Forcing multiple parse stacks to 'reduce'

[Derek M Jones]

All,

I have written a parser for C that processes
a single statement or declaration at a time.
So after each statement/declaration yyparse
returns.

Originally I used various ad-hoc rules in yylex
to figure out which was the last token of a
statement/declaration and then returned a zero
value as the subsequent token.

These ad-hoc rules are complicated (for instance,
a ; can occur in a for loop header and a { can
occur in a cast) and I had the 'great' idea of allowing
the token following the last token of a statement/declaration
to 'naturally' cause yyparse to generate the outstanding
reductions.  So in:

w=3;
y=5;

when yylex returns id (because it has encountered y)
the sequence w=3; reduces to statement and yyparse
returns (yes, I need to hang onto y in order to correctly
parse the next statement).

In some cases multiple parse stacks are outstanding,
and in most cases the following token causes the expected
return to deterministic parsing and subsequent reduction to
the accept state.  But in some cases it does not.  For instance,
in

typedef x y;

by the time the ; is encountered there are three parse stacks.
Depending on the token that follows the ; these three parse stacks
may or may not be reduced to a single stack (which then reduces
to a declaration).

In the case:

typedef x, y;
typedef i, j;

the second typedef token is shifted onto all three stacks and
subsequent tokens are processed like a declaration (which they
do form part of)!  So I don't get a parse of a single declaration
(in fact yyparse eventually reports an ambiguity).

I know that for some grammars the optimizations performed by
Bison result in ambiguities being reported (i.e., the BOGUS
example in 5.7 of bison.info).

In my case no ambiguity is being reported (I am in glr mode
after all ;-).  But the parse stacks are not being reduced as
expected (all three are in the state I would expect them to be
in when the second typedef token is encountered).

Does anybody have any ideas on how I can force yyparse
to consider reductions in the number of parse stacks and
thence reductions to the accept state?

derek

--
Derek M Jones                                           tel: +44 (0) 1252 520 
667
Knowledge Software Ltd                            mailto:address@hidden
Applications Standards Conformance Testing   http://www.knosof.co.uk