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
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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 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
-
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-
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-
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|
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
<EMAIL_ADDRESS>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
.