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?

sepp2k sepp2k · Accepted Answer · 2017-10-16T20:34:18

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.

Undefined reference to 'yylex' bison error

1 Answers