gnu1987/bison-1.16/man/bison.info-4

Info file bison.info, produced by Makeinfo, -*- Text -*- from input
file bison.texinfo.

   This file documents the Bison parser generator.

   Copyright (C) 1988, 1989, 1990 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License" and
"Conditions for Using Bison" are included exactly as in the original,
and provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License", "Conditions for Using Bison" and this permission notice may
be included in translations approved by the Free Software Foundation
instead of in the original English.


File: bison.info,  Node: Debugging,  Next: Invocation,  Prev: Context Dependency,  Up: Top

Debugging Your Parser
*********************

   If a Bison grammar compiles properly but doesn't do what you want
when it runs, the `yydebug' parser-trace feature can help you figure
out why.

   To enable compilation of trace facilities, you must define the macro
`YYDEBUG' when you compile the parser.  You could use `-DYYDEBUG=1' as
a compiler option or you could put `#define YYDEBUG 1' in the C
declarations section of the grammar file (*note C Declarations::.). 
Alternatively, use the `-t' option when you run Bison (*note
Invocation::.).  We always define `YYDEBUG' so that debugging is
always possible.

   The trace facility uses `stderr', so you must add
`#include <stdio.h>' to the C declarations section unless it is
already there.

   Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable `yydebug'. 
You can do this by making the C code do it (in `main', perhaps), or
you can alter the value with a C debugger.

   Each step taken by the parser when `yydebug' is nonzero produces a
line or two of trace information, written on `stderr'.  The trace
messages tell you these things:

   * Each time the parser calls `yylex', what kind of token was read.

   * Each time a token is shifted, the depth and complete contents of
     the state stack (*note Parser States::.).

   * Each time a rule is reduced, which rule it is, and the complete
     contents of the state stack afterward.

   To make sense of this information, it helps to refer to the listing
file produced by the Bison `-v' option (*note Invocation::.).  This
file shows the meaning of each state in terms of positions in various
rules, and also what each state will do with each possible input
token.  As you read the successive trace messages, you can see that
the parser is functioning according to its specification in the
listing file.  Eventually you will arrive at the place where something
undesirable happens, and you will see which parts of the grammar are
to blame.

   The parser file is a C program and you can use C debuggers on it,
but it's not easy to interpret what it is doing.  The parser function
is a finite-state machine interpreter, and aside from the actions it
executes the same code over and over.  Only the values of variables
show where in the grammar it is working.

   The debugging information normally gives the token type of each
token read, but not its semantic value.  You can optionally define a
macro named `YYPRINT' to provide a way to print the value.  If you
define `YYPRINT', it should take three arguments.  The parser will
pass a standard I/O stream, the numeric code for the token type, and
the token value (from `yylval').

   Here is an example of `YYPRINT' suitable for the multi-function
calculator (*note Mfcalc Decl::.):

     #define YYPRINT(file, type, value)   yyprint (file, type, value)
     
     static void
     yyprint (file, type, value)
          FILE *file;
          int type;
          YYSTYPE value;
     {
       if (type == VAR)
         fprintf (file, " %s", value.tptr->name);
       else if (type == NUM)
         fprintf (file, " %d", value.val);
     }


File: bison.info,  Node: Invocation,  Next: Table of Symbols,  Prev: Debugging,  Up: Top

Invoking Bison
**************

   The usual way to invoke Bison is as follows:

     bison INFILE

   Here INFILE is the grammar file name, which usually ends in `.y'. 
The parser file's name is made by replacing the `.y' with `.tab.c'. 
Thus, the `bison foo.y' filename yields `foo.tab.c', and the `bison
hack/foo.y' filename yields `hack/foo.tab.c'.

   Bison supports both traditional single-letter options and mnemonic
long option names.  Long option names are indicated with `--' instead
of `-'.  Abbreviations for option names are allowed as long as they
are unique.  When a long option takes an argument, like
`--file-prefix', connect the option name and the argument with `='.

   These options can be used with Bison:

`-b FILE-PREFIX'
`--file-prefix=PREFIX'
     Specify a prefix to use for all Bison output file names.  The
     names are chosen as if the input file were named `PREFIX.c'.

`-d'
`--defines'
     Write an extra output file containing macro definitions for the
     token type names defined in the grammar and the semantic value
     type `YYSTYPE', as well as a few `extern' variable declarations.

     If the parser output file is named `NAME.c' then this file is
     named `NAME.h'.

     This output file is essential if you wish to put the definition of
     `yylex' in a separate source file, because `yylex' needs to be
     able to refer to token type codes and the variable `yylval'. 
     *Note Token Values::.

`-l'
`--no-lines'
     Don't put any `#line' preprocessor commands in the parser file. 
     Ordinarily Bison puts them in the parser file so that the C
     compiler and debuggers will associate errors with your source
     file, the grammar file.  This option causes them to associate
     errors with the parser file, treating it an independent source
     file in its own right.

`-o OUTFILE'
`--output-file=OUTFILE'
     Specify the name OUTFILE for the parser file.

     The other output files' names are constructed from OUTFILE as
     described under the `-v' and `-d' switches.

`-p PREFIX'
`--name-prefix=PREFIX'
     Rename the external symbols used in the parser so that they start
     with PREFIX instead of `yy'.  The precise list of symbols renamed
     is `yyparse', `yylex', `yyerror', `yylval', `yychar' and
     `yydebug'.

     For example, if you use `-p c', the names become `cparse',
     `clex', and so on.

     *Note Multiple Parsers::.

`-t'
`--debug'
     Output a definition of the macro `YYDEBUG' into the parser file,
     so that the debugging facilities are compiled.  *Note Debugging::.

`-v'
`--verbose'
     Write an extra output file containing verbose descriptions of the
     parser states and what is done for each type of look-ahead token
     in that state.

     This file also describes all the conflicts, both those resolved by
     operator precedence and the unresolved ones.

     The file's name is made by removing `.tab.c' or `.c' from the
     parser output file name, and adding `.output' instead.

     Therefore, if the input file is `foo.y', then the parser file is
     called `foo.tab.c' by default.  As a consequence, the verbose
     output file is called `foo.output'.

`-V'
`--version'
     Print the version number of Bison.

`-y'
`--yacc'
`--fixed-output-files'
     Equivalent to `-o y.tab.c'; the parser output file is called
     `y.tab.c', and the other outputs are called `y.output' and
     `y.tab.h'.  The purpose of this switch is to imitate Yacc's output
     file name conventions.  Thus, the following shell script can
     substitute for Yacc:

          bison -y $*


File: bison.info,  Node: Table of Symbols,  Next: Glossary,  Prev: Invocation,  Up: Top

Bison Symbols
*************

`error'
     A token name reserved for error recovery.  This token may be used
     in grammar rules so as to allow the Bison parser to recognize an
     error in the grammar without halting the process.  In effect, a
     sentence containing an error may be recognized as valid.  On a
     parse error, the token `error' becomes the current look-ahead
     token.  Actions corresponding to `error' are then executed, and
     the look-ahead token is reset to the token that originally caused
     the violation.  *Note Error Recovery::.

`YYABORT'
     Macro to pretend that an unrecoverable syntax error has occurred,
     by making `yyparse' return 1 immediately.  The error reporting
     function `yyerror' is not called.  *Note Parser Function::.

`YYACCEPT'
     Macro to pretend that a complete utterance of the language has
     been read, by making `yyparse' return 0 immediately.  *Note
     Parser Function::.

`YYBACKUP'
     Macro to discard a value from the parser stack and fake a
     look-ahead token.  *Note Action Features::.

`YYERROR'
     Macro to pretend that a syntax error has just been detected: call
     `yyerror' and then perform normal error recovery if possible
     (*note Error Recovery::.), or (if recovery is impossible) make
     `yyparse' return 1.  *Note Error Recovery::.

`YYINITDEPTH'
     Macro for specifying the initial size of the parser stack.  *Note
     Stack Overflow::.

`YYLTYPE'
     Macro for the data type of `yylloc'; a structure with four
     members.  *Note Token Positions::.

`YYMAXDEPTH'
     Macro for specifying the maximum size of the parser stack.  *Note
     Stack Overflow::.

`YYRECOVERING'
     Macro whose value indicates whether the parser is recovering from
     a syntax error.  *Note Action Features::.

`YYSTYPE'
     Macro for the data type of semantic values; `int' by default. 
     *Note Value Type::.

`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 `yyparse'.)  Error-recovery rule actions may
     examine this variable.  *Note Action Features::.

`yyclearin'
     Macro used in error-recovery rule actions.  It clears the previous
     look-ahead token.  *Note Error Recovery::.

`yydebug'
     External integer variable set to zero by default.  If `yydebug'
     is given a nonzero value, the parser will output information on
     input symbols and parser action.  *Note Debugging::.

`yyerrok'
     Macro to cause parser to recover immediately to its normal mode
     after a parse error.  *Note Error Recovery::.

`yyerror'
     User-supplied function to be called by `yyparse' on error.  The
     function receives one argument, a pointer to a character string
     containing an error message.  *Note Error Reporting::.

`yylex'
     User-supplied lexical analyzer function, called with no arguments
     to get the next token.  *Note Lexical::.

`yylval'
     External variable in which `yylex' should place the semantic
     value associated with a token.  (In a pure parser, it is a local
     variable within `yyparse', and its address is passed to `yylex'.)
      *Note Token Values::.

`yylloc'
     External variable in which `yylex' should place the line and
     column numbers associated with a token.  (In a pure parser, it is
     a local variable within `yyparse', and its address is passed to
     `yylex'.)  You can ignore this variable if you don't use the `@'
     feature in the grammar actions.  *Note Token Positions::.

`yynerrs'
     Global variable which Bison increments each time there is a parse
     error.  (In a pure parser, it is a local variable within
     `yyparse'.)  *Note Error Reporting::.

`yyparse'
     The parser function produced by Bison; call this function to start
     parsing.  *Note Parser Function::.

`%left'
     Bison declaration to assign left associativity to token(s). 
     *Note Precedence Decl::.

`%nonassoc'
     Bison declaration to assign nonassociativity to token(s).  *Note
     Precedence Decl::.

`%prec'
     Bison declaration to assign a precedence to a specific rule. 
     *Note Contextual Precedence::.

`%pure_parser'
     Bison declaration to request a pure (reentrant) parser.  *Note
     Pure Decl::.

`%right'
     Bison declaration to assign right associativity to token(s). 
     *Note Precedence Decl::.

`%start'
     Bison declaration to specify the start symbol.  *Note Start
     Decl::.

`%token'
     Bison declaration to declare token(s) without specifying
     precedence.  *Note Token Decl::.

`%type'
     Bison declaration to declare nonterminals.  *Note Type Decl::.

`%union'
     Bison declaration to specify several possible data types for
     semantic values.  *Note Union Decl::.

   These are the punctuation and delimiters used in Bison input:

`%%'
     Delimiter used to separate the grammar rule section from the
     Bison declarations section or the additional C code section. 
     *Note Grammar Layout::.

`%{ %}'
     All code listed between `%{' and `%}' is copied directly to the
     output file uninterpreted.  Such code forms the "C declarations"
     section of the input file.  *Note Grammar Outline::.

`/*...*/'
     Comment delimiters, as in C.

`:'
     Separates a rule's result from its components.  *Note Rules::.

`;'
     Terminates a rule.  *Note Rules::.

`|'
     Separates alternate rules for the same result nonterminal.  *Note
     Rules::.


File: bison.info,  Node: Glossary,  Next: Index,  Prev: Table of Symbols,  Up: top

Glossary
********

Backus-Naur Form (BNF)
     Formal method of specifying context-free grammars.  BNF was first
     used in the `ALGOL-60' report, 1963.  *Note Language and
     Grammar::.

Context-free grammars
     Grammars specified as rules that can be applied regardless of
     context.  Thus, if there is a rule which says that an integer can
     be used as an expression, integers are allowed *anywhere* an
     expression is permitted.  *Note Language and Grammar::.

Dynamic allocation
     Allocation of memory that occurs during execution, rather than at
     compile time or on entry to a function.

Empty string
     Analogous to the empty set in set theory, the empty string is a
     character string of length zero.

Finite-state stack machine
     A "machine" that has discrete states in which it is said to exist
     at each instant in time.  As input to the machine is processed,
     the machine moves from state to state as specified by the logic
     of the machine.  In the case of the parser, the input is the
     language being parsed, and the states correspond to various
     stages in the grammar rules.  *Note Algorithm::.

Grouping
     A language construct that is (in general) grammatically divisible;
     for example, `expression' or `declaration' in C.  *Note Language
     and Grammar::.

Infix operator
     An arithmetic operator that is placed between the operands on
     which it performs some operation.

Input stream
     A continuous flow of data between devices or programs.

Language construct
     One of the typical usage schemas of the language.  For example,
     one of the constructs of the C language is the `if' statement. 
     *Note Language and Grammar::.

Left associativity
     Operators having left associativity are analyzed from left to
     right: `a+b+c' first computes `a+b' and then combines with `c'. 
     *Note Precedence::.

Left recursion
     A rule whose result symbol is also its first component symbol;
     for example, `expseq1 : expseq1 ',' exp;'.  *Note Recursion::.

Left-to-right parsing
     Parsing a sentence of a language by analyzing it token by token
     from left to right.  *Note Algorithm::.

Lexical analyzer (scanner)
     A function that reads an input stream and returns tokens one by
     one.  *Note Lexical::.

Lexical tie-in
     A flag, set by actions in the grammar rules, which alters the way
     tokens are parsed.  *Note Lexical Tie-ins::.

Look-ahead token
     A token already read but not yet shifted.  *Note Look-Ahead::.

LALR(1)
     The class of context-free grammars that Bison (like most other
     parser generators) can handle; a subset of LR(1).  *Note 
     Mysterious Reduce/Reduce Conflicts: Mystery Conflicts.

LR(1)
     The class of context-free grammars in which at most one token of
     look-ahead is needed to disambiguate the parsing of any piece of
     input.

Nonterminal symbol
     A grammar symbol standing for a grammatical construct that can be
     expressed through rules in terms of smaller constructs; in other
     words, a construct that is not a token.  *Note Symbols::.

Parse error
     An error encountered during parsing of an input stream due to
     invalid syntax.  *Note Error Recovery::.

Parser
     A function that recognizes valid sentences of a language by
     analyzing the syntax structure of a set of tokens passed to it
     from a lexical analyzer.

Postfix operator
     An arithmetic operator that is placed after the operands upon
     which it performs some operation.

Reduction
     Replacing a string of nonterminals and/or terminals with a single
     nonterminal, according to a grammar rule.  *Note Algorithm::.

Reentrant
     A reentrant subprogram is a subprogram which can be in invoked any
     number of times in parallel, without interference between the
     various invocations.  *Note Pure Decl::.

Reverse polish notation
     A language in which all operators are postfix operators.

Right recursion
     A rule whose result symbol is also its last component symbol; for
     example, `expseq1: exp ',' expseq1;'.  *Note Recursion::.

Semantics
     In computer languages, the semantics are specified by the actions
     taken for each instance of the language, i.e., the meaning of
     each statement.  *Note Semantics::.

Shift
     A parser is said to shift when it makes the choice of analyzing
     further input from the stream rather than reducing immediately
     some already-recognized rule.  *Note Algorithm::.

Single-character literal
     A single character that is recognized and interpreted as is. 
     *Note Grammar in Bison::.

Start symbol
     The nonterminal symbol that stands for a complete valid utterance
     in the language being parsed.  The start symbol is usually listed
     as the first nonterminal symbol in a language specification. 
     *Note Start Decl::.

Symbol table
     A data structure where symbol names and associated data are stored
     during parsing to allow for recognition and use of existing
     information in repeated uses of a symbol.  *Note Multi-function
     Calc::.

Token
     A basic, grammatically indivisible unit of a language.  The symbol
     that describes a token in the grammar is a terminal symbol.  The
     input of the Bison parser is a stream of tokens which comes from
     the lexical analyzer.  *Note Symbols::.

Terminal symbol
     A grammar symbol that has no rules in the grammar and therefore
     is grammatically indivisible.  The piece of text it represents is
     a token.  *Note Language and Grammar::.


File: bison.info,  Node: Index,  Prev: Glossary,  Up: top

Index
*****

* Menu:

* $$:                                   Actions.
* $N:                                   Actions.
* %expect:                              Expect Decl.
* %left:                                Using Precedence.
* %nonassoc:                            Using Precedence.
* %prec:                                Contextual Precedence.
* %pure_parser:                         Pure Decl.
* %right:                               Using Precedence.
* %start:                               Start Decl.
* %token:                               Token Decl.
* %type:                                Type Decl.
* %union:                               Union Decl.
* @N:                                   Action Features.
* calc:                                 Infix Calc.
* else, dangling:                       Shift/Reduce.
* mfcalc:                               Multi-function Calc.
* rpcalc:                               RPN Calc.
* BNF:                                  Language and Grammar.
* Backus-Naur form:                     Language and Grammar.
* Bison declaration summary:            Decl Summary.
* Bison declarations:                   Declarations.
* Bison declarations (introduction):    Bison Declarations.
* Bison grammar:                        Grammar in Bison.
* Bison invocation:                     Invocation.
* Bison parser:                         Bison Parser.
* Bison parser algorithm:               Algorithm.
* Bison symbols, table of:              Table of Symbols.
* Bison utility:                        Bison Parser.
* C code, section for additional:       C Code.
* C declarations section:               C Declarations.
* C-language interface:                 Interface.
* LALR(1):                              Mystery Conflicts.
* LR(1):                                Mystery Conflicts.
* YYABORT:                              Parser Function.
* YYACCEPT:                             Parser Function.
* YYBACKUP:                             Action Features.
* YYDEBUG:                              Debugging.
* YYEMPTY:                              Action Features.
* YYERROR:                              Action Features.
* YYINITDEPTH:                          Stack Overflow.
* YYMAXDEPTH:                           Stack Overflow.
* YYPRINT:                              Debugging.
* YYRECOVERING:                         Error Recovery.
* action:                               Actions.
* action data types:                    Action Types.
* action features summary:              Action Features.
* actions in mid-rule:                  Mid-Rule Actions.
* actions, semantic:                    Semantic Actions.
* additional C code section:            C Code.
* algorithm of parser:                  Algorithm.
* associativity:                        Why Precedence.
* calculator, infix notation:           Infix Calc.
* calculator, multi-function:           Multi-function Calc.
* calculator, simple:                   RPN Calc.
* character token:                      Symbols.
* compiling the parser:                 Rpcalc Compile.
* conflicts:                            Shift/Reduce.
* conflicts, reduce/reduce:             Reduce/Reduce.
* conflicts, suppressing warnings of:   Expect Decl.
* context-dependent precedence:         Contextual Precedence.
* context-free grammar:                 Language and Grammar.
* controlling function:                 Rpcalc Main.
* dangling else:                        Shift/Reduce.
* data types in actions:                Action Types.
* data types of semantic values:        Value Type.
* debugging:                            Debugging.
* declaration summary:                  Decl Summary.
* declarations, Bison:                  Declarations.
* declarations, Bison (introduction):   Bison Declarations.
* declarations, C:                      C Declarations.
* declaring operator precedence:        Precedence Decl.
* declaring the start symbol:           Start Decl.
* declaring token type names:           Token Decl.
* declaring value types:                Union Decl.
* declaring value types, nonterminals:  Type Decl.
* defining language semantics:          Semantics.
* error:                                Error Recovery.
* error recovery:                       Error Recovery.
* error recovery, simple:               Simple Error Recovery.
* error reporting function:             Error Reporting.
* error reporting routine:              Rpcalc Error.
* examples, simple:                     Examples.
* exercises:                            Exercises.
* file format:                          Grammar Layout.
* finite-state machine:                 Parser States.
* formal grammar:                       Grammar in Bison.
* format of grammar file:               Grammar Layout.
* glossary:                             Glossary.
* grammar file:                         Grammar Layout.
* grammar rule syntax:                  Rules.
* grammar rules section:                Grammar Rules.
* grammar, Bison:                       Grammar in Bison.
* grammar, context-free:                Language and Grammar.
* grouping, syntactic:                  Language and Grammar.
* infix notation calculator:            Infix Calc.
* interface:                            Interface.
* introduction:                         Introduction.
* invoking Bison:                       Invocation.
* language semantics, defining:         Semantics.
* layout of Bison grammar:              Grammar Layout.
* left recursion:                       Recursion.
* lexical analyzer:                     Lexical.
* lexical analyzer, purpose:            Bison Parser.
* lexical analyzer, writing:            Rpcalc Lexer.
* lexical tie-in:                       Lexical Tie-ins.
* literal token:                        Symbols.
* look-ahead token:                     Look-Ahead.
* main function in simple example:      Rpcalc Main.
* mid-rule actions:                     Mid-Rule Actions.
* multi-function calculator:            Multi-function Calc.
* mutual recursion:                     Recursion.
* nonterminal symbol:                   Symbols.
* operator precedence:                  Precedence.
* operator precedence, declaring:       Precedence Decl.
* options for Bison invocation:         Invocation.
* overflow of parser stack:             Stack Overflow.
* parse error:                          Error Reporting.
* parser:                               Bison Parser.
* parser stack:                         Algorithm.
* parser stack overflow:                Stack Overflow.
* parser state:                         Parser States.
* polish notation calculator:           RPN Calc.
* precedence declarations:              Precedence Decl.
* precedence of operators:              Precedence.
* precedence, context-dependent:        Contextual Precedence.
* precedence, unary operator:           Contextual Precedence.
* preventing warnings about conflicts:  Expect Decl.
* pure parser:                          Pure Decl.
* recovery from errors:                 Error Recovery.
* recursive rule:                       Recursion.
* reduce/reduce conflict:               Reduce/Reduce.
* reduction:                            Algorithm.
* reentrant parser:                     Pure Decl.
* reverse polish notation:              RPN Calc.
* right recursion:                      Recursion.
* rule syntax:                          Rules.
* rules section for grammar:            Grammar Rules.
* running Bison (introduction):         Rpcalc Gen.
* semantic actions:                     Semantic Actions.
* semantic value:                       Semantic Values.
* semantic value type:                  Value Type.
* shift/reduce conflicts:               Shift/Reduce.
* shifting:                             Algorithm.
* simple examples:                      Examples.
* single-character literal:             Symbols.
* stack overflow:                       Stack Overflow.
* stack, parser:                        Algorithm.
* stages in using Bison:                Stages.
* start symbol:                         Language and Grammar.
* start symbol, declaring:              Start Decl.
* state (of parser):                    Parser States.
* summary, Bison declaration:           Decl Summary.
* summary, action features:             Action Features.
* suppressing conflict warnings:        Expect Decl.
* symbol:                               Symbols.
* symbol table example:                 Mfcalc Symtab.
* symbols (abstract):                   Language and Grammar.
* symbols in Bison, table of:           Table of Symbols.
* syntactic grouping:                   Language and Grammar.
* syntax error:                         Error Reporting.
* syntax of grammar rules:              Rules.
* terminal symbol:                      Symbols.
* token:                                Language and Grammar.
* token type:                           Symbols.
* token type names, declaring:          Token Decl.
* tracing the parser:                   Debugging.
* unary operator precedence:            Contextual Precedence.
* using Bison:                          Stages.
* value type, semantic:                 Value Type.
* value types, declaring:               Union Decl.
* value types, nonterminals, declaring: Type Decl.
* value, semantic:                      Semantic Values.
* warnings, preventing:                 Expect Decl.
* writing a lexical analyzer:           Rpcalc Lexer.
* yychar:                               Look-Ahead.
* yyclearin:                            Error Recovery.
* yydebug:                              Debugging.
* yyerrok:                              Error Recovery.
* yyerror:                              Error Reporting.
* yylex:                                Lexical.
* yylloc:                               Token Positions.
* yylval:                               Token Values.
* yynerrs:                              Error Reporting.
* yyparse:                              Parser Function.
* |:                                    Rules.