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.
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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.
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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 The Bison parser is actually a C
function named You call the function
The lexical analyzer
function, The value that This interface has been designed so that the output from the
In an ordinary (nonreentrant) parser, the semantic value
of the token must be stored into the global variable When you are using multiple data types, then the code in
If you are using the
`@n'-feature (see section Special
Features for Use in Actions) in actions to keep track of the textual
locations of tokens and groupings, then you must provide this information in
When you use the Bison declaration If the grammar file does not use the `@' constructs to refer
to textual positions, then the type Then call the parser like this: In the grammar actions, use expressions like this to refer to the data: If you wish to pass the additional parameter data to
You should then define The Bison parser detects a parse error or syntax
error whenever it reads a token which cannot satisfy any syntax rule. A
action in the grammar can also explicitly proclaim an error, using the macro
After Here is a table of Bison constructs,
variables and macros that are useful in actions.
As Bison reads tokens,
it pushes them onto a stack along with their semantic values. The stack is
called the parser stack. Pushing a token is traditionally called
shifting. For example, suppose the infix calculator has read `1
+ 5 *', with a `3' to come. The stack will have four
elements, one for each token that was shifted. But the stack does not always
have an element for each token read. When the last n tokens and
groupings shifted match the components of a grammar rule, they can be combined
according to that rule. This is called reduction. Those tokens and
groupings are replaced on the stack by a single grouping whose symbol is the
result (left hand side) of that rule. Running the rule's action is part of the
process of reduction, because this is what computes the semantic value of the
resulting grouping. For example, if the infix calculator's parser stack
contains this: and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule: Then the stack contains just these three elements: At this point, another reduction can be made, resulting in the single value
16. Then the newline token can be shifted. The parser tries, by shifts and
reductions, to reduce the entire input down to a single grouping whose symbol
is the grammar's start-symbol (see section Languages
and Context-Free Grammars). This kind of parser is known in the literature
as a bottom-up parser.
The Bison parser does not always reduce
immediately as soon as the last n tokens and groupings match a
rule. This is because such a simple strategy is inadequate to handle most
languages. Instead, when a reduction is possible, the parser sometimes "looks
ahead" at the next token in order to decide what to do. When a token is read,
it is not immediately shifted; first it becomes the look-ahead token,
which is not on the stack. Now the parser can perform one or more reductions
of tokens and groupings on the stack, while the look-ahead token remains off
to the side. When no more reductions should take place, the look-ahead token
is shifted onto the stack. This does not mean that all possible reductions
have been done; depending on the token type of the look-ahead token, some
rules may choose to delay their application. Here is a simple case where
look-ahead is needed. These three rules define expressions which contain
binary addition operators and postfix unary factorial operators
(`!'), and allow parentheses for grouping. Suppose that the tokens `1 + 2' have been read and shifted;
what should be done? If the following token is `)', then the
first three tokens must be reduced to form an Suppose we are parsing a language which has if-then and
if-then-else statements, with a pair of rules like this: Here we assume that But if the parser chose to reduce when possible rather than shift, the
result would be to attach the else-clause to the outermost if-statement,
making these two inputs equivalent: The conflict exists because the grammar as written is ambiguous: either
parsing of the simple nested if-statement is legitimate. The established
convention is that these ambiguities are resolved by attaching the else-clause
to the innermost if-statement; this is what Bison accomplishes by choosing to
shift rather than reduce. (It would ideally be cleaner to write an unambiguous
grammar, but that is very hard to do in this case.) This particular ambiguity
was first encountered in the specifications of Algol 60 and is called the
"dangling
Another situation where shift/reduce
conflicts appear is in arithmetic expressions. Here shifting is not always the
preferred resolution; the Bison declarations for operator precedence allow you
to specify when to shift and when to reduce.
Consider the following ambiguous grammar fragment (ambiguous because the
input `1 - 2 * 3' can be parsed in two different ways): Suppose the parser has seen the tokens `1', `-'
and `2'; should it reduce them via the rule for the addition
operator? It depends on the next token. Of course, if the next token is
`)', we must reduce; shifting is invalid because no single rule
can reduce the token sequence `- 2 )' or anything starting with
that. But if the next token is `*' or `<', we
have a choice: either shifting or reduction would allow the parse to complete,
but with different results. To decide which one Bison should do, we must
consider the results. If the next operator token op is shifted,
then it must be reduced first in order to permit another opportunity to reduce
the sum. The result is (in effect) `1 - (2 op 3)'. On
the other hand, if the subtraction is reduced before shifting op,
the result is `(1 - 2) op 3'. Clearly, then, the
choice of shift or reduce should depend on the relative precedence of the
operators `-' and op: `*' should be
shifted first, but not `<'. What about
input such as `1 - 2 - 5'; should this be `(1 - 2) -
5' or should it be `1 - (2 - 5)'? For most operators we
prefer the former, which is called left association. The latter
alternative, right association, is desirable for assignment
operators. The choice of left or right association is a matter of whether the
parser chooses to shift or reduce when the stack contains `1 - 2'
and the look-ahead token is `-': shifting makes
right-associativity.
Bison allows you
to specify these choices with the operator precedence declarations
In our example, we would want the following declarations: In a more complete example, which supports other operators as well, we
would declare them in groups of equal precedence. For example,
(Here The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from the
last terminal symbol mentioned in the components. (You can also specify
explicitly the precedence of a rule. See section Context-Dependent
Precedence.) Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the look-ahead token. If
the token's precedence is higher, the choice is to shift. If the rule's
precedence is higher, the choice is to reduce. If they have equal precedence,
the choice is made based on the associativity of that precedence level. The
verbose output file made by `-v' (see section Invoking
Bison) says how each conflict was resolved. Not all rules and not all
tokens have precedence. If either the rule or the look-ahead token has no
precedence, then the default is to shift.
Often the precedence of an operator depends
on the context. This sounds outlandish at first, but it is really very common.
For example, a minus sign typically has a very high precedence as a unary
operator, and a somewhat lower precedence (lower than multiplication) as a
binary operator. The Bison precedence declarations, and it is written after the components of the rule. Its effect is to assign
the rule the precedence of terminal-symbol, overriding the
precedence that would be deduced for it in the ordinary way. The altered rule
precedence then affects how conflicts involving that rule are resolved (see
section Operator
Precedence). Here is how Now the precedence of
The function
A reduce/reduce conflict occurs if
there are two or more rules that apply to the same sequence of input. This
usually indicates a serious error in the grammar. For example, here is an
erroneous attempt to define a sequence of zero or more The error is an ambiguity: there is more than one way to parse a single
Here is another common error that yields a reduce/reduce conflict: The intention here is to define a sequence which can contain either
Second, to prevent either a
Sometimes reduce/reduce conflicts can occur that don't look warranted. Here
is an example: It would seem that this grammar can be parsed with only a single token of
look-ahead: when a This corrects the problem because it introduces the possibility of an
additional active rule in the context after the
The Bison parser
stack can overflow if too many tokens are shifted and not reduced. When this
happens, the parser function It is not usually acceptable to have
a program terminate on a parse error. For example, a compiler should recover
sufficiently to parse the rest of the input file and check it for errors; a
calculator should accept another expression. In a simple interactive command
parser where each input is one line, it may be sufficient to allow
The fourth rule in this example says that an error followed by a newline
makes a valid addition to any It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed. Otherwise the close-delimiter
will probably appear to be unmatched, and generate another, spurious error
message: Error recovery strategies are necessarily guesses. When they guess wrong,
one syntax error often leads to another. In the above example, the error
recovery rule guesses that an error is due to bad input within one
The Bison paradigm is to parse tokens first, then group them into larger
syntactic units. In many languages, the meaning of a token is affected by its
context. Although this violates the Bison paradigm, certain techniques (known
as kludges) may enable you to write Bison parsers for such languages.
(Actually, "kludge" means any technique that gets its job done but is neither
clean nor robust.)
The C language has a context dependency: the way an identifier is used
depends on what its current meaning is. For example, consider this: This looks like a function call statement, but if Unfortunately, the name being declared is separated from the declaration
construct itself by a complicated syntactic structure--the "declarator". As a
result, the part of Bison parser for C needs to be duplicated, with all the
nonterminal names changed: once for parsing a declaration in which a typedef
name can be redefined, and once for parsing a declaration in which that can't
be done. Here is a part of the duplication, with actions omitted for brevity: Here One way to handle context-dependency is the lexical
tie-in: a flag which is set by Bison actions, whose purpose is to alter
the way tokens are parsed. For example, suppose we have a language vaguely
like C, but with a special construct `hex (hex-expr)'.
After the keyword Here we assume that Lexical tie-ins make strict demands on any error recovery rules you have.
See section Error
Recovery. The reason for this is that the purpose of an error recovery
rule is to abort the parsing of one construct and resume in some larger
construct. For example, in C-like languages, a typical error recovery rule is
to skip tokens until the next semicolon, and then start a new statement, like
this: If there is a syntax error in the middle of a `hex
(expr)' construct, this error rule will apply, and then the
action for the completed `hex (expr)' will never run.
So If this rule acts within the If a Bison grammar compiles properly but doesn't do what you
want when it runs, the To make sense of this information, it helps to refer to the listing file
produced by the Bison `-v' option (see section Invoking
Bison). This file shows the meaning of each state in terms of positions in
various rules, and also what each state will do with each possible input
token. As you read the successive trace messages, you can see that the parser
is functioning according to its specification in the listing file. Eventually
you will arrive at the place where something undesirable happens, and you will
see which parts of the grammar are to blame. The parser file is a C program
and you can use C debuggers on it, but it's not easy to interpret what it is
doing. The parser function is a finite-state machine interpreter, and aside
from the actions it executes the same code over and over. Only the values of
variables show where in the grammar it is working. The
debugging information normally gives the token type of each token read, but
not its semantic value. You can optionally define a macro named
The usual way to
invoke Bison is as follows: Here infile is the grammar file name, which usually ends in
`.y'. The parser file's name is made by replacing the
`.y' with `.tab.c'. Thus, the `bison
foo.y' filename yields `foo.tab.c', and the `bison
hack/foo.y' filename yields `hack/foo.tab.c'.
Bison supports both traditional single-letter options and mnemonic long
option names. Long option names are indicated with `--' instead
of `-'. Abbreviations for option names are allowed as long as
they are unique. When a long option takes an argument, like
`--file-prefix', connect the option name and the argument with
`='. Here is a list of options that can be used with Bison,
alphabetized by short option. It is followed by a cross key alphabetized by
long option.
Here is a list of options, alphabetized by long option, to help you find
the corresponding short option.
The command line syntax for Bison on
VMS is a variant of the usual Bison command syntax--adapted to fit VMS
conventions. To find the VMS equivalent for any Bison option, start with the
long option, and substitute a `/' for the leading
`--', and substitute a `_' for each `-'
in the name of the long option. For example, the following invocation under
VMS: is equivalent to the following command under POSIX. The VMS file system does not permit filenames such as `foo.tab.c'.
In the above example, the output file would instead be named
`foo_tab.c'.
These are the punctuation and delimiters used in Bison input:
Jump to: $ - % - @ - a - b - c - d - e - f - g - i - l - m - n - o - p - r - s - t - u - v - w - y - |
This document was generated on 2 October 1998 using the texi2html translator version
1.52.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
rpcalc
input: /* 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
input
input
:
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
line
line
:
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
$$
. As a consequence, the semantic value associated with the
line
is uninitialized (its value will be unpredictable). This
would be a bug if that value were ever used, but we don't use it: once rpcalc
has printed the value of the user's input line, that value is no longer
needed.
Explanation
of
expr
exp
grouping has several rules, one for each kind of
expression. The first rule handles the simplest expressions: those that are
just numbers. The second handles an addition-expression, which looks like two
expressions followed by a plus-sign. The third handles subtraction, and so on.
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
...
;
exp
,
but we could equally well have written them separately:
exp: NUM ;
exp: exp exp '+' { $$ = $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:
mfcalc
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 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
mfcalc
calc
; 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.
Parser
C-Language Interface
yyparse
. Here we describe the interface
conventions of yyparse
and the other functions that it needs to
use. Keep in mind that the parser uses many C identifiers starting with
`yy' and `YY' for internal purposes. If you use such
an identifier (aside from those in this manual) in an action or in additional
C code in the grammar file, you are likely to run into trouble.
The Parser
Function
yyparse
yyparse
to cause
parsing to occur. This function reads tokens, executes actions, and ultimately
returns when it encounters end-of-input or an unrecoverable syntax error. You
can also write an action which directs yyparse
to return
immediately without reading further. The value returned by
yyparse
is 0 if parsing was successful (return is due to
end-of-input). The value is 1 if parsing failed (return is due to a syntax
error). In an action, you can cause immediate return from yyparse
by using these macros:
YYACCEPT
YYABORT
The Lexical
Analyzer Function
yylex
yylex
, recognizes tokens from the input stream and
returns them to the parser. Bison does not create this function automatically;
you must write it so that yyparse
can call it. The function is
sometimes referred to as a lexical scanner. In simple programs,
yylex
is often defined at the end of the Bison grammar file. If
yylex
is defined in a separate source file, you need to arrange
for the token-type macro definitions to be available there. To do this, 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.
Calling
Convention for
yylex
yylex
returns must be the numeric code for the
type of token it has just found, or 0 for end-of-input. When a token is
referred to in the grammar rules by a name, that name in the parser file
becomes a C macro whose definition is the proper numeric code for that token
type. So yylex
can use the name to indicate that type. See
section Symbols,
Terminal and Nonterminal. When a token is referred to in the grammar rules
by a character literal, the numeric code for that character is also the code
for the token type. So yylex
can simply return that character
code. The null character must not be used this way, because its code is zero
and that is what signifies end-of-input. Here is an example showing these
things: yylex ()
{
...
if (c == EOF) /* Detect end of file. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for `+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
lex
utility can be used without change as the definition of
yylex
.
Semantic
Values of Tokens
yylval
. When
you are using just one data type for semantic values, yylval
has
that type. Thus, if the type is int
(the default), you might
write this in yylex
: ...
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
yylval
's type is a
union made from the %union
declaration (see section The
Collection of Value Types). So when you store a token's value, you must
use the proper member of the union. If the %union
declaration
looks like this: %union {
int intval;
double val;
symrec *tptr;
}
yylex
might look like this: ...
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
Textual
Positions of Tokens
yylex
. The function yyparse
expects to find the
textual location of a token just parsed in the global variable
yylloc
. So yylex
must store the proper data in that
variable. The value of yylloc
is a structure and you need only
initialize the members that are going to be used by the actions. The four
members are called first_line
, first_column
,
last_line
and last_column
. Note that the use of this
feature makes the parser noticeably slower. The data type
of yylloc
has the name YYLTYPE
.
Calling
Conventions for Pure Parsers
%pure_parser
to request a
pure, reentrant parser, the global communication variables yylval
and yylloc
cannot be used. (See section A Pure
(Reentrant) Parser.) In such parsers the two global variables are replaced
by pointers passed as arguments to yylex
. You must declare them
as shown here, and pass the information back by storing it through those
pointers. yylex (lvalp, llocp)
YYSTYPE *lvalp;
YYLTYPE *llocp;
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
YYLTYPE
will not be defined.
In this case, omit the second argument; yylex
will be called with
only one argument. You can pass parameter information to a
reentrant parser in a reentrant way. Define the macro
YYPARSE_PARAM
as a variable name. The resulting
yyparse
function then accepts one argument, of type void
*
, with that name. When you call yyparse
, pass the address
of an object, casting the address to void *
. The grammar actions
can refer to the contents of the object by casting the pointer value back to
its proper type and then dereferencing it. Here's an example. Write this in
the parser: %{
struct parser_control
{
int nastiness;
int randomness;
};
#define YYPARSE_PARAM parm
%}
struct parser_control
{
int nastiness;
int randomness;
};
...
{
struct parser_control foo;
... /* Store proper data in
foo
. */
value = yyparse ((void *) &foo);
...
}
((struct parser_control *) parm)->randomness
yylex
, define the macro YYLEX_PARAM
just like
YYPARSE_PARAM
, as shown here: %{
struct parser_control
{
int nastiness;
int randomness;
};
#define YYPARSE_PARAM parm
#define YYLEX_PARAM parm
%}
yylex
to accept one additional
argument--the value of parm
. (This makes either two or three
arguments in total, depending on whether an argument of type
YYLTYPE
is passed.) You can declare the argument as a pointer to
the proper object type, or you can declare it as void *
and
access the contents as shown above.
The Error
Reporting Function
yyerror
YYERROR
(see section Special
Features for Use in Actions). The Bison parser expects to report the error
by calling an error reporting function named yyerror
, which you
must supply. It is called by yyparse
whenever a syntax error is
found, and it receives one argument. For a parse error, the string is normally
"parse error"
. If you define the macro
YYERROR_VERBOSE
in the Bison declarations section (see section The Bison
Declarations Section), then Bison provides a more verbose and specific
error message string instead of just plain "parse error"
. It
doesn't matter what definition you use for YYERROR_VERBOSE
, just
whether you define it. The parser can detect one other kind of error: stack
overflow. This happens when the input contains constructions that are very
deeply nested. It isn't likely you will encounter this, since the Bison parser
extends its stack automatically up to a very large limit. But if overflow
happens, yyparse
calls yyerror
in the usual fashion,
except that the argument string is "parser stack overflow"
. The
following definition suffices in simple programs: yyerror (s)
char *s;
{
fprintf (stderr, "%s\n", s);
}
yyerror
returns to yyparse
, the latter will
attempt error recovery if you have written suitable error recovery grammar
rules (see section Error
Recovery). If recovery is impossible, yyparse
will
immediately return 1. The variable yynerrs
contains the number of syntax errors encountered so far. Normally this
variable is global; but if you request a pure parser (see section A Pure
(Reentrant) Parser) then it is a local variable which only the actions can
access.
Special
Features for Use in Actions
$$
but specifies alternative typealt in the
union specified by the %union
declaration. See section Data
Types of Values in Actions.
$n
but specifies alternative
typealt in the union specified by the %union
declaration. See section Data
Types of Values in Actions.
yyparse
, indicating failure. See
section The
Parser Function yyparse
.
yyparse
, indicating success. See
section The
Parser Function yyparse
.
yychar
when there is no
look-ahead token.
yyerror
, and does not print any
message. If you want to print an error message, call yyerror
explicitly before the `YYERROR;' statement. See section Error
Recovery.
yyparse
.) When there
is no look-ahead token, the value YYEMPTY
is stored in the
variable. See section Look-Ahead
Tokens.
struct {
int first_line, last_line;
int first_column, last_column;
};
Thus, to get the starting line number of the third component, use
`@3.first_line'. In order for the members of this structure to
contain valid information, you must make yylex
supply this
information about each token. If you need only certain members, then
yylex
need only fill in those members. The use of this feature
makes the parser noticeably slower. The Bison
Parser Algorithm
1 + 5 * 3
expr: expr '*' expr;
1 + 15
Look-Ahead
Tokens
expr: term '+' expr
| term
;
term: '(' expr ')'
| term '!'
| NUMBER
;
expr
. This is the
only valid course, because shifting the `)' would produce a
sequence of symbols term ')'
, and no rule allows this. If the
following token is `!', then it must be shifted immediately so
that `2 !' can be reduced to make a term
. If instead
the parser were to reduce before shifting, `1 + 2' would become
an expr
. It would then be impossible to shift the
`!' because doing so would produce on the stack the sequence of
symbols expr '!'
. No rule allows that sequence. The current look-ahead token is stored in the variable
yychar
. See section Special
Features for Use in Actions.
Shift/Reduce
Conflicts
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
IF
, THEN
and
ELSE
are terminal symbols for specific keyword tokens. When the
ELSE
token is read and becomes the look-ahead token, the contents
of the stack (assuming the input is valid) are just right for reduction by the
first rule. But it is also legitimate to shift the ELSE
, because
that would lead to eventual reduction by the second rule. This situation,
where either a shift or a reduction would be valid, is called a
shift/reduce conflict. Bison is designed to resolve these conflicts
by choosing to shift, unless otherwise directed by operator precedence
declarations. To see the reason for this, let's contrast it with the other
alternative. Since the parser prefers to shift the ELSE
, the
result is to attach the else-clause to the innermost if-statement, making
these two inputs equivalent: if x then if y then win (); else lose;
if x then do; if y then win (); else lose; end;
if x then if y then win (); else lose;
if x then do; if y then win (); end; else lose;
else
" ambiguity. To avoid warnings from Bison about
predictable, legitimate shift/reduce conflicts, use the %expect
n
declaration. There will be no warning as long as the
number of shift/reduce conflicts is exactly n. See section Suppressing
Conflict Warnings. The definition of if_stmt
above is solely
to blame for the conflict, but the conflict does not actually appear without
additional rules. Here is a complete Bison input file that actually manifests
the conflict: %token IF THEN ELSE variable
%%
stmt: expr
| if_stmt
;
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
expr: variable
;
Operator
Precedence
When
Precedence is Needed
expr: expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
...
;
Specifying
Operator Precedence
%left
and %right
. Each such declaration contains a
list of tokens, which are operators whose precedence and associativity is
being declared. The %left
declaration makes all those operators
left-associative and the %right
declaration makes them
right-associative. A third alternative is %nonassoc
, which
declares that it is a syntax error to find the same operator twice "in a row".
The relative precedence of different operators is controlled by the order in
which they are declared. The first %left
or %right
declaration in the file declares the operators whose precedence is lowest, the
next such declaration declares the operators whose precedence is a little
higher, and so on.
Precedence
Examples
%left '<'
%left '-'
%left '*'
'+'
is declared with '-'
: %left '<' '>' '=' NE LE GE
%left '+' '-'
%left '*' '/'
NE
and so on stand for the operators for "not equal" and
so on. We assume that these tokens are more than one character long and
therefore are represented by names, not character literals.)
How
Precedence Works
Context-Dependent
Precedence
%left
,
%right
and %nonassoc
, can only be used once for a
given token; so a token has only one precedence declared in this way. For
context-dependent precedence, you need to use an additional mechanism: the
%prec
modifier for rules. The %prec
modifier
declares the precedence of a particular rule by specifying a terminal symbol
whose precedence should be used for that rule. It's not necessary for that
symbol to appear otherwise in the rule. The modifier's syntax is: %prec terminal-symbol
%prec
solves the problem of unary
minus. First, declare a precedence for a fictitious terminal symbol named
UMINUS
. There are no tokens of this type, but the symbol serves
to stand for its precedence: ...
%left '+' '-'
%left '*'
%left UMINUS
UMINUS
can be used in specific rules: exp: ...
| exp '-' exp
...
| '-' exp %prec UMINUS
Parser
States
yyparse
is implemented using a finite-state machine. The values
pushed on the parser stack are not simply token type codes; they represent the
entire sequence of terminal and nonterminal symbols at or near the top of the
stack. The current state collects all the information about previous input
which is relevant to deciding what to do next. Each time a look-ahead token is
read, the current parser state together with the type of look-ahead token are
looked up in a table. This table entry can say, "Shift the look-ahead token."
In this case, it also specifies the new parser state, which is pushed onto the
top of the parser stack. Or it can say, "Reduce using rule number
n." This means that a certain number of tokens or groupings are
taken off the top of the stack, and replaced by one grouping. In other words,
that number of states are popped from the stack, and one new state is pushed.
There is one other alternative: the table can say that the look-ahead token is
erroneous in the current state. This causes error processing to begin (see
section Error
Recovery).
Reduce/Reduce
Conflicts
word
groupings. sequence: /* empty */
{ printf ("empty sequence\n"); }
| maybeword
| sequence word
{ printf ("added word %s\n", $2); }
;
maybeword: /* empty */
{ printf ("empty maybeword\n"); }
| word
{ printf ("single word %s\n", $1); }
;
word
into a sequence
. It could be reduced to a
maybeword
and then into a sequence
via the second
rule. Alternatively, nothing-at-all could be reduced into a
sequence
via the first rule, and this could be combined with the
word
using the third rule for sequence
. There is
also more than one way to reduce nothing-at-all into a sequence
.
This can be done directly via the first rule, or indirectly via
maybeword
and then the second rule. You might think that this is
a distinction without a difference, because it does not change whether any
particular input is valid or not. But it does affect which actions are run.
One parsing order runs the second rule's action; the other runs the first
rule's action and the third rule's action. In this example, the output of the
program changes. Bison resolves a reduce/reduce conflict by choosing to use
the rule that appears first in the grammar, but it is very risky to rely on
this. Every reduce/reduce conflict must be studied and usually eliminated.
Here is the proper way to define sequence
: sequence: /* empty */
{ printf ("empty sequence\n"); }
| sequence word
{ printf ("added word %s\n", $2); }
;
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
word
or redirect
groupings. The individual
definitions of sequence
, words
and
redirects
are error-free, but the three together make a subtle
ambiguity: even an empty input can be parsed in infinitely many ways!
Consider: nothing-at-all could be a words
. Or it could be two
words
in a row, or three, or any number. It could equally well be
a redirects
, or two, or any number. Or it could be a
words
followed by three redirects
and another
words
. And so on. Here are two ways to correct these rules.
First, to make it a single level of sequence: sequence: /* empty */
| sequence word
| sequence redirect
;
words
or a redirects
from being empty: sequence: /* empty */
| sequence words
| sequence redirects
;
words: word
| words word
;
redirects:redirect
| redirects redirect
;
Mysterious
Reduce/Reduce Conflicts
%token ID
%%
def: param_spec return_spec ','
;
param_spec:
type
| name_list ':' type
;
return_spec:
type
| name ':' type
;
type: ID
;
name: ID
;
name_list:
name
| name ',' name_list
;
param_spec
is being read, an ID
is a name
if a comma or colon follows, or a type
if
another ID
follows. In other words, this grammar is LR(1). However, Bison, like most parser
generators, cannot actually handle all LR(1) grammars. In this grammar, two
contexts, that after an ID
at the beginning of a
param_spec
and likewise at the beginning of a
return_spec
, are similar enough that Bison assumes they are the
same. They appear similar because the same set of rules would be active--the
rule for reducing to a name
and that for reducing to a
type
. Bison is unable to determine at that stage of processing
that the rules would require different look-ahead tokens in the two contexts,
so it makes a single parser state for them both. Combining the two contexts
causes a conflict later. In parser terminology, this occurrence means that the
grammar is not LALR(1). In general, it is better to fix deficiencies than to
document them. But this particular deficiency is intrinsically hard to fix;
parser generators that can handle LR(1) grammars are hard to write and tend to
produce parsers that are very large. In practice, Bison is more useful as it
is now. When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make them look
distinct. In the above example, adding one rule to return_spec
as
follows makes the problem go away: %token BOGUS
...
%%
...
return_spec:
type
| name ':' type
/* This rule is never used. */
| ID BOGUS
;
ID
at the
beginning of return_spec
. This rule is not active in the
corresponding context in a param_spec
, so the two contexts
receive distinct parser states. As long as the token BOGUS
is
never generated by yylex
, the added rule cannot alter the way
actual input is parsed. In this particular example, there is another way to
solve the problem: rewrite the rule for return_spec
to use
ID
directly instead of via name
. This also causes
the two confusing contexts to have different sets of active rules, because the
one for return_spec
activates the altered rule for
return_spec
rather than the one for name
. param_spec:
type
| name_list ':' type
;
return_spec:
type
| ID ':' type
;
Stack
Overflow, and How to Avoid It
yyparse
returns a nonzero value,
pausing only to call yyerror
to report the overflow. By defining the macro YYMAXDEPTH
, you can control
how deep the parser stack can become before a stack overflow occurs. Define
the macro with a value that is an integer. This value is the maximum number of
tokens that can be shifted (and not reduced) before overflow. It must be a
constant expression whose value is known at compile time. The stack space
allowed is not necessarily allocated. If you specify a large value for
YYMAXDEPTH
, the parser actually allocates a small stack at first,
and then makes it bigger by stages as needed. This increasing allocation
happens automatically and silently. Therefore, you do not need to make
YYMAXDEPTH
painfully small merely to save space for ordinary
inputs that do not need much stack. The default value of
YYMAXDEPTH
, if you do not define it, is 10000. You can control how much stack is allocated initially by
defining the macro YYINITDEPTH
. This value too must be a
compile-time constant integer. The default is 200.
Error
Recovery
yyparse
to return 1 on error and have the caller ignore the rest
of the input line when that happens (and then call yyparse
again). But this is inadequate for a compiler, because it forgets all the
syntactic context leading up to the error. A syntax error deep within a
function in the compiler input should not cause the compiler to treat the
following line like the beginning of a source file. You can
define how to recover from a syntax error by writing rules to recognize the
special token error
. This is a terminal symbol that is always
defined (you need not declare it) and reserved for error handling. The Bison
parser generates an error
token whenever a syntax error happens;
if you have provided a rule to recognize this token in the current context,
the parse can continue. For example: stmnts: /* empty string */
| stmnts '\n'
| stmnts exp '\n'
| stmnts error '\n'
stmnts
. What happens if a syntax
error occurs in the middle of an exp
? The error recovery rule,
interpreted strictly, applies to the precise sequence of a
stmnts
, an error
and a newline. If an error occurs
in the middle of an exp
, there will probably be some additional
tokens and subexpressions on the stack after the last stmnts
, and
there will be tokens to read before the next newline. So the rule is not
applicable in the ordinary way. But Bison can force the situation to fit the
rule, by discarding part of the semantic context and part of the input. First
it discards states and objects from the stack until it gets back to a state in
which the error
token is acceptable. (This means that the
subexpressions already parsed are discarded, back to the last complete
stmnts
.) At this point the error
token can be
shifted. Then, if the old look-ahead token is not acceptable to be shifted
next, the parser reads tokens and discards them until it finds a token which
is acceptable. In this example, Bison reads and discards input until the next
newline so that the fourth rule can apply. The choice of error rules in the
grammar is a choice of strategies for error recovery. A simple and useful
strategy is simply to skip the rest of the current input line or current
statement if an error is detected: stmnt: error ';' /* on error, skip until ';' is read */
primary: '(' expr ')'
| '(' error ')'
...
;
stmnt
. Suppose that instead a spurious semicolon is inserted in
the middle of a valid stmnt
. After the error recovery rule
recovers from the first error, another syntax error will be found
straightaway, since the text following the spurious semicolon is also an
invalid stmnt
. To prevent an outpouring of error messages, the
parser will output no error message for another syntax error that happens
shortly after the first; only after three consecutive input tokens have been
successfully shifted will error messages resume. Note that rules which accept
the error
token may have actions, just as any other rules can. You can make error messages resume immediately by using the
macro yyerrok
in an action. If you do this in the error rule's
action, no error messages will be suppressed. This macro requires no
arguments; `yyerrok;' is a valid C statement. The previous look-ahead token is reanalyzed immediately after
an error. If this is unacceptable, then the macro yyclearin
may
be used to clear this token. Write the statement `yyclearin;' in
the error rule's action. For example, suppose that on a parse error, an error
handling routine is called that advances the input stream to some point where
parsing should once again commence. The next symbol returned by the lexical
scanner is probably correct. The previous look-ahead token ought to be
discarded with `yyclearin;'. The macro
YYRECOVERING
stands for an expression that has the value 1 when
the parser is recovering from a syntax error, and 0 the rest of the time. A
value of 1 indicates that error messages are currently suppressed for new
syntax errors.
Handling
Context Dependencies
Semantic
Info in Token Types
foo (x);
foo
is a
typedef name, then this is actually a declaration of x
. How can a
Bison parser for C decide how to parse this input? The method used in GNU C is
to have two different token types, IDENTIFIER
and
TYPENAME
. When yylex
finds an identifier, it looks
up the current declaration of the identifier in order to decide which token
type to return: TYPENAME
if the identifier is declared as a
typedef, IDENTIFIER
otherwise. The grammar rules can then express
the context dependency by the choice of token type to recognize.
IDENTIFIER
is accepted as an expression, but
TYPENAME
is not. TYPENAME
can start a declaration,
but IDENTIFIER
cannot. In contexts where the meaning of the
identifier is not significant, such as in declarations that can
shadow a typedef name, either TYPENAME
or IDENTIFIER
is accepted--there is one rule for each of the two token types. This technique
is simple to use if the decision of which kinds of identifiers to allow is
made at a place close to where the identifier is parsed. But in C this is not
always so: C allows a declaration to redeclare a typedef name provided an
explicit type has been specified earlier: typedef int foo, bar, lose;
static foo (bar); /* redeclare
bar
as static variable */
static int foo (lose); /* redeclare foo
as function */
initdcl:
declarator maybeasm '='
init
| declarator maybeasm
;
notype_initdcl:
notype_declarator maybeasm '='
init
| notype_declarator maybeasm
;
initdcl
can redeclare a typedef name, but
notype_initdcl
cannot. The distinction between
declarator
and notype_declarator
is the same sort of
thing. There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis is
changed during parsing by other parts of the program. The difference is here
the information is global, and is used for other purposes in the program. A
true lexical tie-in has a special-purpose flag controlled by the syntactic
context.
Lexical
Tie-ins
hex
comes an expression in parentheses in which
all integers are hexadecimal. In particular, the token `a1b' must
be treated as an integer rather than as an identifier if it appears in that
context. Here is how you can do it: %{
int hexflag;
%}
%%
...
expr: IDENTIFIER
| constant
| HEX '('
{ hexflag = 1; }
expr ')'
{ hexflag = 0;
$$ = $4; }
| expr '+' expr
{ $$ = make_sum ($1, $3); }
...
;
constant:
INTEGER
| STRING
;
yylex
looks at the value of
hexflag
; when it is nonzero, all integers are parsed in
hexadecimal, and tokens starting with letters are parsed as integers if
possible. The declaration of hexflag
shown in the C declarations
section of the parser file is needed to make it accessible to the actions (see
section The C
Declarations Section). You must also write the code in yylex
to obey the flag.
Lexical
Tie-ins and Error Recovery
stmt: expr ';'
| IF '(' expr ')' stmt { ... }
...
error ';'
{ hexflag = 0; }
;
hexflag
would remain set for the entire rest of the input, or
until the next hex
keyword, causing identifiers to be
misinterpreted as integers. To avoid this problem the error recovery rule
itself clears hexflag
. There may also be an error recovery rule
that works within expressions. For example, there could be a rule which
applies within parentheses and skips to the close-parenthesis: expr: ...
| '(' expr ')'
{ $$ = $2; }
| '(' error ')'
...
hex
construct, it is not going to
abort that construct (since it applies to an inner level of parentheses within
the construct). Therefore, it should not clear the flag: the rest of the
hex
construct should be parsed with the flag still in effect.
What if there is an error recovery rule which might abort out of the
hex
construct or might not, depending on circumstances? There is
no way you can write the action to determine whether a hex
construct is being aborted or not. So if you are using a lexical tie-in, you
had better make sure your error recovery rules are not of this kind. Each rule
must be such that you can be sure that it always will, or always won't, have
to clear the flag.
Debugging
Your Parser
yydebug
parser-trace feature can help you
figure out why. To enable compilation of trace facilities, you must define the
macro YYDEBUG
when you compile the parser. You could use
`-DYYDEBUG=1' as a compiler option or you could put
`#define YYDEBUG 1' in the C declarations section of the grammar
file (see section The C
Declarations Section). Alternatively, use the `-t' option
when you run Bison (see section Invoking
Bison). We always define YYDEBUG
so that debugging is always
possible. The trace facility uses stderr
, so you must add
#include <stdio.h>
to the C declarations section unless it
is already there. Once you have compiled the program with trace facilities,
the way to request a trace is to store a nonzero value in the variable
yydebug
. You can do this by making the C code do it (in
main
, perhaps), or you can alter the value with a C debugger.
Each step taken by the parser when yydebug
is nonzero produces a
line or two of trace information, written on stderr
. The trace
messages tell you these things:
yylex
, what kind of token was
read.
YYPRINT
to provide a way to print the value. If you define
YYPRINT
, it should take three arguments. The parser will pass a
standard I/O stream, the numeric code for the token type, and the token value
(from yylval
). Here is an example of YYPRINT
suitable for the multi-function calculator (see section Declarations
for mfcalc
): #define YYPRINT(file, type, value) yyprint (file, type, value)
static void
yyprint (file, type, value)
FILE *file;
int type;
YYSTYPE value;
{
if (type == VAR)
fprintf (file, " %s", value.tptr->name);
else if (type == NUM)
fprintf (file, " %d", value.val);
}
Invoking
Bison
bison infile
Bison
Options
YYSTYPE
, as well as a few extern
variable
declarations. If the parser output file is named
`name.c' then this file is named
`name.h'. This output file is essential if you wish to
put the definition of yylex
in a separate source file, because
yylex
needs to be able to refer to token type codes and the
variable yylval
. See section Semantic
Values of Tokens.
#line
preprocessor commands in the parser
file. Ordinarily Bison puts them in the parser file so that the C compiler
and debuggers will associate errors with your source file, the grammar file.
This option causes them to associate errors with the parser file, treating
it an independent source file in its own right.
yyparse
, yylex
, yyerror
,
yynerrs
, yylval
, yychar
and
yydebug
. For example, if you use `-p c', the names
become cparse
, clex
, and so on. See section Multiple
Parsers in the Same Program.
YYDEBUG
into the parser
file, so that the debugging facilities are compiled. See section Debugging
Your Parser.
bison -y $*
Option
Cross Key
Invoking
Bison under VMS
bison /debug/name_prefix=bar foo.y
bison --debug --name-prefix=bar foo.y
Bison
Symbols
error
error
becomes the current look-ahead token. Actions
corresponding to error
are then executed, and the look-ahead
token is reset to the token that originally caused the violation. See
section Error
Recovery.
YYABORT
yyparse
return 1 immediately. The error reporting
function yyerror
is not called. See section The
Parser Function yyparse
.
YYACCEPT
yyparse
return 0 immediately. See section The
Parser Function yyparse
.
YYBACKUP
YYERROR
yyerror
and then perform normal error recovery if possible (see
section Error
Recovery), or (if recovery is impossible) make yyparse
return 1. See section Error
Recovery.
YYERROR_VERBOSE
#define
in the Bison
declarations section to request verbose, specific error message strings when
yyerror
is called.
YYINITDEPTH
YYLEX_PARAM
yyparse
to pass to yylex
. See section Calling
Conventions for Pure Parsers.
YYLTYPE
yylloc
; a structure with four
members. See section Textual
Positions of Tokens.
YYMAXDEPTH
YYPARSE_PARAM
yyparse
should accept. See section Calling
Conventions for Pure Parsers.
YYRECOVERING
YYSTYPE
int
by default.
See section Data
Types of Semantic Values.
yychar
yyparse
.) Error-recovery rule actions may examine this
variable. See section Special
Features for Use in Actions.
yyclearin
yydebug
yydebug
is given a nonzero value, the parser will output
information on input symbols and parser action. See section Debugging
Your Parser.
yyerrok
yyerror
yyparse
on error.
The function receives one argument, a pointer to a character string
containing an error message. See section The Error
Reporting Function yyerror
.
yylex
yylex
.
yylval
yylex
should place the semantic
value associated with a token. (In a pure parser, it is a local variable
within yyparse
, and its address is passed to
yylex
.) See section Semantic
Values of Tokens.
yylloc
yylex
should place the line and
column numbers associated with a token. (In a pure parser, it is a local
variable within yyparse
, and its address is passed to
yylex
.) You can ignore this variable if you don't use the
`@' feature in the grammar actions. See section Textual
Positions of Tokens.
yynerrs
yyparse
.) See
section The Error
Reporting Function yyerror
.
yyparse
yyparse
.
%left
%nonassoc
%prec
%pure_parser
%right
%start
%token
%type
%union
Glossary
if
statement. See
section Languages
and Context-Free Grammars.
yylex
.
mfcalc
.
Index
$
%
@
a
b
c
calc
d
else
e
else
,
dangling
f
g
i
l
m
mfcalc
n
o
p
r
rpcalc
s
t
u
v
w
y
|