How to Read Multiple Lines of input file for arithmetic yacc program? - yacc

Question

I am new to compilers and learning to make calculator that inputs multiple line equations (one equation each line) from a .txt file. And I am facing the problem of segmentation fault.
YACC Code :
%{
#include <stdio.h>
#include <string.h>
#define YYSTYPE int /* the attribute type for Yacc's stack */
extern int yylval; /* defined by lex, holds attrib of cur token */
extern char yytext[]; /* defined by lex and holds most recent token */
extern FILE * yyin; /* defined by lex; lex reads from this file */
%}
%token NUM
%%
Begin : Line
| Begin Line
;
Line : Calc {printf("%s",$$); }
;
Calc : Expr {printf("Result = %d\n",$1);}
Expr : Fact '+' Expr { $$ = $1 + $3; }
| Fact '-' Expr { $$ = $1 - $3; }
| Fact '*' Expr { $$ = $1 * $3; }
| Fact '/' Expr { $$ = $1 / $3; }
| Fact { $$ = $1; }
| '-' Expr { $$ = -$2; }
;
Fact : '(' Expr ')' { $$ = $2; }
| Id { $$ = $1; }
;
Id : NUM { $$ = yylval; }
;
%%
void yyerror(char *mesg); /* this one is required by YACC */
main(int argc, char* *argv){
char ch;
if(argc != 2) {printf("useage: calc filename \n"); exit(1);}
if( !(yyin = fopen(argv[1],"r")) ){
printf("cannot open file\n");exit(1);
}
yyparse();
}
void yyerror(char *mesg){
printf("Bad Expression : %s\n", mesg);
exit(1); /* stop after the first error */
}
LEX Code :
%{
#include <stdio.h>
#include "y.tab.h"
int yylval; /*declared extern by yacc code. used to pass info to yacc*/
%}
letter [A-Za-z]
digit [0-9]
num ({digit})*
op "+"|"*"|"("|")"|"/"|"-"
ws [ \t\n]
other .
%%
{ws} { /* note, no return */ }
{num} { yylval = atoi(yytext); return NUM;}
{op} { return yytext[0];}
{other} { printf("bad%cbad%d\n",*yytext,*yytext); return '?'; }
%%
/* c functions called in the matching section could go here */
I am trying to print the expression along with result.
Thanks In Advance.

Answer 1

In your parser, you have:
Line : Calc {printf("%s",$$); }
Now $$ is the semantic value which the rule is computing, and you haven't assigned anything to it. So it would not be unreasonable to assume that it is undefined, which would be bad, but in fact it does have a value because of the default rule $$ = $1;. All the same, it would be much more readable to write
printf("%s", $1);
But that's not correct, is it? After all, you have
#define YYSTYPE int
so all semantic types are integers. But you're telling printf that $1 is a string (%s). printf will believe you, so it will go ahead and try to dereference the int as though it were a char*, with predictable results (i.e., a segfault).
You are probably using a compiler which is clever enough to notice the fact that you are trying to print an int with a %s format code. But either you haven't asked the compiler to help you or you are ignoring its advice.
Always compile with warnings enabled. If you are using gcc or clang, that means putting -Wall in the command line. (If you are using some other compiler, find out how to produce warnings. It will be documented.) And then read the warnings and fix them before trying to run the program.
There are several other errors and/or questionable practices in your code. Your grammar is inaccurate (why do you use fact as the left-hand operand of every operator?), and despite your comment, your lexical scanner ignores newline characters, so there is no way the parser can know whether expressions are one per line, two per line, or spread over multiple lines; that will make it hard to use the calculator as a command-line tool.
There is no need to define the lex macro digit; (f)lex recognizes the Posix character class [[:digit:]] (and others, documented here) automatically. Nor is it particularly useful to define the macro num. Overuse of lex macros makes your program harder to read; it is usually better to just write the patterns out in place:
[[:digit:]]+ { yylval = atoi(yytext); return NUM; }
which would be more readable and less work both for you and for anyone reading your code. (If your professor or tutor disagrees, I'd be happy to discuss the matter with them directly.)