\input texinfo @c -*-texinfo-*-
@comment %**start of header
@setfilename bison.info
-@settitle Bison 1.24
+@include version.texi
+@settitle Bison @value{VERSION}
@setchapternewpage odd
-@iftex
@finalout
-@end iftex
-@c SMALL BOOK version
+@c SMALL BOOK version
@c This edition has been formatted so that you can format and print it in
-@c the smallbook format.
+@c the smallbook format.
@c @smallbook
-@c next time, consider using @set for edition number, etc...
-
@c Set following if you have the new `shorttitlepage' command
@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled
@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo
@c titlepage; should NOT be changed in the GPL. --mew
+@c FIXME: I don't understand this `iftex'. Obsolete? --akim.
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@end ifinfo
@comment %**end of header
-@ifinfo
-This file documents the Bison parser generator.
-
-Copyright (C) 1988, 89, 90, 91, 92, 93, 1995 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.
-
-@ignore
-Permission is granted to process this file through Tex and print the
-results, provided the printed document carries copying permission
-notice identical to this one except for the removal of this paragraph
-(this paragraph not being relevant to the printed manual).
-
-@end ignore
-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.
-@end ifinfo
+@copying
+
+This manual is for GNU Bison (version @value{VERSION}, @value{UPDATED}),
+the GNU parser generator.
+
+Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
+1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+
+@quotation
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.1 or
+any later version published by the Free Software Foundation; with no
+Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
+and with the Back-Cover Texts as in (a) below. A copy of the
+license is included in the section entitled ``GNU Free Documentation
+License.''
+
+(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
+this GNU Manual, like GNU software. Copies published by the Free
+Software Foundation raise funds for GNU development.''
+@end quotation
+@end copying
+
+@dircategory GNU programming tools
+@direntry
+* bison: (bison). GNU parser generator (yacc replacement).
+@end direntry
@ifset shorttitlepage-enabled
@shorttitlepage Bison
@titlepage
@title Bison
@subtitle The YACC-compatible Parser Generator
-@subtitle May 1995, Bison Version 1.24
+@subtitle @value{UPDATED}, Bison Version @value{VERSION}
@author by Charles Donnelly and Richard Stallman
@page
@vskip 0pt plus 1filll
-Copyright @copyright{} 1988, 89, 90, 91, 92, 93, 1995 Free Software
-Foundation
-
+@insertcopying
@sp 2
Published by the Free Software Foundation @*
-675 Massachusetts Avenue @*
-Cambridge, MA 02139 USA @*
-Printed copies are available for $15 each.@*
-ISBN-1-882114-30-2
-
-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.
-
-@ignore
-Permission is granted to process this file through TeX and print the
-results, provided the printed document carries copying permission
-notice identical to this one except for the removal of this paragraph
-(this paragraph not being relevant to the printed manual).
-
-@end ignore
-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.
+59 Temple Place, Suite 330 @*
+Boston, MA 02111-1307 USA @*
+Printed copies are available from the Free Software Foundation.@*
+ISBN 1-882114-44-2
@sp 2
Cover art by Etienne Suvasa.
@end titlepage
-@page
-@node Top, Introduction, (dir), (dir)
+@contents
-@ifinfo
-This manual documents version 1.24 of Bison.
-@end ifinfo
+@ifnottex
+@node Top
+@top Bison
+@insertcopying
+@end ifnottex
@menu
-* Introduction::
-* Conditions::
+* Introduction::
+* Conditions::
* Copying:: The GNU General Public License says
how you can copy and share Bison
* 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:: Debugging Bison parsers that parse wrong.
+* 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 Detailed Node Listing ---
+@detailmenu --- The Detailed Node Listing ---
The Concepts of Bison
* Infix Calc:: Infix (algebraic) notation calculator.
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
+* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc:: Calculator with memory and trig functions.
It uses multiple data-types for semantic values.
* Exercises:: Ideas for improving the multi-function calculator.
Reverse Polish Notation Calculator
-* Decls: Rpcalc Decls. Bison and C declarations for rpcalc.
+* 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.
Grammar Rules for @code{rpcalc}
-* Rpcalc Input::
-* Rpcalc Line::
-* Rpcalc Expr::
+* Rpcalc Input::
+* Rpcalc Line::
+* Rpcalc Expr::
+
+Location Tracking Calculator: @code{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: @code{mfcalc}
Outline of a Bison Grammar
-* C Declarations:: Syntax and usage of the C declarations section.
+* 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.
-* C Code:: Syntax and usage of the additional C code section.
+* Epilogue:: Syntax and usage of the epilogue (additional code section).
Defining Language Semantics
Parser C-Language Interface
* Parser Function:: How to call @code{yyparse} and what it returns.
-* Lexical:: You must supply a function @code{yylex}
+* Lexical:: You must supply a function @code{yylex}
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
* Pure Calling:: How the calling convention differs
in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
-The Bison Parser Algorithm
+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.
* 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
* 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,
+* 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.
+
+@end detailmenu
@end menu
-@node Introduction, Conditions, Top, Top
+@node Introduction
@unnumbered Introduction
@cindex introduction
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. This edition corresponds to version 1.24 of Bison.
+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 @value{VERSION} of Bison.
-@node Conditions, Copying, Introduction, Top
+@node Conditions
@unnumbered Conditions for Using Bison
As of Bison version 1.24, we have changed the distribution terms for
-@code{yyparse} to permit using Bison's output in non-free programs.
-Formerly, Bison parsers could be used only in programs that were free
-software.
+@code{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 non-free
+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.
practical conditions for using Bison match the practical conditions for
using the other GNU tools.
-@node Copying, Concepts, Conditions, Top
-@unnumbered GNU GENERAL PUBLIC LICENSE
-@center Version 2, June 1991
-
-@display
-Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
-675 Mass Ave, Cambridge, MA 02139, USA
-
-Everyone is permitted to copy and distribute verbatim copies
-of this license document, but changing it is not allowed.
-@end display
-
-@unnumberedsec 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
-software. If the software is modified by someone else and passed on, we
-want its recipients to know that what they have is not the original, so
-that any problems introduced by others will not reflect on the original
-authors' reputations.
-
- Finally, any free program is threatened constantly by software
-patents. We wish to avoid the danger that redistributors of a free
-program will individually obtain patent licenses, in effect making the
-program proprietary. To prevent this, we have made it clear that any
-patent must be licensed for everyone's free use or not licensed at all.
-
- The precise terms and conditions for copying, distribution and
-modification follow.
-
-@iftex
-@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
-@end iftex
-@ifinfo
-@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
-@end ifinfo
-
-@enumerate 0
-@item
-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 the Program''
-means either the Program or any derivative work under copyright law:
-that is to say, a work containing the Program or a portion of it,
-either verbatim or with modifications and/or 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.
-
-@item
-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.
-
-@item
-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:
-
-@enumerate a
-@item
-You must cause the modified files to carry prominent notices
-stating that you changed the files and the date of any change.
-
-@item
-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.
-
-@item
-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.)
-@end enumerate
-
-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.
-
-@item
-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:
-
-@enumerate a
-@item
-Accompany it with the complete corresponding machine-readable
-source code, which must be distributed under the terms of Sections
-1 and 2 above on a medium customarily used for software interchange; or,
-
-@item
-Accompany it with a written offer, valid for at least three
-years, to give any third party, for a charge no more than your
-cost of physically performing source distribution, a complete
-machine-readable copy of the corresponding source code, to be
-distributed under the terms of Sections 1 and 2 above on a medium
-customarily used for software interchange; or,
-
-@item
-Accompany it with the information you received as to the offer
-to distribute corresponding source code. (This alternative is
-allowed only for noncommercial distribution and only if you
-received the program in object code or executable form with such
-an offer, in accord with Subsection b above.)
-@end enumerate
-
-The source code for a work means the preferred form of the work for
-making modifications to it. For an executable work, complete source
-code means all the source code for all modules it contains, plus any
-associated interface definition files, plus the scripts used to
-control compilation and installation of the executable. However, as a
-special exception, the source code distributed need not include
-anything that is normally distributed (in either source or binary
-form) with the major components (compiler, kernel, and so on) of the
-operating system on which the executable runs, unless that component
-itself accompanies the executable.
-
-If distribution of executable or object code is made by offering
-access to copy from a designated place, then offering equivalent
-access to copy the source code from the same place counts as
-distribution of the source code, even though third parties are not
-compelled to copy the source along with the object code.
-
-@item
-You may not copy, modify, sublicense, or distribute the Program
-except as expressly provided under this License. Any attempt
-otherwise to copy, modify, sublicense or distribute the Program is
-void, and will automatically terminate your rights under this License.
-However, parties who have received copies, or rights, from you under
-this License will not have their licenses terminated so long as such
-parties remain in full compliance.
-
-@item
-You are not required to accept this License, since you have not
-signed it. However, nothing else grants you permission to modify or
-distribute the Program or its derivative works. These actions are
-prohibited by law if you do not accept this License. Therefore, by
-modifying or distributing the Program (or any work based on the
-Program), you indicate your acceptance of this License to do so, and
-all its terms and conditions for copying, distributing or modifying
-the Program or works based on it.
-
-@item
-Each time you redistribute the Program (or any work based on the
-Program), the recipient automatically receives a license from the
-original licensor to copy, distribute or modify the Program subject to
-these terms and conditions. You may not impose any further
-restrictions on the recipients' exercise of the rights granted herein.
-You are not responsible for enforcing compliance by third parties to
-this License.
-
-@item
-If, as a consequence of a court judgment or allegation of patent
-infringement or for any other reason (not limited to patent issues),
-conditions are imposed on you (whether by court order, agreement or
-otherwise) that contradict the conditions of this License, they do not
-excuse you from the conditions of this License. If you cannot
-distribute so as to satisfy simultaneously your obligations under this
-License and any other pertinent obligations, then as a consequence you
-may not distribute the Program at all. For example, if a patent
-license would not permit royalty-free redistribution of the Program by
-all those who receive copies directly or indirectly through you, then
-the only way you could satisfy both it and this License would be to
-refrain entirely from distribution of the Program.
-
-If any portion of this section is held invalid or unenforceable under
-any particular circumstance, the balance of the section is intended to
-apply and the section as a whole is intended to apply in other
-circumstances.
-
-It is not the purpose of this section to induce you to infringe any
-patents or other property right claims or to contest validity of any
-such claims; this section has the sole purpose of protecting the
-integrity of the free software distribution system, which is
-implemented by public license practices. Many people have made
-generous contributions to the wide range of software distributed
-through that system in reliance on consistent application of that
-system; it is up to the author/donor to decide if he or she is willing
-to distribute software through any other system and a 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.
-
-@item
-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 this License.
-
-@item
-The Free Software Foundation may publish revised and/or new versions
-of the General Public License from time to time. Such new versions will
-be similar in spirit to the present version, but may differ in detail to
-address new problems or concerns.
-
-Each version is given a distinguishing version number. If the Program
-specifies a version number of this License which applies to it and ``any
-later version'', you have the option of following the terms and conditions
-either of that version or of any later version published by the Free
-Software Foundation. If the Program does not specify a version number of
-this License, you may choose any version ever published by the Free Software
-Foundation.
-
-@item
-If you wish to incorporate parts of the Program into other free
-programs whose distribution conditions are different, write to the author
-to ask for permission. For software which is copyrighted by the Free
-Software Foundation, write to the Free Software Foundation; we sometimes
-make exceptions for this. Our decision will be guided by the two goals
-of preserving the free status of all derivatives of our free software and
-of promoting the sharing and reuse of software generally.
-
-@iftex
-@heading NO WARRANTY
-@end iftex
-@ifinfo
-@center NO WARRANTY
-@end ifinfo
-
-@item
-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 FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
-TO THE 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.
-
-@item
-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 enumerate
-
-@iftex
-@heading END OF TERMS AND CONDITIONS
-@end iftex
-@ifinfo
-@center END OF TERMS AND CONDITIONS
-@end ifinfo
-
-@page
-@unnumberedsec 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.
+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 @samp{.c} output file by
+inspecting it to see whether it says ``As a special exception, when
+this file is copied by Bison into a Bison output file, you may use
+that output file without restriction.''
-@smallexample
-@var{one line to give the program's name and a brief idea of what it does.}
-Copyright (C) 19@var{yy} @var{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., 675 Mass Ave, Cambridge, MA 02139, USA.
-@end smallexample
-
-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:
-
-@smallexample
-Gnomovision version 69, Copyright (C) 19@var{yy} @var{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.
-@end smallexample
-
-The hypothetical commands @samp{show w} and @samp{show c} should show
-the appropriate parts of the General Public License. Of course, the
-commands you use may be called something other than @samp{show w} and
-@samp{show c}; they could even be mouse-clicks or menu items---whatever
-suits your program.
+@include gpl.texi
-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:
-
-@smallexample
-Yoyodyne, Inc., hereby disclaims all copyright interest in the program
-`Gnomovision' (which makes passes at compilers) written by James Hacker.
-
-@var{signature of Ty Coon}, 1 April 1989
-Ty Coon, President of Vice
-@end smallexample
-
-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.
-
-@node Concepts, Examples, Copying, Top
+@node Concepts
@chapter The Concepts of Bison
This chapter introduces many of the basic concepts without which the
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.
@end menu
-@node Language and Grammar, Grammar in Bison, , Concepts
+@node Language and Grammar
@section Languages and Context-Free Grammars
@cindex context-free grammar
context-free grammar. The input to Bison is essentially machine-readable
BNF.
-Not all context-free languages can be handled by Bison, only those
-that are LALR(1). In brief, this means that it must be possible to
+@cindex LALR(1) grammars
+@cindex LR(1) grammars
+There are various important subclasses of context-free grammar. Although it
+can handle almost all context-free grammars, Bison is optimized for what
+are called LALR(1) grammars.
+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
LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, ,
Mysterious Reduce/Reduce Conflicts}, for more information on this.
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@cindex ambiguous grammars
+@cindex non-deterministic parsing
+Parsers for LALR(1) grammars are @dfn{deterministic}, meaning roughly that
+the next grammar rule to apply at any point in the input is uniquely
+determined by the preceding input and a fixed, finite portion (called
+a @dfn{look-ahead}) of the remaining input.
+A context-free grammar can be @dfn{ambiguous}, meaning that
+there are multiple ways to apply the grammar rules to get the some inputs.
+Even unambiguous grammars can be @dfn{non-deterministic}, meaning that no
+fixed look-ahead always suffices to determine the next grammar rule to apply.
+With the proper declarations, Bison is also able to parse these more general
+context-free grammars, using a technique known as GLR parsing (for
+Generalized LR). Bison's GLR parsers are able to handle any context-free
+grammar for which the number of possible parses of any given string
+is finite.
+
@cindex symbols (abstract)
@cindex token
@cindex syntactic grouping
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token types}. We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
-corresponding to a single nonterminal symbol a @dfn{grouping}.@refill
+corresponding to a single nonterminal symbol a @dfn{grouping}.
We can use the C language as an example of what symbols, terminal and
nonterminal, mean. The tokens of C are identifiers, constants (numeric and
Here is a simple C function subdivided into tokens:
+@ifinfo
@example
int /* @r{keyword `int'} */
-square (x) /* @r{identifier, open-paren,} */
- /* @r{identifier, close-paren} */
- int x; /* @r{keyword `int', identifier, semicolon} */
+square (int x) /* @r{identifier, open-paren, identifier,}
+ @r{identifier, close-paren} */
@{ /* @r{open-brace} */
- return x * x; /* @r{keyword `return', identifier,} */
- /* @r{asterisk, identifier, semicolon} */
+ return x * x; /* @r{keyword `return', identifier, asterisk,
+ identifier, semicolon} */
@} /* @r{close-brace} */
@end example
+@end ifinfo
+@ifnotinfo
+@example
+int /* @r{keyword `int'} */
+square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */
+@{ /* @r{open-brace} */
+ return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
+@} /* @r{close-brace} */
+@end example
+@end ifnotinfo
The syntactic groupings of C include the expression, the statement, the
declaration, and the function definition. These are represented in the
must be a `sequence of definitions and declarations'. If not, the parser
reports a syntax error.
-@node Grammar in Bison, Semantic Values, Language and Grammar, Concepts
+@node Grammar in Bison
@section From Formal Rules to Bison Input
@cindex Bison grammar
@cindex grammar, Bison
@code{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 @code{error} is reserved for error recovery.
-@xref{Symbols}.@refill
+@xref{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. @xref{Symbols}, for more information.
+
The grammar rules also have an expression in Bison syntax. For example,
here is the Bison rule for a C @code{return} statement. The semicolon in
quotes is a literal character token, representing part of the C syntax for
@noindent
@xref{Rules, ,Syntax of Grammar Rules}.
-@node Semantic Values, Semantic Actions, Grammar in Bison, Concepts
+@node Semantic Values
@section Semantic Values
@cindex semantic value
@cindex value, semantic
@emph{any} integer constant is grammatically valid in that position. The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
-grammatical.@refill
+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
@code{INTEGER}, @code{IDENTIFIER} or @code{','}. 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.@refill
+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
language, an expression typically has a semantic value that is a tree
structure describing the meaning of the expression.
-@node Semantic Actions, Bison Parser, Semantic Values, Concepts
+@node Semantic Actions
@section Semantic Actions
@cindex semantic actions
@cindex actions, semantic
rule can have an @dfn{action} made up of C statements. Each time the
parser recognizes a match for that rule, the action is executed.
@xref{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
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.
-@node Bison Parser, Stages, Semantic Actions, Concepts
+@node GLR Parsers
+@section Writing GLR Parsers
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@findex %glr-parser
+@cindex conflicts
+@cindex shift/reduce conflicts
+
+In some grammars, there will be cases where Bison's standard LALR(1)
+parsing algorithm cannot decide whether to apply a certain grammar rule
+at a given point. That is, it may not be able to decide (on the basis
+of the input read so far) which of two possible reductions (applications
+of a grammar rule) applies, or whether to apply a reduction or read more
+of the input and apply a reduction later in the input. These are known
+respectively as @dfn{reduce/reduce} conflicts (@pxref{Reduce/Reduce}),
+and @dfn{shift/reduce} conflicts (@pxref{Shift/Reduce}).
+
+To use a grammar that is not easily modified to be LALR(1), a more
+general parsing algorithm is sometimes necessary. If you include
+@code{%glr-parser} among the Bison declarations in your file
+(@pxref{Grammar Outline}), the result will be a Generalized LR (GLR)
+parser. These parsers handle Bison grammars that contain no unresolved
+conflicts (i.e., after applying precedence declarations) identically to
+LALR(1) parsers. However, when faced with unresolved shift/reduce and
+reduce/reduce conflicts, GLR parsers use the simple expedient of doing
+both, effectively cloning the parser to follow both possibilities. Each
+of the resulting parsers can again split, so that at any given time,
+there can be any number of possible parses being explored. The parsers
+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++.
+
+@example
+%@{
+ #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 ')'
+ ;
+@end example
+
+@noindent
+This models a problematic part of the C++ grammar---the ambiguity between
+certain declarations and statements. For example,
+
+@example
+T (x) = y+z;
+@end example
+
+@noindent
+parses as either an @code{expr} or a @code{stmt}
+(assuming that @samp{T} is recognized as a TYPENAME and @samp{x} as an ID).
+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
+@code{decl}, which implies that @code{x} is a declarator.
+The parser therefore prints
+
+@example
+"x" y z + T <init-declare>
+@end example
+
+Consider a different input string for this parser:
+
+@example
+T (x) + y;
+@end example
+
+@noindent
+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
+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
+
+@example
+x T <cast> 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
+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:
+
+@example
+stmt : expr ';' %merge <stmtMerge>
+ | decl %merge <stmtMerge>
+ ;
+@end example
+
+@noindent
+
+and define the @code{stmtMerge} function as:
+
+@example
+static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1)
+@{
+ printf ("<OR> ");
+ return "";
+@}
+@end example
+
+@noindent
+with an accompanying forward declaration
+in the C declarations at the beginning of the file:
+
+@example
+%@{
+ #define YYSTYPE const char*
+ 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
+
+@example
+"x" y z + T <init-declare> x T <cast> y z + = <OR>
+@end example
+
+
+@node Locations Overview
+@section Locations
+@cindex location
+@cindex textual position
+@cindex position, 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.
+Bison provides a mechanism for handling these locations.
+
+Each token has a semantic value. In a similar fashion, each token has an
+associated location, but the type of locations is the same for all tokens and
+groupings. Moreover, the output parser is equipped with a default data
+structure for storing locations (@pxref{Locations}, for more details).
+
+Like semantic values, locations can be reached in actions using a dedicated
+set of constructs. In the example above, the location of the whole grouping
+is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
+@code{@@3}.
+
+When a rule is matched, a default action is used to compute the semantic value
+of its left hand side (@pxref{Actions}). In the same way, another default
+action is used for locations. However, the action for locations is general
+enough for most cases, meaning there is usually no need to describe for each
+rule how @code{@@$} should be formed. When building a new location for a given
+grouping, the default behavior of the output parser is to take the beginning
+of the first symbol, and the end of the last symbol.
+
+@node Bison Parser
@section Bison Output: the Parser File
@cindex Bison parser
@cindex Bison utility
expressions. As it does this, it runs the actions for the grammar rules it
uses.
-The tokens come from a function called the @dfn{lexical analyzer} that you
-must supply in some fashion (such as by writing it in C). The Bison parser
-calls the lexical analyzer each time it wants a new token. It doesn't know
-what is ``inside'' the tokens (though their semantic values may reflect
-this). Typically the lexical analyzer makes the tokens by parsing
-characters of text, but Bison does not depend on this. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C). The Bison
+parser calls the lexical analyzer each time it wants a new token. It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this). Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
The Bison parser file is C code which defines a function named
@code{yyparse} which implements that grammar. This function does not make
@xref{Interface, ,Parser C-Language Interface}.
Aside from the token type names and the symbols in the actions you
-write, all variable and function names used in the Bison parser file
+write, all symbols defined in the Bison parser file itself
begin with @samp{yy} or @samp{YY}. This includes interface functions
such as the lexical analyzer function @code{yylex}, the error reporting
function @code{yyerror} and the parser function @code{yyparse} itself.
or @samp{YY} in the Bison grammar file except for the ones defined in
this manual.
-@node Stages, Grammar Layout, Bison Parser, Concepts
+In some cases the Bison parser file includes system headers, and in
+those cases your code should respect the identifiers reserved by those
+headers. On some non-@sc{gnu} hosts, @code{<alloca.h>},
+@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
+declare memory allocators and related types. Other system headers may
+be included if you define @code{YYDEBUG} to a nonzero value
+(@pxref{Tracing, ,Tracing Your Parser}).
+
+@node Stages
@section Stages in Using Bison
@cindex stages in using Bison
@cindex using Bison
@enumerate
@item
Formally specify the grammar in a form recognized by Bison
-(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule in the language,
-describe the action that is to be taken when an instance of that rule
-is recognized. The action is described by a sequence of C statements.
+(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule
+in the language, describe the action that is to be taken when an
+instance of that rule is recognized. The action is described by a
+sequence of C statements.
@item
-Write a lexical analyzer to process input and pass tokens to the
-parser. The lexical analyzer may be written by hand in C
-(@pxref{Lexical, ,The Lexical Analyzer Function @code{yylex}}). It could also be produced using Lex, but the use
-of Lex is not discussed in this manual.
+Write a lexical analyzer to process input and pass tokens to the parser.
+The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
+Lexical Analyzer Function @code{yylex}}). It could also be produced
+using Lex, but the use of Lex is not discussed in this manual.
@item
Write a controlling function that calls the Bison-produced parser.
Link the object files to produce the finished product.
@end enumerate
-@node Grammar Layout, , Stages, Concepts
+@node Grammar Layout
@section The Overall Layout of a Bison Grammar
@cindex grammar file
@cindex file format
@example
%@{
-@var{C declarations}
+@var{Prologue}
%@}
@var{Bison declarations}
%%
@var{Grammar rules}
%%
-@var{Additional C code}
+@var{Epilogue}
@end example
@noindent
The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
in every Bison grammar file to separate the sections.
-The C declarations may define types and variables used in the actions.
-You can also use preprocessor commands to define macros used there, and use
+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.
The Bison declarations declare the names of the terminal and nonterminal
The grammar rules define how to construct each nonterminal symbol from its
parts.
-The additional C code can contain any C 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 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.
-@node Examples, Grammar File, Concepts, Top
+@node Examples
@chapter Examples
@cindex simple examples
@cindex examples, simple
* Infix Calc:: Infix (algebraic) notation calculator.
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
+* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc:: Calculator with memory and trig functions.
It uses multiple data-types for semantic values.
* Exercises:: Ideas for improving the multi-function calculator.
@end menu
-@node RPN Calc, Infix Calc, , Examples
+@node RPN Calc
@section Reverse Polish Notation Calculator
@cindex reverse polish notation
@cindex polish notation calculator
@samp{.y} extension is a convention used for Bison input files.
@menu
-* Decls: Rpcalc Decls. Bison and C declarations for rpcalc.
+* 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.
* Comp: Rpcalc Compile. Run the C compiler on the output code.
@end menu
-@node Rpcalc Decls, Rpcalc Rules, , RPN Calc
+@node Rpcalc Decls
@subsection Declarations for @code{rpcalc}
Here are the C and Bison declarations for the reverse polish notation
calculator. As in C, comments are placed between @samp{/*@dots{}*/}.
@example
-/* Reverse polish notation calculator. */
+/* Reverse polish notation calculator. */
%@{
#define YYSTYPE double
%token NUM
-%% /* Grammar rules and actions follow */
+%% /* Grammar rules and actions follow. */
@end example
-The C declarations section (@pxref{C Declarations, ,The C Declarations Section}) contains two
+The declarations section (@pxref{Prologue, , The prologue}) contains two
preprocessor directives.
The @code{#define} directive defines the macro @code{YYSTYPE}, thus
-specifying the C data type for semantic values of both tokens and groupings
-(@pxref{Value Type, ,Data Types of Semantic Values}). The Bison parser will use whatever type
-@code{YYSTYPE} is defined as; if you don't define it, @code{int} is the
-default. Because we specify @code{double}, each token and each expression
-has an associated value, which is a floating point number.
+specifying the C data type for semantic values of both tokens and
+groupings (@pxref{Value Type, ,Data Types of Semantic Values}). The
+Bison parser will use whatever type @code{YYSTYPE} is defined as; if you
+don't define it, @code{int} is the default. Because we specify
+@code{double}, each token and each expression has an associated value,
+which is a floating point number.
The @code{#include} directive is used to declare the exponentiation
function @code{pow}.
-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 single-character literal must be declared here. (Single-character
+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
+single-character literal must be declared here. (Single-character
literals normally don't need to be declared.) In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is @code{NUM}, the token
type for numeric constants.
-@node Rpcalc Rules, Rpcalc Lexer, Rpcalc Decls, RPN Calc
+@node Rpcalc Rules
@subsection Grammar Rules for @code{rpcalc}
Here are the grammar rules for the reverse polish notation calculator.
rule are referred to as @code{$1}, @code{$2}, and so on.
@menu
-* Rpcalc Input::
-* Rpcalc Line::
-* Rpcalc Expr::
+* Rpcalc Input::
+* Rpcalc Line::
+* Rpcalc Expr::
@end menu
-@node Rpcalc Input, Rpcalc Line, , Rpcalc Rules
+@node Rpcalc Input
@subsubsection Explanation of @code{input}
Consider the definition of @code{input}:
The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
-input tokens; we will arrange for the latter to happen at end of file.
+input tokens; we will arrange for the latter to happen at end-of-input.
-@node Rpcalc Line, Rpcalc Expr, Rpcalc Input, Rpcalc Rules
+@node Rpcalc Line
@subsubsection Explanation of @code{line}
Now consider the definition of @code{line}:
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.
-@node Rpcalc Expr, , Rpcalc Line, Rpcalc Rules
+@node Rpcalc Expr
@subsubsection Explanation of @code{expr}
The @code{exp} grouping has several rules, one for each kind of expression.
This is what happens in the first rule (the one that uses @code{NUM}).
The formatting shown here is the recommended convention, but Bison does
-not require it. You can add or change whitespace as much as you wish.
+not require it. You can add or change white space as much as you wish.
For example, this:
@example
@noindent
The latter, however, is much more readable.
-@node Rpcalc Lexer, Rpcalc Main, Rpcalc Rules, RPN Calc
+@node Rpcalc Lexer
@subsection The @code{rpcalc} Lexical Analyzer
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing
-The lexical analyzer's job is low-level parsing: converting characters or
-sequences of characters into tokens. The Bison parser gets its tokens by
-calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The lexical analyzer's job is low-level parsing: converting characters
+or sequences of characters into tokens. The Bison parser gets its
+tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
+Analyzer Function @code{yylex}}.
Only a simple lexical analyzer is needed for the RPN calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
represents a token type. The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways. If the token type is a character literal, then its
-numeric code is the ASCII code for that character; you can use the same
+numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number. In this example,
therefore, @code{NUM} becomes a macro for @code{yylex} to use.
-The semantic value of the token (if it has one) is stored into the global
-variable @code{yylval}, which is where the Bison parser will look for it.
-(The C data type of @code{yylval} is @code{YYSTYPE}, which was defined
-at the beginning of the grammar; @pxref{Rpcalc Decls, ,Declarations for @code{rpcalc}}.)
+The semantic value of the token (if it has one) is stored into the
+global variable @code{yylval}, which is where the Bison parser will look
+for it. (The C data type of @code{yylval} is @code{YYSTYPE}, which was
+defined at the beginning of the grammar; @pxref{Rpcalc Decls,
+,Declarations for @code{rpcalc}}.)
-A token type code of zero is returned if the end-of-file is encountered.
-(Bison recognizes any nonpositive value as indicating the end of the
-input.)
+A token type code of zero is returned if the end-of-input is encountered.
+(Bison recognizes any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
@example
@group
-/* Lexical analyzer returns a double floating point
- number on the stack and the token NUM, or the ASCII
- character read if not a number. Skips all blanks
- and tabs, returns 0 for EOF. */
+/* The lexical analyzer returns a double floating point
+ number on the stack and the token NUM, or the numeric code
+ of the character read if not a number. It skips all blanks
+ and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
@end group
@group
-yylex ()
+int
+yylex (void)
@{
int c;
- /* skip white space */
- while ((c = getchar ()) == ' ' || c == '\t')
+ /* Skip white space. */
+ while ((c = getchar ()) == ' ' || c == '\t')
;
@end group
@group
- /* process numbers */
- if (c == '.' || isdigit (c))
+ /* Process numbers. */
+ if (c == '.' || isdigit (c))
@{
ungetc (c, stdin);
scanf ("%lf", &yylval);
@}
@end group
@group
- /* return end-of-file */
- if (c == EOF)
+ /* Return end-of-input. */
+ if (c == EOF)
return 0;
- /* return single chars */
- return c;
+ /* Return a single char. */
+ return c;
@}
@end group
@end example
-@node Rpcalc Main, Rpcalc Error, Rpcalc Lexer, RPN Calc
+@node Rpcalc Main
@subsection The Controlling Function
@cindex controlling function
@cindex main function in simple example
@example
@group
-main ()
+int
+main (void)
@{
- yyparse ();
+ return yyparse ();
@}
@end group
@end example
-@node Rpcalc Error, Rpcalc Gen, Rpcalc Main, RPN Calc
+@node Rpcalc Error
@subsection The Error Reporting Routine
@cindex error reporting routine
When @code{yyparse} detects a syntax error, it calls the error reporting
-function @code{yyerror} to print an error message (usually but not always
-@code{"parse error"}). It is up to the programmer to supply @code{yyerror}
-(@pxref{Interface, ,Parser C-Language Interface}), so here is the definition we will use:
+function @code{yyerror} to print an error message (usually but not
+always @code{"parse error"}). It is up to the programmer to supply
+@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so
+here is the definition we will use:
@example
@group
#include <stdio.h>
-yyerror (s) /* Called by yyparse on error */
- char *s;
+void
+yyerror (const char *s) /* called by yyparse on error */
@{
printf ("%s\n", s);
@}
(@pxref{Error Recovery}). Otherwise, @code{yyparse} returns nonzero. We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit. This is not clean behavior for a
-real calculator, but it is adequate in the first example.
+real calculator, but it is adequate for the first example.
-@node Rpcalc Gen, Rpcalc Compile, Rpcalc Error, RPN Calc
+@node Rpcalc Gen
@subsection Running Bison to Make the Parser
@cindex running Bison (introduction)
-Before running Bison to produce a parser, we need to decide how to arrange
-all the source code in one or more source files. For such a simple example,
-the easiest thing is to put everything in one file. The definitions of
-@code{yylex}, @code{yyerror} and @code{main} go at the end, in the
-``additional C code'' section of the file (@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).
+Before running Bison to produce a parser, we need to decide how to
+arrange all the source code in one or more source files. For such a
+simple example, the easiest thing is to put everything in one file. The
+definitions of @code{yylex}, @code{yyerror} and @code{main} go at the
+end, in the epilogue of the file
+(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).
For a large project, you would probably have several source files, and use
@code{make} to arrange to recompile them.
@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c},
-removing the @samp{.y} from the original file name. The file output by
+removing the @samp{.y} from the original file name. The file output by
Bison contains the source code for @code{yyparse}. The additional
functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
are copied verbatim to the output.
-@node Rpcalc Compile, , Rpcalc Gen, RPN Calc
+@node Rpcalc Compile
@subsection Compiling the Parser File
@cindex compiling the parser
@example
@group
# @r{List files in current directory.}
-% ls
+$ @kbd{ls}
rpcalc.tab.c rpcalc.y
@end group
@group
# @r{Compile the Bison parser.}
# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
-% cc rpcalc.tab.c -lm -o rpcalc
+$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
@end group
@group
# @r{List files again.}
-% ls
+$ @kbd{ls}
rpcalc rpcalc.tab.c rpcalc.y
@end group
@end example
example session using @code{rpcalc}.
@example
-% rpcalc
-4 9 +
+$ @kbd{rpcalc}
+@kbd{4 9 +}
13
-3 7 + 3 4 5 *+-
+@kbd{3 7 + 3 4 5 *+-}
-13
-3 7 + 3 4 5 * + - n @r{Note the unary minus, @samp{n}}
+@kbd{3 7 + 3 4 5 * + - n} @r{Note the unary minus, @samp{n}}
13
-5 6 / 4 n +
+@kbd{5 6 / 4 n +}
-3.166666667
-3 4 ^ @r{Exponentiation}
+@kbd{3 4 ^} @r{Exponentiation}
81
-^D @r{End-of-file indicator}
-%
+@kbd{^D} @r{End-of-file indicator}
+$
@end example
-@node Infix Calc, Simple Error Recovery, RPN Calc, Examples
+@node Infix Calc
@section Infix Notation Calculator: @code{calc}
@cindex infix notation calculator
@cindex @code{calc}
@end example
@noindent
-The functions @code{yylex}, @code{yyerror} and @code{main} can be the same
-as before.
+The functions @code{yylex}, @code{yyerror} and @code{main} can be the
+same as before.
There are two important new features shown in this code.
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
-by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}.
+by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator
+Precedence}.
-The other important new feature is the @code{%prec} in the grammar section
-for the unary minus operator. The @code{%prec} simply instructs Bison that
-the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this
-case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}.
+The other important new feature is the @code{%prec} in the grammar
+section for the unary minus operator. The @code{%prec} simply instructs
+Bison that the rule @samp{| '-' exp} has the same precedence as
+@code{NEG}---in this case the next-to-highest. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
Here is a sample run of @file{calc.y}:
@need 500
@example
-% calc
-4 + 4.5 - (34/(8*3+-3))
+$ @kbd{calc}
+@kbd{4 + 4.5 - (34/(8*3+-3))}
6.880952381
--56 + 2
+@kbd{-56 + 2}
-54
-3 ^ 2
+@kbd{3 ^ 2}
9
@end example
-@node Simple Error Recovery, Multi-function Calc, Infix Calc, Examples
+@node Simple Error Recovery
@section Simple Error Recovery
@cindex error recovery, simple
Up to this point, this manual has not addressed the issue of @dfn{error
recovery}---how to continue parsing after the parser detects a syntax
-error. All we have handled is error reporting with @code{yyerror}. Recall
-that by default @code{yyparse} returns after calling @code{yyerror}. This
-means that an erroneous input line causes the calculator program to exit.
-Now we show how to rectify this deficiency.
+error. All we have handled is error reporting with @code{yyerror}.
+Recall that by default @code{yyparse} returns after calling
+@code{yyerror}. This means that an erroneous input line causes the
+calculator program to exit. Now we show how to rectify this deficiency.
The Bison language itself includes the reserved word @code{error}, which
may be included in the grammar rules. In the example below it has
@end group
@end example
-This addition to the grammar allows for simple error recovery in the event
-of a parse error. If an expression that cannot be evaluated is read, the
-error will be recognized by the third rule for @code{line}, and parsing
-will continue. (The @code{yyerror} function is still called upon to print
-its message as well.) The action executes the statement @code{yyerrok}, a
-macro defined automatically by Bison; its meaning is that error recovery is
-complete (@pxref{Error Recovery}). Note the difference between
-@code{yyerrok} and @code{yyerror}; neither one is a misprint.@refill
+This addition to the grammar allows for simple error recovery in the
+event of a parse error. If an expression that cannot be evaluated is
+read, the error will be recognized by the third rule for @code{line},
+and parsing will continue. (The @code{yyerror} function is still called
+upon to print its message as well.) The action executes the statement
+@code{yyerrok}, a macro defined automatically by Bison; its meaning is
+that error recovery is complete (@pxref{Error Recovery}). Note the
+difference between @code{yyerrok} and @code{yyerror}; neither one is a
+misprint.
This form of error recovery deals with syntax errors. There are other
kinds of errors; for example, division by zero, which raises an exception
input. We won't discuss this issue further because it is not specific to
Bison programs.
-@node Multi-function Calc, Exercises, Simple Error Recovery, Examples
-@section Multi-Function Calculator: @code{mfcalc}
-@cindex multi-function calculator
-@cindex @code{mfcalc}
-@cindex calculator, multi-function
-
-Now that the basics of Bison have been discussed, it is time to move on to
-a more advanced problem. The above calculators provided only five
-functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}. It would
-be nice to have a calculator that provides other mathematical functions such
-as @code{sin}, @code{cos}, etc.
-
-It is easy to add new operators to the infix calculator as long as they are
-only single-character literals. The lexical analyzer @code{yylex} passes
-back all non-number characters as tokens, so new grammar rules suffice for
-adding a new operator. But we want something more flexible: built-in
-functions whose syntax has this form:
-
-@example
-@var{function_name} (@var{argument})
-@end example
-
-@noindent
-At the same time, we will add memory to the calculator, by allowing you
-to create named variables, store values in them, and use them later.
-Here is a sample session with the multi-function calculator:
-
-@example
-% mfcalc
-pi = 3.141592653589
-3.1415926536
-sin(pi)
-0.0000000000
-alpha = beta1 = 2.3
-2.3000000000
-alpha
-2.3000000000
-ln(alpha)
-0.8329091229
-exp(ln(beta1))
-2.3000000000
-%
-@end example
+@node Location Tracking Calc
+@section Location Tracking Calculator: @code{ltcalc}
+@cindex location tracking calculator
+@cindex @code{ltcalc}
+@cindex calculator, location tracking
-Note that multiple assignment and nested function calls are permitted.
+This example extends the infix notation calculator with location
+tracking. This feature will be used to improve the error messages. For
+the sake of clarity, this example is a simple integer calculator, since
+most of the work needed to use locations will be done in the lexical
+analyzer.
@menu
-* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
-* Rules: Mfcalc Rules. Grammar rules for the calculator.
-* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
+* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
+* Lexer: Ltcalc Lexer. The lexical analyzer.
@end menu
-@node Mfcalc Decl, Mfcalc Rules, , Multi-function Calc
-@subsection Declarations for @code{mfcalc}
+@node Ltcalc Decls
+@subsection Declarations for @code{ltcalc}
-Here are the C and Bison declarations for the multi-function calculator.
+The C and Bison declarations for the location tracking calculator are
+the same as the declarations for the infix notation calculator.
+
+@example
+/* Location tracking calculator. */
-@smallexample
%@{
-#include <math.h> /* For math functions, cos(), sin(), etc. */
-#include "calc.h" /* Contains definition of `symrec' */
+#define YYSTYPE int
+#include <math.h>
%@}
-%union @{
-double val; /* For returning numbers. */
-symrec *tptr; /* For returning symbol-table pointers */
-@}
-%token <val> NUM /* Simple double precision number */
-%token <tptr> VAR FNCT /* Variable and Function */
-%type <val> exp
+/* Bison declarations. */
+%token NUM
-%right '='
%left '-' '+'
%left '*' '/'
-%left NEG /* Negation--unary minus */
-%right '^' /* Exponentiation */
+%left NEG
+%right '^'
-/* Grammar follows */
+%% /* Grammar follows */
+@end example
-%%
-@end smallexample
+@noindent
+Note there are no declarations specific to locations. Defining a data
+type for storing locations is not needed: we will use the type provided
+by default (@pxref{Location Type, ,Data Types of Locations}), which is a
+four member structure with the following integer fields:
+@code{first_line}, @code{first_column}, @code{last_line} and
+@code{last_column}.
-The above grammar introduces only two new features of the Bison language.
-These features allow semantic values to have various data types
-(@pxref{Multiple Types, ,More Than One Value Type}).
+@node Ltcalc Rules
+@subsection Grammar Rules for @code{ltcalc}
-The @code{%union} declaration specifies the entire list of possible types;
-this is instead of defining @code{YYSTYPE}. The allowable types are now
-double-floats (for @code{exp} and @code{NUM}) and pointers to entries in
-the symbol table. @xref{Union Decl, ,The Collection of Value Types}.
+Whether handling locations or not has no effect on the syntax of your
+language. Therefore, grammar rules for this example will be very close
+to those of the previous example: we will only modify them to benefit
+from the new information.
-Since values can now have various types, it is necessary to associate a
-type with each grammar symbol whose semantic value is used. These symbols
+Here, we will use locations to report divisions by zero, and locate the
+wrong expressions or subexpressions.
+
+@example
+@group
+input : /* empty */
+ | input line
+;
+@end group
+
+@group
+line : '\n'
+ | exp '\n' @{ printf ("%d\n", $1); @}
+;
+@end group
+
+@group
+exp : NUM @{ $$ = $1; @}
+ | exp '+' exp @{ $$ = $1 + $3; @}
+ | exp '-' exp @{ $$ = $1 - $3; @}
+ | exp '*' exp @{ $$ = $1 * $3; @}
+@end group
+@group
+ | exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr, "%d.%d-%d.%d: division by zero",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
+@end group
+@group
+ | '-' exp %preg NEG @{ $$ = -$2; @}
+ | exp '^' exp @{ $$ = pow ($1, $3); @}
+ | '(' exp ')' @{ $$ = $2; @}
+@end group
+@end example
+
+This code shows how to reach locations inside of semantic actions, by
+using the pseudo-variables @code{@@@var{n}} for rule components, and the
+pseudo-variable @code{@@$} for groupings.
+
+We don't need to assign a value to @code{@@$}: the output parser does it
+automatically. By default, before executing the C code of each action,
+@code{@@$} is set to range from the beginning of @code{@@1} to the end
+of @code{@@@var{n}}, for a rule with @var{n} components. This behavior
+can be redefined (@pxref{Location Default Action, , Default Action for
+Locations}), and for very specific rules, @code{@@$} can be computed by
+hand.
+
+@node Ltcalc Lexer
+@subsection The @code{ltcalc} Lexical Analyzer.
+
+Until now, we relied on Bison's defaults to enable location
+tracking. The next step is to rewrite the lexical analyzer, and make it
+able to feed the parser with the token locations, as it already does for
+semantic values.
+
+To this end, we must take into account every single character of the
+input text, to avoid the computed locations of being fuzzy or wrong:
+
+@example
+@group
+int
+yylex (void)
+@{
+ int c;
+
+ /* Skip white space. */
+ while ((c = getchar ()) == ' ' || c == '\t')
+ ++yylloc.last_column;
+
+ /* Step. */
+ yylloc.first_line = yylloc.last_line;
+ yylloc.first_column = yylloc.last_column;
+@end group
+
+@group
+ /* Process numbers. */
+ if (isdigit (c))
+ @{
+ yylval = c - '0';
+ ++yylloc.last_column;
+ while (isdigit (c = getchar ()))
+ @{
+ ++yylloc.last_column;
+ yylval = yylval * 10 + c - '0';
+ @}
+ ungetc (c, stdin);
+ return NUM;
+ @}
+@end group
+
+ /* Return end-of-input. */
+ if (c == EOF)
+ return 0;
+
+ /* Return a single char, and update location. */
+ if (c == '\n')
+ @{
+ ++yylloc.last_line;
+ yylloc.last_column = 0;
+ @}
+ else
+ ++yylloc.last_column;
+ return c;
+@}
+@end example
+
+Basically, the lexical analyzer performs the same processing as before:
+it skips blanks and tabs, and reads numbers or single-character tokens.
+In addition, it updates @code{yylloc}, the global variable (of type
+@code{YYLTYPE}) containing the token's location.
+
+Now, each time this function returns a token, the parser has its number
+as well as its semantic value, and its location in the text. The last
+needed change is to initialize @code{yylloc}, for example in the
+controlling function:
+
+@example
+@group
+int
+main (void)
+@{
+ yylloc.first_line = yylloc.last_line = 1;
+ yylloc.first_column = yylloc.last_column = 0;
+ return yyparse ();
+@}
+@end group
+@end example
+
+Remember that computing locations is not a matter of syntax. Every
+character must be associated to a location update, whether it is in
+valid input, in comments, in literal strings, and so on.
+
+@node Multi-function Calc
+@section Multi-Function Calculator: @code{mfcalc}
+@cindex multi-function calculator
+@cindex @code{mfcalc}
+@cindex calculator, multi-function
+
+Now that the basics of Bison have been discussed, it is time to move on to
+a more advanced problem. The above calculators provided only five
+functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}. It would
+be nice to have a calculator that provides other mathematical functions such
+as @code{sin}, @code{cos}, etc.
+
+It is easy to add new operators to the infix calculator as long as they are
+only single-character literals. The lexical analyzer @code{yylex} passes
+back all nonnumber characters as tokens, so new grammar rules suffice for
+adding a new operator. But we want something more flexible: built-in
+functions whose syntax has this form:
+
+@example
+@var{function_name} (@var{argument})
+@end example
+
+@noindent
+At the same time, we will add memory to the calculator, by allowing you
+to create named variables, store values in them, and use them later.
+Here is a sample session with the multi-function calculator:
+
+@example
+$ @kbd{mfcalc}
+@kbd{pi = 3.141592653589}
+3.1415926536
+@kbd{sin(pi)}
+0.0000000000
+@kbd{alpha = beta1 = 2.3}
+2.3000000000
+@kbd{alpha}
+2.3000000000
+@kbd{ln(alpha)}
+0.8329091229
+@kbd{exp(ln(beta1))}
+2.3000000000
+$
+@end example
+
+Note that multiple assignment and nested function calls are permitted.
+
+@menu
+* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
+* Rules: Mfcalc Rules. Grammar rules for the calculator.
+* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+@end menu
+
+@node Mfcalc Decl
+@subsection Declarations for @code{mfcalc}
+
+Here are the C and Bison declarations for the multi-function calculator.
+
+@smallexample
+%@{
+#include <math.h> /* For math functions, cos(), sin(), etc. */
+#include "calc.h" /* Contains definition of `symrec' */
+%@}
+%union @{
+double val; /* For returning numbers. */
+symrec *tptr; /* For returning symbol-table pointers */
+@}
+
+%token <val> NUM /* Simple double precision number */
+%token <tptr> VAR FNCT /* Variable and Function */
+%type <val> exp
+
+%right '='
+%left '-' '+'
+%left '*' '/'
+%left NEG /* Negation--unary minus */
+%right '^' /* Exponentiation */
+
+/* Grammar follows */
+
+%%
+@end smallexample
+
+The above grammar introduces only two new features of the Bison language.
+These features allow semantic values to have various data types
+(@pxref{Multiple Types, ,More Than One Value Type}).
+
+The @code{%union} declaration specifies the entire list of possible types;
+this is instead of defining @code{YYSTYPE}. The allowable types are now
+double-floats (for @code{exp} and @code{NUM}) and pointers to entries in
+the symbol table. @xref{Union Decl, ,The Collection of Value Types}.
+
+Since values can now have various types, it is necessary to associate a
+type with each grammar symbol whose semantic value is used. These symbols
are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}. Their
declarations are augmented with information about their data type (placed
between angle brackets).
-The Bison construct @code{%type} is used for declaring nonterminal symbols,
-just as @code{%token} is used for declaring token types. We have not used
-@code{%type} before because nonterminal symbols are normally declared
-implicitly by the rules that define them. But @code{exp} must be declared
-explicitly so we can specify its value type. @xref{Type Decl, ,Nonterminal Symbols}.
+The Bison construct @code{%type} is used for declaring nonterminal
+symbols, just as @code{%token} is used for declaring token types. We
+have not used @code{%type} before because nonterminal symbols are
+normally declared implicitly by the rules that define them. But
+@code{exp} must be declared explicitly so we can specify its value type.
+@xref{Type Decl, ,Nonterminal Symbols}.
-@node Mfcalc Rules, Mfcalc Symtab, Mfcalc Decl, Multi-function Calc
+@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}
Here are the grammar rules for the multi-function calculator.
%%
@end smallexample
-@node Mfcalc Symtab, , Mfcalc Rules, Multi-function Calc
+@node Mfcalc Symtab
@subsection The @code{mfcalc} Symbol Table
@cindex symbol table example
@smallexample
@group
+/* Function type. */
+typedef double (*func_t) (double);
+
/* Data type for links in the chain of symbols. */
struct symrec
@{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
- union @{
- double var; /* value of a VAR */
- double (*fnctptr)(); /* value of a FNCT */
+ union
+ @{
+ double var; /* value of a VAR */
+ func_t fnctptr; /* value of a FNCT */
@} value;
struct symrec *next; /* link field */
@};
/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
-symrec *putsym ();
-symrec *getsym ();
+symrec *putsym (const char *, func_t);
+symrec *getsym (const char *);
@end group
@end smallexample
@group
#include <stdio.h>
-main ()
+int
+main (void)
@{
init_table ();
- yyparse ();
+ return yyparse ();
@}
@end group
@group
-yyerror (s) /* Called by yyparse on error */
- char *s;
+void
+yyerror (const char *s) /* Called by yyparse on error */
@{
printf ("%s\n", s);
@}
struct init
@{
char *fname;
- double (*fnct)();
+ double (*fnct)(double);
@};
@end group
@group
-struct init arith_fncts[]
- = @{
- "sin", sin,
- "cos", cos,
- "atan", atan,
- "ln", log,
- "exp", exp,
- "sqrt", sqrt,
- 0, 0
- @};
+struct init arith_fncts[] =
+@{
+ "sin", sin,
+ "cos", cos,
+ "atan", atan,
+ "ln", log,
+ "exp", exp,
+ "sqrt", sqrt,
+ 0, 0
+@};
/* The symbol table: a chain of `struct symrec'. */
-symrec *sym_table = (symrec *)0;
+symrec *sym_table = (symrec *) 0;
@end group
@group
-init_table () /* puts arithmetic functions in table. */
+/* Put arithmetic functions in table. */
+void
+init_table (void)
@{
int i;
symrec *ptr;
@smallexample
symrec *
-putsym (sym_name,sym_type)
- char *sym_name;
- int sym_type;
+putsym (char *sym_name, int sym_type)
@{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
- ptr->value.var = 0; /* set value to 0 even if fctn. */
+ ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
@}
symrec *
-getsym (sym_name)
- char *sym_name;
+getsym (const char *sym_name)
@{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
-characters with a leading nondigit are recognized as either variables or
+characters with a leading non-digit are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to @code{getsym} for look up in the symbol table. If
(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}. If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}. Again, a pointer and its type (which must be @code{VAR}) is
-returned to @code{yyparse}.@refill
+returned to @code{yyparse}.
No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.
@smallexample
@group
#include <ctype.h>
-yylex ()
+
+int
+yylex (void)
@{
int c;
- /* Ignore whitespace, get first nonwhite character. */
+ /* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t');
if (c == EOF)
@}
@end group
@group
- while (c != EOF && isalnum (c));
+ while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
@end group
@end smallexample
-This program is both powerful and flexible. You may easily add new
-functions, and it is a simple job to modify this code to install predefined
-variables such as @code{pi} or @code{e} as well.
+This program is both powerful and flexible. You may easily add new
+functions, and it is a simple job to modify this code to install
+predefined variables such as @code{pi} or @code{e} as well.
-@node Exercises, , Multi-function Calc, Examples
+@node Exercises
@section Exercises
@cindex exercises
uninitialized variable in any way except to store a value in it.
@end enumerate
-@node Grammar File, Interface, Examples, Top
+@node Grammar File
@chapter Bison Grammar Files
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.
The Bison grammar input file conventionally has a name ending in @samp{.y}.
+@xref{Invocation, ,Invoking Bison}.
@menu
* Grammar Outline:: Overall layout of the grammar file.
* 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.
@end menu
-@node Grammar Outline, Symbols, , Grammar File
+@node Grammar Outline
@section Outline of a Bison Grammar
A Bison grammar file has four main sections, shown here with the
@example
%@{
-@var{C declarations}
+@var{Prologue}
%@}
@var{Bison declarations}
@var{Grammar rules}
%%
-@var{Additional C code}
+@var{Epilogue}
@end example
Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
@menu
-* C Declarations:: Syntax and usage of the C 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.
-* C Code:: Syntax and usage of the additional C code section.
+* Epilogue:: Syntax and usage of the epilogue.
@end menu
-@node C Declarations, Bison Declarations, , Grammar Outline
-@subsection The C Declarations Section
-@cindex C declarations section
-@cindex declarations, C
+@node Prologue, Bison Declarations, , Grammar Outline
+@subsection The prologue
+@cindex declarations section
+@cindex Prologue
+@cindex declarations
-The @var{C declarations} section contains macro definitions and
+The @var{Prologue} section contains macro definitions and
declarations of functions and variables that are used in the actions in the
grammar rules. These are copied to the beginning of the parser file so
that they precede the definition of @code{yyparse}. You can use
need any C declarations, you may omit the @samp{%@{} and @samp{%@}}
delimiters that bracket this section.
-@node Bison Declarations, Grammar Rules, C Declarations, Grammar Outline
+You may have more than one @var{Prologue} section, intermixed with the
+@var{Bison declarations}. This allows you to have C and Bison
+declarations that refer to each other. For example, the @code{%union}
+declaration may use types defined in a header file, and you may wish to
+prototype functions that take arguments of type @code{YYSTYPE}. This
+can be done with two @var{Prologue} blocks, one before and one after the
+@code{%union} declaration.
+
+@smallexample
+%@{
+#include <stdio.h>
+#include "ptypes.h"
+%@}
+
+%union @{
+ long n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+static void yyprint(FILE *, int, YYSTYPE);
+#define YYPRINT(F, N, L) yyprint(F, N, L)
+%@}
+
+@dots{}
+@end smallexample
+
+@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
@cindex declarations, Bison (introduction)
In some simple grammars you may not need any declarations.
@xref{Declarations, ,Bison Declarations}.
-@node Grammar Rules, C Code, Bison Declarations, Grammar Outline
+@node Grammar Rules
@subsection The Grammar Rules Section
@cindex grammar rules section
@cindex rules section for grammar
@samp{%%} (which precedes the grammar rules) may never be omitted even
if it is the first thing in the file.
-@node C Code, , Grammar Rules, Grammar Outline
-@subsection The Additional C Code Section
+@node Epilogue, , Grammar Rules, Grammar Outline
+@subsection The epilogue
@cindex additional C code section
+@cindex epilogue
@cindex C code, section for additional
-The @var{additional C code} section is copied verbatim to the end of
-the parser file, just as the @var{C declarations} section 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. @xref{Interface, ,Parser C-Language Interface}.
+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.
+@xref{Interface, ,Parser C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.
The Bison parser itself contains many static variables whose names start
with @samp{yy} and many macros whose names start with @samp{YY}. It is a
good idea to avoid using any such names (except those documented in this
-manual) in the additional C code section of the grammar file.
+manual) in the epilogue of the grammar file.
-@node Symbols, Rules, Grammar Outline, Grammar File
+@node Symbols
@section Symbols, Terminal and Nonterminal
@cindex nonterminal symbol
@cindex terminal symbol
Symbol names can contain letters, digits (not at the beginning),
underscores and periods. Periods make sense only in nonterminals.
-There are two ways of writing terminal symbols in the grammar:
+There are three ways of writing terminal symbols in the grammar:
@itemize @bullet
@item
@cindex character token
@cindex literal token
@cindex single-character literal
-A @dfn{character token type} (or @dfn{literal token}) is written in
-the grammar using the same syntax used in C for character constants;
-for example, @code{'+'} is a character token type. A character token
-type doesn't need to be declared unless you need to specify its
-semantic value data type (@pxref{Value Type, ,Data Types of Semantic Values}), associativity, or
-precedence (@pxref{Precedence, ,Operator Precedence}).
+A @dfn{character token type} (or @dfn{literal character token}) is
+written in the grammar using the same syntax used in C for character
+constants; for example, @code{'+'} is a character token type. A
+character token type doesn't need to be declared unless you need to
+specify its semantic value data type (@pxref{Value Type, ,Data Types of
+Semantic Values}), associativity, or precedence (@pxref{Precedence,
+,Operator Precedence}).
By convention, a character token type is used only to represent a
token that consists of that particular character. Thus, the token
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its ASCII code, zero, is the code
-@code{yylex} returns for end-of-input (@pxref{Calling Convention, ,Calling Convention for @code{yylex}}).
+character literal because its numeric code, zero, signifies
+end-of-input (@pxref{Calling Convention, ,Calling Convention
+for @code{yylex}}).
+
+@item
+@cindex string token
+@cindex literal string token
+@cindex multicharacter literal
+A @dfn{literal string token} is written like a C string constant; for
+example, @code{"<="} is a literal string token. A literal string token
+doesn't need to be declared unless you need to specify its semantic
+value data type (@pxref{Value Type}), associativity, or precedence
+(@pxref{Precedence}).
+
+You can associate the literal string token with a symbolic name as an
+alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token
+Declarations}). If you don't do that, the lexical analyzer has to
+retrieve the token number for the literal string token from the
+@code{yytname} table (@pxref{Calling Convention}).
+
+@strong{WARNING}: literal string tokens do not work in Yacc.
+
+By convention, a literal string token is used only to represent a token
+that consists of that particular string. Thus, you should use the token
+type @code{"<="} to represent the string @samp{<=} as a token. Bison
+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. A literal string token must contain two or more
+characters; for a token containing just one character, use a character
+token (see above).
@end itemize
How you choose to write a terminal symbol has no effect on its
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
-The value returned by @code{yylex} is always one of the terminal symbols
-(or 0 for end-of-input). Whichever way you write the token type in the
-grammar rules, you write it the same way in the definition of @code{yylex}.
-The numeric code for a character token type is simply the ASCII code for
-the character, so @code{yylex} can use the identical character constant to
-generate the requisite code. Each named token type becomes a C macro in
+The value returned by @code{yylex} is always one of the terminal
+symbols, except that a zero or negative value signifies end-of-input.
+Whichever way you write the token type in the grammar rules, you write
+it the same way in the definition of @code{yylex}. The numeric code
+for a character token type is simply the positive numeric code of the
+character, so @code{yylex} can use the identical value to generate the
+requisite code, though you may need to convert it to @code{unsigned
+char} to avoid sign-extension on hosts where @code{char} is signed.
+Each named token type becomes a C macro in
the parser file, so @code{yylex} can use the name to stand for the code.
-(This is why periods don't make sense in terminal symbols.)
+(This is why periods don't make sense in terminal symbols.)
@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
If @code{yylex} is defined in a separate file, you need to arrange for the
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
+If you want to write a grammar that is portable to any Standard C
+host, you must use only non-null character tokens taken from the basic
+execution character set of Standard C. This set consists of the ten
+digits, the 52 lower- and upper-case English letters, and the
+characters in the following C-language string:
+
+@example
+"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
+@end example
+
+The @code{yylex} function and Bison must use a consistent character
+set and encoding for character tokens. For example, if you run Bison in an
+@sc{ascii} environment, but then compile and run the resulting program
+in an environment that uses an incompatible character set like
+@sc{ebcdic}, the resulting program may not work because the
+tables generated by Bison will assume @sc{ascii} numeric values for
+character tokens. It is standard
+practice for software distributions to contain C source files that
+were generated by Bison in an @sc{ascii} environment, so installers on
+platforms that are incompatible with @sc{ascii} must rebuild those
+files before compiling them.
+
The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
-In particular, @code{yylex} should never return this value.
+In particular, @code{yylex} should never return this value. The default
+value of the error token is 256, unless you explicitly assigned 256 to
+one of your tokens with a @code{%token} declaration.
-@node Rules, Recursion, Symbols, Grammar File
+@node Rules
@section Syntax of Grammar Rules
@cindex rule syntax
@cindex grammar rule syntax
@end example
@noindent
-where @var{result} is the nonterminal symbol that this rule describes
+where @var{result} is the nonterminal symbol that this rule describes,
and @var{components} are various terminal and nonterminal symbols that
-are put together by this rule (@pxref{Symbols}).
+are put together by this rule (@pxref{Symbols}).
For example,
says that two groupings of type @code{exp}, with a @samp{+} token in between,
can be combined into a larger grouping of type @code{exp}.
-Whitespace in rules is significant only to separate symbols. You can add
-extra whitespace as you wish.
+White space in rules is significant only to separate symbols. You can add
+extra white space as you wish.
Scattered among the components can be @var{actions} that determine
the semantics of the rule. An action looks like this:
It is customary to write a comment @samp{/* empty */} in each rule
with no components.
-@node Recursion, Semantics, Rules, Grammar File
+@node Recursion
@section Recursive Rules
@cindex recursive rule
A rule is called @dfn{recursive} when its @var{result} nonterminal appears
also on its right hand side. Nearly all Bison grammars need to use
recursion, because that is the only way to define a sequence of any number
-of somethings. Consider this recursive definition of a comma-separated
-sequence of one or more expressions:
+of a particular thing. Consider this recursive definition of a
+comma-separated sequence of one or more expressions:
@example
@group
@end example
@noindent
-Any kind of sequence can be defined using either left recursion or
-right recursion, but you should always use left recursion, because it
-can parse a sequence of any number of elements with bounded stack
-space. Right recursion uses up space on the Bison stack in proportion
-to the number of elements in the sequence, because all the elements
-must be shifted onto the stack before the rule can be applied even
-once. @xref{Algorithm, ,The Bison Parser Algorithm }, for
-further explanation of this.
+Any kind of sequence can be defined using either left recursion or right
+recursion, but you should always use left recursion, because it can
+parse a sequence of any number of elements with bounded stack space.
+Right recursion uses up space on the Bison stack in proportion to the
+number of elements in the sequence, because all the elements must be
+shifted onto the stack before the rule can be applied even once.
+@xref{Algorithm, ,The Bison Parser Algorithm}, for further explanation
+of this.
@cindex mutual recursion
@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
rule does not appear directly on its right hand side, but does appear
in rules for other nonterminals which do appear on its right hand
-side.
+side.
For example:
defines two mutually-recursive nonterminals, since each refers to the
other.
-@node Semantics, Declarations, Recursion, Grammar File
+@node Semantics
@section Defining Language Semantics
@cindex defining language semantics
-@cindex language semantics, defining
+@cindex language semantics, defining
The grammar rules for a language determine only the syntax. The semantics
are determined by the semantic values associated with various tokens and
action in the middle of a rule.
@end menu
-@node Value Type, Multiple Types, , Semantics
+@node Value Type
@subsection Data Types of Semantic Values
@cindex semantic value type
@cindex value type, semantic
In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs. This was true in the
-RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish Notation Calculator}).
+RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
+Notation Calculator}).
Bison's default is to use type @code{int} for all semantic values. To
specify some other type, define @code{YYSTYPE} as a macro, like this:
@end example
@noindent
-This macro definition must go in the C declarations section of the grammar
-file (@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
+This macro definition must go in the prologue of the grammar file
+(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
-@node Multiple Types, Actions, Value Type, Semantics
+@node Multiple Types
@subsection More Than One Value Type
In most programs, you will need different data types for different kinds
@itemize @bullet
@item
Specify the entire collection of possible data types, with the
-@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of Value Types}).
+@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
+Value Types}).
@item
-Choose one of those types for each symbol (terminal or nonterminal)
-for which semantic values are used. This is done for tokens with the
-@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names}) and for groupings
-with the @code{%type} Bison declaration (@pxref{Type Decl, ,Nonterminal Symbols}).
+Choose one of those types for each symbol (terminal or nonterminal) for
+which semantic values are used. This is done for tokens with the
+@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names})
+and for groupings with the @code{%type} Bison declaration (@pxref{Type
+Decl, ,Nonterminal Symbols}).
@end itemize
-@node Actions, Action Types, Multiple Types, Semantics
+@node Actions
@subsection Actions
@cindex action
@vindex $$
semantic values associated with tokens or smaller groupings.
An action consists of C statements surrounded by braces, much like a
-compound statement in C. It can be placed at any position in the rule; it
-is executed at that position. Most rules have just one action at the end
-of the rule, following all the components. Actions in the middle of a rule
-are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}).
+compound statement in C. It can be placed at any position in the rule;
+it is executed at that position. Most rules have just one action at the
+end of the rule, following all the components. Actions in the middle of
+a rule are tricky and used only for special purposes (@pxref{Mid-Rule
+Actions, ,Actions in Mid-Rule}).
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
The sum is stored into @code{$$} so that it becomes the semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
-referred to as @code{$2}.@refill
+referred to as @code{$2}.
+
+Note that the vertical-bar character @samp{|} is really a rule
+separator, and actions are attached to a single rule. This is a
+difference with tools like Flex, for which @samp{|} stands for either
+``or'', or ``the same action as that of the next rule''. In the
+following example, the action is triggered only when @samp{b} is found:
+
+@example
+@group
+a-or-b: 'a'|'b' @{ a_or_b_found = 1; @};
+@end group
+@end example
@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.
-@node Action Types, Mid-Rule Actions, Actions, Semantics
+@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
@cindex data types in actions
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
-in the rule. In this example,@refill
+in the rule. In this example,
@example
@group
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}. If
@code{$2} were used, it would have the data type declared for the
-terminal symbol @code{'+'}, whatever that might be.@refill
+terminal symbol @code{'+'}, whatever that might be.
Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
then you can write @code{$<itype>1} to refer to the first subunit of the
rule as an integer, or @code{$<dtype>1} to refer to it as a double.
-@node Mid-Rule Actions, , Action Types, Semantics
+@node Mid-Rule Actions
@subsection Actions in Mid-Rule
@cindex actions in mid-rule
@cindex mid-rule actions
its value with an assignment to @code{$$}, and actions later in the rule
can refer to the value using @code{$@var{n}}. Since there is no symbol
to name the action, there is no way to declare a data type for the value
-in advance, so you must use the @samp{$<@dots{}>} construct to specify a
-data type each time you refer to this value.
+in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to
+specify a data type each time you refer to this value.
There is no way to set the value of the entire rule with a mid-rule
action, because assignments to @code{$$} do not have that effect. The
converted to an end-of-rule action in this way, and this is what Bison
actually does to implement mid-rule actions.
-@node Declarations, Multiple Parsers, Semantics, Grammar File
+@node Locations
+@section Tracking Locations
+@cindex location
+@cindex textual position
+@cindex position, 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.
+
+@menu
+* 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.
+@end menu
+
+@node Location Type
+@subsection Data Type of Locations
+@cindex data type of locations
+@cindex default location type
+
+Defining a data type for locations is much simpler than for semantic values,
+since all tokens and groupings always use the same type.
+
+The type of locations is specified by defining a macro called @code{YYLTYPE}.
+When @code{YYLTYPE} is not defined, Bison uses a default structure type with
+four members:
+
+@example
+struct
+@{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+@}
+@end example
+
+@node Actions and Locations
+@subsection Actions and Locations
+@cindex location actions
+@cindex actions, location
+@vindex @@$
+@vindex @@@var{n}
+
+Actions are not only useful for defining language semantics, but also for
+describing the behavior of the output parser with locations.
+
+The most obvious way for building locations of syntactic groupings is very
+similar to the way semantic values are computed. In a given rule, several
+constructs can be used to access the locations of the elements being matched.
+The location of the @var{n}th component of the right hand side is
+@code{@@@var{n}}, while the location of the left hand side grouping is
+@code{@@$}.
+
+Here is a basic example using the default data type for locations:
+
+@example
+@group
+exp: @dots{}
+ | exp '/' exp
+ @{
+ @@$.first_column = @@1.first_column;
+ @@$.first_line = @@1.first_line;
+ @@$.last_column = @@3.last_column;
+ @@$.last_line = @@3.last_line;
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ printf("Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
+@end group
+@end example
+
+As for semantic values, there is a default action for locations that is
+run each time a rule is matched. It sets the beginning of @code{@@$} to the
+beginning of the first symbol, and the end of @code{@@$} to the end of the
+last symbol.
+
+With this default action, the location tracking can be fully automatic. The
+example above simply rewrites this way:
+
+@example
+@group
+exp: @dots{}
+ | exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ 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);
+ @}
+ @}
+@end group
+@end example
+
+@node Location Default Action
+@subsection Default Action for Locations
+@vindex YYLLOC_DEFAULT
+
+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.
+
+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.
+
+By default, it is defined this way for simple LALR(1) parsers:
+
+@example
+@group
+#define YYLLOC_DEFAULT(Current, Rhs, N) \
+ Current.first_line = Rhs[1].first_line; \
+ Current.first_column = Rhs[1].first_column; \
+ Current.last_line = Rhs[N].last_line; \
+ Current.last_column = Rhs[N].last_column;
+@end group
+@end example
+
+@noindent
+and like this for GLR parsers:
+
+@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
+
+When defining @code{YYLLOC_DEFAULT}, you should consider that:
+
+@itemize @bullet
+@item
+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}.
+@end itemize
+
+@node Declarations
@section Bison Declarations
@cindex declarations, Bison
@cindex Bison declarations
The first rule in the file also specifies the start symbol, by default.
If you want some other symbol to be the start symbol, you must declare
-it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
+it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
+Grammars}).
@menu
* Token Decl:: Declaring terminal symbols.
* Decl Summary:: Table of all Bison declarations.
@end menu
-@node Token Decl, Precedence Decl, , Declarations
+@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
@cindex token type names, declaring
+@cindex declaring literal string tokens
@findex %token
The basic way to declare a token type name (terminal symbol) is as follows:
the parser, so that the function @code{yylex} (if it is in this file)
can use the name @var{name} to stand for this token type's code.
-Alternatively, you can use @code{%left}, @code{%right}, or @code{%nonassoc}
-instead of @code{%token}, if you wish to specify precedence.
-@xref{Precedence Decl, ,Operator Precedence}.
+Alternatively, you can use @code{%left}, @code{%right}, or
+@code{%nonassoc} instead of @code{%token}, if you wish to specify
+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:
@noindent
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't conflict
-with each other or with ASCII characters.
+with each other or with normal characters.
In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
-alternative delimited by angle-brackets (@pxref{Multiple Types, ,More Than One Value Type}).
+alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
+Than One Value Type}).
For example:
@end group
@end example
-@node Precedence Decl, Union Decl, Token Decl, Declarations
+You can associate a literal string token with a token type name by
+writing the literal string at the end of a @code{%token}
+declaration which declares the name. For example:
+
+@example
+%token arrow "=>"
+@end example
+
+@noindent
+For example, a grammar for the C language might specify these names with
+equivalent literal string tokens:
+
+@example
+%token <operator> OR "||"
+%token <operator> LE 134 "<="
+%left OR "<="
+@end example
+
+@noindent
+Once you equate the literal string and the token name, you can use them
+interchangeably in further declarations or the grammar rules. The
+@code{yylex} function can use the token name or the literal string to
+obtain the token type code number (@pxref{Calling Convention}).
+
+@node Precedence Decl
@subsection Operator Precedence
@cindex precedence declarations
@cindex declaring operator precedence
Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
declare a token and specify its precedence and associativity, all at
once. These are called @dfn{precedence declarations}.
-@xref{Precedence, ,Operator Precedence}, for general information on operator precedence.
+@xref{Precedence, ,Operator Precedence}, for general information on
+operator precedence.
The syntax of a precedence declaration is the same as that of
@code{%token}: either
the one declared later has the higher precedence and is grouped first.
@end itemize
-@node Union Decl, Type Decl, Precedence Decl, Declarations
+@node Union Decl
@subsection The Collection of Value Types
@cindex declaring value types
@cindex value types, declaring
The @code{%union} declaration specifies the entire collection of possible
data types for semantic values. The keyword @code{%union} is followed by a
pair of braces containing the same thing that goes inside a @code{union} in
-C.
+C.
For example:
Note that, unlike making a @code{union} declaration in C, you do not write
a semicolon after the closing brace.
-@node Type Decl, Expect Decl, Union Decl, Declarations
+@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@cindex value types, nonterminals, declaring
@end example
@noindent
-Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type}
-is the name given in the @code{%union} to the alternative that you want
-(@pxref{Union Decl, ,The Collection of Value Types}). You can give any number of nonterminal symbols in
-the same @code{%type} declaration, if they have the same value type. Use
-spaces to separate the symbol names.
-
-@node Expect Decl, Start Decl, Type Decl, Declarations
+Here @var{nonterminal} is the name of a nonterminal symbol, and
+@var{type} is the name given in the @code{%union} to the alternative
+that you want (@pxref{Union Decl, ,The Collection of Value Types}). You
+can give any number of nonterminal symbols in the same @code{%type}
+declaration, if they have the same value type. Use spaces to separate
+the symbol names.
+
+You can also declare the value type of a terminal symbol. To do this,
+use the same @code{<@var{type}>} construction in a declaration for the
+terminal symbol. All kinds of token declarations allow
+@code{<@var{type}>}.
+
+@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex preventing warnings about conflicts
@findex %expect
Bison normally warns if there are any conflicts in the grammar
-(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars have harmless shift/reduce
-conflicts which are resolved in a predictable way and would be difficult to
-eliminate. It is desirable to suppress the warning about these conflicts
-unless the number of conflicts changes. You can do this with the
-@code{%expect} declaration.
+(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
+have harmless shift/reduce conflicts which are resolved in a predictable
+way and would be difficult to eliminate. It is desirable to suppress
+the warning about these conflicts unless the number of conflicts
+changes. You can do this with the @code{%expect} declaration.
The declaration looks like this:
%expect @var{n}
@end example
-Here @var{n} is a decimal integer. The declaration says there should be no
-warning if there are @var{n} shift/reduce conflicts and no reduce/reduce
-conflicts. The usual warning is given if there are either more or fewer
-conflicts, or if there are any reduce/reduce conflicts.
+Here @var{n} is a decimal integer. The declaration says there should be
+no warning if there are @var{n} shift/reduce conflicts and no
+reduce/reduce conflicts. An error, instead of the usual warning, is
+given if there are either more or fewer conflicts, or if there are any
+reduce/reduce conflicts.
In general, using @code{%expect} involves these steps:
it will warn you again if changes in the grammar result in additional
conflicts.
-@node Start Decl, Pure Decl, Expect Decl, Declarations
+@node Start Decl
@subsection The Start-Symbol
@cindex declaring the start symbol
@cindex start symbol, declaring
%start @var{symbol}
@end example
-@node Pure Decl, Decl Summary, Start Decl, Declarations
+@node Pure Decl
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
-@findex %pure_parser
+@findex %pure-parser
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code. Reentrancy is important whenever asynchronous execution is possible;
-for example, a nonreentrant program may not be safe to call from a signal
-handler. In systems with multiple threads of control, a nonreentrant
+for example, a non-reentrant program may not be safe to call from a signal
+handler. In systems with multiple threads of control, a non-reentrant
program must be called only within interlocks.
-The Bison parser is not normally a reentrant program, because it uses
-statically allocated variables for communication with @code{yylex}. These
-variables include @code{yylval} and @code{yylloc}.
+Normally, Bison generates a parser which is not reentrant. This is
+suitable for most uses, and it permits compatibility with YACC. (The
+standard YACC interfaces are inherently nonreentrant, because they use
+statically allocated variables for communication with @code{yylex},
+including @code{yylval} and @code{yylloc}.)
-The Bison declaration @code{%pure_parser} says that you want the parser
-to be reentrant. It looks like this:
+Alternatively, you can generate a pure, reentrant parser. The Bison
+declaration @code{%pure-parser} says that you want the parser to be
+reentrant. It looks like this:
@example
-%pure_parser
+%pure-parser
@end example
-The effect is that the two communication variables become local
-variables in @code{yyparse}, and a different calling convention is used
-for the lexical analyzer function @code{yylex}. @xref{Pure Calling,
-,Calling Conventions for Pure Parsers}, for the details of this. The
-variable @code{yynerrs} also becomes local in @code{yyparse}
-(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
-The convention for calling @code{yyparse} itself is unchanged.
+The result is that the communication variables @code{yylval} and
+@code{yylloc} become local variables in @code{yyparse}, and a different
+calling convention is used for the lexical analyzer function
+@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
+Parsers}, for the details of this. The variable @code{yynerrs} also
+becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}). The convention for calling
+@code{yyparse} itself is unchanged.
-@node Decl Summary, , Pure Decl, Declarations
+Whether the parser is pure has nothing to do with the grammar rules.
+You can generate either a pure parser or a nonreentrant parser from any
+valid grammar.
+
+@node Decl Summary
@subsection Bison Declaration Summary
@cindex Bison declaration summary
@cindex declaration summary
@cindex summary, Bison declaration
-Here is a summary of all Bison declarations:
+Here is a summary of the declarations used to define a grammar:
@table @code
@item %union
(@pxref{Type Decl, ,Nonterminal Symbols}).
@item %start
-Specify the grammar's start symbol (@pxref{Start Decl, ,The Start-Symbol}).
+Specify the grammar's start symbol (@pxref{Start Decl, ,The
+Start-Symbol}).
@item %expect
Declare the expected number of shift-reduce conflicts
(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
-
-@item %pure_parser
-Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
@end table
-@node Multiple Parsers, , Declarations, Grammar File
-@section Multiple Parsers in the Same Program
+@sp 1
+@noindent
+In order to change the behavior of @command{bison}, use the following
+directives:
-Most programs that use Bison parse only one language and therefore contain
-only one Bison parser. But what if you want to parse more than one
+@table @code
+@item %debug
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Tracing, ,Tracing Your Parser}.
+
+@item %defines
+Write an extra output file containing macro definitions for the token
+type names defined in the grammar and the semantic value type
+@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
+
+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}.
+
+@item %file-prefix="@var{prefix}"
+Specify a prefix to use for all Bison output file names. The names are
+chosen as if the input file were named @file{@var{prefix}.y}.
+
+@c @item %header-extension
+@c Specify the extension of the parser header file generated when
+@c @code{%define} or @samp{-d} are used.
+@c
+@c For example, a grammar file named @file{foo.ypp} and containing a
+@c @code{%header-extension .hh} directive will produce a header file
+@c named @file{foo.tab.hh}
+
+@item %locations
+Generate the code processing the locations (@pxref{Action Features,
+,Special Features for Use in Actions}). This mode is enabled as soon as
+the grammar uses the special @samp{@@@var{n}} tokens, but if your
+grammar does not use it, using @samp{%locations} allows for more
+accurate parse error messages.
+
+@item %name-prefix="@var{prefix}"
+Rename the external symbols used in the parser so that they start with
+@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
+is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
+@code{yylval}, @code{yychar}, @code{yydebug}, and possible
+@code{yylloc}. For example, if you use @samp{%name-prefix="c_"}, the
+names become @code{c_parse}, @code{c_lex}, and so on. @xref{Multiple
+Parsers, ,Multiple Parsers in the Same Program}.
+
+@item %no-parser
+Do not include any C code in the parser file; generate tables only. The
+parser file contains just @code{#define} directives and static variable
+declarations.
+
+This option also tells Bison to write the C code for the grammar actions
+into a file named @file{@var{filename}.act}, in the form of a
+brace-surrounded body fit for a @code{switch} statement.
+
+@item %no-lines
+Don't generate any @code{#line} preprocessor commands in the parser
+file. Ordinarily Bison writes these commands in the parser file so that
+the C compiler and debuggers will associate errors and object code with
+your source file (the grammar file). This directive causes them to
+associate errors with the parser file, treating it an independent source
+file in its own right.
+
+@item %output="@var{filename}"
+Specify the @var{filename} for the parser file.
+
+@item %pure-parser
+Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
+(Reentrant) Parser}).
+
+@c @item %source-extension
+@c Specify the extension of the parser output file.
+@c
+@c For example, a grammar file named @file{foo.yy} and containing a
+@c @code{%source-extension .cpp} directive will produce a parser file
+@c named @file{foo.tab.cpp}
+
+@item %token-table
+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"},
+@code{"error"}, and @code{"$undefined"}; after these come the symbols
+defined in the grammar file.
+
+For single-character literal tokens and literal string tokens, the name
+in the table includes the single-quote or double-quote characters: for
+example, @code{"'+'"} is a single-character literal and @code{"\"<=\""}
+is a literal string token. All the characters of the literal string
+token appear verbatim in the string found in the table; even
+double-quote characters are not escaped. For example, if the token
+consists of three characters @samp{*"*}, its string in @code{yytname}
+contains @samp{"*"*"}. (In C, that would be written as
+@code{"\"*\"*\""}).
+
+When you specify @code{%token-table}, Bison also generates macro
+definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
+@code{YYNRULES}, and @code{YYNSTATES}:
+
+@table @code
+@item YYNTOKENS
+The highest token number, plus one.
+@item YYNNTS
+The number of nonterminal symbols.
+@item YYNRULES
+The number of grammar rules,
+@item YYNSTATES
+The number of parser states (@pxref{Parser States}).
+@end table
+
+@item %verbose
+Write an extra output file containing verbose descriptions of the
+parser states and what is done for each type of look-ahead token in
+that state. @xref{Understanding, , Understanding Your Parser}, for more
+information.
+
+
+
+@item %yacc
+Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
+including its naming conventions. @xref{Bison Options}, for more.
+@end table
+
+
+
+
+@node Multiple Parsers
+@section Multiple Parsers in the Same Program
+
+Most programs that use Bison parse only one language and therefore contain
+only one Bison parser. But what if you want to parse more than one
language with the same program? Then you need to avoid a name conflict
between different definitions of @code{yyparse}, @code{yylval}, and so on.
The easy way to do this is to use the option @samp{-p @var{prefix}}
-(@pxref{Invocation, ,Invoking Bison}). This renames the interface functions and
-variables of the Bison parser to start with @var{prefix} instead of
-@samp{yy}. You can use this to give each parser distinct names that do
-not conflict.
+(@pxref{Invocation, ,Invoking Bison}). This renames the interface
+functions and variables of the Bison parser to start with @var{prefix}
+instead of @samp{yy}. You can use this to give each parser distinct
+names that do not conflict.
The precise list of symbols renamed is @code{yyparse}, @code{yylex},
@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar} and
@code{@var{prefix}parse}, and so on. This effectively substitutes one
name for the other in the entire parser file.
-@node Interface, Algorithm, Grammar File, Top
+@node Interface
@chapter Parser C-Language Interface
@cindex C-language interface
@cindex interface
Keep in mind that the parser uses many C identifiers starting with
@samp{yy} and @samp{YY} for internal purposes. If you use such an
-identifier (aside from those in this manual) in an action or in additional
-C code in the grammar file, you are likely to run into trouble.
+identifier (aside from those in this manual) in an action or in epilogue
+in the grammar file, you are likely to run into trouble.
@menu
* Parser Function:: How to call @code{yyparse} and what it returns.
-* Lexical:: You must supply a function @code{yylex}
+* Lexical:: You must supply a function @code{yylex}
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
@end menu
-@node Parser Function, Lexical, , Interface
+@node Parser Function
@section The Parser Function @code{yyparse}
@findex yyparse
You call the function @code{yyparse} to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
-write an action which directs @code{yyparse} to return immediately without
-reading further.
+write an action which directs @code{yyparse} to return immediately
+without reading further.
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).
Return immediately with value 1 (to report failure).
@end table
-@node Lexical, Error Reporting, Parser Function, Interface
+@node Lexical
@section The Lexical Analyzer Function @code{yylex}
@findex yylex
@cindex lexical analyzer
To do this, use the @samp{-d} option when you run Bison, so that it will
write these macro definitions into a separate header file
@file{@var{name}.tab.h} which you can include in the other source files
-that need it. @xref{Invocation, ,Invoking Bison}.@refill
+that need it. @xref{Invocation, ,Invoking Bison}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
@end menu
-@node Calling Convention, Token Values, , Lexical
+@node Calling Convention
@subsection Calling Convention for @code{yylex}
-The value that @code{yylex} returns must be the numeric code for the type
-of token it has just found, or 0 for end-of-input.
+The value that @code{yylex} returns must be the positive numeric code
+for the type of token it has just found; a zero or negative value
+signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
-So @code{yylex} can simply return that character code. The null character
-must not be used this way, because its code is zero and that is what
+So @code{yylex} can simply return that character code, possibly converted
+to @code{unsigned char} to avoid sign-extension. The null character
+must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
@example
-yylex ()
+int
+yylex (void)
@{
@dots{}
- if (c == EOF) /* Detect end of file. */
+ if (c == EOF) /* Detect end-of-input. */
return 0;
@dots{}
if (c == '+' || c == '-')
- return c; /* Assume token type for `+' is '+'. */
+ return c; /* Assume token type for `+' is '+'. */
@dots{}
- return INT; /* Return the type of the token. */
+ return INT; /* Return the type of the token. */
@dots{}
@}
@end example
This interface has been designed so that the output from the @code{lex}
utility can be used without change as the definition of @code{yylex}.
-@node Token Values, Token Positions, Calling Convention, Lexical
+If the grammar uses literal string tokens, there are two ways that
+@code{yylex} can determine the token type codes for them:
+
+@itemize @bullet
+@item
+If the grammar defines symbolic token names as aliases for the
+literal string tokens, @code{yylex} can use these symbolic names like
+all others. In this case, the use of the literal string tokens in
+the grammar file has no effect on @code{yylex}.
+
+@item
+@code{yylex} can find the multicharacter token in the @code{yytname}
+table. The index of the token in the table is the token type's code.
+The name of a multicharacter token is recorded in @code{yytname} with a
+double-quote, the token's characters, and another double-quote. The
+token's characters are not escaped in any way; they appear verbatim in
+the contents of the string in the table.
+
+Here's code for looking up a token in @code{yytname}, assuming that the
+characters of the token are stored in @code{token_buffer}.
+
+@smallexample
+for (i = 0; i < YYNTOKENS; i++)
+ @{
+ if (yytname[i] != 0
+ && yytname[i][0] == '"'
+ && ! strncmp (yytname[i] + 1, token_buffer,
+ strlen (token_buffer))
+ && yytname[i][strlen (token_buffer) + 1] == '"'
+ && yytname[i][strlen (token_buffer) + 2] == 0)
+ break;
+ @}
+@end smallexample
+
+The @code{yytname} table is generated only if you use the
+@code{%token-table} declaration. @xref{Decl Summary}.
+@end itemize
+
+@node Token Values
@subsection Semantic Values of Tokens
@vindex yylval
-In an ordinary (nonreentrant) parser, the semantic value of the token must
+In an ordinary (non-reentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}. When you are using
just one data type for semantic values, @code{yylval} has that type.
Thus, if the type is @code{int} (the default), you might write this in
@example
@group
@dots{}
- yylval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
When you are using multiple data types, @code{yylval}'s type is a union
-made from the @code{%union} declaration (@pxref{Union Decl, ,The Collection of Value Types}). So when
-you store a token's value, you must use the proper member of the union.
-If the @code{%union} declaration looks like this:
+made from the @code{%union} declaration (@pxref{Union Decl, ,The
+Collection of Value Types}). So when you store a token's value, you
+must use the proper member of the union. If the @code{%union}
+declaration looks like this:
@example
@group
@example
@group
@dots{}
- yylval.intval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval.intval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
-@node Token Positions, Pure Calling, Token Values, Lexical
+@node Token Positions
@subsection Textual Positions of Tokens
@vindex yylloc
-If you are using the @samp{@@@var{n}}-feature (@pxref{Action Features, ,Special Features for Use in Actions}) in
-actions to keep track of the textual locations of tokens and groupings,
-then you must provide this information in @code{yylex}. The function
-@code{yyparse} expects to find the textual location of a token just parsed
-in the global variable @code{yylloc}. So @code{yylex} must store the
-proper data in that variable. The value of @code{yylloc} is a structure
-and you need only initialize the members that are going to be used by the
-actions. The four members are called @code{first_line},
-@code{first_column}, @code{last_line} and @code{last_column}. Note that
-the use of this feature makes the parser noticeably slower.
+If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
+Tracking Locations}) in actions to keep track of the
+textual locations of tokens and groupings, then you must provide this
+information in @code{yylex}. The function @code{yyparse} expects to
+find the textual location of a token just parsed in the global variable
+@code{yylloc}. So @code{yylex} must store the proper data in that
+variable.
+
+By default, the value of @code{yylloc} is a structure and you need only
+initialize the members that are going to be used by the actions. The
+four members are called @code{first_line}, @code{first_column},
+@code{last_line} and @code{last_column}. Note that the use of this
+feature makes the parser noticeably slower.
@tindex YYLTYPE
The data type of @code{yylloc} has the name @code{YYLTYPE}.
-@node Pure Calling, , Token Positions, Lexical
+@node Pure Calling
@subsection Calling Conventions for Pure Parsers
-When you use the Bison declaration @code{%pure_parser} to request a
+When you use the Bison declaration @code{%pure-parser} to request a
pure, reentrant parser, the global communication variables @code{yylval}
and @code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant)
Parser}.) In such parsers the two global variables are replaced by
pointers.
@example
-yylex (lvalp, llocp)
- YYSTYPE *lvalp;
- YYLTYPE *llocp;
+int
+yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
@{
@dots{}
*lvalp = value; /* Put value onto Bison stack. */
only one argument.
@vindex YYPARSE_PARAM
-You can pass parameter information to a reentrant parser in a reentrant
-way. Define the macro @code{YYPARSE_PARAM} as a variable name. The
-resulting @code{yyparse} function then accepts one argument, of type
-@code{void *}, with that name.
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way. To do so, define the
+macro @code{YYPARSE_PARAM} as a variable name. This modifies the
+@code{yyparse} function to accept one argument, of type @code{void *},
+with that name.
When you call @code{yyparse}, pass the address of an object, casting the
address to @code{void *}. The grammar actions can refer to the contents
the proper object type, or you can declare it as @code{void *} and
access the contents as shown above.
-@node Error Reporting, Action Features, Lexical, Interface
+You can use @samp{%pure-parser} to request a reentrant parser without
+also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse}
+with no arguments, as usual.
+
+@node Error Reporting
@section The Error Reporting Function @code{yyerror}
@cindex error reporting function
@findex yyerror
@cindex syntax error
The Bison parser detects a @dfn{parse error} or @dfn{syntax error}
-whenever it reads a token which cannot satisfy any syntax rule. A
+whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
-macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use in Actions}).
+macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use
+in Actions}).
The Bison parser expects to report the error by calling an error
reporting function named @code{yyerror}, which you must supply. It is
@findex YYERROR_VERBOSE
If you define the macro @code{YYERROR_VERBOSE} in the Bison declarations
-section (@pxref{Bison Declarations, ,The Bison Declarations Section}), then Bison provides a more verbose
-and specific error message string instead of just plain @w{@code{"parse
-error"}}. It doesn't matter what definition you use for
-@code{YYERROR_VERBOSE}, just whether you define it.
+section (@pxref{Bison Declarations, ,The Bison Declarations Section}),
+then Bison provides a more verbose and specific error message string
+instead of just plain @w{@code{"parse error"}}. It doesn't matter what
+definition you use for @code{YYERROR_VERBOSE}, just whether you define
+it.
The parser can detect one other kind of error: stack overflow. This
happens when the input contains constructions that are very deeply
@example
@group
-yyerror (s)
- char *s;
+void
+yyerror (char *s)
@{
@end group
@group
@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
encountered so far. Normally this variable is global; but if you
-request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable
-which only the actions can access.
+request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
+then it is a local variable which only the actions can access.
-@node Action Features, , Error Reporting, Interface
+@node Action Features
@section Special Features for Use in Actions
@cindex summary, action features
@cindex action features summary
@item $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
-specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}.
+specified by the @code{%union} declaration. @xref{Action Types, ,Data
+Types of Values in Actions}.
@item $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
-union specified by the @code{%union} declaration.
-@xref{Action Types, ,Data Types of Values in Actions}.@refill
+union specified by the @code{%union} declaration.
+@xref{Action Types, ,Data Types of Values in Actions}.
@item YYABORT;
Return immediately from @code{yyparse}, indicating failure.
@findex YYBACKUP
Unshift a token. This macro is allowed only for rules that reduce
a single value, and only when there is no look-ahead token.
+It is also disallowed in GLR parsers.
It installs a look-ahead token with token type @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.
@item yyerrok;
Resume generating error messages immediately for subsequent syntax
-errors. This is useful primarily in error rules.
+errors. This is useful primarily in error rules.
@xref{Error Recovery}.
-@item @@@var{n}
-@findex @@@var{n}
-Acts like a structure variable containing information on the line
-numbers and column numbers of the @var{n}th component of the current
-rule. The structure has four members, like this:
+@item @@$
+@findex @@$
+Acts like a structure variable containing information on the textual position
+of the grouping made by the current rule. @xref{Locations, ,
+Tracking Locations}.
-@example
-struct @{
- int first_line, last_line;
- int first_column, last_column;
-@};
-@end example
+@c Check if those paragraphs are still useful or not.
+
+@c @example
+@c struct @{
+@c int first_line, last_line;
+@c int first_column, last_column;
+@c @};
+@c @end example
-Thus, to get the starting line number of the third component, use
-@samp{@@3.first_line}.
+@c Thus, to get the starting line number of the third component, you would
+@c use @samp{@@3.first_line}.
-In order for the members of this structure to contain valid information,
-you must make @code{yylex} supply this information about each token.
-If you need only certain members, then @code{yylex} need only fill in
-those members.
+@c In order for the members of this structure to contain valid information,
+@c you must make @code{yylex} supply this information about each token.
+@c If you need only certain members, then @code{yylex} need only fill in
+@c those members.
+
+@c The use of this feature makes the parser noticeably slower.
+
+@item @@@var{n}
+@findex @@@var{n}
+Acts like a structure variable containing information on the textual position
+of the @var{n}th component of the current rule. @xref{Locations, ,
+Tracking Locations}.
-The use of this feature makes the parser noticeably slower.
@end table
-@node Algorithm, Error Recovery, Interface, Top
-@chapter The Bison Parser Algorithm
-@cindex Bison parser algorithm
+@node Algorithm
+@chapter The Bison Parser Algorithm
+@cindex Bison parser algorithm
@cindex algorithm of parser
@cindex shifting
@cindex reduction
* 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.
@end menu
-@node Look-Ahead, Shift/Reduce, , Algorithm
+@node Look-Ahead
@section Look-Ahead Tokens
@cindex look-ahead token
The current look-ahead token is stored in the variable @code{yychar}.
@xref{Action Features, ,Special Features for Use in Actions}.
-@node Shift/Reduce, Precedence, Look-Ahead, Algorithm
+@node Shift/Reduce
@section Shift/Reduce Conflicts
@cindex conflicts
@cindex shift/reduce conflicts
;
@end example
-@node Precedence, Contextual Precedence, Shift/Reduce, Algorithm
+@node Precedence
@section Operator Precedence
@cindex operator precedence
@cindex precedence of operators
* How Precedence:: How they work.
@end menu
-@node Why Precedence, Using Precedence, , Precedence
+@node Why Precedence
@subsection When Precedence is Needed
Consider the following ambiguous grammar fragment (ambiguous because the
@noindent
Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
-should it reduce them via the rule for the addition operator? It depends
-on the next token. Of course, if the next token is @samp{)}, we must
-reduce; shifting is invalid because no single rule can reduce the token
-sequence @w{@samp{- 2 )}} or anything starting with that. But if the next
-token is @samp{*} or @samp{<}, we have a choice: either shifting or
-reduction would allow the parse to complete, but with different
-results.
-
-To decide which one Bison should do, we must consider the
-results. If the next operator token @var{op} is shifted, then it
-must be reduced first in order to permit another opportunity to
-reduce the sum. The result is (in effect) @w{@samp{1 - (2
-@var{op} 3)}}. On the other hand, if the subtraction is reduced
-before shifting @var{op}, the result is @w{@samp{(1 - 2) @var{op}
-3}}. Clearly, then, the choice of shift or reduce should depend
-on the relative precedence of the operators @samp{-} and
-@var{op}: @samp{*} should be shifted first, but not @samp{<}.
+should it reduce them via the rule for the subtraction operator? It
+depends on the next token. Of course, if the next token is @samp{)}, we
+must reduce; shifting is invalid because no single rule can reduce the
+token sequence @w{@samp{- 2 )}} or anything starting with that. But if
+the next token is @samp{*} or @samp{<}, we have a choice: either
+shifting or reduction would allow the parse to complete, but with
+different results.
+
+To decide which one Bison should do, we must consider the results. If
+the next operator token @var{op} is shifted, then it must be reduced
+first in order to permit another opportunity to reduce the difference.
+The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}. On the other
+hand, if the subtraction is reduced before shifting @var{op}, the result
+is @w{@samp{(1 - 2) @var{op} 3}}. Clearly, then, the choice of shift or
+reduce should depend on the relative precedence of the operators
+@samp{-} and @var{op}: @samp{*} should be shifted first, but not
+@samp{<}.
@cindex associativity
What about input such as @w{@samp{1 - 2 - 5}}; should this be
-@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For
-most operators we prefer the former, which is called @dfn{left
-association}. The latter alternative, @dfn{right association}, is
-desirable for assignment operators. The choice of left or right
-association is a matter of whether the parser chooses to shift or
-reduce when the stack contains @w{@samp{1 - 2}} and the look-ahead
-token is @samp{-}: shifting makes right-associativity.
-
-@node Using Precedence, Precedence Examples, Why Precedence, Precedence
+@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For most
+operators we prefer the former, which is called @dfn{left association}.
+The latter alternative, @dfn{right association}, is desirable for
+assignment operators. The choice of left or right association is a
+matter of whether the parser chooses to shift or reduce when the stack
+contains @w{@samp{1 - 2}} and the look-ahead token is @samp{-}: shifting
+makes right-associativity.
+
+@node Using Precedence
@subsection Specifying Operator Precedence
@findex %left
@findex %right
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.
-@node Precedence Examples, How Precedence, Using Precedence, Precedence
+@node Precedence Examples
@subsection Precedence Examples
In our example, we would want the following declarations:
and so on. We assume that these tokens are more than one character long
and therefore are represented by names, not character literals.)
-@node How Precedence, , Precedence Examples, Precedence
+@node How Precedence
@subsection How Precedence Works
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
-precedence levels to certain rules: each rule gets its precedence from the
-last terminal symbol mentioned in the components. (You can also specify
-explicitly the precedence of a rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}.)
-
-Finally, the resolution of conflicts works by comparing the
-precedence of the rule being considered with that of the
-look-ahead token. If the token's precedence is higher, the
-choice is to shift. If the rule's precedence is higher, the
-choice is to reduce. If they have equal precedence, the choice
-is made based on the associativity of that precedence level. The
-verbose output file made by @samp{-v} (@pxref{Invocation, ,Invoking Bison}) says
-how each conflict was resolved.
+precedence levels to certain rules: each rule gets its precedence from
+the last terminal symbol mentioned in the components. (You can also
+specify explicitly the precedence of a rule. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.)
+
+Finally, the resolution of conflicts works by comparing the precedence
+of the rule being considered with that of the look-ahead token. If the
+token's precedence is higher, the choice is to shift. If the rule's
+precedence is higher, the choice is to reduce. If they have equal
+precedence, the choice is made based on the associativity of that
+precedence level. The verbose output file made by @samp{-v}
+(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
+resolved.
Not all rules and not all tokens have precedence. If either the rule or
the look-ahead token has no precedence, then the default is to shift.
-@node Contextual Precedence, Parser States, Precedence, Algorithm
+@node Contextual Precedence
@section Context-Dependent Precedence
@cindex context-dependent precedence
@cindex unary operator precedence
@code{%nonassoc}, can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
-modifier for rules.@refill
+modifier for rules.
The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
@end group
@end example
-@node Parser States, Reduce/Reduce, Contextual Precedence, Algorithm
+@node Parser States
@section Parser States
@cindex finite-state machine
@cindex parser state
is erroneous in the current state. This causes error processing to begin
(@pxref{Error Recovery}).
-@node Reduce/Reduce, Mystery Conflicts, Parser States, Algorithm
+@node Reduce/Reduce
@section Reduce/Reduce Conflicts
@cindex reduce/reduce conflict
@cindex conflicts, reduce/reduce
;
@end example
-@node Mystery Conflicts, Stack Overflow, Reduce/Reduce, Algorithm
+@node Mystery Conflicts
@section Mysterious Reduce/Reduce Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
@end example
It would seem that this grammar can be parsed with only a single token
-of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
+of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows. In other words, this grammar is LR(1).
;
@end example
-@node Stack Overflow, , Mystery Conflicts, Algorithm
+@node Generalized LR Parsing
+@section Generalized LR (GLR) Parsing
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@cindex ambiguous grammars
+@cindex non-deterministic parsing
+
+Bison produces @emph{deterministic} parsers that choose uniquely
+when to reduce and which reduction to apply
+based on a summary of the preceding input and on one extra token of lookahead.
+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
+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
+summarize the input seen so far loses necessary information.
+
+When you use the @samp{%glr-parser} declaration in your grammar file,
+Bison generates a parser that uses a different algorithm, called
+Generalized LR (or GLR). A Bison GLR parser uses the same basic
+algorithm for parsing as an ordinary Bison parser, but behaves
+differently in cases where there is a shift-reduce conflict that has not
+been resolved by precedence rules (@pxref{Precedence}) or a
+reduce-reduce conflict. When a GLR parser encounters such a situation, it
+effectively @emph{splits} into a several parsers, one for each possible
+shift or reduction. These parsers then proceed as usual, consuming
+tokens in lock-step. Some of the stacks may encounter other conflicts
+and split further, with the result that instead of a sequence of states,
+a Bison GLR parsing stack is what is in effect a tree of states.
+
+In effect, each stack represents a guess as to what the proper parse
+is. Additional input may indicate that a guess was wrong, in which case
+the appropriate stack silently disappears. Otherwise, the semantics
+actions generated in each stack are saved, rather than being executed
+immediately. When a stack disappears, its saved semantic actions never
+get executed. When a reduction causes two stacks to become equivalent,
+their sets of semantic actions are both saved with the state that
+results from the reduction. We say that two stacks are equivalent
+when they both represent the same sequence of states,
+and each pair of corresponding states represents a
+grammar symbol that produces the same segment of the input token
+stream.
+
+Whenever the parser makes a transition from having multiple
+states to having one, it reverts to the normal LALR(1) parsing
+algorithm, after resolving and executing the saved-up actions.
+At this transition, some of the states on the stack will have semantic
+values that are sets (actually multisets) of possible actions. The
+parser tries to pick one of the actions by first finding one whose rule
+has the highest dynamic precedence, as set by the @samp{%dprec}
+declaration. Otherwise, if the alternative actions are not ordered by
+precedence, but there the same merging function is declared for both
+rules by the @samp{%merge} declaration,
+Bison resolves and evaluates both and then calls the merge function on
+the result. Otherwise, it reports an ambiguity.
+
+It is possible to use a data structure for the GLR parsing tree that
+permits the processing of any LALR(1) grammar in linear time (in the
+size of the input), any unambiguous (not necessarily LALR(1)) grammar in
+quadratic worst-case time, and any general (possibly ambiguous)
+context-free grammar in cubic worst-case time. However, Bison currently
+uses a simpler data structure that requires time proportional to the
+length of the input times the maximum number of stacks required for any
+prefix of the input. Thus, really ambiguous or non-deterministic
+grammars can require exponential time and space to process. Such badly
+behaving examples, however, are not generally of practical interest.
+Usually, non-determinism in a grammar is local---the parser is ``in
+doubt'' only for a few tokens at a time. Therefore, the current data
+structure should generally be adequate. On LALR(1) portions of a
+grammar, in particular, it is only slightly slower than with the default
+Bison parser.
+
+@node Stack Overflow
@section Stack Overflow, and How to Avoid It
@cindex stack overflow
@cindex parser stack overflow
returns a nonzero value, pausing only to call @code{yyerror} to report
the overflow.
+Becaue Bison parsers have growing stacks, hitting the upper limit
+usually results from using a right recursion instead of a left
+recursion, @xref{Recursion, ,Recursive Rules}.
+
@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
parser stack can become before a stack overflow occurs. Define the
macro @code{YYINITDEPTH}. This value too must be a compile-time
constant integer. The default is 200.
-@node Error Recovery, Context Dependency, Algorithm, Top
+@c FIXME: C++ output.
+Because of semantical differences between C and C++, the LALR(1) parsers
+in C produced by Bison by compiled as C++ cannot grow. In this precise
+case (compiling a C parser as C++) you are suggested to grow
+@code{YYINITDEPTH}. In the near future, a C++ output output will be
+provided which addresses this issue.
+
+@node Error Recovery
@chapter Error Recovery
@cindex error recovery
@cindex recovery from errors
is always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an @code{error} token whenever a
syntax error happens; if you have provided a rule to recognize this token
-in the current context, the parse can continue.
+in the current context, the parse can continue.
For example:
the current input line or current statement if an error is detected:
@example
-stmnt: error ';' /* on error, skip until ';' is read */
+stmnt: error ';' /* On error, skip until ';' is read. */
@end example
It is also useful to recover to the matching close-delimiter of an
rest of the time. A value of 1 indicates that error messages are
currently suppressed for new syntax errors.
-@node Context Dependency, Debugging, Error Recovery, Top
+@node Context Dependency
@chapter Handling Context Dependencies
The Bison paradigm is to parse tokens first, then group them into larger
(Actually, ``kludge'' means any technique that gets its job done but is
neither clean nor robust.)
-@node Semantic Tokens, Lexical Tie-ins, , Context Dependency
+@node Semantic Tokens
@section Semantic Info in Token Types
The C language has a context dependency: the way an identifier is used
Unfortunately, the name being declared is separated from the declaration
construct itself by a complicated syntactic structure---the ``declarator''.
-As a result, the part of Bison parser for C needs to be duplicated, with
-all the nonterminal names changed: once for parsing a declaration in which
-a typedef name can be redefined, and once for parsing a declaration in
-which that can't be done. Here is a part of the duplication, with actions
-omitted for brevity:
+As a result, part of the Bison parser for C needs to be duplicated, with
+all the nonterminal names changed: once for parsing a declaration in
+which a typedef name can be redefined, and once for parsing a
+declaration in which that can't be done. Here is a part of the
+duplication, with actions omitted for brevity:
@example
initdcl:
program. A true lexical tie-in has a special-purpose flag controlled by
the syntactic context.
-@node Lexical Tie-ins, Tie-in Recovery, Semantic Tokens, Context Dependency
+@node Lexical Tie-ins
@section Lexical Tie-ins
@cindex lexical tie-in
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
-The declaration of @code{hexflag} shown in the C declarations section of
-the parser file is needed to make it accessible to the actions
-(@pxref{C Declarations, ,The C Declarations Section}). You must also write the code in @code{yylex}
-to obey the flag.
+The declaration of @code{hexflag} shown in the prologue of the parser file
+is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}).
+You must also write the code in @code{yylex} to obey the flag.
-@node Tie-in Recovery, , Lexical Tie-ins, Context Dependency
+@node Tie-in Recovery
@section Lexical Tie-ins and Error Recovery
Lexical tie-ins make strict demands on any error recovery rules you have.
be such that you can be sure that it always will, or always won't, have to
clear the flag.
-@node Debugging, Invocation, Context Dependency, Top
+@c ================================================== Debugging Your Parser
+
+@node Debugging
@chapter Debugging Your Parser
-@findex YYDEBUG
+
+Developing a parser can be a challenge, especially if you don't
+understand the algorithm (@pxref{Algorithm, ,The Bison Parser
+Algorithm}). Even so, sometimes a detailed description of the automaton
+can help (@pxref{Understanding, , Understanding Your Parser}), or
+tracing the execution of the parser can give some insight on why it
+behaves improperly (@pxref{Tracing, , Tracing Your Parser}).
+
+@menu
+* Understanding:: Understanding the structure of your parser.
+* Tracing:: Tracing the execution of your parser.
+@end menu
+
+@node Understanding
+@section Understanding Your Parser
+
+As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm})
+Bison parsers are @dfn{shift/reduce automata}. In some cases (much more
+frequent than one would hope), looking at this automaton is required to
+tune or simply fix a parser. Bison provides two different
+representation of it, either textually or graphically (as a @sc{vcg}
+file).
+
+The textual file is generated when the options @option{--report} or
+@option{--verbose} are specified, see @xref{Invocation, , Invoking
+Bison}. Its name is made by removing @samp{.tab.c} or @samp{.c} from
+the parser output file name, and adding @samp{.output} instead.
+Therefore, if the input file is @file{foo.y}, then the parser file is
+called @file{foo.tab.c} by default. As a consequence, the verbose
+output file is called @file{foo.output}.
+
+The following grammar file, @file{calc.y}, will be used in the sequel:
+
+@example
+%token NUM STR
+%left '+' '-'
+%left '*'
+%%
+exp: exp '+' exp
+ | exp '-' exp
+ | exp '*' exp
+ | exp '/' exp
+ | NUM
+ ;
+useless: STR;
+%%
+@end example
+
+@command{bison} reports:
+
+@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.
+@end example
+
+When given @option{--report=state}, in addition to @file{calc.tab.c}, it
+creates a file @file{calc.output} with contents detailed below. The
+order of the output and the exact presentation might vary, but the
+interpretation is the same.
+
+The first section includes details on conflicts that were solved thanks
+to precedence and/or associativity:
+
+@example
+Conflict in state 8 between rule 2 and token '+' resolved as reduce.
+Conflict in state 8 between rule 2 and token '-' resolved as reduce.
+Conflict in state 8 between rule 2 and token '*' resolved as shift.
+@exdent @dots{}
+@end example
+
+@noindent
+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.
+@end example
+
+@noindent
+@cindex token, useless
+@cindex useless token
+@cindex nonterminal, useless
+@cindex useless nonterminal
+@cindex rule, useless
+@cindex useless rule
+The next section reports useless tokens, nonterminal and rules. Useless
+nonterminals and rules are removed in order to produce a smaller parser,
+but useless tokens are preserved, since they might be used by the
+scanner (note the difference between ``useless'' and ``not used''
+below):
+
+@example
+Useless nonterminals:
+ useless
+
+Terminals which are not used:
+ STR
+
+Useless rules:
+#6 useless: STR;
+@end example
+
+@noindent
+The next section reproduces the exact grammar that Bison used:
+
+@example
+Grammar
+
+ Number, Line, Rule
+ 0 5 $accept -> exp $end
+ 1 5 exp -> exp '+' exp
+ 2 6 exp -> exp '-' exp
+ 3 7 exp -> exp '*' exp
+ 4 8 exp -> exp '/' exp
+ 5 9 exp -> NUM
+@end example
+
+@noindent
+and reports the uses of the symbols:
+
+@example
+Terminals, with rules where they appear
+
+$end (0) 0
+'*' (42) 3
+'+' (43) 1
+'-' (45) 2
+'/' (47) 4
+error (256)
+NUM (258) 5
+
+Nonterminals, with rules where they appear
+
+$accept (8)
+ on left: 0
+exp (9)
+ on left: 1 2 3 4 5, on right: 0 1 2 3 4
+@end example
+
+@noindent
+@cindex item
+@cindex pointed rule
+@cindex rule, pointed
+Bison then proceeds onto the automaton itself, describing each state
+with it set of @dfn{items}, also known as @dfn{pointed rules}. Each
+item is a production rule together with a point (marked by @samp{.})
+that the input cursor.
+
+@example
+state 0
+
+ $accept -> . exp $ (rule 0)
+
+ NUM shift, and go to state 1
+
+ exp go to state 2
+@end example
+
+This reads as follows: ``state 0 corresponds to being at the very
+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
+the parse stack, and the control flow jumps to state 1. Any other
+lookahead triggers a parse 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
+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
+@option{--report=itemset} to list all the items, include those that can
+be derived:
+
+@example
+state 0
+
+ $accept -> . exp $ (rule 0)
+ exp -> . exp '+' exp (rule 1)
+ exp -> . exp '-' exp (rule 2)
+ exp -> . exp '*' exp (rule 3)
+ exp -> . exp '/' exp (rule 4)
+ exp -> . NUM (rule 5)
+
+ NUM shift, and go to state 1
+
+ exp go to state 2
+@end example
+
+@noindent
+In the state 1...
+
+@example
+state 1
+
+ exp -> NUM . (rule 5)
+
+ $default reduce using rule 5 (exp)
+@end example
+
+@noindent
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead
+(@samp{$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}).
+
+@example
+state 2
+
+ $accept -> exp . $ (rule 0)
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ $ shift, and go to state 3
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+@end example
+
+@noindent
+In state 2, the automaton can only shift a symbol. For instance,
+because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
+@samp{+}, it will be shifted on the parse stack, and the automaton
+control will jump to state 4, corresponding to the item @samp{exp -> exp
+'+' . exp}. Since there is no default action, any other token than
+those listed above will trigger a parse error.
+
+The state 3 is named the @dfn{final state}, or the @dfn{accepting
+state}:
+
+@example
+state 3
+
+ $accept -> exp $ . (rule 0)
+
+ $default accept
+@end example
+
+@noindent
+the initial rule is completed (the start symbol and the end
+of input were read), the parsing exits successfully.
+
+The interpretation of states 4 to 7 is straightforward, and is left to
+the reader.
+
+@example
+state 4
+
+ exp -> exp '+' . exp (rule 1)
+
+ NUM shift, and go to state 1
+
+ exp go to state 8
+
+state 5
+
+ exp -> exp '-' . exp (rule 2)
+
+ NUM shift, and go to state 1
+
+ exp go to state 9
+
+state 6
+
+ exp -> exp '*' . exp (rule 3)
+
+ NUM shift, and go to state 1
+
+ exp go to state 10
+
+state 7
+
+ exp -> exp '/' . exp (rule 4)
+
+ NUM shift, and go to state 1
+
+ exp go to state 11
+@end example
+
+As was announced in beginning of the report, @samp{State 8 contains 1
+shift/reduce conflict}:
+
+@example
+state 8
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp '+' exp . (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
+@end example
+
+Indeed, there are two actions associated to the lookahead @samp{/}:
+either shifting (and going to state 7), or reducing rule 1. The
+conflict means that either the grammar is ambiguous, or the parser lacks
+information to make the right decision. Indeed the grammar is
+ambiguous, as, since we did not specify the precedence of @samp{/}, the
+sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
+NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
+NUM}, which corresponds to reducing rule 1.
+
+Because in LALR(1) parsing a single decision can be made, Bison
+arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
+Shift/Reduce Conflicts}. Discarded actions are reported in between
+square brackets.
+
+Note that all the previous states had a single possible action: either
+shifting the next token and going to the corresponding state, or
+reducing a single rule. In the other cases, i.e., when shifting
+@emph{and} reducing is possible or when @emph{several} reductions are
+possible, the lookahead is required to select the action. State 8 is
+one such state: if the lookahead is @samp{*} or @samp{/} then the action
+is shifting, otherwise the action is reducing rule 1. In other words,
+the first two items, corresponding to rule 1, are not eligible when the
+lookahead is @samp{*}, since we specified that @samp{*} has higher
+precedence that @samp{+}. More generally, some items are eligible only
+with some set of possible lookaheads. When run with
+@option{--report=lookahead}, Bison specifies these lookaheads:
+
+@example
+state 8
+
+ exp -> exp . '+' exp [$, '+', '-', '/'] (rule 1)
+ exp -> exp '+' exp . [$, '+', '-', '/'] (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
+@end example
+
+The remaining states are similar:
+
+@example
+state 9
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp '-' exp . (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 2 (exp)]
+ $default reduce using rule 2 (exp)
+
+state 10
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp '*' exp . (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 3 (exp)]
+ $default reduce using rule 3 (exp)
+
+state 11
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+ exp -> exp '/' exp . (rule 4)
+
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '+' [reduce using rule 4 (exp)]
+ '-' [reduce using rule 4 (exp)]
+ '*' [reduce using rule 4 (exp)]
+ '/' [reduce using rule 4 (exp)]
+ $default reduce using rule 4 (exp)
+@end example
+
+@noindent
+Observe that state 11 contains conflicts due to the lack of precedence
+of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the
+associativity of @samp{/} is not specified.
+
+
+@node Tracing
+@section Tracing Your Parser
@findex yydebug
@cindex debugging
@cindex tracing the parser
If a Bison grammar compiles properly but doesn't do what you want when it
runs, the @code{yydebug} parser-trace feature can help you figure out why.
-To enable compilation of trace facilities, you must define the macro
-@code{YYDEBUG} when you compile the parser. You could use
+There are several means to enable compilation of trace facilities:
+
+@table @asis
+@item the macro @code{YYDEBUG}
+@findex YYDEBUG
+Define the macro @code{YYDEBUG} to a nonzero value when you compile the
+parser. This is compliant with POSIX Yacc. You could use
@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
-YYDEBUG 1} in the C declarations section of the grammar file
-(@pxref{C Declarations, ,The C Declarations Section}). Alternatively, use the @samp{-t} option when
-you run Bison (@pxref{Invocation, ,Invoking Bison}). We always define @code{YYDEBUG} so that
-debugging is always possible.
+YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
+Prologue}).
+
+@item the option @option{-t}, @option{--debug}
+Use the @samp{-t} option when you run Bison (@pxref{Invocation,
+,Invoking Bison}). This is POSIX compliant too.
+
+@item the directive @samp{%debug}
+@findex %debug
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
+Declaration Summary}). This is a Bison extension, which will prove
+useful when Bison will output parsers for languages that don't use a
+preprocessor. Useless POSIX and Yacc portability matter to you, this is
+the preferred solution.
+@end table
+
+We suggest that you always enable the debug option so that debugging is
+always possible.
-The trace facility uses @code{stderr}, so you must add @w{@code{#include
-<stdio.h>}} to the C declarations section unless it is already there.
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and
+arguments. If you define @code{YYDEBUG} to a nonzero value but do not
+define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
+and @code{YYPRINTF} is defined to @code{fprintf}.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable @code{yydebug}.
@end itemize
To make sense of this information, it helps to refer to the listing file
-produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking Bison}). 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.
+produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
+Bison}). 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
#define YYPRINT(file, type, value) yyprint (file, type, value)
static void
-yyprint (file, type, value)
- FILE *file;
- int type;
- YYSTYPE value;
+yyprint (FILE *file, int type, YYSTYPE value)
@{
if (type == VAR)
fprintf (file, " %s", value.tptr->name);
@}
@end smallexample
-@node Invocation, Table of Symbols, Debugging, Top
+@c ================================================= Invoking Bison
+
+@node Invocation
@chapter Invoking Bison
@cindex invoking Bison
@cindex Bison invocation
@samp{.y}. The parser file's name is made by replacing the @samp{.y}
with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}.@refill
+@file{hack/foo.tab.c}. It's also possible, in case you are writing
+C++ code instead of C in your grammar file, to name it @file{foo.ypp}
+or @file{foo.y++}. Then, the output files will take an extension like
+the given one as input (respectively @file{foo.tab.cpp} and
+@file{foo.tab.c++}).
+This feature takes effect with all options that manipulate filenames like
+@samp{-o} or @samp{-d}.
+
+For example :
+
+@example
+bison -d @var{infile.yxx}
+@end example
+@noindent
+will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and
+
+@example
+bison -d -o @var{output.c++} @var{infile.y}
+@end example
+@noindent
+will produce @file{output.c++} and @file{outfile.h++}.
@menu
-* Bison Options:: All the options described in detail,
+* 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.
@end menu
-@node Bison Options, Option Cross Key, , Invocation
+@node Bison Options
@section Bison Options
Bison supports both traditional single-letter options and mnemonic long
short option. It is followed by a cross key alphabetized by long
option.
-@table @samp
-@item -b @var{file-prefix}
-@itemx --file-prefix=@var{prefix}
-Specify a prefix to use for all Bison output file names. The names are
-chosen as if the input file were named @file{@var{prefix}.c}.
+@c Please, keep this ordered as in `bison --help'.
+@noindent
+Operations modes:
+@table @option
+@item -h
+@itemx --help
+Print a summary of the command-line options to Bison and exit.
-@item -d
-@itemx --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.
+@item -V
+@itemx --version
+Print the version number of Bison and exit.
-If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.@refill
+@need 1750
+@item -y
+@itemx --yacc
+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:
-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}.@refill
+@example
+bison -y $*
+@end example
+@end table
+
+@noindent
+Tuning the parser:
+
+@table @option
+@item -S @var{file}
+@itemx --skeleton=@var{file}
+Specify the skeleton to use. You probably don't need this option unless
+you are developing Bison.
+
+@item -t
+@itemx --debug
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Tracing, ,Tracing Your Parser}.
+
+@item --locations
+Pretend that @code{%locations} was specified. @xref{Decl Summary}.
+
+@item -p @var{prefix}
+@itemx --name-prefix=@var{prefix}
+Pretend that @code{%name-prefix="@var{prefix}"} was specified.
+@xref{Decl Summary}.
@item -l
@itemx --no-lines
grammar file. This option causes them to associate errors with the
parser file, treating it as an independent source file in its own right.
-@item -o @var{outfile}
-@itemx --output-file=@var{outfile}
-Specify the name @var{outfile} for the parser file.
+@item -n
+@itemx --no-parser
+Pretend that @code{%no-parser} was specified. @xref{Decl Summary}.
-The other output files' names are constructed from @var{outfile}
-as described under the @samp{-v} and @samp{-d} switches.
+@item -k
+@itemx --token-table
+Pretend that @code{%token-table} was specified. @xref{Decl Summary}.
+@end table
-@item -p @var{prefix}
-@itemx --name-prefix=@var{prefix}
-Rename the external symbols used in the parser so that they start with
-@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
-is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@code{yylval}, @code{yychar} and @code{yydebug}.
+@noindent
+Adjust the output:
-For example, if you use @samp{-p c}, the names become @code{cparse},
-@code{clex}, and so on.
+@table @option
+@item -d
+@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}.
-@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+@item --defines=@var{defines-file}
+Same as above, but save in the file @var{defines-file}.
-@item -t
-@itemx --debug
-Output a definition of the macro @code{YYDEBUG} into the parser file,
-so that the debugging facilities are compiled. @xref{Debugging, ,Debugging Your Parser}.
-
-@item -v
-@itemx --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.
+@item -b @var{file-prefix}
+@itemx --file-prefix=@var{prefix}
+Pretend that @code{%verbose} was specified, i.e, specify prefix to use
+for all Bison output file names. @xref{Decl Summary}.
-This file also describes all the conflicts, both those resolved by
-operator precedence and the unresolved ones.
+@item -r @var{things}
+@itemx --report=@var{things}
+Write an extra output file containing verbose description of the comma
+separated list of @var{things} among:
-The file's name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.@refill
+@table @code
+@item state
+Description of the grammar, conflicts (resolved and unresolved), and
+LALR automaton.
-Therefore, if the input file is @file{foo.y}, then the parser file is
-called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.@refill
+@item lookahead
+Implies @code{state} and augments the description of the automaton with
+each rule's lookahead set.
-@item -V
-@itemx --version
-Print the version number of Bison and exit.
+@item itemset
+Implies @code{state} and augments the description of the automaton with
+the full set of items for each state, instead of its core only.
+@end table
-@item -h
-@itemx --help
-Print a summary of the command-line options to Bison and exit.
+For instance, on the following grammar
-@need 1750
-@item -y
-@itemx --yacc
-@itemx --fixed-output-files
-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 switch is to imitate Yacc's output
-file name conventions. Thus, the following shell script can substitute
-for Yacc:@refill
-
-@example
-bison -y $*
-@end example
+@item -v
+@itemx --verbose
+Pretend that @code{%verbose} was specified, i.e, write an extra output
+file containing verbose descriptions of the grammar and
+parser. @xref{Decl Summary}.
+
+@item -o @var{filename}
+@itemx --output=@var{filename}
+Specify the @var{filename} for the parser file.
+
+The other output files' names are constructed from @var{filename} as
+described under the @samp{-v} and @samp{-d} options.
+
+@item -g
+Output a VCG definition of the LALR(1) grammar automaton computed by
+Bison. If the grammar file is @file{foo.y}, the VCG output file will
+be @file{foo.vcg}.
+
+@item --graph=@var{graph-file}
+The behavior of @var{--graph} is the same than @samp{-g}. The only
+difference is that it has an optional argument which is the name of
+the output graph filename.
@end table
-@node Option Cross Key, VMS Invocation, Bison Options, Invocation
+@node Option Cross Key
@section Option Cross Key
Here is a list of options, alphabetized by long option, to help you find
\line{ --debug \leaderfill -t}
\line{ --defines \leaderfill -d}
\line{ --file-prefix \leaderfill -b}
-\line{ --fixed-output-files \leaderfill -y}
+\line{ --graph \leaderfill -g}
\line{ --help \leaderfill -h}
\line{ --name-prefix \leaderfill -p}
\line{ --no-lines \leaderfill -l}
-\line{ --output-file \leaderfill -o}
+\line{ --no-parser \leaderfill -n}
+\line{ --output \leaderfill -o}
+\line{ --token-table \leaderfill -k}
\line{ --verbose \leaderfill -v}
\line{ --version \leaderfill -V}
\line{ --yacc \leaderfill -y}
@ifinfo
@example
--debug -t
---defines -d
+--defines=@var{defines-file} -d
--file-prefix=@var{prefix} -b @var{file-prefix}
---fixed-output-files --yacc -y
+--graph=@var{graph-file} -d
--help -h
---name-prefix -p
+--name-prefix=@var{prefix} -p @var{name-prefix}
--no-lines -l
---output-file=@var{outfile} -o @var{outfile}
+--no-parser -n
+--output=@var{outfile} -o @var{outfile}
+--token-table -k
--verbose -v
--version -V
+--yacc -y
@end example
@end ifinfo
-@node VMS Invocation, , Option Cross Key, Invocation
+@node VMS Invocation
@section Invoking Bison under VMS
@cindex invoking Bison under VMS
@cindex VMS
@file{foo.tab.c}. In the above example, the output file
would instead be named @file{foo_tab.c}.
-@node Table of Symbols, Glossary, Invocation, Top
+@c ================================================= Invoking Bison
+
+@node FAQ
+@chapter Frequently Asked Questions
+@cindex frequently asked questions
+@cindex questions
+
+Several questions about Bison come up occasionally. Here some of them
+are addressed.
+
+@menu
+* Parser Stack Overflow:: Breaking the Stack Limits
+@end menu
+
+@node Parser Stack Overflow
+@section Parser Stack Overflow
+
+@display
+My parser returns with error with a @samp{parser stack overflow}
+message. What can I do?
+@end display
+
+This question is already addressed elsewhere, @xref{Recursion,
+,Recursive Rules}.
+
+@c ================================================= Table of Symbols
+
+@node Table of Symbols
@appendix Bison Symbols
@cindex Bison symbols, table of
@cindex symbols in Bison, table of
@table @code
+@item @@$
+In an action, the location of the left-hand side of the rule.
+@xref{Locations, , Locations Overview}.
+
+@item @@@var{n}
+In an action, the location of the @var{n}-th symbol of the right-hand
+side of the rule. @xref{Locations, , Locations Overview}.
+
+@item $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+
+@item $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule. @xref{Actions}.
+
+@item $accept
+The predefined nonterminal whose only rule is @samp{$accept: @var{start}
+$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
+Start-Symbol}. It cannot be used in the grammar.
+
+@item $end
+The predefined token marking the end of the token stream. It cannot be
+used in the grammar.
+
+@item $undefined
+The predefined token onto which all undefined values returned by
+@code{yylex} are mapped. It cannot be used in the grammar, rather, use
+@code{error}.
+
@item error
A token name reserved for error recovery. This token may be used in
grammar rules so as to allow the Bison parser to recognize an error in
@item YYABORT
Macro to pretend that an unrecoverable syntax error has occurred, by
making @code{yyparse} return 1 immediately. The error reporting
-function @code{yyerror} is not called. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+function @code{yyerror} is not called. @xref{Parser Function, ,The
+Parser Function @code{yyparse}}.
@item YYACCEPT
Macro to pretend that a complete utterance of the language has been
-read, by making @code{yyparse} return 0 immediately.
+read, by making @code{yyparse} return 0 immediately.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
@item YYBACKUP
Macro to discard a value from the parser stack and fake a look-ahead
token. @xref{Action Features, ,Special Features for Use in Actions}.
+@item YYDEBUG
+Macro to define to equip the parser with tracing code. @xref{Tracing,
+,Tracing Your Parser}.
+
@item YYERROR
Macro to pretend that a syntax error has just been detected: call
@code{yyerror} and then perform normal error recovery if possible
@item YYLTYPE
Macro for the data type of @code{yylloc}; a structure with four
-members. @xref{Token Positions, ,Textual Positions of Tokens}.
+members. @xref{Location Type, , Data Types of Locations}.
+
+@item yyltype
+Default value for YYLTYPE.
@item YYMAXDEPTH
Macro for specifying the maximum size of the parser stack.
Macro whose value indicates whether the parser is recovering from a
syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@item YYSTACK_USE_ALLOCA
+Macro used to control the use of @code{alloca}. If defined to @samp{0},
+the parser will not use @code{alloca} but @code{malloc} when trying to
+grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
+to anything else.
+
@item YYSTYPE
Macro for the data type of semantic values; @code{int} by default.
@xref{Value Type, ,Data Types of Semantic Values}.
@item yychar
-External integer variable that contains the integer value of the
-current look-ahead token. (In a pure parser, it is a local variable
-within @code{yyparse}.) Error-recovery rule actions may examine this
-variable. @xref{Action Features, ,Special Features for Use in Actions}.
+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}.
@item yyclearin
Macro used in error-recovery rule actions. It clears the previous
@item yydebug
External integer variable set to zero by default. If @code{yydebug}
is given a nonzero value, the parser will output information on input
-symbols and parser action. @xref{Debugging, ,Debugging Your Parser}.
+symbols and parser action. @xref{Tracing, ,Tracing Your Parser}.
@item yyerrok
Macro to cause parser to recover immediately to its normal mode
@item yyerror
User-supplied function to be called by @code{yyparse} on error. The
function receives one argument, a pointer to a character string
-containing an error message. @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+containing an error message. @xref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}.
@item yylex
-User-supplied lexical analyzer function, called with no arguments
-to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+User-supplied lexical analyzer function, called with no arguments to get
+the next token. @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
@item yylval
External variable in which @code{yylex} should place the semantic
@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
@item yylloc
-External variable in which @code{yylex} should place the line and
-column numbers associated with a token. (In a pure parser, it is a
-local variable within @code{yyparse}, and its address is passed to
+External variable in which @code{yylex} should place the line and column
+numbers associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
@code{yylex}.) You can ignore this variable if you don't use the
-@samp{@@} feature in the grammar actions. @xref{Token Positions, ,Textual Positions of Tokens}.
+@samp{@@} feature in the grammar actions. @xref{Token Positions,
+,Textual Positions of Tokens}.
@item yynerrs
-Global variable which Bison increments each time there is a parse
-error. (In a pure parser, it is a local variable within
-@code{yyparse}.) @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+Global variable which Bison increments each time there is a parse error.
+(In a pure parser, it is a local variable within @code{yyparse}.)
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
@item yyparse
The parser function produced by Bison; call this function to start
parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@item %debug
+Equip the parser for debugging. @xref{Decl Summary}.
+
+@item %defines
+Bison declaration to create a header file meant for the scanner.
+@xref{Decl Summary}.
+
+@item %dprec
+Bison declaration to assign a precedence to a rule that is used at parse
+time to resolve reduce/reduce conflicts. @xref{GLR Parsers}.
+
+@item %file-prefix="@var{prefix}"
+Bison declaration to set the prefix of the output files. @xref{Decl
+Summary}.
+
+@item %glr-parser
+Bison declaration to produce a GLR parser. @xref{GLR Parsers}.
+
+@c @item %source-extension
+@c Bison declaration to specify the generated parser output file extension.
+@c @xref{Decl Summary}.
+@c
+@c @item %header-extension
+@c Bison declaration to specify the generated parser header file extension
+@c if required. @xref{Decl Summary}.
+
@item %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@item %merge
+Bison declaration to assign a merging function to a rule. If there is a
+reduce/reduce conflict with a rule having the same merging function, the
+function is applied to the two semantic values to get a single result.
+@xref{GLR Parsers}.
+
+@item %name-prefix="@var{prefix}"
+Bison declaration to rename the external symbols. @xref{Decl Summary}.
+
+@item %no-lines
+Bison declaration to avoid generating @code{#line} directives in the
+parser file. @xref{Decl Summary}.
+
@item %nonassoc
-Bison declaration to assign nonassociativity to token(s).
+Bison declaration to assign non-associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@item %output="@var{filename}"
+Bison declaration to set the name of the parser file. @xref{Decl
+Summary}.
+
@item %prec
Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
-@item %pure_parser
+@item %pure-parser
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
@xref{Precedence Decl, ,Operator Precedence}.
@item %start
-Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}.
+Bison declaration to specify the start symbol. @xref{Start Decl, ,The
+Start-Symbol}.
@item %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
+@item %token-table
+Bison declaration to include a token name table in the parser file.
+@xref{Decl Summary}.
+
@item %type
-Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
+Bison declaration to declare nonterminals. @xref{Type Decl,
+,Nonterminal Symbols}.
@item %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
@end table
+@sp 1
+
These are the punctuation and delimiters used in Bison input:
@table @samp
@item %%
Delimiter used to separate the grammar rule section from the
-Bison declarations section or the additional C code section.
+Bison declarations section or the epilogue.
@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
@item %@{ %@}
-All code listed between @samp{%@{} and @samp{%@}} is copied directly
-to the output file uninterpreted. Such code forms the ``C
-declarations'' section of the input file. @xref{Grammar Outline, ,Outline of a Bison Grammar}.
+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}.
@item /*@dots{}*/
Comment delimiters, as in C.
@item :
-Separates a rule's result from its components. @xref{Rules, ,Syntax of Grammar Rules}.
+Separates a rule's result from its components. @xref{Rules, ,Syntax of
+Grammar Rules}.
@item ;
Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
@xref{Rules, ,Syntax of Grammar Rules}.
@end table
-@node Glossary, Index, Table of Symbols, Top
+@node Glossary
@appendix Glossary
@cindex glossary
@table @asis
@item Backus-Naur Form (BNF)
Formal method of specifying context-free grammars. BNF was first used
-in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar,
+,Languages and Context-Free Grammars}.
@item 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 @emph{anywhere} an expression is
-permitted. @xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+permitted. @xref{Language and Grammar, ,Languages and Context-Free
+Grammars}.
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
parsed, and the states correspond to various stages in the grammar
rules. @xref{Algorithm, ,The Bison Parser Algorithm }.
+@item Generalized LR (GLR)
+A parsing algorithm that can handle all context-free grammars, including those
+that are not LALR(1). It resolves situations that Bison's usual LALR(1)
+algorithm cannot by effectively splitting off multiple parsers, trying all
+possible parsers, and discarding those that fail in the light of additional
+right context. @xref{Generalized LR Parsing, ,Generalized LR Parsing}.
+
@item Grouping
A language construct that is (in general) grammatically divisible;
-for example, `expression' or `declaration' in C.
+for example, `expression' or `declaration' in C.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@item Infix operator
@samp{c}. @xref{Precedence, ,Operator Precedence}.
@item Left recursion
-A rule whose result symbol is also its first component symbol;
-for example, @samp{expseq1 : expseq1 ',' exp;}. @xref{Recursion, ,Recursive Rules}.
+A rule whose result symbol is also its first component symbol; for
+example, @samp{expseq1 : expseq1 ',' exp;}. @xref{Recursion, ,Recursive
+Rules}.
@item Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from
A flag, set by actions in the grammar rules, which alters the way
tokens are parsed. @xref{Lexical Tie-ins}.
+@item Literal string token
+A token which consists of two or more fixed characters. @xref{Symbols}.
+
@item Look-ahead token
-A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}.
+A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead
+Tokens}.
@item LALR(1)
The class of context-free grammars that Bison (like most other parser
@item Reduction
Replacing a string of nonterminals and/or terminals with a single
-nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison Parser Algorithm }.
+nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison
+Parser Algorithm }.
@item Reentrant
A reentrant subprogram is a subprogram which can be in invoked any
A language in which all operators are postfix operators.
@item Right recursion
-A rule whose result symbol is also its last component symbol;
-for example, @samp{expseq1: exp ',' expseq1;}. @xref{Recursion, ,Recursive Rules}.
+A rule whose result symbol is also its last component symbol; for
+example, @samp{expseq1: exp ',' expseq1;}. @xref{Recursion, ,Recursive
+Rules}.
@item Semantics
In computer languages, the semantics are specified by the actions
@item 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.
+first nonterminal symbol in a language specification.
@xref{Start Decl, ,The Start-Symbol}.
@item Symbol table
the lexical analyzer. @xref{Symbols}.
@item 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. @xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+A grammar symbol that has no rules in the grammar and therefore is
+grammatically indivisible. The piece of text it represents is a token.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@end table
-@node Index, , Glossary, Top
-@unnumbered Index
-
-@printindex cp
-
-@contents
-
-@bye
-
+@node Copying This Manual
+@appendix Copying This Manual
-\f
-
-@c old menu
+@menu
+* GNU Free Documentation License:: License for copying this manual.
+@end menu
-* Introduction::
-* Conditions::
-* Copying:: The GNU General Public License says
- how you can copy and share Bison
+@include fdl.texi
-Tutorial sections:
-* Concepts:: Basic concepts for understanding Bison.
-* Examples:: Three simple explained examples of using Bison.
+@node Index
+@unnumbered Index
-Reference sections:
-* Grammar File:: Writing Bison declarations and rules.
-* Interface:: C-language interface to the parser function @code{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:: Debugging Bison parsers that parse wrong.
-* 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.
-* Index:: Cross-references to the text.
+@printindex cp
+@bye
projects / bison.git / blobdiff