@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled
+@c Set following if you want to document %default-prec and %no-default-prec.
+@c This feature is experimental and may change in future Bison versions.
+@c @set defaultprec
+
@c ISPELL CHECK: done, 14 Jan 1993 --bob
@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo
@copying
-This manual is for GNU Bison (version @value{VERSION}, @value{UPDATED}),
-the GNU parser generator.
+This manual is for @acronym{GNU} Bison (version @value{VERSION},
+@value{UPDATED}), the @acronym{GNU} parser generator.
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
-under the terms of the GNU Free Documentation License, Version 1.1 or
-any later version published by the Free Software Foundation; with no
-Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
-and with the Back-Cover Texts as in (a) below. A copy of the
-license is included in the section entitled ``GNU Free Documentation
-License.''
-
-(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
-this GNU Manual, like GNU software. Copies published by the Free
-Software Foundation raise funds for GNU development.''
+under the terms of the @acronym{GNU} Free Documentation License,
+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
+``@acronym{GNU} Free Documentation License.''
+
+(a) The @acronym{FSF}'s Back-Cover Text is: ``You have freedom to copy
+and modify this @acronym{GNU} Manual, like @acronym{GNU} software.
+Copies published by the Free Software Foundation raise funds for
+@acronym{GNU} development.''
@end quotation
@end copying
-@dircategory GNU programming tools
+@dircategory Software development
@direntry
-* bison: (bison). GNU parser generator (yacc replacement).
+* bison: (bison). @acronym{GNU} parser generator (Yacc replacement).
@end direntry
@ifset shorttitlepage-enabled
@end ifset
@titlepage
@title Bison
-@subtitle The YACC-compatible Parser Generator
+@subtitle The Yacc-compatible Parser Generator
@subtitle @value{UPDATED}, Bison Version @value{VERSION}
@author by Charles Donnelly and Richard Stallman
@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.@*
-ISBN 1-882114-44-2
+@acronym{ISBN} 1-882114-44-2
@sp 2
Cover art by Etienne Suvasa.
@end titlepage
@menu
* Introduction::
* Conditions::
-* Copying:: The GNU General Public License says
+* Copying:: The @acronym{GNU} General Public License says
how you can copy and share Bison
Tutorial sections:
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.
-@detailmenu --- The Detailed Node Listing ---
+@detailmenu
+ --- The Detailed Node Listing ---
The Concepts of Bison
a semantic value (the value of an integer,
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.
+* 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.
+* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
+
Examples
* RPN Calc:: Reverse polish notation calculator;
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
-* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
+* Multi-function Calc:: Calculator with memory and trig functions.
+ It uses multiple data-types for semantic values.
* Exercises:: Ideas for improving the multi-function calculator.
Reverse Polish Notation Calculator
* Rules:: How to write grammar rules.
* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
+* Locations:: Locations and actions.
* Declarations:: All kinds of Bison declarations are described here.
* Multiple Parsers:: Putting more than one Bison parser in one program.
Outline of a Bison Grammar
-* Prologue:: Syntax and usage of the prologue (declarations section).
+* Prologue:: Syntax and usage of the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
-* Epilogue:: Syntax and usage of the epilogue (additional code section).
+* Epilogue:: Syntax and usage of the epilogue.
Defining Language Semantics
This says when, why and how to use the exceptional
action in the middle of a rule.
+Tracking Locations
+
+* Location Type:: Specifying a data type for locations.
+* Actions and Locations:: Using locations in actions.
+* Location Default Action:: Defining a general way to compute locations.
+
Bison Declarations
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* 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.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* 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.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
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}
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
* Token Values:: How @code{yylex} must return the semantic value
of the token it has read.
-* Token Positions:: How @code{yylex} must return the text position
+* Token Locations:: How @code{yylex} must return the text location
(line number, etc.) of the token, if the
- actions want that.
+ actions want that.
* Pure Calling:: How the calling convention differs
in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
* 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
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.
-Understanding or Debugging Your Parser
+Debugging Your Parser
* Understanding:: Understanding the structure of your parser.
* Tracing:: Tracing the execution of your parser.
Invoking Bison
* Bison Options:: All the options described in detail,
- in alphabetical order by short options.
+ in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
-* VMS Invocation:: Bison command syntax on VMS.
+* 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
+* Implementing Gotos/Loops:: Control Flow in the Calculator
Copying This Manual
@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts a
-grammar description for an LALR(1) context-free grammar into a C
+grammar description for an @acronym{LALR}(1) context-free grammar into a C
program to parse that grammar. Once you are proficient with Bison,
you may use it to develop a wide range of language parsers, from those
used in simple desk calculators to complex programming languages.
As of Bison version 1.24, we have changed the distribution terms for
@code{yyparse} to permit using Bison's output in nonfree programs when
-Bison is generating C code for LALR(1) parsers. Formerly, these
+Bison is generating C code for @acronym{LALR}(1) parsers. Formerly, these
parsers could be used only in programs that were free software.
-The other GNU programming tools, such as the GNU C compiler, have never
+The other @acronym{GNU} programming tools, such as the @acronym{GNU} C
+compiler, have never
had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
verbatim copy of a sizable piece of Bison, which is the code for the
@code{yyparse} function. (The actions from your grammar are inserted
into this function at one point, but the rest of the function is not
-changed.) When we applied the GPL terms to the code for @code{yyparse},
+changed.) When we applied the @acronym{GPL} terms to the code for
+@code{yyparse},
the effect was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to
concluded that limiting Bison's use to free software was doing little to
encourage people to make other software free. So we decided to make the
practical conditions for using Bison match the practical conditions for
-using the other GNU tools.
+using the other @acronym{GNU} tools.
-This exception applies only when Bison is generating C code for a
-LALR(1) parser; otherwise, the GPL terms operate as usual. You can
+This exception applies only when Bison is generating C code for an
+@acronym{LALR}(1) parser; otherwise, the @acronym{GPL} terms operate
+as usual. You can
tell whether the exception applies to your @samp{.c} output file by
inspecting it to see whether it says ``As a special exception, when
this file is copied by Bison into a Bison output file, you may use
a semantic value (the value of an integer,
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
+* GLR Parsers:: Writing parsers for general context-free languages.
* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
recursive, but there must be at least one rule which leads out of the
recursion.
-@cindex BNF
+@cindex @acronym{BNF}
@cindex Backus-Naur form
The most common formal system for presenting such rules for humans to read
-is @dfn{Backus-Naur Form} or ``BNF'', which was developed in order to
-specify the language Algol 60. Any grammar expressed in BNF is a
-context-free grammar. The input to Bison is essentially machine-readable
-BNF.
+is @dfn{Backus-Naur Form} or ``@acronym{BNF}'', which was developed in
+order to specify the language Algol 60. Any grammar expressed in
+@acronym{BNF} is a context-free grammar. The input to Bison is
+essentially machine-readable @acronym{BNF}.
-@cindex LALR(1) grammars
-@cindex LR(1) grammars
+@cindex @acronym{LALR}(1) grammars
+@cindex @acronym{LR}(1) grammars
There are various important subclasses of context-free grammar. Although it
can handle almost all context-free grammars, Bison is optimized for what
-are called LALR(1) grammars.
+are called @acronym{LALR}(1) grammars.
In brief, in these grammars, it must be possible to
tell how to parse any portion of an input string with just a single
token of look-ahead. Strictly speaking, that is a description of an
-LR(1) grammar, and LALR(1) involves additional restrictions that are
+@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
+restrictions that are
hard to explain simply; but it is rare in actual practice to find an
-LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, ,
-Mysterious Reduce/Reduce Conflicts}, for more information on this.
+@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1).
+@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
+more information on this.
-@cindex GLR parsing
-@cindex generalized LR (GLR) parsing
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
-@cindex non-deterministic parsing
-Parsers for LALR(1) grammars are @dfn{deterministic}, meaning roughly that
-the next grammar rule to apply at any point in the input is 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 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 general
-context-free grammars, using a technique known as GLR parsing (for
-Generalized LR). Bison's GLR parsers are able to handle any context-free
-grammar for which the number of possible parses of any given string
-is finite.
+@cindex nondeterministic parsing
+
+Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
+roughly that the next grammar rule to apply at any point in the input is
+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 same inputs. Even unambiguous
+grammars can be @dfn{nondeterministic}, 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
+general context-free grammars, using a technique known as @acronym{GLR}
+parsing (for Generalized @acronym{LR}). Bison's @acronym{GLR} parsers
+are able to handle any context-free grammar for which the number of
+possible parses of any given string is finite.
@cindex symbols (abstract)
@cindex token
@cindex syntactic grouping
@cindex grouping, syntactic
-In the formal grammatical rules for a language, each kind of syntactic unit
-or grouping is named by a @dfn{symbol}. Those which are built by grouping
-smaller constructs according to grammatical rules are called
+In the formal grammatical rules for a language, each kind of syntactic
+unit or grouping is named by a @dfn{symbol}. Those which are built by
+grouping smaller constructs according to grammatical rules are called
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token types}. We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
corresponding to a single nonterminal symbol a @dfn{grouping}.
We can use the C language as an example of what symbols, terminal and
-nonterminal, mean. The tokens of C are identifiers, constants (numeric and
-string), and the various keywords, arithmetic operators and punctuation
-marks. So the terminal symbols of a grammar for C include `identifier',
-`number', `string', plus one symbol for each keyword, operator or
-punctuation mark: `if', `return', `const', `static', `int', `char',
-`plus-sign', `open-brace', `close-brace', `comma' and many more. (These
-tokens can be subdivided into characters, but that is a matter of
+nonterminal, mean. The tokens of C are identifiers, constants (numeric
+and string), and the various keywords, arithmetic operators and
+punctuation marks. So the terminal symbols of a grammar for C include
+`identifier', `number', `string', plus one symbol for each keyword,
+operator or punctuation mark: `if', `return', `const', `static', `int',
+`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
+(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
@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} */
a @dfn{Bison grammar} file. @xref{Grammar File, ,Bison Grammar Files}.
A nonterminal symbol in the formal grammar is represented in Bison input
-as an identifier, like an identifier in C. By convention, it should be
+as an identifier, like an identifier in C@. By convention, it should be
in lower case, such as @code{expr}, @code{stmt} or @code{declaration}.
The Bison representation for a terminal symbol is also called a @dfn{token
But the precise value is very important for what the input means once it is
parsed. A compiler is useless if it fails to distinguish between 4, 1 and
3989 as constants in the program! Therefore, each token in a Bison grammar
-has both a token type and a @dfn{semantic value}. @xref{Semantics, ,Defining Language Semantics},
+has both a token type and a @dfn{semantic value}. @xref{Semantics,
+,Defining Language Semantics},
for details.
The token type is a terminal symbol defined in the grammar, such as
from the values of the two subexpressions.
@node GLR Parsers
-@section Writing GLR Parsers
-@cindex GLR parsing
-@cindex generalized LR (GLR) parsing
+@section Writing @acronym{GLR} Parsers
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@findex %glr-parser
@cindex conflicts
@cindex shift/reduce conflicts
-
-In some grammars, there will be cases where Bison's standard 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 reductions (applications
-of a grammar rule) applies, or whether to apply a reduction or read more
-of the input and apply a reduction later in the input. These are known
-respectively as @dfn{reduce/reduce} conflicts (@pxref{Reduce/Reduce}),
-and @dfn{shift/reduce} conflicts (@pxref{Shift/Reduce}).
-
-To use a grammar that is not easily modified to be LALR(1), a more
-general parsing algorithm is sometimes necessary. If you include
+@cindex reduce/reduce conflicts
+
+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
+reductions (applications of a grammar rule) applies, or whether to apply
+a reduction or read more of the input and apply a reduction later in the
+input. These are known respectively as @dfn{reduce/reduce} conflicts
+(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
+(@pxref{Shift/Reduce}).
+
+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 LR (GLR)
-parser. These parsers handle Bison grammars that contain no unresolved
-conflicts (i.e., after applying precedence declarations) identically to
-LALR(1) parsers. However, when faced with unresolved shift/reduce and
-reduce/reduce conflicts, GLR parsers use the simple expedient of doing
-both, effectively cloning the parser to follow both possibilities. Each
-of the resulting parsers can again split, so that at any given time,
-there can be any number of possible parses being explored. The parsers
+(@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
+faced with unresolved shift/reduce and reduce/reduce conflicts,
+@acronym{GLR} parsers use the simple expedient of doing both,
+effectively cloning the parser to follow both possibilities. Each of
+the resulting parsers can again split, so that at any given time, there
+can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
user-defined function on the resulting values to produce an arbitrary
merged result.
-Let's consider an example, vastly simplified from C++.
+@menu
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
+* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* 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
%@{
- #define YYSTYPE const char*
+ #include <stdio.h>
+ #define YYSTYPE char const *
+ int yylex (void);
+ void yyerror (char const *);
%@}
%token TYPENAME ID
| decl %dprec 2
;
-expr : ID @{ printf ("%s ", $$); @}
+expr : ID @{ printf ("%s ", $$); @}
| TYPENAME '(' expr ')'
- @{ printf ("%s <cast> ", $1); @}
- | expr '+' expr @{ printf ("+ "); @}
- | expr '=' expr @{ printf ("= "); @}
+ @{ printf ("%s <cast> ", $1); @}
+ | expr '+' expr @{ printf ("+ "); @}
+ | expr '=' expr @{ printf ("= "); @}
;
decl : TYPENAME declarator ';'
- @{ printf ("%s <declare> ", $1); @}
+ @{ printf ("%s <declare> ", $1); @}
| TYPENAME declarator '=' expr ';'
- @{ printf ("%s <init-declare> ", $1); @}
+ @{ printf ("%s <init-declare> ", $1); @}
;
-declarator : ID @{ printf ("\"%s\" ", $1); @}
+declarator : ID @{ printf ("\"%s\" ", $1); @}
| '(' declarator ')'
;
@end example
@noindent
parses as either an @code{expr} or a @code{stmt}
-(assuming that @samp{T} is recognized as a TYPENAME and @samp{x} as an ID).
+(assuming that @samp{T} is recognized as a @code{TYPENAME} and
+@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
-static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1)
+static YYSTYPE
+stmtMerge (YYSTYPE x0, YYSTYPE x1)
@{
printf ("<OR> ");
return "";
@example
%@{
- #define YYSTYPE const char*
+ #define YYSTYPE char const *
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%@}
@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
+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.
+
+@node GLR Semantic Actions
+@subsection GLR Semantic Actions
+
+@cindex deferred semantic actions
+By definition, a deferred semantic action is not performed at the same time as
+the associated reduction.
+This raises caveats for several Bison features you might use in a semantic
+action in a @acronym{GLR} parser.
+
+@vindex yychar
+@cindex @acronym{GLR} parsers and @code{yychar}
+@vindex yylval
+@cindex @acronym{GLR} parsers and @code{yylval}
+@vindex yylloc
+@cindex @acronym{GLR} parsers and @code{yylloc}
+In any semantic action, you can examine @code{yychar} to determine the type of
+the look-ahead token present at the time of the associated reduction.
+After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
+you can then examine @code{yylval} and @code{yylloc} to determine the
+look-ahead token's semantic value and location, if any.
+In a nondeferred semantic action, you can also modify any of these variables to
+influence syntax analysis.
+@xref{Look-Ahead, ,Look-Ahead Tokens}.
+
+@findex yyclearin
+@cindex @acronym{GLR} parsers and @code{yyclearin}
+In a deferred semantic action, it's too late to influence syntax analysis.
+In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
+shallow copies of the values they had at the time of the associated reduction.
+For this reason alone, modifying them is dangerous.
+Moreover, the result of modifying them is undefined and subject to change with
+future versions of Bison.
+For example, if a semantic action might be deferred, you should never write it
+to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
+memory referenced by @code{yylval}.
+
+@findex YYERROR
+@cindex @acronym{GLR} parsers and @code{YYERROR}
+Another Bison feature requiring special consideration is @code{YYERROR}
+(@pxref{Action Features}), which you can invoke in any semantic action to
+initiate error recovery.
+During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
+the same as its effect in an @acronym{LALR}(1) parser.
+In a deferred semantic action, its effect is undefined.
+@c The effect is probably a syntax error at the split point.
+
+@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
+up to the user of these parsers to handle
+portability issues. For instance, if using Autoconf and the Autoconf
+macro @code{AC_C_INLINE}, a mere
+
+@example
+%@{
+ #include <config.h>
+%@}
+@end example
+
+@noindent
+will suffice. Otherwise, we suggest
+
+@example
+%@{
+ #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
+ #define inline
+ #endif
+%@}
+@end example
@node Locations Overview
@section Locations
@cindex location
-@cindex textual position
-@cindex position, textual
+@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 position}, or @dfn{location}, of each syntactic construct.
+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
This also includes numerous identifiers used for internal purposes.
Therefore, you should avoid using C identifiers starting with @samp{yy}
or @samp{YY} in the Bison grammar file except for the ones defined in
-this manual.
+this manual. Also, you should avoid using the C identifiers
+@samp{malloc} and @samp{free} for anything other than their usual
+meanings.
In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
-headers. On some non-@sc{gnu} hosts, @code{<alloca.h>},
+headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.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 prologue may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
@code{#include} to include header files that do any of these things.
+You need to declare the lexical analyzer @code{yylex} and the error
+printer @code{yyerror} here, along with any other global identifiers
+used by the actions in the grammar rules.
The Bison declarations declare the names of the terminal and nonterminal
symbols, and may also describe operator precedence and the data types of
The grammar rules define how to construct each nonterminal symbol from its
parts.
-The epilogue can contain any code you want to use. Often the definition of
-the lexical analyzer @code{yylex} goes here, plus subroutines called by the
-actions in the grammar rules. In a simple program, all the rest of the
-program can go here.
+The epilogue can contain any code you want to use. Often the
+definitions of functions declared in the prologue go here. In a
+simple program, all the rest of the program can go here.
@node Examples
@chapter Examples
/* Reverse polish notation calculator. */
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <math.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
%token NUM
@end example
The declarations section (@pxref{Prologue, , The prologue}) contains two
-preprocessor directives.
+preprocessor directives and two forward declarations.
The @code{#define} directive defines the macro @code{YYSTYPE}, thus
specifying the C data type for semantic values of both tokens and
The @code{#include} directive is used to declare the exponentiation
function @code{pow}.
+The forward declarations for @code{yylex} and @code{yyerror} are
+needed because the C language requires that functions be declared
+before they are used. These functions will be defined in the
+epilogue, but the parser calls them so they must be declared in the
+prologue.
+
The second section, Bison declarations, provides information to Bison
about the token types (@pxref{Bison Declarations, ,The Bison
Declarations Section}). Each terminal symbol that is not a
;
line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+ | exp '\n' @{ printf ("\t%.10g\n", $1); @}
;
-exp: NUM @{ $$ = $1; @}
- | exp exp '+' @{ $$ = $1 + $2; @}
- | exp exp '-' @{ $$ = $1 - $2; @}
- | exp exp '*' @{ $$ = $1 * $2; @}
- | exp exp '/' @{ $$ = $1 / $2; @}
- /* Exponentiation */
- | exp exp '^' @{ $$ = pow ($1, $2); @}
- /* Unary minus */
- | exp 'n' @{ $$ = -$1; @}
+exp: NUM @{ $$ = $1; @}
+ | exp exp '+' @{ $$ = $1 + $2; @}
+ | exp exp '-' @{ $$ = $1 - $2; @}
+ | exp exp '*' @{ $$ = $1 * $2; @}
+ | exp exp '/' @{ $$ = $1 / $2; @}
+ /* Exponentiation */
+ | exp exp '^' @{ $$ = pow ($1, $2); @}
+ /* Unary minus */
+ | exp 'n' @{ $$ = -$1; @}
;
%%
@end example
For example, this:
@example
-exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{}
+exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example
@noindent
exp: NUM
| exp exp '+' @{ $$ = $1 + $2; @}
| @dots{}
+;
@end example
@noindent
tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
Analyzer Function @code{yylex}}.
-Only a simple lexical analyzer is needed for the RPN calculator. This
+Only a simple lexical analyzer is needed for the @acronym{RPN}
+calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{NUM} tokens. Any other character
that isn't part of a number is a separate token. Note that the token-code
When @code{yyparse} detects a syntax error, it calls the error reporting
function @code{yyerror} to print an error message (usually but not
-always @code{"parse error"}). It is up to the programmer to supply
+always @code{"syntax error"}). It is up to the programmer to supply
@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so
here is the definition we will use:
@group
#include <stdio.h>
+/* Called by yyparse on error. */
void
-yyerror (const char *s) /* called by yyparse on error */
+yyerror (char const *s)
@{
- printf ("%s\n", s);
+ fprintf (stderr, "%s\n", s);
@}
@end group
@end example
convert it into a parser file:
@example
-bison @var{file_name}.y
+bison @var{file}.y
@end example
@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
-CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c},
+@sc{calc}ulator''). Bison produces a file named @file{@var{file}.tab.c},
removing the @samp{.y} from the original file name. The file output by
Bison contains the source code for @code{yyparse}. The additional
functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
@group
# @r{Compile the Bison parser.}
# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
-$ @kbd{cc rpcalc.tab.c -lm -o rpcalc}
+$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
@end group
@group
@file{calc.y}, an infix desk-top calculator.
@example
-/* Infix notation calculator--calc */
+/* Infix notation calculator. */
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <math.h>
+ #include <stdio.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
-/* BISON Declarations */
+/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
-%right '^' /* exponentiation */
+%right '^' /* exponentiation */
-/* Grammar follows */
-%%
-input: /* empty string */
+%% /* The grammar follows. */
+input: /* empty */
| input line
;
@end example
This addition to the grammar allows for simple error recovery in the
-event of a parse error. If an expression that cannot be evaluated is
+event of a syntax error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for @code{line},
and parsing will continue. (The @code{yyerror} function is still called
upon to print its message as well.) The action executes the statement
/* Location tracking calculator. */
%@{
-#define YYSTYPE int
-#include <math.h>
+ #define YYSTYPE int
+ #include <math.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
/* Bison declarations. */
%left NEG
%right '^'
-%% /* Grammar follows */
+%% /* The grammar follows. */
@end example
@noindent
yylex (void)
@{
int c;
+@end group
+@group
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
+@end group
+@group
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
It is easy to add new operators to the infix calculator as long as they are
only single-character literals. The lexical analyzer @code{yylex} passes
-back all nonnumber characters as tokens, so new grammar rules suffice for
+back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
Here are the C and Bison declarations for the multi-function calculator.
@smallexample
+@group
%@{
-#include <math.h> /* For math functions, cos(), sin(), etc. */
-#include "calc.h" /* Contains definition of `symrec' */
+ #include <math.h> /* For math functions, cos(), sin(), etc. */
+ #include "calc.h" /* Contains definition of `symrec'. */
+ int yylex (void);
+ void yyerror (char const *);
%@}
+@end group
+@group
%union @{
-double val; /* For returning numbers. */
-symrec *tptr; /* For returning symbol-table pointers */
+ double val; /* For returning numbers. */
+ symrec *tptr; /* For returning symbol-table pointers. */
@}
-
-%token <val> NUM /* Simple double precision number */
-%token <tptr> VAR FNCT /* Variable and Function */
+@end group
+%token <val> NUM /* Simple double precision number. */
+%token <tptr> VAR FNCT /* Variable and Function. */
%type <val> exp
+@group
%right '='
%left '-' '+'
%left '*' '/'
-%left NEG /* Negation--unary minus */
-%right '^' /* Exponentiation */
-
-/* Grammar follows */
-
-%%
+%left NEG /* negation--unary minus */
+%right '^' /* exponentiation */
+@end group
+%% /* The grammar follows. */
@end smallexample
The above grammar introduces only two new features of the Bison language.
those which mention @code{VAR} or @code{FNCT}, are new.
@smallexample
+@group
input: /* empty */
| input line
;
+@end group
+@group
line:
'\n'
| exp '\n' @{ printf ("\t%.10g\n", $1); @}
| error '\n' @{ yyerrok; @}
;
+@end group
+@group
exp: NUM @{ $$ = $1; @}
| VAR @{ $$ = $1->value.var; @}
| VAR '=' exp @{ $$ = $3; $1->value.var = $3; @}
| exp '^' exp @{ $$ = pow ($1, $3); @}
| '(' exp ')' @{ $$ = $2; @}
;
-/* End of grammar */
+@end group
+/* End of grammar. */
%%
@end smallexample
@smallexample
@group
-/* Function type. */
+/* Function type. */
typedef double (*func_t) (double);
+@end group
-/* Data type for links in the chain of symbols. */
+@group
+/* Data type for links in the chain of symbols. */
struct symrec
@{
- char *name; /* name of symbol */
+ char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
@{
- double var; /* value of a VAR */
- func_t fnctptr; /* value of a FNCT */
+ double var; /* value of a VAR */
+ func_t fnctptr; /* value of a FNCT */
@} value;
- struct symrec *next; /* link field */
+ struct symrec *next; /* link field */
@};
@end group
@group
typedef struct symrec symrec;
-/* The symbol table: a chain of `struct symrec'. */
+/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
-symrec *putsym (const char *, func_t);
-symrec *getsym (const char *);
+symrec *putsym (char const *, int);
+symrec *getsym (char const *);
@end group
@end smallexample
@code{init_table} as well:
@smallexample
-@group
#include <stdio.h>
-int
-main (void)
-@{
- init_table ();
- return yyparse ();
-@}
-@end group
-
@group
+/* Called by yyparse on error. */
void
-yyerror (const char *s) /* Called by yyparse on error */
+yyerror (char const *s)
@{
printf ("%s\n", s);
@}
+@end group
+@group
struct init
@{
- char *fname;
- double (*fnct)(double);
+ char const *fname;
+ double (*fnct) (double);
@};
@end group
@group
-struct init arith_fncts[] =
+struct init const arith_fncts[] =
@{
"sin", sin,
"cos", cos,
"sqrt", sqrt,
0, 0
@};
+@end group
+@group
/* The symbol table: a chain of `struct symrec'. */
-symrec *sym_table = (symrec *) 0;
+symrec *sym_table;
@end group
@group
@}
@}
@end group
+
+@group
+int
+main (void)
+@{
+ init_table ();
+ return yyparse ();
+@}
+@end group
@end smallexample
By simply editing the initialization list and adding the necessary include
@smallexample
symrec *
-putsym (char *sym_name, int sym_type)
+putsym (char const *sym_name, int sym_type)
@{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
@}
symrec *
-getsym (const char *sym_name)
+getsym (char const *sym_name)
@{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
-characters with a leading non-digit are recognized as either variables or
+characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to @code{getsym} for look up in the symbol table. If
@smallexample
@group
#include <ctype.h>
+@end group
+@group
int
yylex (void)
@{
if (i == length)
@{
length *= 2;
- symbuf = (char *)realloc (symbuf, length + 1);
+ symbuf = (char *) realloc (symbuf, length + 1);
@}
/* Add this character to the buffer. */
symbuf[i++] = c;
@example
%@{
-@var{Prologue}
+ @var{Prologue}
%@}
@var{Bison declarations}
@end example
Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
+As a @acronym{GNU} extension, @samp{//} introduces a comment that
+continues until end of line.
@menu
* Prologue:: Syntax and usage of the prologue.
* Epilogue:: Syntax and usage of the epilogue.
@end menu
-@node Prologue, Bison Declarations, , Grammar Outline
+@node Prologue
@subsection The prologue
@cindex declarations section
@cindex Prologue
@cindex declarations
-The @var{Prologue} section contains macro definitions and
-declarations of functions and variables that are used in the actions in the
-grammar rules. These are copied to the beginning of the parser file so
-that they precede the definition of @code{yyparse}. You can use
-@samp{#include} to get the declarations from a header file. If you don't
-need any C declarations, you may omit the @samp{%@{} and @samp{%@}}
-delimiters that bracket this section.
+The @var{Prologue} section contains macro definitions and declarations
+of functions and variables that are used in the actions in the grammar
+rules. These are copied to the beginning of the parser file so that
+they precede the definition of @code{yyparse}. You can use
+@samp{#include} to get the declarations from a header file. If you
+don't need any C declarations, you may omit the @samp{%@{} and
+@samp{%@}} delimiters that bracket this section.
You may have more than one @var{Prologue} section, intermixed with the
@var{Bison declarations}. This allows you to have C and Bison
@smallexample
%@{
-#include <stdio.h>
-#include "ptypes.h"
+ #include <stdio.h>
+ #include "ptypes.h"
%@}
%union @{
- long n;
+ long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
%@{
-static void yyprint(FILE *, int, YYSTYPE);
-#define YYPRINT(F, N, L) yyprint(F, N, L)
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
%@}
@dots{}
@samp{%%} (which precedes the grammar rules) may never be omitted even
if it is the first thing in the file.
-@node Epilogue, , Grammar Rules, Grammar Outline
+@node Epilogue
@subsection The epilogue
@cindex additional C code section
@cindex epilogue
the @var{Prologue} is copied to the beginning. This is the most convenient
place to put anything that you want to have in the parser file but which need
not come before the definition of @code{yyparse}. For example, the
-definitions of @code{yylex} and @code{yyerror} often go here.
+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 in the Epilogue.
@xref{Interface, ,Parser C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.
-The Bison parser itself contains many static variables whose names start
-with @samp{yy} and many macros whose names start with @samp{YY}. It is a
-good idea to avoid using any such names (except those documented in this
-manual) in the epilogue of the grammar file.
+The Bison parser itself contains many macros and identifiers whose names
+start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
+any such names (except those documented in this manual) in the epilogue
+of the grammar file.
@node Symbols
@section Symbols, Terminal and Nonterminal
class of syntactically equivalent tokens. You use the symbol in grammar
rules to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the @code{yylex}
-function returns a token type code to indicate what kind of token has been
-read. You don't need to know what the code value is; you can use the
-symbol to stand for it.
+function returns a token type code to indicate what kind of token has
+been read. You don't need to know what the code value is; you can use
+the symbol to stand for it.
-A @dfn{nonterminal symbol} stands for a class of syntactically equivalent
-groupings. The symbol name is used in writing grammar rules. By convention,
-it should be all lower case.
+A @dfn{nonterminal symbol} stands for a class of syntactically
+equivalent groupings. The symbol name is used in writing grammar rules.
+By convention, it should be all lower case.
Symbol names can contain letters, digits (not at the beginning),
underscores and periods. Periods make sense only in nonterminals.
@itemize @bullet
@item
A @dfn{named token type} is written with an identifier, like an
-identifier in C. By convention, it should be all upper case. Each
+identifier in C@. By convention, it should be all upper case. Each
such name must be defined with a Bison declaration such as
@code{%token}. @xref{Token Decl, ,Token Type Names}.
used in Bison as well, but you must not use the null character as a
character literal because its numeric code, zero, signifies
end-of-input (@pxref{Calling Convention, ,Calling Convention
-for @code{yylex}}).
+for @code{yylex}}). Also, unlike standard C, trigraphs have no
+special meaning in Bison character literals, nor is backslash-newline
+allowed.
@item
@cindex string token
retrieve the token number for the literal string token from the
@code{yytname} table (@pxref{Calling Convention}).
-@strong{WARNING}: literal string tokens do not work in Yacc.
+@strong{Warning}: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a token
that consists of that particular string. Thus, you should use the token
read your program will be confused.
All the escape sequences used in string literals in C can be used in
-Bison as well. A literal string token must contain two or more
-characters; for a token containing just one character, use a character
-token (see above).
+Bison as well, except that you must not use a null character within a
+string literal. Also, unlike Standard C, trigraphs have no special
+meaning in Bison string literals, nor is backslash-newline allowed. A
+literal string token must contain two or more characters; for a token
+containing just one character, use a character token (see above).
@end itemize
How you choose to write a terminal symbol has no effect on its
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
If you want to write a grammar that is portable to any Standard C
-host, you must use only non-null character tokens taken from the basic
-execution character set of Standard C. This set consists of the ten
+host, you must use only nonnull character tokens taken from the basic
+execution character set of Standard C@. This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
@end example
-The @code{yylex} function and Bison must use a consistent character
-set and encoding for character tokens. For example, if you run Bison in an
-@sc{ascii} environment, but then compile and run the resulting program
-in an environment that uses an incompatible character set like
-@sc{ebcdic}, the resulting program may not work because the
-tables generated by Bison will assume @sc{ascii} numeric values for
-character tokens. It is standard
-practice for software distributions to contain C source files that
-were generated by Bison in an @sc{ascii} environment, so installers on
-platforms that are incompatible with @sc{ascii} must rebuild those
-files before compiling them.
+The @code{yylex} function and Bison must use a consistent character set
+and encoding for character tokens. For example, if you run Bison in an
+@acronym{ASCII} environment, but then compile and run the resulting
+program in an environment that uses an incompatible character set like
+@acronym{EBCDIC}, the resulting program may not work because the tables
+generated by Bison will assume @acronym{ASCII} numeric values for
+character tokens. It is standard practice for software distributions to
+contain C source files that were generated by Bison in an
+@acronym{ASCII} environment, so installers on platforms that are
+incompatible with @acronym{ASCII} must rebuild those files before
+compiling them.
The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
@section Recursive Rules
@cindex recursive rule
-A rule is called @dfn{recursive} when its @var{result} nonterminal appears
-also on its right hand side. Nearly all Bison grammars need to use
-recursion, because that is the only way to define a sequence of any number
-of a particular thing. Consider this recursive definition of a
+A rule is called @dfn{recursive} when its @var{result} nonterminal
+appears also on its right hand side. Nearly all Bison grammars need to
+use recursion, because that is the only way to define a sequence of any
+number of a particular thing. Consider this recursive definition of a
comma-separated sequence of one or more expressions:
@example
In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs. This was true in the
-RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
+@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
Notation Calculator}).
Bison's default is to use type @code{int} for all semantic values. To
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
-@code{int} or @code{long}, while a string constant needs type @code{char *},
-and an identifier might need a pointer to an entry in the symbol table.
+@code{int} or @code{long int}, while a string constant needs type
+@code{char *}, and an identifier might need a pointer to an entry in the
+symbol table.
To use more than one data type for semantic values in one parser, Bison
requires you to do two things:
semantic values associated with tokens or smaller groupings.
An action consists of C statements surrounded by braces, much like a
-compound statement in C. It can be placed at any position in the rule;
+compound statement in C@. An action can contain any sequence of C
+statements. Bison does not look for trigraphs, though, so if your C
+code uses trigraphs you should ensure that they do not affect the
+nesting of braces or the boundaries of comments, strings, or character
+literals.
+
+An action can be placed at any position in the rule;
it is executed at that position. Most rules have just one action at the
end of the rule, following all the components. Actions in the middle of
a rule are tricky and used only for special purposes (@pxref{Mid-Rule
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. (Bison translates both of these constructs
-into array element references when it copies the actions into the parser
-file.)
+being constructed is @code{$$}. Bison translates both of these
+constructs into expressions of the appropriate type when it copies the
+actions into the parser file. @code{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
Here is a typical example:
@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
-@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes
-the value of the whole rule. Of course, the default rule is valid only
-if the two data types match. There is no meaningful default action for
-an empty rule; every empty rule must have an explicit action unless the
-rule's value does not matter.
+@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule
+becomes the value of the whole rule. Of course, the default action is
+valid only if the two data types match. There is no meaningful default
+action for an empty rule; every empty rule must have an explicit action
+unless the rule's value does not matter.
@code{$@var{n}} with @var{n} zero or negative is allowed for reference
to tokens and groupings on the stack @emph{before} those that match the
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.
+@vindex yylval
+It is also possible to access the semantic value of the look-ahead token, if
+any, from a semantic action.
+This semantic value is stored in @code{yylval}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
@node Locations
@section Tracking Locations
@cindex location
-@cindex textual position
-@cindex position, textual
+@cindex textual location
+@cindex location, textual
Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional information,
especially symbol locations.
-@c (terminal or not) ?
-
The way locations are handled is defined by providing a data type, and
actions to take when rules are matched.
four members:
@example
-struct
+typedef struct YYLTYPE
@{
int first_line;
int first_column;
int last_line;
int last_column;
-@}
+@} YYLTYPE;
@end example
@node Actions and Locations
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
@end example
+@vindex yylloc
+It is also possible to access the location of the look-ahead token, if any,
+from a semantic action.
+This location is stored in @code{yylloc}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
-matched, before the associated action is run.
+matched, before the associated action is run. It is also invoked
+while processing a syntax error, to compute the error's location.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
-the location of the grouping (the result of the computation). The second one
-is an array holding locations of all right hand side elements of the rule
-being matched. The last one is the size of the right hand side rule.
+the location of the grouping (the result of the computation). When a
+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 identifies locations of
+the symbols that were discarded during error processing, and the third
+parameter is the number of discarded symbols.
-By default, it is defined this way for simple 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 GLR parsers:
+@end smallexample
-@example
-@group
-#define YYLLOC_DEFAULT(Current, Rhs, 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;
-@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
+actual arguments may not be surrounded by parentheses. Also, your
+macro should expand to something that can be used as a single
+statement when it is followed by a semicolon.
@end itemize
@node Declarations
Grammars}).
@menu
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* 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.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* 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.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
@end menu
+@node Require Decl
+@subsection Require a Version of Bison
+@cindex version requirement
+@cindex requiring a version of Bison
+@findex %require
+
+You may require the minimum version of Bison to process the grammar. If
+the requirement is not met, @command{bison} exits with an error (exit
+status 63).
+
+@example
+%require "@var{version}"
+@end example
+
@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
Precedence}.
You can explicitly specify the numeric code for a token type by appending
-an integer value in the field immediately following the token name:
+a decimal or hexadecimal integer value in the field immediately
+following the token name:
@example
%token NUM 300
+%token XNUM 0x12d // a GNU extension
@end example
@noindent
in the @code{%token} and @code{%type} declarations to pick one of the types
for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}).
-Note that, unlike making a @code{union} declaration in C, you do not write
+As an extension to @acronym{POSIX}, a tag is allowed after the
+@code{union}. For example:
+
+@example
+@group
+%union value @{
+ double val;
+ symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+specifies the union tag @code{value}, so the corresponding C type is
+@code{union value}. If you do not specify a tag, it defaults to
+@code{YYSTYPE}.
+
+As another extension to @acronym{POSIX}, you may specify multiple
+@code{%union} declarations; their contents are concatenated. However,
+only the first @code{%union} declaration can specify a tag.
+
+Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.
@node Type Decl
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{@@$} --- initial value and location of the look-ahead --- and the
+@code{%parse-param}.
+@end deffn
+
+For instance, if your locations use a file name, you may use
+
+@example
+%parse-param @{ char const *file_name @};
+%initial-action
+@{
+ @@$.initialize (file_name);
+@};
+@end example
+
+
+@node Destructor Decl
+@subsection Freeing Discarded Symbols
+@cindex freeing discarded symbols
+@findex %destructor
+
+During error recovery (@pxref{Error Recovery}), symbols already pushed
+on the stack and tokens coming from the rest of the file are discarded
+until the parser falls on its feet. If the parser runs out of memory,
+or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
+symbols on the stack must be discarded. Even if the parser succeeds, it
+must discard the start symbol.
+
+When discarded symbols convey heap based information, this memory is
+lost. While this behavior can be tolerable for batch parsers, such as
+in traditional compilers, it is unacceptable for programs like shells or
+protocol implementations that may parse and execute indefinitely.
+
+The @code{%destructor} directive defines code that is called when a
+symbol is automatically discarded.
+
+@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
+@findex %destructor
+Invoke @var{code} whenever the parser discards one of the @var{symbols}.
+Within @var{code}, @code{$$} designates the semantic value associated
+with the discarded symbol. The additional parser parameters are also
+available (@pxref{Parser Function, , The Parser Function
+@code{yyparse}}).
+@end deffn
+
+For instance:
+
+@smallexample
+%union
+@{
+ char *string;
+@}
+%token <string> STRING
+%type <string> string
+%destructor @{ free ($$); @} STRING string
+@end smallexample
+
+@noindent
+guarantees that when a @code{STRING} or a @code{string} is discarded,
+its associated memory will be freed.
+
+@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 (except the current
+right-hand side symbols) when the parser returns immediately, and
+@item
+the start symbol, when the parser succeeds.
+@end itemize
+
+The parser can @dfn{return immediately} because of an explicit call to
+@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
+exhaustion.
+
+Right-hand size symbols of a rule that explicitly triggers a syntax
+error via @code{YYERROR} are not discarded automatically. As a rule
+of thumb, destructors are invoked only when user actions cannot manage
+the memory.
+
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
+@findex %expect-rr
Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
%expect @var{n}
@end example
-Here @var{n} is a decimal integer. The declaration says there should be
-no warning if there are @var{n} shift/reduce conflicts and no
-reduce/reduce conflicts. An error, instead of the usual warning, is
-given if there are either more or fewer conflicts, or if there are any
-reduce/reduce conflicts.
+Here @var{n} is a decimal integer. The declaration says there should
+be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
+Bison reports an error if the number of shift/reduce conflicts differs
+from @var{n}, or if there are any reduce/reduce conflicts.
+
+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 @acronym{GLR}
+parsers, however, both kinds of conflicts are routine; otherwise,
+there 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
+%expect-rr @var{n}
+@end example
In general, using @code{%expect} involves these steps:
@item
Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed.
+number which Bison printed. With @acronym{GLR} parsers, add an
+@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will stop annoying you about the conflicts you have checked, but
-it will warn you again if changes in the grammar result in additional
-conflicts.
+Now Bison will warn you if you introduce an unexpected conflict, but
+will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code. Reentrancy is important whenever asynchronous execution is possible;
-for example, a non-reentrant program may not be safe to call from a signal
-handler. In systems with multiple threads of control, a non-reentrant
+for example, a nonreentrant program may not be safe to call from a signal
+handler. In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
-suitable for most uses, and it permits compatibility with YACC. (The
-standard YACC interfaces are inherently nonreentrant, because they use
+suitable for most uses, and it permits compatibility with Yacc. (The
+standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with @code{yylex},
including @code{yylval} and @code{yylloc}.)
Here is a summary of the declarations used to define a grammar:
-@table @code
-@item %union
+@deffn {Directive} %union
Declare the collection of data types that semantic values may have
(@pxref{Union Decl, ,The Collection of Value Types}).
+@end deffn
-@item %token
+@deffn {Directive} %token
Declare a terminal symbol (token type name) with no precedence
or associativity specified (@pxref{Token Decl, ,Token Type Names}).
+@end deffn
-@item %right
+@deffn {Directive} %right
Declare a terminal symbol (token type name) that is right-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %left
+@deffn {Directive} %left
Declare a terminal symbol (token type name) that is left-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %nonassoc
+@deffn {Directive} %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
-(using it in a way that would be associative is a syntax error)
(@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules lacking an explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
+@end ifset
-@item %type
+@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
(@pxref{Type Decl, ,Nonterminal Symbols}).
+@end deffn
-@item %start
+@deffn {Directive} %start
Specify the grammar's start symbol (@pxref{Start Decl, ,The
Start-Symbol}).
+@end deffn
-@item %expect
+@deffn {Directive} %expect
Declare the expected number of shift-reduce conflicts
(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
-@end table
+@end deffn
+
@sp 1
@noindent
In order to change the behavior of @command{bison}, use the following
directives:
-@table @code
-@item %debug
+@deffn {Directive} %debug
In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
already defined, so that the debugging facilities are compiled.
+@end deffn
@xref{Tracing, ,Tracing Your Parser}.
-@item %defines
-Write an extra output file containing macro definitions for the token
-type names defined in the grammar and the semantic value type
-@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
-
+@deffn {Directive} %defines
+Write a header file containing macro definitions for the token type
+names defined in the grammar as well as a few other declarations.
If the parser output file is named @file{@var{name}.c} then this file
is named @file{@var{name}.h}.
-This output file is essential if you wish to put the definition of
-@code{yylex} in a separate source file, because @code{yylex} needs to
-be able to refer to token type codes and the variable
-@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.
+Unless @code{YYSTYPE} is already defined as a macro, the output header
+declares @code{YYSTYPE}. Therefore, if you are using a @code{%union}
+(@pxref{Multiple Types, ,More Than One Value Type}) with components that
+require other definitions, or if you have defined a @code{YYSTYPE} macro
+(@pxref{Value Type, ,Data Types of Semantic Values}), you need to
+arrange for these definitions to be propagated to all modules, e.g., by
+putting them in a prerequisite header that is included both by your
+parser and by any other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the output header declares @code{yylval}
+as an external variable. @xref{Pure Decl, ,A Pure (Reentrant)
+Parser}.
+
+If you have also used locations, the output header declares
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
+@code{YYSTYPE} and @code{yylval}. @xref{Locations, ,Tracking
+Locations}.
+
+This output file is normally essential if you wish to put the definition
+of @code{yylex} in a separate source file, because @code{yylex}
+typically needs to be able to refer to the above-mentioned declarations
+and to the token type codes. @xref{Token Values, ,Semantic Values of
+Tokens}.
+@end deffn
+
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
-@item %file-prefix="@var{prefix}"
+@deffn {Directive} %file-prefix="@var{prefix}"
Specify a prefix to use for all Bison output file names. The names are
chosen as if the input file were named @file{@var{prefix}.y}.
+@end deffn
-@c @item %header-extension
-@c Specify the extension of the parser header file generated when
-@c @code{%define} or @samp{-d} are used.
-@c
-@c For example, a grammar file named @file{foo.ypp} and containing a
-@c @code{%header-extension .hh} directive will produce a header file
-@c named @file{foo.tab.hh}
-
-@item %locations
+@deffn {Directive} %locations
Generate the code processing the locations (@pxref{Action Features,
,Special Features for Use in Actions}). This mode is enabled as soon as
the grammar uses the special @samp{@@@var{n}} tokens, but if your
grammar does not use it, using @samp{%locations} allows for more
-accurate parse error messages.
+accurate syntax error messages.
+@end deffn
-@item %name-prefix="@var{prefix}"
+@deffn {Directive} %name-prefix="@var{prefix}"
Rename the external symbols used in the parser so that they start with
@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@code{yylval}, @code{yychar}, @code{yydebug}, and possible
-@code{yylloc}. For example, if you use @samp{%name-prefix="c_"}, the
-names become @code{c_parse}, @code{c_lex}, and so on. @xref{Multiple
-Parsers, ,Multiple Parsers in the Same Program}.
+@code{yylval}, @code{yylloc}, @code{yychar}, @code{yydebug}, and
+possible @code{yylloc}. For example, if you use
+@samp{%name-prefix="c_"}, the names become @code{c_parse}, @code{c_lex},
+and so on. @xref{Multiple Parsers, ,Multiple Parsers in the Same
+Program}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
-@item %no-parser
+@deffn {Directive} %no-parser
Do not include any C code in the parser file; generate tables only. The
parser file contains just @code{#define} directives and static variable
declarations.
This option also tells Bison to write the C code for the grammar actions
-into a file named @file{@var{filename}.act}, in the form of a
+into a file named @file{@var{file}.act}, in the form of a
brace-surrounded body fit for a @code{switch} statement.
+@end deffn
-@item %no-lines
+@deffn {Directive} %no-lines
Don't generate any @code{#line} preprocessor commands in the parser
file. Ordinarily Bison writes these commands in the parser file so that
the C compiler and debuggers will associate errors and object code with
your source file (the grammar file). This directive causes them to
associate errors with the parser file, treating it an independent source
file in its own right.
+@end deffn
-@item %output="@var{filename}"
-Specify the @var{filename} for the parser file.
+@deffn {Directive} %output="@var{file}"
+Specify @var{file} for the parser file.
+@end deffn
-@item %pure-parser
+@deffn {Directive} %pure-parser
Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
(Reentrant) Parser}).
+@end deffn
-@c @item %source-extension
-@c Specify the extension of the parser output file.
-@c
-@c For example, a grammar file named @file{foo.yy} and containing a
-@c @code{%source-extension .cpp} directive will produce a parser file
-@c named @file{foo.tab.cpp}
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
-@item %token-table
+@deffn {Directive} %token-table
Generate an array of token names in the parser file. The name of the
array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
token whose internal Bison token code number is @var{i}. The first
-three elements of @code{yytname} are always @code{"$end"},
+three elements of @code{yytname} correspond to the predefined tokens
+@code{"$end"},
@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
@item YYNSTATES
The number of parser states (@pxref{Parser States}).
@end table
+@end deffn
-@item %verbose
+@deffn {Directive} %verbose
Write an extra output file containing verbose descriptions of the
parser states and what is done for each type of look-ahead token in
that state. @xref{Understanding, , Understanding Your Parser}, for more
information.
+@end deffn
-
-
-@item %yacc
+@deffn {Directive} %yacc
Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
including its naming conventions. @xref{Bison Options}, for more.
-@end table
-
-
+@end deffn
@node Multiple Parsers
names that do not conflict.
The precise list of symbols renamed is @code{yyparse}, @code{yylex},
-@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar} and
-@code{yydebug}. For example, if you use @samp{-p c}, the names become
-@code{cparse}, @code{clex}, and so on.
+@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc},
+@code{yychar} and @code{yydebug}. For example, if you use @samp{-p c},
+the names become @code{cparse}, @code{clex}, and so on.
@strong{All the other variables and macros associated with Bison are not
renamed.} These others are not global; there is no conflict if the same
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
write an action which directs @code{yyparse} to return immediately
without reading further.
+
+@deftypefun int yyparse (void)
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).
-The value is 1 if parsing failed (return is due to a syntax error).
+The value is 1 if parsing failed because of invalid input, i.e., input
+that contains a syntax error or that causes @code{YYABORT} to be
+invoked.
+
+The value is 2 if parsing failed due to memory exhaustion.
+@end deftypefun
In an action, you can cause immediate return from @code{yyparse} by using
these macros:
-@table @code
-@item YYACCEPT
+@defmac YYACCEPT
@findex YYACCEPT
Return immediately with value 0 (to report success).
+@end defmac
-@item YYABORT
+@defmac YYABORT
@findex YYABORT
Return immediately with value 1 (to report failure).
-@end table
+@end defmac
+
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way. To do so, use the
+declaration @code{%parse-param}:
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+@findex %parse-param
+Declare that an argument declared by @code{argument-declaration} is an
+additional @code{yyparse} argument.
+The @var{argument-declaration} is used when declaring
+functions or prototypes. The last identifier in
+@var{argument-declaration} must be the argument name.
+@end deffn
+
+Here's an example. Write this in the parser:
+
+@example
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+Then call the parser like this:
+
+@example
+@{
+ int nastiness, randomness;
+ @dots{} /* @r{Store proper data in @code{nastiness} and @code{randomness}.} */
+ value = yyparse (&nastiness, &randomness);
+ @dots{}
+@}
+@end example
+
+@noindent
+In the grammar actions, use expressions like this to refer to the data:
+
+@example
+exp: @dots{} @{ @dots{}; *randomness += 1; @dots{} @}
+@end example
+
@node Lexical
@section The Lexical Analyzer Function @code{yylex}
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
* Token Values:: How @code{yylex} must return the semantic value
of the token it has read.
-* Token Positions:: How @code{yylex} must return the text position
+* Token Locations:: How @code{yylex} must return the text location
(line number, etc.) of the token, if the
actions want that.
* Pure Calling:: How the calling convention differs
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++)
@{
if (yytname[i] != 0
&& yytname[i][0] == '"'
- && strncmp (yytname[i] + 1, token_buffer,
- strlen (token_buffer))
+ && ! strncmp (yytname[i] + 1, token_buffer,
+ strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
@subsection Semantic Values of Tokens
@vindex yylval
-In an ordinary (non-reentrant) parser, the semantic value of the token must
+In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}. When you are using
just one data type for semantic values, @code{yylval} has that type.
Thus, if the type is @code{int} (the default), you might write this in
@end group
@end example
-@node Token Positions
-@subsection Textual Positions of Tokens
+@node Token Locations
+@subsection Textual Locations of Tokens
@vindex yylloc
If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
-Tracking Locations}) in actions to keep track of the
-textual locations of tokens and groupings, then you must provide this
-information in @code{yylex}. The function @code{yyparse} expects to
-find the textual location of a token just parsed in the global variable
-@code{yylloc}. So @code{yylex} must store the proper data in that
-variable.
+Tracking Locations}) in actions to keep track of the textual locations
+of tokens and groupings, then you must provide this information in
+@code{yylex}. The function @code{yyparse} expects to find the textual
+location of a token just parsed in the global variable @code{yylloc}.
+So @code{yylex} must store the proper data in that variable.
By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions. The
@end example
If the grammar file does not use the @samp{@@} constructs to refer to
-textual positions, then the type @code{YYLTYPE} will not be defined. In
+textual locations, then the type @code{YYLTYPE} will not be defined. In
this case, omit the second argument; @code{yylex} will be called with
only one argument.
-@vindex YYPARSE_PARAM
-If you use a reentrant parser, you can optionally pass additional
-parameter information to it in a reentrant way. To do so, define the
-macro @code{YYPARSE_PARAM} as a variable name. This modifies the
-@code{yyparse} function to accept one argument, of type @code{void *},
-with that name.
-
-When you call @code{yyparse}, pass the address of an object, casting the
-address to @code{void *}. The grammar actions can refer to the contents
-of the object by casting the pointer value back to its proper type and
-then dereferencing it. Here's an example. Write this in the parser:
-@example
-%@{
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
+If you wish to pass the additional parameter data to @code{yylex}, use
+@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
+Function}).
-#define YYPARSE_PARAM parm
-%@}
-@end example
+@deffn {Directive} lex-param @{@var{argument-declaration}@}
+@findex %lex-param
+Declare that @code{argument-declaration} is an additional @code{yylex}
+argument declaration.
+@end deffn
-@noindent
-Then call the parser like this:
+For instance:
@example
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
-
-@dots{}
-
-@{
- struct parser_control foo;
- @dots{} /* @r{Store proper data in @code{foo}.} */
- value = yyparse ((void *) &foo);
- @dots{}
-@}
+%parse-param @{int *nastiness@}
+%lex-param @{int *nastiness@}
+%parse-param @{int *randomness@}
@end example
@noindent
-In the grammar actions, use expressions like this to refer to the data:
+results in the following signature:
@example
-((struct parser_control *) parm)->randomness
+int yylex (int *nastiness);
+int yyparse (int *nastiness, int *randomness);
@end example
-@vindex YYLEX_PARAM
-If you wish to pass the additional parameter data to @code{yylex},
-define the macro @code{YYLEX_PARAM} just like @code{YYPARSE_PARAM}, as
-shown here:
+If @code{%pure-parser} is added:
@example
-%@{
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
-
-#define YYPARSE_PARAM parm
-#define YYLEX_PARAM parm
-%@}
+int yylex (YYSTYPE *lvalp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
@end example
-You should then define @code{yylex} to accept one additional
-argument---the value of @code{parm}. (This makes either two or three
-arguments in total, depending on whether an argument of type
-@code{YYLTYPE} is passed.) You can declare the argument as a pointer to
-the proper object type, or you can declare it as @code{void *} and
-access the contents as shown above.
+@noindent
+and finally, if both @code{%pure-parser} and @code{%locations} are used:
-You can use @samp{%pure-parser} to request a reentrant parser without
-also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse}
-with no arguments, as usual.
+@example
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+@end example
@node Error Reporting
@section The Error Reporting Function @code{yyerror}
@cindex parse error
@cindex syntax error
-The Bison parser detects a @dfn{parse error} or @dfn{syntax error}
+The Bison parser detects a @dfn{syntax error} or @dfn{parse error}
whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use
The Bison parser expects to report the error by calling an error
reporting function named @code{yyerror}, which you must supply. It is
called by @code{yyparse} whenever a syntax error is found, and it
-receives one argument. For a parse error, the string is normally
-@w{@code{"parse error"}}.
-
-@findex YYERROR_VERBOSE
-If you define the macro @code{YYERROR_VERBOSE} in the Bison declarations
-section (@pxref{Bison Declarations, ,The Bison Declarations Section}),
-then Bison provides a more verbose and specific error message string
-instead of just plain @w{@code{"parse error"}}. It doesn't matter what
-definition you use for @code{YYERROR_VERBOSE}, just whether you define
-it.
-
-The parser can detect one other kind of error: stack overflow. This
-happens when the input contains constructions that are very deeply
+receives one argument. For a syntax error, the string is normally
+@w{@code{"syntax error"}}.
+
+@findex %error-verbose
+If you invoke the directive @code{%error-verbose} in the Bison
+declarations section (@pxref{Bison Declarations, ,The Bison Declarations
+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: 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:
@example
@group
void
-yyerror (char *s)
+yyerror (char const *s)
@{
@end group
@group
(@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will
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 @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:
+
+@example
+void yyerror (char const *msg); /* Yacc parsers. */
+void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
+@end example
+
+If @samp{%parse-param @{int *nastiness@}} is used, then:
+
+@example
+void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */
+void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
+@end example
+
+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.:
+
+@example
+/* Location tracking. */
+%locations
+/* Pure yylex. */
+%pure-parser
+%lex-param @{int *nastiness@}
+/* Pure yyparse. */
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+results in the following signatures for all the parser kinds:
+
+@example
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+void yyerror (YYLTYPE *locp,
+ int *nastiness, int *randomness,
+ char const *msg);
+@end example
+
+@noindent
+The prototypes are only indications of how the code produced by Bison
+uses @code{yyerror}. Bison-generated code always ignores the returned
+value, so @code{yyerror} can return any type, including @code{void}.
+Also, @code{yyerror} can be a variadic function; that is why the
+message is always passed last.
+
+Traditionally @code{yyerror} returns an @code{int} that is always
+ignored, but this is purely for historical reasons, and @code{void} is
+preferable since it more accurately describes the return type for
+@code{yyerror}.
+
@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
-encountered so far. Normally this variable is global; but if you
+reported so far. Normally this variable is global; but if you
request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
then it is a local variable which only the actions can access.
Here is a table of Bison constructs, variables and macros that
are useful in actions.
-@table @samp
-@item $$
+@deffn {Variable} $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule. @xref{Actions}.
+@end deffn
-@item $@var{n}
+@deffn {Variable} $@var{n}
Acts like a variable that contains the semantic value for the
@var{n}th component of the current rule. @xref{Actions}.
+@end deffn
-@item $<@var{typealt}>$
+@deffn {Variable} $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
specified by the @code{%union} declaration. @xref{Action Types, ,Data
Types of Values in Actions}.
+@end deffn
-@item $<@var{typealt}>@var{n}
+@deffn {Variable} $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
@xref{Action Types, ,Data Types of Values in Actions}.
+@end deffn
-@item YYABORT;
+@deffn {Macro} YYABORT;
Return immediately from @code{yyparse}, indicating failure.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
-@item YYACCEPT;
+@deffn {Macro} YYACCEPT;
Return immediately from @code{yyparse}, indicating success.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
-@item YYBACKUP (@var{token}, @var{value});
+@deffn {Macro} YYBACKUP (@var{token}, @var{value});
@findex YYBACKUP
Unshift a token. This macro is allowed only for rules that reduce
a single value, and only when there is no look-ahead token.
-It is also disallowed in GLR parsers.
+It is also disallowed in @acronym{GLR} parsers.
It installs a look-ahead token with token type @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.
recovery.
In either case, the rest of the action is not executed.
+@end deffn
-@item YYEMPTY
+@deffn {Macro} YYEMPTY
@vindex YYEMPTY
Value stored in @code{yychar} when there is no look-ahead token.
+@end deffn
+
+@deffn {Macro} YYEOF
+@vindex YYEOF
+Value stored in @code{yychar} when the look-ahead is the end of the input
+stream.
+@end deffn
-@item YYERROR;
+@deffn {Macro} YYERROR;
@findex YYERROR
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call @code{yyerror}, and does not print any message. If you
want to print an error message, call @code{yyerror} explicitly before
the @samp{YYERROR;} statement. @xref{Error Recovery}.
+@end deffn
-@item YYRECOVERING
+@deffn {Macro} YYRECOVERING
This macro stands for an expression that has the value 1 when the parser
is recovering from a syntax error, and 0 the rest of the time.
@xref{Error Recovery}.
-
-@item yychar
-Variable containing the current look-ahead token. (In a pure parser,
-this is actually a local variable within @code{yyparse}.) When there is
-no look-ahead token, the value @code{YYEMPTY} is stored in the variable.
+@end deffn
+
+@deffn {Variable} yychar
+Variable containing either the look-ahead token, or @code{YYEOF} when the
+look-ahead is the end of the input stream, or @code{YYEMPTY} when no look-ahead
+has been performed so the next token is not yet known.
+Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
@xref{Look-Ahead, ,Look-Ahead Tokens}.
+@end deffn
-@item yyclearin;
+@deffn {Macro} yyclearin;
Discard the current look-ahead token. This is useful primarily in
-error rules. @xref{Error Recovery}.
+error rules.
+Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
+Semantic Actions}).
+@xref{Error Recovery}.
+@end deffn
-@item yyerrok;
+@deffn {Macro} yyerrok;
Resume generating error messages immediately for subsequent syntax
errors. This is useful primarily in error rules.
@xref{Error Recovery}.
-
-@item @@$
+@end deffn
+
+@deffn {Variable} yylloc
+Variable containing the look-ahead token location when @code{yychar} is not set
+to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+Variable containing the look-ahead token semantic value when @code{yychar} is
+not set to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions, ,Actions}.
+@end deffn
+
+@deffn {Value} @@$
@findex @@$
-Acts like a structure variable containing information on the textual position
+Acts like a structure variable containing information on the textual location
of the grouping made by the current rule. @xref{Locations, ,
Tracking Locations}.
@c those members.
@c The use of this feature makes the parser noticeably slower.
+@end deffn
-@item @@@var{n}
+@deffn {Value} @@@var{n}
@findex @@@var{n}
-Acts like a structure variable containing information on the textual position
+Acts like a structure variable containing information on the textual location
of the @var{n}th component of the current rule. @xref{Locations, ,
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
-@end table
@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
'!'}. No rule allows that sequence.
@vindex yychar
-The current look-ahead token is stored in the variable @code{yychar}.
+@vindex yylval
+@vindex yylloc
+The look-ahead token is stored in the variable @code{yychar}.
+Its semantic value and location, if any, are stored in the variables
+@code{yylval} and @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Shift/Reduce
@end group
@end example
+@ifset defaultprec
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%no-default-prec;} declaration makes it easier to discover
+this kind of problem systematically. It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence. This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%no-default-prec;} can be reversed by giving
+@code{%default-prec;}, which is the default.
+@end ifset
+
@node Parser States
@section Parser States
@cindex finite-state machine
It would seem that this grammar can be parsed with only a single token
of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
-@code{ID} follows. In other words, this grammar is LR(1).
+@code{ID} follows. In other words, this grammar is @acronym{LR}(1).
-@cindex LR(1)
-@cindex LALR(1)
+@cindex @acronym{LR}(1)
+@cindex @acronym{LALR}(1)
However, Bison, like most parser generators, cannot actually handle all
-LR(1) grammars. In this grammar, two contexts, that after an @code{ID}
+@acronym{LR}(1) grammars. In this grammar, two contexts, that after
+an @code{ID}
at the beginning of a @code{param_spec} and likewise at the beginning of
a @code{return_spec}, are similar enough that Bison assumes they are the
same. They appear similar because the same set of rules would be
that the rules would require different look-ahead tokens in the two
contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
-occurrence means that the grammar is not LALR(1).
+occurrence means that the grammar is not @acronym{LALR}(1).
In general, it is better to fix deficiencies than to document them. But
this particular deficiency is intrinsically hard to fix; parser
-generators that can handle LR(1) grammars are hard to write and tend to
+generators that can handle @acronym{LR}(1) grammars are hard to write
+and tend to
produce parsers that are very large. In practice, Bison is more useful
as it is now.
;
@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 LR (GLR) Parsing
-@cindex GLR parsing
-@cindex generalized LR (GLR) parsing
+@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
-@cindex non-deterministic parsing
+@cindex nondeterministic parsing
Bison produces @emph{deterministic} parsers that choose uniquely
when to reduce and which reduction to apply
-based on a summary of the preceding input and on one extra token of lookahead.
+based on a summary of the preceding input and on one extra token of look-ahead.
As a result, normal Bison handles a proper subset of the family of
context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
-lookahead, since the parser lacks the information necessary to make a
+look-ahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (@pxref{Mystery Conflicts}),
there are languages where Bison's particular choice of how to
When you use the @samp{%glr-parser} declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
-Generalized LR (or GLR). A Bison GLR parser uses the same basic
+Generalized @acronym{LR} (or @acronym{GLR}). A Bison @acronym{GLR}
+parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (@pxref{Precedence}) or a
-reduce-reduce conflict. When a GLR parser encounters such a situation, it
+reduce-reduce conflict. When a @acronym{GLR} parser encounters such a
+situation, it
effectively @emph{splits} into a several parsers, one for each possible
shift or reduction. These parsers then proceed as usual, consuming
tokens in lock-step. Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
-a Bison GLR parsing stack is what is in effect a tree of states.
+a Bison @acronym{GLR} parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse
is. Additional input may indicate that a guess was wrong, in which case
stream.
Whenever the parser makes a transition from having multiple
-states to having one, it reverts to the normal LALR(1) parsing
+states to having one, it reverts to the normal @acronym{LALR}(1) parsing
algorithm, after resolving and executing the saved-up actions.
At this transition, some of the states on the stack will have semantic
values that are sets (actually multisets) of possible actions. The
Bison resolves and evaluates both and then calls the merge function on
the result. Otherwise, it reports an ambiguity.
-It is possible to use a data structure for the GLR parsing tree that
-permits the processing of any LALR(1) grammar in linear time (in the
-size of the input), any unambiguous (not necessarily LALR(1)) grammar in
+It is possible to use a data structure for the @acronym{GLR} parsing tree that
+permits the processing of any @acronym{LALR}(1) grammar in linear time (in the
+size of the input), any unambiguous (not necessarily
+@acronym{LALR}(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time. However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
-prefix of the input. Thus, really ambiguous or non-deterministic
+prefix of the input. Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process. Such badly
behaving examples, however, are not generally of practical interest.
-Usually, non-determinism in a grammar is local---the parser is ``in
+Usually, nondeterminism in a grammar is local---the parser is ``in
doubt'' only for a few tokens at a time. Therefore, the current data
-structure should generally be adequate. On LALR(1) portions of a
+structure should generally be adequate. On @acronym{LALR}(1) portions of a
grammar, in particular, it is only slightly slower than with the default
Bison parser.
-@node Stack Overflow
-@section Stack Overflow, and How to Avoid It
+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 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.
-Becaue Bison parsers have growing stacks, hitting the upper limit
+Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, @xref{Recursion, ,Recursive Rules}.
@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 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.
+Because of semantical differences between C and C++, the
+@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
@cindex error recovery
@cindex recovery from errors
-It is not usually acceptable to have a program terminate on a parse
+It is not usually acceptable to have a program terminate on a syntax
error. For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should accept
another expression.
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part of
-the semantic context and part of the input. First it discards states and
-objects from the stack until it gets back to a state in which the
+the semantic context and part of the input. First it discards states
+and objects from the stack until it gets back to a state in which the
@code{error} token is acceptable. (This means that the subexpressions
-already parsed are discarded, back to the last complete @code{stmnts}.) At
-this point the @code{error} token can be shifted. Then, if the old
+already parsed are discarded, back to the last complete @code{stmnts}.)
+At this point the @code{error} token can be shifted. Then, if the old
look-ahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
-this example, Bison reads and discards input until the next newline
-so that the fourth rule can apply.
+this example, Bison reads and discards input until the next newline so
+that the fourth rule can apply. Note that discarded symbols are
+possible sources of memory leaks, see @ref{Destructor Decl, , Freeing
+Discarded Symbols}, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies for
error recovery. A simple and useful strategy is simply to skip the rest of
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token. Write the statement @samp{yyclearin;} in the error rule's
action.
+@xref{Action Features, ,Special Features for Use in Actions}.
-For example, suppose that on a parse error, an error handling routine is
+For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence. The next symbol returned by the lexical scanner is
probably correct. The previous look-ahead token ought to be discarded
name, then this is actually a declaration of @code{x}. How can a Bison
parser for C decide how to parse this input?
-The method used in GNU C is to have two different token types,
+The method used in @acronym{GNU} C is to have two different token types,
@code{IDENTIFIER} and @code{TYPENAME}. When @code{yylex} finds an
identifier, it looks up the current declaration of the identifier in order
to decide which token type to return: @code{TYPENAME} if the identifier is
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
@example
@group
%@{
-int hexflag;
+ int hexflag;
+ int yylex (void);
+ void yyerror (char const *);
%@}
%%
@dots{}
Bison parsers are @dfn{shift/reduce automata}. In some cases (much more
frequent than one would hope), looking at this automaton is required to
tune or simply fix a parser. Bison provides two different
-representation of it, either textually or graphically (as a @sc{vcg}
+representation of it, either textually or graphically (as a @acronym{VCG}
file).
The textual file is generated when the options @option{--report} or
@example
calc.y: warning: 1 useless nonterminal and 1 useless rule
calc.y:11.1-7: warning: useless nonterminal: useless
-calc.y:11.8-12: warning: useless rule: useless: STR
-calc.y contains 7 shift/reduce conflicts.
+calc.y:11.10-12: warning: useless rule: useless: STR
+calc.y: conflicts: 7 shift/reduce
@end example
When given @option{--report=state}, in addition to @file{calc.tab.c}, it
The next section lists states that still have conflicts.
@example
-State 8 contains 1 shift/reduce conflict.
-State 9 contains 1 shift/reduce conflict.
-State 10 contains 1 shift/reduce conflict.
-State 11 contains 4 shift/reduce conflicts.
+State 8 conflicts: 1 shift/reduce
+State 9 conflicts: 1 shift/reduce
+State 10 conflicts: 1 shift/reduce
+State 11 conflicts: 4 shift/reduce
@end example
@noindent
$accept -> . exp $ (rule 0)
- NUM shift, and go to state 1
+ NUM shift, and go to state 1
- exp go to state 2
+ exp go to state 2
@end example
This reads as follows: ``state 0 corresponds to being at the very
symbol (here, @code{exp}). When the parser returns to this state right
after having reduced a rule that produced an @code{exp}, the control
flow jumps to state 2. If there is no such transition on a nonterminal
-symbol, and the lookahead is a @code{NUM}, then this token is shifted on
+symbol, and the look-ahead is a @code{NUM}, then this token is shifted on
the parse stack, and the control flow jumps to state 1. Any other
-lookahead triggers a parse error.''
+look-ahead triggers a syntax error.''
@cindex core, item set
@cindex item set core
@cindex kernel, item set
@cindex item set core
Even though the only active rule in state 0 seems to be rule 0, the
-report lists @code{NUM} as a lookahead symbol because @code{NUM} can be
+report lists @code{NUM} as a look-ahead token because @code{NUM} can be
at the beginning of any rule deriving an @code{exp}. By default Bison
reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
you want to see more detail you can invoke @command{bison} with
exp -> NUM . (rule 5)
- $default reduce using rule 5 (exp)
+ $default reduce using rule 5 (exp)
@end example
@noindent
-the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the look-ahead token
(@samp{$default}), the parser will reduce it. If it was coming from
state 0, then, after this reduction it will return to state 0, and will
jump to state 2 (@samp{exp: go to state 2}).
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
- $ shift, and go to state 3
- '+' shift, and go to state 4
- '-' shift, and go to state 5
- '*' shift, and go to state 6
- '/' shift, and go to state 7
+ $ shift, and go to state 3
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
@end example
@noindent
In state 2, the automaton can only shift a symbol. For instance,
-because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
+because of the item @samp{exp -> exp . '+' exp}, if the look-ahead if
@samp{+}, it will be shifted on the parse stack, and the automaton
control will jump to state 4, corresponding to the item @samp{exp -> exp
'+' . exp}. Since there is no default action, any other token than
-those listed above will trigger a parse error.
+those listed above will trigger a syntax error.
The state 3 is named the @dfn{final state}, or the @dfn{accepting
state}:
$accept -> exp $ . (rule 0)
- $default accept
+ $default accept
@end example
@noindent
exp -> exp '+' . exp (rule 1)
- NUM shift, and go to state 1
+ NUM shift, and go to state 1
- exp go to state 8
+ exp go to state 8
state 5
exp -> exp '-' . exp (rule 2)
- NUM shift, and go to state 1
+ NUM shift, and go to state 1
- exp go to state 9
+ exp go to state 9
state 6
exp -> exp '*' . exp (rule 3)
- NUM shift, and go to state 1
+ NUM shift, and go to state 1
- exp go to state 10
+ exp go to state 10
state 7
exp -> exp '/' . exp (rule 4)
- NUM shift, and go to state 1
+ NUM shift, and go to state 1
- exp go to state 11
+ exp go to state 11
@end example
-As was announced in beginning of the report, @samp{State 8 contains 1
-shift/reduce conflict}:
+As was announced in beginning of the report, @samp{State 8 conflicts:
+1 shift/reduce}:
@example
state 8
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
- '*' shift, and go to state 6
- '/' shift, and go to state 7
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
- '/' [reduce using rule 1 (exp)]
- $default reduce using rule 1 (exp)
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
@end example
-Indeed, there are two actions associated to the lookahead @samp{/}:
+Indeed, there are two actions associated to the look-ahead @samp{/}:
either shifting (and going to state 7), or reducing rule 1. The
conflict means that either the grammar is ambiguous, or the parser lacks
information to make the right decision. Indeed the grammar is
NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
NUM}, which corresponds to reducing rule 1.
-Because in LALR(1) parsing a single decision can be made, Bison
+Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
Shift/Reduce Conflicts}. Discarded actions are reported in between
square brackets.
shifting the next token and going to the corresponding state, or
reducing a single rule. In the other cases, i.e., when shifting
@emph{and} reducing is possible or when @emph{several} reductions are
-possible, the lookahead is required to select the action. State 8 is
-one such state: if the lookahead is @samp{*} or @samp{/} then the action
+possible, the look-ahead is required to select the action. State 8 is
+one such state: if the look-ahead is @samp{*} or @samp{/} then the action
is shifting, otherwise the action is reducing rule 1. In other words,
the first two items, corresponding to rule 1, are not eligible when the
-lookahead is @samp{*}, since we specified that @samp{*} has higher
-precedence that @samp{+}. More generally, some items are eligible only
-with some set of possible lookaheads. When run with
-@option{--report=lookahead}, Bison specifies these lookaheads:
+look-ahead token is @samp{*}, since we specified that @samp{*} has higher
+precedence than @samp{+}. More generally, some items are eligible only
+with some set of possible look-ahead tokens. When run with
+@option{--report=look-ahead}, Bison specifies these look-ahead tokens:
@example
state 8
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
- '*' shift, and go to state 6
- '/' shift, and go to state 7
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
- '/' [reduce using rule 2 (exp)]
- $default reduce using rule 2 (exp)
+ '/' [reduce using rule 2 (exp)]
+ $default reduce using rule 2 (exp)
state 10
exp -> exp '*' exp . (rule 3)
exp -> exp . '/' exp (rule 4)
- '/' shift, and go to state 7
+ '/' shift, and go to state 7
- '/' [reduce using rule 3 (exp)]
- $default reduce using rule 3 (exp)
+ '/' [reduce using rule 3 (exp)]
+ $default reduce using rule 3 (exp)
state 11
exp -> exp . '/' exp (rule 4)
exp -> exp '/' exp . (rule 4)
- '+' shift, and go to state 4
- '-' shift, and go to state 5
- '*' shift, and go to state 6
- '/' shift, and go to state 7
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
- '+' [reduce using rule 4 (exp)]
- '-' [reduce using rule 4 (exp)]
- '*' [reduce using rule 4 (exp)]
- '/' [reduce using rule 4 (exp)]
- $default reduce using rule 4 (exp)
+ '+' [reduce using rule 4 (exp)]
+ '-' [reduce using rule 4 (exp)]
+ '*' [reduce using rule 4 (exp)]
+ '/' [reduce using rule 4 (exp)]
+ $default reduce using rule 4 (exp)
@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.
@item the macro @code{YYDEBUG}
@findex YYDEBUG
Define the macro @code{YYDEBUG} to a nonzero value when you compile the
-parser. This is compliant with POSIX Yacc. You could use
+parser. This is compliant with @acronym{POSIX} Yacc. You could use
@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
Prologue}).
@item the option @option{-t}, @option{--debug}
Use the @samp{-t} option when you run Bison (@pxref{Invocation,
-,Invoking Bison}). This is POSIX compliant too.
+,Invoking Bison}). This is @acronym{POSIX} compliant too.
@item the directive @samp{%debug}
@findex %debug
Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
Declaration Summary}). This is a Bison extension, which will prove
useful when Bison will output parsers for languages that don't use a
-preprocessor. Useless POSIX and Yacc portability matter to you, this is
+preprocessor. Unless @acronym{POSIX} and Yacc portability matter to
+you, this is
the preferred solution.
@end table
calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):
@smallexample
-#define YYPRINT(file, type, value) yyprint (file, type, value)
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(file, type, value) print_token_value (file, type, value)
+%@}
+
+@dots{} %% @dots{} %% @dots{}
static void
-yyprint (FILE *file, int type, YYSTYPE value)
+print_token_value (FILE *file, int type, YYSTYPE value)
@{
if (type == VAR)
- fprintf (file, " %s", value.tptr->name);
+ fprintf (file, "%s", value.tptr->name);
else if (type == NUM)
- fprintf (file, " %d", value.val);
+ fprintf (file, "%d", value.val);
@}
@end smallexample
Here @var{infile} is the grammar file name, which usually ends in
@samp{.y}. The parser file's name is made by replacing the @samp{.y}
-with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
-@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}. It's also possible, in case you are writing
+with @samp{.tab.c} and removing any leading directory. Thus, the
+@samp{bison foo.y} file name yields
+@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields
+@file{foo.tab.c}. It's also possible, in case you are writing
C++ code instead of C in your grammar file, to name it @file{foo.ypp}
or @file{foo.y++}. Then, the output files will take an extension like
the given one as input (respectively @file{foo.tab.cpp} and
@file{foo.tab.c++}).
-This feature takes effect with all options that manipulate filenames like
+This feature takes effect with all options that manipulate file names like
@samp{-o} or @samp{-d}.
For example :
will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and
@example
-bison -d @var{infile.y} -o @var{output.c++}
+bison -d -o @var{output.c++} @var{infile.y}
@end example
@noindent
will produce @file{output.c++} and @file{outfile.h++}.
+For compatibility with @acronym{POSIX}, the standard Bison
+distribution also contains a shell script called @command{yacc} that
+invokes Bison with the @option{-y} option.
+
@menu
* Bison Options:: All the options described in detail,
- in alphabetical order by short options.
+ in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
-* VMS Invocation:: Bison command syntax on VMS.
+* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
@end menu
@node Bison Options
@itemx --version
Print the version number of Bison and exit.
-@need 1750
+@item --print-localedir
+Print the name of the directory containing locale-dependent data.
+
@item -y
@itemx --yacc
-Equivalent to @samp{-o y.tab.c}; the parser output file is called
+Act more like the traditional Yacc command. This can cause
+different diagnostics to be generated, and may change behavior in
+other minor ways. Most importantly, imitate Yacc's output
+file name conventions, so that the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
-@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
-file name conventions. Thus, the following shell script can substitute
-for Yacc:
+@file{y.tab.h}. Thus, the following shell script can substitute
+for Yacc, and the Bison distribution contains such a script for
+compatibility with @acronym{POSIX}:
@example
-bison -y $*
+#! /bin/sh
+bison -y "$@@"
@end example
+
+The @option{-y}/@option{--yacc} option is intended for use with
+traditional Yacc grammars. If your grammar uses a Bison extension
+like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
+this option is specified.
+
@end table
@noindent
@itemx --defines
Pretend that @code{%defines} was specified, i.e., write an extra output
file containing macro definitions for the token type names defined in
-the grammar and the semantic value type @code{YYSTYPE}, as well as a few
-@code{extern} variable declarations. @xref{Decl Summary}.
+the grammar, as well as a few other declarations. @xref{Decl Summary}.
@item --defines=@var{defines-file}
Same as above, but save in the file @var{defines-file}.
@table @code
@item state
Description of the grammar, conflicts (resolved and unresolved), and
-LALR automaton.
+@acronym{LALR} automaton.
-@item lookahead
+@item look-ahead
Implies @code{state} and augments the description of the automaton with
-each rule's lookahead set.
+each rule's look-ahead set.
@item itemset
Implies @code{state} and augments the description of the automaton with
file containing verbose descriptions of the grammar and
parser. @xref{Decl Summary}.
-@item -o @var{filename}
-@itemx --output=@var{filename}
-Specify the @var{filename} for the parser file.
+@item -o @var{file}
+@itemx --output=@var{file}
+Specify the @var{file} for the parser file.
-The other output files' names are constructed from @var{filename} as
+The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.
@item -g
-Output a VCG definition of the LALR(1) grammar automaton computed by
-Bison. If the grammar file is @file{foo.y}, the VCG output file will
+Output a @acronym{VCG} definition of the @acronym{LALR}(1) grammar
+automaton computed by Bison. If the grammar file is @file{foo.y}, the
+@acronym{VCG} output file will
be @file{foo.vcg}.
@item --graph=@var{graph-file}
The behavior of @var{--graph} is the same than @samp{-g}. The only
difference is that it has an optional argument which is the name of
-the output graph filename.
+the output graph file.
@end table
@node Option Cross Key
\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
@end example
@end ifinfo
-@node VMS Invocation
-@section Invoking Bison under VMS
-@cindex invoking Bison under VMS
-@cindex VMS
+@node Yacc Library
+@section Yacc Library
-The command line syntax for Bison on VMS is a variant of the usual
-Bison command syntax---adapted to fit VMS conventions.
+The Yacc library contains default implementations of the
+@code{yyerror} and @code{main} functions. These default
+implementations are normally not useful, but @acronym{POSIX} requires
+them. To use the Yacc library, link your program with the
+@option{-ly} option. Note that Bison's implementation of the Yacc
+library is distributed under the terms of the @acronym{GNU} General
+Public License (@pxref{Copying}).
-To find the VMS equivalent for any Bison option, start with the long
-option, and substitute a @samp{/} for the leading @samp{--}, and
-substitute a @samp{_} for each @samp{-} in the name of the long option.
-For example, the following invocation under VMS:
+If you use the Yacc library's @code{yyerror} function, you should
+declare @code{yyerror} as follows:
@example
-bison /debug/name_prefix=bar foo.y
+int yyerror (char const *);
@end example
-@noindent
-is equivalent to the following command under POSIX.
+Bison ignores the @code{int} value returned by this @code{yyerror}.
+If you use the Yacc library's @code{main} function, your
+@code{yyparse} function should have the following type signature:
@example
-bison --debug --name-prefix=bar foo.y
+int yyparse (void);
@end example
-The VMS file system does not permit filenames such as
-@file{foo.tab.c}. In the above example, the output file
-would instead be named @file{foo_tab.c}.
+@c ================================================= C++ Bison
-@c ================================================= Invoking Bison
+@node C++ Language Interface
+@chapter C++ Language Interface
-@node FAQ
-@chapter Frequently Asked Questions
-@cindex frequently asked questions
-@cindex questions
+@menu
+* C++ Parsers:: The interface to generate C++ parser classes
+* A Complete C++ Example:: Demonstrating their use
+@end menu
-Several questions about Bison come up occasionally. Here some of them
-are addressed.
+@node C++ Parsers
+@section C++ Parsers
@menu
-* Parser Stack Overflow:: Breaking the Stack Limits
+* 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 Parser Stack Overflow
-@section Parser Stack Overflow
+@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{file}.hh
+@itemx @var{file}.cc
+The declaration and implementation of the C++ parser class.
+@var{file} is the name of the output file. It follows the same
+rules as with regular C parsers.
+
+Note that @file{@var{file}.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
-@display
-My parser returns with error with a @samp{parser stack overflow}
-message. What can I do?
-@end display
+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 (similar to Boost variants) might be implemented to
+alleviate these issues.}, which have a few specific features in C++.
+@itemize @minus
+@item
+The type @code{YYSTYPE} is defined but its use is discouraged: rather
+you should refer to the parser's encapsulated type
+@code{yy::parser::semantic_type}.
+@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
-This question is already addressed elsewhere, @xref{Recursion,
-,Recursive Rules}.
+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*} file
+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{file}:@var{line}.@var{column}}, or
+@samp{@var{line}.@var{column}} if @var{file} 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 detailed 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 nonzero, 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.
-@c ================================================= Table of Symbols
+@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 Table of Symbols
-@appendix Bison Symbols
-@cindex Bison symbols, table of
-@cindex symbols in Bison, table of
+@node Calc++ --- C++ Calculator
+@subsection Calc++ --- C++ Calculator
-@table @code
-@item @@$
-In an action, the location of the left-hand side of the rule.
-@xref{Locations, , Locations Overview}.
+Of course the grammar is dedicated to arithmetics, a single
+expression, possibly 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.
-@item @@@var{n}
-In an action, the location of the @var{n}-th symbol of the right-hand
-side of the rule. @xref{Locations, , Locations Overview}.
+@example
+three := 3
+seven := one + two * three
+seven * seven
+@end example
-@item $$
-In an action, the semantic value of the left-hand side of the rule.
-@xref{Actions}.
+@node Calc++ Parsing Driver
+@subsection Calc++ Parsing Driver
+@c - An env
+@c - A place to store error messages
+@c - A place for the result
-@item $@var{n}
-In an action, the semantic value of the @var{n}-th symbol of the
-right-hand side of the rule. @xref{Actions}.
+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.
-@item $accept
-The predefined nonterminal whose only rule is @samp{$accept: @var{start}
-$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
-Start-Symbol}. It cannot be used in the grammar.
+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, and the declaration of the parser
+class.
-@item $end
-The predefined token marking the end of the token stream. It cannot be
-used in the grammar.
+@comment file: calc++-driver.hh
+@example
+#ifndef CALCXX_DRIVER_HH
+# define CALCXX_DRIVER_HH
+# include <string>
+# include <map>
+# include "calc++-parser.hh"
+@end example
-@item $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}.
-@item 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 parse 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}.
+@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.
-@item 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}}.
+@comment file: calc++-driver.hh
+@example
+// Announce to Flex the prototype we want for lexing function, ...
+# define YY_DECL \
+ int yylex (yy::calcxx_parser::semantic_type* yylval, \
+ yy::calcxx_parser::location_type* yylloc, \
+ calcxx_driver& driver)
+// ... and declare it for the parser's sake.
+YY_DECL;
+@end example
-@item 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}}.
+@noindent
+The @code{calcxx_driver} class is then declared with its most obvious
+members.
-@item 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}.
+@comment file: calc++-driver.hh
+@example
+// Conducting the whole scanning and parsing of Calc++.
+class calcxx_driver
+@{
+public:
+ calcxx_driver ();
+ virtual ~calcxx_driver ();
-@item YYDEBUG
-Macro to define to equip the parser with tracing code. @xref{Tracing,
-,Tracing Your Parser}.
+ std::map<std::string, int> variables;
-@item 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}.
+ int result;
+@end example
-@item YYERROR_VERBOSE
-Macro that you define with @code{#define} in the Bison declarations
-section to request verbose, specific error message strings when
-@code{yyerror} is called.
+@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.
-@item YYINITDEPTH
-Macro for specifying the initial size of the parser stack.
-@xref{Stack Overflow}.
+@comment file: calc++-driver.hh
+@example
+ // Handling the scanner.
+ void scan_begin ();
+ void scan_end ();
+ bool trace_scanning;
+@end example
-@item YYLEX_PARAM
-Macro for specifying an extra argument (or list of extra arguments) for
-@code{yyparse} to pass to @code{yylex}. @xref{Pure Calling,, Calling
-Conventions for Pure Parsers}.
+@noindent
+Similarly for the parser itself.
-@item YYLTYPE
-Macro for the data type of @code{yylloc}; a structure with four
-members. @xref{Location Type, , Data Types of Locations}.
+@comment file: calc++-driver.hh
+@example
+ // Handling the parser.
+ void parse (const std::string& f);
+ std::string file;
+ bool trace_parsing;
+@end example
-@item yyltype
-Default value for YYLTYPE.
+@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.
-@item YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack.
-@xref{Stack Overflow}.
+@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
-@item YYPARSE_PARAM
-Macro for specifying the name of a parameter that @code{yyparse} should
-accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+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.
-@item YYRECOVERING
-Macro whose value indicates whether the parser is recovering from a
-syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@comment file: calc++-driver.cc
+@example
+#include "calc++-driver.hh"
+#include "calc++-parser.hh"
-@item 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.
+calcxx_driver::calcxx_driver ()
+ : trace_scanning (false), trace_parsing (false)
+@{
+ variables["one"] = 1;
+ variables["two"] = 2;
+@}
-@item YYSTYPE
-Macro for the data type of semantic values; @code{int} by default.
-@xref{Value Type, ,Data Types of Semantic Values}.
+calcxx_driver::~calcxx_driver ()
+@{
+@}
-@item 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}.
+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 ();
+@}
-@item yyclearin
-Macro used in error-recovery rule actions. It clears the previous
-look-ahead token. @xref{Error Recovery}.
+void
+calcxx_driver::error (const yy::location& l, const std::string& m)
+@{
+ std::cerr << l << ": " << m << std::endl;
+@}
-@item 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}.
+void
+calcxx_driver::error (const std::string& m)
+@{
+ std::cerr << m << std::endl;
+@}
+@end example
-@item yyerrok
-Macro to cause parser to recover immediately to its normal mode
-after a parse error. @xref{Error Recovery}.
+@node Calc++ Parser
+@subsection Calc++ Parser
-@item yyerror
-User-supplied function to be called by @code{yyparse} on error. The
-function receives one argument, a pointer to a character string
-containing an error message. @xref{Error Reporting, ,The Error
-Reporting Function @code{yyerror}}.
+The parser definition file @file{calc++-parser.yy} starts by asking for
+the C++ LALR(1) skeleton, the creation of the parser header file, and
+specifies the name of the parser class. Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
-@item yylex
-User-supplied lexical analyzer function, called with no arguments to get
-the next token. @xref{Lexical, ,The Lexical Analyzer Function
-@code{yylex}}.
+@comment file: calc++-parser.yy
+@example
+%skeleton "lalr1.cc" /* -*- C++ -*- */
+%require "2.1a"
+%defines
+%define "parser_class_name" "calcxx_parser"
+@end example
-@item 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}.
+@noindent
+Then come the declarations/inclusions needed to define the
+@code{%union}. Because the parser uses the parsing driver and
+reciprocally, both cannot include the header of the other. Because the
+driver's header needs detailed knowledge about the parser class (in
+particular its inner types), it is the parser's header which will simply
+use a forward declaration of the driver.
-@item 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 Positions,
-,Textual Positions of Tokens}.
+@comment file: calc++-parser.yy
+@example
+%@{
+# include <string>
+class calcxx_driver;
+%@}
+@end example
-@item yynerrs
-Global variable which Bison increments each time there is a parse error.
-(In a pure parser, it is a local variable within @code{yyparse}.)
-@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@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.
-@item yyparse
-The parser function produced by Bison; call this function to start
-parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@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 = @@$.end.filename = &driver.file;
+@};
+@end example
+
+@noindent
+Use the two following directives to enable parser tracing and verbose
+error messages.
+
+@comment file: calc++-parser.yy
+@example
+%debug
+%error-verbose
+@end example
+
+@noindent
+Semantic values cannot use ``real'' objects, but only pointers to
+them.
+
+@comment file: calc++-parser.yy
+@example
+// Symbols.
+%union
+@{
+ int ival;
+ std::string *sval;
+@};
+@end example
+
+@noindent
+The code between @samp{%@{} and @samp{%@}} after the introduction of the
+@samp{%union} is output in the @file{*.cc} file; it needs detailed
+knowledge about the driver.
+
+@comment file: calc++-parser.yy
+@example
+%@{
+# include "calc++-driver.hh"
+%@}
+@end example
+
+
+@noindent
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$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 END 0 "end of file"
+%token ASSIGN ":="
+%token <sval> IDENTIFIER "identifier"
+%token <ival> 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 () << $$; @} "number" "expression"
+@end example
+
+@noindent
+The grammar itself is straightforward.
+
+@comment file: calc++-parser.yy
+@example
+%%
+%start unit;
+unit: assignments exp @{ driver.result = $2; @};
+
+assignments: assignments assignment @{@}
+ | /* Nothing. */ @{@};
+
+assignment: "identifier" ":=" exp @{ driver.variables[*$1] = $3; @};
+
+%left '+' '-';
+%left '*' '/';
+exp: exp '+' exp @{ $$ = $1 + $3; @}
+ | exp '-' exp @{ $$ = $1 - $3; @}
+ | exp '*' exp @{ $$ = $1 * $3; @}
+ | exp '/' exp @{ $$ = $1 / $3; @}
+ | "identifier" @{ $$ = driver.variables[*$1]; @}
+ | "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 <cstdlib>
+# include <errno.h>
+# include <limits.h>
+# 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 accurately. 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.
+It is convenient to use a typedef to shorten
+@code{yy::calcxx_parser::token::identifier} into
+@code{token::identifier} for instance.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+ typedef yy::calcxx_parser::token token;
+%@}
+
+[-+*/] return yytext[0];
+":=" return token::ASSIGN;
+@{int@} @{
+ errno = 0;
+ long n = strtol (yytext, NULL, 10);
+ if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
+ driver.error (*yylloc, "integer is out of range");
+ yylval->ival = n;
+ 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, 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}.
+
+@node How Can I Reset the Parser
+@section How Can I Reset the Parser
+
+The following phenomenon has several symptoms, resulting in the
+following typical questions:
+
+@display
+I invoke @code{yyparse} several times, and on correct input it works
+properly; but when a parse error is found, all the other calls fail
+too. How can I reset the error flag of @code{yyparse}?
+@end display
+
+@noindent
+or
+
+@display
+My parser includes support for an @samp{#include}-like feature, in
+which case I run @code{yyparse} from @code{yyparse}. This fails
+although I did specify I needed a @code{%pure-parser}.
+@end display
+
+These problems typically come not from Bison itself, but from
+Lex-generated scanners. Because these scanners use large buffers for
+speed, they might not notice a change of input file. As a
+demonstration, consider the following source file,
+@file{first-line.l}:
+
+@verbatim
+%{
+#include <stdio.h>
+#include <stdlib.h>
+%}
+%%
+.*\n ECHO; return 1;
+%%
+int
+yyparse (char const *file)
+{
+ yyin = fopen (file, "r");
+ if (!yyin)
+ exit (2);
+ /* One token only. */
+ yylex ();
+ if (fclose (yyin) != 0)
+ exit (3);
+ return 0;
+}
+
+int
+main (void)
+{
+ yyparse ("input");
+ yyparse ("input");
+ return 0;
+}
+@end verbatim
+
+@noindent
+If the file @file{input} contains
+
+@verbatim
+input:1: Hello,
+input:2: World!
+@end verbatim
+
+@noindent
+then instead of getting the first line twice, you get:
+
+@example
+$ @kbd{flex -ofirst-line.c first-line.l}
+$ @kbd{gcc -ofirst-line first-line.c -ll}
+$ @kbd{./first-line}
+input:1: Hello,
+input:2: World!
+@end example
+
+Therefore, whenever you change @code{yyin}, you must tell the
+Lex-generated scanner to discard its current buffer and switch to the
+new one. This depends upon your implementation of Lex; see its
+documentation for more. For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your
+Flex-generated scanner needs to read from several input streams to
+handle features like include files, you might consider using Flex
+functions like @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
+
+@node Strings are Destroyed
+@section Strings are Destroyed
+
+@display
+My parser seems to destroy old strings, or maybe it loses track of
+them. Instead of reporting @samp{"foo", "bar"}, it reports
+@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
+@end display
+
+This error is probably the single most frequent ``bug report'' sent to
+Bison lists, but is only concerned with a misunderstanding of the role
+of scanner. Consider the following Lex code:
+
+@verbatim
+%{
+#include <stdio.h>
+char *yylval = NULL;
+%}
+%%
+.* yylval = yytext; return 1;
+\n /* IGNORE */
+%%
+int
+main ()
+{
+ /* Similar to using $1, $2 in a Bison action. */
+ char *fst = (yylex (), yylval);
+ char *snd = (yylex (), yylval);
+ printf ("\"%s\", \"%s\"\n", fst, snd);
+ return 0;
+}
+@end verbatim
+
+If you compile and run this code, you get:
+
+@example
+$ @kbd{flex -osplit-lines.c split-lines.l}
+$ @kbd{gcc -osplit-lines split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"one
+two", "two"
+@end example
+
+@noindent
+this is because @code{yytext} is a buffer provided for @emph{reading}
+in the action, but if you want to keep it, you have to duplicate it
+(e.g., using @code{strdup}). Note that the output may depend on how
+your implementation of Lex handles @code{yytext}. For instance, when
+given the Lex compatibility option @option{-l} (which triggers the
+option @samp{%array}) Flex generates a different behavior:
+
+@example
+$ @kbd{flex -l -osplit-lines.c split-lines.l}
+$ @kbd{gcc -osplit-lines split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"two", "two"
+@end example
+
+
+@node Implementing Gotos/Loops
+@section Implementing Gotos/Loops
+
+@display
+My simple calculator supports variables, assignments, and functions,
+but how can I implement gotos, or loops?
+@end display
+
+Although very pedagogical, the examples included in the document blur
+the distinction to make between the parser---whose job is to recover
+the structure of a text and to transmit it to subsequent modules of
+the program---and the processing (such as the execution) of this
+structure. This works well with so called straight line programs,
+i.e., precisely those that have a straightforward execution model:
+execute simple instructions one after the others.
+
+@cindex abstract syntax tree
+@cindex @acronym{AST}
+If you want a richer model, you will probably need to use the parser
+to construct a tree that does represent the structure it has
+recovered; this tree is usually called the @dfn{abstract syntax tree},
+or @dfn{@acronym{AST}} for short. Then, walking through this tree,
+traversing it in various ways, will enable treatments such as its
+execution or its translation, which will result in an interpreter or a
+compiler.
+
+This topic is way beyond the scope of this manual, and the reader is
+invited to consult the dedicated literature.
+
+
+
+@c ================================================= Table of Symbols
+
+@node Table of Symbols
+@appendix Bison Symbols
+@cindex Bison symbols, table of
+@cindex symbols in Bison, table of
+
+@deffn {Variable} @@$
+In an action, the location of the left-hand side of the rule.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} @@@var{n}
+In an action, the location of the @var{n}-th symbol of the right-hand
+side of the rule. @xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+@end deffn
+
+@deffn {Variable} $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule. @xref{Actions}.
+@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}.
+@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}.
+@end deffn
+
+@deffn {Construct} /*@dots{}*/
+Comment delimiters, as in C.
+@end deffn
+
+@deffn {Delimiter} :
+Separates a rule's result from its components. @xref{Rules, ,Syntax of
+Grammar Rules}.
+@end deffn
+
+@deffn {Delimiter} ;
+Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
+
+@deffn {Delimiter} |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
+@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.
+@end deffn
-@item %debug
+@deffn {Directive} %debug
Equip the parser for debugging. @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
-@item %defines
+@deffn {Directive} %defines
Bison declaration to create a header file meant for the scanner.
@xref{Decl Summary}.
+@end deffn
-@item %dprec
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} %dprec
Bison declaration to assign a precedence to a rule that is used at parse
-time to resolve reduce/reduce conflicts. @xref{GLR Parsers}.
+time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing
+@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
-@item %file-prefix="@var{prefix}"
+@deffn {Directive} %error-verbose
+Bison declaration to request verbose, specific error message strings
+when @code{yyerror} is called.
+@end deffn
+
+@deffn {Directive} %file-prefix="@var{prefix}"
Bison declaration to set the prefix of the output files. @xref{Decl
Summary}.
+@end deffn
-@item %glr-parser
-Bison declaration to produce a GLR parser. @xref{GLR Parsers}.
+@deffn {Directive} %glr-parser
+Bison declaration to produce a @acronym{GLR} parser. @xref{GLR
+Parsers, ,Writing @acronym{GLR} Parsers}.
+@end deffn
-@c @item %source-extension
-@c Bison declaration to specify the generated parser output file extension.
-@c @xref{Decl Summary}.
-@c
-@c @item %header-extension
-@c Bison declaration to specify the generated parser header file extension
-@c if required. @xref{Decl Summary}.
+@deffn {Directive} %initial-action
+Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}.
+@end deffn
-@item %left
+@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{argument-declaration}@}
+Bison declaration to specifying an additional parameter that
+@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions
+for Pure Parsers}.
+@end deffn
-@item %merge
+@deffn {Directive} %merge
Bison declaration to assign a merging function to a rule. If there is a
reduce/reduce conflict with a rule having the same merging function, the
function is applied to the two semantic values to get a single result.
-@xref{GLR Parsers}.
+@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}.
+@end deffn
-@item %name-prefix="@var{prefix}"
+@deffn {Directive} %name-prefix="@var{prefix}"
Bison declaration to rename the external symbols. @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
-@item %no-lines
+@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
parser file. @xref{Decl Summary}.
+@end deffn
-@item %nonassoc
-Bison declaration to assign non-associativity to token(s).
+@deffn {Directive} %nonassoc
+Bison declaration to assign nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %output="@var{filename}"
+@deffn {Directive} %output="@var{file}"
Bison declaration to set the name of the parser file. @xref{Decl
Summary}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+Bison declaration to specifying an additional parameter that
+@code{yyparse} should accept. @xref{Parser Function,, The Parser
+Function @code{yyparse}}.
+@end deffn
-@item %prec
+@deffn {Directive} %prec
Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
+@end deffn
-@item %pure-parser
+@deffn {Directive} %pure-parser
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+@end deffn
-@item %right
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %start
+@deffn {Directive} %start
Bison declaration to specify the start symbol. @xref{Start Decl, ,The
Start-Symbol}.
+@end deffn
-@item %token
+@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
+@end deffn
-@item %token-table
+@deffn {Directive} %token-table
Bison declaration to include a token name table in the parser file.
@xref{Decl Summary}.
+@end deffn
-@item %type
+@deffn {Directive} %type
Bison declaration to declare nonterminals. @xref{Type Decl,
,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
-@item %union
+@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
-@end table
+@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
-@table @samp
-@item %%
-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
-@item %@{ %@}
-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
+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
-@item /*@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
-@item :
-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
-@item ;
-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
-@item |
-Separates alternate rules for the same result nonterminal.
-@xref{Rules, ,Syntax of Grammar Rules}.
-@end table
+@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}. 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 {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}.
+In semantic actions, it stores the location of the look-ahead token.
+@xref{Actions and Locations, ,Actions and Locations}.
+@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}.
+In semantic actions, it stores the semantic value of the look-ahead token.
+@xref{Actions, ,Actions}.
+@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 it reports 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.
+
+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
@appendix Glossary
@cindex glossary
@table @asis
-@item Backus-Naur Form (BNF)
-Formal method of specifying context-free grammars. BNF was first used
-in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar,
-,Languages and Context-Free Grammars}.
+@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
+Formal method of specifying context-free grammars originally proposed
+by John Backus, and slightly improved by Peter Naur in his 1960-01-02
+committee document contributing to what became the Algol 60 report.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
machine moves from state to state as specified by the logic of the
machine. In the case of the parser, the input is the language being
parsed, and the states correspond to various stages in the grammar
-rules. @xref{Algorithm, ,The Bison Parser Algorithm }.
+rules. @xref{Algorithm, ,The Bison Parser Algorithm}.
-@item Generalized LR (GLR)
+@item Generalized @acronym{LR} (@acronym{GLR})
A parsing algorithm that can handle all context-free grammars, including those
-that are not LALR(1). It resolves situations that Bison's usual LALR(1)
+that are not @acronym{LALR}(1). It resolves situations that Bison's
+usual @acronym{LALR}(1)
algorithm cannot by effectively splitting off multiple parsers, trying all
possible parsers, and discarding those that fail in the light of additional
-right context. @xref{Generalized LR Parsing, ,Generalized LR Parsing}.
+right context. @xref{Generalized LR Parsing, ,Generalized
+@acronym{LR} Parsing}.
@item Grouping
A language construct that is (in general) grammatically divisible;
-for example, `expression' or `declaration' in C.
+for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item Infix operator
@item Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from
-left to right. @xref{Algorithm, ,The Bison Parser Algorithm }.
+left to right. @xref{Algorithm, ,The Bison Parser Algorithm}.
@item Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one.
A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead
Tokens}.
-@item LALR(1)
+@item @acronym{LALR}(1)
The class of context-free grammars that Bison (like most other parser
-generators) can handle; a subset of LR(1). @xref{Mystery Conflicts, ,
-Mysterious Reduce/Reduce Conflicts}.
+generators) can handle; a subset of @acronym{LR}(1). @xref{Mystery
+Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
-@item LR(1)
+@item @acronym{LR}(1)
The class of context-free grammars in which at most one token of
look-ahead is needed to disambiguate the parsing of any piece of input.
be expressed through rules in terms of smaller constructs; in other
words, a construct that is not a token. @xref{Symbols}.
-@item Parse error
-An error encountered during parsing of an input stream due to invalid
-syntax. @xref{Error Recovery}.
-
@item Parser
A function that recognizes valid sentences of a language by analyzing
the syntax structure of a set of tokens passed to it from a lexical
@item Reduction
Replacing a string of nonterminals and/or terminals with a single
nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison
-Parser Algorithm }.
+Parser Algorithm}.
@item Reentrant
A reentrant subprogram is a subprogram which can be in invoked any
@item Shift
A parser is said to shift when it makes the choice of analyzing
further input from the stream rather than reducing immediately some
-already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm }.
+already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm}.
@item Single-character literal
A single character that is recognized and interpreted as is.
during parsing to allow for recognition and use of existing
information in repeated uses of a symbol. @xref{Multi-function Calc}.
+@item Syntax error
+An error encountered during parsing of an input stream due to invalid
+syntax. @xref{Error Recovery}.
+
@item Token
A basic, grammatically indivisible unit of a language. The symbol
that describes a token in the grammar is a terminal symbol.
@printindex cp
@bye
+
+@c LocalWords: texinfo setfilename settitle setchapternewpage finalout
+@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex
+@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry
+@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa
+@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc
+@c LocalWords: rpcalc Lexer Gen Comp Expr ltcalc mfcalc Decl Symtab yylex
+@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref
+@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex
+@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge
+@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG
+@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit
+@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok
+@c LocalWords: longjmp fprintf stderr preg yylloc YYLTYPE cos ln
+@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym
+@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof
+@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum
+@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype
+@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless typefull yynerrs
+@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES
+@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param
+@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
+@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
+@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
+@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
+@c LocalWords: yyrestart nbar yytext fst snd osplit ntwo strdup AST
+@c LocalWords: YYSTACK DVI fdl printindex
projects / bison.git / blobdiff