Bison is a general-purpose parser generator that converts a grammar description for an LALR(1) context-free grammar into a C program to parse that grammar. Once you are proficient with Bison, you may use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little trouble. You need to be fluent in C programming in order to use Bison or to understand this manual.
We begin with tutorial chapters that explain the basic concepts of using Bison and show three explained examples, each building on the last. If you don't know Bison or Yacc, start by reading these chapters. Reference chapters follow which describe specific aspects of Bison in detail.
Bison was written primarily by Robert Corbett; Richard Stallman made it Yacc-compatible. This edition corresponds to version 1.24 of Bison.
As of Bison version 1.24, we have changed the distribution terms for
yyparse to permit using Bison's output in non-free programs.
Formerly, Bison parsers could be used only in programs that were free
software.
The other GNU programming tools, such as the GNU C compiler, have never had such a requirement. They could always be used for non-free software. The reason Bison was different was not due to a special policy decision; it resulted from applying the usual General Public License to all of the Bison source code.
The output of the Bison utility--the Bison parser file--contains a verbatim
copy of a sizable piece of Bison, which is the code for the
yyparse function. (The actions from your grammar are inserted
into this function at one point, but the rest of the function is not changed.)
When we applied the GPL terms to the code for yyparse, the effect
was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to make software proprietary. Software should be free. But we concluded that limiting Bison's use to free software was doing little to encourage people to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for using the other GNU tools.
Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USAEveryone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
NO WARRANTY
END OF TERMS AND CONDITIONS
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) 19yy name of authorThis program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Also add information on how to contact you by electronic and paper mail. If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker.signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
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.
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.
Not all context-free languages can be handled by Bison, only those that are
LALR(1). In brief, this means that it must be possible to tell how to parse
any portion of an input string with just a single token of look-ahead.
Strictly speaking, that is a description of an LR(1) grammar, and LALR(1)
involves additional restrictions that are hard to explain simply; but it is
rare in actual practice to find an LR(1) grammar that fails to be LALR(1). See
section Mysterious
Reduce/Reduce Conflicts, for more information on this.
In the
formal grammatical rules for a language, each kind of syntactic unit or
grouping is named by a symbol. Those which are built by grouping
smaller constructs according to grammatical rules are called nonterminal
symbols; those which can't be subdivided are called terminal
symbols or token types. We call a piece of input corresponding
to a single terminal symbol a token, and a piece corresponding to a
single nonterminal symbol a grouping. We can use the C language as an
example of what symbols, terminal and nonterminal, mean. The tokens of C are
identifiers, constants (numeric and string), and the various keywords,
arithmetic operators and punctuation marks. So the terminal symbols of a
grammar for C include `identifier', `number', `string', plus one symbol for
each keyword, operator or punctuation mark: `if', `return', `const', `static',
`int', `char', `plus-sign', `open-brace', `close-brace', `comma' and many
more. (These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
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
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.
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 section 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 See section 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.
The grammar rules also have an expression in Bison syntax. For example,
here is the Bison rule for a C
See section Syntax of
Grammar Rules.
A formal grammar selects tokens only by
their classifications: for example, if a rule mentions the terminal symbol
`integer constant', it means that any integer constant is
grammatically valid in that position. The precise value of the constant is
irrelevant to how to parse the input: if `x+4' is grammatical
then `x+1' or `x+3989' is equally grammatical.
But the precise value is very important for what the input means once it is
parsed. A compiler is useless if it fails to distinguish between 4, 1 and 3989
as constants in the program! Therefore, each token in a Bison grammar has both
a token type and a semantic value. See section Defining
Language Semantics, for details.
The token type is a terminal symbol defined in the grammar, such as
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 For example, an input token might be classified as token type
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.
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 section 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:
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.
When you run Bison, you give it a Bison grammar file as input.
The output is a C source file that parses the language described by the
grammar. This file is called a Bison parser. 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 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 section The Lexical
Analyzer Function The actual language-design process
using Bison, from grammar specification to a working compiler or interpreter,
has these parts:
To turn this source code as written into a runnable program, you must
follow these steps:
The
input file for the Bison utility is a Bison grammar file. The general
form of a Bison grammar file is as follows: The `%%', `%{' and `%}' are
punctuation that appears in every Bison grammar file to separate the sections.
The C declarations may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
The grammar rules define how to construct each nonterminal symbol from its
parts.
The additional C code can contain any C code you want to use. Often the
definition of the lexical analyzer
Now we show and explain three sample
programs written using Bison: a reverse polish notation calculator, an
algebraic (infix) notation calculator, and a multi-function calculator. All
three have been tested under BSD Unix 4.3; each produces a usable, though
limited, interactive desk-top calculator.
These examples are simple, but Bison grammars for real programming
languages are written the same way.
The
first example 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 input files.
Here are the C and Bison declarations for the reverse polish notation
calculator. As in C, comments are placed between `/*...*/'.
The C declarations section (see section The C
Declarations Section) contains two preprocessor directives.
The The The second section, Bison declarations, provides information to Bison about
the token types (see section The Bison
Declarations Section). Each terminal symbol that is not a single-character
literal must be declared here. (Single-character literals normally don't need
to be declared.) In this example, all the arithmetic operators are designated
by single-character literals, so the only terminal symbol that needs to be
declared is
Here are the grammar rules for the reverse polish notation calculator.
The groupings of the rpcalc "language" defined here are the expression
(given the name 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 section 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
Consider the definition of
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 The first alternative is empty because there are no symbols between the
colon and the first `|'; this means that It's conventional to put an empty alternative first and write the comment
`/* empty */' in it.
The second alternate rule ( The parser function Now consider the definition of 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
The We have used `|' to join all the rules for 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,
means the same thing as this: The latter, however, is much more readable.
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 section The Lexical
Analyzer 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
When After Before running Bison to produce a parser, we need to
decide how to arrange all the source code in one or more source files. For
such a simple example, the easiest thing is to put everything in one file. The
definitions of In this example the file was called `rpcalc.y' (for "Reverse
Polish CALCulator"). Bison produces a file named
`file_name.tab.c', removing the `.y' from the
original file name. The file output by Bison contains the source code for
Here is how to compile and run the parser file: The file `rpcalc' now contains the executable code. Here is an
example session using
We now modify rpcalc
to handle infix operators instead of postfix. 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. The functions
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
This addition to the grammar allows for simple error recovery in the event
of a parse error. If an expression that cannot be evaluated is read, the error
will be recognized by the third rule for Now that the basics
of Bison have been discussed, it is time to move on to a more advanced
problem. The above calculators provided only five functions, `+',
`-', `*', `/' and `^'. It
would be nice to have a calculator that provides other mathematical functions
such as 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: Note that multiple assignment and nested function calls are permitted.
Here are the C and Bison declarations for the multi-function calculator. The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types (see section
More
Than One Value Type). The Here are the grammar rules for the multi-function calculator. Most of them
are copied directly from
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. The new version of 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 The function 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
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar. The
Bison grammar input file conventionally has a name ending in
`.y'.
A Bison grammar file has four main sections, shown here with the
appropriate delimiters: Comments enclosed in `/* ... */' may appear in any of the
sections.
The C declarations section
contains macro definitions and declarations of functions and variables that
are used in the actions in the grammar rules. These are copied to the
beginning of the parser file so that they precede the definition of
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 section Bison
Declarations.
The grammar rules section
contains one or more Bison grammar rules, and nothing else. See section Syntax of
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.
The additional C code
section is copied verbatim to the end of the parser file, just as the C
declarations section is copied to the beginning. This is the most
convenient place to put anything that you want to have in the parser file but
which need not come before the definition of Symbols in Bison grammars represent the grammatical
classifications of the language. A terminal symbol (also known as a
token type) 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 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 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 section Symbols,
Terminal and Nonterminal). For example, says that two groupings of type Usually there is only one action and it follows the components. See section
Actions.
Multiple rules for the same result can be written
separately or can be joined with the vertical-bar character `|'
as follows: They are still considered distinct rules even when joined in this way. If
components in a rule is empty, it means that result can
match the empty string. For example, here is how to define a comma-separated
sequence of zero or more It is customary to write a comment `/* empty */' in each rule
with no components.
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 somethings. Consider this recursive definition of a
comma-separated sequence of one or more expressions: Since the recursive use of
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 section 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: defines two mutually-recursive nonterminals, since each refers to the
other.
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.
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 section Reverse
Polish Notation Calculator). Bison's default is to use type
This macro definition must go in the C declarations section of the grammar
file (see section Outline of
a Bison Grammar).
In most programs, you will need different data types for different kinds of
tokens and groupings. For example, a numeric constant may need type
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 C statements
surrounded by braces, much like a compound statement in C. It can be placed at
any position in the rule; it is executed at that position. Most rules have
just one action at the end of the rule, following all the components. Actions
in the middle of a rule are tricky and used only for special purposes (see
section Actions in
Mid-Rule). The C code in an action can refer to the semantic values of the
components matched by the rule with the construct This rule constructs an As long as If you have chosen a single data type
for semantic values, the then you can write 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. A mid-rule action may refer to the components
preceding it using 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
But when we add a mid-rule action as follows, the rules become
nonfunctional: Now the parser is forced to decide whether to run the mid-rule 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 look-ahead
token at this time, since the parser is still deciding what to do about it.
See section Look-Ahead
Tokens.) You might think that you could correct the problem by putting
identical actions into the two rules, like this: 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: 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: Now Bison can execute the action in the rule for The Bison declarations section
of a Bison grammar defines the symbols used in formulating the grammar and the
data types of semantic values. See section Symbols,
Terminal and Nonterminal. All token type names (but not single-character
literal tokens such as The basic way to
declare a token type name (terminal symbol) is as follows: Bison will convert this into a It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't conflict
with each other or with ASCII characters. In the event that the stack type is
a union, you must augment the
Use the
or And indeed any of these declarations serves the purposes of
The
This says that the two alternative types are When you use
Here nonterminal is the name of a nonterminal symbol, and
type is the name given in the Bison normally warns if there are any
conflicts in the grammar (see section 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 Here n is a decimal integer. The declaration says there should
be no warning if there are n shift/reduce conflicts and no
reduce/reduce conflicts. The usual warning is given if there are either more
or fewer conflicts, or if there are any reduce/reduce conflicts. In general,
using Now Bison will stop annoying you about the conflicts you have checked, but
it will warn you again if changes in the grammar result in additional
conflicts.
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
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. The Bison parser is not normally a reentrant
program, because it uses statically allocated variables for communication with
The effect is that the two communication variables become local variables
in Here is a summary
of all Bison declarations:
Most programs that use Bison parse only one language and therefore contain
only one Bison parser. But what if you want to parse more than one language
with the same program? Then you need to avoid a name conflict between
different definitions of int /* keyword `int' */
square (x) /* identifier, open-paren, */
/* identifier, close-paren */
int x; /* keyword `int', identifier, semicolon */
{ /* open-brace */
return x * x; /* keyword `return', identifier, */
/* asterisk, identifier, semicolon */
} /* close-brace */
return statement; this would be described with a grammar rule
which reads informally as follows:
From Formal
Rules to Bison Input
expr, stmt or
declaration. The Bison representation for a terminal symbol is
also called a token type. Token types as well can be represented as
C-like identifiers. By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, INTEGER,
IDENTIFIER, IF or RETURN. A terminal
symbol that stands for a particular keyword in the language should be named
after that keyword converted to upper case. The terminal symbol
error is reserved for error recovery.
return statement. The semicolon in
quotes is a literal character token, representing part of the C syntax for the
statement; the naked semicolon, and the colon, are Bison punctuation used in
every rule.
stmt: RETURN expr ';'
;
Semantic
Values
INTEGER, IDENTIFIER or ','. It tells
everything you need to know to decide where the token may validly appear and
how to group it with other tokens. The grammar rules know nothing about tokens
except their types.
',' which is just punctuation doesn't need to have
any semantic value.)
INTEGER and have the semantic value 4. Another input token might
have the same token type INTEGER but value 3989. When a grammar
rule says that INTEGER is allowed, either of these tokens is
acceptable because each is an INTEGER. When the parser accepts
the token, it keeps track of the token's semantic value.
Semantic
Actions
expr: expr '+' expr { $$ = $1 + $3; }
;
Bison
Output: the Parser File
yylex. The Bison parser 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 section Parser
C-Language Interface. Aside from the token type names and the symbols in
the actions you write, all variable and function names used in the Bison
parser file 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.
Stages in
Using Bison
yylex). It could also be produced
using Lex, but the use of Lex is not discussed in this manual.
The Overall
Layout of a Bison Grammar
%{
C declarations
%}
Bison declarations
%%
Grammar rules
%%
Additional C code
#include to include header files that do any of these things. 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.
yylex goes here, plus
subroutines called by the actions in the grammar rules. In a simple program,
all the rest of the program can go here.
Examples
Reverse
Polish Notation Calculator
Declarations
for
rpcalc/* Reverse polish notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
%}
%token NUM
%% /* Grammar rules and actions follow */
#define directive defines the macro YYSTYPE,
thus specifying the C data type for semantic values of both tokens and
groupings (see section Data Types
of Semantic Values). The Bison parser will use whatever type
YYSTYPE 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.
#include directive is used to declare the exponentiation
function pow.
NUM, the token type for numeric constants.
Grammar
Rules for
rpcalcinput: /* empty */
| input line
;
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
| exp exp '*' { $$ = $1 * $2; }
| exp exp '/' { $$ = $1 / $2; }
/* Exponentiation */
| exp exp '^' { $$ = pow ($1, $2); }
/* Unary minus */
| exp 'n' { $$ = -$1; }
;
%%
exp), the line of input (line), and
the complete input transcript (input). Each of these nonterminal
symbols has several alternate rules, joined by the `|' punctuator
which is read as "or". The following sections explain what these rules mean.
$$ stands for the semantic value for the grouping that the rule
is going to construct. Assigning a value to $$ 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
inputinput:
input: /* empty */
| input line
;
input appears always as the leftmost symbol
in the sequence. See section Recursive
Rules.
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.
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.
yyparse continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more input
tokens; we will arrange for the latter to happen at end of file.
Explanation
of
lineline:
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
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
$$. As a consequence, the semantic value associated with the
line is uninitialized (its value will be unpredictable). This
would be a bug if that value were ever used, but we don't use it: once rpcalc
has printed the value of the user's input line, that value is no longer
needed.
Explanation
of
exprexp grouping has several rules, one for each kind of
expression. The first rule handles the simplest expressions: those that are
just numbers. The second handles an addition-expression, which looks like two
expressions followed by a plus-sign. The third handles subtraction, and so on.
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
...
;
exp,
but we could equally well have written them separately:
exp: NUM ;
exp: exp exp '+' { $$ = $1 + $2; } ;
exp: exp exp '-' { $$ = $1 - $2; } ;
...
$1 refers to the first component exp and
$2 refers to the second one. The third component,
'+', has no meaningful associated semantic value, but if it had
one you could refer to it as $3. When yyparse
recognizes a sum expression using this rule, the sum of the two
subexpressions' values is produced as the value of the entire expression. See
section Actions.
You don't have to give an action for every rule. When a rule has no action,
Bison by default copies the value of $1 into $$.
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 whitespace as much as you wish. For example,
this: exp : NUM | exp exp '+' {$$ = $1 + $2; } | ...
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| ...
The
rpcalc Lexical Analyzeryylex. 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 type. The same text used in Bison rules
to stand for this token type is also a C expression for the numeric code for
the type. This works in two ways. If the token type is a character literal,
then its numeric code is the ASCII code for that character; you can use the
same character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C macro
whose definition is the appropriate number. In this example, therefore,
NUM becomes a macro 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, which was defined at
the beginning of the grammar; see section Declarations
for rpcalc.) A token type code of zero is returned if the
end-of-file is encountered. (Bison recognizes any nonpositive value as
indicating the end of the input.) Here is the code for the lexical analyzer: /* Lexical analyzer returns a double floating point
number on the stack and the token NUM, or the ASCII
character read if not a number. Skips all blanks
and tabs, returns 0 for EOF. */
#include <ctype.h>
yylex ()
{
int c;
/* skip white space */
while ((c = getchar ()) == ' ' || c == '\t')
;
/* process numbers */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval);
return NUM;
}
/* return end-of-file */
if (c == EOF)
return 0;
/* return single chars */
return c;
}
The
Controlling Function
yyparse to start the process of
parsing. main ()
{
yyparse ();
}
The Error
Reporting Routine
yyparse detects a syntax error, it
calls the error reporting function yyerror to print an error
message (usually but not always "parse error"). It is up to the
programmer to supply yyerror (see section Parser
C-Language Interface), so here is the definition we will use: #include <stdio.h>
yyerror (s) /* Called by yyparse on error */
char *s;
{
printf ("%s\n", s);
}
yyerror returns, the Bison parser may recover from the
error and continue parsing if the grammar contains a suitable error rule (see
section 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 in the first example.
Running
Bison to Make the Parser
yylex, yyerror and main
go at the end, in the "additional C code" section of the file (see section 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 a single file, you use the following command to convert
it into a parser file: bison file_name.y
yyparse. The additional functions in the input file
(yylex, yyerror and main) are copied
verbatim to the output.
Compiling
the Parser 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 rpcalc.tab.c -lm -o rpcalc
# List files again.
% ls
rpcalc rpcalc.tab.c rpcalc.y
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
%
Infix
Notation Calculator:
calc/* Infix notation calculator--calc */
%{
#define YYSTYPE double
#include <math.h>
%}
/* BISON Declarations */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
/* Grammar follows */
%%
input: /* empty string */
| input line
;
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp '+' exp { $$ = $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; }
;
%%
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 types 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 type name without associativity. (These tokens are single-character
literals, which ordinarily don't need to be declared. We declare them here to
specify the associativity.) 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. See
section 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 section
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
Simple
Error Recovery
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; }
;
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 section 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.
Multi-Function
Calculator:
mfcalcsin, 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 non-number
characters as tokens, so new grammar rules suffice for adding a new operator.
But we want something more flexible: built-in functions whose syntax has this
form: function_name (argument)
% 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
%
Declarations
for
mfcalc%{
#include <math.h> /* For math functions, cos(), sin(), etc. */
#include "calc.h" /* Contains definition of `symrec' */
%}
%union {
double val; /* For returning numbers. */
symrec *tptr; /* For returning symbol-table pointers */
}
%token <val> NUM /* Simple double precision number */
%token <tptr> VAR FNCT /* Variable and Function */
%type <val> exp
%right '='
%left '-' '+'
%left '*' '/'
%left NEG /* Negation--unary minus */
%right '^' /* Exponentiation */
/* Grammar follows */
%%
%union declaration specifies the
entire list of possible types; this is instead of defining
YYSTYPE. The allowable types are now double-floats (for
exp and NUM) and pointers to entries in the symbol
table. See section The
Collection of Value Types. Since values can now have various types, it is
necessary to associate a type with each grammar symbol whose semantic value is
used. These symbols are NUM, VAR, FNCT,
and exp. Their declarations are augmented with information about
their data type (placed between angle brackets). The Bison construct
%type is used for declaring nonterminal symbols, just as
%token is used for declaring token types. We have not used
%type 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 section Nonterminal
Symbols.
Grammar
Rules for
mfcalccalc; three rules, those which mention
VAR or FNCT, are new. input: /* empty */
| input line
;
line:
'\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;
exp: NUM { $$ = $1; }
| VAR { $$ = $1->value.var; }
| VAR '=' exp { $$ = $3; $1->value.var = $3; }
| FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($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 */
%%
The
mfcalc Symbol Table/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union {
double var; /* value of a VAR */
double (*fnctptr)(); /* value of a FNCT */
} value;
struct symrec *next; /* link field */
};
typedef struct symrec symrec;
/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
symrec *putsym ();
symrec *getsym ();
main includes a call to
init_table, a function that initializes the symbol table. Here it
is, and init_table as well: #include <stdio.h>
main ()
{
init_table ();
yyparse ();
}
yyerror (s) /* Called by yyparse on error */
char *s;
{
printf ("%s\n", s);
}
struct init
{
char *fname;
double (*fnct)();
};
struct init arith_fncts[]
= {
"sin", sin,
"cos", cos,
"atan", atan,
"ln", log,
"exp", exp,
"sqrt", sqrt,
0, 0
};
/* The symbol table: a chain of `struct symrec'. */
symrec *sym_table = (symrec *)0;
init_table () /* puts arithmetic functions in table. */
{
int i;
symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
{
ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
}
}
putsym is passed a name and the type (VAR
or FNCT) 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. symrec *
putsym (sym_name,sym_type)
char *sym_name;
int sym_type;
{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
ptr->value.var = 0; /* set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
}
symrec *
getsym (sym_name)
char *sym_name;
{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
if (strcmp (ptr->name,sym_name) == 0)
return ptr;
return 0;
}
yylex must now recognize variables, numeric
values, and the single-character arithmetic operators. Strings of alphanumeric
characters with a leading nondigit are recognized as either variables or
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 FNCT) 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>
yylex ()
{
int c;
/* Ignore whitespace, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t');
if (c == EOF)
return 0;
/* Char starts a number => parse the number. */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval.val);
return NUM;
}
/* Char starts an identifier => read the name. */
if (isalpha (c))
{
symrec *s;
static char *symbuf = 0;
static int length = 0;
int i;
/* Initially make the buffer long enough
for a 40-character symbol name. */
if (length == 0)
length = 40, symbuf = (char *)malloc (length + 1);
i = 0;
do
{
/* If buffer is full, make it bigger. */
if (i == length)
{
length *= 2;
symbuf = (char *)realloc (symbuf, length + 1);
}
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
}
while (c != EOF && isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
s = getsym (symbuf);
if (s == 0)
s = putsym (symbuf, VAR);
yylval.tptr = s;
return s->type;
}
/* Any other character is a token by itself. */
return c;
}
pi or e as well.
Exercises
init_table to add these constants to the symbol table. It will
be easiest to give the constants type VAR.
Bison
Grammar Files
Outline of
a Bison Grammar
%{
C declarations
%}
Bison declarations
%%
Grammar rules
%%
Additional C code
The C
Declarations Section
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 Bison
Declarations Section
The Grammar
Rules Section
The
Additional C Code Section
yyparse. For
example, the definitions of yylex and yyerror often
go here. See section 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 static variables whose names start with `yy'
and many macros whose names start with `YY'. It is a good idea to
avoid using any such names (except those documented in this manual) in the
additional C code section of the grammar file.
Symbols,
Terminal and Nonterminal
yylex function returns a token type 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, digits (not at the beginning), underscores and
periods. Periods make sense only in nonterminals. There are two ways of
writing terminal symbols in the grammar:
%token. See
section Token
Type Names.
'+' is a character token type. A character token type doesn't
need to be declared unless you need to specify its semantic value data type
(see section Data
Types of Semantic Values), associativity, or precedence (see section Operator
Precedence). By convention, a character token type is used only to
represent a token that consists of that particular character. Thus, the
token type '+' 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 ASCII code, zero, is the code yylex returns for
end-of-input (see section Calling
Convention for yylex). yylex is
always one of the terminal symbols (or 0 for end-of-input). Whichever way you
write the token type in the grammar rules, you write it the same way in the
definition of yylex. The numeric code for a character token type
is simply the ASCII code for the character, so yylex can use the
identical character constant to generate the requisite code. Each named token
type becomes a C macro in the parser file, so yylex can use the
name to stand for the code. (This is why periods don't make sense in terminal
symbols.) See section Calling
Convention for yylex. If yylex is defined in a
separate file, you need to arrange for the token-type macro definitions to be
available there. Use the `-d' option when you run Bison, so that
it will write these macro definitions into a separate header file
`name.tab.h' which you can include in the other source
files that need it. See section Invoking
Bison. The symbol error is a terminal symbol reserved for
error recovery (see section Error
Recovery); you shouldn't use it for any other purpose. In particular,
yylex should never return this value.
Syntax of
Grammar Rules
result: components...
;
exp: exp '+' exp
;
exp, with a `+'
token in between, can be combined into a larger grouping of type
exp. Whitespace in rules is significant only to separate symbols.
You can add extra whitespace as you wish. Scattered among the components can
be actions that determine the semantics of the rule. An action
looks like this: {C statements}
result: rule1-components...
| rule2-components...
...
;
exp groupings: expseq: /* empty */
| expseq1
;
expseq1: exp
| expseq1 ',' exp
;
Recursive
Rules
expseq1: exp
| expseq1 ',' exp
;
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
;
expr: primary
| primary '+' primary
;
primary: constant
| '(' expr ')'
;
Defining
Language Semantics
Data Types
of Semantic Values
int for all semantic values. To specify some other type, define
YYSTYPE as a macro, like this: #define YYSTYPE double
More Than
One Value 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:
%union Bison declaration (see section The
Collection of Value Types).
%token Bison declaration (see section Token
Type Names) and for groupings with the %type Bison
declaration (see section Nonterminal
Symbols). Actions
$n,
which stands for the value of the nth component. The semantic value
for the grouping being constructed is $$. (Bison translates both
of these constructs into array element references when it copies the actions
into the parser file.) Here is a typical example: exp: ...
| exp '+' exp
{ $$ = $1 + $3; }
exp from two smaller exp
groupings connected by a plus-sign token. In the action, $1 and
$3 refer to the semantic values of the two component
exp groupings, which are the first and third symbols on the right
hand side of the rule. The sum is stored into $$ 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. If you don't specify an action for a rule, Bison supplies a
default: $$ = $1. Thus, the value of the first symbol in the rule
becomes the value of the whole rule. Of course, the default rule is valid only
if the two data types match. There is no meaningful default action for an
empty rule; every empty rule must have an explicit action unless the rule's
value does not matter. $n with n zero or
negative is allowed for reference to tokens and groupings on the stack
before those that match the current rule. This is a very risky
practice, and to use it reliably you must be certain of the context in which
the rule is applied. Here is a case in which you can use this reliably: foo: expr bar '+' expr { ... }
| expr bar '-' expr { ... }
;
bar: /* empty */
{ previous_expr = $0; }
;
bar is used only in the fashion shown here,
$0 always refers to the expr which precedes
bar in the definition of foo.
Data Types
of Values in Actions
$$ 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;
}
$<itype>1 to refer to the first
subunit of the rule as an integer, or $<dtype>1 to refer to
it as a double.
Actions in
Mid-Rule
$n, but it may not refer to
subsequent components because it is run before they are parsed. The mid-rule
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 mid-rule action can also have a semantic
value. The action can set its value with an assignment to $$, and
actions later in the rule can refer to the value using
$n. Since there is no symbol to name the action, there
is no way to declare a data type for the value in advance, so you must use the
`$<...>' construct to specify a data type each time you
refer to this value. There is no way to set the value of the entire rule with
a mid-rule action, because assignments to $$ 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); }
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 mid-rule action is component number 5, so the
`stmt' is component number 6. 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. 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
mid-rule 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 '}'
;
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
;
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
;
compound: '{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
;
subroutine: /* empty */
{ prepare_for_local_variables (); }
;
compound: subroutine
'{' declarations statements '}'
| subroutine
'{' statements '}'
;
subroutine
without deciding which rule for compound it will eventually use.
Note that the action is now at the end of its rule. Any mid-rule action can be
converted to an end-of-rule action in this way, and this is what Bison
actually does to implement mid-rule actions.
Bison
Declarations
'+' and '*') must be
declared. Nonterminal symbols must be declared if you need to specify which
data type to use for the semantic value (see section More Than
One Value Type). The first rule in the file also specifies the start
symbol, by default. If you want some other symbol to be the start symbol, you
must declare it explicitly (see section Languages
and Context-Free Grammars).
Token Type
Names
%token name
#define directive in the
parser, so that the function yylex (if it is in this file) can
use the name name to stand for this token type's code.
Alternatively, you can use %left, %right, or
%nonassoc instead of %token, if you wish to specify
precedence. See section Operator
Precedence. You can explicitly specify the numeric code for a token type
by appending an integer value in the field immediately following the token
name: %token NUM 300
%token or other token declaration
to include the data type alternative delimited by angle-brackets (see section
More
Than One Value Type). For example: %union { /* define stack type */
double val;
symrec *tptr;
}
%token <val> NUM /* define token NUM and its type */
Operator
Precedence
%left, %right or %nonassoc declaration
to declare a token and specify its precedence and associativity, all at once.
These are called precedence declarations. See section Operator
Precedence, for general information on operator precedence. The syntax of
a precedence declaration is the same as that of %token: either %left symbols...
%left <type> symbols...
%token. But in addition, they specify the associativity and
relative precedence for all the symbols:
%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.
The
Collection of Value Types
%union declaration specifies the entire collection of possible
data types for semantic values. The keyword %union is followed by
a pair of braces containing the same thing that goes inside a
union in C. For example: %union {
double val;
symrec *tptr;
}
double and
symrec *. They are given names val and
tptr; these names are used in the %token and
%type declarations to pick one of the types for a terminal or
nonterminal symbol (see section Nonterminal
Symbols). Note that, unlike making a union declaration in C,
you do not write a semicolon after the closing brace.
Nonterminal
Symbols
%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...
%union to the
alternative that you want (see section The
Collection of Value Types). 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.
Suppressing
Conflict Warnings
%expect
declaration. The declaration looks like this: %expect n
%expect involves these steps:
%expect. Use the
`-v' option to get a verbose list of where the conflicts occur.
Bison will also print the number of conflicts.
%expect declaration, copying the number n
from the number which Bison printed. The
Start-Symbol
%start
declaration as follows: %start symbol
A Pure
(Reentrant) Parser
yylex. These variables include yylval and
yylloc. The Bison declaration %pure_parser says that
you want the parser to be reentrant. It looks like this: %pure_parser
yyparse, and a different calling convention is used for the
lexical analyzer function yylex. See section Calling
Conventions for Pure Parsers, for the details of this. The variable
yynerrs also becomes local in yyparse (see section
The
Error Reporting Function yyerror). The convention for calling
yyparse itself is unchanged.
Bison
Declaration Summary
%union
%token
%right
%left
%nonassoc
%type
%start
%expect
%pure_parser
Multiple
Parsers in the Same Program
yyparse, yylval, and so on.
The easy way to do this is to use the option `-p
prefix' (see section Invoking
Bison). This renames the interface functions and variables of the Bison
parser to start with prefix instead of `yy'. You can
use this to give each parser distinct names that do not conflict. The precise
list of symbols renamed is yyparse, yylex,
yyerror, yynerrs, yylval,
yychar and yydebug. For example, if you use
`-p c', the names become cparse, clex,
and so on. All the other variables and macros associated with Bison
are not renamed. These others are not global; there is no conflict if
the same name is used in different parsers. For example, YYSTYPE
is not renamed, but defining this in different ways in different parsers
causes no trouble (see section Data Types
of Semantic Values). The `-p' option works by adding macro
definitions to the beginning of the parser source file, defining
yyparse as prefixparse, and so on. This
effectively substitutes one name for the other in the entire parser file.