How does flex support bison-location exactly?

Question

I\'m trying to use flex and bison to create a filter, because I want get certain grammar elements from a complex language. My plan is to use flex + bison to recognise the gr

Answer 1

The yylex declaration probably changed because you used a reentrant or pure-parser. Seems like many documents around the web suggest it's required if you want bison locations to work but it's not required.

I needed line numbers too and found the Bison documentation confusing in that regard. The simple solution (using the global var yylloc): In your Bison file just add the %locations directive:

%{
...
%}
%locations
...
%%
...

in your lexer:

%{
...
#include "yourprser.tab.h"  /* This is where it gets the definition for yylloc from */
#define YY_USER_ACTION yylloc.first_line = yylloc.last_line = yylineno;
%}
%option yylineno
...
%%
...

The YY_USER_ACTION macro is "called" before each of your token actions and updates yylloc. Now you can use the @N/@$ rules like this:

statement : error ';'   { fprintf(stderr, "Line %d: Bad statement.\n", @1.first_line); }

, or use the yylloc global var:

void yyerror(char *s)
{
  fprintf(stderr, "ERROR line %d: %s\n", yylloc.first_line, s);
}