@value{UPDATED}), the @acronym{GNU} parser generator.
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
+1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the @acronym{GNU} Free Documentation License,
-Version 1.1 or any later version published by the Free Software
+Version 1.2 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in
(a) below. A copy of the license is included in the section entitled
@end quotation
@end copying
-@dircategory GNU programming tools
+@dircategory Software development
@direntry
* bison: (bison). @acronym{GNU} parser generator (Yacc replacement).
@end direntry
@insertcopying
@sp 2
Published by the Free Software Foundation @*
-59 Temple Place, Suite 330 @*
-Boston, MA 02111-1307 USA @*
+51 Franklin Street, Fifth Floor @*
+Boston, MA 02110-1301 USA @*
Printed copies are available from the Free Software Foundation.@*
@acronym{ISBN} 1-882114-44-2
@sp 2
messy for Bison to handle straightforwardly.
* Debugging:: Understanding or debugging Bison parsers.
* Invocation:: How to run Bison (to produce the parser source file).
+* C++ Language Interface:: Creating C++ parser objects.
+* FAQ:: Frequently Asked Questions
* Table of Symbols:: All the keywords of the Bison language are explained.
* Glossary:: Basic concepts are explained.
-* FAQ:: Frequently Asked Questions
* Copying This Manual:: License for copying this manual.
* Index:: Cross-references to the text.
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
-* Simple GLR Parsers:: Using GLR in its simplest form.
* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
* Grammar Layout:: Overall structure of a Bison grammar file.
+Writing @acronym{GLR} Parsers
+
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+
Examples
* RPN Calc:: Reverse polish notation calculator;
* 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.
+* Initial Action Decl:: Code run before parsing starts.
* Destructor Decl:: Declaring how symbols are freed.
* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
The Lexical Analyzer Function @code{yylex}
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
-* Stack Overflow:: What happens when stack gets full. How to avoid it.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
Operator Precedence
* Option Cross Key:: Alphabetical list of long options.
* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
+C++ Language Interface
+
+* C++ Parsers:: The interface to generate C++ parser classes
+* A Complete C++ Example:: Demonstrating their use
+
+C++ Parsers
+
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+
+A Complete C++ Example
+
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+
Frequently Asked Questions
-* Parser Stack Overflow:: Breaking the Stack Limits
+* Memory Exhausted:: Breaking the Stack Limits
* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
-* C++ Parsers:: Compiling Parsers with C++ Compilers
-* Implementing Loops:: Control Flow in the Calculator
+* Implementing Gotos/Loops:: Control Flow in the Calculator
Copying This Manual
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
-* Simple GLR Parsers:: Using GLR in its simplest form.
* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
uniquely determined by the preceding input and a fixed, finite portion
(called a @dfn{look-ahead}) of the remaining input. A context-free
grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
-apply the grammar rules to get the some inputs. Even unambiguous
+apply the grammar rules to get the same inputs. Even unambiguous
grammars can be @dfn{non-deterministic}, meaning that no fixed
look-ahead always suffices to determine the next grammar rule to apply.
With the proper declarations, Bison is also able to parse these more
@ifinfo
@example
int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, identifier,}
+square (int x) /* @r{identifier, open-paren, keyword `int',}
@r{identifier, close-paren} */
@{ /* @r{open-brace} */
return x * x; /* @r{keyword `return', identifier, asterisk,
@ifnotinfo
@example
int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */
+square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */
@{ /* @r{open-brace} */
return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
@} /* @r{close-brace} */
@findex %glr-parser
@cindex conflicts
@cindex shift/reduce conflicts
+@cindex reduce/reduce conflicts
-In some grammars, there will be cases where Bison's standard
+In some grammars, Bison's standard
@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
certain grammar rule at a given point. That is, it may not be able to
decide (on the basis of the input read so far) which of two possible
To use a grammar that is not easily modified to be @acronym{LALR}(1), a
more general parsing algorithm is sometimes necessary. If you include
@code{%glr-parser} among the Bison declarations in your file
-(@pxref{Grammar Outline}), the result will be a Generalized @acronym{LR}
+(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR}
(@acronym{GLR}) parser. These parsers handle Bison grammars that
contain no unresolved conflicts (i.e., after applying precedence
declarations) identically to @acronym{LALR}(1) parsers. However, when
user-defined function on the resulting values to produce an arbitrary
merged result.
+@menu
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+@end menu
+
+@node Simple GLR Parsers
+@subsection Using @acronym{GLR} on Unambiguous Grammars
+@cindex @acronym{GLR} parsing, unambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
+@findex %glr-parser
+@findex %expect-rr
+@cindex conflicts
+@cindex reduce/reduce conflicts
+@cindex shift/reduce conflicts
+
+In the simplest cases, you can use the @acronym{GLR} algorithm
+to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
+Such grammars typically require more than one symbol of look-ahead,
+or (in rare cases) fall into the category of grammars in which the
+@acronym{LALR}(1) algorithm throws away too much information (they are in
+@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+
+Consider a problem that
+arises in the declaration of enumerated and subrange types in the
+programming language Pascal. Here are some examples:
+
+@example
+type subrange = lo .. hi;
+type enum = (a, b, c);
+@end example
+
+@noindent
+The original language standard allows only numeric
+literals and constant identifiers for the subrange bounds (@samp{lo}
+and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC}
+10206) and many other
+Pascal implementations allow arbitrary expressions there. This gives
+rise to the following situation, containing a superfluous pair of
+parentheses:
+
+@example
+type subrange = (a) .. b;
+@end example
+
+@noindent
+Compare this to the following declaration of an enumerated
+type with only one value:
+
+@example
+type enum = (a);
+@end example
+
+@noindent
+(These declarations are contrived, but they are syntactically
+valid, and more-complicated cases can come up in practical programs.)
+
+These two declarations look identical until the @samp{..} token.
+With normal @acronym{LALR}(1) one-token look-ahead it is not
+possible to decide between the two forms when the identifier
+@samp{a} is parsed. It is, however, desirable
+for a parser to decide this, since in the latter case
+@samp{a} must become a new identifier to represent the enumeration
+value, while in the former case @samp{a} must be evaluated with its
+current meaning, which may be a constant or even a function call.
+
+You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
+to be resolved later, but this typically requires substantial
+contortions in both semantic actions and large parts of the
+grammar, where the parentheses are nested in the recursive rules for
+expressions.
+
+You might think of using the lexer to distinguish between the two
+forms by returning different tokens for currently defined and
+undefined identifiers. But if these declarations occur in a local
+scope, and @samp{a} is defined in an outer scope, then both forms
+are possible---either locally redefining @samp{a}, or using the
+value of @samp{a} from the outer scope. So this approach cannot
+work.
+
+A simple solution to this problem is to declare the parser to
+use the @acronym{GLR} algorithm.
+When the @acronym{GLR} parser reaches the critical state, it
+merely splits into two branches and pursues both syntax rules
+simultaneously. Sooner or later, one of them runs into a parsing
+error. If there is a @samp{..} token before the next
+@samp{;}, the rule for enumerated types fails since it cannot
+accept @samp{..} anywhere; otherwise, the subrange type rule
+fails since it requires a @samp{..} token. So one of the branches
+fails silently, and the other one continues normally, performing
+all the intermediate actions that were postponed during the split.
+
+If the input is syntactically incorrect, both branches fail and the parser
+reports a syntax error as usual.
+
+The effect of all this is that the parser seems to ``guess'' the
+correct branch to take, or in other words, it seems to use more
+look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
+for. In this example, @acronym{LALR}(2) would suffice, but also some cases
+that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+
+In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
+and the current Bison parser even takes exponential time and space
+for some grammars. In practice, this rarely happens, and for many
+grammars it is possible to prove that it cannot happen.
+The present example contains only one conflict between two
+rules, and the type-declaration context containing the conflict
+cannot be nested. So the number of
+branches that can exist at any time is limited by the constant 2,
+and the parsing time is still linear.
+
+Here is a Bison grammar corresponding to the example above. It
+parses a vastly simplified form of Pascal type declarations.
+
+@example
+%token TYPE DOTDOT ID
+
+@group
+%left '+' '-'
+%left '*' '/'
+@end group
+
+%%
+
+@group
+type_decl : TYPE ID '=' type ';'
+ ;
+@end group
+
+@group
+type : '(' id_list ')'
+ | expr DOTDOT expr
+ ;
+@end group
+
+@group
+id_list : ID
+ | id_list ',' ID
+ ;
+@end group
+
+@group
+expr : '(' expr ')'
+ | expr '+' expr
+ | expr '-' expr
+ | expr '*' expr
+ | expr '/' expr
+ | ID
+ ;
+@end group
+@end example
+
+When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+about one reduce/reduce conflict. In the conflicting situation the
+parser chooses one of the alternatives, arbitrarily the one
+declared first. Therefore the following correct input is not
+recognized:
+
+@example
+type t = (a) .. b;
+@end example
+
+The parser can be turned into a @acronym{GLR} parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, by
+adding these two declarations to the Bison input file (before the first
+@samp{%%}):
+
+@example
+%glr-parser
+%expect-rr 1
+@end example
+
+@noindent
+No change in the grammar itself is required. Now the
+parser recognizes all valid declarations, according to the
+limited syntax above, transparently. In fact, the user does not even
+notice when the parser splits.
+
+So here we have a case where we can use the benefits of @acronym{GLR}, almost
+without disadvantages. Even in simple cases like this, however, there
+are at least two potential problems to beware.
+First, always analyze the conflicts reported by
+Bison to make sure that @acronym{GLR} splitting is only done where it is
+intended. A @acronym{GLR} parser splitting inadvertently may cause
+problems less obvious than an @acronym{LALR} parser statically choosing the
+wrong alternative in a conflict.
+Second, consider interactions with the lexer (@pxref{Semantic Tokens})
+with great care. Since a split parser consumes tokens
+without performing any actions during the split, the lexer cannot
+obtain information via parser actions. Some cases of
+lexer interactions can be eliminated by using @acronym{GLR} to
+shift the complications from the lexer to the parser. You must check
+the remaining cases for correctness.
+
+In our example, it would be safe for the lexer to return tokens
+based on their current meanings in some symbol table, because no new
+symbols are defined in the middle of a type declaration. Though it
+is possible for a parser to define the enumeration
+constants as they are parsed, before the type declaration is
+completed, it actually makes no difference since they cannot be used
+within the same enumerated type declaration.
+
+@node Merging GLR Parses
+@subsection Using @acronym{GLR} to Resolve Ambiguities
+@cindex @acronym{GLR} parsing, ambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars
+@findex %dprec
+@findex %merge
+@cindex conflicts
+@cindex reduce/reduce conflicts
+
Let's consider an example, vastly simplified from a C++ grammar.
@example
@samp{x} as an @code{ID}).
Bison detects this as a reduce/reduce conflict between the rules
@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
-time it encounters @code{x} in the example above. The two @code{%dprec}
-declarations, however, give precedence to interpreting the example as a
+time it encounters @code{x} in the example above. Since this is a
+@acronym{GLR} parser, it therefore splits the problem into two parses, one for
+each choice of resolving the reduce/reduce conflict.
+Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
+however, neither of these parses ``dies,'' because the grammar as it stands is
+ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and
+the other reduces @code{stmt : decl}, after which both parsers are in an
+identical state: they've seen @samp{prog stmt} and have the same unprocessed
+input remaining. We say that these parses have @dfn{merged.}
+
+At this point, the @acronym{GLR} parser requires a specification in the
+grammar of how to choose between the competing parses.
+In the example above, the two @code{%dprec}
+declarations specify that Bison is to give precedence
+to the parse that interprets the example as a
@code{decl}, which implies that @code{x} is a declarator.
The parser therefore prints
"x" y z + T <init-declare>
@end example
-Consider a different input string for this parser:
+The @code{%dprec} declarations only come into play when more than one
+parse survives. Consider a different input string for this parser:
@example
T (x) + y;
@end example
@noindent
+This is another example of using @acronym{GLR} to parse an unambiguous
+construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters @code{x}, it does not
have enough information to resolve the reduce/reduce conflict (again,
between @code{x} as an @code{expr} or a @code{declarator}). In this
-case, no precedence declaration is used. Instead, the parser splits
+case, no precedence declaration is used. Again, the parser splits
into two, one assuming that @code{x} is an @code{expr}, and the other
assuming @code{x} is a @code{declarator}. The second of these parsers
then vanishes when it sees @code{+}, and the parser prints
@end example
Suppose that instead of resolving the ambiguity, you wanted to see all
-the possibilities. For this purpose, we must @dfn{merge} the semantic
+the possibilities. For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other. To do so, you could change the declaration of @code{stmt} as
follows:
@end example
@noindent
-
and define the @code{stmtMerge} function as:
@example
@end example
@noindent
-With these declarations, the resulting parser will parse the first example
-as both an @code{expr} and a @code{decl}, and print
+With these declarations, the resulting parser parses the first example
+as both an @code{expr} and a @code{decl}, and prints
@example
"x" y z + T <init-declare> x T <cast> y z + = <OR>
@end example
-@sp 1
+Bison requires that all of the
+productions that participate in any particular merge have identical
+@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable,
+and the parser will report an error during any parse that results in
+the offending merge.
-@cindex @code{incline}
+@node Compiler Requirements
+@subsection Considerations when Compiling @acronym{GLR} Parsers
+@cindex @code{inline}
@cindex @acronym{GLR} parsers and @code{inline}
+
The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or
later. In addition, they use the @code{inline} keyword, which is not
C89, but is C99 and is a common extension in pre-C99 compilers. It is
%@}
@end example
-@node Simple GLR Parsers
-@section Using @acronym{GLR} in its Simplest Form
-@cindex @acronym{GLR} parsing, unambiguous grammars
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
-@findex %glr-parser
-@findex %expect-rr
-@cindex conflicts
-@cindex reduce/reduce conflicts
+@node Locations Overview
+@section Locations
+@cindex location
+@cindex textual location
+@cindex location, textual
-The C++ example for @acronym{GLR} (@pxref{GLR Parsers}) explains how to use
-the @acronym{GLR} parsing algorithm with some advanced features such as
-@samp{%dprec} and @samp{%merge} to handle syntactically ambiguous
-grammars. However, the @acronym{GLR} algorithm can also be used in a simpler
-way to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
-Such grammars typically require more than one symbol of lookahead,
-or (in rare cases) fall into the category of grammars in which the
-@acronym{LALR}(1) algorithm throws away too much information (they are in
-@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+Many applications, like interpreters or compilers, have to produce verbose
+and useful error messages. To achieve this, one must be able to keep track of
+the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
+Bison provides a mechanism for handling these locations.
-Here is an example of this situation, using a problem that
-arises in the declaration of enumerated and subrange types in the
-programming language Pascal. These declarations look like this:
+Each token has a semantic value. In a similar fashion, each token has an
+associated location, but the type of locations is the same for all tokens and
+groupings. Moreover, the output parser is equipped with a default data
+structure for storing locations (@pxref{Locations}, for more details).
-@example
-type subrange = lo .. hi;
-type enum = (a, b, c);
-@end example
+Like semantic values, locations can be reached in actions using a dedicated
+set of constructs. In the example above, the location of the whole grouping
+is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
+@code{@@3}.
-@noindent
-The original language standard allows only numeric
-literals and constant identifiers for the subrange bounds (@samp{lo}
-and @samp{hi}), but Extended Pascal (ISO/IEC 10206:1990) and many other
-Pascal implementations allow arbitrary expressions there. This gives
-rise to the following situation, containing a superfluous pair of
-parentheses:
+When a rule is matched, a default action is used to compute the semantic value
+of its left hand side (@pxref{Actions}). In the same way, another default
+action is used for locations. However, the action for locations is general
+enough for most cases, meaning there is usually no need to describe for each
+rule how @code{@@$} should be formed. When building a new location for a given
+grouping, the default behavior of the output parser is to take the beginning
+of the first symbol, and the end of the last symbol.
-@example
-type subrange = (a) .. b;
-@end example
+@node Bison Parser
+@section Bison Output: the Parser File
+@cindex Bison parser
+@cindex Bison utility
+@cindex lexical analyzer, purpose
+@cindex parser
-@noindent
-Compare this to the following declaration of an enumerated
-type with only one value:
+When you run Bison, you give it a Bison grammar file as input. The output
+is a C source file that parses the language described by the grammar.
+This file is called a @dfn{Bison parser}. Keep in mind that the Bison
+utility and the Bison parser are two distinct programs: the Bison utility
+is a program whose output is the Bison parser that becomes part of your
+program.
-@example
-type enum = (a);
-@end example
+The job of the Bison parser is to group tokens into groupings according to
+the grammar rules---for example, to build identifiers and operators into
+expressions. As it does this, it runs the actions for the grammar rules it
+uses.
-@noindent
-(These declarations are contrived, but they are syntactically
-valid, and more-complicated cases can come up in practical programs.)
-
-These two declarations look identical until the @samp{..} token.
-With normal @acronym{LALR}(1) one-token look-ahead it is not
-possible to decide between the two forms when the identifier
-@samp{a} is parsed. It is, however, desirable
-for a parser to decide this, since in the latter case
-@samp{a} must become a new identifier to represent the enumeration
-value, while in the former case @samp{a} must be evaluated with its
-current meaning, which may be a constant or even a function call.
-
-You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
-to be resolved later, but this typically requires substantial
-contortions in both semantic actions and large parts of the
-grammar, where the parentheses are nested in the recursive rules for
-expressions.
-
-You might think of using the lexer to distinguish between the two
-forms by returning different tokens for currently defined and
-undefined identifiers. But if these declarations occur in a local
-scope, and @samp{a} is defined in an outer scope, then both forms
-are possible---either locally redefining @samp{a}, or using the
-value of @samp{a} from the outer scope. So this approach cannot
-work.
-
-A solution to this problem is to use a @acronym{GLR} parser in its simplest
-form, i.e., without using special features such as @samp{%dprec} and
-@samp{%merge}. When the @acronym{GLR} parser reaches the critical state, it
-simply splits into two branches and pursues both syntax rules
-simultaneously. Sooner or later, one of them runs into a parsing
-error. If there is a @samp{..} token before the next
-@samp{;}, the rule for enumerated types fails since it cannot
-accept @samp{..} anywhere; otherwise, the subrange type rule
-fails since it requires a @samp{..} token. So one of the branches
-fails silently, and the other one continues normally, performing
-all the intermediate actions that were postponed during the split.
-
-If the input is syntactically incorrect, both branches fail and the parser
-reports a syntax error as usual.
-
-The effect of all this is that the parser seems to ``guess'' the
-correct branch to take, or in other words, it seems to use more
-look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
-for. In this example, @acronym{LALR}(2) would suffice, but also some cases
-that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
-
-Since there can be only two branches and at least one of them
-must fail, you need not worry about merging the branches by
-using dynamic precedence or @samp{%merge}.
-
-Another potential problem of @acronym{GLR} does not arise here, either. In
-general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
-and the current Bison parser even takes exponential time and space
-for some grammars. In practice, this rarely happens, and for many
-grammars it is possible to prove that it cannot happen. In
-in the present example, there is only one conflict between two
-rules, and the type-declaration context where the conflict
-arises cannot be nested. So the number of
-branches that can exist at any time is limited by the constant 2,
-and the parsing time is still linear.
-
-So here we have a case where we can use the benefits of @acronym{GLR}, almost
-without disadvantages. There are two things to note, though.
-First, one should carefully analyze the conflicts reported by
-Bison to make sure that @acronym{GLR} splitting is done only where it is
-intended to be. A @acronym{GLR} parser splitting inadvertently may cause
-problems less obvious than an @acronym{LALR} parser statically choosing the
-wrong alternative in a conflict.
-
-Second, interactions with the lexer (@pxref{Semantic Tokens}) must
-be considered with great care. Since a split parser consumes tokens
-without performing any actions during the split, the lexer cannot
-obtain information via parser actions. Some cases of
-lexer interactions can simply be eliminated by using @acronym{GLR}, i.e.,
-shifting the complications from the lexer to the parser. Remaining
-cases have to be checked for safety.
-
-In our example, it would be safe for the lexer to return tokens
-based on their current meanings in some symbol table, because no new
-symbols are defined in the middle of a type declaration. Though it
-is possible for a parser to define the enumeration
-constants as they are parsed, before the type declaration is
-completed, it actually makes no difference since they cannot be used
-within the same enumerated type declaration.
-
-Here is a Bison grammar corresponding to the example above. It
-parses a vastly simplified form of Pascal type declarations.
-
-@example
-%token TYPE DOTDOT ID
-
-@group
-%left '+' '-'
-%left '*' '/'
-@end group
-
-%%
-
-@group
-type_decl:
- TYPE ID '=' type ';'
-;
-@end group
-
-@group
-type: '(' id_list ')'
- | expr DOTDOT expr
-;
-@end group
-
-@group
-id_list: ID
- | id_list ',' ID
-;
-@end group
-
-@group
-expr: '(' expr ')'
- | expr '+' expr
- | expr '-' expr
- | expr '*' expr
- | expr '/' expr
- | ID
-;
-@end group
-@end example
-
-When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
-about one reduce/reduce conflict. In the conflicting situation the
-parser chooses one of the alternatives, arbitrarily the one
-declared first. Therefore the following correct input is not
-recognized:
-
-@example
-type t = (a) .. b;
-@end example
-
-The parser can be turned into a @acronym{GLR} parser, while also telling Bison
-to be silent about the one known reduce/reduce conflict, simply by
-adding these two declarations to the Bison input file:
-
-@example
-%glr-parser
-%expect-rr 1
-@end example
-
-@noindent
-No change in the grammar itself is required. Now the
-parser recognizes all valid declarations, according to the
-limited syntax above, transparently. In fact, the user does not even
-notice when the parser splits.
-
-@node Locations Overview
-@section Locations
-@cindex location
-@cindex textual location
-@cindex location, textual
-
-Many applications, like interpreters or compilers, have to produce verbose
-and useful error messages. To achieve this, one must be able to keep track of
-the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
-Bison provides a mechanism for handling these locations.
-
-Each token has a semantic value. In a similar fashion, each token has an
-associated location, but the type of locations is the same for all tokens and
-groupings. Moreover, the output parser is equipped with a default data
-structure for storing locations (@pxref{Locations}, for more details).
-
-Like semantic values, locations can be reached in actions using a dedicated
-set of constructs. In the example above, the location of the whole grouping
-is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
-@code{@@3}.
-
-When a rule is matched, a default action is used to compute the semantic value
-of its left hand side (@pxref{Actions}). In the same way, another default
-action is used for locations. However, the action for locations is general
-enough for most cases, meaning there is usually no need to describe for each
-rule how @code{@@$} should be formed. When building a new location for a given
-grouping, the default behavior of the output parser is to take the beginning
-of the first symbol, and the end of the last symbol.
-
-@node Bison Parser
-@section Bison Output: the Parser File
-@cindex Bison parser
-@cindex Bison utility
-@cindex lexical analyzer, purpose
-@cindex parser
-
-When you run Bison, you give it a Bison grammar file as input. The output
-is a C source file that parses the language described by the grammar.
-This file is called a @dfn{Bison parser}. Keep in mind that the Bison
-utility and the Bison parser are two distinct programs: the Bison utility
-is a program whose output is the Bison parser that becomes part of your
-program.
-
-The job of the Bison parser is to group tokens into groupings according to
-the grammar rules---for example, to build identifiers and operators into
-expressions. As it does this, it runs the actions for the grammar rules it
-uses.
-
-The tokens come from a function called the @dfn{lexical analyzer} that
-you must supply in some fashion (such as by writing it in C). The Bison
-parser calls the lexical analyzer each time it wants a new token. It
-doesn't know what is ``inside'' the tokens (though their semantic values
-may reflect this). Typically the lexical analyzer makes the tokens by
-parsing characters of text, but Bison does not depend on this.
-@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C). The Bison
+parser calls the lexical analyzer each time it wants a new token. It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this). Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
The Bison parser file is C code which defines a function named
@code{yyparse} which implements that grammar. This function does not make
those cases your code should respect the identifiers reserved by those
headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>},
@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
-declare memory allocators and related types. Other system headers may
+declare memory allocators and related types. @code{<libintl.h>} is
+included if message translation is in use
+(@pxref{Internationalization}). Other system headers may
be included if you define @code{YYDEBUG} to a nonzero value
(@pxref{Tracing, ,Tracing Your Parser}).
/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
-symrec *putsym (char const *, func_t);
+symrec *putsym (char const *, int);
symrec *getsym (char const *);
@end group
@end smallexample
definitions of @code{yylex} and @code{yyerror} often go here. Because
C requires functions to be declared before being used, you often need
to declare functions like @code{yylex} and @code{yyerror} in the Prologue,
-even if you define them int he Epilogue.
+even if you define them in the Epilogue.
@xref{Interface, ,Parser C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
the location of the grouping (the result of the computation). When a
-rule is matched, the second parameter is an array holding locations of
+rule is matched, the second parameter identifies locations of
all right hand side elements of the rule being matched, and the third
parameter is the size of the rule's right hand side. When processing
-a syntax error, the second parameter is an array holding locations of
+a syntax error, the second parameter identifies locations of
the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.
-By default, @code{YYLLOC_DEFAULT} is defined this way for simple
-@acronym{LALR}(1) parsers:
+By default, @code{YYLLOC_DEFAULT} is defined this way:
-@example
+@smallexample
@group
-# define YYLLOC_DEFAULT(Current, Rhs, N) \
- ((Current).first_line = (Rhs)[1].first_line, \
- (Current).first_column = (Rhs)[1].first_column, \
- (Current).last_line = (Rhs)[N].last_line, \
- (Current).last_column = (Rhs)[N].last_column)
+# define YYLLOC_DEFAULT(Current, Rhs, N) \
+ do \
+ if (N) \
+ @{ \
+ (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \
+ (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \
+ (Current).last_line = YYRHSLOC(Rhs, N).last_line; \
+ (Current).last_column = YYRHSLOC(Rhs, N).last_column; \
+ @} \
+ else \
+ @{ \
+ (Current).first_line = (Current).last_line = \
+ YYRHSLOC(Rhs, 0).last_line; \
+ (Current).first_column = (Current).last_column = \
+ YYRHSLOC(Rhs, 0).last_column; \
+ @} \
+ while (0)
@end group
-@end example
-
-@noindent
-and like this for @acronym{GLR} parsers:
+@end smallexample
-@example
-@group
-# define YYLLOC_DEFAULT(yyCurrent, yyRhs, YYN) \
- ((yyCurrent).first_line = YYRHSLOC(yyRhs, 1).first_line, \
- (yyCurrent).first_column = YYRHSLOC(yyRhs, 1).first_column, \
- (yyCurrent).last_line = YYRHSLOC(yyRhs, YYN).last_line, \
- (yyCurrent).last_column = YYRHSLOC(yyRhs, YYN).last_column)
-@end group
-@end example
+where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
+in @var{rhs} when @var{k} is positive, and the location of the symbol
+just before the reduction when @var{k} and @var{n} are both zero.
When defining @code{YYLLOC_DEFAULT}, you should consider that:
result) should be modified by @code{YYLLOC_DEFAULT}.
@item
-For consistency with semantic actions, valid indexes for the location
-array range from 1 to @var{n}.
+For consistency with semantic actions, valid indexes within the
+right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a
+valid index, and it refers to the symbol just before the reduction.
+During error processing @var{n} is always positive.
@item
Your macro should parenthesize its arguments, if need be, since 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.
+* Initial Action Decl:: Code run before parsing starts.
* Destructor Decl:: Declaring how symbols are freed.
* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
terminal symbol. All kinds of token declarations allow
@code{<@var{type}>}.
+@node Initial Action Decl
+@subsection Performing Actions before Parsing
+@findex %initial-action
+
+Sometimes your parser needs to perform some initializations before
+parsing. The @code{%initial-action} directive allows for such arbitrary
+code.
+
+@deffn {Directive} %initial-action @{ @var{code} @}
+@findex %initial-action
+Declare that the @var{code} must be invoked before parsing each time
+@code{yyparse} is called. The @var{code} may use @code{$$} and
+@code{@@$} --- initial value and location of the look-ahead --- and the
+@code{%parse-param}.
+@end deffn
+
+For instance, if your locations use a file name, you may use
+
+@example
+%parse-param @{ const char *filename @};
+%initial-action
+@{
+ @@$.begin.filename = @@$.end.filename = filename;
+@};
+@end example
+
+
@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.
+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.
-The @code{%destructor} directive allows for the definition of code that
-is called when a symbol is thrown away.
+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.
@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 available
+Invoke @var{code} whenever the parser discards one of the
+@var{symbols}. Within @var{code}, @code{$$} designates the semantic
+value associated with the discarded symbol. The additional
+parser parameters are also available
(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
-@strong{Warning:} as of Bison 1.875, this feature is still considered as
-experimental, as there was not enough user feedback. In particular,
+@strong{Warning:} as of Bison 2.1, this feature is still
+experimental, as there has not been enough user feedback. In particular,
the syntax might still change.
@end deffn
@end smallexample
@noindent
-guarantees that when a @code{STRING} or a @code{string} will be discarded,
+guarantees that when a @code{STRING} or a @code{string} is discarded,
its associated memory will be freed.
Note that in the future, Bison might also consider that right hand side
typefull: string; // $$ = $1 applies, $1 is not destroyed.
@end smallexample
+@sp 1
+
+@cindex discarded symbols
+@dfn{Discarded symbols} are the following:
+
+@itemize
+@item
+stacked symbols popped during the first phase of error recovery,
+@item
+incoming terminals during the second phase of error recovery,
+@item
+the current look-ahead and the entire stack when the parser aborts
+(either via an explicit call to @code{YYABORT}, or as a consequence of
+a failed error recovery or of memory exhaustion), and
+@item
+the start symbol, when the parser succeeds.
+@end itemize
+
+
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
given if there are either more or fewer conflicts, or if there are any
reduce/reduce conflicts.
-For normal LALR(1) parsers, reduce/reduce conflicts are more serious,
+For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more serious,
and should be eliminated entirely. Bison will always report
-reduce/reduce conflicts for these parsers. With GLR parsers, however,
+reduce/reduce conflicts for these parsers. With @acronym{GLR} parsers, however,
both shift/reduce and reduce/reduce are routine (otherwise, there
-would be no need to use GLR parsing). Therefore, it is also possible
-to specify an expected number of reduce/reduce conflicts in GLR
+would be no need to use @acronym{GLR} parsing). Therefore, it is also possible
+to specify an expected number of reduce/reduce conflicts in @acronym{GLR}
parsers, using the declaration:
@example
@end deffn
@deffn {Directive} %destructor
-Specifying how the parser should reclaim the memory associated to
-discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn
@deffn {Directive} %file-prefix="@var{prefix}"
@code{"error"}, and @code{"$undefined"}; after these come the symbols
defined in the grammar file.
-For single-character literal tokens and literal string tokens, the name
-in the table includes the single-quote or double-quote characters: for
-example, @code{"'+'"} is a single-character literal and @code{"\"<=\""}
-is a literal string token. All the characters of the literal string
-token appear verbatim in the string found in the table; even
-double-quote characters are not escaped. For example, if the token
-consists of three characters @samp{*"*}, its string in @code{yytname}
-contains @samp{"*"*"}. (In C, that would be written as
-@code{"\"*\"*\""}).
+The name in the table includes all the characters needed to represent
+the token in Bison. For single-character literals and literal
+strings, this includes the surrounding quoting characters and any
+escape sequences. For example, the Bison single-character literal
+@code{'+'} corresponds to a three-character name, represented in C as
+@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
+corresponds to a five-character name, represented in C as
+@code{"\"\\\\/\""}.
When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
@end menu
@node Parser Function
table. The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in @code{yytname} with a
double-quote, the token's characters, and another double-quote. The
-token's characters are not escaped in any way; they appear verbatim in
-the contents of the string in the table.
+token's characters are escaped as necessary to be suitable as input
+to Bison.
-Here's code for looking up a token in @code{yytname}, assuming that the
-characters of the token are stored in @code{token_buffer}.
+Here's code for looking up a multicharacter token in @code{yytname},
+assuming that the characters of the token are stored in
+@code{token_buffer}, and assuming that the token does not contain any
+characters like @samp{"} that require escaping.
@smallexample
for (i = 0; i < YYNTOKENS; i++)
Section}), then Bison provides a more verbose and specific error message
string instead of just plain @w{@code{"syntax error"}}.
-The parser can detect one other kind of error: stack overflow. This
-happens when the input contains constructions that are very deeply
+The parser can detect one other kind of error: memory exhaustion. This
+can happen when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
-parser extends its stack automatically up to a very large limit. But
-if overflow happens, @code{yyparse} calls @code{yyerror} in the usual
-fashion, except that the argument string is @w{@code{"parser stack
-overflow"}}.
+parser normally extends its stack automatically up to a very large limit. But
+if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
+fashion, except that the argument string is @w{@code{"memory exhausted"}}.
+
+In some cases diagnostics like @w{@code{"syntax error"}} are
+translated automatically from English to some other language before
+they are passed to @code{yyerror}. @xref{Internationalization}.
The following definition suffices in simple programs:
immediately return 1.
Obviously, in location tracking pure parsers, @code{yyerror} should have
-an access to the current location. This is indeed the case for the GLR
+an access to the current location.
+This is indeed the case for the @acronym{GLR}
parsers, but not for the Yacc parser, for historical reasons. I.e., if
@samp{%locations %pure-parser} is passed then the prototypes for
@code{yyerror} are:
void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
@end example
-Finally, GLR and Yacc parsers share the same @code{yyerror} calling
+Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling
convention for absolutely pure parsers, i.e., when the calling
convention of @code{yylex} @emph{and} the calling convention of
@code{%pure-parser} are pure. I.e.:
Tracking Locations}.
@end deffn
+@node Internationalization
+@section Parser Internationalization
+@cindex internationalization
+@cindex i18n
+@cindex NLS
+@cindex gettext
+@cindex bison-po
+
+A Bison-generated parser can print diagnostics, including error and
+tracing messages. By default, they appear in English. However, Bison
+also supports outputting diagnostics in the user's native language.
+To make this work, the user should set the usual environment
+variables. @xref{Users, , The User's View, gettext, GNU
+@code{gettext} utilities}. For
+example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might set
+the user's locale to French Canadian using the @acronym{UTF}-8
+encoding. The exact set of available locales depends on the user's
+installation.
+
+The maintainer of a package that uses a Bison-generated parser enables
+the internationalization of the parser's output through the following
+steps. Here we assume a package that uses @acronym{GNU} Autoconf and
+@acronym{GNU} Automake.
+
+@enumerate
+@item
+@cindex bison-i18n.m4
+Into the directory containing the @acronym{GNU} Autoconf macros used
+by the package---often called @file{m4}---copy the
+@file{bison-i18n.m4} file installed by Bison under
+@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
+For example:
+
+@example
+cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
+@end example
+
+@item
+@findex BISON_I18N
+@vindex BISON_LOCALEDIR
+@vindex YYENABLE_NLS
+In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
+invocation, add an invocation of @code{BISON_I18N}. This macro is
+defined in the file @file{bison-i18n.m4} that you copied earlier. It
+causes @samp{configure} to find the value of the
+@code{BISON_LOCALEDIR} variable, and it defines the source-language
+symbol @code{YYENABLE_NLS} to enable translations in the
+Bison-generated parser.
+
+@item
+In the @code{main} function of your program, designate the directory
+containing Bison's runtime message catalog, through a call to
+@samp{bindtextdomain} with domain name @samp{bison-runtime}.
+For example:
+
+@example
+bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
+@end example
+
+Typically this appears after any other call @code{bindtextdomain
+(PACKAGE, LOCALEDIR)} that your package already has. Here we rely on
+@samp{BISON_LOCALEDIR} to be defined as a string through the
+@file{Makefile}.
+
+@item
+In the @file{Makefile.am} that controls the compilation of the @code{main}
+function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
+either in @samp{DEFS} or in @samp{AM_CPPFLAGS}. For example:
+
+@example
+DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+or:
+
+@example
+AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+@item
+Finally, invoke the command @command{autoreconf} to generate the build
+infrastructure.
+@end enumerate
+
@node Algorithm
@chapter The Bison Parser Algorithm
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
-* Stack Overflow:: What happens when stack gets full. How to avoid it.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
@node Look-Ahead
;
@end example
+For a more detailed exposition of @acronym{LALR}(1) parsers and parser
+generators, please see:
+Frank DeRemer and Thomas Pennello, Efficient Computation of
+@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on
+Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982),
+pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.
+
@node Generalized LR Parsing
@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
@cindex @acronym{GLR} parsing
grammar, in particular, it is only slightly slower than with the default
Bison parser.
-For a more detailed exposition of GLR parsers, please see: Elizabeth
+For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth
Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
Generalised @acronym{LR} Parsers, Royal Holloway, University of
London, Department of Computer Science, TR-00-12,
@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
(2000-12-24).
-@node Stack Overflow
-@section Stack Overflow, and How to Avoid It
+@node Memory Management
+@section Memory Management, and How to Avoid Memory Exhaustion
+@cindex memory exhaustion
+@cindex memory management
@cindex stack overflow
@cindex parser stack overflow
@cindex overflow of parser stack
-The Bison parser stack can overflow if too many tokens are shifted and
+The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced. When this happens, the parser function @code{yyparse}
-returns a nonzero value, pausing only to call @code{yyerror} to report
-the overflow.
+calls @code{yyerror} and then returns 2.
Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
-parser stack can become before a stack overflow occurs. Define the
+parser stack can become before memory is exhausted. Define the
macro with a value that is an integer. This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
-It must be a constant expression whose value is known at compile time.
The stack space allowed is not necessarily allocated. If you specify a
-large value for @code{YYMAXDEPTH}, the parser actually allocates a small
+large value for @code{YYMAXDEPTH}, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
space for ordinary inputs that do not need much stack.
+However, do not allow @code{YYMAXDEPTH} to be a value so large that
+arithmetic overflow could occur when calculating the size of the stack
+space. Also, do not allow @code{YYMAXDEPTH} to be less than
+@code{YYINITDEPTH}.
+
@cindex default stack limit
The default value of @code{YYMAXDEPTH}, if you do not define it, is
10000.
@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
-macro @code{YYINITDEPTH}. This value too must be a compile-time
-constant integer. The default is 200.
+macro @code{YYINITDEPTH} to a positive integer. For the C
+@acronym{LALR}(1) parser, this value must be a compile-time constant
+unless you are assuming C99 or some other target language or compiler
+that allows variable-length arrays. The default is 200.
+
+Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
@c FIXME: C++ output.
Because of semantical differences between C and C++, the
-@acronym{LALR}(1) parsers
-in C produced by Bison by compiled as C++ cannot grow. In this precise
-case (compiling a C parser as C++) you are suggested to grow
-@code{YYINITDEPTH}. In the near future, a C++ output output will be
-provided which addresses this issue.
+@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
+by C++ compilers. In this precise case (compiling a C parser as C++) you are
+suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix
+this deficiency in a future release.
@node Error Recovery
@chapter Error Recovery
earlier:
@example
-typedef int foo, bar, lose;
-static foo (bar); /* @r{redeclare @code{bar} as static variable} */
-static int foo (lose); /* @r{redeclare @code{foo} as function} */
+typedef int foo, bar;
+int baz (void)
+@{
+ static bar (bar); /* @r{redeclare @code{bar} as static variable} */
+ extern foo foo (foo); /* @r{redeclare @code{foo} as function} */
+ return foo (bar);
+@}
@end example
Unfortunately, the name being declared is separated from the declaration
@end example
@noindent
-Observe that state 11 contains conflicts due to the lack of precedence
-of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the
+Observe that state 11 contains conflicts not only due to the lack of
+precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
+@samp{*}, but also because the
associativity of @samp{/} is not specified.
@itemx --version
Print the version number of Bison and exit.
+@item --print-localedir
+Print the name of the directory containing locale-dependent data.
+
@need 1750
@item -y
@itemx --yacc
\line{ --no-lines \leaderfill -l}
\line{ --no-parser \leaderfill -n}
\line{ --output \leaderfill -o}
+\line{ --print-localedir}
\line{ --token-table \leaderfill -k}
\line{ --verbose \leaderfill -v}
\line{ --version \leaderfill -V}
--no-lines -l
--no-parser -n
--output=@var{outfile} -o @var{outfile}
+--print-localedir
--token-table -k
--verbose -v
--version -V
int yyparse (void);
@end example
-@c ================================================= Invoking Bison
-
-@node FAQ
-@chapter Frequently Asked Questions
-@cindex frequently asked questions
-@cindex questions
+@c ================================================= C++ Bison
-Several questions about Bison come up occasionally. Here some of them
-are addressed.
+@node C++ Language Interface
+@chapter C++ Language Interface
@menu
-* Parser Stack Overflow:: Breaking the Stack Limits
-* How Can I Reset the Parser:: @code{yyparse} Keeps some State
-* Strings are Destroyed:: @code{yylval} Loses Track of Strings
-* C++ Parsers:: Compiling Parsers with C++ Compilers
-* Implementing Loops:: Control Flow in the Calculator
+* C++ Parsers:: The interface to generate C++ parser classes
+* A Complete C++ Example:: Demonstrating their use
@end menu
-@node Parser Stack Overflow
-@section Parser Stack Overflow
+@node C++ Parsers
+@section C++ Parsers
-@display
-My parser returns with error with a @samp{parser stack overflow}
-message. What can I do?
-@end display
+@menu
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+@end menu
+
+@node C++ Bison Interface
+@subsection C++ Bison Interface
+@c - %skeleton "lalr1.cc"
+@c - Always pure
+@c - initial action
+
+The C++ parser @acronym{LALR}(1) skeleton is named @file{lalr1.cc}. To select
+it, you may either pass the option @option{--skeleton=lalr1.cc} to
+Bison, or include the directive @samp{%skeleton "lalr1.cc"} in the
+grammar preamble. When run, @command{bison} will create several
+files:
+@table @file
+@item position.hh
+@itemx location.hh
+The definition of the classes @code{position} and @code{location},
+used for location tracking. @xref{C++ Location Values}.
+
+@item stack.hh
+An auxiliary class @code{stack} used by the parser.
+
+@item @var{filename}.hh
+@itemx @var{filename}.cc
+The declaration and implementation of the C++ parser class.
+@var{filename} is the name of the output file. It follows the same
+rules as with regular C parsers.
+
+Note that @file{@var{filename}.hh} is @emph{mandatory}, the C++ cannot
+work without the parser class declaration. Therefore, you must either
+pass @option{-d}/@option{--defines} to @command{bison}, or use the
+@samp{%defines} directive.
+@end table
+
+All these files are documented using Doxygen; run @command{doxygen}
+for a complete and accurate documentation.
+
+@node C++ Semantic Values
+@subsection C++ Semantic Values
+@c - No objects in unions
+@c - YSTYPE
+@c - Printer and destructor
+
+The @code{%union} directive works as for C, see @ref{Union Decl, ,The
+Collection of Value Types}. In particular it produces a genuine
+@code{union}@footnote{In the future techniques to allow complex types
+within pseudo-unions (variants) might be implemented to alleviate
+these issues.}, which have a few specific features in C++.
+@itemize @minus
+@item
+The name @code{YYSTYPE} also denotes @samp{union YYSTYPE}. You may
+forward declare it just with @samp{union YYSTYPE;}.
+@item
+Non POD (Plain Old Data) types cannot be used. C++ forbids any
+instance of classes with constructors in unions: only @emph{pointers}
+to such objects are allowed.
+@end itemize
+
+Because objects have to be stored via pointers, memory is not
+reclaimed automatically: using the @code{%destructor} directive is the
+only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded
+Symbols}.
+
+
+@node C++ Location Values
+@subsection C++ Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+@c - %define "filename_type" "const symbol::Symbol"
+
+When the directive @code{%locations} is used, the C++ parser supports
+location tracking, see @ref{Locations, , Locations Overview}. Two
+auxiliary classes define a @code{position}, a single point in a file,
+and a @code{location}, a range composed of a pair of
+@code{position}s (possibly spanning several files).
+
+@deftypemethod {position} {std::string*} filename
+The name of the file. It will always be handled as a pointer, the
+parser will never duplicate nor deallocate it. As an experimental
+feature you may change it to @samp{@var{type}*} using @samp{%define
+"filename_type" "@var{type}"}.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} line
+The line, starting at 1.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} lines (int @var{height} = 1)
+Advance by @var{height} lines, resetting the column number.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} column
+The column, starting at 0.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} columns (int @var{width} = 1)
+Advance by @var{width} columns, without changing the line number.
+@end deftypemethod
+
+@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width})
+Various forms of syntactic sugar for @code{columns}.
+@end deftypemethod
+
+@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p})
+Report @var{p} on @var{o} like this:
+@samp{@var{filename}:@var{line}.@var{column}}, or
+@samp{@var{line}.@var{column}} if @var{filename} is null.
+@end deftypemethod
+
+@deftypemethod {location} {position} begin
+@deftypemethodx {location} {position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypemethod
+
+@deftypemethod {location} {unsigned int} columns (int @var{width} = 1)
+@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1)
+Advance the @code{end} position.
+@end deftypemethod
+
+@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end})
+@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width})
+@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width})
+Various forms of syntactic sugar.
+@end deftypemethod
+
+@deftypemethod {location} {void} step ()
+Move @code{begin} onto @code{end}.
+@end deftypemethod
+
+
+@node C++ Parser Interface
+@subsection C++ Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c debug_stream.
+@c - Reporting errors
+
+The output files @file{@var{output}.hh} and @file{@var{output}.cc}
+declare and define the parser class in the namespace @code{yy}. The
+class name defaults to @code{parser}, but may be changed using
+@samp{%define "parser_class_name" "@var{name}"}. The interface of
+this class is detailled below. It can be extended using the
+@code{%parse-param} feature: its semantics is slightly changed since
+it describes an additional member of the parser class, and an
+additional argument for its constructor.
+
+@defcv {Type} {parser} {semantic_value_type}
+@defcvx {Type} {parser} {location_value_type}
+The types for semantics value and locations.
+@end defcv
+
+@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
+Build a new parser object. There are no arguments by default, unless
+@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
+@end deftypemethod
+
+@deftypemethod {parser} {int} parse ()
+Run the syntactic analysis, and return 0 on success, 1 otherwise.
+@end deftypemethod
+
+@deftypemethod {parser} {std::ostream&} debug_stream ()
+@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o})
+Get or set the stream used for tracing the parsing. It defaults to
+@code{std::cerr}.
+@end deftypemethod
+
+@deftypemethod {parser} {debug_level_type} debug_level ()
+@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l})
+Get or set the tracing level. Currently its value is either 0, no trace,
+or non-zero, full tracing.
+@end deftypemethod
+
+@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
+The definition for this member function must be supplied by the user:
+the parser uses it to report a parser error occurring at @var{l},
+described by @var{m}.
+@end deftypemethod
+
+
+@node C++ Scanner Interface
+@subsection C++ Scanner Interface
+@c - prefix for yylex.
+@c - Pure interface to yylex
+@c - %lex-param
+
+The parser invokes the scanner by calling @code{yylex}. Contrary to C
+parsers, C++ parsers are always pure: there is no point in using the
+@code{%pure-parser} directive. Therefore the interface is as follows.
+
+@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
+Return the next token. Its type is the return value, its semantic
+value and location being @var{yylval} and @var{yylloc}. Invocations of
+@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
+@end deftypemethod
+
+
+@node A Complete C++ Example
+@section A Complete C++ Example
+
+This section demonstrates the use of a C++ parser with a simple but
+complete example. This example should be available on your system,
+ready to compile, in the directory @dfn{../bison/examples/calc++}. It
+focuses on the use of Bison, therefore the design of the various C++
+classes is very naive: no accessors, no encapsulation of members etc.
+We will use a Lex scanner, and more precisely, a Flex scanner, to
+demonstrate the various interaction. A hand written scanner is
+actually easier to interface with.
+
+@menu
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+@end menu
+
+@node Calc++ --- C++ Calculator
+@subsection Calc++ --- C++ Calculator
+
+Of course the grammar is dedicated to arithmetics, a single
+expression, possibily preceded by variable assignments. An
+environment containing possibly predefined variables such as
+@code{one} and @code{two}, is exchanged with the parser. An example
+of valid input follows.
+
+@example
+three := 3
+seven := one + two * three
+seven * seven
+@end example
+
+@node Calc++ Parsing Driver
+@subsection Calc++ Parsing Driver
+@c - An env
+@c - A place to store error messages
+@c - A place for the result
+
+To support a pure interface with the parser (and the scanner) the
+technique of the ``parsing context'' is convenient: a structure
+containing all the data to exchange. Since, in addition to simply
+launch the parsing, there are several auxiliary tasks to execute (open
+the file for parsing, instantiate the parser etc.), we recommend
+transforming the simple parsing context structure into a fully blown
+@dfn{parsing driver} class.
+
+The declaration of this driver class, @file{calc++-driver.hh}, is as
+follows. The first part includes the CPP guard and imports the
+required standard library components.
+
+@comment file: calc++-driver.hh
+@example
+#ifndef CALCXX_DRIVER_HH
+# define CALCXX_DRIVER_HH
+# include <string>
+# include <map>
+@end example
+
+@noindent
+Then come forward declarations. Because the parser uses the parsing
+driver and reciprocally, simple inclusions of header files will not
+do. Because the driver's declaration is the one that will be imported
+by the rest of the project, it is saner to forward declare the
+parser's information here.
+
+@comment file: calc++-driver.hh
+@example
+// Forward declarations.
+union YYSTYPE;
+namespace yy
+@{
+ class location;
+ class calcxx_parser;
+@}
+class calcxx_driver;
+@end example
+
+@noindent
+Then comes the declaration of the scanning function. Flex expects
+the signature of @code{yylex} to be defined in the macro
+@code{YY_DECL}, and the C++ parser expects it to be declared. We can
+factor both as follows.
+
+@comment file: calc++-driver.hh
+@example
+// Announce to Flex the prototype we want for lexing function, ...
+# define YY_DECL \
+ int yylex (YYSTYPE* yylval, yy::location* yylloc, calcxx_driver& driver)
+// ... and declare it for the parser's sake.
+YY_DECL;
+@end example
+
+@noindent
+The @code{calcxx_driver} class is then declared with its most obvious
+members.
+
+@comment file: calc++-driver.hh
+@example
+// Conducting the whole scanning and parsing of Calc++.
+class calcxx_driver
+@{
+public:
+ calcxx_driver ();
+ virtual ~calcxx_driver ();
+
+ std::map<std::string, int> variables;
+
+ int result;
+@end example
+
+@noindent
+To encapsulate the coordination with the Flex scanner, it is useful to
+have two members function to open and close the scanning phase.
+members.
+
+@comment file: calc++-driver.hh
+@example
+ // Handling the scanner.
+ void scan_begin ();
+ void scan_end ();
+ bool trace_scanning;
+@end example
+
+@noindent
+Similarly for the parser itself.
+
+@comment file: calc++-driver.hh
+@example
+ // Handling the parser.
+ void parse (const std::string& f);
+ std::string file;
+ bool trace_parsing;
+@end example
+
+@noindent
+To demonstrate pure handling of parse errors, instead of simply
+dumping them on the standard error output, we will pass them to the
+compiler driver using the following two member functions. Finally, we
+close the class declaration and CPP guard.
+
+@comment file: calc++-driver.hh
+@example
+ // Error handling.
+ void error (const yy::location& l, const std::string& m);
+ void error (const std::string& m);
+@};
+#endif // ! CALCXX_DRIVER_HH
+@end example
+
+The implementation of the driver is straightforward. The @code{parse}
+member function deserves some attention. The @code{error} functions
+are simple stubs, they should actually register the located error
+messages and set error state.
+
+@comment file: calc++-driver.cc
+@example
+#include "calc++-driver.hh"
+#include "calc++-parser.hh"
+
+calcxx_driver::calcxx_driver ()
+ : trace_scanning (false), trace_parsing (false)
+@{
+ variables["one"] = 1;
+ variables["two"] = 2;
+@}
+
+calcxx_driver::~calcxx_driver ()
+@{
+@}
+
+void
+calcxx_driver::parse (const std::string &f)
+@{
+ file = f;
+ scan_begin ();
+ yy::calcxx_parser parser (*this);
+ parser.set_debug_level (trace_parsing);
+ parser.parse ();
+ scan_end ();
+@}
+
+void
+calcxx_driver::error (const yy::location& l, const std::string& m)
+@{
+ std::cerr << l << ": " << m << std::endl;
+@}
+
+void
+calcxx_driver::error (const std::string& m)
+@{
+ std::cerr << m << std::endl;
+@}
+@end example
+
+@node Calc++ Parser
+@subsection Calc++ Parser
+
+The parser definition file @file{calc++-parser.yy} starts by asking
+for the C++ skeleton, the creation of the parser header file, and
+specifies the name of the parser class. It then includes the required
+headers.
+
+@comment file: calc++-parser.yy
+@example
+%skeleton "lalr1.cc" /* -*- C++ -*- */
+%define "parser_class_name" "calcxx_parser"
+%defines
+%@{
+# include <string>
+# include "calc++-driver.hh"
+%@}
+@end example
+
+@noindent
+The driver is passed by reference to the parser and to the scanner.
+This provides a simple but effective pure interface, not relying on
+global variables.
+
+@comment file: calc++-parser.yy
+@example
+// The parsing context.
+%parse-param @{ calcxx_driver& driver @}
+%lex-param @{ calcxx_driver& driver @}
+@end example
+
+@noindent
+Then we request the location tracking feature, and initialize the
+first location's file name. Afterwards new locations are computed
+relatively to the previous locations: the file name will be
+automatically propagated.
+
+@comment file: calc++-parser.yy
+@example
+%locations
+%initial-action
+@{
+ // Initialize the initial location.
+ @@$.begin.filename = @@$.end.filename = &driver.file;
+@};
+@end example
+
+@noindent
+Use the two following directives to enable parser tracing and verbose
+error messages.
+
+@comment file: calc++-parser.yy
+@example
+%debug
+%error-verbose
+@end example
+
+@noindent
+Semantic values cannot use ``real'' objects, but only pointers to
+them.
+
+@comment file: calc++-parser.yy
+@example
+// Symbols.
+%union
+@{
+ int ival;
+ std::string *sval;
+@};
+@end example
+
+@noindent
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$end''. Similarly user friendly named are provided for each
+symbol. Note that the tokens names are prefixed by @code{TOKEN_} to
+avoid name clashes.
+
+@comment file: calc++-parser.yy
+@example
+%token YYEOF 0 "end of file"
+%token TOKEN_ASSIGN ":="
+%token <sval> TOKEN_IDENTIFIER "identifier"
+%token <ival> TOKEN_NUMBER "number"
+%type <ival> exp "expression"
+@end example
+
+@noindent
+To enable memory deallocation during error recovery, use
+@code{%destructor}.
+
+@comment file: calc++-parser.yy
+@example
+%printer @{ debug_stream () << *$$; @} "identifier"
+%destructor @{ delete $$; @} "identifier"
+
+%printer @{ debug_stream () << $$; @} "number" "expression"
+@end example
+
+@noindent
+The grammar itself is straightforward.
+
+@comment file: calc++-parser.yy
+@example
+%%
+%start unit;
+unit: assignments exp @{ driver.result = $2; @};
+
+assignments: assignments assignment @{@}
+ | /* Nothing. */ @{@};
+
+assignment: TOKEN_IDENTIFIER ":=" exp @{ driver.variables[*$1] = $3; @};
+
+%left '+' '-';
+%left '*' '/';
+exp: exp '+' exp @{ $$ = $1 + $3; @}
+ | exp '-' exp @{ $$ = $1 - $3; @}
+ | exp '*' exp @{ $$ = $1 * $3; @}
+ | exp '/' exp @{ $$ = $1 / $3; @}
+ | TOKEN_IDENTIFIER @{ $$ = driver.variables[*$1]; @}
+ | TOKEN_NUMBER @{ $$ = $1; @};
+%%
+@end example
+
+@noindent
+Finally the @code{error} member function registers the errors to the
+driver.
+
+@comment file: calc++-parser.yy
+@example
+void
+yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
+ const std::string& m)
+@{
+ driver.error (l, m);
+@}
+@end example
+
+@node Calc++ Scanner
+@subsection Calc++ Scanner
+
+The Flex scanner first includes the driver declaration, then the
+parser's to get the set of defined tokens.
+
+@comment file: calc++-scanner.ll
+@example
+%@{ /* -*- C++ -*- */
+# include <string>
+# include "calc++-driver.hh"
+# include "calc++-parser.hh"
+%@}
+@end example
+
+@noindent
+Because there is no @code{#include}-like feature we don't need
+@code{yywrap}, we don't need @code{unput} either, and we parse an
+actual file, this is not an interactive session with the user.
+Finally we enable the scanner tracing features.
+
+@comment file: calc++-scanner.ll
+@example
+%option noyywrap nounput batch debug
+@end example
+
+@noindent
+Abbreviations allow for more readable rules.
+
+@comment file: calc++-scanner.ll
+@example
+id [a-zA-Z][a-zA-Z_0-9]*
+int [0-9]+
+blank [ \t]
+@end example
+
+@noindent
+The following paragraph suffices to track locations acurately. Each
+time @code{yylex} is invoked, the begin position is moved onto the end
+position. Then when a pattern is matched, the end position is
+advanced of its width. In case it matched ends of lines, the end
+cursor is adjusted, and each time blanks are matched, the begin cursor
+is moved onto the end cursor to effectively ignore the blanks
+preceding tokens. Comments would be treated equally.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+# define YY_USER_ACTION yylloc->columns (yyleng);
+%@}
+%%
+%@{
+ yylloc->step ();
+%@}
+@{blank@}+ yylloc->step ();
+[\n]+ yylloc->lines (yyleng); yylloc->step ();
+@end example
+
+@noindent
+The rules are simple, just note the use of the driver to report
+errors.
+
+@comment file: calc++-scanner.ll
+@example
+[-+*/] return yytext[0];
+":=" return TOKEN_ASSIGN;
+@{int@} yylval->ival = atoi (yytext); return TOKEN_NUMBER;
+@{id@} yylval->sval = new std::string (yytext); return TOKEN_IDENTIFIER;
+. driver.error (*yylloc, "invalid character");
+%%
+@end example
+
+@noindent
+Finally, because the scanner related driver's member function depend
+on the scanner's data, it is simpler to implement them in this file.
+
+@comment file: calc++-scanner.ll
+@example
+void
+calcxx_driver::scan_begin ()
+@{
+ yy_flex_debug = trace_scanning;
+ if (!(yyin = fopen (file.c_str (), "r")))
+ error (std::string ("cannot open ") + file);
+@}
+
+void
+calcxx_driver::scan_end ()
+@{
+ fclose (yyin);
+@}
+@end example
+
+@node Calc++ Top Level
+@subsection Calc++ Top Level
+
+The top level file, @file{calc++.cc}, poses no problem.
+
+@comment file: calc++.cc
+@example
+#include <iostream>
+#include "calc++-driver.hh"
+
+int
+main (int argc, const char* argv[])
+@{
+ calcxx_driver driver;
+ for (++argv; argv[0]; ++argv)
+ if (*argv == std::string ("-p"))
+ driver.trace_parsing = true;
+ else if (*argv == std::string ("-s"))
+ driver.trace_scanning = true;
+ else
+ @{
+ driver.parse (*argv);
+ std::cout << driver.result << std::endl;
+ @}
+@}
+@end example
+
+@c ================================================= FAQ
+
+@node FAQ
+@chapter Frequently Asked Questions
+@cindex frequently asked questions
+@cindex questions
+
+Several questions about Bison come up occasionally. Here some of them
+are addressed.
+
+@menu
+* Memory Exhausted:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops:: Control Flow in the Calculator
+@end menu
+
+@node Memory Exhausted
+@section Memory Exhausted
+
+@display
+My parser returns with error with a @samp{memory exhausted}
+message. What can I do?
+@end display
This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.
yyin = fopen (file, "r");
if (!yyin)
exit (2);
- /* One token only. */
+ /* One token only. */
yylex ();
if (fclose (yyin) != 0)
exit (3);
int
main ()
{
- /* Similar to using $1, $2 in a Bison action. */
+ /* Similar to using $1, $2 in a Bison action. */
char *fst = (yylex (), yylval);
char *snd = (yylex (), yylval);
printf ("\"%s\", \"%s\"\n", fst, snd);
@end example
-@node C++ Parsers
-@section C++ Parsers
-
-@display
-How can I generate parsers in C++?
-@end display
-
-We are working on a C++ output for Bison, but unfortunately, for lack
-of time, the skeleton is not finished. It is functional, but in
-numerous respects, it will require additional work which @emph{might}
-break backward compatibility. Since the skeleton for C++ is not
-documented, we do not consider ourselves bound to this interface,
-nevertheless, as much as possible we will try to keep compatibility.
-
-Another possibility is to use the regular C parsers, and to compile
-them with a C++ compiler. This works properly, provided that you bear
-some simple C++ rules in mind, such as not including ``real classes''
-(i.e., structure with constructors) in unions. Therefore, in the
-@code{%union}, use pointers to classes, or better yet, a single
-pointer type to the root of your lexical/syntactic hierarchy.
-
-
-@node Implementing Loops
-@section Implementing Loops
+@node Implementing Gotos/Loops
+@section Implementing Gotos/Loops
@display
My simple calculator supports variables, assignments, and functions,
-but how can I implement loops?
+but how can I implement gotos, or loops?
@end display
Although very pedagogical, the examples included in the document blur
right-hand side of the rule. @xref{Actions}.
@end deffn
-@deffn {Symbol} $accept
-The predefined nonterminal whose only rule is @samp{$accept: @var{start}
-$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
-Start-Symbol}. It cannot be used in the grammar.
+@deffn {Delimiter} %%
+Delimiter used to separate the grammar rule section from the
+Bison declarations section or the epilogue.
+@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
@end deffn
-@deffn {Symbol} $end
-The predefined token marking the end of the token stream. It cannot be
-used in the grammar.
+@c Don't insert spaces, or check the DVI output.
+@deffn {Delimiter} %@{@var{code}%@}
+All code listed between @samp{%@{} and @samp{%@}} is copied directly to
+the output file uninterpreted. Such code forms the prologue of the input
+file. @xref{Grammar Outline, ,Outline of a Bison
+Grammar}.
@end deffn
-@deffn {Symbol} $undefined
-The predefined token onto which all undefined values returned by
-@code{yylex} are mapped. It cannot be used in the grammar, rather, use
-@code{error}.
+@deffn {Construct} /*@dots{}*/
+Comment delimiters, as in C.
@end deffn
-@deffn {Symbol} error
-A token name reserved for error recovery. This token may be used in
-grammar rules so as to allow the Bison parser to recognize an error in
-the grammar without halting the process. In effect, a sentence
-containing an error may be recognized as valid. On a syntax error, the
-token @code{error} becomes the current look-ahead token. Actions
-corresponding to @code{error} are then executed, and the look-ahead
-token is reset to the token that originally caused the violation.
-@xref{Error Recovery}.
+@deffn {Delimiter} :
+Separates a rule's result from its components. @xref{Rules, ,Syntax of
+Grammar Rules}.
@end deffn
-@deffn {Macro} YYABORT
-Macro to pretend that an unrecoverable syntax error has occurred, by
-making @code{yyparse} return 1 immediately. The error reporting
-function @code{yyerror} is not called. @xref{Parser Function, ,The
-Parser Function @code{yyparse}}.
-@end deffn
-
-@deffn {Macro} YYACCEPT
-Macro to pretend that a complete utterance of the language has been
-read, by making @code{yyparse} return 0 immediately.
-@xref{Parser Function, ,The Parser Function @code{yyparse}}.
-@end deffn
-
-@deffn {Macro} YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
-token. @xref{Action Features, ,Special Features for Use in Actions}.
-@end deffn
-
-@deffn {Macro} YYDEBUG
-Macro to define to equip the parser with tracing code. @xref{Tracing,
-,Tracing Your Parser}.
-@end deffn
-
-@deffn {Macro} YYERROR
-Macro to pretend that a syntax error has just been detected: call
-@code{yyerror} and then perform normal error recovery if possible
-(@pxref{Error Recovery}), or (if recovery is impossible) make
-@code{yyparse} return 1. @xref{Error Recovery}.
-@end deffn
-
-@deffn {Macro} YYERROR_VERBOSE
-An obsolete macro that you define with @code{#define} in the prologue
-to request verbose, specific error message strings
-when @code{yyerror} is called. It doesn't matter what definition you
-use for @code{YYERROR_VERBOSE}, just whether you define it. Using
-@code{%error-verbose} is preferred.
-@end deffn
-
-@deffn {Macro} YYINITDEPTH
-Macro for specifying the initial size of the parser stack.
-@xref{Stack Overflow}.
-@end deffn
-
-@deffn {Macro} YYLEX_PARAM
-An obsolete macro for specifying an extra argument (or list of extra
-arguments) for @code{yyparse} to pass to @code{yylex}. he use of this
-macro is deprecated, and is supported only for Yacc like parsers.
-@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
-@end deffn
-
-@deffn {Type} YYLTYPE
-Data type of @code{yylloc}; by default, a structure with four
-members. @xref{Location Type, , Data Types of Locations}.
-@end deffn
-
-@deffn {Macro} YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack. @xref{Stack
-Overflow}.
-@end deffn
-
-@deffn {Macro} YYPARSE_PARAM
-An obsolete macro for specifying the name of a parameter that
-@code{yyparse} should accept. The use of this macro is deprecated, and
-is supported only for Yacc like parsers. @xref{Pure Calling,, Calling
-Conventions for Pure Parsers}.
-@end deffn
-
-@deffn {Macro} YYRECOVERING
-Macro whose value indicates whether the parser is recovering from a
-syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
-@end deffn
-
-@deffn {Macro} YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca}. If defined to @samp{0},
-the parser will not use @code{alloca} but @code{malloc} when trying to
-grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
-to anything else.
-@end deffn
-
-@deffn {Type} YYSTYPE
-Data type of semantic values; @code{int} by default.
-@xref{Value Type, ,Data Types of Semantic Values}.
-@end deffn
-
-@deffn {Variable} yychar
-External integer variable that contains the integer value of the current
-look-ahead token. (In a pure parser, it is a local variable within
-@code{yyparse}.) Error-recovery rule actions may examine this variable.
-@xref{Action Features, ,Special Features for Use in Actions}.
-@end deffn
-
-@deffn {Variable} yyclearin
-Macro used in error-recovery rule actions. It clears the previous
-look-ahead token. @xref{Error Recovery}.
-@end deffn
-
-@deffn {Variable} yydebug
-External integer variable set to zero by default. If @code{yydebug}
-is given a nonzero value, the parser will output information on input
-symbols and parser action. @xref{Tracing, ,Tracing Your Parser}.
-@end deffn
-
-@deffn {Macro} yyerrok
-Macro to cause parser to recover immediately to its normal mode
-after a syntax error. @xref{Error Recovery}.
-@end deffn
-
-@deffn {Function} yyerror
-User-supplied function to be called by @code{yyparse} on error.
-@xref{Error Reporting, ,The Error
-Reporting Function @code{yyerror}}.
-@end deffn
-
-@deffn {Function} yylex
-User-supplied lexical analyzer function, called with no arguments to get
-the next token. @xref{Lexical, ,The Lexical Analyzer Function
-@code{yylex}}.
-@end deffn
-
-@deffn {Variable} yylval
-External variable in which @code{yylex} should place the semantic
-value associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
-@end deffn
-
-@deffn {Variable} yylloc
-External variable in which @code{yylex} should place the line and column
-numbers associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) You can ignore this variable if you don't use the
-@samp{@@} feature in the grammar actions. @xref{Token Locations,
-,Textual Locations of Tokens}.
+@deffn {Delimiter} ;
+Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
@end deffn
-@deffn {Variable} yynerrs
-Global variable which Bison increments each time there is a syntax error.
-(In a pure parser, it is a local variable within @code{yyparse}.)
-@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@deffn {Delimiter} |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
@end deffn
-@deffn {Function} yyparse
-The parser function produced by Bison; call this function to start
-parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@deffn {Symbol} $accept
+The predefined nonterminal whose only rule is @samp{$accept: @var{start}
+$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
+Start-Symbol}. It cannot be used in the grammar.
@end deffn
@deffn {Directive} %debug
@end deffn
@deffn {Directive} %destructor
-Specifying how the parser should reclaim the memory associated to
-discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn
@deffn {Directive} %dprec
@acronym{GLR} Parsers}.
@end deffn
+@deffn {Symbol} $end
+The predefined token marking the end of the token stream. It cannot be
+used in the grammar.
+@end deffn
+
+@deffn {Symbol} error
+A token name reserved for error recovery. This token may be used in
+grammar rules so as to allow the Bison parser to recognize an error in
+the grammar without halting the process. In effect, a sentence
+containing an error may be recognized as valid. On a syntax error, the
+token @code{error} becomes the current look-ahead token. Actions
+corresponding to @code{error} are then executed, and the look-ahead
+token is reset to the token that originally caused the violation.
+@xref{Error Recovery}.
+@end deffn
+
@deffn {Directive} %error-verbose
Bison declaration to request verbose, specific error message strings
when @code{yyerror} is called.
Parsers, ,Writing @acronym{GLR} Parsers}.
@end deffn
+@deffn {Directive} %initial-action
+Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}.
+@end deffn
+
@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
,Nonterminal Symbols}.
@end deffn
+@deffn {Symbol} $undefined
+The predefined token onto which all undefined values returned by
+@code{yylex} are mapped. It cannot be used in the grammar, rather, use
+@code{error}.
+@end deffn
+
@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
@end deffn
-@sp 1
+@deffn {Macro} YYABORT
+Macro to pretend that an unrecoverable syntax error has occurred, by
+making @code{yyparse} return 1 immediately. The error reporting
+function @code{yyerror} is not called. @xref{Parser Function, ,The
+Parser Function @code{yyparse}}.
+@end deffn
-These are the punctuation and delimiters used in Bison input:
+@deffn {Macro} YYACCEPT
+Macro to pretend that a complete utterance of the language has been
+read, by making @code{yyparse} return 0 immediately.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
-@deffn {Delimiter} %%
-Delimiter used to separate the grammar rule section from the
-Bison declarations section or the epilogue.
-@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
+@deffn {Macro} YYBACKUP
+Macro to discard a value from the parser stack and fake a look-ahead
+token. @xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
-@c Don't insert spaces, or check the DVI output.
-@deffn {Delimiter} %@{@var{code}%@}
-All code listed between @samp{%@{} and @samp{%@}} is copied directly to
-the output file uninterpreted. Such code forms the prologue of the input
-file. @xref{Grammar Outline, ,Outline of a Bison
-Grammar}.
+@deffn {Variable} yychar
+External integer variable that contains the integer value of the current
+look-ahead token. (In a pure parser, it is a local variable within
+@code{yyparse}.) Error-recovery rule actions may examine this variable.
+@xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
-@deffn {Construct} /*@dots{}*/
-Comment delimiters, as in C.
+@deffn {Variable} yyclearin
+Macro used in error-recovery rule actions. It clears the previous
+look-ahead token. @xref{Error Recovery}.
@end deffn
-@deffn {Delimiter} :
-Separates a rule's result from its components. @xref{Rules, ,Syntax of
-Grammar Rules}.
+@deffn {Macro} YYDEBUG
+Macro to define to equip the parser with tracing code. @xref{Tracing,
+,Tracing Your Parser}.
@end deffn
-@deffn {Delimiter} ;
-Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@deffn {Variable} yydebug
+External integer variable set to zero by default. If @code{yydebug}
+is given a nonzero value, the parser will output information on input
+symbols and parser action. @xref{Tracing, ,Tracing Your Parser}.
@end deffn
-@deffn {Delimiter} |
-Separates alternate rules for the same result nonterminal.
-@xref{Rules, ,Syntax of Grammar Rules}.
+@deffn {Macro} yyerrok
+Macro to cause parser to recover immediately to its normal mode
+after a syntax error. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYERROR
+Macro to pretend that a syntax error has just been detected: call
+@code{yyerror} and then perform normal error recovery if possible
+(@pxref{Error Recovery}), or (if recovery is impossible) make
+@code{yyparse} return 1. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Function} yyerror
+User-supplied function to be called by @code{yyparse} on error.
+@xref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Macro} YYERROR_VERBOSE
+An obsolete macro that you define with @code{#define} in the prologue
+to request verbose, specific error message strings
+when @code{yyerror} is called. It doesn't matter what definition you
+use for @code{YYERROR_VERBOSE}, just whether you define it. Using
+@code{%error-verbose} is preferred.
+@end deffn
+
+@deffn {Macro} YYINITDEPTH
+Macro for specifying the initial size of the parser stack.
+@xref{Memory Management}.
+@end deffn
+
+@deffn {Function} yylex
+User-supplied lexical analyzer function, called with no arguments to get
+the next token. @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
+@end deffn
+
+@deffn {Macro} YYLEX_PARAM
+An obsolete macro for specifying an extra argument (or list of extra
+arguments) for @code{yyparse} to pass to @code{yylex}. he use of this
+macro is deprecated, and is supported only for Yacc like parsers.
+@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Variable} yylloc
+External variable in which @code{yylex} should place the line and column
+numbers associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.) You can ignore this variable if you don't use the
+@samp{@@} feature in the grammar actions. @xref{Token Locations,
+,Textual Locations of Tokens}.
+@end deffn
+
+@deffn {Type} YYLTYPE
+Data type of @code{yylloc}; by default, a structure with four
+members. @xref{Location Type, , Data Types of Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+External variable in which @code{yylex} should place the semantic
+value associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
+@end deffn
+
+@deffn {Macro} YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack. @xref{Memory
+Management}.
+@end deffn
+
+@deffn {Variable} yynerrs
+Global variable which Bison increments each time there is a syntax error.
+(In a pure parser, it is a local variable within @code{yyparse}.)
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Function} yyparse
+The parser function produced by Bison; call this function to start
+parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Macro} YYPARSE_PARAM
+An obsolete macro for specifying the name of a parameter that
+@code{yyparse} should accept. The use of this macro is deprecated, and
+is supported only for Yacc like parsers. @xref{Pure Calling,, Calling
+Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Macro} YYRECOVERING
+Macro whose value indicates whether the parser is recovering from a
+syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Macro} YYSTACK_USE_ALLOCA
+Macro used to control the use of @code{alloca} when the C
+@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0,
+the parser will use @code{malloc} to extend its stacks. If defined to
+1, the parser will use @code{alloca}. Values other than 0 and 1 are
+reserved for future Bison extensions. If not defined,
+@code{YYSTACK_USE_ALLOCA} defaults to 0.
+
+If you define @code{YYSTACK_USE_ALLOCA} to 1, it is your
+responsibility to make sure that @code{alloca} is visible, e.g., by
+using @acronym{GCC} or by including @code{<stdlib.h>}. Furthermore,
+in the all-too-common case where your code may run on a host with a
+limited stack and with unreliable stack-overflow checking, you should
+set @code{YYMAXDEPTH} to a value that cannot possibly result in
+unchecked stack overflow on any of your target hosts when
+@code{alloca} is called. You can inspect the code that Bison
+generates in order to determine the proper numeric values. This will
+require some expertise in low-level implementation details.
+@end deffn
+
+@deffn {Type} YYSTYPE
+Data type of semantic values; @code{int} by default.
+@xref{Value Type, ,Data Types of Semantic Values}.
@end deffn
@node Glossary
projects / bison.git / blobdiff