Re: Maintaining scope while parsing C with a YACC grammar

There are two independent topics being discussed here.


(1) Scope of variables.
(2) typedef variables.


(1) Scope of variables is solved easily. In your symbol table you
have to keep track of the level for all variables. Every time you
see a '{' you have to increment the level. So the 'a' will be put
into the symbol table as a level 1 variable and the 'b' will be a
level 2. The bottom-up quality of LALR will not affect this. The
'a' will be seen first and the 'b' later, so no problem here.


(2) The infamous 'typedef' problem continues to plague the newbie or
part-time LALR grammar hacker, but it was solved way back in 1987 by
an LALR parser generator which used an integrated symbol table and
semantic grammar symbols (e.i. {typedef}). The state-of-the-art
simple solution works fine with the LRSTAR parser generator (see
http://highperware.com). Here is the simple LALR(1) grammar which
solves this 'typedef' problem:


/* C Typedef Solution. */


      <error> => error();
      <identifier> => lookup(); /* Symbol table lookup. */


/* Rules. */


      Input -> [Declaration]... <eof> +> input_


      Declaration
              -> VarDecl [',' Var ]... ';' +> decl_
              -> typedef VarDecl2 [',' Var2]... ';' +> typedefdecl_


      VarDecl -> Type... Ident


      Var -> [Ptr]... Ident


      VarDecl2 -> Type... Ident2


      Var2 -> [Ptr]... Ident2


      Ident -> <identifier> +> ident_(1)


      Ident2 -> <identifier> +> ident_(1,{typedef})


      Type
                    -> SimpleType +> type_(1)
                    -> Type Ptr


      Ptr -> '*' +> ptr_


      SimpleType
                    -> char
                    -> int
                    -> short
                    -> unsigned
                    -> {typedef}




It handles the input file:


typedef unsigned int UINT, * UINTPTR;
UNIT a, b, c;
UINTPTR x, y, z;


Note, no hacks or kludges are required, just a state-of-the-art parser
generator.


Paul B Mann

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