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<HTML><HEAD><!-- This HTML file has been created by texi2html 1.44 from /opt/src/gnu/bison-1.25/bison.texinfo on 30 June 1997 --><TITLE>Bison 1.25 - Debugging Your Parser</TITLE></HEAD><BODY>Go to the <A HREF="bison_1.html">first</A>, <A HREF="bison_10.html">previous</A>, <A HREF="bison_12.html">next</A>, <A HREF="bison_15.html">last</A> section, <A HREF="index.html">table of contents</A>.<HR><H1><A NAME="SEC86" HREF="index.html#SEC86">Debugging Your Parser</A></H1><P><A NAME="IDX187"></A><A NAME="IDX188"></A><A NAME="IDX189"></A><A NAME="IDX190"></A></P><P>If a Bison grammar compiles properly but doesn't do what you want when itruns, the <CODE>yydebug</CODE> parser-trace feature can help you figure out why.</P><P>To enable compilation of trace facilities, you must define the macro<CODE>YYDEBUG</CODE> when you compile the parser. You could use<SAMP>`-DYYDEBUG=1'</SAMP> as a compiler option or you could put <SAMP>`#defineYYDEBUG 1'</SAMP> in the C declarations section of the grammar file (see section <A HREF="bison_6.html#SEC36">The C Declarations Section</A>). Alternatively, use the <SAMP>`-t'</SAMP> option whenyou run Bison (see section <A HREF="bison_12.html#SEC87">Invoking Bison</A>). We always define <CODE>YYDEBUG</CODE> so thatdebugging is always possible.</P><P>The trace facility uses <CODE>stderr</CODE>, so you must add <CODE>#include<stdio.h></CODE> to the C declarations section unless it is already there.</P><P>Once you have compiled the program with trace facilities, the way torequest a trace is to store a nonzero value in the variable <CODE>yydebug</CODE>.You can do this by making the C code do it (in <CODE>main</CODE>, perhaps), oryou can alter the value with a C debugger.</P><P>Each step taken by the parser when <CODE>yydebug</CODE> is nonzero produces aline or two of trace information, written on <CODE>stderr</CODE>. The tracemessages tell you these things:</P><UL><LI>Each time the parser calls <CODE>yylex</CODE>, what kind of token was read.<LI>Each time a token is shifted, the depth and complete contents of thestate stack (see section <A HREF="bison_8.html#SEC77">Parser States</A>).<LI>Each time a rule is reduced, which rule it is, and the complete contentsof the state stack afterward.</UL><P>To make sense of this information, it helps to refer to the listing fileproduced by the Bison <SAMP>`-v'</SAMP> option (see section <A HREF="bison_12.html#SEC87">Invoking Bison</A>). This fileshows the meaning of each state in terms of positions in various rules, andalso what each state will do with each possible input token. As you readthe successive trace messages, you can see that the parser is functioningaccording to its specification in the listing file. Eventually you willarrive at the place where something undesirable happens, and you will seewhich parts of the grammar are to blame.</P><P>The parser file is a C program and you can use C debuggers on it, but it'snot easy to interpret what it is doing. The parser function is afinite-state machine interpreter, and aside from the actions it executesthe same code over and over. Only the values of variables show where inthe grammar it is working.</P><P><A NAME="IDX191"></A>The debugging information normally gives the token type of each tokenread, but not its semantic value. You can optionally define a macronamed <CODE>YYPRINT</CODE> to provide a way to print the value. If you define<CODE>YYPRINT</CODE>, it should take three arguments. The parser will pass astandard I/O stream, the numeric code for the token type, and the tokenvalue (from <CODE>yylval</CODE>).</P><P>Here is an example of <CODE>YYPRINT</CODE> suitable for the multi-functioncalculator (see section <A HREF="bison_5.html#SEC30">Declarations for <CODE>mfcalc</CODE></A>):</P><PRE>#define YYPRINT(file, type, value) yyprint (file, type, value)static voidyyprint (file, type, value) FILE *file; int type; YYSTYPE value;{ if (type == VAR) fprintf (file, " %s", value.tptr->name); else if (type == NUM) fprintf (file, " %d", value.val);}</PRE><HR>Go to the <A HREF="bison_1.html">first</A>, <A HREF="bison_10.html">previous</A>, <A HREF="bison_12.html">next</A>, <A HREF="bison_15.html">last</A> section, <A HREF="index.html">table of contents</A>.</BODY></HTML>⌨️ 快捷键说明
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