Unexpected behavior from lexer and grammar for simple calculator - c

Question

I'm starting on the whole world of Flex and Bison. So I followed a tutorial to write this l file for flex:
%{
#include <stdio.h>
#include <stdlib.h>
void yyerror(char *);
#include "y.tab.h"
%}
%%
/******************** RULES ********************/
/* One letter variables */
[a-z] {
yylval = *yytext - 'a'; // This is to return a number between 0 and 26 representting the letter variable.
printf("VAR: %s\n",yytext);
return VARIABLE;
}
/* Integer constants */
[0-9]+ {
yylval = atoi(yytext);
printf("INT: %d\n",yylval);
return INTEGER;
}
/* Operators */
[-+()=/*\n]+ { printf("OPR: %s\n",yytext); return *yytext; /*\n is considered an operator because it signals the end of a statement*/ }
/* This skips white space and tab chararcters */
[ \t] ;
/* Anything esle is not allowed */
. yyerror("Invalid character found");
/***************** SUBROUTINES *****************/
%%
int yywrap(void){
return 1;
}
And this is the grammar:
/***************** DEFINITIONS *****************/
%token INTEGER VARIABLE
%left '+' '-'
%left '*' '/'
%{
void yyerror(char *);
int yylex(void);
int sym[26];
%}
%%
/******************** RULES ********************/
program:
program statement '\n'
|
;
statement:
expr { printf("EXPR: %d\n", $1); }
| VARIABLE '=' expr { sym[$1] = $3; }
;
expr:
INTEGER
| VARIABLE { $$ = sym[$1]; }
| expr '+' expr { $$ = $1 + $3; }
| expr '-' expr { $$ = $1 - $3; }
| expr '*' expr { $$ = $1 * $3; }
| expr '/' expr { $$ = $1 / $3; }
| '(' expr ')' { $$ = $2; }
;
%%
/***************** SUBROUTINES *****************/
void yyerror(char *s){
printf("%s\n",s);
}
int main(void) {
yyparse();
return 0;
}
And serveral question arise. The first comes when compiling. This is how I compile:
bison -d bas.y -o y.tab.c
flex bas.l
gcc y.tab.h lex.yy.c y.tab.c -o bas_fe
Which gives me two warnings like this:
bas.y:24:7: warning: incompatible implicit declaration of built-in function ‘printf’
expr { printf("EXPR: %d\n", $1); }
^
bas.y: In function ‘yyerror’:
bas.y:39:4: warning: incompatible implicit declaration of built-in function ‘printf’
printf("%s\n",s);
Now, they are warnings and the print work, but I found it odd, since I have clearly included the libraries for use of the printf function.
My real question arises from my interaction with the program. This is the console output:
x = (3+5)
VAR: x
OPR: =
OPR: (
INT: 3
OPR: +
INT: 5
x
OPR: )
VAR: x
syntax error
Several questions arise from this.
1) Upon inputting x = (3+5) the program printout does not include the ')' Why?
2) When inputting x (expected output would have been 8) only then the ')' appears. Why?
3) And then there is the "syntax error" message. I'm assuming the message is automatically generated within the code of y.tab.c. Can it be changed to somthing more meaningful? Am I right in assuming that the syntax error is because the program found ) and newline and the variable and that this DOES NOT correspond to a program statement, as defined by the grammar?

Answer 1

I have clearly included the libraries for use of the printf function.
You included stdio.h in your flex file, but not in your bison file. And the warnings about printf being undeclared are from your bison file, not your flex file.
When you compile multiple files with gcc (or any other C compiler), the files are compiled independently and then linked together. So your command
gcc y.tab.h lex.yy.c y.tab.c -o bas_fe
does not concatenate the three files and compile them as a single unit. Rather, it compiles the three files independently, including uselessly compiling the header file y.tab.h.
What you should do is add a prolog block including #include <stdio.h> to your bas.y file.
[-+()=/*\n]+ {... return *yytext; ...}
This flex pattern matches any number of characters from the set [-+()=/*\n]. So in the input x=(3+5)\n, the )\n is being matched as a single token. However, the action returns *yytext, the first character of yytext, effectively ignoring the \n. Since your grammar requires \n, that creates a syntax error.
Simply remove the repetition operator from the pattern.
Can the error message be changed to something more meaningful?
If you have a reasonably modern bison, add the declaration
%error-verbose
to the beginning of your bison file.