Flex input buffer reset after error

Question

I'm using flex & bison to parse a custom language and I'm in the situation described here: http://www.gnu.org/software/bison/manual/html_node/How-Can-I-Reset-the-Parser.html. To be more precise

I invoke yyparse several times, and on correct input it works properly; but when a parse error is found, all the other calls fail too. How can I reset the error flag of yyparse?

My parser and scanner run inside a separate thread, but there is only one thread working with the input file. In my understanding I don't need to write a reentrant scanner since there is only one thread working with the input file. In that page the problem is clearly explained but the solution is not clear to me. It says:

Therefore, whenever you change yyin, you must tell the Lex-generated scanner to discard its current buffer and switch to the new one. This depends upon your implementation of Lex; see its documentation for more. For Flex, it suffices to call ‘YY_FLUSH_BUFFER’ after each change to yyin. If your Flex-generated scanner needs to read from several input streams to handle features like include files, you might consider using Flex functions like ‘yy_switch_to_buffer’ that manipulate multiple input buffers

My parser thread calls yyparse in order to build my AST. What is not clear to me is when and where I have to call yy_flush_buffer to fix the problem. In my understanding the scanner code (generated by Flex) is called by the parser code (generated by Bison). The Bison generated code is generated by the grammar. As a result the parser code is not under my direct control. This means I cannot include the call to yy_flush_buffer into the parser code since it would be overwritten every time I generate the parser code by the grammar. It means that I should put the yy_flush_buffer in the grammr file somewhere. But where?

Salvatore Salvatore · Accepted Answer · 2013-07-12T15:00:41

I fixed the problem by doing:

...
FILE *f = fopen(_filename, "r");
yyrestart(f);
yyparse();
...

I leave the question since it could be useful for other people.

Flex input buffer reset after error

1 Answers