Bug 20172
| Summary: | yacc(1) byacc 1.9 fails to generate $default transitions for non-terminals | ||
|---|---|---|---|
| Product: | Base System | Reporter: | bertho <bertho> |
| Component: | bin | Assignee: | freebsd-bugs (Nobody) <bugs> |
| Status: | Closed FIXED | ||
| Severity: | Affects Only Me | ||
| Priority: | Normal | ||
| Version: | Unspecified | ||
| Hardware: | Any | ||
| OS: | Any | ||
Two comments:
1) The example should use a slightly different action for one rule to
make it more clear what happens. The yyclearin should eliminate the
lookahead and then all should continue as planned. However, byacc will
generate a syntax error on the input. Here is the rule as intended:
line: tTOK xpr ',' xpr {
if(yychar == tNL) {
printf("Success: Got tNL\n");
yyclearin;
}
}
2) After some email exchange with the original author it has become
apparent that he does not seem to consider the problem a bug. This means
that it is likely that byacc will be unusable for all heavy yacc-parsers
that manipulate lookahead tokens in an above fassion.
Greetings Bertho
State Changed From-To: open->closed yacc author does not consider this a bug |
Transitions on declared, but unused tokens are not included into state transitions. You normally would get $default transitions, but byacc insists on specifying all tokens. This leads to wrong results because a syntax-error is generated on a perfectly legal input. Version of byacc: yysccsid[] = \"@(#)yaccpar 1.9 (Berkeley) 02/21/93\";" The bug was also sent to the maintainer as mentioned in the source's README file. However, it is promised in that file that no rapid response is to be expected. Fix: You have to define a rule that captures all non-terminal tokens (i.e. all tokens in %token, %left and %right declarations that are otherwise unused). This can be tricky if such a rule has grammatical side-effects. A real fix requires a patch in byacc (but I am not familiar with its internals). OTOH, GNU bison does not suffer from this bug and can be used as an alternative. How-To-Repeat: Try this source, once without the tNL rule, and once with. A syntax error is generated if the rule is not implemented. When the rule is there, then the proper result is printed. To compile (use attached source below): $ byacc byacc-bug.y $ gcc -o byacc-bug y.tab.c $ ./byacc-bug ---- file byacc-bug.y ---- %{ #include <stdio.h> #include <stdlib.h> %} %token tTOK tNUM tNL %left '+' %% lines : line | lines line ; line : tTOK xpr ',' xpr { if(yychar == tNL) printf("Success: Got tNL\n"); } /* Declare NON-terminal as used */ /* | tNL */ ; xpr : xpr '+' xpr | tNUM ; %% int yylex(void) { static int tok[] = {tTOK, tNUM, ',', tNUM, tNL, 0}; static int idx = 0; #define NTOK (sizeof(tok)/sizeof(tok[0])) if(idx < NTOK) return tok[idx++]; return 0; } void yyerror(char *s) { printf("yyerror: %s\n", s); exit(1); } int main(void) { return yyparse(); }