Syntax error, unexpected TRUE, expecting '{'

Question

I'm writing a parser in Bison for a basic compiler (and then expand it to contain subroutines and dynamic memory allocation). The grammar is defined in Appendix A of the dragon book. My Flex scanner works--I ran my test files through it and it printed out all the correct tokens it found. Sorry about the strange formatting below. My bison and flex modes in emacs are a little haywire so I'm just using C mode until I fix.

%{
#include <stdio.h>

#define YYERROR_VERBOSE 1066

  extern FILE* yyin ;
  extern int yylineno ;
  extern char* yytext ;
  extern int yylex() ;
  extern void yyerror() ;
  int YYDEBUG = 1 ;

%}

/* Tokens */
%token AND BASIC BREAK DO ELSE EQ FALSE
%token GREQ ID IF INDEX LEEQ MINUS NOEQ NUM OR REAL TEMP TRUE WHILE

/* Grammar rules from Appendix A */
%%
program: block { printf( "Matched program\n" ) ; }
;

block: '{' decls stmts '}' { printf( " Matched block\n" ) ; }
;

decls: decls decl |
;

decl: type ID ';'
;

type: type '[' NUM ']' | BASIC
;

stmts: stmts stmt |
;

stmt: loc '=' bool ';'
| IF '(' bool ')' stmt
| IF '(' bool ')' stmt ELSE stmt
| WHILE '(' bool ')' stmt
| DO stmt WHILE '(' bool ')' ';'
| BREAK ';'
| block
;

loc: loc '[' bool ']' | ID
;

bool: bool OR join | join
;

join: join AND equality | equality
;

equality: equality EQ rel | equality NOEQ rel | rel
;

rel: expr '<' expr | expr LEEQ expr | expr GREQ expr | expr '>' expr | expr
;

expr: expr '+' term | expr '-' term | term
;

term: term '*' unary | term '/' unary | unary
;

unary: '!' unary | '-' unary | factor
;

factor: '(' bool ')' | loc | NUM | REAL | TRUE | FALSE
;

%%

/*
 * Additional C Code
 * Main Routine
 * yyerror()
 */
int main( int argc, char *argv[] ) {

  int i ;

  if ( argc < 2 ) {
    fprintf( stderr, "No input files.\n\n" ) ;
    return 1 ;
  }

  for ( i = 0 ; i < argc ; i++ ) {

    yyin = fopen( argv[i], "r" ) ;

    if ( !yyin ) {
      fprintf( stderr, "Error opening file: %s.\n\n", argv[i] ) ;
      return 1 ;
    }

    yyparse() ;
  }
  return 0 ; 
}

void yyerror( char *s ) {

  /* fprintf( stderr, "Error parsing - %d: %s at %s\n", yylineno, s, yytext ) ; */
  fprintf( stderr, "Error parsing - %d: %s at %s\n", yylineno, s, yytext ) ;

}

I feel like I might be missing something important. I don't think it's the rules. I set yyin to be the input files provided in argv[]. The errors are

Error parsing - 1: syntax error, unexpected TRUE, expecting '{' at

Error parsing - 1: syntax error, unexpected FALSE, expecting '{' at ELF

Any help would be greatly appreciated!

EDIT: If I change the main function to not set yyin (so yyparse just reads from stdin), I get this:

{ int x; }

Error parsing - 1: syntax error, unexpected TRUE, expecting '{' at {

I don't understand how that is wrong...

The error occurs at the very beginning of input. I get those same errors with every input file, some of which are as simple as { int x; } or { do (x) while (true); } Which is syntactically correct but not semantically. — Kizaru
How did that compile with retrun 0? Is this your exact code? — jamesdlin
Not sure I'd use "FALSE" like that. Won't bison #define FALSE to something like 264? (Assuming you're using bison -d.) Any situation where FALSE is, well, not false, seems like asking for trouble. — bstpierre
(I stand by that statement for TRUE too. Too common a name, you'll get in trouble with it.) — bstpierre

bstpierre bstpierre · Accepted Answer · 2010-08-05T19:07:32

When I run your sample input above using a stub yylex, the input program matches. I'm making the assumption that "int" tokenizes as BASIC. (You also need to fix "retrun".)

You need to debug your lexer. Either attach a debugger so you can see what it is returning, or put a print statement at the end of yylex.

This replaces everything in the bottom section:

%%
FILE* yyin = NULL;
int yylineno = 0;
char* yytext = NULL;
int main()
{
  yyparse() ;
  return 0 ; 
}

void yyerror( char *s )
{
  fprintf( stderr, "Error parsing - %d: %s at %s\n", yylineno, s, yytext ) ;
}

int yylex()
{
    static int i = 0;
    static int tokens[] = { '{', BASIC, ID, ';', '}' };

    int tok = tokens[i];
    yylineno++;
    i++;
    return tok;
}

Syntax error, unexpected TRUE, expecting '{'

2 Answers