Node:Pure Calling, Previous:Token Positions, Up:Lexical

Calling Conventions for Pure Parsers

When you use the Bison declaration %pure_parser to request a pure, reentrant parser, the global communication variables yylval and yylloc cannot be used. (See A Pure (Reentrant) Parser.) In such parsers the two global variables are replaced by pointers passed as arguments to yylex. You must declare them as shown here, and pass the information back by storing it through those pointers.

int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
  ...
  *lvalp = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  ...
}

If the grammar file does not use the @ constructs to refer to textual positions, then the type YYLTYPE will not be defined. In this case, omit the second argument; yylex will be called with only one argument.

If you use a reentrant parser, you can optionally pass additional parameter information to it in a reentrant way. To do so, define the macro YYPARSE_PARAM as a variable name. This modifies the yyparse function to accept one argument, of type void *, with that name.

When you call yyparse, pass the address of an object, casting the address to void *. The grammar actions can refer to the contents of the object by casting the pointer value back to its proper type and then dereferencing it. Here's an example. Write this in the parser:

%{
struct parser_control
{
  int nastiness;
  int randomness;
};

#define YYPARSE_PARAM parm
%}

Then call the parser like this:

struct parser_control
{
  int nastiness;
  int randomness;
};

...

{
  struct parser_control foo;
  ...  /* Store proper data in foo.  */
  value = yyparse ((void *) &foo);
  ...
}

In the grammar actions, use expressions like this to refer to the data:

((struct parser_control *) parm)->randomness

If you wish to pass the additional parameter data to yylex, define the macro YYLEX_PARAM just like YYPARSE_PARAM, as shown here:

%{
struct parser_control
{
  int nastiness;
  int randomness;
};

#define YYPARSE_PARAM parm
#define YYLEX_PARAM parm
%}

You should then define yylex to accept one additional argument--the value of parm. (This makes either two or three arguments in total, depending on whether an argument of type YYLTYPE is passed.) You can declare the argument as a pointer to the proper object type, or you can declare it as void * and access the contents as shown above.

You can use %pure_parser to request a reentrant parser without also using YYPARSE_PARAM. Then you should call yyparse with no arguments, as usual.