* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
+* Destructor Decl:: Declaring how symbols are freed.
* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
%right '^' /* Exponentiation */
/* Grammar follows */
-
%%
@end smallexample
@group
/* Function type. */
typedef double (*func_t) (double);
+@end group
+@group
/* Data type for links in the chain of symbols. */
struct symrec
@{
@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
-@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes
-the value of the whole rule. Of course, the default rule is valid only
-if the two data types match. There is no meaningful default action for
-an empty rule; every empty rule must have an explicit action unless the
-rule's value does not matter.
+@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule
+becomes the value of the whole rule. Of course, the default action is
+valid only if the two data types match. There is no meaningful default
+action for an empty rule; every empty rule must have an explicit action
+unless the rule's value does not matter.
@code{$@var{n}} with @var{n} zero or negative is allowed for reference
to tokens and groupings on the stack @emph{before} those that match the
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
+* Destructor Decl:: Declaring how symbols are freed.
* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
terminal symbol. All kinds of token declarations allow
@code{<@var{type}>}.
+@node Destructor Decl
+@subsection Freeing Discarded Symbols
+@cindex freeing discarded symbols
+@findex %destructor
+
+Some symbols can be discarded by the parser, typically during error
+recovery (@pxref{Error Recovery}). Basically, during error recovery,
+embarrassing symbols already pushed on the stack, and embarrassing
+tokens coming from the rest of the file are thrown away until the parser
+falls on its feet. If these symbols convey heap based information, this
+memory is lost. While this behavior is tolerable for batch parsers,
+such as in compilers, it is unacceptable for parsers that can
+possibility ``never end'' such as shells, or implementations of
+communication protocols.
+
+The @code{%destructor} directive allows for the definition of code that
+is called when a symbol is thrown away.
+
+@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
+@findex %destructor
+Declare that the @var{code} must be invoked for each of the
+@var{symbols} that will be discarded by the parser. The @var{code}
+should use @code{$$} to designate the semantic value associated to the
+@var{symbols}. The additional parser parameters are also avaible
+(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
+
+@strong{Warning:} as of Bison 1.875, this feature is still considered as
+experimental, as there was not enough users feedback. In particular,
+the syntax might still change, and for the time being, only the default
+@acronym{LALR}(1) skeleton supports this feature.
+@end deffn
+
+For instance:
+
+@smallexample
+%union
+@{
+ char *string;
+@}
+%token <string> STRING
+%type <string> string
+%destructor @{ free ($$); @} STRING string
+@end smallexample
+
+@noindent
+guarantees that when a @code{STRING} or a @code{string} will be 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
+
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
be able to refer to token type codes and the variable
@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.
+@item %destructor
+Specifying how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+
@item %file-prefix="@var{prefix}"
Specify a prefix to use for all Bison output file names. The names are
chosen as if the input file were named @file{@var{prefix}.y}.
that state. @xref{Understanding, , Understanding Your Parser}, for more
information.
-
-
@item %yacc
Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
including its naming conventions. @xref{Bison Options}, for more.
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part of
-the semantic context and part of the input. First it discards states and
-objects from the stack until it gets back to a state in which the
+the semantic context and part of the input. First it discards states
+and objects from the stack until it gets back to a state in which the
@code{error} token is acceptable. (This means that the subexpressions
-already parsed are discarded, back to the last complete @code{stmnts}.) At
-this point the @code{error} token can be shifted. Then, if the old
+already parsed are discarded, back to the last complete @code{stmnts}.)
+At this point the @code{error} token can be shifted. Then, if the old
look-ahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
-this example, Bison reads and discards input until the next newline
-so that the fourth rule can apply.
+this example, Bison reads and discards input until the next newline so
+that the fourth rule can apply. Note that discarded symbols are
+possible sources of memory leaks, see @ref{Destructor Decl, , Freeing
+Discarded Symbols}, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies for
error recovery. A simple and useful strategy is simply to skip the rest of
Bison declaration to create a header file meant for the scanner.
@xref{Decl Summary}.
+@item %destructor
+Specifying how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+
@item %dprec
Bison declaration to assign a precedence to a rule that is used at parse
time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing
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