@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
@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 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
* 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.
+* Simple GLR Parsers:: Using GLR in its simplest form.
+* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
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
* Token Decl:: Declaring terminal symbols.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* 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.
* 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}).
* 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.
* Bison Options:: All the options described in detail,
in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
+* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
Frequently Asked Questions
* Parser Stack Overflow:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Loops:: Control Flow in the Calculator
Copying This Manual
practical conditions for using Bison match the practical conditions for
using the other @acronym{GNU} tools.
-This exception applies only when Bison is generating C code for a
+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
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.
+* Simple GLR Parsers:: Using GLR in its simplest form.
* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
@cindex non-deterministic 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 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 @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.
+
+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 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 @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:
@cindex conflicts
@cindex shift/reduce conflicts
-In some grammars, there will be cases where 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
+In some grammars, there will be cases where 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
-@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
+(@pxref{Grammar Outline}), the result will be 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
@example
%@{
- #define YYSTYPE const char*
+ #include <stdio.h>
+ #define YYSTYPE char const *
+ int yylex (void);
+ void yyerror (char const *);
%@}
%token TYPENAME ID
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
"x" y z + T <init-declare> x T <cast> y z + = <OR>
@end example
+@sp 1
+
+@cindex @code{incline}
+@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 Simple GLR Parsers
+@section Using @acronym{GLR} in its Simplest Form
+@cindex @acronym{GLR} parsing, unambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
+@findex %glr-parser
+@findex %expect-rr
+@cindex conflicts
+@cindex reduce/reduce conflicts
+
+The C++ example for @acronym{GLR} (@pxref{GLR Parsers}) explains how to use
+the @acronym{GLR} parsing algorithm with some advanced features such as
+@samp{%dprec} and @samp{%merge} to handle syntactically ambiguous
+grammars. However, the @acronym{GLR} algorithm can also be used in a simpler
+way to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
+Such grammars typically require more than one symbol of lookahead,
+or (in rare cases) fall into the category of grammars in which the
+@acronym{LALR}(1) algorithm throws away too much information (they are in
+@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+
+Here is an example of this situation, using a problem that
+arises in the declaration of enumerated and subrange types in the
+programming language Pascal. These declarations look like this:
+
+@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 (ISO/IEC 10206:1990) and many other
+Pascal implementations allow arbitrary expressions there. This gives
+rise to the following situation, containing a superfluous pair of
+parentheses:
+
+@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 solution to this problem is to use a @acronym{GLR} parser in its simplest
+form, i.e., without using special features such as @samp{%dprec} and
+@samp{%merge}. When the @acronym{GLR} parser reaches the critical state, it
+simply splits into two branches and pursues both syntax rules
+simultaneously. Sooner or later, one of them runs into a parsing
+error. If there is a @samp{..} token before the next
+@samp{;}, the rule for enumerated types fails since it cannot
+accept @samp{..} anywhere; otherwise, the subrange type rule
+fails since it requires a @samp{..} token. So one of the branches
+fails silently, and the other one continues normally, performing
+all the intermediate actions that were postponed during the split.
+
+If the input is syntactically incorrect, both branches fail and the parser
+reports a syntax error as usual.
+
+The effect of all this is that the parser seems to ``guess'' the
+correct branch to take, or in other words, it seems to use more
+look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
+for. In this example, @acronym{LALR}(2) would suffice, but also some cases
+that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+
+Since there can be only two branches and at least one of them
+must fail, you need not worry about merging the branches by
+using dynamic precedence or @samp{%merge}.
+
+Another potential problem of @acronym{GLR} does not arise here, either. In
+general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
+and the current Bison parser even takes exponential time and space
+for some grammars. In practice, this rarely happens, and for many
+grammars it is possible to prove that it cannot happen. In
+in the present example, there is only one conflict between two
+rules, and the type-declaration context where the conflict
+arises cannot be nested. So the number of
+branches that can exist at any time is limited by the constant 2,
+and the parsing time is still linear.
+
+So here we have a case where we can use the benefits of @acronym{GLR}, almost
+without disadvantages. There are two things to note, though.
+First, one should carefully analyze the conflicts reported by
+Bison to make sure that @acronym{GLR} splitting is done only where it is
+intended to be. A @acronym{GLR} parser splitting inadvertently may cause
+problems less obvious than an @acronym{LALR} parser statically choosing the
+wrong alternative in a conflict.
+
+Second, interactions with the lexer (@pxref{Semantic Tokens}) must
+be considered with great care. Since a split parser consumes tokens
+without performing any actions during the split, the lexer cannot
+obtain information via parser actions. Some cases of
+lexer interactions can simply be eliminated by using @acronym{GLR}, i.e.,
+shifting the complications from the lexer to the parser. Remaining
+cases have to be checked for safety.
+
+In our example, it would be safe for the lexer to return tokens
+based on their current meanings in some symbol table, because no new
+symbols are defined in the middle of a type declaration. Though it
+is possible for a parser to define the enumeration
+constants as they are parsed, before the type declaration is
+completed, it actually makes no difference since they cannot be used
+within the same enumerated type declaration.
+
+Here is a Bison grammar corresponding to the example above. It
+parses a vastly simplified form of Pascal type declarations.
+
+@example
+%token TYPE DOTDOT ID
+
+@group
+%left '+' '-'
+%left '*' '/'
+@end group
+
+%%
+
+@group
+type_decl:
+ TYPE ID '=' type ';'
+;
+@end group
+
+@group
+type: '(' id_list ')'
+ | expr DOTDOT expr
+;
+@end group
+
+@group
+id_list: ID
+ | id_list ',' ID
+;
+@end group
+
+@group
+expr: '(' expr ')'
+ | expr '+' expr
+ | expr '-' expr
+ | expr '*' expr
+ | expr '/' expr
+ | ID
+;
+@end group
+@end example
+
+When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+about one reduce/reduce conflict. In the conflicting situation the
+parser chooses one of the alternatives, arbitrarily the one
+declared first. Therefore the following correct input is not
+recognized:
+
+@example
+type t = (a) .. b;
+@end example
+
+The parser can be turned into a @acronym{GLR} parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, simply by
+adding these two declarations to the Bison input file:
+
+@example
+%glr-parser
+%expect-rr 1
+@end example
+
+@noindent
+No change in the grammar itself is required. Now the
+parser recognizes all valid declarations, according to the
+limited syntax above, transparently. In fact, the user does not even
+notice when the parser splits.
@node Locations Overview
@section Locations
@cindex location
-@cindex textual 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
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
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
@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;
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
@group
-/* Data type for links in the chain of symbols. */
+/* 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 *, func_t);
+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;
@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}
* 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
@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 int he 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
+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.
read your program will be confused.
All the escape sequences used in string literals in C can be used in
-Bison as well. However, unlike Standard C, trigraphs have no special
+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).
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 *},
+@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
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{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
Here is a typical example:
@node Locations
@section Tracking Locations
@cindex location
-@cindex textual position
-@cindex position, textual
+@cindex textual location
+@cindex location, textual
Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional information,
especially symbol locations.
-@c (terminal or not) ?
-
The way locations are handled is defined by providing a data type, and
actions to take when rules are matched.
four members:
@example
-struct
+typedef struct YYLTYPE
@{
int first_line;
int first_column;
int last_line;
int last_column;
-@}
+@} YYLTYPE;
@end example
@node Actions and Locations
else
@{
$$ = 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
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 is an array holding locations of
+all right hand side elements of the rule being matched, and the third
+parameter is the size of the rule's right hand side. When processing
+a syntax error, the second parameter is an array holding locations of
+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 @acronym{LALR}(1) parsers:
+By default, @code{YYLLOC_DEFAULT} is defined this way for simple
+@acronym{LALR}(1) parsers:
@example
@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) \
+ ((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)
@end group
@end example
@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;
+# define YYLLOC_DEFAULT(yyCurrent, yyRhs, YYN) \
+ ((yyCurrent).first_line = YYRHSLOC(yyRhs, 1).first_line, \
+ (yyCurrent).first_column = YYRHSLOC(yyRhs, 1).first_column, \
+ (yyCurrent).last_line = YYRHSLOC(yyRhs, YYN).last_line, \
+ (yyCurrent).last_column = YYRHSLOC(yyRhs, YYN).last_column)
@end group
@end example
@item
For consistency with semantic actions, valid indexes for the location
array range from 1 to @var{n}.
+
+@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
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* 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.
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
+
+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}.
+
+Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.
@node Type Decl
Declare that the @var{code} must be invoked for each of the
@var{symbols} that will be discarded by the parser. The @var{code}
should use @code{$$} to designate the semantic value associated to the
-@var{symbols}. The additional parser parameters are also avaible
+@var{symbols}. The additional parser parameters are also available
(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
@strong{Warning:} as of Bison 1.875, this feature is still considered as
-experimental, as there was not enough users feedback. In particular,
-the syntax might still change, and for the time being, only the default
-@acronym{LALR}(1) skeleton supports this feature.
+experimental, as there was not enough user feedback. In particular,
+the syntax might still change.
@end deffn
For instance:
@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
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
+reduce/reduce conflicts. The usual warning is
given if there are either more or fewer conflicts, or if there are any
reduce/reduce conflicts.
+For normal LALR(1) parsers, reduce/reduce conflicts are more serious,
+and should be eliminated entirely. Bison will always report
+reduce/reduce conflicts for these parsers. With GLR parsers, however,
+both shift/reduce and reduce/reduce are routine (otherwise, there
+would be no need to use GLR parsing). Therefore, it is also possible
+to specify an expected number of reduce/reduce conflicts in GLR
+parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
+
In general, using @code{%expect} involves these steps:
@itemize @bullet
number which Bison printed.
@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 stop annoying you if you do not change the number of
+conflicts, but it will warn you again if changes in the grammar result
+in more or fewer conflicts.
@node Start Decl
@subsection The Start-Symbol
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
-@item %type
+@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
+
+@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
-@item %destructor
+@deffn {Directive} %destructor
Specifying 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},
@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
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}"
+@deffn {Directive} %output="@var{filename}"
Specify the @var{filename} 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}
-
-@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.
@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
parameter information to it in a reentrant way. To do so, use the
declaration @code{%parse-param}:
-@deffn {Directive} %parse-param @var{argument-declaration} @var{argument-name}
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
@findex %parse-param
-Declare that @code{argument-name} is an additional @code{yyparse}
-argument. This argument is also passed to @code{yyerror}. The
-@var{argument-declaration} is used when declaring functions or
-prototypes.
+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" "nastiness"
-%parse-param "int *randomness" "randomness"
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
@end example
@noindent
* 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
@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, ,
@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.
@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
Function}).
-@deffn {Directive} lex-param @var{argument-declaration} @var{argument-name}
+@deffn {Directive} lex-param @{@var{argument-declaration}@}
@findex %lex-param
-Declare that @code{argument-name} is an additional @code{yylex}
-argument.
+Declare that @code{argument-declaration} is an additional @code{yylex}
+argument declaration.
@end deffn
For instance:
@example
-%parse-param "int *nastiness" "nastiness"
-%lex-param "int *nastiness" "nastiness"
-%parse-param "int *randomness" "randomness"
+%parse-param @{int *nastiness@}
+%lex-param @{int *nastiness@}
+%parse-param @{int *randomness@}
@end example
@noindent
@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"}}.
+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{"parse error"}}.
+string instead of just plain @w{@code{"syntax error"}}.
The parser can detect one other kind of error: stack overflow. This
happens when the input contains constructions that are very deeply
@example
@group
void
-yyerror (const char *s)
+yyerror (char const *s)
@{
@end group
@group
@code{yyerror} are:
@example
-void yyerror (const char *msg); /* Yacc parsers. */
-void yyerror (YYLTYPE *locp, const char *msg); /* GLR parsers. */
+void yyerror (char const *msg); /* Yacc parsers. */
+void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
@end example
-If @samp{%parse-param "int *nastiness" "nastiness"} is used, then:
+If @samp{%parse-param @{int *nastiness@}} is used, then:
@example
-void yyerror (int *randomness, const char *msg); /* Yacc parsers. */
-void yyerror (int *randomness, const char *msg); /* GLR parsers. */
+void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */
+void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
@end example
Finally, GLR and Yacc parsers share the same @code{yyerror} calling
%locations
/* Pure yylex. */
%pure-parser
-%lex-param "int *nastiness" "nastiness"
+%lex-param @{int *nastiness@}
/* Pure yyparse. */
-%parse-param "int *nastiness" "nastiness"
-%parse-param "int *randomness" "randomness"
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
@end example
@noindent
int yyparse (int *nastiness, int *randomness);
void yyerror (YYLTYPE *locp,
int *nastiness, int *randomness,
- const char *msg);
+ char const *msg);
@end example
@noindent
-Please, note that the prototypes are only indications of how the code
-produced by Bison will use @code{yyerror}; you still have freedom on the
-exit value, and even on making @code{yyerror} a variadic function. It
-is precisely to enable this that the message is always passed last.
+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
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.
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
-@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}.
+@end deffn
-@item yychar
+@deffn {Variable} 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.
@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}.
+@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}.
+@end deffn
-@item @@$
+@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
-@end table
@node Algorithm
@chapter The Bison Parser Algorithm
@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
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
grammar, in particular, it is only slightly slower than with the default
Bison parser.
+For a more detailed exposition of GLR parsers, please see: Elizabeth
+Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
+Generalised @acronym{LR} Parsers, Royal Holloway, University of
+London, Department of Computer Science, TR-00-12,
+@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
+(2000-12-24).
+
@node Stack Overflow
@section Stack Overflow, and How to Avoid It
@cindex stack overflow
@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.
this token. Write the statement @samp{yyclearin;} in the error rule's
action.
-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
@example
@group
%@{
-int hexflag;
+ int hexflag;
+ int yylex (void);
+ void yyerror (char const *);
%@}
%%
@dots{}
@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
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
@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}).
@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}:
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
$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
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
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
@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.
* Option Cross Key:: Alphabetical list of long options.
+* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
@end menu
@node Bison Options
@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:
+for Yacc, and the Bison distribution contains such a script for
+compatibility with @acronym{POSIX}:
@example
-bison -y $*
+#! /bin/sh
+bison -y "$@@"
@end example
@end table
@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}.
Description of the grammar, conflicts (resolved and unresolved), and
@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
@end example
@end ifinfo
+@node Yacc Library
+@section Yacc Library
+
+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}).
+
+If you use the Yacc library's @code{yyerror} function, you should
+declare @code{yyerror} as follows:
+
+@example
+int yyerror (char const *);
+@end example
+
+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
+int yyparse (void);
+@end example
+
@c ================================================= Invoking Bison
@node FAQ
@menu
* Parser Stack Overflow:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Loops:: Control Flow in the Calculator
@end menu
@node Parser Stack Overflow
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 C++ Parsers
+@section C++ Parsers
+
+@display
+How can I generate parsers in C++?
+@end display
+
+We are working on a C++ output for Bison, but unfortunately, for lack
+of time, the skeleton is not finished. It is functional, but in
+numerous respects, it will require additional work which @emph{might}
+break backward compatibility. Since the skeleton for C++ is not
+documented, we do not consider ourselves bound to this interface,
+nevertheless, as much as possible we will try to keep compatibility.
+
+Another possibility is to use the regular C parsers, and to compile
+them with a C++ compiler. This works properly, provided that you bear
+some simple C++ rules in mind, such as not including ``real classes''
+(i.e., structure with constructors) in unions. Therefore, in the
+@code{%union}, use pointers to classes, or better yet, a single
+pointer type to the root of your lexical/syntactic hierarchy.
+
+
+@node Implementing Loops
+@section Implementing Loops
+
+@display
+My simple calculator supports variables, assignments, and functions,
+but how can I implement 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
@cindex Bison symbols, table of
@cindex symbols in Bison, table of
-@table @code
-@item @@$
+@deffn {Variable} @@$
In an action, the location of the left-hand side of the rule.
@xref{Locations, , Locations Overview}.
+@end deffn
-@item @@@var{n}
+@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
-@item $$
+@deffn {Variable} $$
In an action, the semantic value of the left-hand side of the rule.
@xref{Actions}.
+@end deffn
-@item $@var{n}
+@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
-@item $accept
+@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 $end
+@deffn {Symbol} $end
The predefined token marking the end of the token stream. It cannot be
used in the grammar.
+@end deffn
-@item $undefined
+@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 error
+@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 parse error, the
+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 YYABORT
+@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
-@item YYACCEPT
+@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
-@item YYBACKUP
+@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 YYDEBUG
+@deffn {Macro} YYDEBUG
Macro to define to equip the parser with tracing code. @xref{Tracing,
,Tracing Your Parser}.
+@end deffn
-@item YYERROR
+@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
-@item YYERROR_VERBOSE
-An obsolete macro that you define with @code{#define} in the Bison
-declarations section to request verbose, specific error message strings
+@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
-@item YYINITDEPTH
+@deffn {Macro} YYINITDEPTH
Macro for specifying the initial size of the parser stack.
@xref{Stack Overflow}.
+@end deffn
-@item YYLEX_PARAM
+@deffn {Macro} YYLEX_PARAM
An obsolete macro for specifying an extra argument (or list of extra
arguments) for @code{yyparse} to pass to @code{yylex}. he use of this
macro is deprecated, and is supported only for Yacc like parsers.
@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+@end deffn
-@item YYLTYPE
-Macro for the data type of @code{yylloc}; a structure with four
+@deffn {Type} YYLTYPE
+Data type of @code{yylloc}; by default, a structure with four
members. @xref{Location Type, , Data Types of Locations}.
+@end deffn
-@item yyltype
-Default value for YYLTYPE.
-
-@item YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack.
-@xref{Stack Overflow}.
+@deffn {Macro} YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack. @xref{Stack
+Overflow}.
+@end deffn
-@item YYPARSE_PARAM
+@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
-@item YYRECOVERING
+@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
-@item YYSTACK_USE_ALLOCA
+@deffn {Macro} YYSTACK_USE_ALLOCA
Macro used to control the use of @code{alloca}. If defined to @samp{0},
the parser will not use @code{alloca} but @code{malloc} when trying to
grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
to anything else.
+@end deffn
-@item YYSTYPE
-Macro for the data type of semantic values; @code{int} by default.
+@deffn {Type} YYSTYPE
+Data type of semantic values; @code{int} by default.
@xref{Value Type, ,Data Types of Semantic Values}.
+@end deffn
-@item yychar
+@deffn {Variable} yychar
External integer variable that contains the integer value of the current
look-ahead token. (In a pure parser, it is a local variable within
@code{yyparse}.) Error-recovery rule actions may examine this variable.
@xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
-@item yyclearin
+@deffn {Variable} yyclearin
Macro used in error-recovery rule actions. It clears the previous
look-ahead token. @xref{Error Recovery}.
+@end deffn
-@item yydebug
+@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 yyerrok
+@deffn {Macro} yyerrok
Macro to cause parser to recover immediately to its normal mode
-after a parse error. @xref{Error Recovery}.
+after a syntax error. @xref{Error Recovery}.
+@end deffn
-@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
+@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
-@item yylex
+@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
-@item yylval
+@deffn {Variable} yylval
External variable in which @code{yylex} should place the semantic
value associated with a token. (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
+@end deffn
-@item yylloc
+@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 Positions,
-,Textual Positions of Tokens}.
+@samp{@@} feature in the grammar actions. @xref{Token Locations,
+,Textual Locations of Tokens}.
+@end deffn
-@item yynerrs
-Global variable which Bison increments each time there is a parse error.
+@deffn {Variable} yynerrs
+Global variable which Bison increments each time there is a syntax error.
(In a pure parser, it is a local variable within @code{yyparse}.)
@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@end deffn
-@item yyparse
+@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
-@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 %destructor
+@deffn {Directive} %destructor
Specifying how the parser should reclaim the memory associated to
discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
-@item %dprec
+@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, ,Writing
@acronym{GLR} Parsers}.
+@end deffn
-@item %error-verbose
+@deffn {Directive} %error-verbose
Bison declaration to request verbose, specific error message strings
when @code{yyerror} is called.
+@end deffn
-@item %file-prefix="@var{prefix}"
+@deffn {Directive} %file-prefix="@var{prefix}"
Bison declaration to set the prefix of the output files. @xref{Decl
Summary}.
+@end deffn
-@item %glr-parser
+@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}.
-
-@item %left
+@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %lex-param "@var{argument-declaration}" "@var{argument-name}"
+@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, ,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
+@deffn {Directive} %nonassoc
Bison declaration to assign non-associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %output="@var{filename}"
+@deffn {Directive} %output="@var{filename}"
Bison declaration to set the name of the parser file. @xref{Decl
Summary}.
+@end deffn
-@item %parse-param "@var{argument-declaration}" "@var{argument-name}"
+@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} %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
-@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
These are the punctuation and delimiters used in Bison input:
-@table @samp
-@item %%
+@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
-@item %@{ %@}
+@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
-@item /*@dots{}*/
+@deffn {Construct} /*@dots{}*/
Comment delimiters, as in C.
+@end deffn
-@item :
+@deffn {Delimiter} :
Separates a rule's result from its components. @xref{Rules, ,Syntax of
Grammar Rules}.
+@end deffn
-@item ;
+@deffn {Delimiter} ;
Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
-@item |
+@deffn {Delimiter} |
Separates alternate rules for the same result nonterminal.
@xref{Rules, ,Syntax of Grammar Rules}.
-@end table
+@end deffn
@node Glossary
@appendix Glossary
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
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 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
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