Boolean Expressions Evaluator Error - c

Question

I am working on making a boolean expression evaluator in lex and c, however I am having problem finding the error in my code.
When I run the code the parser.c file throws the error("expected end-of-file"). This means that the program is not reading the end of file character, and I cannot find where this is going wrong.
I have attached the problematic code below. If, to solve this issue you need to see some more of the code please let me know, and I will be happy to post them also. I have been stuck on this problem for several weeks not, any help would be greatly appreciated.
Lexer.h
#ifndef ____lexer__
#define ____lexer__
#include <stdio.h>
#define AND 258
#define OR 259
#define NOT 260
#define TRUE 261
#define FALSE 262
#define DONE 300
#define NONE (-1)
int lexan();
extern int value;
extern int lineNo;
extern char lexbuf[];
extern FILE *fileSource;
#endif /* defined(____lexer__) */
lexer.lex
%{
#include <ctype.h>
#include <unistd.h>
#include "lexer.h"
#include "error.h"
int value;
int lineNo;
%}
%option noyywrap
%%
['''\t']* {}
['\n']* { lineNo++; }
<<EOF>> {
return DONE;
}
"True" {return (TRUE);}
"False" {return (FALSE);}
"or" {return (OR);}
"and" {return (AND);}
"not" {return (NOT);}
.|\n {
value = NONE;
int temp = (int)(yytext[0]);
return (temp);
}
%%
int lexan()
{
yyin = fileSource;
int result = yylex();
return result;
}
parser.c
#include <stdlib.h>
#include "parser.h"
#include "lexer.h"
#include "error.h"
#include "interpreter.h"
static int lookahead;
static void stmts();
static void stmt();
static void assign();
static void expr();
static void match();
void parse()
{
lookahead = lexan();
stmts();
lookahead = lexan();
if(lookahead != DONE)
error("expected end-of-file");
}
static void stmts()
{
while (lookahead != DONE)
{
if(lookahead == AND || lookahead == OR || lookahead == NOT || lookahead == TRUE || lookahead == FALSE)
{
stmt();
}
else
break;
}
}
static void stmt()
{
switch (lookahead)
{
case AND:
emit(AND);
match(AND);
break;
case OR:
emit(OR);
match(OR);
break;
case NOT:
emit(NOT);
match(NOT);
break;
default:
assign();
}
}
static void assign()
{
switch (lookahead)
{
case TRUE:
emit(TRUE);
match(TRUE);
break;
case FALSE:
emit(FALSE);
match(FALSE);
default:
error("syntax error");
}
}
void match(int t)
{
if (lookahead == t)
{
lookahead = lexan();
}
else
error("syntax error");
}

Answer 1

Your immediate problem is that lookahead is already DONE if stmts() returns successfully. Calling yylex again after it hits EOF is undefined behaviour, but you really don't want to do the call because the logic is incorrect; lookahead hasn't yet been matched when stmts() returns so the caller should attempt to match it, not overwrite it with a new token.
Fixing that is the least of your problems, though. The logic of stmts and stmt are also wrong; you need to reread whatever guide/text you are using to write a recursive descent parser.
By the way, if you don't have an <<EOF>> rule, yylex will return 0 when it hits the end of input. You should consider making use of this behaviour rather than producing a custom return code.