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8.4.1 Enabling Traces

There are several means to enable compilation of trace facilities:

the macro YYDEBUG
Define the macro 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 The Prologue).

If the %define variable api.prefix is used (see Multiple Parsers in the Same Program), for instance ‘%define api.prefix x’, then if CDEBUG is defined, its value controls the tracing feature (enabled if and only if nonzero); otherwise tracing is enabled if and only if YYDEBUG is nonzero.

the option -t (POSIX Yacc compliant)
the option --debug (Bison extension)
Use the ‘-t’ option when you run Bison (see Invoking Bison). With ‘%define api.prefix {c}’, it defines CDEBUG to 1, otherwise it defines YYDEBUG to 1.
the directive ‘%debug
Add the %debug directive (see Bison Declaration Summary). This Bison extension is maintained for backward compatibility with previous versions of Bison.
the variable ‘parse.trace
Add the ‘%define parse.trace’ directive (see parse.trace), or pass the -Dparse.trace option (see Bison Options). This is a Bison extension, which is especially useful for languages that don't use a preprocessor. Unless POSIX and Yacc portability matter to you, this is the preferred solution.

We suggest that you always enable the trace 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 variadic arguments. If you define YYDEBUG to a nonzero value but do not define YYFPRINTF, <stdio.h> is automatically included and YYFPRINTF 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:

To make sense of this information, it helps to refer to the automaton description file (see Understanding Your Parser). 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 implementation file is a C/C++/Java program and you can use 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.