Manual

Chapter 8: Debugging Your Parser 89
8 Debugging Your Parser
Developing a parser can be a challenge, especially if you don’t understand the algorithm
(see Chapter 5 [The Bison Parser Algorithm], page 71). Even so, sometimes a detailed description
of the automaton can help (see Section 8.1 [Understanding Your Parser], page 89),
or tracing the execution of the parser can give some insight on why it behaves improperly
(see Section 8.2 [Tracing Your Parser], page 95).
8.1 Understanding Your Parser
As documented elsewhere (see Chapter 5 [The Bison Parser Algorithm], page 71) Bison
parsers are shift/reduce automata. In some cases (much more frequent than one would
hope), looking at this automaton is required to tune or simply fix a parser. Bison provides
two different representation of it, either textually or graphically (as a VCG file).
The textual file is generated when the options ‘--report’ or ‘--verbose’ are specified,
see See Chapter 9 [Invoking Bison], page 97. Its name is made by removing ‘.tab.c’ or ‘.c’
from the parser output file name, and adding ‘.output’ instead. Therefore, if the input
file is ‘foo.y’, then the parser file is called ‘foo.tab.c’ by default. As a consequence, the
verbose output file is called ‘foo.output’.
The following grammar file, ‘calc.y’, will be used in the sequel:
%token NUM STR
%left ’+’ ’-’
%left ’*’
%%
exp: exp ’+’ exp
| exp ’-’ exp
| exp ’*’ exp
| exp ’/’ exp
| NUM
;
useless: STR;
%%
bison reports:
calc.y: warning: 1 useless nonterminal and 1 useless rule
calc.y:11.1-7: warning: useless nonterminal: useless
calc.y:11.10-12: warning: useless rule: useless: STR
calc.y: conflicts: 7 shift/reduce
When given ‘--report=state’, in addition to ‘calc.tab.c’, it creates a file
‘calc.output’ with contents detailed below. The order of the output and the exact
presentation might vary, but the interpretation is the same.
The first section includes details on conflicts that were solved thanks to precedence
and/or associativity:
Conflict in state 8 between rule 2 and token ’+’ resolved as reduce.
Conflict in state 8 between rule 2 and token ’-’ resolved as reduce.
Conflict in state 8 between rule 2 and token ’*’ resolved as shift.