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This manual is for GNU Bison (version 1.75, 14 October 2002), the
GNU parser generator.

Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.1 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being "A GNU Manual," and with the Back-Cover Texts as in (a)
below. A copy of the license is included in the section entitled
"GNU Free Documentation License."

(a) The FSF's Back-Cover Text is: "You have freedom to copy and
modify this GNU Manual, like GNU software. Copies published by
the Free Software Foundation raise funds for GNU development."

INFO-DIR-SECTION GNU programming tools
START-INFO-DIR-ENTRY
* bison: (bison). GNU parser generator (yacc replacement).
END-INFO-DIR-ENTRY

File: bison.info, Node: Top, Next: Introduction, Up: (dir)

Bison
*****

This manual is for GNU Bison (version 1.75, 14 October 2002), the
GNU parser generator.

(a) The FSF's Back-Cover Text is: "You have freedom to copy and
modify this GNU Manual, like GNU software. Copies published by
the Free Software Foundation raise funds for GNU development."

* Menu:

* Introduction::
* Conditions::
* Copying:: The GNU General Public License says
how you can copy and share Bison

Tutorial sections:
* Concepts:: Basic concepts for understanding Bison.
* Examples:: Three simple explained examples of using Bison.

Reference sections:
* Grammar File:: Writing Bison declarations and rules.
* Interface:: C-language interface to the parser function `yyparse'.
* Algorithm:: How the Bison parser works at run-time.
* Error Recovery:: Writing rules for error recovery.
* Context Dependency:: What to do if your language syntax is too
messy for Bison to handle straightforwardly.
* Debugging:: Understanding or debugging Bison parsers.
* Invocation:: How to run Bison (to produce the parser source file).
* Table of Symbols:: All the keywords of the Bison language are explained.
* Glossary:: Basic concepts are explained.
* FAQ:: Frequently Asked Questions
* Copying This Manual:: License for copying this manual.
* Index:: Cross-references to the text.

The Concepts of Bison

* Language and Grammar:: Languages and context-free grammars,
as mathematical ideas.
* Grammar in Bison:: How we represent grammars for Bison's sake.
* Semantic Values:: Each token or syntactic grouping can have
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.
* 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.

Examples

* RPN Calc:: Reverse polish notation calculator;
a first example with no operator precedence.
* Infix Calc:: Infix (algebraic) notation calculator.
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* 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

* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc.
* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
* Lexer: Rpcalc Lexer. The lexical analyzer.
* Main: Rpcalc Main. The controlling function.
* Error: Rpcalc Error. The error reporting function.
* Gen: Rpcalc Gen. Running Bison on the grammar file.
* Comp: Rpcalc Compile. Run the C compiler on the output code.

Grammar Rules for `rpcalc'

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::

Location Tracking Calculator: `ltcalc'

* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
* Lexer: Ltcalc Lexer. The lexical analyzer.

Multi-Function Calculator: `mfcalc'

* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
* Rules: Mfcalc Rules. Grammar rules for the calculator.
* Symtab: Mfcalc Symtab. Symbol table management subroutines.

Bison Grammar Files

* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
* Recursion:: Writing recursive rules.
* Semantics:: Semantic values 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).
* 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).

Defining Language Semantics

* Value Type:: Specifying one data type for all semantic values.
* Multiple Types:: Specifying several alternative data types.
* Actions:: An action is the semantic definition of a grammar rule.
* Action Types:: Specifying data types for actions to operate on.
* Mid-Rule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.

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.
* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.

Parser C-Language Interface

* Parser Function:: How to call `yyparse' and what it returns.
* Lexical:: You must supply a function `yylex'
which reads tokens.
* Error Reporting:: You must supply a function `yyerror'.
* Action Features:: Special features for use in actions.

The Lexical Analyzer Function `yylex'

* Calling Convention:: How `yyparse' calls `yylex'.
* Token Values:: How `yylex' must return the semantic value
of the token it has read.
* Token Positions:: How `yylex' must return the text position
(line number, etc.) of the token, if the
actions want that.
* Pure Calling:: How the calling convention differs
in a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.).

The Bison Parser Algorithm

* Look-Ahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Stack Overflow:: What happens when stack gets full. How to avoid it.

Operator Precedence

* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence in Bison grammars.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.

Handling Context Dependencies

* Semantic Tokens:: Token parsing can depend on the semantic context.
* Lexical Tie-ins:: Token parsing can depend on the syntactic context.
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.

Understanding or Debugging Your Parser

* Understanding:: Understanding the structure of your parser.
* Tracing:: Tracing the execution of your parser.

Invoking Bison

* Bison Options:: All the options described in detail,
in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
* VMS Invocation:: Bison command syntax on VMS.

Frequently Asked Questions

* Parser Stack Overflow:: Breaking the Stack Limits

Copying This Manual

* GNU Free Documentation License:: License for copying this manual.

File: bison.info, Node: Introduction, Next: Conditions, Prev: Top, Up: Top

Introduction
************

"Bison" is a general-purpose parser generator that converts a
grammar description for an LALR(1) context-free grammar into a C
program to parse that grammar. Once you are proficient with Bison, you
may use it to develop a wide range of language parsers, from those used
in simple desk calculators to complex programming languages.

Bison is upward compatible with Yacc: all properly-written Yacc
grammars ought to work with Bison with no change. Anyone familiar with
Yacc should be able to use Bison with little trouble. You need to be
fluent in C programming in order to use Bison or to understand this
manual.

We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last. If you don't know Bison or Yacc, start by reading these
chapters. Reference chapters follow which describe specific aspects of
Bison in detail.

Bison was written primarily by Robert Corbett; Richard Stallman made
it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
multi-character string literals and other features.

This edition corresponds to version 1.75 of Bison.

File: bison.info, Node: Conditions, Next: Copying, Prev: Introduction, Up: Top

Conditions for Using Bison
**************************

As of Bison version 1.24, we have changed the distribution terms for
`yyparse' to permit using Bison's output in nonfree programs when Bison
is generating C code for LALR(1) parsers. Formerly, these parsers
could be used only in programs that were free software.

The other GNU programming tools, such as the GNU C compiler, have
never had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.

The output of the Bison utility--the Bison parser file--contains a
verbatim copy of a sizable piece of Bison, which is the code for the
`yyparse' function. (The actions from your grammar are inserted into
this function at one point, but the rest of the function is not
changed.) When we applied the GPL terms to the code for `yyparse', the
effect was to restrict the use of Bison output to free software.

We didn't change the terms because of sympathy for people who want to
make software proprietary. *Software should be free.* But we
concluded that limiting Bison's use to free software was doing little to
encourage people to make other software free. So we decided to make the
practical conditions for using Bison match the practical conditions for
using the other GNU tools.

This exception applies only when Bison is generating C code for a
LALR(1) parser; otherwise, the GPL terms operate as usual. You can
tell whether the exception applies to your `.c' output file by
inspecting it to see whether it says "As a special exception, when this
file is copied by Bison into a Bison output file, you may use that
output file without restriction."

File: bison.info, Node: Copying, Next: Concepts, Prev: Conditions, Up: Top

GNU GENERAL PUBLIC LICENSE
**************************

Version 2, June 1991

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

Preamble
========

The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users. This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it. (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.) You can apply it to
your programs, too.

When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it in
new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
rights.

We protect your rights with two steps: (1) copyright the software,
and (2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
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want its recipients to know that what they have is not the original, so
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Finally, any free program is threatened constantly by software
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The precise terms and conditions for copying, distribution and
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TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. This License applies to any program or other work which contains a
notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License. The "Program",
below, refers to any such program or work, and a "work based on
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translated into another language. (Hereinafter, translation is
included without limitation in the term "modification".) Each
licensee is addressed as "you".

Activities other than copying, distribution and modification are
not covered by this License; they are outside its scope. The act
of running the Program is not restricted, and the output from the
Program is covered only if its contents constitute a work based on
the Program (independent of having been made by running the
Program). Whether that is true depends on what the Program does.

1. You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any
warranty; and give any other recipients of the Program a copy of
this License along with the Program.

You may charge a fee for the physical act of transferring a copy,
and you may at your option offer warranty protection in exchange
for a fee.

2. You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

a. You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

b. You must cause any work that you distribute or publish, that
in whole or in part contains or is derived from the Program
or any part thereof, to be licensed as a whole at no charge
to all third parties under the terms of this License.

c. If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display
an announcement including an appropriate copyright notice and
a notice that there is no warranty (or else, saying that you
provide a warranty) and that users may redistribute the
program under these conditions, and telling the user how to
view a copy of this License. (Exception: if the Program
itself is interactive but does not normally print such an
announcement, your work based on the Program is not required
to print an announcement.)

These requirements apply to the modified work as a whole. If
identifiable sections of that work are not derived from the
Program, and can be reasonably considered independent and separate
works in themselves, then this License, and its terms, do not
apply to those sections when you distribute them as separate
works. But when you distribute the same sections as part of a
whole which is a work based on the Program, the distribution of
the whole must be on the terms of this License, whose permissions
for other licensees extend to the entire whole, and thus to each
and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or
contest your rights to work written entirely by you; rather, the
intent is to exercise the right to control the distribution of
derivative or collective works based on the Program.

In addition, mere aggregation of another work not based on the
Program with the Program (or with a work based on the Program) on
a volume of a storage or distribution medium does not bring the
other work under the scope of this License.

3. You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms
of Sections 1 and 2 above provided that you also do one of the
following:

a. Accompany it with the complete corresponding machine-readable
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b. Accompany it with a written offer, valid for at least three
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compelled to copy the source along with the object code.

4. You may not copy, modify, sublicense, or distribute the Program
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way you could satisfy both it and this License would be to refrain
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If any portion of this section is held invalid or unenforceable
under any particular circumstance, the balance of the section is
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licensee cannot impose that choice.

This section is intended to make thoroughly clear what is believed
to be a consequence of the rest of this License.

8. If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces,
the original copyright holder who places the Program under this
License may add an explicit geographical distribution limitation
excluding those countries, so that distribution is permitted only
in or among countries not thus excluded. In such case, this
License incorporates the limitation as if written in the body of
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9. The Free Software Foundation may publish revised and/or new
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10. If you wish to incorporate parts of the Program into other free
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NO WARRANTY

11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO
WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE
LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT
WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT
NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
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QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY
SERVICING, REPAIR OR CORRECTION.

12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY
MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL,
INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR
INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU
OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY
OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

END OF TERMS AND CONDITIONS

Appendix: How to Apply These Terms to Your New Programs
=======================================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.

ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
Copyright (C) YYYY NAME OF AUTHOR

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

Also add information on how to contact you by electronic and paper
mail.

If the program is interactive, make it output a short notice like
this when it starts in an interactive mode:

Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.

The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License. Of course, the
commands you use may be called something other than `show w' and `show
c'; they could even be mouse-clicks or menu items--whatever suits your
program.

You should also get your employer (if you work as a programmer) or
your school, if any, to sign a "copyright disclaimer" for the program,
if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

SIGNATURE OF TY COON, 1 April 1989
Ty Coon, President of Vice

This General Public License does not permit incorporating your
program into proprietary programs. If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library. If this is what you want to do, use the
GNU Library General Public License instead of this License.

File: bison.info, Node: Concepts, Next: Examples, Prev: Copying, Up: Top

The Concepts of Bison
*********************

This chapter introduces many of the basic concepts without which the
details of Bison will not make sense. If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter
carefully.

* Menu:

* Language and Grammar:: Languages and context-free grammars,
as mathematical ideas.
* Grammar in Bison:: How we represent grammars for Bison's sake.
* Semantic Values:: Each token or syntactic grouping can have
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.

File: bison.info, Node: Language and Grammar, Next: Grammar in Bison, Up: Concepts

Languages and Context-Free Grammars
===================================

In order for Bison to parse a language, it must be described by a
"context-free grammar". This means that you specify one or more
"syntactic groupings" and give rules for constructing them from their
parts. For example, in the C language, one kind of grouping is called
an `expression'. One rule for making an expression might be, "An
expression can be made of a minus sign and another expression".
Another would be, "An expression can be an integer". As you can see,
rules are often recursive, but there must be at least one rule which
leads out of the recursion.

The most common formal system for presenting such rules for humans
to read is "Backus-Naur Form" or "BNF", which was developed in order to
specify the language Algol 60. Any grammar expressed in BNF is a
context-free grammar. The input to Bison is essentially
machine-readable BNF.

There are various important subclasses of context-free grammar.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LALR(1) grammars. In brief, in these
grammars, it must be possible to tell how to parse any portion of an
input string with just a single token of look-ahead. Strictly
speaking, that is a description of an LR(1) grammar, and LALR(1)
involves additional restrictions that are hard to explain simply; but
it is rare in actual practice to find an LR(1) grammar that fails to be
LALR(1). *Note Mysterious Reduce/Reduce Conflicts: Mystery Conflicts,
for more information on this.

Parsers for LALR(1) grammars are "deterministic", meaning roughly
that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
(called a "look-ahead") of the remaining input. A context-free grammar
can be "ambiguous", meaning that there are multiple ways to apply the
grammar rules to get the some inputs. Even unambiguous grammars can be
"non-deterministic", meaning that no fixed look-ahead always suffices
to determine the next grammar rule to apply. With the proper
declarations, Bison is also able to parse these more general
context-free grammars, using a technique known as GLR parsing (for
Generalized LR). Bison's GLR parsers are able to handle any
context-free grammar for which the number of possible parses of any
given string is finite.

In the formal grammatical rules for a language, each kind of
syntactic unit or grouping is named by a "symbol". Those which are
built by grouping smaller constructs according to grammatical rules are
called "nonterminal symbols"; those which can't be subdivided are called
"terminal symbols" or "token types". We call a piece of input
corresponding to a single terminal symbol a "token", and a piece
corresponding to a single nonterminal symbol a "grouping".

We can use the C language as an example of what symbols, terminal and
nonterminal, mean. The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks. So the terminal symbols of a grammar for C include
`identifier', `number', `string', plus one symbol for each keyword,
operator or punctuation mark: `if', `return', `const', `static', `int',
`char', `plus-sign', `open-brace', `close-brace', `comma' and many
more. (These tokens can be subdivided into characters, but that is a
matter of lexicography, not grammar.)

Here is a simple C function subdivided into tokens:

int /* keyword `int' */
square (int x) /* identifier, open-paren, identifier,
identifier, close-paren */
{ /* open-brace */
return x * x; /* keyword `return', identifier, asterisk,
identifier, semicolon */
} /* close-brace */

The syntactic groupings of C include the expression, the statement,
the declaration, and the function definition. These are represented in
the grammar of C by nonterminal symbols `expression', `statement',
`declaration' and `function definition'. The full grammar uses dozens
of additional language constructs, each with its own nonterminal
symbol, in order to express the meanings of these four. The example
above is a function definition; it contains one declaration, and one
statement. In the statement, each `x' is an expression and so is `x *
x'.

Each nonterminal symbol must have grammatical rules showing how it
is made out of simpler constructs. For example, one kind of C
statement is the `return' statement; this would be described with a
grammar rule which reads informally as follows:

A `statement' can be made of a `return' keyword, an `expression'
and a `semicolon'.

There would be many other rules for `statement', one for each kind of
statement in C.

One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language. It is called the "start
symbol". In a compiler, this means a complete input program. In the C
language, the nonterminal symbol `sequence of definitions and
declarations' plays this role.

For example, `1 + 2' is a valid C expression--a valid part of a C
program--but it is not valid as an _entire_ C program. In the
context-free grammar of C, this follows from the fact that `expression'
is not the start symbol.

The Bison parser reads a sequence of tokens as its input, and groups
the tokens using the grammar rules. If the input is valid, the end
result is that the entire token sequence reduces to a single grouping
whose symbol is the grammar's start symbol. If we use a grammar for C,
the entire input must be a `sequence of definitions and declarations'.
If not, the parser reports a syntax error.

File: bison.info, Node: Grammar in Bison, Next: Semantic Values, Prev: Language and Grammar, Up: Concepts

From Formal Rules to Bison Input
================================

A formal grammar is a mathematical construct. To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a "Bison grammar" file. *Note Bison Grammar Files: Grammar File.

A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C. By convention, it
should be in lower case, such as `expr', `stmt' or `declaration'.

The Bison representation for a terminal symbol is also called a
"token type". Token types as well can be represented as C-like
identifiers. By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, `INTEGER',
`IDENTIFIER', `IF' or `RETURN'. A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case. The terminal symbol `error' is reserved for
error recovery. *Note Symbols::.

A terminal symbol can also be represented as a character literal,
just like a C character constant. You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that
same character in a literal as the terminal symbol for that token.

A third way to represent a terminal symbol is with a C string
constant containing several characters. *Note Symbols::, for more
information.

The grammar rules also have an expression in Bison syntax. For
example, here is the Bison rule for a C `return' statement. The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.

stmt: RETURN expr ';'
;

*Note Syntax of Grammar Rules: Rules.

File: bison.info, Node: Semantic Values, Next: Semantic Actions, Prev: Grammar in Bison, Up: Concepts

Semantic Values
===============

A formal grammar selects tokens only by their classifications: for
example, if a rule mentions the terminal symbol `integer constant', it
means that _any_ integer constant is grammatically valid in that
position. The precise value of the constant is irrelevant to how to
parse the input: if `x+4' is grammatical then `x+1' or `x+3989' is
equally grammatical.

But the precise value is very important for what the input means
once it is parsed. A compiler is useless if it fails to distinguish
between 4, 1 and 3989 as constants in the program! Therefore, each
token in a Bison grammar has both a token type and a "semantic value".
*Note Defining Language Semantics: Semantics, for details.

The token type is a terminal symbol defined in the grammar, such as
`INTEGER', `IDENTIFIER' or `',''. It tells everything you need to know
to decide where the token may validly appear and how to group it with
other tokens. The grammar rules know nothing about tokens except their
types.

The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier. (A token such as `','' which is just punctuation doesn't
need to have any semantic value.)

For example, an input token might be classified as token type
`INTEGER' and have the semantic value 4. Another input token might
have the same token type `INTEGER' but value 3989. When a grammar rule
says that `INTEGER' is allowed, either of these tokens is acceptable
because each is an `INTEGER'. When the parser accepts the token, it
keeps track of the token's semantic value.

Each grouping can also have a semantic value as well as its
nonterminal symbol. For example, in a calculator, an expression
typically has a semantic value that is a number. In a compiler for a
programming language, an expression typically has a semantic value that
is a tree structure describing the meaning of the expression.

File: bison.info, Node: Semantic Actions, Next: GLR Parsers, Prev: Semantic Values, Up: Concepts

Semantic Actions
================

In order to be useful, a program must do more than parse input; it
must also produce some output based on the input. In a Bison grammar,
a grammar rule can have an "action" made up of C statements. Each time
the parser recognizes a match for that rule, the action is executed.
*Note Actions::.

Most of the time, the purpose of an action is to compute the
semantic value of the whole construct from the semantic values of its
parts. For example, suppose we have a rule which says an expression
can be the sum of two expressions. When the parser recognizes such a
sum, each of the subexpressions has a semantic value which describes
how it was built up. The action for this rule should create a similar
sort of value for the newly recognized larger expression.

For example, here is a rule that says an expression can be the sum of
two subexpressions:

expr: expr '+' expr { $$ = $1 + $3; }
;

The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.

File: bison.info, Node: GLR Parsers, Next: Locations Overview, Prev: Semantic Actions, Up: Concepts

Writing GLR Parsers
===================

In some grammars, there will be cases where Bison's standard LALR(1)
parsing algorithm cannot decide whether to apply a certain grammar rule
at a given point. That is, it may not be able to decide (on the basis
of the input read so far) which of two possible reductions (applications
of a grammar rule) applies, or whether to apply a reduction or read more
of the input and apply a reduction later in the input. These are known
respectively as "reduce/reduce" conflicts (*note Reduce/Reduce::), and
"shift/reduce" conflicts (*note Shift/Reduce::).

To use a grammar that is not easily modified to be LALR(1), a more
general parsing algorithm is sometimes necessary. If you include
`%glr-parser' among the Bison declarations in your file (*note Grammar
Outline::), the result will be a Generalized LR (GLR) parser. These
parsers handle Bison grammars that contain no unresolved conflicts
(i.e., after applying precedence declarations) identically to LALR(1)
parsers. However, when faced with unresolved shift/reduce and
reduce/reduce conflicts, GLR parsers use the simple expedient of doing
both, effectively cloning the parser to follow both possibilities. Each
of the resulting parsers can again split, so that at any given time,
there can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.

During the time that there are multiple parsers, semantic actions are
recorded, but not performed. When a parser disappears, its recorded
semantic actions disappear as well, and are never performed. When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions. Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.

Let's consider an example, vastly simplified from C++.

%{
#define YYSTYPE const char*
%}

%token TYPENAME ID

%right '='
%left '+'

%glr-parser

prog :
| prog stmt { printf ("\n"); }
;

stmt : expr ';' %dprec 1
| decl %dprec 2
;

expr : ID { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
{ printf ("%s <cast> ", $1); }
| expr '+' expr { printf ("+ "); }
| expr '=' expr { printf ("= "); }
;

decl : TYPENAME declarator ';'
{ printf ("%s <declare> ", $1); }
| TYPENAME declarator '=' expr ';'
{ printf ("%s <init-declare> ", $1); }
;

declarator : ID { printf ("\"%s\" ", $1); }
| '(' declarator ')'
;

This models a problematic part of the C++ grammar--the ambiguity between
certain declarations and statements. For example,

T (x) = y+z;

parses as either an `expr' or a `stmt' (assuming that `T' is recognized
as a TYPENAME and `x' as an ID). Bison detects this as a reduce/reduce
conflict between the rules `expr : ID' and `declarator : ID', which it
cannot resolve at the time it encounters `x' in the example above. The
two `%dprec' declarations, however, give precedence to interpreting the
example as a `decl', which implies that `x' is a declarator. The
parser therefore prints

"x" y z + T <init-declare>

Consider a different input string for this parser:

T (x) + y;

Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters `x', it does not have
enough information to resolve the reduce/reduce conflict (again,
between `x' as an `expr' or a `declarator'). In this case, no
precedence declaration is used. Instead, the parser splits into two,
one assuming that `x' is an `expr', and the other assuming `x' is a
`declarator'. The second of these parsers then vanishes when it sees
`+', and the parser prints

x T <cast> y +

Suppose that instead of resolving the ambiguity, you wanted to see
all the possibilities. For this purpose, we 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 `stmt' as follows:

stmt : expr ';' %merge <stmtMerge>
| decl %merge <stmtMerge>
;

and define the `stmtMerge' function as:

static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
printf ("<OR> ");
return "";
}

with an accompanying forward declaration in the C declarations at the
beginning of the file:

%{
#define YYSTYPE const char*
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%}

With these declarations, the resulting parser will parse the first
example as both an `expr' and a `decl', and print

"x" y z + T <init-declare> x T <cast> y z + = <OR>