yacc.html

unix 下的C开发手册,还用详细的例程。

<i>yylval</i>.
<p>
If the parser and
<i>yylex()</i>
do not agree on these token numbers,
reliable communication between them cannot occur.
For (one character) literals, the token is simply the numeric
value of the character in the current character set.
The numbers for other tokens can either be chosen by
<i>yacc</i>,
or chosen by the user.
In either case, the
<b>#define</b>
construct of C is used to allow
<i>yylex()</i>
to return these numbers symbolically.
The
<b>#define</b>
statements are put into
the code file, and the header file
if that file is requested.
The set of characters permitted by
<i>yacc</i>
in an identifier is larger
than that permitted by C.
Token names found to contain such characters
will not be included in the
<b>#define</b>
declarations.
<p>
If the token numbers are chosen by
<i>yacc</i>,
the tokens other than literals will be assigned
numbers greater than 256,
although no order is implied.
A token can be explicitly assigned a number by following
its first appearance in the
declarations section with a number.
Names and literals not defined this way retain their default definition.
All assigned token numbers will be unique and distinct from the
token numbers used for literals.
If duplicate token numbers
cause conflicts in parser generation,
<i>yacc</i>
will report an error;
otherwise, it is unspecified whether the token assignment is
accepted or an error is reported.
<p>
The end of the input is marked by a special token called the
<i>endmarker ,</i>
which has a token number that is zero or negative.
(These values are invalid for any other token.)
All lexical analysers will return zero or negative as a token number
upon reaching the end of their input.
If the tokens up to, but excluding, the endmarker form a structure that
matches the start symbol, the parser will accept the input.
If the endmarker is seen in any other context, it will be considered an error.
<h5><a name = "tag_001_014_2954_010">&nbsp;</a>Completing the Program</h5>
In addition to
<i>yyparse()</i>
and
<i>yylex()</i>,
the functions
<i>yyerror()</i>
and
<i>main()</i>
are required to make a complete program.
The application can supply
<i>main()</i>
and
<i>yyerror()</i>,
or those routines can be obtained from the
<i>yacc</i>
library.
<h5><a name = "tag_001_014_2954_011">&nbsp;</a>Yacc Library</h5>
<xref type="5" name="yacclib"></xref>
The following functions appear only in the
<i>yacc</i>
library accessible through the
<b>-l&nbsp;y</b>
operand to
<i><a href="cc.html">cc</a></i>
or
<i><a href="c89.html">c89</a></i>;
they can therefore be redefined by a portable application:
<dl compact>

<dt><b>int main(void)</b><dd>
This function will call
<i>yyparse()</i>
and exit with an unspecified value.
Other actions within this function are unspecified.

<dt><b>int yyerror(const char *<i>s</i></b><dd>
This function
will write the NUL-terminated argument to standard error, followed by a
newline character.

</dl>
<p>
The order of the
<b>-l&nbsp;y</b>
and
<b>-l&nbsp;l</b>
operands given to
<i><a href="cc.html">cc</a></i>
or
<i><a href="c89.html">c89</a></i>
is significant;
the application must either provide its own
<i>main()</i>
function or ensure that
<b>-l&nbsp;y</b>
precedes
<b>-l&nbsp;l</b>.
<h5><a name = "tag_001_014_2954_012">&nbsp;</a>Debugging the Parser</h5>
The parser generated by
<i>yacc</i>
will have diagnostic facilities in it that can be
optionally enabled at either compile time or at run time (if enabled
at compile time).
The compilation of the runtime debugging code is under the control of
YYDEBUG,
a preprocessor symbol.
If
YYDEBUG
has a non-zero value, the debugging code will be included.
If its value is zero, the code will not be included.
<p>
In parsers where the debugging code has been included, the external
int yydebug
can be used to turn debugging on (with a non-zero value) and off (zero value)
at run time.
The initial value of
<i>yydebug</i>
will be zero.
<p>
When
<b>-t</b>
is specified,
the code file will be built such that, if
YYDEBUG
is not already defined at compilation time (using the
<i><a href="c89.html">c89</a></i>
<b>-D</b>
<b>YYDEBUG</b>
option, for example),
YYDEBUG
will be set explicitly to 1.
When
<b>-t</b>
is not specified,
the code file will be built such that, if
YYDEBUG
is not already defined, it will be set explicitly to zero.
<p>
The format of the debugging output is unspecified but
includes at least enough information to determine the shift and reduce actions,
and the input symbols.
It also provides information
about error recovery.
<h5><a name = "tag_001_014_2954_013">&nbsp;</a>Algorithms</h5>
<xref type="5" name="yaccalg"></xref>
The parser constructed by
<i>yacc</i>
implements an
LALR(1)
parsing algorithm as
documented in the literature.
It is unspecified whether the parser is table-driven
or direct-coded.
<p>
A parser generated by
<b>yacc</b>
will never request an input symbol from
<i>yylex()</i>
while in a state where the only actions other than
the error action are reductions by a single rule.
<p>
The literature of parsing theory defines these concepts.
<h5><a name = "tag_001_014_2954_014">&nbsp;</a>Limits</h5>
The
<i>yacc</i>
utility may have several internal tables.
The minimum maximums for these tables are shown in
the following table.
The exact meaning of these values is implementation-dependent.
The implementation will define the relationship between these
values and between them and any error messages that the
implementation may generate should it run out of space for any
internal structure.
An implementation may combine groups of these
resources into a single pool as long as the total available to
the user does not fall below the
sum of the sizes specified by this section.
<pre>
<table  bordercolor=#000000 border=1 align=center>
<tr valign=top><th align=center><b>Limit</b>
<th align=center><b>Minimum<br>Maximum</b>
<th align=center><b>Description</b>
<tr valign=top><td align=left>{NTERMS}
<td align=left>126
<td align=left>Number of tokens.
<tr valign=top><td align=left>{NNONTERM}
<td align=left>200
<td align=left>Number of non-terminals.
<tr valign=top><td align=left>{NPROD}
<td align=left>300
<td align=left>Number of rules.
<tr valign=top><td align=left>{NSTATES}
<td align=left>600
<td align=left>Number of states.
<tr valign=top><td align=left>{MEMSIZE}
<td align=left>5200
<td align=left> Length of rules. The total length, in names (tokens and non-terminals), of all the rules of the grammar. The left-hand side is counted for each rule, even if it is not explicitly repeated, as specified in 
<tr valign=top><td align=left>{ACTSIZE}
<td align=left>4000
<td align=left> Number of actions. &quot;Actions&quot; here (and in the description file) refer to parser actions (shift, reduce, and so on) not to semantic actions defined in 
</table>
</pre>
<h6 align=center><xref table="Internal Limits in <I>yacc</i>"></xref>Table: Internal Limits in <i>yacc</i></h6>
<p>
</blockquote><h4><a name = "tag_001_014_2955">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>Successful completion.

<dt>&gt;0<dd>An error occurred.

</dl>
</blockquote><h4><a name = "tag_001_014_2956">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
If any errors are encountered, the run is aborted and
<i>yacc</i>
exits with a non-zero status.
Partial code files and header files files may be produced.
The summary information in the description file
will always be produced if the
<b>-v</b>
flag is present.
</blockquote><h4><a name = "tag_001_014_2957">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
Historical implementations experience name conflicts on the names
yacc.tmp,
yacc.acts,
yacc.debug,
y.tab.c,
y.tab.h
and
y.output
if more
than one copy of
<i>yacc</i>
is running in a single directory at one time.
The
<b>-b</b>
option was added to overcome this problem.
The related problem of allowing multiple
<i>yacc</i>
parsers to be placed in the same file was addressed
by adding a
<b>-p</b>
option to override the previously hard-coded
yy
variable prefix.
<p>
The description of the
<b>-p</b>
option specifies the minimal set of function and variable names
that cause conflict when multiple parsers are linked together.
YYSTYPE
does not need to be changed.
Instead, the programmer can use
<b>-b</b>
to give the
header files for different parsers different names, and then the file
with the
<i>yylex()</i>
for a given parser can include the header for that parser.
Names such as
<i>yyclearerr</i>
do not need to be changed because
they are used only in the actions;
they do not have linkage.
It is possible that an implementation will have other names,
either internal ones for implementing things such as
<i>yyclearerr</i>,
or providing non-standard features that it wants to change with
<b>-p</b>.
<p>
Unary operators that are the same token as a binary operator in general
need their precedence adjusted.
This is handled by the
<b>%prec</b>
advisory symbol associated with the particular grammar rule defining
that unary operator.
See
<xref href=yaccgram><a href="#tag_001_014_2954_004">
Grammar Rules in yacc
</a></xref>.
Applications are not required to use this operator for unary
operators, but the grammars that do not require it are rare.
<br>
</blockquote><h4><a name = "tag_001_014_2958">&nbsp;</a>EXAMPLES</h4><blockquote>
Access to the
<i>yacc</i>
library is obtained with library search operands to
<i><a href="cc.html">cc</a></i>
or
<i><a href="c89.html">c89</a></i>.
To use the
<i>yacc</i>
library
<i>main()</i>:
<pre>
<code>
c89 y.tab.c -l y
</code>
</pre>
<p>
Both the
<i><a href="lex.html">lex</a></i>
library and the
<i>yacc</i>
library contain
<i>main()</i>.
To access the
<i>yacc</i>
<i>main()</i>:
<pre>
<code>
c89 y.tab.c lex.yy.c -l y -l l
</code>
</pre>
<br>
This ensures that the
<i>yacc</i>
library is searched first, so that its
<i>main()</i>
is used.
<p>
The historical
<i>yacc</i>
libraries have contained two simple functions that are
normally coded by the application programmer.
These library functions are similar to the following code:
<pre>
<code>
#include &lt;locale.h&gt;
int main(void)
{
    extern int yyparse();

    setlocale(LC_ALL, "");

    /* If the following parser is one created by lex, the
       application must be careful to ensure that LC_CTYPE
       and LC_COLLATE are set to the POSIX locale.  */
    (void) yyparse();
    return (0);
}

#include &lt;stdio.h&gt;

int yyerror(const char *msg)
{
    (void) fprintf(stderr, "%s\n", msg);
    return (0);
}
</code>
</pre>
</blockquote><h4><a name = "tag_001_014_2959">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about
parts of this interface definition to the IEEE PASC Shell and Utilities Working Group
which is identifying the corrections.
A future revision of this specification will align with
IEEE Std. 1003.2b when finalised.
</blockquote><h4><a name = "tag_001_014_2960">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="cc.html">cc</a></i>,
<i><a href="c89.html">c89</a></i>,
<i><a href="lex.html">lex</a></i>.
</blockquote><hr size=2 noshade>
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