X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/feeb0edaf114181320636090d082cef9cd144da2..cb530ce9e2687c6b1e51b77a29a9062ad52166b4:/doc/bison.texinfo diff --git a/doc/bison.texinfo b/doc/bison.texinfo index ef803899..a684e851 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -16,6 +16,10 @@ @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 @@ -40,12 +44,12 @@ This manual is for @acronym{GNU} Bison (version @value{VERSION}, @value{UPDATED}), the @acronym{GNU} parser generator. Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, -1999, 2000, 2001, 2002 Free Software Foundation, Inc. +1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. @quotation Permission is granted to copy, distribute and/or modify this document under the terms of the @acronym{GNU} Free Documentation License, -Version 1.1 or any later version published by the Free Software +Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled @@ -119,7 +123,8 @@ Reference sections: * 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 @@ -130,11 +135,19 @@ The Concepts of Bison a semantic value (the value of an integer, the name of an identifier, etc.). * Semantic Actions:: Each rule can have an action containing C code. +* GLR Parsers:: Writing parsers for general context-free languages. +* Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? * Stages:: Stages in writing and running Bison grammars. * Grammar Layout:: Overall structure of a Bison grammar file. +Writing @acronym{GLR} Parsers + +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler + Examples * RPN Calc:: Reverse polish notation calculator; @@ -143,8 +156,8 @@ Examples 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 @@ -182,15 +195,16 @@ Bison Grammar Files * 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 @@ -202,14 +216,21 @@ 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. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Initial Action Decl:: Code run before parsing starts. * Destructor Decl:: Declaring how symbols are freed. -* Expect Decl:: Suppressing warnings about 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. @@ -227,9 +248,9 @@ The Lexical Analyzer Function @code{yylex} * Calling Convention:: How @code{yyparse} calls @code{yylex}. * Token Values:: How @code{yylex} must return the semantic value of the token it has read. -* Token Positions:: How @code{yylex} must return the text position +* Token Locations:: How @code{yylex} must return the text location (line number, etc.) of the token, if the - actions want that. + actions want that. * Pure Calling:: How the calling convention differs in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). @@ -259,7 +280,7 @@ Handling Context Dependencies * 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. @@ -269,10 +290,15 @@ Invoking Bison * 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 Gotos/Loops:: Control Flow in the Calculator Copying This Manual @@ -337,7 +363,7 @@ encourage people to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for using the other @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 @@ -362,7 +388,7 @@ use Bison or Yacc, we suggest you start by reading this chapter carefully. a semantic value (the value of an integer, the name of an identifier, etc.). * Semantic Actions:: Each rule can have an action containing C code. -* GLR Parsers:: Writing parsers for general context-free languages +* GLR Parsers:: Writing parsers for general context-free languages. * Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? @@ -454,7 +480,7 @@ Here is a simple C function subdivided into tokens: @ifinfo @example int /* @r{keyword `int'} */ -square (int x) /* @r{identifier, open-paren, identifier,} +square (int x) /* @r{identifier, open-paren, keyword `int',} @r{identifier, close-paren} */ @{ /* @r{open-brace} */ return x * x; /* @r{keyword `return', identifier, asterisk, @@ -465,7 +491,7 @@ square (int x) /* @r{identifier, open-paren, identifier,} @ifnotinfo @example int /* @r{keyword `int'} */ -square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */ +square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */ @{ /* @r{open-brace} */ return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */ @} /* @r{close-brace} */ @@ -640,8 +666,9 @@ from the values of the two subexpressions. @findex %glr-parser @cindex conflicts @cindex shift/reduce conflicts +@cindex reduce/reduce conflicts -In some grammars, there will be cases where Bison's standard +In some grammars, Bison's standard @acronym{LALR}(1) parsing algorithm cannot decide whether to apply a certain grammar rule at a given point. That is, it may not be able to decide (on the basis of the input read so far) which of two possible @@ -654,7 +681,7 @@ input. These are known respectively as @dfn{reduce/reduce} conflicts To use a grammar that is not easily modified to be @acronym{LALR}(1), a more general parsing algorithm is sometimes necessary. If you include @code{%glr-parser} among the Bison declarations in your file -(@pxref{Grammar Outline}), the result will be a Generalized @acronym{LR} +(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR} (@acronym{GLR}) parser. These parsers handle Bison grammars that contain no unresolved conflicts (i.e., after applying precedence declarations) identically to @acronym{LALR}(1) parsers. However, when @@ -681,11 +708,225 @@ involved, or by performing both actions, and then calling a designated user-defined function on the resulting values to produce an arbitrary merged result. +@menu +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler +@end menu + +@node Simple GLR Parsers +@subsection Using @acronym{GLR} on Unambiguous Grammars +@cindex @acronym{GLR} parsing, unambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars +@findex %glr-parser +@findex %expect-rr +@cindex conflicts +@cindex reduce/reduce conflicts +@cindex shift/reduce conflicts + +In the simplest cases, you can use the @acronym{GLR} algorithm +to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1). +Such grammars typically require more than one symbol of look-ahead, +or (in rare cases) fall into the category of grammars in which the +@acronym{LALR}(1) algorithm throws away too much information (they are in +@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}). + +Consider a problem that +arises in the declaration of enumerated and subrange types in the +programming language Pascal. Here are some examples: + +@example +type subrange = lo .. hi; +type enum = (a, b, c); +@end example + +@noindent +The original language standard allows only numeric +literals and constant identifiers for the subrange bounds (@samp{lo} +and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC} +10206) and many other +Pascal implementations allow arbitrary expressions there. This gives +rise to the following situation, containing a superfluous pair of +parentheses: + +@example +type subrange = (a) .. b; +@end example + +@noindent +Compare this to the following declaration of an enumerated +type with only one value: + +@example +type enum = (a); +@end example + +@noindent +(These declarations are contrived, but they are syntactically +valid, and more-complicated cases can come up in practical programs.) + +These two declarations look identical until the @samp{..} token. +With normal @acronym{LALR}(1) one-token look-ahead it is not +possible to decide between the two forms when the identifier +@samp{a} is parsed. It is, however, desirable +for a parser to decide this, since in the latter case +@samp{a} must become a new identifier to represent the enumeration +value, while in the former case @samp{a} must be evaluated with its +current meaning, which may be a constant or even a function call. + +You could parse @samp{(a)} as an ``unspecified identifier in parentheses'', +to be resolved later, but this typically requires substantial +contortions in both semantic actions and large parts of the +grammar, where the parentheses are nested in the recursive rules for +expressions. + +You might think of using the lexer to distinguish between the two +forms by returning different tokens for currently defined and +undefined identifiers. But if these declarations occur in a local +scope, and @samp{a} is defined in an outer scope, then both forms +are possible---either locally redefining @samp{a}, or using the +value of @samp{a} from the outer scope. So this approach cannot +work. + +A simple solution to this problem is to declare the parser to +use the @acronym{GLR} algorithm. +When the @acronym{GLR} parser reaches the critical state, it +merely splits into two branches and pursues both syntax rules +simultaneously. Sooner or later, one of them runs into a parsing +error. If there is a @samp{..} token before the next +@samp{;}, the rule for enumerated types fails since it cannot +accept @samp{..} anywhere; otherwise, the subrange type rule +fails since it requires a @samp{..} token. So one of the branches +fails silently, and the other one continues normally, performing +all the intermediate actions that were postponed during the split. + +If the input is syntactically incorrect, both branches fail and the parser +reports a syntax error as usual. + +The effect of all this is that the parser seems to ``guess'' the +correct branch to take, or in other words, it seems to use more +look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows +for. In this example, @acronym{LALR}(2) would suffice, but also some cases +that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way. + +In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time, +and the current Bison parser even takes exponential time and space +for some grammars. In practice, this rarely happens, and for many +grammars it is possible to prove that it cannot happen. +The present example contains only one conflict between two +rules, and the type-declaration context containing the conflict +cannot be nested. So the number of +branches that can exist at any time is limited by the constant 2, +and the parsing time is still linear. + +Here is a Bison grammar corresponding to the example above. It +parses a vastly simplified form of Pascal type declarations. + +@example +%token TYPE DOTDOT ID + +@group +%left '+' '-' +%left '*' '/' +@end group + +%% + +@group +type_decl : TYPE ID '=' type ';' + ; +@end group + +@group +type : '(' id_list ')' + | expr DOTDOT expr + ; +@end group + +@group +id_list : ID + | id_list ',' ID + ; +@end group + +@group +expr : '(' expr ')' + | expr '+' expr + | expr '-' expr + | expr '*' expr + | expr '/' expr + | ID + ; +@end group +@end example + +When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains +about one reduce/reduce conflict. In the conflicting situation the +parser chooses one of the alternatives, arbitrarily the one +declared first. Therefore the following correct input is not +recognized: + +@example +type t = (a) .. b; +@end example + +The parser can be turned into a @acronym{GLR} parser, while also telling Bison +to be silent about the one known reduce/reduce conflict, by +adding these two declarations to the Bison input file (before the first +@samp{%%}): + +@example +%glr-parser +%expect-rr 1 +@end example + +@noindent +No change in the grammar itself is required. Now the +parser recognizes all valid declarations, according to the +limited syntax above, transparently. In fact, the user does not even +notice when the parser splits. + +So here we have a case where we can use the benefits of @acronym{GLR}, almost +without disadvantages. Even in simple cases like this, however, there +are at least two potential problems to beware. +First, always analyze the conflicts reported by +Bison to make sure that @acronym{GLR} splitting is only done where it is +intended. A @acronym{GLR} parser splitting inadvertently may cause +problems less obvious than an @acronym{LALR} parser statically choosing the +wrong alternative in a conflict. +Second, consider interactions with the lexer (@pxref{Semantic Tokens}) +with great care. Since a split parser consumes tokens +without performing any actions during the split, the lexer cannot +obtain information via parser actions. Some cases of +lexer interactions can be eliminated by using @acronym{GLR} to +shift the complications from the lexer to the parser. You must check +the remaining cases for correctness. + +In our example, it would be safe for the lexer to return tokens +based on their current meanings in some symbol table, because no new +symbols are defined in the middle of a type declaration. Though it +is possible for a parser to define the enumeration +constants as they are parsed, before the type declaration is +completed, it actually makes no difference since they cannot be used +within the same enumerated type declaration. + +@node Merging GLR Parses +@subsection Using @acronym{GLR} to Resolve Ambiguities +@cindex @acronym{GLR} parsing, ambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars +@findex %dprec +@findex %merge +@cindex conflicts +@cindex reduce/reduce conflicts + Let's consider an example, vastly simplified from a C++ grammar. @example %@{ - #define YYSTYPE const char* + #include + #define YYSTYPE char const * + int yylex (void); + void yyerror (char const *); %@} %token TYPENAME ID @@ -737,8 +978,21 @@ parses as either an @code{expr} or a @code{stmt} @samp{x} as an @code{ID}). Bison detects this as a reduce/reduce conflict between the rules @code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the -time it encounters @code{x} in the example above. The two @code{%dprec} -declarations, however, give precedence to interpreting the example as a +time it encounters @code{x} in the example above. Since this is a +@acronym{GLR} parser, it therefore splits the problem into two parses, one for +each choice of resolving the reduce/reduce conflict. +Unlike the example from the previous section (@pxref{Simple GLR Parsers}), +however, neither of these parses ``dies,'' because the grammar as it stands is +ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and +the other reduces @code{stmt : decl}, after which both parsers are in an +identical state: they've seen @samp{prog stmt} and have the same unprocessed +input remaining. We say that these parses have @dfn{merged.} + +At this point, the @acronym{GLR} parser requires a specification in the +grammar of how to choose between the competing parses. +In the example above, the two @code{%dprec} +declarations specify that Bison is to give precedence +to the parse that interprets the example as a @code{decl}, which implies that @code{x} is a declarator. The parser therefore prints @@ -746,18 +1000,21 @@ The parser therefore prints "x" y z + T @end example -Consider a different input string for this parser: +The @code{%dprec} declarations only come into play when more than one +parse survives. Consider a different input string for this parser: @example T (x) + y; @end example @noindent +This is another example of using @acronym{GLR} to parse an unambiguous +construct, as shown in the previous section (@pxref{Simple GLR Parsers}). Here, there is no ambiguity (this cannot be parsed as a declaration). However, at the time the Bison parser encounters @code{x}, it does not have enough information to resolve the reduce/reduce conflict (again, between @code{x} as an @code{expr} or a @code{declarator}). In this -case, no precedence declaration is used. Instead, the parser splits +case, no precedence declaration is used. Again, the parser splits into two, one assuming that @code{x} is an @code{expr}, and the other assuming @code{x} is a @code{declarator}. The second of these parsers then vanishes when it sees @code{+}, and the parser prints @@ -767,7 +1024,7 @@ x T y + @end example Suppose that instead of resolving the ambiguity, you wanted to see all -the possibilities. For this purpose, we must @dfn{merge} the semantic +the possibilities. For this purpose, you must merge the semantic actions of the two possible parsers, rather than choosing one over the other. To do so, you could change the declaration of @code{stmt} as follows: @@ -779,11 +1036,11 @@ stmt : expr ';' %merge @end example @noindent - and define the @code{stmtMerge} function as: @example -static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1) +static YYSTYPE +stmtMerge (YYSTYPE x0, YYSTYPE x1) @{ printf (" "); return ""; @@ -796,32 +1053,40 @@ in the C declarations at the beginning of the file: @example %@{ - #define YYSTYPE const char* + #define YYSTYPE char const * static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1); %@} @end example @noindent -With these declarations, the resulting parser will parse the first example -as both an @code{expr} and a @code{decl}, and print +With these declarations, the resulting parser parses the first example +as both an @code{expr} and a @code{decl}, and prints @example "x" y z + T x T y z + = @end example -@sp 1 +Bison requires that all of the +productions that participate in any particular merge have identical +@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable, +and the parser will report an error during any parse that results in +the offending merge. -@cindex @code{incline} +@node Compiler Requirements +@subsection Considerations when Compiling @acronym{GLR} Parsers +@cindex @code{inline} @cindex @acronym{GLR} parsers and @code{inline} -Note that the @acronym{GLR} parsers require an ISO C89 compiler. In -addition, they use the @code{inline} keyword, which is not C89, but a -common extension. It is up to the user of these parsers to handle + +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 + #include %@} @end example @@ -830,21 +1095,21 @@ will suffice. Otherwise, we suggest @example %@{ -#if ! defined __GNUC__ && ! defined inline -# define inline -#endif + #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline + #define inline + #endif %@} @end example @node Locations Overview @section Locations @cindex location -@cindex textual position -@cindex position, textual +@cindex textual location +@cindex location, textual Many applications, like interpreters or compilers, have to produce verbose and useful error messages. To achieve this, one must be able to keep track of -the @dfn{textual position}, or @dfn{location}, of each syntactic construct. +the @dfn{textual location}, or @dfn{location}, of each syntactic construct. Bison provides a mechanism for handling these locations. Each token has a semantic value. In a similar fashion, each token has an @@ -992,6 +1257,9 @@ in every Bison grammar file to separate the sections. 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 @@ -1000,10 +1268,9 @@ semantic values of various symbols. 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 @@ -1070,8 +1337,10 @@ calculator. As in C, comments are placed between @samp{/*@dots{}*/}. /* Reverse polish notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + int yylex (void); + void yyerror (char const *); %@} %token NUM @@ -1080,7 +1349,7 @@ calculator. As in C, comments are placed between @samp{/*@dots{}*/}. @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 @@ -1093,6 +1362,12 @@ which is a floating point number. 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 @@ -1256,7 +1531,7 @@ not require it. You can add or change white space as much as you wish. For example, this: @example -exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} +exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ; @end example @noindent @@ -1266,6 +1541,7 @@ means the same thing as this: exp: NUM | exp exp '+' @{ $$ = $1 + $2; @} | @dots{} +; @end example @noindent @@ -1381,10 +1657,11 @@ here is the definition we will use: @group #include +/* 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 @@ -1482,23 +1759,25 @@ parentheses nested to arbitrary depth. Here is the Bison code for @file{calc.y}, an infix desk-top calculator. @example -/* Infix notation calculator--calc */ +/* Infix notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + #include + 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 ; @@ -1628,8 +1907,10 @@ the same as the declarations for the infix notation calculator. /* Location tracking calculator. */ %@{ -#define YYSTYPE int -#include + #define YYSTYPE int + #include + int yylex (void); + void yyerror (char const *); %@} /* Bison declarations. */ @@ -1640,7 +1921,7 @@ the same as the declarations for the infix notation calculator. %left NEG %right '^' -%% /* Grammar follows */ +%% /* The grammar follows. */ @end example @noindent @@ -1863,29 +2144,30 @@ Here are the C and Bison declarations for the multi-function calculator. @smallexample @group %@{ -#include /* For math functions, cos(), sin(), etc. */ -#include "calc.h" /* Contains definition of `symrec' */ + #include /* 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. */ @} @end group -%token NUM /* Simple double precision number. */ -%token VAR FNCT /* Variable and Function. */ +%token NUM /* Simple double precision number. */ +%token VAR FNCT /* Variable and Function. */ %type exp @group %right '=' %left '-' '+' %left '*' '/' -%left NEG /* Negation--unary minus */ -%right '^' /* Exponentiation */ +%left NEG /* negation--unary minus */ +%right '^' /* exponentiation */ @end group -/* Grammar follows */ -%% +%% /* The grammar follows. */ @end smallexample The above grammar introduces only two new features of the Bison language. @@ -1946,7 +2228,7 @@ exp: NUM @{ $$ = $1; @} | '(' exp ')' @{ $$ = $2; @} ; @end group -/* End of grammar */ +/* End of grammar. */ %% @end smallexample @@ -1965,33 +2247,33 @@ provides for either functions or variables to be placed in the table. @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 @@ -2003,17 +2285,9 @@ function that initializes the symbol table. Here it is, and #include @group -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); @} @@ -2022,13 +2296,13 @@ yyerror (const char *s) /* Called by yyparse on error. */ @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, @@ -2042,7 +2316,7 @@ struct init arith_fncts[] = @group /* The symbol table: a chain of `struct symrec'. */ -symrec *sym_table = (symrec *) 0; +symrec *sym_table; @end group @group @@ -2059,6 +2333,15 @@ init_table (void) @} @} @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 @@ -2073,7 +2356,7 @@ found, a pointer to that symbol is returned; otherwise zero is returned. @smallexample symrec * -putsym (char *sym_name, int sym_type) +putsym (char const *sym_name, int sym_type) @{ symrec *ptr; ptr = (symrec *) malloc (sizeof (symrec)); @@ -2087,7 +2370,7 @@ putsym (char *sym_name, int sym_type) @} symrec * -getsym (const char *sym_name) +getsym (char const *sym_name) @{ symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; @@ -2245,7 +2528,7 @@ appropriate delimiters: @example %@{ -@var{Prologue} + @var{Prologue} %@} @var{Bison declarations} @@ -2268,7 +2551,7 @@ continues until end of line. * 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 @@ -2292,18 +2575,18 @@ can be done with two @var{Prologue} blocks, one before and one after the @smallexample %@{ -#include -#include "ptypes.h" + #include + #include "ptypes.h" %@} %union @{ - long n; + long int n; tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ @} %@{ -static void print_token_value (FILE *, int, YYSTYPE); -#define YYPRINT(F, N, L) print_token_value (F, N, L) + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) %@} @dots{} @@ -2331,7 +2614,7 @@ There must always be at least one grammar rule, and the first @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 @@ -2341,14 +2624,17 @@ The @var{Epilogue} is copied verbatim to the end of the parser file, just as 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. @@ -2437,7 +2723,8 @@ does not enforce this convention, but if you depart from it, people who 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). @@ -2710,7 +2997,7 @@ This macro definition must go in the prologue of the grammar file 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 @@ -2757,9 +3044,10 @@ Actions, ,Actions in Mid-Rule}). The C code in an action can refer to the semantic values of the components matched by the rule with the construct @code{$@var{n}}, which stands for the value of the @var{n}th component. The semantic value for the grouping -being constructed is @code{$$}. (Bison translates both of these constructs -into array element references when it copies the actions into the parser -file.) +being constructed is @code{$$}. Bison translates both of these +constructs into expressions of the appropriate type when it copies the +actions into the parser file. @code{$$} is translated to a modifiable +lvalue, so it can be assigned to. Here is a typical example: @@ -3035,15 +3323,13 @@ actually does to implement mid-rule actions. @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. @@ -3066,13 +3352,13 @@ When @code{YYLTYPE} is not defined, Bison uses a default structure type with 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 @@ -3108,9 +3394,10 @@ exp: @dots{} else @{ $$ = 1; - printf("Division by zero, l%d,c%d-l%d,c%d", - @@3.first_line, @@3.first_column, - @@3.last_line, @@3.last_column); + fprintf (stderr, + "Division by zero, l%d,c%d-l%d,c%d", + @@3.first_line, @@3.first_column, + @@3.last_line, @@3.last_column); @} @} @end group @@ -3134,9 +3421,10 @@ exp: @dots{} else @{ $$ = 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 @@ -3150,40 +3438,48 @@ Actually, actions are not the best place to compute locations. Since locations are much more general than semantic values, there is room in the output parser to redefine the default action to take for each rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is -matched, before the associated action is run. +matched, before the associated action is run. It is also invoked +while processing a syntax error, to compute the error's location. Most of the time, this macro is general enough to suppress location dedicated code from semantic actions. The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is -the location of the grouping (the result of the computation). The second one -is an array holding locations of all right hand side elements of the rule -being matched. The last one is the size of the right hand side rule. +the location of the grouping (the result of the computation). When a +rule is matched, the second parameter identifies locations of +all right hand side elements of the rule being matched, and the third +parameter is the size of the rule's right hand side. When processing +a syntax error, the second parameter identifies locations of +the symbols that were discarded during error processing, and the third +parameter is the number of discarded symbols. -By default, it is defined this way for simple @acronym{LALR}(1) parsers: +By default, @code{YYLLOC_DEFAULT} is defined this way: -@example +@smallexample @group -#define YYLLOC_DEFAULT(Current, Rhs, N) \ - Current.first_line = Rhs[1].first_line; \ - Current.first_column = Rhs[1].first_column; \ - Current.last_line = Rhs[N].last_line; \ - Current.last_column = Rhs[N].last_column; +# define YYLLOC_DEFAULT(Current, Rhs, N) \ + do \ + if (N) \ + @{ \ + (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \ + (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \ + (Current).last_line = YYRHSLOC(Rhs, N).last_line; \ + (Current).last_column = YYRHSLOC(Rhs, N).last_column; \ + @} \ + else \ + @{ \ + (Current).first_line = (Current).last_line = \ + YYRHSLOC(Rhs, 0).last_line; \ + (Current).first_column = (Current).last_column = \ + YYRHSLOC(Rhs, 0).last_column; \ + @} \ + while (0) @end group -@end example - -@noindent -and like this for @acronym{GLR} parsers: +@end smallexample -@example -@group -#define YYLLOC_DEFAULT(Current, Rhs, N) \ - Current.first_line = YYRHSLOC(Rhs,1).first_line; \ - Current.first_column = YYRHSLOC(Rhs,1).first_column; \ - Current.last_line = YYRHSLOC(Rhs,N).last_line; \ - Current.last_column = YYRHSLOC(Rhs,N).last_column; -@end group -@end example +where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol +in @var{rhs} when @var{k} is positive, and the location of the symbol +just before the reduction when @var{k} and @var{n} are both zero. When defining @code{YYLLOC_DEFAULT}, you should consider that: @@ -3193,8 +3489,16 @@ All arguments are free of side-effects. However, only the first one (the result) should be modified by @code{YYLLOC_DEFAULT}. @item -For consistency with semantic actions, valid indexes for the location -array range from 1 to @var{n}. +For consistency with semantic actions, valid indexes within the +right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a +valid index, and it refers to the symbol just before the reduction. +During error processing @var{n} is always positive. + +@item +Your macro should parenthesize its arguments, if need be, since the +actual arguments may not be surrounded by parentheses. Also, your +macro should expand to something that can be used as a single +statement when it is followed by a semicolon. @end itemize @node Declarations @@ -3221,8 +3525,9 @@ Grammars}). * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Initial Action Decl:: Code run before parsing starts. * 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. @@ -3251,10 +3556,12 @@ associativity and precedence. @xref{Precedence Decl, ,Operator 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 @@ -3381,7 +3688,23 @@ This says that the two alternative types are @code{double} and @code{symrec 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 @@ -3412,19 +3735,45 @@ use the same @code{<@var{type}>} construction in a declaration for the terminal symbol. All kinds of token declarations allow @code{<@var{type}>}. +@node Initial Action Decl +@subsection Performing Actions before Parsing +@findex %initial-action + +Sometimes your parser needs to perform some initializations before +parsing. The @code{%initial-action} directive allows for such arbitrary +code. + +@deffn {Directive} %initial-action @{ @var{code} @} +@findex %initial-action +Declare that the @var{code} must be invoked before parsing each time +@code{yyparse} is called. The @var{code} may use @code{$$} and +@code{@@$} --- initial value and location of the look-ahead --- and the +@code{%parse-param}. +@end deffn + +For instance, if your locations use a file name, you may use + +@example +%parse-param @{ const char *filename @}; +%initial-action +@{ + @@$.begin.filename = @@$.end.filename = filename; +@}; +@end example + + @node Destructor Decl @subsection Freeing Discarded Symbols @cindex freeing discarded symbols @findex %destructor -Some symbols can be discarded by the parser, typically during error -recovery (@pxref{Error Recovery}). Basically, during error recovery, -embarrassing symbols already pushed on the stack, and embarrassing -tokens coming from the rest of the file are thrown away until the parser -falls on its feet. If these symbols convey heap based information, this -memory is lost. While this behavior is tolerable for batch parsers, -such as in compilers, it is unacceptable for parsers that can -possibility ``never end'' such as shells, or implementations of +Some symbols can be discarded by the parser. For instance, during error +recovery (@pxref{Error Recovery}), embarrassing symbols already pushed +on the stack, and embarrassing tokens coming from the rest of the file +are thrown away until the parser falls on its feet. If these symbols +convey heap based information, this memory is lost. While this behavior +can be tolerable for batch parsers, such as in compilers, it is not for +possibly ``never ending'' parsers such as shells, or implementations of communication protocols. The @code{%destructor} directive allows for the definition of code that @@ -3435,11 +3784,11 @@ is called when a symbol is thrown away. Declare that the @var{code} must be invoked for each of the @var{symbols} that will be discarded by the parser. The @var{code} should use @code{$$} 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, +experimental, as there was not enough user feedback. In particular, the syntax might still change. @end deffn @@ -3477,6 +3826,22 @@ typeless: string; // $$ = $1 does not apply; $1 is destroyed. typefull: string; // $$ = $1 applies, $1 is not destroyed. @end smallexample +@sp 1 + +@cindex discarded symbols +@dfn{Discarded symbols} are the following: + +@itemize +@item +stacked symbols popped during the first phase of error recovery, +@item +incoming terminals during the second phase of error recovery, +@item +the current look-ahead when the parser aborts (either via an explicit +call to @code{YYABORT}, or as a consequence of a failed error recovery). +@end itemize + + @node Expect Decl @subsection Suppressing Conflict Warnings @cindex suppressing conflict warnings @@ -3484,6 +3849,7 @@ typefull: string; // $$ = $1 applies, $1 is not destroyed. @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 @@ -3500,10 +3866,22 @@ The declaration looks like this: 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 @acronym{LALR}(1) parsers, reduce/reduce conflicts are more serious, +and should be eliminated entirely. Bison will always report +reduce/reduce conflicts for these parsers. With @acronym{GLR} parsers, however, +both shift/reduce and reduce/reduce are routine (otherwise, there +would be no need to use @acronym{GLR} parsing). Therefore, it is also possible +to specify an expected number of reduce/reduce conflicts in @acronym{GLR} +parsers, using the declaration: + +@example +%expect-rr @var{n} +@end example + In general, using @code{%expect} involves these steps: @itemize @bullet @@ -3522,9 +3900,9 @@ Add an @code{%expect} declaration, copying the number @var{n} from the 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 @@ -3611,9 +3989,16 @@ Declare a terminal symbol (token type name) that is left-associative @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) -@end deffn (@pxref{Precedence Decl, ,Operator Precedence}). +Using it in a way that would be associative is a syntax error. +@end deffn + +@ifset defaultprec +@deffn {Directive} %default-prec +Assign a precedence to rules lacking an explicit @code{%prec} modifier +(@pxref{Contextual Precedence, ,Context-Dependent Precedence}). +@end deffn +@end ifset @deffn {Directive} %type Declare the type of semantic values for a nonterminal symbol @@ -3643,22 +4028,40 @@ already defined, so that the debugging facilities are compiled. @xref{Tracing, ,Tracing Your Parser}. @deffn {Directive} %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. - +Write a header file containing macro definitions for the token type +names defined in the grammar as well as a few other declarations. If the parser output file is named @file{@var{name}.c} then this file is named @file{@var{name}.h}. -This output file is essential if you wish to put the definition of -@code{yylex} in a separate source file, because @code{yylex} needs to -be able to refer to token type codes and the variable -@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}. +Unless @code{YYSTYPE} is already defined as a macro, the output header +declares @code{YYSTYPE}. Therefore, if you are using a @code{%union} +(@pxref{Multiple Types, ,More Than One Value Type}) with components +that require other definitions, or if you have defined a +@code{YYSTYPE} macro (@pxref{Value Type, ,Data Types of Semantic +Values}), you need to arrange for these definitions to be propagated to +all modules, e.g., by putting them in a +prerequisite header that is included both by your parser and by any +other module that needs @code{YYSTYPE}. + +Unless your parser is pure, the output header declares @code{yylval} +as an external variable. @xref{Pure Decl, ,A Pure (Reentrant) +Parser}. + +If you have also used locations, the output header declares +@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of +@code{YYSTYPE} and @code{yylval}. @xref{Locations, ,Tracking +Locations}. + +This output file is normally essential if you wish to put the +definition of @code{yylex} in a separate source file, because +@code{yylex} typically needs to be able to refer to the +above-mentioned declarations and to the token type codes. +@xref{Token Values, ,Semantic Values of Tokens}. @end deffn @deffn {Directive} %destructor Specifying how the parser should reclaim the memory associated to -discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. @end deffn @deffn {Directive} %file-prefix="@var{prefix}" @@ -3685,6 +4088,14 @@ 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 + @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 @@ -3717,7 +4128,8 @@ Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure 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. @@ -3850,8 +4262,8 @@ declaration @code{%parse-param}: @deffn {Directive} %parse-param @{@var{argument-declaration}@} @findex %parse-param Declare that an argument declared by @code{argument-declaration} is an -additional @code{yyparse} argument. This argument is also passed to -@code{yyerror}. The @var{argument-declaration} is used when declaring +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 @@ -3905,7 +4317,7 @@ that need it. @xref{Invocation, ,Invoking Bison}. * 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 @@ -4038,8 +4450,8 @@ then the code in @code{yylex} might look like this: @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, , @@ -4082,7 +4494,7 @@ yylex (YYSTYPE *lvalp, YYLTYPE *llocp) @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. @@ -4166,7 +4578,7 @@ The following definition suffices in simple programs: @example @group void -yyerror (const char *s) +yyerror (char const *s) @{ @end group @group @@ -4181,24 +4593,25 @@ error recovery if you have written suitable error recovery grammar rules immediately return 1. Obviously, in location tracking pure parsers, @code{yyerror} should have -an access to the current location. This is indeed the case for the GLR +an access to the current location. +This is indeed the case for the @acronym{GLR} parsers, but not for the Yacc parser, for historical reasons. I.e., if @samp{%locations %pure-parser} is passed then the prototypes for @code{yyerror} are: @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@}} 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 +Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling convention for absolutely pure parsers, i.e., when the calling convention of @code{yylex} @emph{and} the calling convention of @code{%pure-parser} are pure. I.e.: @@ -4222,14 +4635,20 @@ int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness); 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 @@ -4334,7 +4753,7 @@ errors. This is useful primarily in error rules. @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}. @@ -4360,7 +4779,7 @@ Tracking Locations}. @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 @@ -4776,6 +5195,28 @@ exp: @dots{} @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 @@ -5036,13 +5477,13 @@ return_spec: 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 @@ -5106,6 +5547,13 @@ structure should generally be adequate. On @acronym{LALR}(1) portions of a grammar, in particular, it is only slightly slower than with the default Bison parser. +For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth +Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style +Generalised @acronym{LR} Parsers, Royal Holloway, University of +London, Department of Computer Science, TR-00-12, +@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}, +(2000-12-24). + @node Stack Overflow @section Stack Overflow, and How to Avoid It @cindex stack overflow @@ -5126,7 +5574,6 @@ By defining the macro @code{YYMAXDEPTH}, you can control how deep the parser stack can become before a stack overflow occurs. Define the macro with a value that is an integer. This value is the maximum number of tokens that can be shifted (and not reduced) before overflow. -It must be a constant expression whose value is known at compile time. The stack space allowed is not necessarily allocated. If you specify a large value for @code{YYMAXDEPTH}, the parser actually allocates a small @@ -5135,22 +5582,33 @@ increasing allocation happens automatically and silently. Therefore, you do not need to make @code{YYMAXDEPTH} painfully small merely to save space for ordinary inputs that do not need much stack. +However, do not allow @code{YYMAXDEPTH} to be a value so large that +arithmetic overflow could occur when calculating the size of the stack +space. Also, do not allow @code{YYMAXDEPTH} to be less than +@code{YYINITDEPTH}. + @cindex default stack limit The default value of @code{YYMAXDEPTH}, if you do not define it, is 10000. @vindex YYINITDEPTH You can control how much stack is allocated initially by defining the -macro @code{YYINITDEPTH}. This value too must be a compile-time -constant integer. The default is 200. +macro @code{YYINITDEPTH} to a positive integer. For the C +@acronym{LALR}(1) parser, this value must be a compile-time constant +unless you are assuming C99 or some other target language or compiler +that allows variable-length arrays. The default is 200. + +Do not allow @code{YYINITDEPTH} to be a value so large that arithmetic +overflow would occur when calculating the size of the stack space. +Also, do not allow @code{YYINITDEPTH} to be greater than +@code{YYMAXDEPTH}. @c FIXME: C++ output. Because of semantical differences between C and C++, the -@acronym{LALR}(1) parsers -in C produced by Bison by compiled as C++ cannot grow. In this precise -case (compiling a C parser as C++) you are suggested to grow -@code{YYINITDEPTH}. In the near future, a C++ output output will be -provided which addresses this issue. +@acronym{LALR}(1) parsers in C produced by Bison by compiled as C++ +cannot grow. In this precise case (compiling a C parser as C++) you are +suggested to grow @code{YYINITDEPTH}. In the near future, a C++ output +output will be provided which addresses this issue. @node Error Recovery @chapter Error Recovery @@ -5383,7 +5841,9 @@ as an identifier if it appears in that context. Here is how you can do it: @example @group %@{ -int hexflag; + int hexflag; + int yylex (void); + void yyerror (char const *); %@} %% @dots{} @@ -5531,8 +5991,8 @@ useless: STR; @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 @@ -5554,10 +6014,10 @@ Conflict in state 8 between rule 2 and token '*' resolved as shift. 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 @@ -5645,16 +6105,16 @@ beginning of the parsing, in the initial rule, right before the start 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 syntax 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 @@ -5688,7 +6148,7 @@ state 1 @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}). @@ -5711,7 +6171,7 @@ 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 @@ -5769,8 +6229,8 @@ state 7 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 @@ -5788,7 +6248,7 @@ 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 @@ -5806,14 +6266,14 @@ Note that all the previous states had a single possible action: either 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 @@ -5882,8 +6342,9 @@ state 11 @end example @noindent -Observe that state 11 contains conflicts due to the lack of precedence -of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the +Observe that state 11 contains conflicts not only due to the lack of +precedence of @samp{/} with respect to @samp{+}, @samp{-}, and +@samp{*}, but also because the associativity of @samp{/} is not specified. @@ -5981,7 +6442,12 @@ Here is an example of @code{YYPRINT} suitable for the multi-function calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}): @smallexample -#define YYPRINT(file, type, value) print_token_value (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 print_token_value (FILE *file, int type, YYSTYPE value) @@ -6033,10 +6499,15 @@ bison -d -o @var{output.c++} @var{infile.y} @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 @@ -6072,10 +6543,12 @@ Equivalent to @samp{-o y.tab.c}; the parser output file is called @file{y.tab.c}, and the other outputs are called @file{y.output} and @file{y.tab.h}. The purpose of this option is to imitate Yacc's output file name conventions. Thus, the following shell script can substitute -for Yacc: +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 @@ -6127,8 +6600,7 @@ Adjust the output: @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}. @@ -6148,9 +6620,9 @@ separated list of @var{things} among: 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 @@ -6228,6 +6700,32 @@ the corresponding short option. @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 @@ -6240,6 +6738,10 @@ are addressed. @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 Gotos/Loops:: Control Flow in the Calculator @end menu @node Parser Stack Overflow @@ -6253,216 +6755,285 @@ message. What can I do? This question is already addressed elsewhere, @xref{Recursion, ,Recursive Rules}. -@c ================================================= Table of Symbols +@node How Can I Reset the Parser +@section How Can I Reset the Parser -@node Table of Symbols -@appendix Bison Symbols -@cindex Bison symbols, table of -@cindex symbols in Bison, table of +The following phenomenon has several symptoms, resulting in the +following typical questions: -@deffn {Variable} @@$ -In an action, the location of the left-hand side of the rule. -@xref{Locations, , Locations Overview}. -@end deffn +@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 -@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 +@noindent +or -@deffn {Variable} $$ -In an action, the semantic value of the left-hand side of the rule. -@xref{Actions}. -@end deffn +@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 -@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 +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}: -@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 +@verbatim +%{ +#include +#include +%} +%% +.*\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; +} -@deffn {Symbol} $end -The predefined token marking the end of the token stream. It cannot be -used in the grammar. -@end deffn +int +main (void) +{ + yyparse ("input"); + yyparse ("input"); + return 0; +} +@end verbatim -@deffn {Symbol} $undefined -The predefined token onto which all undefined values returned by -@code{yylex} are mapped. It cannot be used in the grammar, rather, use -@code{error}. -@end deffn +@noindent +If the file @file{input} contains -@deffn {Symbol} error -A token name reserved for error recovery. This token may be used in -grammar rules so as to allow the Bison parser to recognize an error in -the grammar without halting the process. In effect, a sentence -containing an error may be recognized as valid. On a syntax error, the -token @code{error} becomes the current look-ahead token. Actions -corresponding to @code{error} are then executed, and the look-ahead -token is reset to the token that originally caused the violation. -@xref{Error Recovery}. -@end deffn +@verbatim +input:1: Hello, +input:2: World! +@end verbatim -@deffn {Macro} YYABORT -Macro to pretend that an unrecoverable syntax error has occurred, by -making @code{yyparse} return 1 immediately. The error reporting -function @code{yyerror} is not called. @xref{Parser Function, ,The -Parser Function @code{yyparse}}. -@end deffn +@noindent +then instead of getting the first line twice, you get: -@deffn {Macro} YYACCEPT -Macro to pretend that a complete utterance of the language has been -read, by making @code{yyparse} return 0 immediately. -@xref{Parser Function, ,The Parser Function @code{yyparse}}. -@end deffn +@example +$ @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 -@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 +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. -@deffn {Macro} YYDEBUG -Macro to define to equip the parser with tracing code. @xref{Tracing, -,Tracing Your Parser}. -@end deffn +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)}. -@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 +@node Strings are Destroyed +@section Strings are Destroyed -@deffn {Macro} YYERROR_VERBOSE -An obsolete macro that you define with @code{#define} in the Bison -declarations section 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 +@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 -@deffn {Macro} YYINITDEPTH -Macro for specifying the initial size of the parser stack. -@xref{Stack Overflow}. -@end deffn +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: -@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 +@verbatim +%{ +#include +char *yylval = NULL; +%} +%% +.* yylval = yytext; return 1; +\n /* IGNORE */ +%% +int +main () +{ + /* Similar to using $1, $2 in a Bison action. */ + char *fst = (yylex (), yylval); + char *snd = (yylex (), yylval); + printf ("\"%s\", \"%s\"\n", fst, snd); + return 0; +} +@end verbatim -@deffn {Macro} YYLTYPE -Macro for the data type of @code{yylloc}; a structure with four -members. @xref{Location Type, , Data Types of Locations}. -@end deffn +If you compile and run this code, you get: -@deffn {Type} yyltype -Default value for YYLTYPE. -@end deffn +@example +$ @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 -@deffn {Macro} YYMAXDEPTH -Macro for specifying the maximum size of the parser stack. @xref{Stack -Overflow}. -@end deffn +@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: -@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 +@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 -@deffn {Macro} YYRECOVERING -Macro whose value indicates whether the parser is recovering from a -syntax error. @xref{Action Features, ,Special Features for Use in Actions}. -@end deffn -@deffn {Macro} YYSTACK_USE_ALLOCA -Macro used to control the use of @code{alloca}. If defined to @samp{0}, -the parser will not use @code{alloca} but @code{malloc} when trying to -grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA} -to anything else. -@end deffn +@node C++ Parsers +@section C++ Parsers -@deffn {Macro} YYSTYPE -Macro for the data type of semantic values; @code{int} by default. -@xref{Value Type, ,Data Types of Semantic Values}. +@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. + + +@node Implementing Gotos/Loops +@section Implementing Gotos/Loops + +@display +My simple calculator supports variables, assignments, and functions, +but how can I implement gotos, or loops? +@end display + +Although very pedagogical, the examples included in the document blur +the distinction to make between the parser---whose job is to recover +the structure of a text and to transmit it to subsequent modules of +the program---and the processing (such as the execution) of this +structure. This works well with so called straight line programs, +i.e., precisely those that have a straightforward execution model: +execute simple instructions one after the others. + +@cindex abstract syntax tree +@cindex @acronym{AST} +If you want a richer model, you will probably need to use the parser +to construct a tree that does represent the structure it has +recovered; this tree is usually called the @dfn{abstract syntax tree}, +or @dfn{@acronym{AST}} for short. Then, walking through this tree, +traversing it in various ways, will enable treatments such as its +execution or its translation, which will result in an interpreter or a +compiler. + +This topic is way beyond the scope of this manual, and the reader is +invited to consult the dedicated literature. + + + +@c ================================================= Table of Symbols + +@node Table of Symbols +@appendix Bison Symbols +@cindex Bison symbols, table of +@cindex symbols in Bison, table of + +@deffn {Variable} @@$ +In an action, the location of the left-hand side of the rule. +@xref{Locations, , Locations Overview}. @end deffn -@deffn {Variable} 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}. +@deffn {Variable} @@@var{n} +In an action, the location of the @var{n}-th symbol of the right-hand +side of the rule. @xref{Locations, , Locations Overview}. @end deffn -@deffn {Variable} yyclearin -Macro used in error-recovery rule actions. It clears the previous -look-ahead token. @xref{Error Recovery}. +@deffn {Variable} $$ +In an action, the semantic value of the left-hand side of the rule. +@xref{Actions}. @end deffn -@deffn {Variable} yydebug -External integer variable set to zero by default. If @code{yydebug} -is given a nonzero value, the parser will output information on input -symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. +@deffn {Variable} $@var{n} +In an action, the semantic value of the @var{n}-th symbol of the +right-hand side of the rule. @xref{Actions}. @end deffn -@deffn {Macro} yyerrok -Macro to cause parser to recover immediately to its normal mode -after a syntax error. @xref{Error Recovery}. +@deffn {Delimiter} %% +Delimiter used to separate the grammar rule section from the +Bison declarations section or the epilogue. +@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. @end deffn -@deffn {Function} yyerror -User-supplied function to be called by @code{yyparse} on error. The -function receives one argument, a pointer to a character string -containing an error message. @xref{Error Reporting, ,The Error -Reporting Function @code{yyerror}}. +@c Don't insert spaces, or check the DVI output. +@deffn {Delimiter} %@{@var{code}%@} +All code listed between @samp{%@{} and @samp{%@}} is copied directly to +the output file uninterpreted. Such code forms the prologue of the input +file. @xref{Grammar Outline, ,Outline of a Bison +Grammar}. @end deffn -@deffn {Function} yylex -User-supplied lexical analyzer function, called with no arguments to get -the next token. @xref{Lexical, ,The Lexical Analyzer Function -@code{yylex}}. +@deffn {Construct} /*@dots{}*/ +Comment delimiters, as in C. @end deffn -@deffn {Variable} yylval -External variable in which @code{yylex} should place the semantic -value associated with a token. (In a pure parser, it is a local -variable within @code{yyparse}, and its address is passed to -@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}. +@deffn {Delimiter} : +Separates a rule's result from its components. @xref{Rules, ,Syntax of +Grammar Rules}. @end deffn -@deffn {Variable} yylloc -External variable in which @code{yylex} should place the line and column -numbers associated with a token. (In a pure parser, it is a local -variable within @code{yyparse}, and its address is passed to -@code{yylex}.) You can ignore this variable if you don't use the -@samp{@@} feature in the grammar actions. @xref{Token Positions, -,Textual Positions of Tokens}. +@deffn {Delimiter} ; +Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. @end deffn -@deffn {Variable} yynerrs -Global variable which Bison increments each time there is a syntax error. -(In a pure parser, it is a local variable within @code{yyparse}.) -@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@deffn {Delimiter} | +Separates alternate rules for the same result nonterminal. +@xref{Rules, ,Syntax of Grammar Rules}. @end deffn -@deffn {Function} yyparse -The parser function produced by Bison; call this function to start -parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@deffn {Symbol} $accept +The predefined nonterminal whose only rule is @samp{$accept: @var{start} +$end}, where @var{start} is the start symbol. @xref{Start Decl, , The +Start-Symbol}. It cannot be used in the grammar. @end deffn @deffn {Directive} %debug 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 + @deffn {Directive} %defines Bison declaration to create a header file meant for the scanner. @xref{Decl Summary}. @@ -6470,7 +7041,7 @@ Bison declaration to create a header file meant for the scanner. @deffn {Directive} %destructor Specifying how the parser should reclaim the memory associated to -discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. @end deffn @deffn {Directive} %dprec @@ -6479,6 +7050,22 @@ time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. @end deffn +@deffn {Symbol} $end +The predefined token marking the end of the token stream. It cannot be +used in the grammar. +@end deffn + +@deffn {Symbol} error +A token name reserved for error recovery. This token may be used in +grammar rules so as to allow the Bison parser to recognize an error in +the grammar without halting the process. In effect, a sentence +containing an error may be recognized as valid. On a syntax error, the +token @code{error} becomes the current look-ahead token. Actions +corresponding to @code{error} are then executed, and the look-ahead +token is reset to the token that originally caused the violation. +@xref{Error Recovery}. +@end deffn + @deffn {Directive} %error-verbose Bison declaration to request verbose, specific error message strings when @code{yyerror} is called. @@ -6494,6 +7081,10 @@ Bison declaration to produce a @acronym{GLR} parser. @xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. @end deffn +@deffn {Directive} %initial-action +Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}. +@end deffn + @deffn {Directive} %left Bison declaration to assign left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @@ -6516,6 +7107,14 @@ function is applied to the two semantic values to get a single result. 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 + @deffn {Directive} %no-lines Bison declaration to avoid generating @code{#line} directives in the parser file. @xref{Decl Summary}. @@ -6572,45 +7171,174 @@ Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}. @end deffn +@deffn {Symbol} $undefined +The predefined token onto which all undefined values returned by +@code{yylex} are mapped. It cannot be used in the grammar, rather, use +@code{error}. +@end deffn + @deffn {Directive} %union Bison declaration to specify several possible data types for semantic values. @xref{Union Decl, ,The Collection of Value Types}. @end deffn -@sp 1 +@deffn {Macro} YYABORT +Macro to pretend that an unrecoverable syntax error has occurred, by +making @code{yyparse} return 1 immediately. The error reporting +function @code{yyerror} is not called. @xref{Parser Function, ,The +Parser Function @code{yyparse}}. +@end deffn -These are the punctuation and delimiters used in Bison input: +@deffn {Macro} YYACCEPT +Macro to pretend that a complete utterance of the language has been +read, by making @code{yyparse} return 0 immediately. +@xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@deffn {Delimiter} %% -Delimiter used to separate the grammar rule section from the -Bison declarations section or the epilogue. -@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. +@deffn {Macro} YYBACKUP +Macro to discard a value from the parser stack and fake a look-ahead +token. @xref{Action Features, ,Special Features for Use in Actions}. @end deffn -@c Don't insert spaces, or check the DVI output. -@deffn {Delimiter} %@{@var{code}%@} -All code listed between @samp{%@{} and @samp{%@}} is copied directly to -the output file uninterpreted. Such code forms the prologue of the input -file. @xref{Grammar Outline, ,Outline of a Bison -Grammar}. +@deffn {Variable} yychar +External integer variable that contains the integer value of the current +look-ahead token. (In a pure parser, it is a local variable within +@code{yyparse}.) Error-recovery rule actions may examine this variable. +@xref{Action Features, ,Special Features for Use in Actions}. @end deffn -@deffn {Construct} /*@dots{}*/ -Comment delimiters, as in C. +@deffn {Variable} yyclearin +Macro used in error-recovery rule actions. It clears the previous +look-ahead token. @xref{Error Recovery}. @end deffn -@deffn {Delimiter} : -Separates a rule's result from its components. @xref{Rules, ,Syntax of -Grammar Rules}. +@deffn {Macro} YYDEBUG +Macro to define to equip the parser with tracing code. @xref{Tracing, +,Tracing Your Parser}. @end deffn -@deffn {Delimiter} ; -Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Variable} yydebug +External integer variable set to zero by default. If @code{yydebug} +is given a nonzero value, the parser will output information on input +symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. @end deffn -@deffn {Delimiter} | -Separates alternate rules for the same result nonterminal. -@xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Macro} yyerrok +Macro to cause parser to recover immediately to its normal mode +after a syntax error. @xref{Error Recovery}. +@end deffn + +@deffn {Macro} YYERROR +Macro to pretend that a syntax error has just been detected: call +@code{yyerror} and then perform normal error recovery if possible +(@pxref{Error Recovery}), or (if recovery is impossible) make +@code{yyparse} return 1. @xref{Error Recovery}. +@end deffn + +@deffn {Function} yyerror +User-supplied function to be called by @code{yyparse} on error. +@xref{Error Reporting, ,The Error +Reporting Function @code{yyerror}}. +@end deffn + +@deffn {Macro} YYERROR_VERBOSE +An obsolete macro that you define with @code{#define} in the prologue +to request verbose, specific error message strings +when @code{yyerror} is called. It doesn't matter what definition you +use for @code{YYERROR_VERBOSE}, just whether you define it. Using +@code{%error-verbose} is preferred. +@end deffn + +@deffn {Macro} YYINITDEPTH +Macro for specifying the initial size of the parser stack. +@xref{Stack Overflow}. +@end deffn + +@deffn {Function} yylex +User-supplied lexical analyzer function, called with no arguments to get +the next token. @xref{Lexical, ,The Lexical Analyzer Function +@code{yylex}}. +@end deffn + +@deffn {Macro} YYLEX_PARAM +An obsolete macro for specifying an extra argument (or list of extra +arguments) for @code{yyparse} to pass to @code{yylex}. he use of this +macro is deprecated, and is supported only for Yacc like parsers. +@xref{Pure Calling,, Calling Conventions for Pure Parsers}. +@end deffn + +@deffn {Variable} yylloc +External variable in which @code{yylex} should place the line and column +numbers associated with a token. (In a pure parser, it is a local +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) You can ignore this variable if you don't use the +@samp{@@} feature in the grammar actions. @xref{Token Locations, +,Textual Locations of Tokens}. +@end deffn + +@deffn {Type} YYLTYPE +Data type of @code{yylloc}; by default, a structure with four +members. @xref{Location Type, , Data Types of Locations}. +@end deffn + +@deffn {Variable} yylval +External variable in which @code{yylex} should place the semantic +value associated with a token. (In a pure parser, it is a local +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}. +@end deffn + +@deffn {Macro} YYMAXDEPTH +Macro for specifying the maximum size of the parser stack. @xref{Stack +Overflow}. +@end deffn + +@deffn {Variable} yynerrs +Global variable which Bison increments each time there is a syntax error. +(In a pure parser, it is a local variable within @code{yyparse}.) +@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@end deffn + +@deffn {Function} yyparse +The parser function produced by Bison; call this function to start +parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn + +@deffn {Macro} YYPARSE_PARAM +An obsolete macro for specifying the name of a parameter that +@code{yyparse} should accept. The use of this macro is deprecated, and +is supported only for Yacc like parsers. @xref{Pure Calling,, Calling +Conventions for Pure Parsers}. +@end deffn + +@deffn {Macro} YYRECOVERING +Macro whose value indicates whether the parser is recovering from a +syntax error. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn + +@deffn {Macro} YYSTACK_USE_ALLOCA +Macro used to control the use of @code{alloca} when the C +@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0, +the parser will use @code{malloc} to extend its stacks. If defined to +1, the parser will use @code{alloca}. Values other than 0 and 1 are +reserved for future Bison extensions. If not defined, +@code{YYSTACK_USE_ALLOCA} defaults to 0. + +If you define @code{YYSTACK_USE_ALLOCA} to 1, it is your +responsibility to make sure that @code{alloca} is visible, e.g., by +using @acronym{GCC} or by including @code{}. Furthermore, +in the all-too-common case where your code may run on a host with a +limited stack and with unreliable stack-overflow checking, you should +set @code{YYMAXDEPTH} to a value that cannot possibly result in +unchecked stack overflow on any of your target hosts when +@code{alloca} is called. You can inspect the code that Bison +generates in order to determine the proper numeric values. This will +require some expertise in low-level implementation details. +@end deffn + +@deffn {Type} YYSTYPE +Data type of semantic values; @code{int} by default. +@xref{Value Type, ,Data Types of Semantic Values}. @end deffn @node Glossary @@ -6799,3 +7527,33 @@ grammatically indivisible. The piece of text it represents is a token. @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