I'm getting an error: undefined reference to yyparse() - windows

Question

I've run the following code in flex and got yyparse() error, The target output I'm hoping to get is :
P
font-size: 8px
font-family: arial
color: red
TD
background-color: yellow
border: 1px solid black
Here is my program:
%{
#include<stdio.h>
#include"yousuf.h"
int yylineno;
%}
%%
"<"[^>]*> ;
[P] {printf("P\n",yytext);}
"font-size: 8 px" {printf("font-size: 8 px\n"); return FONTSIZE;}
"font-family: arial" {printf("font-family: arial\n"); return FONTFAMILY;}
"color: red" {printf("color: red\n"); return COLOR;}
[TD] {printf("TD\n",yytext);}
"background-color: yellow" {printf("background-color: yellow\n"); return BACKGROUNDCOLOR;}
"border: 1px solid black" {printf("border: 1px solid black"); return BORDER;}
[ \t\n] ;
. ;
%%
int main(int argc, char **argv)
{
char string[]= "<style>P{font-size: 8px; font-family: arial; color: red;}TD{background-color: yellow; border: 1px solid black;}</style>";
YY_BUFFER_STATE buffer= yy_scan_string(string);
yyparse();
yy_delete_buffer(buffer);
return 0;
}
int yywrap(void)
{
return 0;
}
Also, is there any other way to feed input to flex other than fopen("filename.txt","r"); because it doesn't seem to read the file and take it as input.

Answer 1

The function defined by (f)lex is called yylex. yyparse is generated by bison/yacc, which you do not appear to be using.
Flex takes its input from yyin, declared as:
extern FILE* yyin;
So it's not sufficient to just call fopen; you need to assign the return value to yyin. You should also make sure that the fopen call succeeded:
yyin = fopen("filename.txt","r");
if (yyin == NULL) {
fprintf(stderr, "Could not open 'filename.txt': %s\n", strerror(errno));
exit(1);
}
By the way, [TD] recognises either a T or a D. So with input TD, your rule will print TD twice.