Re: How to restart FLEX scanners?

On 3 Feb 1999 23:55:26 -0500, jenglish@flightlab.com (Joe English)
wrote:
>The language has a "load file" command, which leads to the usual
>re-entrancy problems. I'm solving this in the usual way: the grammar
>is written to return one statement at a time, and the read-eval-print
>loop looks something like this:
>
>
> extern PARSE_TREE parse_tree; /* set by yyparse() %start rule */
> int parse_file(FILE *fp)
> {
> YY_BUFFER_STATE old = YY_CURRENT_BUFFER;
> YY_BUFFER_STATE new = yy_create_buffer(fp, ...);
> yy_switch_to_buffer(new);
> while (yyparse() != EOF_SIGNAL)
> evaluate(parse_tree);
> yy_delete_buffer(new);
> if (old != NULL)
> yy_switch_to_buffer(old);
> }
>


This won't work for either yyparse or yylex unless the "load file"
command functions as an inline substitution - i.e. the scanner's input
file is changed but the parser simply continues on with the parse
sequence.


Neither yyparse nor yylex can normally be called recursively. The GNU
project has a Yacc work-alike called Bison which can generate a
reentrant parser compatible with yylex, but as yet there is no
corresponding reentrant scanner tool.




George Neuner
Dynamic Resolutions, Inc.

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