Re: how to work with Bison locations?

[Luca]

Location are created by lexer and "reduced" by the parser, so you have 
to create them using the lexer:

#define YY_USER_ACTION {yylloc.first_line = yylineno; 
yylloc.first_column = colnum; colnum=colnum+yyleng; 
yylloc.last_column=colnum; yylloc.last_line = yylineno;}

this macro is executed at each token recognized by lex. Insert it in 
bug.l inside %{}%; you have to define and manage colnum, yylineno is 
automatically defined by flex.
You can access to location using @ from parser actions, or using yylloc 
from flex.


You can also redefine, if you need, YYLTYPE (user custom locations, for 
example to add a field to the struct to track also file location), 
YYLLOC_DEFAULT (to merge locations) and YY_LOCATION_PRINT (to print 
locations) without any trouble.
You have to redefine those macro BEFORE to include y.tab.h  otherwise 
the compiler will complain.

Luca

Kynn Jones ha scritto:
> Hi.  I'm working with Bison (and Flex) for the first time, and it's rough going.
>
> I can't get Bison locations to work.  The following toy parser
> illustrates the problem.
>
> Here's the file for Flex, bug.l:
>
> -------------------------------------------------------------------------------
>
> %{
> #include "bug.tab.h"
> %}
>
> %option noyywrap nounput outfile="bug.yy.c" header-file="bug.yy.h"
>
> %%
>    /* skip over whitespace */
> [ \t\r\n]+
>
>    /* bomb anywhere else */
> <*>.                            { return TINVALID; }
>
> %%
>
> -------------------------------------------------------------------------------
>
> Here's the file for Bison, bug.y:
>
> -------------------------------------------------------------------------------
>
> %{
> #include "bug.tab.h"
> #include "bug.yy.h"
>
> void yyerror ( char const *s ) {}
> %}
>
> %defines
> %token TINVALID
>
> %%
>
> bug: error {
>               fprintf( stderr,
>                        "@$: %d:%d-%d:%d\n",
>                        @$.first_line,
>                        @$.first_column,
>                        @$.last_line,
>                        @$.last_column );
>               return -1;
>             }
>     ;
> %%
>
> -------------------------------------------------------------------------------
>
>
> Here's the file parsebug.c:
>
> -------------------------------------------------------------------------------
> #include <stdio.h>
> #include "bug.tab.h"
> #include "bug.yy.h"
>
> extern int yyparse();
>
> int main (int argc, char **argv) {
>
>   yyout = fopen( "/dev/null", "w" );
>   yyin = argc > 1 ? fopen( argv[ 1 ], "r" ) : stdin;
>
>   return yyparse();
> }
> -------------------------------------------------------------------------------
>
>
>
> Here's what I run:
>
> % bison bug.y
> % flex bug.l
> % gcc -Wall --pedantic bug.tab.c bug.yy.c parsebug.c -o bug
> % ( echo "\n\nfoo\n\n" | bug ) || echo "exited with error"
> @$: 1:1-1:1
> exited with error
>
>
> So the parser fails, as expected, but no matter what the input is, the
> first line of the output is always "@$: 1:1-1:1\n".  This makes no
> sense to me.  If the "location" is going to be of any use, I'd expect
> that it would reflect the position in the input where the parser
> and/or scanner first got stuck.  I.e. I expect that, with the input
> above, at least one of the fields in @$ would be greater than 1.
>
> Am I misunderstanding the meaning of Bisons "locations"?  If not, what
> am I doing wrong?
>
> BTW, what's the standard way of implementing the handling of a
> "scanner error" condition (i.e. to cover cases when the scanner finds
> something that it does not expect).
>
> Thanks!
>
> kynn
>
>
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>
>