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Bison The YACC-compatible Parser Generator November  , Bison Version . by Charles Donnelly and Richard Stallman
Copyright c  ,  , , , , ,   Free Software Foundation Published by the Free Software Foundation  Temple Place, Suite  Boston, MA - USA Printed copies are available for $ each. ISBN --- Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modied 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 lan- guage, under the above conditions for modied 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. Cover art by Etienne Suvasa.
Introduction  Introduction Bison is a general-purpose parser generator that converts a grammar description for an LALR() context-free grammar into a C program to parse that grammar. Once you are procient with Bison, you may use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages. Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little trouble. You need to be uent in C programming in order to use Bison or to understand this manual. We begin with tutorial chapters that explain the basic concepts of using Bison and show three explained examples, each building on the last. If you don’t know Bison or Yacc, start by reading these chapters. Reference chapters follow which describe specic aspects of Bison in detail. Bison was written primarily by Robert Corbett; Richard Stallman made it Yacc- compatible. Wilfred Hansen of Carnegie Mellon University added multicharacter string literals and other features. This edition corresponds to version . of Bison.
 Bison .
Conditions for Using Bison  Conditions for Using Bison As of Bison version ., we have changed the distribution terms for yyparse to permit using Bison’s output in non-free programs. Formerly, Bison 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 software. The reason Bison was dierent was not due to a special policy decision; it resulted from applying the usual General Public License to all of the Bison source code. The output of the Bison utility|the Bison parser le|contains a verbatim copy of a sizable piece of Bison, which is the code for the yyparse function. (The actions from your grammar are inserted into this function at one point, but the rest of the function is not changed.) When we applied the GPL terms to the code for yyparse, the eect was to restrict the use of Bison output to free software. We didn’t change the terms because of sympathy for people who want to make software proprietary. Software should be free. But we concluded that limiting Bison’s use to free software was doing little to encourage people to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for using the other GNU tools.
 Bison .
GNU GENERAL PUBLIC LICENSE  GNU GENERAL PUBLIC LICENSE Version , June   Copyright c   ,   Free Software Foundation, Inc.  Mass Ave, Cambridge, MA  , USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software|to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation’s software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things. To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it. For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. We protect your rights with two steps: () copyright the software, and () oer 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 modied 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 reect 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 eect 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 modication follow.
 Bison . TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION . 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 modications and/or translated into another language. (Hereinafter, translation is included without limitation in the term \modication".) Each licensee is addressed as \you". Activities other than copying, distribution and modication 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. . 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 oer warranty protection in exchange for a fee. . 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 modications or work under the terms of Section  above, provided that you also meet all of these conditions: a. You must cause the modied les to carry prominent notices stating that you changed the les and the date of any change. b. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License. c. If the modied program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.) These requirements apply to the modied work as a whole. If identiable 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
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