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YYACCEPT, YYERROR, and YYABORT, as user actions, should not

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destroy the RHS symbols of a rule.
* data/yacc.c (yylen): Initialize to 0.
Keep its value to the number of items to possibly shift.
In particular, a regular successful parse that ends on YYFINAL by
a (internal) YYACCEPT must not have yylen != 0.
(yyerrorlab, yyreturn): Pop the RHS.
Reorder a bit to emphasize the `shifting' bits of code.
(YYPOPSTACK): Now accept a number of items to pop.
* data/lalr1.cc: Likewise.
* data/glr.c: Formatting changes.
Use goto instead of fall through.
* doc/bison.texinfo (Destructor Decl): Complete.
This commit is contained in:
Akim Demaille
2005-12-21 15:28:30 +00:00
parent e14d0ab687
commit a85284cfbd
6 changed files with 89 additions and 79 deletions
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49
doc/bison.texinfo
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@@ -3810,20 +3810,20 @@ For instance, if your locations use a file name, you may use
@cindex freeing discarded symbols
@findex %destructor
Some symbols can be discarded by the parser. During error recovery
(@pxref{Error Recovery}), symbols already pushed on the stack and tokens
coming from the rest of the file are discarded until the parser falls on
its feet. If the parser runs out of memory, all the symbols on the
stack must be discarded. Even if the parser succeeds, it must discard
the start symbol.
During error recovery (@pxref{Error Recovery}), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet. If the parser runs out of memory,
or if the parsing is cut by @code{YYACCEPT} or @code{YYABORT}, all the
symbols on the stack must be discarded. Even if the parser succeeds, it
must discard the start symbol.
When discarded symbols convey heap based information, this memory is
lost. While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.
The @code{%destructor} directive defines code that
is called when a symbol is discarded.
The @code{%destructor} directive defines code that is called when a
symbol is automatically discarded.
@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
@findex %destructor
@@ -3832,10 +3832,6 @@ Within @var{code}, @code{$$} designates the semantic value associated
with the discarded symbol. The additional parser parameters are also
available (@pxref{Parser Function, , The Parser Function
@code{yyparse}}).
@strong{Warning:} as of Bison 2.1, this feature is still
experimental, as there has not been enough user feedback. In particular,
the syntax might still change.
@end deffn
For instance:
@@ -3854,24 +3850,6 @@ For instance:
guarantees that when a @code{STRING} or a @code{string} is discarded,
its associated memory will be freed.
Note that in the future, Bison might also consider that right hand side
members that are not mentioned in the action can be destroyed. For
instance, in:
@smallexample
comment: "/*" STRING "*/";
@end smallexample
@noindent
the parser is entitled to destroy the semantic value of the
@code{string}. Of course, this will not apply to the default action;
compare:
@smallexample
typeless: string; // $$ = $1 does not apply; $1 is destroyed.
typefull: string; // $$ = $1 applies, $1 is not destroyed.
@end smallexample
@sp 1
@cindex discarded symbols
@@ -3883,13 +3861,18 @@ stacked symbols popped during the first phase of error recovery,
@item
incoming terminals during the second phase of error recovery,
@item
the current look-ahead and the entire stack when the parser aborts
(either via an explicit call to @code{YYABORT}, or as a consequence of
a failed error recovery or of memory exhaustion), and
the current look-ahead and the entire stack (except the current
right-hand side symbols) when the parser aborts (either via an explicit
call to @code{YYABORT} or @code{YYACCEPT}, or as a consequence of a
failed error recovery or of memory exhaustion), and
@item
the start symbol, when the parser succeeds.
@end itemize
Note that right-hand size symbols of a rule that explicitly triggers a
syntax error via @code{YYERROR} are not reclaimed. As a thumb rule,
destructors are invoked only when you do not have other means to manage
the memory.
@node Expect Decl
@subsection Suppressing Conflict Warnings