What does return 0 do in the code and why is the yywrap function written without a body? - yacc

Question

I AM USING BISON AND FLEX.
What does return 0 do in case of the kcalc.l file that I have posted?
And I am not getting the use of yywrap without a body (i mean not literally but an empty body).The code is of a calculator without any variable managing and basic operations that can be done like addition subtraction multiplication division and handling of unary minus operator. I have been studying through the lex and yacc specifications but did not get any answer for the query I asked .
Kcal.y
%{
#include <stdio.h>
%}
%token Number
%left '-' '+'
%left '*' '/'
%nonassoc UMINUS
%%
statement: expression
{ printf(" result = %d\n", $1);} ;
expression: expression '+' expression
{ $$ = $1 + $3;
printf("Recognised'+'expression\n");
}
| expression '-' expression
{ $$ = $1 - $3;
printf("Recognised '-' expression\n");
}
| expression '*' expression
{ $$ = $1 * $3;
printf("Recognised '*' expression\n");
}
| expression '/' expression
{ if ($3 == 0)
printf ("divide by zero\n");
else
$$ = $1 / $3;
printf("Recognised '/' expression\n");
}
| '-' expression %prec UMINUS
{
$$ = - $2;
printf("Recognised paranthesized expression\n");
}
| '(' expression ')'
{
$$ = $2;
printf("Recognised paranthesized expression");
}
| Number { $$ = $1;
printf("Recognised a no.\n");
}
;
%%
int main(void)
{
return yyparse();
}
int yyerror (char *msg)
{
return fprintf(stderr,"Yacc :%s", msg);
}
yywrap()
{
}
 
kcalc.l
%{
#include "y.tab.h"
extern int yylval;
%}
%%
[0-9]+ { yylval = atoi(yytext);
printf("accepted the number : %d\n", yylval);
return Number; }
[ \t] { printf("skipped whitespace \n");}
\n { printf("reached end of line\n");
**return 0;**
}
. { printf("found other data \" %s\n", yytext);
return yytext[0];
}
%%

Answer 1

The return 0 notifies the end-of-input to the parser, so apparently the expression should be contained on a single line. The empty body of yywrap is just wrong. If you use -Wall with the gcc compiler it will give two warnings for yywrap:
kcal.y:54: warning: return type defaults to ‘int’
kcal.y:55: warning: control reaches end of non-void function
The first one because no result type for the function is specified (K&R style C), so it is assumed it should return an int. The second warning because it lacks a return statement for such an int.
Since a newline terminates the input, the chances of yywrap ever being called are slim. But it will be called if the input does not contain a newline. If by sheer accident the (more or less random) return value of yywrap were to be interpreted as 0 the tokenizer would end up in an infinite loop of repeatedly calling yywrap.