Manual

Chapter 8: Debugging Your Parser 95
’+’ shift, and go to state 4
’-’ shift, and go to state 5
’*’ shift, and go to state 6
’/’ shift, and go to state 7
’+’ [reduce using rule 4 (exp)]
’-’ [reduce using rule 4 (exp)]
’*’ [reduce using rule 4 (exp)]
’/’ [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts not only due to the lack of precedence of ‘/’ with
respect to ‘+’, ‘-’, and ‘*’, but also because the associativity of ‘/’ is not specified.
8.2 Tracing Your Parser
If a Bison grammar compiles properly but doesn’t do what you want when it runs, the
yydebug parser-trace feature can help you figure out why.
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 Section 3.1.1 [The Prologue], page 41).
the option ‘-t’, ‘--debug’
Use the ‘-t’ option when you run Bison (see Chapter 9 [Invoking Bison],
page 97). This is POSIX compliant too.
the directive ‘%debug’
Add the %debug directive (see Section 3.7.11 [Bison Declaration Summary],
page 59). This is a Bison extension, which will prove useful when Bison will
output parsers 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 debug 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 arguments. If you
define YYDEBUG to a nonzero value but do not define YYFPRINTF, is automatically
included and YYPRINTF 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:
• Each time the parser calls yylex, what kind of token was read.
• Each time a token is shifted, the depth and complete contents of the state stack (see
Section 5.5 [Parser States], page 75).