bison.texinfo

GNU的词法/语法分析器bison源码

Here are the C and Bison declarations for the reverse polish notation
calculator.  As in C, comments are placed between @samp{/*@dots{}*/}.

@example
/* Reverse polish notation calculator.  */

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

%token NUM

%% /* Grammar rules and actions follow.  */
@end example

The declarations section (@pxref{Prologue, , The prologue}) contains two
preprocessor directives and two forward declarations.

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

The @code{#include} directive is used to declare the exponentiation
function @code{pow}.

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

The second section, Bison declarations, provides information to Bison
about the token types (@pxref{Bison Declarations, ,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 @code{NUM}, the token
type for numeric constants.

@node Rpcalc Rules
@subsection Grammar Rules for @code{rpcalc}

Here are the grammar rules for the reverse polish notation calculator.

@example
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;          @}
;
%%
@end example

The groupings of the rpcalc ``language'' defined here are the expression
(given the name @code{exp}), the line of input (@code{line}), and the
complete input transcript (@code{input}).  Each of these nonterminal
symbols has several alternate rules, joined by the @samp{|} 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.  @xref{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 @code{$$} stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to @code{$$} is the
main job of most actions.  The semantic values of the components of the
rule are referred to as @code{$1}, @code{$2}, and so on.

@menu
* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::
@end menu

@node Rpcalc Input
@subsubsection Explanation of @code{input}

Consider the definition of @code{input}:

@example
input:    /* empty */
        | input line
;
@end example

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 @dfn{left recursive} since @code{input} appears always as the
leftmost symbol in the sequence.  @xref{Recursion, ,Recursive Rules}.

The first alternative is empty because there are no symbols between the
colon and the first @samp{|}; this means that @code{input} can match an
empty string of input (no tokens).  We write the rules this way because it
is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
It's conventional to put an empty alternative first and write the comment
@samp{/* empty */} in it.

The second alternate rule (@code{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 @code{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.

@node Rpcalc Line
@subsubsection Explanation of @code{line}

Now consider the definition of @code{line}:

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

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 @code{exp} grouping is the value of @code{$1} because the @code{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 @code{$$}.  As
a consequence, the semantic value associated with the @code{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.

@node Rpcalc Expr
@subsubsection Explanation of @code{expr}

The @code{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.

@example
exp:      NUM
        | exp exp '+'     @{ $$ = $1 + $2;    @}
        | exp exp '-'     @{ $$ = $1 - $2;    @}
        @dots{}
        ;
@end example

We have used @samp{|} to join all the rules for @code{exp}, but we could
equally well have written them separately:

@example
exp:      NUM ;
exp:      exp exp '+'     @{ $$ = $1 + $2;    @} ;
exp:      exp exp '-'     @{ $$ = $1 - $2;    @} ;
        @dots{}
@end example

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,
@code{$1} refers to the first component @code{exp} and @code{$2} refers to
the second one.  The third component, @code{'+'}, has no meaningful
associated semantic value, but if it had one you could refer to it as
@code{$3}.  When @code{yyparse} recognizes a sum expression using this
rule, the sum of the two subexpressions' values is produced as the value of
the entire expression.  @xref{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 @code{$1} into @code{$$}.
This is what happens in the first rule (the one that uses @code{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:

@example
exp   : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example

@noindent
means the same thing as this:

@example
exp:      NUM
        | exp exp '+'    @{ $$ = $1 + $2; @}
        | @dots{}
;
@end example

@noindent
The latter, however, is much more readable.

@node Rpcalc Lexer
@subsection The @code{rpcalc} Lexical Analyzer
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing

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.  @xref{Lexical, ,The Lexical
Analyzer Function @code{yylex}}.

Only a simple lexical analyzer is needed for the @acronym{RPN}
calculator.  This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{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, @code{NUM} becomes a macro for @code{yylex} to use.

The semantic value of the token (if it has one) is stored into the
global variable @code{yylval}, which is where the Bison parser will look
for it.  (The C data type of @code{yylval} is @code{YYSTYPE}, which was
defined at the beginning of the grammar; @pxref{Rpcalc Decls,
,Declarations for @code{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:

@example
@group
/* 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>
@end group

@group
int
yylex (void)
@{
  int c;

  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == '\t')
    ;
@end group
@group
  /* Process numbers.  */
  if (c == '.' || isdigit (c))
    @{
      ungetc (c, stdin);
      scanf ("%lf", &yylval);
      return NUM;
    @}
@end group
@group
  /* Return end-of-input.  */
  if (c == EOF)
    return 0;
  /* Return a single char.  */
  return c;
@}
@end group
@end example

@node Rpcalc Main
@subsection The Controlling Function
@cindex controlling function
@cindex main function in simple example

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

@example
@group
int
main (void)
@{
  return yyparse ();
@}
@end group
@end example

@node Rpcalc Error
@subsection The Error Reporting Routine
@cindex error reporting routine

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

@example
@group
#include <stdio.h>

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

After @code{yyerror} returns, the Bison parser may recover from the error
and