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. Wilfred Hansen of Carnegie Mellon University added multi-character string literals and other features.
This edition corresponds to version 1.75 of Bison.
As of Bison version 1.24, we have changed the distribution terms for
yyparse to permit using Bison's output in nonfree programs when
Bison is generating C code for LALR(1) parsers. Formerly, these
parsers could be used only in programs that were free software.
The other GNU programming tools, such as the GNU C compiler, have never had such a requirement. They could always be used for nonfree software. The reason Bison was different was not due to a special policy decision; it resulted from applying the usual General Public License to all of the Bison source code.
The 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.
This exception applies only when Bison is generating C code for a LALR(1) parser; otherwise, the GPL terms operate as usual. You can tell whether the exception applies to your `.c' output file by inspecting it to see whether it says "As a special exception, when this file is copied by Bison into a Bison output file, you may use that output file without restriction."
Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
NO WARRANTY
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) yyyy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
This chapter introduces many of the basic concepts without which the details of Bison will not make sense. If you do not already know how to use Bison or Yacc, we suggest you start by reading this chapter carefully.
In order for Bison to parse a language, it must be described by a context-free grammar. This means that you specify one or more syntactic groupings and give rules for constructing them from their parts. For example, in the C language, one kind of grouping is called an `expression'. One rule for making an expression might be, "An expression can be made of a minus sign and another expression". Another would be, "An expression can be an integer". As you can see, rules are often recursive, but there must be at least one rule which leads out of the recursion.
The most common formal system for presenting such rules for humans to read is Backus-Naur Form or "BNF", which was developed in order to specify the language Algol 60. Any grammar expressed in BNF is a context-free grammar. The input to Bison is essentially machine-readable BNF.
There are various important subclasses of context-free grammar. Although it can handle almost all context-free grammars, Bison is optimized for what are called LALR(1) grammars. In brief, in these grammars, it must be possible to tell how to parse any portion of an input string with just a single token of 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.
Parsers for LALR(1) grammars are deterministic, meaning roughly that the next grammar rule to apply at any point in the input is uniquely determined by the preceding input and a fixed, finite portion (called a look-ahead) of the remaining input. A context-free grammar can be ambiguous, meaning that there are multiple ways to apply the grammar rules to get the some inputs. Even unambiguous grammars can be non-deterministic, meaning that no fixed look-ahead always suffices to determine the next grammar rule to apply. With the proper declarations, Bison is also able to parse these more general context-free grammars, using a technique known as GLR parsing (for Generalized LR). Bison's GLR parsers are able to handle any context-free grammar for which the number of possible parses of any given string is finite.
In the formal grammatical rules for a language, each kind of syntactic unit or grouping is named by a symbol. Those which are built by grouping smaller constructs according to grammatical rules are called nonterminal symbols; those which can't be subdivided are called terminal symbols or token types. We call a piece of input corresponding to a single terminal symbol a token, and a piece corresponding to a single nonterminal symbol a grouping.
We can use the C language as an example of what symbols, terminal and nonterminal, mean. The tokens of C are identifiers, constants (numeric and string), and the various keywords, arithmetic operators and punctuation marks. So the terminal symbols of a grammar for C include `identifier', `number', `string', plus one symbol for each keyword, operator or punctuation mark: `if', `return', `const', `static', `int', `char', `plus-sign', `open-brace', `close-brace', `comma' and many more. (These tokens can be subdivided into characters, but that is a matter of lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
@ifnotinfo
int /* keyword `int' */
square (int x) /* identifier, open-paren, identifier, identifier, close-paren */
{ /* open-brace */
return x * x; /* keyword `return', identifier, asterisk, identifier, semicolon */
} /* close-brace */
The syntactic groupings of C include the expression, the statement, the declaration, and the function definition. These are represented in the grammar of C by nonterminal symbols `expression', `statement', `declaration' and `function definition'. The full grammar uses dozens of additional language constructs, each with its own nonterminal symbol, in order to express the meanings of these four. The example above is a function definition; it contains one declaration, and one statement. In the statement, each `x' is an expression and so is `x * x'.
Each nonterminal symbol must have grammatical rules showing how it is made
out of simpler constructs. For example, one kind of C statement is the
return statement; this would be described with a grammar rule which
reads informally as follows:
A `statement' can be made of a `return' keyword, an `expression' and a `semicolon'.
There would be many other rules for `statement', one for each kind of statement in C.
One nonterminal symbol must be distinguished as the special one which defines a complete utterance in the language. It is called the start symbol. In a compiler, this means a complete input program. In the C language, the nonterminal symbol `sequence of definitions and declarations' plays this role.
For example, `1 + 2' is a valid C expression--a valid part of a C program--but it is not valid as an entire C program. In the context-free grammar of C, this follows from the fact that `expression' is not the start symbol.
The Bison parser reads a sequence of tokens as its input, and groups the tokens using the grammar rules. If the input is valid, the end result is that the entire token sequence reduces to a single grouping whose symbol is the grammar's start symbol. If we use a grammar for C, the entire input must be a `sequence of definitions and declarations'. If not, the parser reports a syntax error.
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 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.
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.
A third way to represent a terminal symbol is with a C string constant containing several characters. See section Symbols, Terminal and Nonterminal, for more information.
The grammar rules also have an expression in Bison syntax. For example,
here is the Bison rule for a C return statement. The semicolon in
quotes is a literal character token, representing part of the C syntax for
the statement; the naked semicolon, and the colon, are Bison punctuation
used in every rule.
stmt: RETURN expr ';'
;
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
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.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier. (A token such as ',' which is just punctuation doesn't
need to have any semantic value.)
For example, an input token might be classified as token type
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.
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:
expr: expr '+' expr { $$ = $1 + $3; }
;
The action says how to produce the semantic value of the sum expression from the values of the two subexpressions.
In some grammars, there will be cases where Bison's standard LALR(1) parsing algorithm cannot decide whether to apply a certain grammar rule at a given point. That is, it may not be able to decide (on the basis of the input read so far) which of two possible reductions (applications of a grammar rule) applies, or whether to apply a reduction or read more of the input and apply a reduction later in the input. These are known respectively as reduce/reduce conflicts (see section Reduce/Reduce Conflicts), and shift/reduce conflicts (see section Shift/Reduce Conflicts).
To use a grammar that is not easily modified to be LALR(1), a more
general parsing algorithm is sometimes necessary. If you include
%glr-parser among the Bison declarations in your file
(see section Outline of a Bison Grammar), the result will be a Generalized LR (GLR)
parser. These parsers handle Bison grammars that contain no unresolved
conflicts (i.e., after applying precedence declarations) identically to
LALR(1) parsers. However, when faced with unresolved shift/reduce and
reduce/reduce conflicts, GLR parsers use the simple expedient of doing
both, effectively cloning the parser to follow both possibilities. Each
of the resulting parsers can again split, so that at any given time,
there can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.
During the time that there are multiple parsers, semantic actions are recorded, but not performed. When a parser disappears, its recorded semantic actions disappear as well, and are never performed. When a reduction makes two parsers identical, causing them to merge, Bison records both sets of semantic actions. Whenever the last two parsers merge, reverting to the single-parser case, Bison resolves all the outstanding actions either by precedences given to the grammar rules involved, or by performing both actions, and then calling a designated user-defined function on the resulting values to produce an arbitrary merged result.
Let's consider an example, vastly simplified from C++.
%{
#define YYSTYPE const char*
%}
%token TYPENAME ID
%right '='
%left '+'
%glr-parser
%%
prog :
| prog stmt { printf ("\n"); }
;
stmt : expr ';' %dprec 1
| decl %dprec 2
;
expr : ID { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
{ printf ("%s <cast> ", $1); }
| expr '+' expr { printf ("+ "); }
| expr '=' expr { printf ("= "); }
;
decl : TYPENAME declarator ';'
{ printf ("%s <declare> ", $1); }
| TYPENAME declarator '=' expr ';'
{ printf ("%s <init-declare> ", $1); }
;
declarator : ID { printf ("\"%s\" ", $1); }
| '(' declarator ')'
;
This models a problematic part of the C++ grammar--the ambiguity between certain declarations and statements. For example,
T (x) = y+z;
parses as either an expr or a stmt
(assuming that `T' is recognized as a TYPENAME and `x' as an ID).
Bison detects this as a reduce/reduce conflict between the rules
expr : ID and declarator : ID, which it cannot resolve at the
time it encounters x in the example above. The two %dprec
declarations, however, give precedence to interpreting the example as a
decl, which implies that x is a declarator.
The parser therefore prints
"x" y z + T <init-declare>
Consider a different input string for this parser:
T (x) + y;
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters x, it does not
have enough information to resolve the reduce/reduce conflict (again,
between x as an expr or a declarator). In this
case, no precedence declaration is used. Instead, the parser splits
into two, one assuming that x is an expr, and the other
assuming x is a declarator. The second of these parsers
then vanishes when it sees +, and the parser prints
x T <cast> y +
Suppose that instead of resolving the ambiguity, you wanted to see all
the possibilities. For this purpose, we must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other. To do so, you could change the declaration of stmt as
follows:
stmt : expr ';' %merge <stmtMerge>
| decl %merge <stmtMerge>
;
and define the stmtMerge function as:
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
printf ("<OR> ");
return "";
}
with an accompanying forward declaration in the C declarations at the beginning of the file:
%{
#define YYSTYPE const char*
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%}
With these declarations, the resulting parser will parse the first example
as both an expr and a decl, and print
"x" y z + T <init-declare> x T <cast> y z + = <OR>
Many applications, like interpreters or compilers, have to produce verbose and useful error messages. To achieve this, one must be able to keep track of the textual position, or location, of each syntactic construct. Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token has an associated location, but the type of locations is the same for all tokens and groupings. Moreover, the output parser is equipped with a default data structure for storing locations (see section Tracking Locations, for more details).
Like semantic values, locations can be reached in actions using a dedicated
set of constructs. In the example above, the location of the whole grouping
is @$, while the locations of the subexpressions are @1 and
@3.
When a rule is matched, a default action is used to compute the semantic value
of its left hand side (see section Actions). In the same way, another default
action is used for locations. However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
rule how @$ should be formed. When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.
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 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 symbols defined in the Bison parser file itself
begin with `yy' or `YY'. This includes interface functions
such as the lexical analyzer function yylex, the error reporting
function yyerror and the parser function yyparse itself.
This also includes numerous identifiers used for internal purposes.
Therefore, you should avoid using C identifiers starting with `yy'
or `YY' in the Bison grammar file except for the ones defined in
this manual.
In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
headers. On some non-GNU hosts, <alloca.h>,
<stddef.h>, and <stdlib.h> are included as needed to
declare memory allocators and related types. Other system headers may
be included if you define YYDEBUG to a nonzero value
(see section Tracing Your Parser).
The actual language-design process using Bison, from grammar specification to a working compiler or interpreter, has these parts:
yylex). It could also be produced
using Lex, but the use of Lex is not discussed in this manual.
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:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
The `%%', `%{' and `%}' are punctuation that appears in every Bison grammar file to separate the sections.
The prologue may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
#include to include header files that do any of these things.
The Bison declarations declare the names of the terminal and nonterminal symbols, and may also describe operator precedence and the data types of semantic values of various symbols.
The grammar rules define how to construct each nonterminal symbol from its parts.
The epilogue can contain any code you want to use. Often the definition of
the lexical analyzer 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.
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.
rpcalcHere are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between `/*...*/'.
/* Reverse polish notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
%}
%token NUM
%% /* Grammar rules and actions follow. */
The declarations section (see section The prologue) contains two preprocessor directives.
The #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.
The #include directive is used to declare the exponentiation
function pow.
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 NUM, the token
type for numeric constants.
rpcalcHere are the grammar rules for the reverse polish notation calculator.
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' { $$ = -$1; }
;
%%
The groupings of the rpcalc "language" defined here are the expression
(given the name exp), the line of input (line), and the
complete input transcript (input). Each of these nonterminal
symbols has several alternate rules, joined by the `|' punctuator
which is read as "or". The following sections explain what these rules
mean.
The semantics of the language is determined by the actions taken when a grouping is recognized. The actions are the C code that appears inside braces. See section Actions.
You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules. In each action, the
pseudo-variable $$ 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.
input
Consider the definition of input:
input: /* empty */
| input line
;
This definition reads as follows: "A complete input is either an empty
string, or a complete input followed by an input line". Notice that
"complete input" is defined in terms of itself. This definition is said
to be left recursive since input appears always as the
leftmost symbol in the sequence. See section Recursive Rules.
The first alternative is empty because there are no symbols between the
colon and the first `|'; this means that input can match an
empty string of input (no tokens). We write the rules this way because it
is legitimate to type Ctrl-d right after you start the calculator.
It's conventional to put an empty alternative first and write the comment
`/* empty */' in it.
The second alternate rule (input line) handles all nontrivial input.
It means, "After reading any number of lines, read one more line if
possible." The left recursion makes this rule into a loop. Since the
first alternative matches empty input, the loop can be executed zero or
more times.
The parser function yyparse continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.
line
Now consider the definition of line:
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
The first alternative is a token which is a newline character; this means
that rpcalc accepts a blank line (and ignores it, since there is no
action). The second alternative is an expression followed by a newline.
This is the alternative that makes rpcalc useful. The semantic value of
the exp grouping is the value of $1 because the exp in
question is the first symbol in the alternative. The action prints this
value, which is the result of the computation the user asked for.
This action is unusual because it does not assign a value to $$. As
a consequence, the semantic value associated with the line is
uninitialized (its value will be unpredictable). This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.
expr
The exp grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just numbers.
The second handles an addition-expression, which looks like two expressions
followed by a plus-sign. The third handles subtraction, and so on.
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
...
;
We have used `|' to join all the rules for exp, but we could
equally well have written them separately:
exp: NUM ;
exp: exp exp '+' { $$ = $1 + $2; } ;
exp: exp exp '-' { $$ = $1 - $2; } ;
...
Most of the rules have actions that compute the value of the expression in
terms of the value of its parts. For example, in the rule for addition,
$1 refers to the first component exp and $2 refers to
the second one. The third component, '+', has no meaningful
associated semantic value, but if it had one you could refer to it as
$3. 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 $$.
This is what happens in the first rule (the one that uses NUM).
The formatting shown here is the recommended convention, but Bison does not require it. You can add or change white space as much as you wish. For example, this:
exp : NUM | exp exp '+' {$$ = $1 + $2; } | ...
means the same thing as this:
exp: NUM
| exp exp '+' { $$ = $1 + $2; }
| ...
The latter, however, is much more readable.
rpcalc Lexical Analyzer
The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens. The Bison parser gets its
tokens by calling the lexical analyzer. See section The Lexical Analyzer Function yylex.
Only a simple lexical analyzer is needed for the RPN calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
double and returns them as NUM tokens. Any other character
that isn't part of a number is a separate token. Note that the token-code
for such a single-character token is the character itself.
The return value of the lexical analyzer function is a numeric code which
represents a token 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 that of the 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-input is encountered. (Bison recognizes any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
of the character read if not a number. It skips all blanks
and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
int
yylex (void)
{
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-input. */
if (c == EOF)
return 0;
/* Return a single char. */
return c;
}
In keeping with the spirit of this example, the controlling function is
kept to the bare minimum. The only requirement is that it call
yyparse to start the process of parsing.
int
main (void)
{
return yyparse ();
}
When 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>
void
yyerror (const char *s) /* called by yyparse on error */
{
printf ("%s\n", s);
}
After yyerror returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(see 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 for the first example.
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 yylex, yyerror and main go at the
end, in the epilogue 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
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 yyparse. The additional
functions in the input file (yylex, yyerror and main)
are copied verbatim to the output.
Here is how to compile and run 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 -lm -o rpcalc rpcalc.tab.c
# List files again.
$ ls
rpcalc rpcalc.tab.c rpcalc.y
The file `rpcalc' now contains the executable code. Here is an
example session using rpcalc.
$ rpcalc 4 9 + 13 3 7 + 3 4 5 *+- -13 3 7 + 3 4 5 * + - n Note the unary minus, `n' 13 5 6 / 4 n + -3.166666667 3 4 ^ Exponentiation 81 ^D End-of-file indicator $
calcWe 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.
/* 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; }
;
%%
The functions yylex, yyerror and main can be the
same as before.
There are two important new features shown in this code.
In the second section (Bison declarations), %left declares token
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
Up to this point, this manual has not addressed the issue of error
recovery---how to continue parsing after the parser detects a syntax
error. All we have handled is error reporting with yyerror.
Recall that by default yyparse returns after calling
yyerror. This means that an erroneous input line causes the
calculator program to exit. Now we show how to rectify this deficiency.
The Bison language itself includes the reserved word error, which
may be included in the grammar rules. In the example below it has
been added to one of the alternatives for line:
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;
This addition to the grammar allows for simple error recovery in the
event of a parse error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for line,
and parsing will continue. (The yyerror function is still called
upon to print its message as well.) The action executes the statement
yyerrok, a macro defined automatically by Bison; its meaning is
that error recovery is complete (see 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.
ltcalcThis example extends the infix notation calculator with location tracking. This feature will be used to improve the error messages. For the sake of clarity, this example is a simple integer calculator, since most of the work needed to use locations will be done in the lexical analyzer.
ltcalcThe C and Bison declarations for the location tracking calculator are the same as the declarations for the infix notation calculator.
/* Location tracking calculator. */
%{
#define YYSTYPE int
#include <math.h>
%}
/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG
%right '^'
%% /* Grammar follows */
Note there are no declarations specific to locations. Defining a data
type for storing locations is not needed: we will use the type provided
by default (see section Data Type of Locations), which is a
four member structure with the following integer fields:
first_line, first_column, last_line and
last_column.
ltcalcWhether handling locations or not has no effect on the syntax of your language. Therefore, grammar rules for this example will be very close to those of the previous example: we will only modify them to benefit from the new information.
Here, we will use locations to report divisions by zero, and locate the wrong expressions or subexpressions.
input : /* empty */
| input line
;
line : '\n'
| exp '\n' { printf ("%d\n", $1); }
;
exp : NUM { $$ = $1; }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
| '-' exp %preg NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables @n for rule components, and the
pseudo-variable @$ for groupings.
We don't need to assign a value to @$: the output parser does it
automatically. By default, before executing the C code of each action,
@$ is set to range from the beginning of @1 to the end
of @n, for a rule with n components. This behavior
can be redefined (see section Default Action for Locations), and for very specific rules, @$ can be computed by
hand.
ltcalc Lexical Analyzer.Until now, we relied on Bison's defaults to enable location tracking. The next step is to rewrite the lexical analyzer, and make it able to feed the parser with the token locations, as it already does for semantic values.
To this end, we must take into account every single character of the input text, to avoid the computed locations of being fuzzy or wrong:
int
yylex (void)
{
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
/* Process numbers. */
if (isdigit (c))
{
yylval = c - '0';
++yylloc.last_column;
while (isdigit (c = getchar ()))
{
++yylloc.last_column;
yylval = yylval * 10 + c - '0';
}
ungetc (c, stdin);
return NUM;
}
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char, and update location. */
if (c == '\n')
{
++yylloc.last_line;
yylloc.last_column = 0;
}
else
++yylloc.last_column;
return c;
}
Basically, the lexical analyzer performs the same processing as before:
it skips blanks and tabs, and reads numbers or single-character tokens.
In addition, it updates yylloc, the global variable (of type
YYLTYPE) containing the token's location.
Now, each time this function returns a token, the parser has its number
as well as its semantic value, and its location in the text. The last
needed change is to initialize yylloc, for example in the
controlling function:
int
main (void)
{
yylloc.first_line = yylloc.last_line = 1;
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
}
Remember that computing locations is not a matter of syntax. Every character must be associated to a location update, whether it is in valid input, in comments, in literal strings, and so on.
mfcalc
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 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 nonnumber 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)
At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator:
$ mfcalc pi = 3.141592653589 3.1415926536 sin(pi) 0.0000000000 alpha = beta1 = 2.3 2.3000000000 alpha 2.3000000000 ln(alpha) 0.8329091229 exp(ln(beta1)) 2.3000000000 $
Note that multiple assignment and nested function calls are permitted.
mfcalcHere are the C and Bison declarations for the multi-function calculator.
%{
#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 */
%%
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 %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.
mfcalc
Here are the grammar rules for the multi-function calculator.
Most of them are copied directly from calc; three rules,
those which mention VAR or 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 */
%%
mfcalc Symbol TableThe multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its definition, which is kept in the header `calc.h', is as follows. It provides for either functions or variables to be placed in the table.
/* Function type. */
typedef double (*func_t) (double);
/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
{
double var; /* value of a VAR */
func_t 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 (const char *, func_t);
symrec *getsym (const char *);
The new version of 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>
int
main (void)
{
init_table ();
return yyparse ();
}
void
yyerror (const char *s) /* Called by yyparse on error */
{
printf ("%s\n", s);
}
struct init
{
char *fname;
double (*fnct)(double);
};
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;
/* Put arithmetic functions in table. */
void
init_table (void)
{
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;
}
}
By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator.
Two important functions allow look-up and installation of symbols in the
symbol table. The function putsym is passed a name and the 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 (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 (const 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;
}
The function yylex must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
characters with a leading non-digit 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>
int
yylex (void)
{
int c;
/* Ignore white space, 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 (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;
}
This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as pi or e as well.
init_table to add these constants to the symbol table.
It will be easiest to give the constants type VAR.
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'. See section Invoking Bison.
A Bison grammar file has four main sections, shown here with the appropriate delimiters:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
Comments enclosed in `/* ... */' may appear in any of the sections.
The Prologue section contains macro definitions and
declarations of functions and variables that are used in the actions in the
grammar rules. These are copied to the beginning of the parser file so
that they precede the definition of yyparse. You can use
`#include' to get the declarations from a header file. If you don't
need any C declarations, you may omit the `%{' and `%}'
delimiters that bracket this section.
You may have more than one Prologue section, intermixed with the
Bison declarations. This allows you to have C and Bison
declarations that refer to each other. For example, the %union
declaration may use types defined in a header file, and you may wish to
prototype functions that take arguments of type YYSTYPE. This
can be done with two Prologue blocks, one before and one after the
%union declaration.
%{
#include <stdio.h>
#include "ptypes.h"
%}
%union {
long n;
tree t; /* tree is defined in `ptypes.h'. */
}
%{
static void yyprint(FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) yyprint(F, N, L)
%}
...
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 Epilogue is copied verbatim to the end of the parser file, just as
the Prologue is copied to the beginning. This is the most convenient
place to put anything that you want to have in the parser file but which need
not come before the definition of 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 epilogue of the grammar file.
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 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 three 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 numeric code, zero, signifies
end-of-input (see section Calling Convention for yylex).
"<=" is a literal string token. A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (see section Data Types of Semantic Values), associativity, or precedence
(see section Operator Precedence).
You can associate the literal string token with a symbolic name as an
alias, using the %token declaration (see section Token Type Names). If you don't do that, the lexical analyzer has to
retrieve the token number for the literal string token from the
yytname table (see section Calling Convention for yylex).
WARNING: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a token
that consists of that particular string. Thus, you should use the token
type "<=" to represent the string `<=' as a token. Bison
does not enforce this convention, but if you depart from it, people who
read your program will be confused.
All the escape sequences used in string literals in C can be used in
Bison as well. A literal string token must contain two or more
characters; for a token containing just one character, use a character
token (see above).
How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol.
The value returned by yylex is always one of the terminal
symbols, except that a zero or negative value signifies end-of-input.
Whichever way you write the token 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 positive numeric code of the
character, so yylex can use the identical value to generate the
requisite code, though you may need to convert it to unsigned
char to avoid sign-extension on hosts where char is signed.
Each named token 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.
If you want to write a grammar that is portable to any Standard C host, you must use only non-null character tokens taken from the basic execution character set of Standard C. This set consists of the ten digits, the 52 lower- and upper-case English letters, and the characters in the following C-language string:
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"
The yylex function and Bison must use a consistent character
set and encoding for character tokens. For example, if you run Bison in an
ASCII environment, but then compile and run the resulting program
in an environment that uses an incompatible character set like
EBCDIC, the resulting program may not work because the
tables generated by Bison will assume ASCII numeric values for
character tokens. It is standard
practice for software distributions to contain C source files that
were generated by Bison in an ASCII environment, so installers on
platforms that are incompatible with ASCII must rebuild those
files before compiling them.
The symbol error is a terminal symbol reserved for error recovery
(see section Error Recovery); you shouldn't use it for any other purpose.
In particular, yylex should never return this value. The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a %token declaration.
A Bison grammar rule has the following general form:
result: components...
;
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,
exp: exp '+' exp
;
says that two groupings of type exp, with a `+' token in between,
can be combined into a larger grouping of type exp.
White space in rules is significant only to separate symbols. You can add extra white space as you wish.
Scattered among the components can be actions that determine the semantics of the rule. An action looks like this:
{C statements}
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:
result: rule1-components...
| rule2-components...
...
;
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 exp groupings:
expseq: /* empty */
| expseq1
;
expseq1: exp
| expseq1 ',' exp
;
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 a particular thing. Consider this recursive definition of a comma-separated sequence of one or more expressions:
expseq1: exp
| expseq1 ',' exp
;
Since the recursive use of expseq1 is the leftmost symbol in the
right hand side, we call this left recursion. By contrast, here
the same construct is defined using right recursion:
expseq1: exp
| exp ',' expseq1
;
Any kind of sequence can be defined using either left recursion or right recursion, but you should always use left recursion, because it can parse a sequence of any number of elements with bounded stack space. Right recursion uses up space on the Bison stack in proportion to the number of elements in the sequence, because all the elements must be shifted onto the stack before the rule can be applied even once. See 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:
expr: primary
| primary '+' primary
;
primary: constant
| '(' expr ')'
;
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 int for all semantic values. To
specify some other type, define YYSTYPE as a macro, like this:
#define YYSTYPE double
This macro definition must go in the prologue 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
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).
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 $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
{ $$ = $1 + $3; }
This rule constructs an exp from two smaller exp groupings
connected by a plus-sign token. In the action, $1 and $3
refer to the semantic values of the two component exp groupings,
which are the first and third symbols on the right hand side of the rule.
The sum is stored into $$ 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.
Note that the vertical-bar character `|' is really a rule separator, and actions are attached to a single rule. This is a difference with tools like Flex, for which `|' stands for either "or", or "the same action as that of the next rule". In the following example, the action is triggered only when `b' is found:
a-or-b: 'a'|'b' { a_or_b_found = 1; };
If you don't specify an action for a rule, Bison supplies a default:
$$ = $1. Thus, the value of the first symbol in the rule becomes
the value of the whole rule. Of course, the default rule is valid only
if the two data types match. There is no meaningful default action for
an empty rule; every empty rule must have an explicit action unless the
rule's value does not matter.
$n with n zero or negative is allowed for reference
to tokens and groupings on the stack before those that match the
current rule. This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied. Here
is a case in which you can use this reliably:
foo: expr bar '+' expr { ... }
| expr bar '-' expr { ... }
;
bar: /* empty */
{ previous_expr = $0; }
;
As long as bar is used only in the fashion shown here, $0
always refers to the expr which precedes bar in the
definition of foo.
If you have chosen a single data type for semantic values, the $$
and $n constructs always have that data type.
If you have used %union to specify a variety of data types, then you
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use $$ or
$n, its data type is determined by which symbol it refers to
in the rule. In this example,
exp: ...
| exp '+' exp
{ $$ = $1 + $3; }
$1 and $3 refer to instances of exp, so they all
have the data type declared for the nonterminal symbol exp. If
$2 were used, it would have the data type declared for the
terminal symbol '+', whatever that might be.
Alternatively, you can specify the data type when you refer to the value, by inserting `<type>' after the `$' at the beginning of the reference. For example, if you have defined types as shown here:
%union {
int itype;
double dtype;
}
then you can write $<itype>1 to refer to the first subunit of the
rule as an integer, or $<dtype>1 to refer to it as a double.
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
$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 $$, and actions later in the rule
can refer to the value using $n. Since there is no symbol
to name the action, there is no way to declare a data type for the value
in advance, so you must use the `$<...>n' construct to
specify a data type each time you refer to this value.
There is no way to set the value of the entire rule with a mid-rule
action, because assignments to $$ 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); }
As soon as `let (variable)' has been recognized, the first
action is run. It saves a copy of the current semantic context (the
list of accessible variables) as its semantic value, using alternative
context in the data-type union. Then it calls
declare_variable to add the new variable to that list. Once the
first action is finished, the embedded statement stmt can be
parsed. Note that the 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 '}'
;
But when we add a mid-rule action as follows, the rules become nonfunctional:
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
;
Now the parser is forced to decide whether to run the 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:
compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
;
But this does not help, because Bison does not realize that the two actions are identical. (Bison never tries to understand the C code in an action.)
If the grammar is such that a declaration can be distinguished from a statement by the first token (which is true in C), then one solution which does work is to put the action after the open-brace, like this:
compound: '{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
;
Now the first token of the following declaration or statement, which would in any case tell Bison which rule to use, can still do so.
Another solution is to bury the action inside a nonterminal symbol which serves as a subroutine:
subroutine: /* empty */
{ prepare_for_local_variables (); }
;
compound: subroutine
'{' declarations statements '}'
| subroutine
'{' statements '}'
;
Now Bison can execute the action in the rule for subroutine without
deciding which rule for compound it will eventually use. 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.
Though grammar rules and semantic actions are enough to write a fully functional parser, it can be useful to process some additional information, especially symbol locations.
The way locations are handled is defined by providing a data type, and actions to take when rules are matched.
Defining a data type for locations is much simpler than for semantic values, since all tokens and groupings always use the same type.
The type of locations is specified by defining a macro called YYLTYPE.
When YYLTYPE is not defined, Bison uses a default structure type with
four members:
struct
{
int first_line;
int first_column;
int last_line;
int last_column;
}
Actions are not only useful for defining language semantics, but also for describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is very
similar to the way semantic values are computed. In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the nth component of the right hand side is
@n, while the location of the left hand side grouping is
@$.
Here is a basic example using the default data type for locations:
exp: ...
| exp '/' exp
{
@$.first_column = @1.first_column;
@$.first_line = @1.first_line;
@$.last_column = @3.last_column;
@$.last_line = @3.last_line;
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
printf("Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
As for semantic values, there is a default action for locations that is
run each time a rule is matched. It sets the beginning of @$ to the
beginning of the first symbol, and the end of @$ to the end of the
last symbol.
With this default action, the location tracking can be fully automatic. The example above simply rewrites this way:
exp: ...
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
printf("Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule. The YYLLOC_DEFAULT macro is invoked each time a rule is
matched, before the associated action is run.
Most of the time, this macro is general enough to suppress location dedicated code from semantic actions.
The YYLLOC_DEFAULT macro takes three parameters. The first one is
the location of the grouping (the result of the computation). The second one
is an array holding locations of all right hand side elements of the rule
being matched. The last one is the size of the right hand side rule.
By default, it is defined this way for simple LALR(1) parsers:
#define YYLLOC_DEFAULT(Current, Rhs, N) \ Current.first_line = Rhs[1].first_line; \ Current.first_column = Rhs[1].first_column; \ Current.last_line = Rhs[N].last_line; \ Current.last_column = Rhs[N].last_column;
and like this for GLR parsers:
#define YYLLOC_DEFAULT(Current, Rhs, N) \ Current.first_line = YYRHSLOC(Rhs,1).first_line; \ Current.first_column = YYRHSLOC(Rhs,1).first_column; \ Current.last_line = YYRHSLOC(Rhs,N).last_line; \ Current.last_column = YYRHSLOC(Rhs,N).last_column;
When defining YYLLOC_DEFAULT, you should consider that:
YYLLOC_DEFAULT.
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
'+' 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).
The basic way to declare a token type name (terminal symbol) is as follows:
%token name
Bison will convert this into a #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
associativity and 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
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 normal characters.
In the event that the stack type is a union, you must augment the
%token or other token declaration to include the data type
alternative delimited by angle-brackets (see 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 */
You can associate a literal string token with a token type name by
writing the literal string at the end of a %token
declaration which declares the name. For example:
%token arrow "=>"
For example, a grammar for the C language might specify these names with equivalent literal string tokens:
%token <operator> OR "||" %token <operator> LE 134 "<=" %left OR "<="
Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules. The
yylex function can use the token name or the literal string to
obtain the token type code number (see section Calling Convention for yylex).
Use the %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...
or
%left <type> symbols...
And indeed any of these declarations serves the purposes of %token.
But in addition, they specify the associativity and relative precedence for
all the symbols:
%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 %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;
}
This says that the two alternative types are double and symrec
*. They are given names val and tptr; these names are used
in the %token 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.
When you use %union to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used. This is done with a %type declaration, like this:
%type <type> nonterminal...
Here nonterminal is the name of a nonterminal symbol, and
type is the name given in the %union to the alternative
that you want (see 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.
You can also declare the value type of a terminal symbol. To do this,
use the same <type> construction in a declaration for the
terminal symbol. All kinds of token declarations allow
<type>.
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 %expect declaration.
The declaration looks like this:
%expect n
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. An error, instead of the usual warning, is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts.
In general, using %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.
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 %start declaration as follows:
%start symbol
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 non-reentrant program may not be safe to call from a signal handler. In systems with multiple threads of control, a non-reentrant program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
suitable for most uses, and it permits compatibility with YACC. (The
standard YACC interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with yylex,
including yylval and yylloc.)
Alternatively, you can generate a pure, reentrant parser. The Bison
declaration %pure-parser says that you want the parser to be
reentrant. It looks like this:
%pure-parser
The result is that the communication variables yylval and
yylloc become local variables in yyparse, and a different
calling convention is used for the lexical analyzer function
yylex. See 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.
Whether the parser is pure has nothing to do with the grammar rules. You can generate either a pure parser or a nonreentrant parser from any valid grammar.
Here is a summary of the declarations used to define a grammar:
%union
%token
%right
%left
%nonassoc
%type
%start
%expect
In order to change the behavior of @command{bison}, use the following directives:
%debug
YYDEBUG to 1 if it is not
already defined, so that the debugging facilities are compiled.
See section Tracing Your Parser.
%defines
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.
%file-prefix="prefix"
%locations
%name-prefix="prefix"
yyparse, yylex, yyerror, yynerrs,
yylval, yychar, yydebug, and possible
yylloc. For example, if you use `%name-prefix="c_"', the
names become c_parse, c_lex, and so on. See section Multiple Parsers in the Same Program.
%no-parser
#define directives and static variable
declarations.
This option also tells Bison to write the C code for the grammar actions
into a file named `filename.act', in the form of a
brace-surrounded body fit for a switch statement.
%no-lines
#line preprocessor commands in the parser
file. Ordinarily Bison writes these commands in the parser file so that
the C compiler and debuggers will associate errors and object code with
your source file (the grammar file). This directive causes them to
associate errors with the parser file, treating it an independent source
file in its own right.
%output="filename"
%pure-parser
%token-table
yytname; yytname[i] is the name of the
token whose internal Bison token code number is i. The first
three elements of yytname are always "$end",
"error", and "$undefined"; after these come the symbols
defined in the grammar file.
For single-character literal tokens and literal string tokens, the name
in the table includes the single-quote or double-quote characters: for
example, "'+'" is a single-character literal and "\"<=\""
is a literal string token. All the characters of the literal string
token appear verbatim in the string found in the table; even
double-quote characters are not escaped. For example, if the token
consists of three characters `*"*', its string in yytname
contains `"*"*"'. (In C, that would be written as
"\"*\"*\"").
When you specify %token-table, Bison also generates macro
definitions for macros YYNTOKENS, YYNNTS, and
YYNRULES, and YYNSTATES:
YYNTOKENS
YYNNTS
YYNRULES
YYNSTATES
%verbose
%yacc
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 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.
The Bison parser is actually a C function named 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 epilogue in the grammar file, you are likely to run into trouble.
yyparse
You call the function 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
yylex
The lexical analyzer function, 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.
yylex
The value that yylex returns must be the positive numeric code
for the type of token it has just found; a zero or negative value
signifies 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, possibly converted
to unsigned char to avoid sign-extension. The null character
must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
int
yylex (void)
{
...
if (c == EOF) /* Detect end-of-input. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for `+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the lex
utility can be used without change as the definition of yylex.
If the grammar uses literal string tokens, there are two ways that
yylex can determine the token type codes for them:
yylex can use these symbolic names like
all others. In this case, the use of the literal string tokens in
the grammar file has no effect on yylex.
yylex can find the multicharacter token in the yytname
table. The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in yytname with a
double-quote, the token's characters, and another double-quote. The
token's characters are not escaped in any way; they appear verbatim in
the contents of the string in the table.
Here's code for looking up a token in yytname, assuming that the
characters of the token are stored in token_buffer.
for (i = 0; i < YYNTOKENS; i++)
{
if (yytname[i] != 0
&& yytname[i][0] == '"'
&& ! strncmp (yytname[i] + 1, token_buffer,
strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
}
The yytname table is generated only if you use the
%token-table declaration. See section Bison Declaration Summary.
In an ordinary (non-reentrant) parser, the semantic value of the token must
be stored into the global variable 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. */ ...
When you are using multiple data types, 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;
}
then the code in yylex might look like this:
... yylval.intval = value; /* Put value onto Bison stack. */ return INT; /* Return the type of the token. */ ...
If you are using the `@n'-feature (see section Tracking Locations) in actions to keep track of the
textual locations of tokens and groupings, then you must provide this
information in 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.
By default, 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.
When you use the Bison declaration %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.
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
If the grammar file does not use the `@' constructs to refer to
textual positions, then the type YYLTYPE will not be defined. In
this case, omit the second argument; yylex will be called with
only one argument.
If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way. To do so, define the
macro YYPARSE_PARAM as a variable name. This modifies the
yyparse function to accept 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
%}
Then call the parser like this:
struct parser_control
{
int nastiness;
int randomness;
};
...
{
struct parser_control foo;
... /* Store proper data in foo. */
value = yyparse ((void *) &foo);
...
}
In the grammar actions, use expressions like this to refer to the data:
((struct parser_control *) parm)->randomness
If you wish to pass the additional parameter data to 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
%}
You should then define 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.
You can use `%pure-parser' to request a reentrant parser without
also using YYPARSE_PARAM. Then you should call yyparse
with no arguments, as usual.
yyerror
The Bison parser detects a parse error or syntax error
whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
macro 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:
void
yyerror (char *s)
{
fprintf (stderr, "%s\n", s);
}
After 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.
Here is a table of Bison constructs, variables and macros that are useful 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.
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:
1 + 5 * 3
and the next input token is a newline character, then the last three elements can be reduced to 15 via the rule:
expr: expr '*' expr;
Then the stack contains just these three elements:
1 + 15
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.
expr: term '+' expr
| term
;
term: '(' expr ')'
| term '!'
| NUMBER
;
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 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.
Suppose we are parsing a language which has if-then and if-then-else statements, with a pair of rules like this:
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
Here we assume that 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;
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:
if x then if y then win (); else lose; if x then do; if y then win (); end; else lose;
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 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
;
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):
expr: expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
...
;
Suppose the parser has seen the tokens `1', `-' and `2'; should it reduce them via the rule for the subtraction 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 difference. 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 %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.
In our example, we would want the following declarations:
%left '<' %left '-' %left '*'
In a more complete example, which supports other operators as well, we
would declare them in groups of equal precedence. For example, '+' is
declared with '-':
%left '<' '>' '=' NE LE GE %left '+' '-' %left '*' '/'
(Here 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.)
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, %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
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 %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
Now the precedence of UMINUS can be used in specific rules:
exp: ...
| exp '-' exp
...
| '-' exp %prec UMINUS
The function 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).
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 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); }
;
The error is an ambiguity: there is more than one way to parse a single
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); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
The intention here is to define a sequence which can contain either
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
;
Second, to prevent either a words or a redirects
from being empty:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: word
| words word
;
redirects:redirect
| redirects redirect
;
Sometimes reduce/reduce conflicts can occur that don't look warranted. Here is an example:
%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
;
It would seem that this grammar can be parsed with only a single token
of look-ahead: when a 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
;
This corrects the problem because it introduces the possibility of an
additional active rule in the context after the 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
;
Bison produces deterministic parsers that choose uniquely when to reduce and which reduction to apply based on a summary of the preceding input and on one extra token of lookahead. As a result, normal Bison handles a proper subset of the family of context-free languages. Ambiguous grammars, since they have strings with more than one possible sequence of reductions cannot have deterministic parsers in this sense. The same is true of languages that require more than one symbol of lookahead, since the parser lacks the information necessary to make a decision at the point it must be made in a shift-reduce parser. Finally, as previously mentioned (see section Mysterious Reduce/Reduce Conflicts), there are languages where Bison's particular choice of how to summarize the input seen so far loses necessary information.
When you use the `%glr-parser' declaration in your grammar file, Bison generates a parser that uses a different algorithm, called Generalized LR (or GLR). A Bison GLR parser uses the same basic algorithm for parsing as an ordinary Bison parser, but behaves differently in cases where there is a shift-reduce conflict that has not been resolved by precedence rules (see section Operator Precedence) or a reduce-reduce conflict. When a GLR parser encounters such a situation, it effectively splits into a several parsers, one for each possible shift or reduction. These parsers then proceed as usual, consuming tokens in lock-step. Some of the stacks may encounter other conflicts and split further, with the result that instead of a sequence of states, a Bison GLR parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse is. Additional input may indicate that a guess was wrong, in which case the appropriate stack silently disappears. Otherwise, the semantics actions generated in each stack are saved, rather than being executed immediately. When a stack disappears, its saved semantic actions never get executed. When a reduction causes two stacks to become equivalent, their sets of semantic actions are both saved with the state that results from the reduction. We say that two stacks are equivalent when they both represent the same sequence of states, and each pair of corresponding states represents a grammar symbol that produces the same segment of the input token stream.
Whenever the parser makes a transition from having multiple states to having one, it reverts to the normal LALR(1) parsing algorithm, after resolving and executing the saved-up actions. At this transition, some of the states on the stack will have semantic values that are sets (actually multisets) of possible actions. The parser tries to pick one of the actions by first finding one whose rule has the highest dynamic precedence, as set by the `%dprec' declaration. Otherwise, if the alternative actions are not ordered by precedence, but there the same merging function is declared for both rules by the `%merge' declaration, Bison resolves and evaluates both and then calls the merge function on the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the GLR parsing tree that permits the processing of any LALR(1) grammar in linear time (in the size of the input), any unambiguous (not necessarily LALR(1)) grammar in quadratic worst-case time, and any general (possibly ambiguous) context-free grammar in cubic worst-case time. However, Bison currently uses a simpler data structure that requires time proportional to the length of the input times the maximum number of stacks required for any prefix of the input. Thus, really ambiguous or non-deterministic grammars can require exponential time and space to process. Such badly behaving examples, however, are not generally of practical interest. Usually, non-determinism in a grammar is local--the parser is "in doubt" only for a few tokens at a time. Therefore, the current data structure should generally be adequate. On LALR(1) portions of a grammar, in particular, it is only slightly slower than with the default Bison parser.
The Bison parser stack can overflow if too many tokens are shifted and
not reduced. When this happens, the parser function yyparse
returns a nonzero value, pausing only to call yyerror to report
the overflow.
Becaue Bison parsers have growing stacks, hitting the upper limit usually results from using a right recursion instead of a left recursion, See section Recursive Rules.
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.
Because of semantical differences between C and C++, the LALR(1) parsers
in C produced by Bison by compiled as C++ cannot grow. In this precise
case (compiling a C parser as C++) you are suggested to grow
YYINITDEPTH. In the near future, a C++ output output will be
provided which addresses this issue.
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 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'
The fourth rule in this example says that an error followed by a newline
makes a valid addition to any 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. */
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:
primary: '(' expr ')'
| '(' error ')'
...
;
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
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.
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:
foo (x);
This looks like a function call statement, but if 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); /* redeclarebaras static variable */ static int foo (lose); /* redeclarefooas function */
Unfortunately, the name being declared is separated from the declaration construct itself by a complicated syntactic structure--the "declarator".
As a result, part of the 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:
initdcl:
declarator maybeasm '='
init
| declarator maybeasm
;
notype_initdcl:
notype_declarator maybeasm '='
init
| notype_declarator maybeasm
;
Here 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.
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 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
;
Here we assume that 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 prologue of the parser file
is needed to make it accessible to the actions (see section The prologue).
You must also write the code in yylex to obey the flag.
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:
stmt: expr ';'
| IF '(' expr ')' stmt { ... }
...
error ';'
{ hexflag = 0; }
;
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 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 ')'
...
If this rule acts within the 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.
Developing a parser can be a challenge, especially if you don't understand the algorithm (see section The Bison Parser Algorithm). Even so, sometimes a detailed description of the automaton can help (see section Understanding Your Parser), or tracing the execution of the parser can give some insight on why it behaves improperly (see section Tracing Your Parser).
As documented elsewhere (see section The Bison Parser Algorithm) Bison parsers are shift/reduce automata. In some cases (much more frequent than one would hope), looking at this automaton is required to tune or simply fix a parser. Bison provides two different representation of it, either textually or graphically (as a VCG file).
The textual file is generated when the options @option{--report} or @option{--verbose} are specified, see See section Invoking Bison. Its name is made by removing `.tab.c' or `.c' from the parser output file name, and adding `.output' instead. Therefore, if the input file is `foo.y', then the parser file is called `foo.tab.c' by default. As a consequence, the verbose output file is called `foo.output'.
The following grammar file, `calc.y', will be used in the sequel:
%token NUM STR %left '+' '-' %left '*' %% exp: exp '+' exp | exp '-' exp | exp '*' exp | exp '/' exp | NUM ; useless: STR; %%
@command{bison} reports:
calc.y: warning: 1 useless nonterminal and 1 useless rule calc.y:11.1-7: warning: useless nonterminal: useless calc.y:11.8-12: warning: useless rule: useless: STR calc.y contains 7 shift/reduce conflicts.
When given @option{--report=state}, in addition to `calc.tab.c', it creates a file `calc.output' with contents detailed below. The order of the output and the exact presentation might vary, but the interpretation is the same.
The first section includes details on conflicts that were solved thanks to precedence and/or associativity:
Conflict in state 8 between rule 2 and token '+' resolved as reduce. Conflict in state 8 between rule 2 and token '-' resolved as reduce. Conflict in state 8 between rule 2 and token '*' resolved as shift. ...
The next section lists states that still have conflicts.
State 8 contains 1 shift/reduce conflict. State 9 contains 1 shift/reduce conflict. State 10 contains 1 shift/reduce conflict. State 11 contains 4 shift/reduce conflicts.
The next section reports useless tokens, nonterminal and rules. Useless nonterminals and rules are removed in order to produce a smaller parser, but useless tokens are preserved, since they might be used by the scanner (note the difference between "useless" and "not used" below):
Useless nonterminals: useless Terminals which are not used: STR Useless rules: #6 useless: STR;
The next section reproduces the exact grammar that Bison used:
Grammar
Number, Line, Rule
0 5 $accept -> exp $end
1 5 exp -> exp '+' exp
2 6 exp -> exp '-' exp
3 7 exp -> exp '*' exp
4 8 exp -> exp '/' exp
5 9 exp -> NUM
and reports the uses of the symbols:
Terminals, with rules where they appear
$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM (258) 5
Nonterminals, with rules where they appear
$accept (8)
on left: 0
exp (9)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
Bison then proceeds onto the automaton itself, describing each state with it set of items, also known as pointed rules. Each item is a production rule together with a point (marked by `.') that the input cursor.
state 0
$accept -> . exp $ (rule 0)
NUM shift, and go to state 1
exp go to state 2
This reads as follows: "state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, exp). When the parser returns to this state right
after having reduced a rule that produced an exp, the control
flow jumps to state 2. If there is no such transition on a nonterminal
symbol, and the lookahead is a NUM, then this token is shifted on
the parse stack, and the control flow jumps to state 1. Any other
lookahead triggers a parse error."
Even though the only active rule in state 0 seems to be rule 0, the
report lists NUM as a lookahead symbol because NUM can be
at the beginning of any rule deriving an exp. By default Bison
reports the so-called core or kernel of the item set, but if
you want to see more detail you can invoke @command{bison} with
@option{--report=itemset} to list all the items, include those that can
be derived:
state 0
$accept -> . exp $ (rule 0)
exp -> . exp '+' exp (rule 1)
exp -> . exp '-' exp (rule 2)
exp -> . exp '*' exp (rule 3)
exp -> . exp '/' exp (rule 4)
exp -> . NUM (rule 5)
NUM shift, and go to state 1
exp go to state 2
In the state 1...
state 1
exp -> NUM . (rule 5)
$default reduce using rule 5 (exp)
the rule 5, `exp: NUM;', is completed. Whatever the lookahead (`$default'), the parser will reduce it. If it was coming from state 0, then, after this reduction it will return to state 0, and will jump to state 2 (`exp: go to state 2').
state 2
$accept -> exp . $ (rule 0)
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
$ shift, and go to state 3
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
In state 2, the automaton can only shift a symbol. For instance, because of the item `exp -> exp . '+' exp', if the lookahead if `+', it will be shifted on the parse stack, and the automaton control will jump to state 4, corresponding to the item `exp -> exp '+' . exp'. Since there is no default action, any other token than those listed above will trigger a parse error.
The state 3 is named the final state, or the accepting state:
state 3
$accept -> exp $ . (rule 0)
$default accept
the initial rule is completed (the start symbol and the end of input were read), the parsing exits successfully.
The interpretation of states 4 to 7 is straightforward, and is left to the reader.
state 4
exp -> exp '+' . exp (rule 1)
NUM shift, and go to state 1
exp go to state 8
state 5
exp -> exp '-' . exp (rule 2)
NUM shift, and go to state 1
exp go to state 9
state 6
exp -> exp '*' . exp (rule 3)
NUM shift, and go to state 1
exp go to state 10
state 7
exp -> exp '/' . exp (rule 4)
NUM shift, and go to state 1
exp go to state 11
As was announced in beginning of the report, `State 8 contains 1 shift/reduce conflict':
state 8
exp -> exp . '+' exp (rule 1)
exp -> exp '+' exp . (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Indeed, there are two actions associated to the lookahead `/': either shifting (and going to state 7), or reducing rule 1. The conflict means that either the grammar is ambiguous, or the parser lacks information to make the right decision. Indeed the grammar is ambiguous, as, since we did not specify the precedence of `/', the sentence `NUM + NUM / NUM' can be parsed as `NUM + (NUM / NUM)', which corresponds to shifting `/', or as `(NUM + NUM) / NUM', which corresponds to reducing rule 1.
Because in LALR(1) parsing a single decision can be made, Bison arbitrarily chose to disable the reduction, see section Shift/Reduce Conflicts. Discarded actions are reported in between square brackets.
Note that all the previous states had a single possible action: either shifting the next token and going to the corresponding state, or reducing a single rule. In the other cases, i.e., when shifting and reducing is possible or when several reductions are possible, the lookahead is required to select the action. State 8 is one such state: if the lookahead is `*' or `/' then the action is shifting, otherwise the action is reducing rule 1. In other words, the first two items, corresponding to rule 1, are not eligible when the lookahead is `*', since we specified that `*' has higher precedence that `+'. More generally, some items are eligible only with some set of possible lookaheads. When run with @option{--report=lookahead}, Bison specifies these lookaheads:
state 8
exp -> exp . '+' exp [$, '+', '-', '/'] (rule 1)
exp -> exp '+' exp . [$, '+', '-', '/'] (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
The remaining states are similar:
state 9
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp '-' exp . (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
state 10
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp '*' exp . (rule 3)
exp -> exp . '/' exp (rule 4)
'/' shift, and go to state 7
'/' [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
state 11
exp -> exp . '+' exp (rule 1)
exp -> exp . '-' exp (rule 2)
exp -> exp . '*' exp (rule 3)
exp -> exp . '/' exp (rule 4)
exp -> exp '/' exp . (rule 4)
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
'+' [reduce using rule 4 (exp)]
'-' [reduce using rule 4 (exp)]
'*' [reduce using rule 4 (exp)]
'/' [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts due to the lack of precedence of `/' wrt `+', `-', and `*', but also because the associativity of `/' is not specified.
If a Bison grammar compiles properly but doesn't do what you want when it
runs, the yydebug parser-trace feature can help you figure out why.
There are several means to enable compilation of trace facilities:
YYDEBUG
YYDEBUG to a nonzero value when you compile the
parser. This is compliant with POSIX Yacc. You could use
`-DYYDEBUG=1' as a compiler option or you could put `#define
YYDEBUG 1' in the prologue of the grammar file (see section The prologue).
%debug directive (see section Bison Declaration Summary). This is a Bison extension, which will prove
useful when Bison will output parsers for languages that don't use a
preprocessor. Useless POSIX and Yacc portability matter to you, this is
the preferred solution.
We suggest that you always enable the debug option so that debugging is always possible.
The trace facility outputs messages with macro calls of the form
YYFPRINTF (stderr, format, args) where
format and args are the usual printf format and
arguments. If you define YYDEBUG to a nonzero value but do not
define YYFPRINTF, <stdio.h> is automatically included
and YYPRINTF is defined to fprintf.
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.
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 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 *file, int type, YYSTYPE value)
{
if (type == VAR)
fprintf (file, " %s", value.tptr->name);
else if (type == NUM)
fprintf (file, " %d", value.val);
}
The usual way to invoke Bison is as follows:
bison infile
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'. It's also possible, in case you are writing C++ code instead of C in your grammar file, to name it `foo.ypp' or `foo.y++'. Then, the output files will take an extension like the given one as input (respectively `foo.tab.cpp' and `foo.tab.c++'). This feature takes effect with all options that manipulate filenames like `-o' or `-d'.
For example :
bison -d infile.yxx
will produce `infile.tab.cxx' and `infile.tab.hxx', and
bison -d -o output.c++ infile.y
will produce `output.c++' and `outfile.h++'.
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.
Operations modes:
bison -y $*
Tuning the parser:
YYDEBUG to 1 if it is not
already defined, so that the debugging facilities are compiled.
See section Tracing Your Parser.
%locations was specified. See section Bison Declaration Summary.
%name-prefix="prefix" was specified.
See section Bison Declaration Summary.
#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 as an independent source file in its own right.
%no-parser was specified. See section Bison Declaration Summary.
%token-table was specified. See section Bison Declaration Summary.
Adjust the output:
%defines was specified, i.e., write an extra output
file containing macro definitions for the token type names defined in
the grammar and the semantic value type YYSTYPE, as well as a few
extern variable declarations. See section Bison Declaration Summary.
%verbose was specified, i.e, specify prefix to use
for all Bison output file names. See section Bison Declaration Summary.
state
lookahead
state and augments the description of the automaton with
each rule's lookahead set.
itemset
state and augments the description of the automaton with
the full set of items for each state, instead of its core only.
%verbose was specified, i.e, write an extra output
file containing verbose descriptions of the grammar and
parser. See section Bison Declaration Summary.
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:
bison /debug/name_prefix=bar foo.y
is equivalent to the following command under POSIX.
bison --debug --name-prefix=bar foo.y
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'.
Several questions about Bison come up occasionally. Here some of them are addressed.
My parser returns with error with a `parser stack overflow' message. What can I do?
This question is already addressed elsewhere, See section Recursive Rules.
@$
@n
$$
$n
$accept
$end
$undefined
yylex are mapped. It cannot be used in the grammar, rather, use
error.
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
YYDEBUG
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 Data Type of Locations.
yyltype
YYMAXDEPTH
YYPARSE_PARAM
yyparse should
accept. See section Calling Conventions for Pure Parsers.
YYRECOVERING
YYSTACK_USE_ALLOCA
alloca. If defined to `0',
the parser will not use alloca but malloc when trying to
grow its internal stacks. Do not define YYSTACK_USE_ALLOCA
to anything else.
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 Tracing 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.
%debug
%defines
%dprec
%file-prefix="prefix"
%glr-parser
%left
%merge
%name-prefix="prefix"
%no-lines
#line directives in the
parser file. See section Bison Declaration Summary.
%nonassoc
%output="filename"
%prec
%pure-parser
%right
%start
%token
%token-table
%type
%union
These are the punctuation and delimiters used in Bison input:
if statement.
See section Languages and Context-Free Grammars.
yylex.
mfcalc.
Copyright (C) 2000 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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else
else, dangling
ltcalc
mfcalc
rpcalc
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