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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 additional C code 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 numeric code for the type
of token it has just found, or 0 for end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
numeric code for that token type. So yylex can use the name
to indicate that type. See section Symbols, Terminal and Nonterminal.
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
So yylex can simply return that character code. The null character
must not be used this way, because its code is zero and that is what
signifies end-of-input.
Here is an example showing these things:
yylex ()
{
...
if (c == EOF) /* Detect end of file. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for `+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the lex
utility can be used without change as the definition of yylex.
In an ordinary (nonreentrant) 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 Special Features for Use in Actions) in
actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in yylex. The function
yyparse expects to find the textual location of a token just parsed
in the global variable yylloc. So yylex must store the
proper data in that variable. The value of yylloc is a structure
and you need only initialize the members that are going to be used by the
actions. The four members are called first_line,
first_column, last_line and last_column. Note that
the use of this feature makes the parser noticeably slower.
The data type of yylloc has the name YYLTYPE.
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.
yylex (lvalp, llocp)
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.
yyerror
The Bison parser detects a parse error or syntax error
whenever it reads a token which cannot satisfy any syntax rule. A
action in the grammar can also explicitly proclaim an error, using the
macro YYERROR (see section Special Features for Use in Actions).
The Bison parser expects to report the error by calling an error
reporting function named yyerror, which you must supply. It is
called by yyparse whenever a syntax error is found, and it
receives one argument. For a parse error, the string is normally
"parse error".
If you define the macro YYERROR_VERBOSE in the Bison declarations
section (see section The Bison Declarations Section), then Bison provides a more verbose
and specific error message string instead of just plain "parse
error". It doesn't matter what definition you use for
YYERROR_VERBOSE, just whether you define it.
The parser can detect one other kind of error: stack overflow. This
happens when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
parser extends its stack automatically up to a very large limit. But
if overflow happens, yyparse calls yyerror in the usual
fashion, except that the argument string is "parser stack
overflow".
The following definition suffices in simple programs:
yyerror (s)
char *s;
{
fprintf (stderr, "%s\n", s);
}
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.
If the macro is used when it is not valid, such as when there is a look-ahead token already, then it reports a syntax error with a message `cannot back up' and performs ordinary error recovery.
In either case, the rest of the action is not executed.
yychar when there is no look-ahead token.
yyerror, and does not print any message. If you
want to print an error message, call yyerror explicitly before
the `YYERROR;' statement. See section Error Recovery.
yyparse.) When there is
no look-ahead token, the value YYEMPTY is stored in the variable.
See section Look-Ahead Tokens.
struct {
int first_line, last_line;
int first_column, last_column;
};
Thus, to get the starting line number of the third component, use `@3.first_line'.
In order for the members of this structure to contain valid information,
you must make yylex supply this information about each token.
If you need only certain members, then yylex need only fill in
those members.
The use of this feature makes the parser noticeably slower.
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