Date: Mon, 28 Apr 1997 01:06:47 -0400 (EDT) From: Tim.Vanderhoek@X2296 To: FreeBSD-gnats-submit@freebsd.org Cc: robert.corbett@eng.Sun.COM Subject: bin/3403: yacc doesn't well parse recursivish errors Message-ID: <199704280506.BAA00878@X2296> Resent-Message-ID: <199704280520.WAA22852@hub.freebsd.org>
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>Number: 3403
>Category: bin
>Synopsis: yacc doesn't well parse recursivish errors
>Confidential: no
>Severity: serious
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Sun Apr 27 22:20:01 PDT 1997
>Last-Modified:
>Originator: Tim Vanderhoek
>Organization:
The league of those who hack with yacc when they should be
studying for their upcoming Bible test...
>Release: FreeBSD 2.2-961006-SNAP i386
>Environment:
Reasonably current yacc from FreeBSD-current... The yacc is more current
than the Release line of this pr would suggest...
>Description:
It would seem that yacc doesn't properly handle rules along the lines
of
a: a error
when multiple errors occur.
Cc'd to robert.corbett@eng.Sun.COM (although I don't know if send-pr
will strip that or not...)
>How-To-Repeat:
The following program should print out have the following output:
--
Parsed #1
Parsed #0
--
However, it does not. Rather, only the first line ("Parsed #1") is
printed... This is not what I want the program to do.
It works correctly when one uses Bison (--version: GNU Bison version 1.25)
to generate the parser. (If you try this, don't forget to give the -t
argument to bison).
Follows brief explanation of what goes wrong...
It correctly shifts into state 1 (a : X . a error) for the first X,
and then shifts into state 1 again for the second X. The Y token is
returned and it correctly reduces an `a', and then an error, and finally
reduces from state 5 (X a error .) all the way back to state 4 (X a . error)
to finish off the first X statement. The error recovery correctly
generates an error, however, this time it _DOES NOT_ shift from state 4
to state 5 on the error.
Bison does exactly the same thing, except for the fact that it does shift
on the second error the way it is supposed to... :)
--
%{
#include <stdio.h>
%}
%union {
int exp;
}
%token <exp> X
%token Y
%token NOSUCHTOKEN
%token ERROR
%%
a: Y {} /* Hmm... can't use a simple "empty" here... */
| X a NOSUCHTOKEN {}
| X a error { fprintf (stderr, "Parsed #%d\n", $1); }
;
%%
main () {
yydebug = atoi(getenv("YYDEBUG") ? getenv("YYDEBUG") : ""); /* Bison */
yyparse ();
}
yylex () {
static p = 0;
const int tok[] = {X, X, Y, ERROR, ERROR, ERROR, 0};
if (tok[p] == X) yylval.exp = p;
return tok[p++];
}
yyerror() {}
--
>Fix:
Heh. My head's still spinning after spending most of today tracing this
one down...
>Audit-Trail:
>Unformatted:
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