems to define the 'frame' of syntax values available for
$[0-9]-substitution in the current action. The negative indexing looks a
little weird but is perfectly legal and even portable.
-- 
      Eric S. Raymond                     (the mad mastermind of TMN-Netnews)
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