Re: How to restart FLEX scanners? (George Neuner)
12 Feb 1999 02:56:05 -0500

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From: (George Neuner)
Newsgroups: comp.compilers
Date: 12 Feb 1999 02:56:05 -0500
Organization: Dynamic ReSolutions, Inc.
References: 99-02-013
Keywords: lex

On 3 Feb 1999 23:55:26 -0500, (Joe English)
>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 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

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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