There is a parser stack, which in each entry contains a state, a
semantic value, plus some more, like location value, if called for.
So no values are confused: they are being built bottom-up in the
parse tree. Just build $$ from the $k. For details on how a LALR(1)
parser works, see for example Aho, Sethi & Ullman,
"Compilers..." (the "dragon book"), or perhaps the FAQ of the Usenet
newsgroup comp.compilers, posted there monthly, or perhaps <http://
en.wikipedia.org/wiki/LALR_parser>.
Hans Aberg
On 16 Jan 2007, at 23:05, Roland Nilsson wrote:
Ok, I'll try to find an more recent version. Thanks.
But in any case, does the yylval variable carry the value of the most
recent $$ variable? Or is this perhaps overwritten with each new token
scanned?
// Roland
2007/1/16, Hans Aberg <[EMAIL PROTECTED]>:
Pick down some later version of Bison (2.3+); then, if you use the C
parser, there is a %destructor command, that can be used to handle
clean-up during error recovery. Another way is to use a C++ parser,
and a semantic type that does cleanup via the C++ language, but one
cannot then use %union.
Hans Aberg
On 15 Jan 2007, at 23:12, Roland Nilsson wrote:
> Hello,
>
> [apologies if this is a common question, I didn't find any FAQ for
> this list]
>
> I am using Bison (v1.24, a windows port I found on the web; I'm
not on
> linux) for a simple command-line interpreter of my own experimental
> language. The parser actions are used to recursively build up a
kind
> of syntax tree, so they allocate various objects along the way.
This
> works fine, except when there are parse errors; in which case I
need
> to intercept and deallocate whatever objects have been allocated so
> far, to avoid memory leaks.
>
> To do this cleanup I need to find the thing produced by the last
> reduction before the error. I first thought I could use yylval
inside
> the yyerror() routine for this, but I constantly find that
yylval is
> empty (null) when entering yyerror(). So how do I access the
value of
> the last reduction (i.e. the last thing assigned to $$) ??
>
> By the way, yylval is null also after successful parsing. I thought
> this was strange too, from what I gathered from the Bison docs it
> should contain the $$ result at all the time? I got around that by
> inserting a "top level" rule that simply copies $$ to a different
> variable prior to accepting, but it seems like a weird work-
around ...
> ?
>
> Thanks for any help!
>
> Best,
> Roland Nilsson
> Linköping, Sweden
>
>
> _______________________________________________
> help-bison@gnu.org http://lists.gnu.org/mailman/listinfo/help-bison
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