Undefined reference to 'yylex' bison error

Question

Hi I am new to bison and flex and I am trying to create a simple calculator however I seem to be having errors when trying to compile.

The following is my flex .l file (named a.l):

%{
#include "a.tab.h"
%}
number  [0-9]+

%%

"+"     {return ADD;}
"-"     {return SUB;}
"*"     {return MUL;}
"/"     {return DIV;}
"|"     {return ABS;}
{number}     { return NUMBER;}
\n      {return EOF;}
[ \t]   { }
.    {printf("Mystery Character %s\n", yytext); }

%%

and the following is my bison .y file(named a.y):

%{
#include <stdio.h>
int yyparse(void);
%}

%token NUMBER ADD SUB MUL DIV ABS EOL

%%

calclist: /*nothing*/ | calclist exp EOL { printf("=%d\n, $1") };

exp: factor
    | exp ADD factor { $$ = $1 + $3; }
    | exp SUB factor { $$ = $1 - $3; }
    ;

factor: term |
        | factor MUL term { $$ = $1 * $3; }
        | factor DIV term { $$ = $1 / $3; }

term: NUMBER
    | ABS term { $$ = $2 >= 0? $2 : - $2; }
    ;

%%

int main(void)
{
    return(yyparse());
}

void yyerror(char *s)
{
    fprintf(stderr, "Error : Exiting %s\n", s);
}

This is what I write in the console:

flex a.l
bison a.y
gcc a.tab.c -lfl -o a.exe

The error I get is:

a.tab.c:(.text+0x1f2): undefined reference to `yylex'
collect2.exe: error: ld returned 1 exit status

I also get the following warnings:

a.tab.c: In function 'yyparse':
a.tab.c:595:16: warning: implicit declaration of function 'yylex' [-Wimplicit-function-declaration]
# define YYLEX yylex ()
               ^
a.tab.c:1240:16: note: in expansion of macro 'YYLEX'
       yychar = YYLEX;
                ^~~~~
a.tab.c:1396:7: warning: implicit declaration of function 'yyerror' [-Wimplicit-function-declaration]
       yyerror (YY_("syntax error"));
       ^~~~~~~
a.y: At top level:
a.y:32:6: warning: conflicting types for 'yyerror'
 void yyerror(char *s)
      ^~~~~~~
a.tab.c:1396:7: note: previous implicit declaration of 'yyerror' was here
       yyerror (YY_("syntax error"));
       ^~~~~~~

Would anybody be able to explain to me why these errors/warnings may be occuring?

Answer 1

There will be two generated C files, one generated by flex and one by bison. The one created by flex will be called "lex.yy.c" and you'll need to compile that one as well.