Re: Destructor trouble

[Andrew Chanler]

Howdy,
im new to the help-bison mailing list and now exactly sure how to make
a post but i hope this works!

i thought i would give an example of how i was using the %destructor
feature and a problem i ran into.
heres a few lines of code:
//////////////////////////////////////
%union {
        double double_val;
        char* string_val;
}

%token <string_val> IDENTIFIER
%type <double_val> exp
%type <string_val> line
%type <string_val> any_identifier


%destructor {if($$){
                        delete [] $$;
                        $$=NULL;
                        }
                } IDENTIFIER any_identifier line

%destructor { if((defining_func) &&(yyvaluep->string_val)) {
                        delete [] yyvaluep->string_val;
                        yyvaluep->string_val = NULL;
                         }
                } exp

////////////////////////////////

now in my rules, every time i allocated memory for strings i used:
$$ = new char[strlen + 1];
when i set the flag defining_func to true, i always access exp as a
string_val and allocate memory to it in the same way.
as you can see, my destructor rule for 'exp' is kind of odd. above it,
i declared exp as double_val type and that prevented me for accessing
exp as string_val in the destructor. i thought that i should have been
able to write my destructor rule for exp like this:

%destructor { if((defining_func) && ($<string_val>$) {
                        delete [] $<string_val>$;
                        $<string_val>$ = NULL;
                         }
                } exp

but bison did not like $<string_val>$ in the destructor code, so i went
and loaded my output file(the .tab.c file) and found the line of code
that was being used in the other destructor. 
and it was
yyvaluep->string_val
to access the string value.
so i put that into my destructor rule for exp and then everything
worked like a charm.

i did this project in microsoft visual c++ and used its debugger to
check for memory leaks.
after i got those destructor rules working, all of my memory leaks were
cleared up.

but i thought it was pretty anouying that i couldn't access
$<string_val>$ in the destructor rule.

i hope this helps. %destructor is pretty simple. and i figured out how
to use it with the bison 1.875 manual that comes with the the download
of the binary.

and im pretty new to all the bison/flex stuff(i have been
studying/using it for half of this semester at college) so i hope i
didn't do this incorrectly.


Andrew Chanler



--- Hans Yperman <address@hidden> wrote:
> 
> Hello everyone,
> 
> i have some trouble with understanding the %destructor feature: if I
> have
> a rule:
> 
> %type<pointertype> a b c d
> %destructor {do_something} pointertype
> 
> a: b c d {
>               if(trouble) YYERROR
>       }
> 
> Say b,c and d are pointers that should be freed.
> If trouble happens, should i free b, c and d myself, or are they
> cleaned
> by  YYERROR?  And is it possible to call the destructor in the bison
> code,
> say with something like:
> 
> if(trouble){ YYDESTRUCT(b); ...; YYERROR;}
> 
> Also, if i write yyerror() so that it longjump()s out of the parser,
> how
> can i clean up the stack? This is an issue, as in my program,
> yyerror()
> is called by some other modules that also should clean up the stack
> (e.g. error in the input stream or the lexer)
> 
> Another issue:  I run both bison 1.35 (from Debian) and bison 1.875
> (GNU).  The last one is so immensely slow compared by the first one,
> spending almost 30 seconds running m4. Does someone know why?
> 
> Oh, and one more thing.  The bison html documentation on the gnu site
> is
> not up-to-date.  I can't find anything about %destructor in it.  If
> there
> are syntax errors in my example, i'm sorry (But you get what i mean
> with it, i guess).
> 
> Thanks in advance,
> 
> Hans.
> 


__________________________________
Do you Yahoo!?
The New Yahoo! Search - Faster. Easier. Bingo.
http://search.yahoo.com