Newbie problems with Yacc

[suszterpatt]

As much as I hate asking for help with homework, I feel it's justified in the light that nobody ever bothered to teach us a single word about Flex or Yacc. As such, I'm completely in the dark, and I'm going on whatever tutorials I can scrape together from the internet.

I'm supposed to write a lexer and syntax checker for a simple, self-made programming language. I started with Yacc's input file, pasted at the end of the post (code tags aren't working :\).

When I run yacc (bison to be exact), it says "start symbol program does not derive a sentence", and proceeds to tell me that all my nonterminals and rules are useless. Being a complete beginner, I'm stuck.

Any help is appreciated: the more the merrier, since I don't really have any idea of what I'm supposed to do and why what I'm doing is good or bad.

Thanks.



%token VALUE VAR IF ELSIF ELSE ENDIF CASE OF DEFAULT ENDCASE FOR TO BY ENDFOR WHILE ENDWHILE PRINT
%left EQ NE '<' '>' LE GE
%left '+' '-'
%left '*' '/' '%'
%{
#include <stdio.h>
void yyerror(char *);
int yylex(void);
int sym[260];
%}
%%
program:
program stment '\n' { printf("%d\n", $1); }
;
stment:
assign
| if
| case
| floop
| wloop
| print
|
;
assign:
VAR '=' expr ';' { }
;
if:
IF cond program stment ifend { }
;
ifend:
ENDIF
| ELSIF cond program stment ifend
| ELSE program stment ENDIF
;
case:
CASE expr OF VALUE ':' program stment casend
;
casend:
ENDCASE
| OF VALUE ':' program stment casend
| DEFAULT ':' program stment ENDCASE
;
floop:
fromto step program stment ENDFOR
;
fromto:
FOR VAR '=' VALUE TO VALUE
;
step:
BY VALUE
|
;
wloop:
WHILE cond program stment ENDWHILE
;
print:
PRINT expr
;
expr:
liter
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
| expr '%' expr
| '(' expr ')'
;
cond:
expr EQ expr
| expr NE expr
| expr '<' expr
| expr '>' expr
| expr LE expr
| expr GE expr
;
liter:
VAR
| VALUE
;
%%
void yyerror(char *s) {
fprintf(stderr, "%s\n", s);
return 0;
}
int main(void) {
yyparse();
return 0;
}

 

[degibson]

Thats a bloody bummer. I learned everything I know about yacc and bison (I know VERY little about them) from this tutorial.

Offhand, I don't notice any C-language directives after your grammar rules, but it could just be the lousy forum formatting.

 

[suszterpatt]

I actually found the solution: I messed up the grammar.

program:
program stment '\n' { printf("%d\n", $1); }
;

There's no way out of the recursion, so it will keep generating more and more statements ad infinitum. 🙂


There's no C code yet because I want to get the program to at least compile properly, then I can worry about getting stuff done.


However, I have a new problem now. bison and flex both compile their respective inputs without errors, but when I go to link them together:

cc lex.yy.c syndata.tab.c -o comp

I get 40 or so of errors that all look like this:

lex.yy.c:640: error: called object '260' is not a function
lex.yy.c:641: error: expected ';' before '{' token
lex.yy.c: In function 'yy_get_next_buffer':
lex.yy.c:922: error: called object '260' is not a function
lex.yy.c:923: error: expected ';' before 'yy_fatal_error'
lex.yy.c:926: error: called object '260' is not a function
lex.yy.c:927: error: expected ';' before '{' token

and so on. I'm completely baffled here, since for instance, line 640 in lex.yy.c is as follows:

if ( !(yy_init) )

nothing more. While yy_int is indeed not a function (instead a static int), that should not be a problem for an if statement.

 

[srishti]

how to write a code for checking syntax error for end of message and end of chunk

Bumping a nearly five year-old thread isn't going to get you many responses. Suggest you post this as a new question, and add more detail.

Markbnj
Programming mod