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* doc/bison.texinfo (Destructor Decl): New.

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Akim Demaille
2002-11-12 08:35:00 +00:00
parent b1ae9233bc
commit 72f889cca3
3 changed files with 100 additions and 15 deletions
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6
ChangeLog
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@@ -1,3 +1,7 @@
2002-11-12 Akim Demaille <akim@epita.fr>
* doc/bison.texinfo (Destructor Decl): New.
2002-11-12 Akim Demaille <akim@epita.fr>
* src/tables.c (tables_generate): Use free for pointers that
@@ -6,7 +10,7 @@
* src/state.c (state_new): Likewise.
* src/reader.c (reader): Likewise.
* src/lalr.c (set_goto_map): Likewise.
* src/location.h (LOCATION_PRINT): If first_line is 0, just issue
* src/location.h (LOCATION_PRINT): If first_line is 0, just issue
the file name.
2002-11-12 Akim Demaille <akim@epita.fr>

4
NEWS
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@@ -3,6 +3,10 @@ Bison News
Changes in version 1.75b:
* %destructor
It is now possible to reclaim the memory associated to symbols
discarded during error recovery. This feature is still experimental.
* Bison now parses C99 lexical constructs like digraphs, UCNs, and
backslash-newline within C escape sequences, as POSIX now requires.

105
doc/bison.texinfo
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@@ -208,6 +208,7 @@ Bison Declarations
* 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.
@@ -1851,7 +1852,6 @@ symrec *tptr; /* For returning symbol-table pointers */
%right '^' /* Exponentiation */
/* Grammar follows */
%%
@end smallexample
@@ -1928,7 +1928,9 @@ provides for either functions or variables to be placed in the table.
@group
/* Function type. */
typedef double (*func_t) (double);
@end group
@group
/* Data type for links in the chain of symbols. */
struct symrec
@{
@@ -2748,11 +2750,11 @@ a-or-b: 'a'|'b' @{ a_or_b_found = 1; @};
@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
@@ -3174,6 +3176,7 @@ Grammars}).
* 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.
@@ -3364,6 +3367,72 @@ use the same @code{<@var{type}>} construction in a declaration for the
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
@@ -3535,6 +3604,10 @@ This output file is essential if you wish to put the definition of
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}.
@@ -3634,8 +3707,6 @@ parser states and what is done for each type of look-ahead token in
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.
@@ -5069,15 +5140,17 @@ will be tokens to read before the next newline. So the rule is not
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
@@ -6302,6 +6375,10 @@ Equip the parser for debugging. @xref{Decl Summary}.
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