Write in English Sentences That Describe the Following Declarations C++

Bison

This manual (10 September 2021) is for GNU Bison (version 3.8.1), the GNU parser generator.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 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 the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom."

Introduction
Conditions for Using Bison
GNU GENERAL PUBLIC LICENSE
1 The Concepts of Bison
- 1.1 Languages and Context-Free Grammars
- 1.2 From Formal Rules to Bison Input
- 1.3 Semantic Values
- 1.4 Semantic Actions
- 1.5 Writing GLR Parsers
  - 1.5.1 Using GLR on Unambiguous Grammars
  - 1.5.2 Using GLR to Resolve Ambiguities
  - 1.5.3 GLR Semantic Actions
    - 1.5.3.1 Deferred semantic actions
    - 1.5.3.2 YYERROR
    - 1.5.3.3 Restrictions on semantic values and locations
  - 1.5.4 Controlling a Parse with Arbitrary Predicates
- 1.6 Locations
- 1.7 Bison Output: the Parser Implementation File
- 1.8 Stages in Using Bison
- 1.9 The Overall Layout of a Bison Grammar
2 Examples
- 2.1 Reverse Polish Notation Calculator
  - 2.1.1 Declarations for rpcalc
  - 2.1.2 Grammar Rules for rpcalc
    - 2.1.2.1 Explanation of input
    - 2.1.2.2 Explanation of line
    - 2.1.2.3 Explanation of exp
  - 2.1.3 The rpcalc Lexical Analyzer
  - 2.1.4 The Controlling Function
  - 2.1.5 The Error Reporting Routine
  - 2.1.6 Running Bison to Make the Parser
  - 2.1.7 Compiling the Parser Implementation File
- 2.2 Infix Notation Calculator: calc
- 2.3 Simple Error Recovery
- 2.4 Location Tracking Calculator: ltcalc
  - 2.4.1 Declarations for ltcalc
  - 2.4.2 Grammar Rules for ltcalc
  - 2.4.3 The ltcalc Lexical Analyzer.
- 2.5 Multi-Function Calculator: mfcalc
  - 2.5.1 Declarations for mfcalc
  - 2.5.2 Grammar Rules for mfcalc
  - 2.5.3 The mfcalc Symbol Table
  - 2.5.4 The mfcalc Lexer
  - 2.5.5 The mfcalc Main
- 2.6 Exercises
3 Bison Grammar Files
- 3.1 Outline of a Bison Grammar
  - 3.1.1 The prologue
  - 3.1.2 Prologue Alternatives
  - 3.1.3 The Bison Declarations Section
  - 3.1.4 The Grammar Rules Section
  - 3.1.5 The epilogue
- 3.2 Symbols, Terminal and Nonterminal
- 3.3 Grammar Rules
  - 3.3.1 Syntax of Grammar Rules
  - 3.3.2 Empty Rules
  - 3.3.3 Recursive Rules
- 3.4 Defining Language Semantics
  - 3.4.1 Data Types of Semantic Values
  - 3.4.2 More Than One Value Type
  - 3.4.3 Generating the Semantic Value Type
  - 3.4.4 The Union Declaration
  - 3.4.5 Providing a Structured Semantic Value Type
  - 3.4.6 Actions
  - 3.4.7 Data Types of Values in Actions
  - 3.4.8 Actions in Midrule
    - 3.4.8.1 Using Midrule Actions
    - 3.4.8.2 Typed Midrule Actions
    - 3.4.8.3 Midrule Action Translation
    - 3.4.8.4 Conflicts due to Midrule Actions
- 3.5 Tracking Locations
  - 3.5.1 Data Type of Locations
  - 3.5.2 Actions and Locations
  - 3.5.3 Printing Locations
  - 3.5.4 Default Action for Locations
- 3.6 Named References
- 3.7 Bison Declarations
  - 3.7.1 Require a Version of Bison
  - 3.7.2 Token Kind Names
  - 3.7.3 Operator Precedence
  - 3.7.4 Nonterminal Symbols
  - 3.7.5 Syntax of Symbol Declarations
  - 3.7.6 Performing Actions before Parsing
  - 3.7.7 Freeing Discarded Symbols
  - 3.7.8 Printing Semantic Values
  - 3.7.9 Suppressing Conflict Warnings
  - 3.7.10 The Start-Symbol
  - 3.7.11 A Pure (Reentrant) Parser
  - 3.7.12 A Push Parser
  - 3.7.13 Bison Declaration Summary
  - 3.7.14 %define Summary
  - 3.7.15 %code Summary
- 3.8 Multiple Parsers in the Same Program
4 Parser C-Language Interface
- 4.1 The Parser Function yyparse
- 4.2 Push Parser Interface
- 4.3 The Lexical Analyzer Function yylex
  - 4.3.1 Calling Convention for yylex
  - 4.3.2 Special Tokens
  - 4.3.3 Finding Tokens by String Literals
  - 4.3.4 Semantic Values of Tokens
  - 4.3.5 Textual Locations of Tokens
  - 4.3.6 Calling Conventions for Pure Parsers
- 4.4 Error Reporting
  - 4.4.1 The Error Reporting Function yyerror
  - 4.4.2 The Syntax Error Reporting Function yyreport_syntax_error
- 4.5 Special Features for Use in Actions
- 4.6 Parser Internationalization
  - 4.6.1 Enabling Internationalization
  - 4.6.2 Token Internationalization
5 The Bison Parser Algorithm
- 5.1 Lookahead Tokens
- 5.2 Shift/Reduce Conflicts
- 5.3 Operator Precedence
  - 5.3.1 When Precedence is Needed
  - 5.3.2 Specifying Operator Precedence
  - 5.3.3 Specifying Precedence Only
  - 5.3.4 Precedence Examples
  - 5.3.5 How Precedence Works
  - 5.3.6 Using Precedence For Non Operators
- 5.4 Context-Dependent Precedence
- 5.5 Parser States
- 5.6 Reduce/Reduce Conflicts
- 5.7 Mysterious Conflicts
- 5.8 Tuning LR
  - 5.8.1 LR Table Construction
  - 5.8.2 Default Reductions
  - 5.8.3 LAC
  - 5.8.4 Unreachable States
- 5.9 Generalized LR (GLR) Parsing
- 5.10 Memory Management, and How to Avoid Memory Exhaustion
6 Error Recovery
7 Handling Context Dependencies
- 7.1 Semantic Info in Token Kinds
- 7.2 Lexical Tie-ins
- 7.3 Lexical Tie-ins and Error Recovery
8 Debugging Your Parser
- 8.1 Generation of Counterexamples
- 8.2 Understanding Your Parser
- 8.3 Visualizing Your Parser
- 8.4 Visualizing your parser in multiple formats
- 8.5 Tracing Your Parser
  - 8.5.1 Enabling Traces
  - 8.5.2 Enabling Debug Traces for mfcalc
9 Invoking Bison
- 9.1 Bison Options
  - 9.1.1 Operation Modes
  - 9.1.2 Diagnostics
  - 9.1.3 Tuning the Parser
  - 9.1.4 Output Files
- 9.2 Option Cross Key
- 9.3 Yacc Library
10 Parsers Written In Other Languages
- 10.1 C++ Parsers
  - 10.1.1 A Simple C++ Example
  - 10.1.2 C++ Bison Interface
  - 10.1.3 C++ Parser Interface
  - 10.1.4 C++ Semantic Values
    - 10.1.4.1 C++ Unions
    - 10.1.4.2 C++ Variants
  - 10.1.5 C++ Location Values
    - 10.1.5.1 C++ position
    - 10.1.5.2 C++ location
    - 10.1.5.3 Exposing the Location Classes
    - 10.1.5.4 User Defined Location Type
  - 10.1.6 C++ Parser Context
  - 10.1.7 C++ Scanner Interface
    - 10.1.7.1 Split Symbols
    - 10.1.7.2 Complete Symbols
  - 10.1.8 A Complete C++ Example
    - 10.1.8.1 Calc++ — C++ Calculator
    - 10.1.8.2 Calc++ Parsing Driver
    - 10.1.8.3 Calc++ Parser
    - 10.1.8.4 Calc++ Scanner
    - 10.1.8.5 Calc++ Top Level
- 10.2 D Parsers
  - 10.2.1 D Bison Interface
  - 10.2.2 D Semantic Values
  - 10.2.3 D Location Values
  - 10.2.4 D Parser Interface
  - 10.2.5 D Parser Context Interface
  - 10.2.6 D Scanner Interface
  - 10.2.7 Special Features for Use in D Actions
  - 10.2.8 D Push Parser Interface
  - 10.2.9 D Complete Symbols
- 10.3 Java Parsers
  - 10.3.1 Java Bison Interface
  - 10.3.2 Java Semantic Values
  - 10.3.3 Java Location Values
  - 10.3.4 Java Parser Interface
  - 10.3.5 Java Parser Context Interface
  - 10.3.6 Java Scanner Interface
  - 10.3.7 Special Features for Use in Java Actions
  - 10.3.8 Java Push Parser Interface
  - 10.3.9 Differences between C/C++ and Java Grammars
  - 10.3.10 Java Declarations Summary
11 A Brief History of the Greater Ungulates
- 11.1 The ancestral Yacc
- 11.2 yacchack
- 11.3 Berkeley Yacc
- 11.4 Bison
- 11.5 Other Ungulates
12 Bison Version Compatibility: Best Practices
13 Frequently Asked Questions
- 13.1 Memory Exhausted
- 13.2 How Can I Reset the Parser
- 13.3 Strings are Destroyed
- 13.4 Implementing Gotos/Loops
- 13.5 Multiple start-symbols
- 13.6 Secure? Conform?
- 13.7 Enabling Relocatability
- 13.8 I can't build Bison
- 13.9 Where can I find help?
- 13.10 Bug Reports
- 13.11 More Languages
- 13.12 Beta Testing
- 13.13 Mailing Lists
Appendix A Bison Symbols
Appendix B Glossary
Appendix C GNU Free Documentation License
Bibliography
Index of Terms

Introduction

Bison is a general-purpose parser generator that converts an annotated context-free grammar into a deterministic LR or generalized LR (GLR) parser employing LALR(1), IELR(1) or canonical LR(1) parser tables. Once you are proficient with Bison, you can use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages.

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

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

Bison was written originally by Robert Corbett. Richard Stallman made it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added multi-character string literals and other features. Since then, Bison has grown more robust and evolved many other new features thanks to the hard work of a long list of volunteers. For details, see the THANKS and ChangeLog files included in the Bison distribution.

This edition corresponds to version 3.8.1 of Bison.

Conditions for Using Bison

The distribution terms for Bison-generated parsers permit using the parsers in nonfree programs. Before Bison version 2.2, these extra permissions applied only when Bison was generating LALR(1) parsers in C. And before Bison version 1.24, Bison-generated parsers could be used only in programs that were free software.

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

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

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

This exception applies when Bison is generating code for a parser. You can tell whether the exception applies to a Bison output file by inspecting the file for text beginning with "As a special exception…". The text spells out the exact terms of the exception.

GNU GENERAL PUBLIC LICENSE

Version 3, 29 June 2007

Copyright © 2007 Free Software Foundation, Inc. https://fsf.org/  Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

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The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program—to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too.

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To protect your rights, we need to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.

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1 The Concepts of Bison

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

Languages and Context-Free Grammars
From Formal Rules to Bison Input
Semantic Values
Semantic Actions
Writing GLR Parsers
Locations
Bison Output: the Parser Implementation File
Stages in Using Bison
The Overall Layout of a Bison Grammar

1.1 Languages and Context-Free Grammars

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

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

There are various important subclasses of context-free grammars. Although it can handle almost all context-free grammars, Bison is optimized for what are called LR(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 lookahead. For historical reasons, Bison by default is limited by the additional restrictions of LALR(1), which is hard to explain simply. See Mysterious Conflicts, for more information on this. You can escape these additional restrictions by requesting IELR(1) or canonical LR(1) parser tables. See LR Table Construction, to learn how.

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

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

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

Here is a simple C function subdivided into tokens:

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

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

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

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

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

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

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

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

1.2 From Formal Rules to Bison Input

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

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

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

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. See Symbols, Terminal and Nonterminal, for more information.

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

See Grammar Rules.

1.3 Semantic Values

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

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

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

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

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

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

1.4 Semantic Actions

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

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

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

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

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

1.5 Writing GLR Parsers

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

To use a grammar that is not easily modified to be LR(1), a more general parsing algorithm is sometimes necessary. If you include %glr-parser among the Bison declarations in your file (see Outline of a Bison Grammar), the result is a Generalized LR (GLR) parser. These parsers handle Bison grammars that contain no unresolved conflicts (i.e., after applying precedence declarations) identically to deterministic 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.

Using GLR on Unambiguous Grammars
Using GLR to Resolve Ambiguities
GLR Semantic Actions
Controlling a Parse with Arbitrary Predicates

1.5.1 Using GLR on Unambiguous Grammars

In the simplest cases, you can use the GLR algorithm to parse grammars that are unambiguous but fail to be LR(1). Such grammars typically require more than one symbol of lookahead.

Consider a problem that arises in the declaration of enumerated and subrange types in the programming language Pascal. Here are some examples:

type subrange = lo .. hi; type enum = (a, b, c);

The original language standard allows only numeric literals and constant identifiers for the subrange bounds ('lo' and 'hi'), but Extended Pascal (ISO/IEC 10206) and many other Pascal implementations allow arbitrary expressions there. This gives rise to the following situation, containing a superfluous pair of parentheses:

type subrange = (a) .. b;

Compare this to the following declaration of an enumerated type with only one value:

(These declarations are contrived, but they are syntactically valid, and more-complicated cases can come up in practical programs.)

These two declarations look identical until the '..' token. With normal LR(1) one-token lookahead it is not possible to decide between the two forms when the identifier 'a' is parsed. It is, however, desirable for a parser to decide this, since in the latter case 'a' must become a new identifier to represent the enumeration value, while in the former case 'a' must be evaluated with its current meaning, which may be a constant or even a function call.

You could parse '(a)' as an "unspecified identifier in parentheses", to be resolved later, but this typically requires substantial contortions in both semantic actions and large parts of the grammar, where the parentheses are nested in the recursive rules for expressions.

You might think of using the lexer to distinguish between the two forms by returning different tokens for currently defined and undefined identifiers. But if these declarations occur in a local scope, and 'a' is defined in an outer scope, then both forms are possible—either locally redefining 'a', or using the value of 'a' from the outer scope. So this approach cannot work.

A simple solution to this problem is to declare the parser to use the GLR algorithm. When the GLR parser reaches the critical state, it merely splits into two branches and pursues both syntax rules simultaneously. Sooner or later, one of them runs into a parsing error. If there is a '..' token before the next ';', the rule for enumerated types fails since it cannot accept '..' anywhere; otherwise, the subrange type rule fails since it requires a '..' token. So one of the branches fails silently, and the other one continues normally, performing all the intermediate actions that were postponed during the split.

If the input is syntactically incorrect, both branches fail and the parser reports a syntax error as usual.

The effect of all this is that the parser seems to "guess" the correct branch to take, or in other words, it seems to use more lookahead than the underlying LR(1) algorithm actually allows for. In this example, LR(2) would suffice, but also some cases that are not LR(k) for any k can be handled this way.

In general, a GLR parser can take quadratic or cubic worst-case time, and the current Bison parser even takes exponential time and space for some grammars. In practice, this rarely happens, and for many grammars it is possible to prove that it cannot happen. The present example contains only one conflict between two rules, and the type-declaration context containing the conflict cannot be nested. So the number of branches that can exist at any time is limited by the constant 2, and the parsing time is still linear.

Here is a Bison grammar corresponding to the example above. It parses a vastly simplified form of Pascal type declarations.

%token TYPE DOTDOT ID

%left '+' '-' %left '*' '/'

                    %% type_decl: TYPE ID '=' type ';' ;

type:   '(' id_list ')' | expr DOTDOT expr ;

id_list:   ID | id_list ',' ID ;

expr:   '(' expr ')' | expr '+' expr | expr '-' expr | expr '*' expr | expr '/' expr | ID ;

When used as a normal LR(1) grammar, Bison correctly complains about one reduce/reduce conflict. In the conflicting situation the parser chooses one of the alternatives, arbitrarily the one declared first. Therefore the following correct input is not recognized:

The parser can be turned into a GLR parser, while also telling Bison to be silent about the one known reduce/reduce conflict, by adding these two declarations to the Bison grammar file (before the first '%%'):

No change in the grammar itself is required. Now the parser recognizes all valid declarations, according to the limited syntax above, transparently. In fact, the user does not even notice when the parser splits.

So here we have a case where we can use the benefits of GLR, almost without disadvantages. Even in simple cases like this, however, there are at least two potential problems to beware. First, always analyze the conflicts reported by Bison to make sure that GLR splitting is only done where it is intended. A GLR parser splitting inadvertently may cause problems less obvious than an LR parser statically choosing the wrong alternative in a conflict. Second, consider interactions with the lexer (see Semantic Info in Token Kinds) with great care. Since a split parser consumes tokens without performing any actions during the split, the lexer cannot obtain information via parser actions. Some cases of lexer interactions can be eliminated by using GLR to shift the complications from the lexer to the parser. You must check the remaining cases for correctness.

In our example, it would be safe for the lexer to return tokens based on their current meanings in some symbol table, because no new symbols are defined in the middle of a type declaration. Though it is possible for a parser to define the enumeration constants as they are parsed, before the type declaration is completed, it actually makes no difference since they cannot be used within the same enumerated type declaration.

1.5.2 Using GLR to Resolve Ambiguities

Let's consider an example, vastly simplified from a C++ grammar.¹

%{   #include <stdio.h>   int yylex (void);   void yyerror (char const *); %}  %define api.value.type {char const *}  %token TYPENAME ID  %right '=' %left '+'  %glr-parser  %%  prog:   %empty | prog stmt   { printf ("\n"); } ;  stmt:   expr ';'  %dprec 1 | decl      %dprec 2 ;  expr:   ID               { printf ("%s ", $$); } | TYPENAME '(' expr ')'                    { printf ("%s <cast> ", $1); } | expr '+' expr    { printf ("+ "); } | expr '=' expr    { printf ("= "); } ;  decl:   TYPENAME declarator ';'                    { printf ("%s <declare> ", $1); } | TYPENAME declarator '=' expr ';'                    { printf ("%s <init-declare> ", $1); } ;  declarator:   ID               { printf ("\"%s\" ", $1); } | '(' declarator ')' ;

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

parses as either an expr or a stmt (assuming that 'T' is recognized as a TYPENAME and 'x' as an ID). Bison detects this as a reduce/reduce conflict between the rules expr : ID and declarator : ID, which it cannot resolve at the time it encounters x in the example above. Since this is a GLR parser, it therefore splits the problem into two parses, one for each choice of resolving the reduce/reduce conflict. Unlike the example from the previous section (see Using GLR on Unambiguous Grammars), however, neither of these parses "dies," because the grammar as it stands is ambiguous. One of the parsers eventually reduces stmt : expr ';' and the other reduces stmt : decl, after which both parsers are in an identical state: they've seen 'prog stmt' and have the same unprocessed input remaining. We say that these parses have merged.

At this point, the GLR parser requires a specification in the grammar of how to choose between the competing parses. In the example above, the two %dprec declarations specify that Bison is to give precedence to the parse that interprets the example as a decl, which implies that x is a declarator. The parser therefore prints

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

The %dprec declarations only come into play when more than one parse survives. Consider a different input string for this parser:

This is another example of using GLR to parse an unambiguous construct, as shown in the previous section (see Using GLR on Unambiguous Grammars). Here, there is no ambiguity (this cannot be parsed as a declaration). However, at the time the Bison parser encounters x, it does not have enough information to resolve the reduce/reduce conflict (again, between x as an expr or a declarator). In this case, no precedence declaration is used. Again, the parser splits into two, one assuming that x is an expr, and the other assuming x is a declarator. The second of these parsers then vanishes when it sees +, and the parser prints

Suppose that instead of resolving the ambiguity, you wanted to see all the possibilities. For this purpose, you must merge the semantic actions of the two possible parsers, rather than choosing one over the other. To do so, you could change the declaration of stmt as follows:

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

and define the stmt_merge function as:

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

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

%{   static YYSTYPE stmt_merge (YYSTYPE x0, YYSTYPE x1); %}

With these declarations, the resulting parser parses the first example as both an expr and a decl, and prints

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

Bison requires that all of the productions that participate in any particular merge have identical '%merge' clauses. Otherwise, the ambiguity would be unresolvable, and the parser will report an error during any parse that results in the offending merge.

The signature of the merger depends on the type of the symbol. In the previous example, the merged-to symbol (stmt) does not have a specific type, and the merger is

YYSTYPE stmt_merge (YYSTYPE x0, YYSTYPE x1);

However, if stmt had a declared type, e.g.,

%union {   Node *node;   ... };

%type <node> stmt;

then the prototype of the merger must be:

Node *stmt_merge (YYSTYPE x0, YYSTYPE x1);

(This signature might be a mistake originally, and maybe it should have been 'Node *stmt_merge (Node *x0, Node *x1)'. If you have an opinion about it, please let us know.)

1.5.3 GLR Semantic Actions

The nature of GLR parsing and the structure of the generated parsers give rise to certain restrictions on semantic values and actions.

Deferred semantic actions
YYERROR
Restrictions on semantic values and locations

1.5.3.1 Deferred semantic actions

By definition, a deferred semantic action is not performed at the same time as the associated reduction. This raises caveats for several Bison features you might use in a semantic action in a GLR parser.

In any semantic action, you can examine yychar to determine the kind of the lookahead token present at the time of the associated reduction. After checking that yychar is not set to YYEMPTY or YYEOF, you can then examine yylval and yylloc to determine the lookahead token's semantic value and location, if any. In a nondeferred semantic action, you can also modify any of these variables to influence syntax analysis. See Lookahead Tokens.

In a deferred semantic action, it's too late to influence syntax analysis. In this case, yychar, yylval, and yylloc are set to shallow copies of the values they had at the time of the associated reduction. For this reason alone, modifying them is dangerous. Moreover, the result of modifying them is undefined and subject to change with future versions of Bison. For example, if a semantic action might be deferred, you should never write it to invoke yyclearin (see Special Features for Use in Actions) or to attempt to free memory referenced by yylval.

1.5.3.2 YYERROR

Another Bison feature requiring special consideration is YYERROR (see Special Features for Use in Actions), which you can invoke in a semantic action to initiate error recovery. During deterministic GLR operation, the effect of YYERROR is the same as its effect in a deterministic parser. The effect in a deferred action is similar, but the precise point of the error is undefined; instead, the parser reverts to deterministic operation, selecting an unspecified stack on which to continue with a syntax error. In a semantic predicate (see Controlling a Parse with Arbitrary Predicates) during nondeterministic parsing, YYERROR silently prunes the parse that invoked the test.

1.5.3.3 Restrictions on semantic values and locations

GLR parsers require that you use POD (Plain Old Data) types for semantic values and location types when using the generated parsers as C++ code.

1.5.4 Controlling a Parse with Arbitrary Predicates

In addition to the %dprec and %merge directives, GLR parsers allow you to reject parses on the basis of arbitrary computations executed in user code, without having Bison treat this rejection as an error if there are alternative parses. For example,

widget:   %?{  new_syntax } "widget" id new_args  { $$ = f($3, $4); } | %?{ !new_syntax } "widget" id old_args  { $$ = f($3, $4); } ;

is one way to allow the same parser to handle two different syntaxes for widgets. The clause preceded by %? is treated like an ordinary midrule action, except that its text is handled as an expression and is always evaluated immediately (even when in nondeterministic mode). If the expression yields 0 (false), the clause is treated as a syntax error, which, in a nondeterministic parser, causes the stack in which it is reduced to die. In a deterministic parser, it acts like YYERROR.

As the example shows, predicates otherwise look like semantic actions, and therefore you must take them into account when determining the numbers to use for denoting the semantic values of right-hand side symbols. Predicate actions, however, have no defined value, and may not be given labels.

There is a subtle difference between semantic predicates and ordinary actions in nondeterministic mode, since the latter are deferred. For example, we could try to rewrite the previous example as

widget:   { if (!new_syntax) YYERROR; }     "widget" id new_args  { $$ = f($3, $4); } |  { if (new_syntax) YYERROR; }     "widget" id old_args  { $$ = f($3, $4); } ;

(reversing the sense of the predicate tests to cause an error when they are false). However, this does not have the same effect if new_args and old_args have overlapping syntax. Since the midrule actions testing new_syntax are deferred, a GLR parser first encounters the unresolved ambiguous reduction for cases where new_args and old_args recognize the same string before performing the tests of new_syntax. It therefore reports an error.

Finally, be careful in writing predicates: deferred actions have not been evaluated, so that using them in a predicate will have undefined effects.

1.6 Locations

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 textual location, or 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 (see Tracking 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 @$, while the locations of the subexpressions are @1 and @3.

When a rule is matched, a default action is used to compute the semantic value of its left hand side (see 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 @$ 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.

1.7 Bison Output: the Parser Implementation File

When you run Bison, you give it a Bison grammar file as input. The most important output is a C source file that implements a parser for the language described by the grammar. This parser is called a Bison parser, and this file is called a Bison parser implementation file. Keep in mind that the Bison utility and the Bison parser are two distinct programs: the Bison utility is a program whose output is the Bison parser implementation file that becomes part of your program.

The job of the Bison parser is to group tokens into groupings according to the grammar rules—for example, to build identifiers and operators into expressions. As it does this, it runs the actions for the grammar rules it uses.

The tokens come from a function called the 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. See The Lexical Analyzer Function yylex.

The Bison parser implementation file is C code which defines a function named yyparse which implements that grammar. This function does not make a complete C program: you must supply some additional functions. One is the lexical analyzer. Another is an error-reporting function which the parser calls to report an error. In addition, a complete C program must start with a function called main; you have to provide this, and arrange for it to call yyparse or the parser will never run. See Parser C-Language Interface.

Aside from the token kind names and the symbols in the actions you write, all symbols defined in the Bison parser implementation file itself begin with 'yy' or 'YY'. This includes interface functions such as the lexical analyzer function yylex, the error reporting function yyerror and the parser function yyparse itself. This also includes numerous identifiers used for internal purposes. Therefore, you should avoid using C identifiers starting with 'yy' or 'YY' in the Bison grammar file except for the ones defined in this manual. Also, you should avoid using the C identifiers 'malloc' and 'free' for anything other than their usual meanings.

In some cases the Bison parser implementation file includes system headers, and in those cases your code should respect the identifiers reserved by those headers. On some non-GNU hosts, <limits.h>, <stddef.h>, <stdint.h> (if available), and <stdlib.h> are included to declare memory allocators and integer types and constants. <libintl.h> is included if message translation is in use (see Parser Internationalization). Other system headers may be included if you define YYDEBUG (see Tracing Your Parser) or YYSTACK_USE_ALLOCA (see Bison Symbols) to a nonzero value.

1.8 Stages in Using Bison

The actual language-design process using Bison, from grammar specification to a working compiler or interpreter, has these parts:

Formally specify the grammar in a form recognized by Bison (see 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.
Write a lexical analyzer to process input and pass tokens to the parser. The lexical analyzer may be written by hand in C (see The Lexical Analyzer Function yylex). It could also be produced using Lex, but the use of Lex is not discussed in this manual.
Write a controlling function that calls the Bison-produced parser.
Write error-reporting routines.

To turn this source code as written into a runnable program, you must follow these steps:

Run Bison on the grammar to produce the parser.
Compile the code output by Bison, as well as any other source files.
Link the object files to produce the finished product.

1.9 The Overall Layout of a Bison Grammar

The input file for the Bison utility is a Bison grammar file. The general form of a Bison grammar file is as follows:

%{                Prologue                %}                Bison declarations                %%                Grammar rules                %%                Epilogue

The '%%', '%{' and '%}' are punctuation that appears in every Bison grammar file to separate the sections.

The prologue may define types and variables used in the actions. You can also use preprocessor commands to define macros used there, and use #include to include header files that do any of these things. You need to declare the lexical analyzer yylex and the error printer yyerror here, along with any other global identifiers used by the actions in the grammar rules.

The Bison declarations declare the names of the terminal and nonterminal symbols, and may also describe operator precedence and the data types of semantic values of various symbols.

The grammar rules define how to construct each nonterminal symbol from its parts.

The epilogue can contain any code you want to use. Often the definitions of functions declared in the prologue go here. In a simple program, all the rest of the program can go here.

2 Examples

Now we show and explain several sample programs written using Bison: a Reverse Polish Notation calculator, an algebraic (infix) notation calculator — later extended to track "locations" — and a multi-function calculator. All produce usable, though limited, interactive desk-top calculators.

These examples are simple, but Bison grammars for real programming languages are written the same way. You can copy these examples into a source file to try them.

Bison comes with several examples (including for the different target languages). If this package is properly installed, you shall find them in prefix/share/doc/bison/examples, where prefix is the root of the installation, probably something like /usr/local or /usr.

Reverse Polish Notation Calculator
Infix Notation Calculator: calc
Simple Error Recovery
Location Tracking Calculator: ltcalc
Multi-Function Calculator: mfcalc
Exercises

2.1 Reverse Polish Notation Calculator

The first example² is that of a simple double-precision Reverse Polish Notation calculator (a calculator using postfix operators). This example provides a good starting point, since operator precedence is not an issue. The second example will illustrate how operator precedence is handled.

The source code for this calculator is named rpcalc.y. The '.y' extension is a convention used for Bison grammar files.

Declarations for rpcalc
Grammar Rules for rpcalc
The rpcalc Lexical Analyzer
The Controlling Function
The Error Reporting Routine
Running Bison to Make the Parser
Compiling the Parser Implementation File

2.1.1 Declarations for `rpcalc`

Here are the C and Bison declarations for the Reverse Polish Notation calculator. As in C, comments are placed between '/*…*/' or after '//'.

/* Reverse Polish Notation calculator. */

%{   #include <stdio.h>   #include <math.h>   int yylex (void);   void yyerror (char const *); %}

                    %define api.value.type {double} %token NUM  %% /* Grammar rules and actions follow. */

The declarations section (see The prologue) contains two preprocessor directives and two forward declarations.

The #include directive is used to declare the exponentiation function pow.

The forward declarations for yylex and yyerror are needed because the C language requires that functions be declared before they are used. These functions will be defined in the epilogue, but the parser calls them so they must be declared in the prologue.

The second section, Bison declarations, provides information to Bison about the tokens and their types (see The Bison Declarations Section).

The %define directive defines the variable api.value.type, thus specifying the C data type for semantic values of both tokens and groupings (see Data Types of Semantic Values). The Bison parser will use whatever type api.value.type is defined as; if you don't define it, int is the default. Because we specify '{double}', each token and each expression has an associated value, which is a floating point number. C code can use YYSTYPE to refer to the value api.value.type.

Each terminal symbol that is not a single-character literal must be declared. (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 NUM, the token kind for numeric constants.

2.1.2 Grammar Rules for `rpcalc`

Here are the grammar rules for the Reverse Polish Notation calculator.

input:   %empty | input line ;

line:   '\n' | exp '\n'      { printf ("%.10g\n", $1); } ;

exp:   NUM | exp exp '+'   { $$ = $1 + $2;      } | exp exp '-'   { $$ = $1 - $2;      } | exp exp '*'   { $$ = $1 * $2;      } | exp exp '/'   { $$ = $1 / $2;      } | exp exp '^'   { $$ = pow ($1, $2); }  /* Exponentiation */ | exp 'n'       { $$ = -$1;          }  /* Unary minus   */ ;

%%

The groupings of the rpcalc "language" defined here are the expression (given the name exp), the line of input (line), and the complete input transcript (input). Each of these nonterminal symbols has several alternate rules, joined by the vertical bar '|' which is read as "or". The following sections explain what these rules mean.

The semantics of the language is determined by the actions taken when a grouping is recognized. The actions are the C code that appears inside braces. See Actions.

You must specify these actions in C, but Bison provides the means for passing semantic values between the rules. In each action, the pseudo-variable $$ stands for the semantic value for the grouping that the rule is going to construct. Assigning a value to $$ is the main job of most actions. The semantic values of the components of the rule are referred to as $1, $2, and so on.

Explanation of input
Explanation of line
Explanation of exp

2.1.2.1 Explanation of `input`

Consider the definition of input:

input:   %empty | input line ;

This definition reads as follows: "A complete input is either an empty string, or a complete input followed by an input line". Notice that "complete input" is defined in terms of itself. This definition is said to be left recursive since input appears always as the leftmost symbol in the sequence. See Recursive Rules.

The first alternative is empty because there are no symbols between the colon and the first '|'; this means that input can match an empty string of input (no tokens). We write the rules this way because it is legitimate to type Ctrl-d right after you start the calculator. It's conventional to put an empty alternative first and to use the (optional) %empty directive, or to write the comment '/* empty */' in it (see Empty Rules).

The second alternate rule (input line) handles all nontrivial input. It means, "After reading any number of lines, read one more line if possible." The left recursion makes this rule into a loop. Since the first alternative matches empty input, the loop can be executed zero or more times.

The parser function 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-input.

2.1.2.2 Explanation of `line`

Now consider the definition of line:

line:   '\n' | exp '\n'  { printf ("%.10g\n", $1); } ;

The first alternative is a token which is a newline character; this means that rpcalc accepts a blank line (and ignores it, since there is no action). The second alternative is an expression followed by a newline. This is the alternative that makes rpcalc useful. The semantic value of the exp grouping is the value of $1 because the exp in question is the first symbol in the alternative. The action prints this value, which is the result of the computation the user asked for.

This action is unusual because it does not assign a value to $$. As a consequence, the semantic value associated with the line is uninitialized (its value will be unpredictable). This would be a bug if 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.

2.1.2.3 Explanation of `exp`

The exp grouping has several rules, one for each kind of expression. The first rule handles the simplest expressions: those that are just numbers. The second handles an addition-expression, which looks like two expressions followed by a plus-sign. The third handles subtraction, and so on.

exp:   NUM | exp exp '+'     { $$ = $1 + $2;    } | exp exp '-'     { $$ = $1 - $2;    } … ;

We have used '|' to join all the rules for exp, but we could equally well have written them separately:

exp: NUM; exp: exp exp '+'  { $$ = $1 + $2; }; exp: exp exp '-'  { $$ = $1 - $2; }; …

Most of the rules have actions that compute the value of the expression in terms of the value of its parts. For example, in the rule for addition, $1 refers to the first component exp and $2 refers to the second one. The third component, '+', has no meaningful associated semantic value, but if it had one you could refer to it as $3. The first rule relies on the implicit default action: '{ $$ = $1; }'.

When yyparse recognizes a sum expression using this rule, the sum of the two subexpressions' values is produced as the value of the entire expression. See Actions.

You don't have to give an action for every rule. When a rule has no action, Bison by default copies the value of $1 into $$. This is what happens in the first rule (the one that uses NUM).

The formatting shown here is the recommended convention, but Bison does not require it. You can add or change white space as much as you wish. For example, this:

exp: NUM | exp exp '+' {$$ = $1 + $2; } | … ;

means the same thing as this:

exp:   NUM | exp exp '+'    { $$ = $1 + $2; } | … ;

The latter, however, is much more readable.

2.1.3 The `rpcalc` Lexical Analyzer

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. See The Lexical Analyzer Function yylex.

Only a simple lexical analyzer is needed for the RPN calculator. This lexical analyzer skips blanks and tabs, then reads in numbers as double and returns them as NUM tokens. Any other character that isn't part of a number is a separate token. Note that the token-code for such a single-character token is the character itself.

The return value of the lexical analyzer function is a numeric code which represents a token kind. The same text used in Bison rules to stand for this token kind is also a C expression for the numeric code of the kind. This works in two ways. If the token kind is a character literal, then its 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 kind is an identifier, that identifier is defined by Bison as a C enum whose definition is the appropriate code. In this example, therefore, NUM becomes an enum for yylex to use.

The semantic value of the token (if it has one) is stored into the global variable yylval, which is where the Bison parser will look for it. (The C data type of yylval is YYSTYPE, whose value was defined at the beginning of the grammar via '%define api.value.type {double}'; see Declarations for rpcalc.)

A token kind 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:

/* 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> #include <stdlib.h>

int yylex (void) {   int c = getchar ();   /* Skip white space. */   while (c == ' ' || c == '\t')     c = getchar ();

                  /* Process numbers. */   if (c == '.' || isdigit (c))     {       ungetc (c, stdin);       if (scanf ("%lf", &yylval) != 1)         abort ();       return NUM;     }

                  /* Return end-of-input. */   else if (c == EOF)     return YYEOF;   /* Return a single char. */   else     return c; }

2.1.4 The Controlling Function

In keeping with the spirit of this example, the controlling function is kept to the bare minimum. The only requirement is that it call yyparse to start the process of parsing.

int main (void) {   return yyparse (); }

2.1.5 The Error Reporting Routine

When yyparse detects a syntax error, it calls the error reporting function yyerror to print an error message (usually but not always "syntax error"). It is up to the programmer to supply yyerror (see Parser C-Language Interface), so here is the definition we will use:

#include <stdio.h>

/* Called by yyparse on error. */ void yyerror (char const *s) {   fprintf (stderr, "%s\n", s); }

After yyerror returns, the Bison parser may recover from the error and continue parsing if the grammar contains a suitable error rule (see Error Recovery). Otherwise, 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 for the first example.

2.1.6 Running Bison to Make the Parser

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 grammar file. The definitions of yylex, yyerror and main go at the end, in the epilogue of the grammar file (see The Overall Layout of a Bison Grammar).

For a large project, you would probably have several source files, and use make to arrange to recompile them.

With all the source in the grammar file, you use the following command to convert it into a parser implementation file:

In this example, the grammar file is called rpcalc.y (for "Reverse Polish CALCulator"). Bison produces a parser implementation file named file.tab.c, removing the '.y' from the grammar file name. The parser implementation file contains the source code for yyparse. The additional functions in the grammar file (yylex, yyerror and main) are copied verbatim to the parser implementation file.

2.1.7 Compiling the Parser Implementation File

Here is how to compile and run the parser implementation file:

#                  List files in current directory.                  $                  ls                  rpcalc.tab.c  rpcalc.y

#                  Compile the Bison parser.                  #                                      -lm                    tells compiler to search math library for                    pow.                  $                  cc -lm -o rpcalc rpcalc.tab.c

#                  List files again.                  $                  ls                  rpcalc  rpcalc.tab.c  rpcalc.y

The file rpcalc now contains the executable code. Here is an example session using rpcalc.

$                  rpcalc                  4 9 +                  ⇒ 13                  3 7 + 3 4 5 *+-                  ⇒ -13                  3 7 + 3 4 5 * + - n                  Note the unary minus, 'n'                  ⇒ 13                  5 6 / 4 n +                  ⇒ -3.166666667                  3 4 ^                  Exponentiation                  ⇒ 81                  ^D                  End-of-file indicator                  $

2.2 Infix Notation Calculator: `calc`

We now modify rpcalc to handle infix operators instead of postfix.³ Infix notation involves the concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is the Bison code for calc.y, an infix desk-top calculator.

/* Infix notation calculator. */

%{   #include <math.h>   #include <stdio.h>   int yylex (void);   void yyerror (char const *); %}

/* Bison declarations. */ %define api.value.type {double} %token NUM %left '-' '+' %left '*' '/' %precedence NEG   /* negation--unary minus */ %right '^'        /* exponentiation */

                  %% /* The grammar follows. */

input:   %empty | input line ;

line:   '\n' | exp '\n'  { printf ("\t%.10g\n", $1); } ;

exp:   NUM | exp '+' exp        { $$ = $1 + $3;      } | exp '-' exp        { $$ = $1 - $3;      } | exp '*' exp        { $$ = $1 * $3;      } | exp '/' exp        { $$ = $1 / $3;      } | '-' exp  %prec NEG { $$ = -$2;          } | exp '^' exp        { $$ = pow ($1, $3); } | '(' exp ')'        { $$ = $2;           } ;

%%

The functions yylex, yyerror and main can be the same as before.

There are two important new features shown in this code.

In the second section (Bison declarations), %left declares token kinds and says they are left-associative operators. The declarations %left and %right (right associativity) take the place of %token which is used to declare a token kind name without associativity/precedence. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify the associativity/precedence.)

Operator precedence is determined by the line ordering of the 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 (NEG) is next, followed by '*' and '/', and so on. Unary minus is not associative, only precedence matters (%precedence. See Operator Precedence.

The other important new feature is the %prec in the grammar section for the unary minus operator. The %prec simply instructs Bison that the rule '| '-' exp' has the same precedence as NEG—in this case the next-to-highest. See Context-Dependent Precedence.

Here is a sample run of calc.y:

$                calc                4 + 4.5 - (34/(8*3+-3))                6.880952381                -56 + 2                -54                3 ^ 2                9

2.3 Simple Error Recovery

Up to this point, this manual has not addressed the issue of error recovery—how to continue parsing after the parser detects a syntax error. All we have handled is error reporting with yyerror. Recall that by default yyparse returns after calling 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 error, which may be included in the grammar rules. In the example below it has been added to one of the alternatives for line:

line:   '\n' | exp '\n'   { printf ("\t%.10g\n", $1); } | error '\n' { yyerrok;                  } ;

This addition to the grammar allows for simple error recovery in the event of a syntax error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for line, and parsing will continue. (The yyerror function is still called upon to print its message as well.) The action executes the statement yyerrok, a macro defined automatically by Bison; its meaning is that error recovery is complete (see Error Recovery). Note the difference between yyerrok and 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 signal that is normally fatal. A real calculator program must handle this signal and use longjmp to return to main and resume parsing input lines; it would also have to discard the rest of the current line of input. We won't discuss this issue further because it is not specific to Bison programs.

2.4 Location Tracking Calculator: `ltcalc`

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.

Declarations for ltcalc
Grammar Rules for ltcalc
The ltcalc Lexical Analyzer.

2.4.1 Declarations for `ltcalc`

The C and Bison declarations for the location tracking calculator are the same as the declarations for the infix notation calculator.

/* Location tracking calculator. */  %{   #include <math.h>   int yylex (void);   void yyerror (char const *); %}  /* Bison declarations. */ %define api.value.type {int} %token NUM  %left '-' '+' %left '*' '/' %precedence NEG %right '^'  %% /* The grammar follows. */

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 (see Data Type of Locations), which is a four member structure with the following integer fields: first_line, first_column, last_line and last_column. By conventions, and in accordance with the GNU Coding Standards and common practice, the line and column count both start at 1.

2.4.2 Grammar Rules for `ltcalc`

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.

Here, we will use locations to report divisions by zero, and locate the wrong expressions or subexpressions.

input:   %empty | input line ;

line:   '\n' | exp '\n' { printf ("%d\n", $1); } ;

exp:   NUM | exp '+' exp   { $$ = $1 + $3; } | exp '-' exp   { $$ = $1 - $3; } | exp '*' exp   { $$ = $1 * $3; }

| 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);         }     }

| '-' exp %prec NEG     { $$ = -$2; } | exp '^' exp           { $$ = pow ($1, $3); } | '(' exp ')'           { $$ = $2; }

This code shows how to reach locations inside of semantic actions, by using the pseudo-variables @n for rule components, and the pseudo-variable @$ for groupings.

We don't need to assign a value to @$: the output parser does it automatically. By default, before executing the C code of each action, @$ is set to range from the beginning of @1 to the end of @n , for a rule with n components. This behavior can be redefined (see Default Action for Locations), and for very specific rules, @$ can be computed by hand.

2.4.3 The `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:

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;

                  /* 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;     }

                    /* Return end-of-input. */   if (c == EOF)     return YYEOF;

                  /* Return a single char, and update location. */   if (c == '\n')     {       ++yylloc.last_line;       yylloc.last_column = 0;     }   else     ++yylloc.last_column;   return c; }

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 yylloc, the global variable (of type YYLTYPE) containing the token's location.

Now, each time this function returns a token, the parser has its kind as well as its semantic value, and its location in the text. The last needed change is to initialize yylloc, for example in the controlling function:

int main (void) {   yylloc.first_line = yylloc.last_line = 1;   yylloc.first_column = yylloc.last_column = 0;   return yyparse (); }

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.

2.5 Multi-Function Calculator: `mfcalc`

Now that the basics of Bison have been discussed, it is time to move on to a more advanced problem.⁴ The above calculators provided only five functions, '+', '-', '*', '/' and '^'. It would be nice to have a calculator that provides other mathematical functions such as sin, 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 yylex passes back all nonnumeric 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:

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:

$                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 $

Note that multiple assignment and nested function calls are permitted.

Declarations for mfcalc
Grammar Rules for mfcalc
The mfcalc Symbol Table
The mfcalc Lexer
The mfcalc Main

2.5.1 Declarations for `mfcalc`

Here are the C and Bison declarations for the multi-function calculator.

%{   #include <stdio.h>  /* For printf, etc. */   #include <math.h>   /* For pow, used in the grammar. */   #include "calc.h"   /* Contains definition of 'symrec'. */   int yylex (void);   void yyerror (char const *); %}

                    %define api.value.type union /* Generate YYSTYPE from these types: */ %token <double>  NUM     /* Double precision number. */ %token <symrec*> VAR FUN /* Symbol table pointer: variable/function. */ %nterm <double>  exp

%precedence '=' %left '-' '+' %left '*' '/' %precedence NEG /* negation--unary minus */ %right '^'      /* exponentiation */

The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (see More Than One Value Type).

The special union value assigned to the %define variable api.value.type specifies that the symbols are defined with their data types. Bison will generate an appropriate definition of YYSTYPE to store these values.

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 NUM, VAR, FUN, and exp. Their declarations are augmented with their data type (placed between angle brackets). For instance, values of NUM are stored in double.

The Bison construct %nterm is used for declaring nonterminal symbols, just as %token is used for declaring token kinds. Previously we did not use %nterm before because nonterminal symbols are normally declared implicitly by the rules that define them. But exp must be declared explicitly so we can specify its value type. See Nonterminal Symbols.

2.5.2 Grammar Rules for `mfcalc`

Here are the grammar rules for the multi-function calculator. Most of them are copied directly from calc; three rules, those which mention VAR or FUN, are new.

%% /* The grammar follows. */

input:   %empty | input line ;

line:   '\n' | exp '\n'   { printf ("%.10g\n", $1); } | error '\n' { yyerrok;                } ;

exp:   NUM | VAR                { $$ = $1->value.var;              } | VAR '=' exp        { $$ = $3; $1->value.var = $3;     } | FUN '(' exp ')'    { $$ = $1->value.fun ($3);         } | exp '+' exp        { $$ = $1 + $3;                    } | exp '-' exp        { $$ = $1 - $3;                    } | exp '*' exp        { $$ = $1 * $3;                    } | exp '/' exp        { $$ = $1 / $3;                    } | '-' exp  %prec NEG { $$ = -$2;                        } | exp '^' exp        { $$ = pow ($1, $3);               } | '(' exp ')'        { $$ = $2;                         } ;

/* End of grammar. */ %%

2.5.3 The `mfcalc` Symbol Table

The multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support.

The symbol table itself consists of a linked list of records. Its definition, which is kept in the header calc.h, is as follows. It provides for either functions or variables to be placed in the table.

/* 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 FUN */   union   {     double var;    /* value of a VAR */     func_t *fun;   /* value of a FUN */   } value;   struct symrec *next;  /* link field */ };

typedef struct symrec symrec;  /* The symbol table: a chain of 'struct symrec'. */ extern symrec *sym_table;  symrec *putsym (char const *name, int sym_type); symrec *getsym (char const *name);

The new version of main will call init_table to initialize the symbol table:

struct init {   char const *name;   func_t *fun; };

struct init const funs[] = {   { "atan", atan },   { "cos",  cos  },   { "exp",  exp  },   { "ln",   log  },   { "sin",  sin  },   { "sqrt", sqrt },   { 0, 0 }, };

/* The symbol table: a chain of 'struct symrec'. */ symrec *sym_table;

/* Put functions in table. */ static void init_table (void)

{   for (int i = 0; funs[i].name; i++)     {       symrec *ptr = putsym (funs[i].name, FUN);       ptr->value.fun = funs[i].fun;     } }

By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator.

Two important functions allow look-up and installation of symbols in the symbol table. The function putsym is passed a name and the kind (VAR or FUN) of the object to be installed. The object is linked to the front of the list, and a pointer to the object is returned. The function getsym is passed the name of the symbol to look up. If found, a pointer to that symbol is returned; otherwise zero is returned.

/* The mfcalc code assumes that malloc and realloc    always succeed, and that integer calculations    never overflow.  Production-quality code should    not make these assumptions.  */ #include <assert.h> #include <stdlib.h> /* malloc, realloc. */ #include <string.h> /* strlen. */

symrec * putsym (char const *name, int sym_type) {   symrec *res = (symrec *) malloc (sizeof (symrec));   res->name = strdup (name);   res->type = sym_type;   res->value.var = 0; /* Set value to 0 even if fun. */   res->next = sym_table;   sym_table = res;   return res; }

symrec * getsym (char const *name) {   for (symrec *p = sym_table; p; p = p->next)     if (strcmp (p->name, name) == 0)       return p;   return NULL; }

2.5.4 The `mfcalc` Lexer

The function yylex must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric characters with a leading letter are recognized as either variables or functions depending on what the symbol table says about them.

The string is passed to getsym for look up in the symbol table. If the name appears in the table, a pointer to its location and its type (VAR or FUN) is returned to yyparse. If it is not already in the table, then it is installed as a VAR using putsym. Again, a pointer and its type (which must be VAR) is returned to yyparse.

No change is needed in the handling of numeric values and arithmetic operators in yylex.

#include <ctype.h> #include <stddef.h>

int yylex (void) {   int c = getchar ();    /* Ignore white space, get first nonwhite character. */   while (c == ' ' || c == '\t')     c = getchar ();    if (c == EOF)     return YYEOF;

                  /* Char starts a number => parse the number. */   if (c == '.' || isdigit (c))     {       ungetc (c, stdin);       if (scanf ("%lf", &yylval.NUM) != 1)         abort ();       return NUM;     }

Bison generated a definition of YYSTYPE with a member named NUM to store value of NUM symbols.

                  /* Char starts an identifier => read the name. */   if (isalpha (c))     {       static ptrdiff_t bufsize = 0;       static char *symbuf = 0;

                  ptrdiff_t i = 0;       do

                  {           /* If buffer is full, make it bigger. */           if (bufsize <= i)             {               bufsize = 2 * bufsize + 40;               symbuf = realloc (symbuf, (size_t) bufsize);             }           /* Add this character to the buffer. */           symbuf[i++] = (char) c;           /* Get another character. */           c = getchar ();         }

                  while (isalnum (c));        ungetc (c, stdin);       symbuf[i] = '\0';

                  symrec *s = getsym (symbuf);       if (!s)         s = putsym (symbuf, VAR);       yylval.VAR = s; /* or yylval.FUN = s. */       return s->type;     }    /* Any other character is a token by itself. */   return c; }

2.5.5 The `mfcalc` Main

The error reporting function is unchanged, and the new version of main includes a call to init_table and sets the yydebug on user demand (See Tracing Your Parser, for details):

/* Called by yyparse on error. */ void yyerror (char const *s) {   fprintf (stderr, "%s\n", s); }

int main (int argc, char const* argv[])

{   /* Enable parse traces on option -p. */   if (argc == 2 && strcmp(argv[1], "-p") == 0)     yydebug = 1;

                  init_table ();   return yyparse (); }

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 pi or e as well.

2.6 Exercises

Add some new functions from math.h to the initialization list.
Add another array that contains constants and their values. Then modify init_table to add these constants to the symbol table. It will be easiest to give the constants type VAR.
Make the program report an error if the user refers to an uninitialized variable in any way except to store a value in it.

3 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 file conventionally has a name ending in '.y'. See Invoking Bison.

Outline of a Bison Grammar
Symbols, Terminal and Nonterminal
Grammar Rules
Defining Language Semantics
Tracking Locations
Named References
Bison Declarations
Multiple Parsers in the Same Program

3.1 Outline of a Bison Grammar

A Bison grammar file has four main sections, shown here with the appropriate delimiters:

%{                Prologue                %}                Bison declarations                %%                Grammar rules                %%                Epilogue

Comments enclosed in '/* … */' may appear in any of the sections. As a GNU extension, '//' introduces a comment that continues until end of line.

The prologue
Prologue Alternatives
The Bison Declarations Section
The Grammar Rules Section
The epilogue

3.1.1 The prologue

The 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 implementation file so that they precede the definition of yyparse. You can use '#include' to get the declarations from a header file. If you don't need any C declarations, you may omit the '%{' and '%}' delimiters that bracket this section.

The Prologue section is terminated by the first occurrence of '%}' that is outside a comment, a string literal, or a character constant.

You may have more than one Prologue section, intermixed with the Bison declarations. This allows you to have C and Bison declarations that refer to each other. For example, the %union declaration may use types defined in a header file, and you may wish to prototype functions that take arguments of type YYSTYPE. This can be done with two Prologue blocks, one before and one after the %union declaration.

%{   #define _GNU_SOURCE   #include <stdio.h>   #include "ptypes.h" %}

%union {   long n;   tree t;  /*                                      tree                    is defined in                    ptypes.h.                  */ }

%{   static void print_token (yytoken_kind_t token, YYSTYPE val); %}

…

When in doubt, it is usually safer to put prologue code before all Bison declarations, rather than after. For example, any definitions of feature test macros like _GNU_SOURCE or _POSIX_C_SOURCE should appear before all Bison declarations, as feature test macros can affect the behavior of Bison-generated #include directives.

3.1.2 Prologue Alternatives

The functionality of Prologue sections can often be subtle and inflexible. As an alternative, Bison provides a %code directive with an explicit qualifier field, which identifies the purpose of the code and thus the location(s) where Bison should generate it. For C/C++, the qualifier can be omitted for the default location, or it can be one of requires, provides, top. See %code Summary.

Look again at the example of the previous section:

%{   #define _GNU_SOURCE   #include <stdio.h>   #include "ptypes.h" %}

%union {   long n;   tree t;  /*                                      tree                    is defined in                    ptypes.h.                  */ }

%{   static void print_token (yytoken_kind_t token, YYSTYPE val); %}

…

Notice that there are two Prologue sections here, but there's a subtle distinction between their functionality. For example, if you decide to override Bison's default definition for YYLTYPE, in which Prologue section should you write your new definition?⁵ You should write it in the first since Bison will insert that code into the parser implementation file before the default YYLTYPE definition. In which Prologue section should you prototype an internal function, trace_token, that accepts YYLTYPE and yytoken_kind_t as arguments? You should prototype it in the second since Bison will insert that code after the YYLTYPE and yytoken_kind_t definitions.

This distinction in functionality between the two Prologue sections is established by the appearance of the %union between them. This behavior raises a few questions. First, why should the position of a %union affect definitions related to YYLTYPE and yytoken_kind_t? Second, what if there is no %union? In that case, the second kind of Prologue section is not available. This behavior is not intuitive.

To avoid this subtle %union dependency, rewrite the example using a %code top and an unqualified %code. Let's go ahead and add the new YYLTYPE definition and the trace_token prototype at the same time:

%code top {   #define _GNU_SOURCE   #include <stdio.h>    /* WARNING: The following code really belongs    * in a '%code requires'; see below. */    #include "ptypes.h"   #define YYLTYPE YYLTYPE   typedef struct YYLTYPE   {     int first_line;     int first_column;     int last_line;     int last_column;     char *filename;   } YYLTYPE; }

%union {   long n;   tree t;  /*                                      tree                    is defined in                    ptypes.h.                  */ }

%code {   static void print_token (yytoken_kind_t token, YYSTYPE val);   static void trace_token (yytoken_kind_t token, YYLTYPE loc); }

…

In this way, %code top and the unqualified %code achieve the same functionality as the two kinds of Prologue sections, but it's always explicit which kind you intend. Moreover, both kinds are always available even in the absence of %union.

The %code top block above logically contains two parts. The first two lines before the warning need to appear near the top of the parser implementation file. The first line after the warning is required by YYSTYPE and thus also needs to appear in the parser implementation file. However, if you've instructed Bison to generate a parser header file (see Bison Declaration Summary), you probably want that line to appear before the YYSTYPE definition in that header file as well. The YYLTYPE definition should also appear in the parser header file to override the default YYLTYPE definition there.

In other words, in the %code top block above, all but the first two lines are dependency code required by the YYSTYPE and YYLTYPE definitions. Thus, they belong in one or more %code requires:

%code top {   #define _GNU_SOURCE   #include <stdio.h> }

%code requires {   #include "ptypes.h" }

%union {   long n;   tree t;  /*                                      tree                    is defined in                    ptypes.h.                  */ }

%code requires {   #define YYLTYPE YYLTYPE   typedef struct YYLTYPE   {     int first_line;     int first_column;     int last_line;     int last_column;     char *filename;   } YYLTYPE; }

%code {   static void print_token (yytoken_kind_t token, YYSTYPE val);   static void trace_token (yytoken_kind_t token, YYLTYPE loc); }

…

Now Bison will insert #include "ptypes.h" and the new YYLTYPE definition before the Bison-generated YYSTYPE and YYLTYPE definitions in both the parser implementation file and the parser header file. (By the same reasoning, %code requires would also be the appropriate place to write your own definition for YYSTYPE.)

When you are writing dependency code for YYSTYPE and YYLTYPE, you should prefer %code requires over %code top regardless of whether you instruct Bison to generate a parser header file. When you are writing code that you need Bison to insert only into the parser implementation file and that has no special need to appear at the top of that file, you should prefer the unqualified %code over %code top. These practices will make the purpose of each block of your code explicit to Bison and to other developers reading your grammar file. Following these practices, we expect the unqualified %code and %code requires to be the most important of the four Prologue alternatives.

At some point while developing your parser, you might decide to provide trace_token to modules that are external to your parser. Thus, you might wish for Bison to insert the prototype into both the parser header file and the parser implementation file. Since this function is not a dependency required by YYSTYPE or YYLTYPE, it doesn't make sense to move its prototype to a %code requires. More importantly, since it depends upon YYLTYPE and yytoken_kind_t, %code requires is not sufficient. Instead, move its prototype from the unqualified %code to a %code provides:

%code top {   #define _GNU_SOURCE   #include <stdio.h> }

%code requires {   #include "ptypes.h" }

%union {   long n;   tree t;  /*                                      tree                    is defined in                    ptypes.h.                  */ }

%code requires {   #define YYLTYPE YYLTYPE   typedef struct YYLTYPE   {     int first_line;     int first_column;     int last_line;     int last_column;     char *filename;   } YYLTYPE; }

%code provides {   void trace_token (yytoken_kind_t token, YYLTYPE loc); }

%code {   static void print_token (FILE *file, int token, YYSTYPE val); }

…

Bison will insert the trace_token prototype into both the parser header file and the parser implementation file after the definitions for yytoken_kind_t, YYLTYPE, and YYSTYPE.

The above examples are careful to write directives in an order that reflects the layout of the generated parser implementation and header files: %code top, %code requires, %code provides, and then %code. While your grammar files may generally be easier to read if you also follow this order, Bison does not require it. Instead, Bison lets you choose an organization that makes sense to you.

You may declare any of these directives multiple times in the grammar file. In that case, Bison concatenates the contained code in declaration order. This is the only way in which the position of one of these directives within the grammar file affects its functionality.

The result of the previous two properties is greater flexibility in how you may organize your grammar file. For example, you may organize semantic-type-related directives by semantic type:

%code requires { #include "type1.h" } %union { type1 field1; } %destructor { type1_free ($$); } <field1> %printer { type1_print (yyo, $$); } <field1>

%code requires { #include "type2.h" } %union { type2 field2; } %destructor { type2_free ($$); } <field2> %printer { type2_print (yyo, $$); } <field2>

You could even place each of the above directive groups in the rules section of the grammar file next to the set of rules that uses the associated semantic type. (In the rules section, you must terminate each of those directives with a semicolon.) And you don't have to worry that some directive (like a %union) in the definitions section is going to adversely affect their functionality in some counter-intuitive manner just because it comes first. Such an organization is not possible using Prologue sections.

This section has been concerned with explaining the advantages of the four Prologue alternatives over the original Yacc Prologue. However, in most cases when using these directives, you shouldn't need to think about all the low-level ordering issues discussed here. Instead, you should simply use these directives to label each block of your code according to its purpose and let Bison handle the ordering. %code is the most generic label. Move code to %code requires, %code provides, or %code top as needed.

3.1.3 The Bison Declarations Section

The Bison declarations section contains declarations that define terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. See Bison Declarations.

3.1.4 The Grammar Rules Section

The grammar rules section contains one or more Bison grammar rules, and nothing else. See Grammar Rules.

There must always be at least one grammar rule, and the first '%%' (which precedes the grammar rules) may never be omitted even if it is the first thing in the file.

3.1.5 The epilogue

The Epilogue is copied verbatim to the end of the parser implementation file, just as the Prologue is copied to the beginning. This is the most convenient place to put anything that you want to have in the parser implementation file but which need not come before the definition of yyparse. For example, the definitions of yylex and yyerror often go here. Because C requires functions to be declared before being used, you often need to declare functions like yylex and yyerror in the Prologue, even if you define them in the Epilogue. See Parser C-Language Interface.

If the last section is empty, you may omit the '%%' that separates it from the grammar rules.

The Bison parser itself contains many macros and identifiers whose names start with 'yy' or 'YY', so it is a good idea to avoid using any such names (except those documented in this manual) in the epilogue of the grammar file.

3.2 Symbols, Terminal and Nonterminal

Symbols in Bison grammars represent the grammatical classifications of the language.

A terminal symbol (also known as a token kind) represents a class of syntactically equivalent tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The symbol is represented in the Bison parser by a numeric code, and the yylex function returns a token kind code to indicate what kind of token has been read. You don't need to know what the code value is; you can use the symbol to stand for it.

A nonterminal symbol stands for a class of syntactically equivalent groupings. The symbol name is used in writing grammar rules. By convention, it should be all lower case.

Symbol names can contain letters, underscores, periods, and non-initial digits and dashes. Dashes in symbol names are a GNU extension, incompatible with POSIX Yacc. Periods and dashes make symbol names less convenient to use with named references, which require brackets around such names (see Named References). Terminal symbols that contain periods or dashes make little sense: since they are not valid symbols (in most programming languages) they are not exported as token names.

There are three ways of writing terminal symbols in the grammar:

A named token kind is written with an identifier, like an identifier in C. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as %token. See Token Kind Names.
A character token kind (or literal character token) is written in the grammar using the same syntax used in C for character constants; for example, '+' is a character token kind. A character token kind doesn't need to be declared unless you need to specify its semantic value data type (see Data Types of Semantic Values), associativity, or precedence (see Operator Precedence).
By convention, a character token kind is used only to represent a token that consists of that particular character. Thus, the token kind '+' is used to represent the character '+' as a token. Nothing enforces this convention, but if you depart from it, your program will confuse other readers.

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 numeric code, zero, signifies end-of-input (see Calling Convention for yylex). Also, unlike standard C, trigraphs have no special meaning in Bison character literals, nor is backslash-newline allowed.
A literal string token is written like a C string constant; for example, "<=" 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 (see Data Types of Semantic Values), associativity, or precedence (see Operator Precedence).
You can associate the literal string token with a symbolic name as an alias, using the %token declaration (see Token Kind Names). If you don't do that, the lexical analyzer has to retrieve the token code for the literal string token from the yytname table (see Calling Convention for yylex).

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 kind "<=" to represent the string '<=' 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, except that you must not use a null character within a string literal. Also, unlike Standard C, trigraphs have no special meaning in Bison string literals, nor is backslash-newline allowed. A literal string token must contain two or more characters; for a token containing just one character, use a character token (see above).

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 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 kind in the grammar rules, you write it the same way in the definition of yylex. The numeric code for a character token kind is simply the positive numeric code of the character, so yylex can use the identical value to generate the requisite code, though you may need to convert it to unsigned char to avoid sign-extension on hosts where char is signed. Each named token kind becomes a C macro in the parser implementation file, so yylex can use the name to stand for the code. (This is why periods don't make sense in terminal symbols.) See Calling Convention for yylex.

If yylex is defined in a separate file, you need to arrange for the token-kind definitions to be available there. Use the -d option when you run Bison, so that it will write these definitions into a separate header file name.tab.h which you can include in the other source files that need it. See Invoking Bison.

If you want to write a grammar that is portable to any Standard C host, you must use only nonnull 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:

"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"

The yylex function and Bison must use a consistent character set and encoding for character tokens. For example, if you run Bison in an ASCII environment, but then compile and run the resulting program in an environment that uses an incompatible character set like EBCDIC, the resulting program may not work because the tables generated by Bison will assume 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 ASCII environment, so installers on platforms that are incompatible with ASCII must rebuild those files before compiling them.

The symbol error is a terminal symbol reserved for error recovery (see Error Recovery); you shouldn't use it for any other purpose. In particular, 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 %token declaration.

3.3 Grammar Rules

A Bison grammar is a list of rules.

Syntax of Grammar Rules
Empty Rules
Recursive Rules

3.3.1 Syntax of Grammar Rules

A Bison grammar rule has the following general form:

where result is the nonterminal symbol that this rule describes, and components are various terminal and nonterminal symbols that are put together by this rule (see Symbols, Terminal and Nonterminal).

For example,

says that two groupings of type exp, with a '+' token in between, can be combined into a larger grouping of type exp.

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 actions that determine the semantics of the rule. An action looks like this:

This is an example of braced code, that is, C code surrounded by braces, much like a compound statement in C. Braced code can contain any sequence of C tokens, so long as its braces are balanced. Bison does not check the braced code for correctness directly; it merely copies the code to the parser implementation file, where the C compiler can check it.

Within braced code, the balanced-brace count is not affected by braces within comments, string literals, or character constants, but it is affected by the C digraphs '<%' and '%>' that represent braces. At the top level braced code must be terminated by '}' and not by a digraph. Bison does not look for trigraphs, so if braced code uses trigraphs you should ensure that they do not affect the nesting of braces or the boundaries of comments, string literals, or character constants.

Usually there is only one action and it follows the components. See Actions.

Multiple rules for the same result can be written separately or can be joined with the vertical-bar character '|' as follows:

                  result:                  rule1-components… |                  rule2-components… … ;

They are still considered distinct rules even when joined in this way.

3.3.2 Empty Rules

A rule is said to be empty if its right-hand side (components) is empty. It means that result in the previous example can match the empty string. As another example, here is how to define an optional semicolon:

It is easy not to see an empty rule, especially when | is used. The %empty directive allows to make explicit that a rule is empty on purpose:

semicolon.opt:   %empty | ";" ;

Flagging a non-empty rule with %empty is an error. If run with -Wempty-rule, bison will report empty rules without %empty. Using %empty enables this warning, unless -Wno-empty-rule was specified.

The %empty directive is a Bison extension, it does not work with Yacc. To remain compatible with POSIX Yacc, it is customary to write a comment '/* empty */' in each rule with no components:

semicolon.opt:   /* empty */ | ";" ;

3.3.3 Recursive Rules

A rule is called recursive when its 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 a particular thing. Consider this recursive definition of a comma-separated sequence of one or more expressions:

expseq1:   exp | expseq1 ',' exp ;

Since the recursive use of expseq1 is the leftmost symbol in the right hand side, we call this left recursion. By contrast, here the same construct is defined using right recursion:

expseq1:   exp | exp ',' expseq1 ;

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. See The Bison Parser Algorithm, for further explanation of this.

Indirect or 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.

For example:

expr:   primary | primary '+' primary ;

primary:   constant | '(' expr ')' ;

defines two mutually-recursive nonterminals, since each refers to the other.

3.4 Defining Language Semantics

The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and groupings, and by the actions taken when various groupings are recognized.

For example, the calculator calculates properly because the value associated with each expression is the proper number; it adds properly because the action for the grouping ' x +y ' is to add the numbers associated with x and y.

Data Types of Semantic Values
More Than One Value Type
Generating the Semantic Value Type
The Union Declaration
Providing a Structured Semantic Value Type
Actions
Data Types of Values in Actions
Actions in Midrule

3.4.1 Data Types of Semantic Values

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 (see Reverse Polish Notation Calculator).

Bison normally uses the type int for semantic values if your program uses the same data type for all language constructs. To specify some other type, define the %define variable api.value.type like this:

%define api.value.type {double}

%define api.value.type {struct semantic_value_type}

The value of api.value.type should be a type name that does not contain parentheses or square brackets.

Alternatively in C, instead of relying of Bison's %define support, you may rely on the C preprocessor and define YYSTYPE as a macro:

This macro definition must go in the prologue of the grammar file (see Outline of a Bison Grammar). If compatibility with POSIX Yacc matters to you, use this. Note however that Bison cannot know YYSTYPE's value, not even whether it is defined, so there are services it cannot provide. Besides this works only for C.

3.4.2 More Than One Value Type

In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type int or long, while a string constant needs type char *, and an identifier might need a pointer to an entry in the symbol table.

To use more than one data type for semantic values in one parser, Bison requires you to do two things:

Specify the entire collection of possible data types. There are several options:
- let Bison compute the union type from the tags you assign to symbols;
- use the %union Bison declaration (see The Union Declaration);
- define the %define variable api.value.type to be a union type whose members are the type tags (see Providing a Structured Semantic Value Type);
- use a typedef or a #define to define YYSTYPE to be a union type whose member names are the type tags.
Choose one of those types for each symbol (terminal or nonterminal) for which semantic values are used. This is done for tokens with the %token Bison declaration (see Token Kind Names) and for groupings with the %nterm/%type Bison declarations (see Nonterminal Symbols).

3.4.3 Generating the Semantic Value Type

The special value union of the %define variable api.value.type instructs Bison that the type tags (used with the %token, %nterm and %type directives) are genuine types, not names of members of YYSTYPE.

For example:

%define api.value.type union %token <int> INT "integer" %token <int> 'n' %nterm <int> expr %token <char const *> ID "identifier"

generates an appropriate value of YYSTYPE to support each symbol type. The name of the member of YYSTYPE for tokens than have a declared identifier id (such as INT and ID above, but not 'n') is id . The other symbols have unspecified names on which you should not depend; instead, relying on C casts to access the semantic value with the appropriate type:

/* For an "integer". */ yylval.INT = 42; return INT;  /* For an 'n', also declared as int. */ *((int*)&yylval) = 42; return 'n';  /* For an "identifier". */ yylval.ID = "42"; return ID;

If the %define variable api.token.prefix is defined (see %define Summary), then it is also used to prefix the union member names. For instance, with '%define api.token.prefix {TOK_}':

/* For an "integer". */ yylval.TOK_INT = 42; return TOK_INT;

This Bison extension cannot work if %yacc (or -y/--yacc) is enabled, as POSIX mandates that Yacc generate tokens as macros (e.g., '#define INT 258', or '#define TOK_INT 258').

A similar feature is provided for C++ that in addition overcomes C++ limitations (that forbid non-trivial objects to be part of a union): '%define api.value.type variant', see C++ Variants.

3.4.4 The Union Declaration

The %union declaration specifies the entire collection of possible data types for semantic values. The keyword %union is followed by braced code containing the same thing that goes inside a union in C.

For example:

%union {   double val;   symrec *tptr; }

This says that the two alternative types are double and symrec *. They are given names val and tptr; these names are used in the %token, %nterm and %type declarations to pick one of the types for a terminal or nonterminal symbol (see Nonterminal Symbols).

As an extension to POSIX, a tag is allowed after the %union. For example:

%union value {   double val;   symrec *tptr; }

specifies the union tag value, so the corresponding C type is union value. If you do not specify a tag, it defaults to YYSTYPE (see %define Summary).

As another extension to POSIX, you may specify multiple %union declarations; their contents are concatenated. However, only the first %union declaration can specify a tag.

Note that, unlike making a union declaration in C, you need not write a semicolon after the closing brace.

3.4.5 Providing a Structured Semantic Value Type

Instead of %union, you can define and use your own union type YYSTYPE if your grammar contains at least one '<type>' tag. For example, you can put the following into a header file parser.h:

union YYSTYPE {   double val;   symrec *tptr; };

and then your grammar can use the following instead of %union:

%{ #include "parser.h" %} %define api.value.type {union YYSTYPE} %nterm <val> expr %token <tptr> ID

Actually, you may also provide a struct rather that a union, which may be handy if you want to track information for every symbol (such as preceding comments).

The type you provide may even be structured and include pointers, in which case the type tags you provide may be composite, with '.' and '->' operators.

3.4.6 Actions

An action accompanies a syntactic rule and contains C code to be executed each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings.

An action consists of braced code containing C statements, and 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 (see Actions in Midrule).

The C code in an action can refer to the semantic values of the components matched by the rule with the construct $n , which stands for the value of the nth component. The semantic value for the grouping being constructed is $$. In addition, the semantic values of symbols can be accessed with the named references construct $name or $[name]. Bison translates both of these constructs into expressions of the appropriate type when it copies the actions into the parser implementation file. $$ (or $name , when it stands for the current grouping) is translated to a modifiable lvalue, so it can be assigned to.

Here is a typical example:

exp: … | exp '+' exp     { $$ = $1 + $3; }

Or, in terms of named references:

exp[result]: … | exp[left] '+' exp[right]  { $result = $left + $right; }

This rule constructs an exp from two smaller exp groupings connected by a plus-sign token. In the action, $1 and $3 ($left and $right) refer to the semantic values of the two component exp groupings, which are the first and third symbols on the right hand side of the rule. The sum is stored into $$ ($result) 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 '+' token, it could be referred to as $2.

See Named References, for more information about using the named references construct.

Note that the vertical-bar character '|' is really a rule separator, and actions are attached to a single rule. This is a difference with tools like Flex, for which '|' stands for either "or", or "the same action as that of the next rule". In the following example, the action is triggered only when 'b' is found:

a-or-b: 'a'|'b'   { a_or_b_found = 1; };

If you don't specify an action for a rule, Bison supplies a default: $$ = $1. Thus, the value of the first symbol in the rule becomes the value of the whole rule. Of course, the default action is valid only if the two data types match. There is no meaningful default action for an empty rule; every empty rule must have an explicit action unless the rule's value does not matter.

$n with n zero or negative is allowed for reference to tokens and groupings on the stack before those that match the current rule. This is a very risky practice, and to use it reliably you must be certain of the context in which the rule is applied. Here is a case in which you can use this reliably:

foo:   expr bar '+' expr  { … } | expr bar '-' expr  { … } ;

bar:   %empty    { previous_expr = $0; } ;

As long as bar is used only in the fashion shown here, $0 always refers to the expr which precedes bar in the definition of foo.

It is also possible to access the semantic value of the lookahead token, if any, from a semantic action. This semantic value is stored in yylval. See Special Features for Use in Actions.

3.4.7 Data Types of Values in Actions

If you have chosen a single data type for semantic values, the $$ and $n constructs always have that data type.

If you have used %union to specify a variety of data types, then you must declare a choice among these types for each terminal or nonterminal symbol that can have a semantic value. Then each time you use $$ or $n , its data type is determined by which symbol it refers to in the rule. In this example,

exp:   … | exp '+' exp    { $$ = $1 + $3; }

$1 and $3 refer to instances of exp, so they all have the data type declared for the nonterminal symbol exp. If $2 were used, it would have the data type declared for the terminal symbol '+', whatever that might be.

Alternatively, you can specify the data type when you refer to the value, by inserting '<type>' after the '$' at the beginning of the reference. For example, if you have defined types as shown here:

%union {   int itype;   double dtype; }

then you can write $<itype>1 to refer to the first subunit of the rule as an integer, or $<dtype>1 to refer to it as a double.

3.4.8 Actions in Midrule

Occasionally it is useful to put an action in the middle of a rule. These actions are written just like usual end-of-rule actions, but they are executed before the parser even recognizes the following components.

Using Midrule Actions
Typed Midrule Actions
Midrule Action Translation
Conflicts due to Midrule Actions

3.4.8.1 Using Midrule Actions

A midrule action may refer to the components preceding it using $n , but it may not refer to subsequent components because it is run before they are parsed.

The midrule action itself counts as one of the components of the rule. This makes a difference when there is another action later in the same rule (and usually there is another at the end): you have to count the actions along with the symbols when working out which number n to use in $n .

The midrule action can also have a semantic value. The action can set its value with an assignment to $$, and actions later in the rule can refer to the value using $n . Since there is no symbol to name the action, there is no way to declare a data type for the value in advance, so you must use the '$<…>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 midrule action, because assignments to $$ do not have that effect. The only way to set the value for the entire rule is with an ordinary action at the end of the rule.

Here is an example from a hypothetical compiler, handling a let statement that looks like 'let (variable) statement ' and serves to create a variable named variable temporarily for the duration of statement. To parse this construct, we must put variable into the symbol table while statement is parsed, then remove it afterward. Here is how it is done:

stmt:   "let" '(' var ')'     {       $<context>$ = push_context ();       declare_variable ($3);     }   stmt     {       $$ = $6;       pop_context ($<context>5);     }

As soon as 'let (variable)' has been recognized, the first action is run. It saves a copy of the current semantic context (the list of accessible variables) as its semantic value, using alternative context in the data-type union. Then it calls declare_variable to add the new variable to that list. Once the first action is finished, the embedded statement stmt can be parsed.

Note that the midrule action is component number 5, so the 'stmt' is component number 6. Named references can be used to improve the readability and maintainability (see Named References):

stmt:   "let" '(' var ')'     {       $<context>let = push_context ();       declare_variable ($3);     }[let]   stmt     {       $$ = $6;       pop_context ($<context>let);     }

After the embedded statement is parsed, its semantic value becomes the value of the entire let-statement. Then the semantic value from the earlier action is used to restore the prior list of variables. This removes the temporary let-variable from the list so that it won't appear to exist while the rest of the program is parsed.

Because the types of the semantic values of midrule actions are unknown to Bison, type-based features (e.g., '%printer', '%destructor') do not work, which could result in memory leaks. They also forbid the use of the variant implementation of the api.value.type in C++ (see C++ Variants).

See Typed Midrule Actions, for one way to address this issue, and Midrule Action Translation, for another: turning mid-action actions into regular actions.

3.4.8.2 Typed Midrule Actions

In the above example, if the parser initiates error recovery (see Error Recovery) while parsing the tokens in the embedded statement stmt, it might discard the previous semantic context $<context>5 without restoring it. Thus, $<context>5 needs a destructor (see Freeing Discarded Symbols), and Bison needs the type of the semantic value (context) to select the right destructor.

As an extension to Yacc's midrule actions, Bison offers a means to type their semantic value: specify its type tag ('<...>' before the midrule action.

Consider the previous example, with an untyped midrule action:

stmt:   "let" '(' var ')'     {       $<context>$ = push_context (); // ***       declare_variable ($3);     }   stmt     {       $$ = $6;       pop_context ($<context>5);     // ***     }

If instead you write:

stmt:   "let" '(' var ')'     <context>{                       // ***       $$ = push_context ();          // ***       declare_variable ($3);     }   stmt     {       $$ = $6;       pop_context ($5);              // ***     }

then %printer and %destructor work properly (no more leaks!), C++ variants can be used, and redundancy is reduced (<context> is specified once).

3.4.8.3 Midrule Action Translation

Midrule actions are actually transformed into regular rules and actions. The various reports generated by Bison (textual, graphical, etc., see Understanding Your Parser) reveal this translation, best explained by means of an example. The following rule:

exp: { a(); } "b" { c(); } { d(); } "e" { f(); };

is translated into:

$@1: %empty { a(); }; $@2: %empty { c(); }; $@3: %empty { d(); }; exp: $@1 "b" $@2 $@3 "e" { f(); };

with new nonterminal symbols $@n , where n is a number.

A midrule action is expected to generate a value if it uses $$, or the (final) action uses $n where n denote the midrule action. In that case its nonterminal is rather named @n :

exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };

is translated into

@1: %empty { a(); }; @2: %empty { $$ = c(); }; $@3: %empty { d(); }; exp: @1 "b" @2 $@3 "e" { f = $1; }

There are probably two errors in the above example: the first midrule action does not generate a value (it does not use $$ although the final action uses it), and the value of the second one is not used (the final action does not use $3). Bison reports these errors when the midrule-value warnings are enabled (see Invoking Bison):

$                    bison -Wmidrule-value mid.y

mid.y:2.6-13:                    warning: unset value: $$     2 | exp:                    { a(); }                    "b" { $$ = c(); } { d(); } "e" { f = $1; };       |                    ^~~~~~~~

mid.y:2.19-31:                    warning: unused value: $3     2 | exp: { a(); } "b"                    { $$ = c(); }                    { d(); } "e" { f = $1; };       |                    ^~~~~~~~~~~~~

It is sometimes useful to turn midrule actions into regular actions, e.g., to factor them, or to escape from their limitations. For instance, as an alternative to typed midrule action, you may bury the midrule action inside a nonterminal symbol and to declare a printer and a destructor for that symbol:

%nterm <context> let %destructor { pop_context ($$); } let %printer { print_context (yyo, $$); } let

%%

stmt:   let stmt     {       $$ = $2;       pop_context ($let);     };

let:   "let" '(' var ')'     {       $let = push_context ();       declare_variable ($var);     };

3.4.8.4 Conflicts due to Midrule Actions

Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without midrule actions, can coexist in a working parser because the parser can shift the open-brace token and look at what follows before deciding whether there is a declaration or not:

compound:   '{' declarations statements '}' | '{' statements '}' ;

But when we add a midrule action as follows, the rules become nonfunctional:

compound:   { prepare_for_local_variables (); }      '{' declarations statements '}'

|    '{' statements '}' ;

Now the parser is forced to decide whether to run the midrule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace token is what is called the lookahead token at this time, since the parser is still deciding what to do about it. See Lookahead Tokens.)

You might think that you could correct the problem by putting identical actions into the two rules, like this:

compound:   { prepare_for_local_variables (); }     '{' declarations statements '}' | { prepare_for_local_variables (); }     '{' statements '}' ;

But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.)

If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this:

compound:   '{' { prepare_for_local_variables (); }     declarations statements '}' | '{' statements '}' ;

Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so.

Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine:

subroutine:   %empty  { prepare_for_local_variables (); } ;

compound:   subroutine '{' declarations statements '}' | subroutine '{' statements '}' ;

Now Bison can execute the action in the rule for subroutine without deciding which rule for compound it will eventually use.

3.5 Tracking Locations

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.

The way locations are handled is defined by providing a data type, and actions to take when rules are matched.

Data Type of Locations
Actions and Locations
Printing Locations
Default Action for Locations

3.5.1 Data Type of Locations

Defining a data type for locations is much simpler than for semantic values, since all tokens and groupings always use the same type. The location type is specified using '%define api.location.type':

%define api.location.type {location_t}

This defines, in the C generated code, the YYLTYPE type name. When YYLTYPE is not defined, Bison uses a default structure type with four members:

typedef struct YYLTYPE {   int first_line;   int first_column;   int last_line;   int last_column; } YYLTYPE;

In C, you may also specify the type of locations by defining a macro called YYLTYPE, just as you can specify the semantic value type by defining a YYSTYPE macro (see Data Types of Semantic Values). However, rather than using macros, we recommend the api.value.type and api.location.type %define variables.

Default locations represent a range in the source file(s), but this is not a requirement. It could be a single point or just a line number, or even more complex structures.

When the default location type is used, Bison initializes all these fields to 1 for yylloc at the beginning of the parsing. To initialize yylloc with a custom location type (or to chose a different initialization), use the %initial-action directive. See Performing Actions before Parsing.

3.5.2 Actions and Locations

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 nth component of the right hand side is @n , while the location of the left hand side grouping is @$.

In addition, the named references construct @name and @[name] may also be used to address the symbol locations. See Named References, for more information about using the named references construct.

Here is a basic example using the default data type for locations:

exp:   … | 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;           fprintf (stderr, "%d.%d-%d.%d: division by zero",                    @3.first_line, @3.first_column,                    @3.last_line, @3.last_column);         }     }

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 @$ to the beginning of the first symbol, and the end of @$ 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:

exp:   … | 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);         }     }

It is also possible to access the location of the lookahead token, if any, from a semantic action. This location is stored in yylloc. See Special Features for Use in Actions.

3.5.3 Printing Locations

When using the default location type, the debug traces report the symbols' location. The generated parser does so using the YYLOCATION_PRINT macro.

Macro: YYLOCATION_PRINT (file, loc); ¶: When traces are enabled, print loc (of type 'YYLTYPE const *') on file (of type 'FILE *'). Do nothing when traces are disabled, or if the location type is user defined.

To get locations in the debug traces with your user-defined location types, define the YYLOCATION_PRINT macro. For instance:

#define YYLOCATION_PRINT   location_print

3.5.4 Default Action for Locations

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 YYLLOC_DEFAULT macro is invoked each time a rule is matched, before the associated action is run. It is also invoked while processing a syntax error, to compute the error's location. Before reporting an unresolvable syntactic ambiguity, a GLR parser invokes YYLLOC_DEFAULT recursively to compute the location of that ambiguity.

Most of the time, this macro is general enough to suppress location dedicated code from semantic actions.

The YYLLOC_DEFAULT macro takes three parameters. The first one is the location of the grouping (the result of the computation). When a rule is matched, the second parameter identifies locations of all right hand side elements of the rule being matched, and the third parameter is the size of the rule's right hand side. When a GLR parser reports an ambiguity, which of multiple candidate right hand sides it passes to YYLLOC_DEFAULT is undefined. When processing a syntax error, the second parameter identifies locations of the symbols that were discarded during error processing, and the third parameter is the number of discarded symbols.

By default, YYLLOC_DEFAULT is defined this way:

# define YYLLOC_DEFAULT(Cur, Rhs, N)                      \ do                                                        \   if (N)                                                  \     {                                                     \       (Cur).first_line   = YYRHSLOC(Rhs, 1).first_line;   \       (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \       (Cur).last_line    = YYRHSLOC(Rhs, N).last_line;    \       (Cur).last_column  = YYRHSLOC(Rhs, N).last_column;  \     }                                                     \   else                                                    \     {                                                     \       (Cur).first_line   = (Cur).last_line   =            \         YYRHSLOC(Rhs, 0).last_line;                       \       (Cur).first_column = (Cur).last_column =            \         YYRHSLOC(Rhs, 0).last_column;                     \     }                                                     \ while (0)

where YYRHSLOC (rhs, k) is the location of the kth symbol in rhs when k is positive, and the location of the symbol just before the reduction when k and n are both zero.

When defining YYLLOC_DEFAULT, you should consider that:

All arguments are free of side-effects. However, only the first one (the result) should be modified by YYLLOC_DEFAULT.
For consistency with semantic actions, valid indexes within the right hand side range from 1 to n. When n is zero, only 0 is a valid index, and it refers to the symbol just before the reduction. During error processing n is always positive.
Your macro should parenthesize its arguments, if need be, since the actual arguments may not be surrounded by parentheses. Also, your macro should expand to something that can be used as a single statement when it is followed by a semicolon.

3.6 Named References

As described in the preceding sections, the traditional way to refer to any semantic value or location is a positional reference, which takes the form $n , $$, @n , and @$. However, such a reference is not very descriptive. Moreover, if you later decide to insert or remove symbols in the right-hand side of a grammar rule, the need to renumber such references can be tedious and error-prone.

To avoid these issues, you can also refer to a semantic value or location using a named reference. First of all, original symbol names may be used as named references. For example:

invocation: op '(' args ')'   { $invocation = new_invocation ($op, $args, @invocation); }

Positional and named references can be mixed arbitrarily. For example:

invocation: op '(' args ')'   { $$ = new_invocation ($op, $args, @$); }

However, sometimes regular symbol names are not sufficient due to ambiguities:

exp: exp '/' exp   { $exp = $exp / $exp; } // $exp is ambiguous.  exp: exp '/' exp   { $$ = $1 / $exp; } // One usage is ambiguous.  exp: exp '/' exp   { $$ = $1 / $3; } // No error.

When ambiguity occurs, explicitly declared names may be used for values and locations. Explicit names are declared as a bracketed name after a symbol appearance in rule definitions. For example:

exp[result]: exp[left] '/' exp[right]   { $result = $left / $right; }

In order to access a semantic value generated by a midrule action, an explicit name may also be declared by putting a bracketed name after the closing brace of the midrule action code:

exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]   { $res = $left + $right; }

In references, in order to specify names containing dots and dashes, an explicit bracketed syntax $[name] and @[name] must be used:

if-stmt: "if" '(' expr ')' "then" then.stmt ';'   { $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }

It often happens that named references are followed by a dot, dash or other C punctuation marks and operators. By default, Bison will read '$name.suffix' as a reference to symbol value $name followed by '.suffix', i.e., an access to the suffix field of the semantic value. In order to force Bison to recognize 'name.suffix' in its entirety as the name of a semantic value, the bracketed syntax '$[name.suffix]' must be used.

3.7 Bison Declarations

The Bison declarations section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. See Symbols, Terminal and Nonterminal.

All token kind names (but not single-character literal tokens such as '+' and '*') must be declared. Nonterminal symbols must be declared if you need to specify which data type to use for the semantic value (see More Than One Value Type).

The first rule in the grammar file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (see Languages and Context-Free Grammars).

Require a Version of Bison
Token Kind Names
Operator Precedence
Nonterminal Symbols
Syntax of Symbol Declarations
Performing Actions before Parsing
Freeing Discarded Symbols
Printing Semantic Values
Suppressing Conflict Warnings
The Start-Symbol
A Pure (Reentrant) Parser
A Push Parser
Bison Declaration Summary
%define Summary
%code Summary

3.7.1 Require a Version of Bison

You may require the minimum version of Bison to process the grammar. If the requirement is not met, bison exits with an error (exit status 63).

Some deprecated behaviors are disabled for some required version:

"3.2" (or better): The C++ deprecated files position.hh and stack.hh are no longer generated.

3.7.2 Token Kind Names

The basic way to declare a token kind name (terminal symbol) is as follows:

Bison will convert this into a definition in the parser, so that the function yylex (if it is in this file) can use the name name to stand for this token kind's code.

Alternatively, you can use %left, %right, %precedence, or %nonassoc instead of %token, if you wish to specify associativity and precedence. See Operator Precedence. However, for clarity, we recommend to use these directives only to declare associativity and precedence, and not to add string aliases, semantic types, etc.

You can explicitly specify the numeric code for a token kind by appending a nonnegative decimal or hexadecimal integer value in the field immediately following the token name:

%token NUM 300 %token XNUM 0x12d // a GNU extension

It is generally best, however, to let Bison choose the numeric codes for all token kinds. Bison will automatically select codes that don't conflict with each other or with normal characters.

In the event that the stack type is a union, you must augment the %token or other token declaration to include the data type alternative delimited by angle-brackets (see More Than One Value Type).

For example:

%union {              /* define stack type */   double val;   symrec *tptr; } %token <val> NUM      /* define token NUM and its type */

You can associate a literal string token with a token kind name by writing the literal string at the end of a %token declaration which declares the name. For example:

For example, a grammar for the C language might specify these names with equivalent literal string tokens:

%token  <operator>  OR      "||" %token  <operator>  LE 134  "<=" %left  OR  "<="

Once you equate the literal string and the token kind name, you can use them interchangeably in further declarations or the grammar rules. The yylex function can use the token name or the literal string to obtain the token kind code (see Calling Convention for yylex).

String aliases allow for better error messages using the literal strings instead of the token names, such as 'syntax error, unexpected ||, expecting number or (' rather than 'syntax error, unexpected OR, expecting NUM or LPAREN'.

String aliases may also be marked for internationalization (see Token Internationalization):

%token     OR     "||"     LPAREN "("     RPAREN ")"     '\n'   _("end of line")   <double>     NUM    _("number")

would produce in French 'erreur de syntaxe, || inattendu, attendait nombre ou (' rather than 'erreur de syntaxe, || inattendu, attendait number ou ('.

3.7.3 Operator Precedence

Use the %left, %right, %nonassoc, or %precedence declaration to declare a token and specify its precedence and associativity, all at once. These are called precedence declarations. See Operator Precedence, for general information on operator precedence.

The syntax of a precedence declaration is nearly the same as that of %token: either

And indeed any of these declarations serves the purposes of %token. But in addition, they specify the associativity and relative precedence for all the symbols:

The associativity of an operator op determines how repeated uses of the operator nest: whether ' x op y op z ' is parsed by grouping x with y first or by grouping y with z first. %left specifies left-associativity (grouping x with y first) and %right specifies right-associativity (grouping y with z first). %nonassoc specifies no associativity, which means that ' x op y op z ' is considered a syntax error.
%precedence gives only precedence to the symbols, and defines no associativity at all. Use this to define precedence only, and leave any potential conflict due to associativity enabled.
The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity. When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first.

For backward compatibility, there is a confusing difference between the argument lists of %token and precedence declarations. Only a %token can associate a literal string with a token kind name. A precedence declaration always interprets a literal string as a reference to a separate token. For example:

%left  OR "<="         // Does not declare an alias. %left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".

3.7.4 Nonterminal Symbols

When you use %union to specify multiple value types, you must declare the value type of each nonterminal symbol for which values are used. This is done with a %type declaration, like this:

%type <type>                  nonterminal…

Here nonterminal is the name of a nonterminal symbol, and type is the name given in the %union to the alternative that you want (see The Union Declaration). You can give any number of nonterminal symbols in the same %type declaration, if they have the same value type. Use spaces to separate the symbol names.

While POSIX Yacc allows %type only for nonterminals, Bison accepts that this directive be also applied to terminal symbols. To declare exclusively nonterminal symbols, use the safer %nterm:

%nterm <type>                  nonterminal…

3.7.5 Syntax of Symbol Declarations

The syntax of the various directives to declare symbols is as follows.

%token                  tag? (                  id                  number?                  string? )+ (                  tag                  (                  id                  number?                  string? )+ )* %left                  tag? (                  id                  number?)+ (                  tag                  (                  id                  number? )+ )* %type                  tag? (                  id                  |                  char                  |                  string                  )+ (                  tag                  (                  id                  |                  char                  |                  string                  )+ )* %nterm                  tag?                  id+ (                  tag                  id+ )*

where tag denotes a type tag such as '<ival>', id denotes an identifier such as 'NUM', number a decimal or hexadecimal integer such as '300' or '0x12d', char a character literal such as ''+'', and string a string literal such as '"number"'. The postfix quantifiers are '?' (zero or one), '*' (zero or more) and '+' (one or more).

The directives %precedence, %right and %nonassoc behave like %left.

3.7.6 Performing Actions before Parsing

Sometimes your parser needs to perform some initializations before parsing. The %initial-action directive allows for such arbitrary code.

Directive: %initial-action { code } ¶: Declare that the braced code must be invoked before parsing each time yyparse is called. The code may use $$ (or $<tag>$ ) and @$ — initial value and location of the lookahead — and the %parse-param.

For instance, if your locations use a file name, you may use

%parse-param { char const *file_name }; %initial-action {   @$.initialize (file_name); };

3.7.7 Freeing Discarded Symbols

During error recovery (see Error Recovery), symbols already pushed on the stack and tokens coming from the rest of the file are discarded until the parser falls on its feet. If the parser runs out of memory, or if it returns via YYABORT, YYACCEPT or YYNOMEM, all the symbols on the stack must be discarded. Even if the parser succeeds, it must discard the start symbol.

When discarded symbols convey heap based information, this memory is lost. While this behavior can be tolerable for batch parsers, such as in traditional compilers, it is unacceptable for programs like shells or protocol implementations that may parse and execute indefinitely.

The %destructor directive defines code that is called when a symbol is automatically discarded.

Directive: %destructor { code } symbols ¶

Invoke the braced code whenever the parser discards one of the symbols. Within code, $$ (or $<tag>$ ) designates the semantic value associated with the discarded symbol, and @$ designates its location. The additional parser parameters are also available (see The Parser Function yyparse).

When a symbol is listed among symbols, its %destructor is called a per-symbol %destructor. You may also define a per-type %destructor by listing a semantic type tag among symbols. In that case, the parser will invoke this code whenever it discards any grammar symbol that has that semantic type tag unless that symbol has its own per-symbol %destructor.

Finally, you can define two different kinds of default %destructors. You can place each of <*> and <> in the symbols list of exactly one %destructor declaration in your grammar file. The parser will invoke the code associated with one of these whenever it discards any user-defined grammar symbol that has no per-symbol and no per-type %destructor. The parser uses the code for <*> in the case of such a grammar symbol for which you have formally declared a semantic type tag (%token, %nterm, and %type count as such a declaration, but $<tag>$ does not). The parser uses the code for <> in the case of such a grammar symbol that has no declared semantic type tag.

For example:

%union { char *string; } %token <string> STRING1 STRING2 %nterm <string> string1 string2 %union { char character; } %token <character> CHR %nterm <character> chr %token TAGLESS  %destructor { } <character> %destructor { free ($$); } <*> %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1 %destructor { printf ("Discarding tagless symbol.\n"); } <>

guarantees that, when the parser discards any user-defined symbol that has a semantic type tag other than <character>, it passes its semantic value to free by default. However, when the parser discards a STRING1 or a string1, it uses the third %destructor, which frees it and prints its line number to stdout (free is invoked only once). Finally, the parser merely prints a message whenever it discards any symbol, such as TAGLESS, that has no semantic type tag.

A Bison-generated parser invokes the default %destructors only for user-defined as opposed to Bison-defined symbols. For example, the parser will not invoke either kind of default %destructor for the special Bison-defined symbols $accept, $undefined, or $end (see Bison Symbols), none of which you can reference in your grammar. It also will not invoke either for the error token (see Bison Symbols), which is always defined by Bison regardless of whether you reference it in your grammar. However, it may invoke one of them for the end token (token 0) if you redefine it from $end to, for example, END:

Finally, Bison will never invoke a %destructor for an unreferenced midrule semantic value (see Actions in Midrule). That is, Bison does not consider a midrule to have a semantic value if you do not reference $$ in the midrule's action or $n (where n is the right-hand side symbol position of the midrule) in any later action in that rule. However, if you do reference either, the Bison-generated parser will invoke the <> %destructor whenever it discards the midrule symbol.

Discarded symbols are the following:

stacked symbols popped during the first phase of error recovery,
incoming terminals during the second phase of error recovery,
the current lookahead and the entire stack (except the current right-hand side symbols) when the parser returns immediately, and
the current lookahead and the entire stack (including the current right-hand side symbols) when the C++ parser (lalr1.cc) catches an exception in parse,
the start symbol, when the parser succeeds.

The parser can return immediately because of an explicit call to YYABORT, YYACCEPT or YYNOMEM, or failed error recovery, or memory exhaustion.

Right-hand side symbols of a rule that explicitly triggers a syntax error via YYERROR are not discarded automatically. As a rule of thumb, destructors are invoked only when user actions cannot manage the memory.

3.7.8 Printing Semantic Values

When run-time traces are enabled (see Tracing Your Parser), the parser reports its actions, such as reductions. When a symbol involved in an action is reported, only its kind is displayed, as the parser cannot know how semantic values should be formatted.

The %printer directive defines code that is called when a symbol is reported. Its syntax is the same as %destructor (see Freeing Discarded Symbols).

Directive: %printer { code } symbols ¶

Invoke the braced code whenever the parser displays one of the symbols. Within code, yyo denotes the output stream (a FILE* in C, an std::ostream& in C++, and stdout in D), $$ (or $<tag>$ ) designates the semantic value associated with the symbol, and @$ its location. The additional parser parameters are also available (see The Parser Function yyparse).

The symbols are defined as for %destructor (see Freeing Discarded Symbols.): they can be per-type (e.g., '<ival>'), per-symbol (e.g., 'exp', 'NUM', '"float"'), typed per-default (i.e., '<*>', or untyped per-default (i.e., '<>').

For example:

%union { char *string; } %token <string> STRING1 STRING2 %nterm <string> string1 string2 %union { char character; } %token <character> CHR %nterm <character> chr %token TAGLESS  %printer { fprintf (yyo, "'%c'", $$); } <character> %printer { fprintf (yyo, "&%p", $$); } <*> %printer { fprintf (yyo, "\"%s\"", $$); } STRING1 string1 %printer { fprintf (yyo, "<>"); } <>

guarantees that, when the parser print any symbol that has a semantic type tag other than <character>, it display the address of the semantic value by default. However, when the parser displays a STRING1 or a string1, it formats it as a string in double quotes. It performs only the second %printer in this case, so it prints only once. Finally, the parser print '<>' for any symbol, such as TAGLESS, that has no semantic type tag. See Enabling Debug Traces for mfcalc, for a complete example.

3.7.9 Suppressing Conflict Warnings

Bison normally warns if there are any conflicts in the grammar (see 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 %expect declaration.

The declaration looks like this:

Here n is a decimal integer. The declaration says there should be n shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an error if the number of shift/reduce conflicts differs from n, or if there are any reduce/reduce conflicts.

For deterministic parsers, reduce/reduce conflicts are more serious, and should be eliminated entirely. Bison will always report reduce/reduce conflicts for these parsers. With GLR parsers, however, both kinds of conflicts are routine; otherwise, there would be no need to use GLR parsing. Therefore, it is also possible to specify an expected number of reduce/reduce conflicts in GLR parsers, using the declaration:

You may wish to be more specific in your specification of expected conflicts. To this end, you can also attach %expect and %expect-rr modifiers to individual rules. The interpretation of these modifiers differs from their use as declarations. When attached to rules, they indicate the number of states in which the rule is involved in a conflict. You will need to consult the output resulting from -v to determine appropriate numbers to use. For example, for the following grammar fragment, the first rule for empty_dims appears in two states in which the '[' token is a lookahead. Having determined that, you can document this fact with an %expect modifier as follows:

dims:   empty_dims | '[' expr ']' dims ;  empty_dims:   %empty   %expect 2 | empty_dims '[' ']' ;

Mid-rule actions generate implicit rules that are also subject to conflicts (see Conflicts due to Midrule Actions). To attach an %expect or %expect-rr annotation to an implicit mid-rule action's rule, put it before the action. For example,

%glr-parser %expect-rr 1  %%  clause:   "condition" %expect-rr 1 { value_mode(); } '(' exprs ')' | "condition" %expect-rr 1 { class_mode(); } '(' types ')' ;

Here, the appropriate mid-rule action will not be determined until after the '(' token is shifted. Thus, the two actions will clash with each other, and we should expect one reduce/reduce conflict for each.

In general, using %expect involves these steps:

Compile your grammar without %expect. Use the -v option to get a verbose list of where the conflicts occur. Bison will also print the number of conflicts.
Check each of the conflicts to make sure that Bison's default resolution is what you really want. If not, rewrite the grammar and go back to the beginning.
Add an %expect declaration, copying the number n from the number that Bison printed. With GLR parsers, add an %expect-rr declaration as well.
Optionally, count up the number of states in which one or more conflicted reductions for particular rules appear and add these numbers to the affected rules as %expect-rr or %expect modifiers as appropriate. Rules that are in conflict appear in the output listing surrounded by square brackets or, in the case of reduce/reduce conflicts, as reductions having the same lookahead symbol as a square-bracketed reduction in the same state.

Now Bison will report an error if you introduce an unexpected conflict, but will keep silent otherwise.

3.7.10 The Start-Symbol

Bison assumes by default that the start symbol for the grammar is the first nonterminal specified in the grammar specification section. The programmer may override this restriction with the %start declaration as follows:

3.7.11 A Pure (Reentrant) Parser

A reentrant program is one which does not alter in the course of execution; in other words, it consists entirely of 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 program must be called only within interlocks.

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 yylex, including yylval and yylloc.)

Alternatively, you can generate a pure, reentrant parser. The Bison declaration '%define api.pure' says that you want the parser to be reentrant. It looks like this:

The result is that the communication variables yylval and yylloc become local variables in yyparse, and a different calling convention is used for the lexical analyzer function yylex. See Calling Conventions for Pure Parsers, for the details of this. The variable yynerrs becomes local in yyparse in pull mode but it becomes a member of yypstate in push mode. (see The Error Reporting Function yyerror). The convention for calling yyparse itself is unchanged.

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.

3.7.12 A Push Parser

A pull parser is called once and it takes control until all its input is completely parsed. A push parser, on the other hand, is called each time a new token is made available.

A push parser is typically useful when the parser is part of a main event loop in the client's application. This is typically a requirement of a GUI, when the main event loop needs to be triggered within a certain time period.

Normally, Bison generates a pull parser. The following Bison declaration says that you want the parser to be a push parser (see %define Summary):

%define api.push-pull push

In almost all cases, you want to ensure that your push parser is also a pure parser (see A Pure (Reentrant) Parser). The only time you should create an impure push parser is to have backwards compatibility with the impure Yacc pull mode interface. Unless you know what you are doing, your declarations should look like this:

%define api.pure full %define api.push-pull push

There is a major notable functional difference between the pure push parser and the impure push parser. It is acceptable for a pure push parser to have many parser instances, of the same type of parser, in memory at the same time. An impure push parser should only use one parser at a time.

When a push parser is selected, Bison will generate some new symbols in the generated parser. yypstate is a structure that the generated parser uses to store the parser's state. yypstate_new is the function that will create a new parser instance. yypstate_delete will free the resources associated with the corresponding parser instance. Finally, yypush_parse is the function that should be called whenever a token is available to provide the parser. A trivial example of using a pure push parser would look like this:

int status; yypstate *ps = yypstate_new (); do {   status = yypush_parse (ps, yylex (), NULL); } while (status == YYPUSH_MORE); yypstate_delete (ps);

If the user decided to use an impure push parser, a few things about the generated parser will change. The yychar variable becomes a global variable instead of a local one in the yypush_parse function. For this reason, the signature of the yypush_parse function is changed to remove the token as a parameter. A nonreentrant push parser example would thus look like this:

extern int yychar; int status; yypstate *ps = yypstate_new (); do {   yychar = yylex ();   status = yypush_parse (ps); } while (status == YYPUSH_MORE); yypstate_delete (ps);

That's it. Notice the next token is put into the global variable yychar for use by the next invocation of the yypush_parse function.

Bison also supports both the push parser interface along with the pull parser interface in the same generated parser. In order to get this functionality, you should replace the '%define api.push-pull push' declaration with the '%define api.push-pull both' declaration. Doing this will create all of the symbols mentioned earlier along with the two extra symbols, yyparse and yypull_parse. yyparse can be used exactly as it normally would be used. However, the user should note that it is implemented in the generated parser by calling yypull_parse. This makes the yyparse function that is generated with the '%define api.push-pull both' declaration slower than the normal yyparse function. If the user calls the yypull_parse function it will parse the rest of the input stream. It is possible to yypush_parse tokens to select a subgrammar and then yypull_parse the rest of the input stream. If you would like to switch back and forth between between parsing styles, you would have to write your own yypull_parse function that knows when to quit looking for input. An example of using the yypull_parse function would look like this:

yypstate *ps = yypstate_new (); yypull_parse (ps); /* Will call the lexer */ yypstate_delete (ps);

Adding the '%define api.pure' declaration does exactly the same thing to the generated parser with '%define api.push-pull both' as it did for '%define api.push-pull push'.

3.7.13 Bison Declaration Summary

Here is a summary of the declarations used to define a grammar:

Directive: %union ¶: Declare the collection of data types that semantic values may have (see The Union Declaration).

Directive: %token ¶: Declare a terminal symbol (token kind name) with no precedence or associativity specified (see Token Kind Names).

Directive: %right ¶: Declare a terminal symbol (token kind name) that is right-associative (see Operator Precedence).

Directive: %left ¶: Declare a terminal symbol (token kind name) that is left-associative (see Operator Precedence).

Directive: %nonassoc ¶: Declare a terminal symbol (token kind name) that is nonassociative (see Operator Precedence). Using it in a way that would be associative is a syntax error.

Directive: %nterm ¶: Declare the type of semantic values for a nonterminal symbol (see Nonterminal Symbols).

Directive: %type ¶: Declare the type of semantic values for a symbol (see Nonterminal Symbols).

Directive: %start ¶: Specify the grammar's start symbol (see The Start-Symbol).

Directive: %expect ¶: Declare the expected number of shift/reduce conflicts, either overall or for a given rule (see Suppressing Conflict Warnings).

Directive: %expect-rr ¶: Declare the expected number of reduce/reduce conflicts, either overall or for a given rule (see Suppressing Conflict Warnings).

In order to change the behavior of bison, use the following directives:

Directive: %code {code} ¶
Directive: %code qualifier {code} ¶: Insert code verbatim into the output parser source at the default location or at the location specified by qualifier. See %code Summary.

Directive: %debug ¶: Instrument the parser for traces. Obsoleted by '%define parse.trace'. See Tracing Your Parser.

Directive: %define variable ¶
Directive: %define variable value ¶
Directive: %define variable {value} ¶
Directive: %define variable "value" ¶: Define a variable to adjust Bison's behavior. See %define Summary.

Directive: %defines ¶
Directive: %defines defines-file ¶: Historical name for %header. See %header.

Directive: %destructor ¶: Specify how the parser should reclaim the memory associated to discarded symbols. See Freeing Discarded Symbols.

Directive: %file-prefix "prefix" ¶: Specify a prefix to use for all Bison output file names. The names are chosen as if the grammar file were named prefix.y.

Write a parser header file containing definitions for the token kind names defined in the grammar as well as a few other declarations. If the parser implementation file is named name.c then the parser header file is named name.h.

For C parsers, the parser header file declares YYSTYPE unless YYSTYPE is already defined as a macro or you have used a <type> tag without using %union. Therefore, if you are using a %union (see More Than One Value Type) with components that require other definitions, or if you have defined a YYSTYPE macro or type definition (see Data Types of Semantic Values), you need to arrange for these definitions to be propagated to all modules, e.g., by putting them in a prerequisite header that is included both by your parser and by any other module that needs YYSTYPE.

Unless your parser is pure, the parser header file declares yylval as an external variable. See A Pure (Reentrant) Parser.

If you have also used locations, the parser header file declares YYLTYPE and yylloc using a protocol similar to that of the YYSTYPE macro and yylval. See Tracking Locations.

This parser header file is normally essential if you wish to put the definition of yylex in a separate source file, because yylex typically needs to be able to refer to the above-mentioned declarations and to the token kind codes. See Semantic Values of Tokens.

If you have declared %code requires or %code provides, the output header also contains their code. See %code Summary.

The generated header is protected against multiple inclusions with a C preprocessor guard: 'YY_PREFIX_FILE_INCLUDED', where PREFIX and FILE are the prefix (see Multiple Parsers in the Same Program) and generated file name turned uppercase, with each series of non alphanumerical characters converted to a single underscore.

For instance with '%define api.prefix {calc}' and '%header "lib/parse.h"', the header will be guarded as follows.

#ifndef YY_CALC_LIB_PARSE_H_INCLUDED # define YY_CALC_LIB_PARSE_H_INCLUDED ...

Write in English Sentences That Describe the Following Declarations C++

Bison

Table of Contents

Introduction

Conditions for Using Bison

GNU GENERAL PUBLIC LICENSE

Preamble

TERMS AND CONDITIONS

END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Programs

1 The Concepts of Bison

1.1 Languages and Context-Free Grammars

1.2 From Formal Rules to Bison Input

1.3 Semantic Values

1.4 Semantic Actions

1.5 Writing GLR Parsers

1.5.1 Using GLR on Unambiguous Grammars

1.5.2 Using GLR to Resolve Ambiguities

1.5.3 GLR Semantic Actions

1.5.3.1 Deferred semantic actions

1.5.3.2 YYERROR

1.5.3.3 Restrictions on semantic values and locations

1.5.4 Controlling a Parse with Arbitrary Predicates

1.6 Locations

1.7 Bison Output: the Parser Implementation File

1.8 Stages in Using Bison

1.9 The Overall Layout of a Bison Grammar

2 Examples

2.1 Reverse Polish Notation Calculator

2.1.1 Declarations for rpcalc

2.1.2 Grammar Rules for rpcalc

2.1.2.1 Explanation of input

2.1.2.2 Explanation of line

2.1.2.3 Explanation of exp

2.1.3 The rpcalc Lexical Analyzer

2.1.4 The Controlling Function

2.1.5 The Error Reporting Routine

2.1.6 Running Bison to Make the Parser

2.1.7 Compiling the Parser Implementation File

2.2 Infix Notation Calculator: calc

2.3 Simple Error Recovery

2.4 Location Tracking Calculator: ltcalc

2.4.1 Declarations for ltcalc

2.4.2 Grammar Rules for ltcalc

2.4.3 The ltcalc Lexical Analyzer.

2.5 Multi-Function Calculator: mfcalc

2.5.1 Declarations for mfcalc

2.5.2 Grammar Rules for mfcalc

2.5.3 The mfcalc Symbol Table

2.5.4 The mfcalc Lexer

2.5.5 The mfcalc Main

2.6 Exercises

3 Bison Grammar Files

3.1 Outline of a Bison Grammar

3.1.1 The prologue

3.1.2 Prologue Alternatives

3.1.3 The Bison Declarations Section

3.1.4 The Grammar Rules Section

3.1.5 The epilogue

3.2 Symbols, Terminal and Nonterminal

3.3 Grammar Rules

3.3.1 Syntax of Grammar Rules

3.3.2 Empty Rules

3.3.3 Recursive Rules

3.4 Defining Language Semantics

3.4.1 Data Types of Semantic Values

3.4.2 More Than One Value Type

3.4.3 Generating the Semantic Value Type

3.4.4 The Union Declaration

3.4.5 Providing a Structured Semantic Value Type

3.4.6 Actions

3.4.7 Data Types of Values in Actions

3.4.8 Actions in Midrule

3.4.8.1 Using Midrule Actions

3.4.8.2 Typed Midrule Actions

3.4.8.3 Midrule Action Translation

3.4.8.4 Conflicts due to Midrule Actions

3.5 Tracking Locations

3.5.1 Data Type of Locations

3.5.2 Actions and Locations

2.1.1 Declarations for `rpcalc`

2.1.2 Grammar Rules for `rpcalc`

2.1.2.1 Explanation of `input`

2.1.2.2 Explanation of `line`

2.1.2.3 Explanation of `exp`

2.1.3 The `rpcalc` Lexical Analyzer

2.2 Infix Notation Calculator: `calc`

2.4 Location Tracking Calculator: `ltcalc`

2.4.1 Declarations for `ltcalc`

2.4.2 Grammar Rules for `ltcalc`

2.4.3 The `ltcalc` Lexical Analyzer.

2.5 Multi-Function Calculator: `mfcalc`

2.5.1 Declarations for `mfcalc`

2.5.2 Grammar Rules for `mfcalc`

2.5.3 The `mfcalc` Symbol Table

2.5.4 The `mfcalc` Lexer

2.5.5 The `mfcalc` Main