:?: what i smeant my linker error in c++.?
why it causes in c++?

In my experience it means the linker isn't able to find a function/variable in any of the header files included. Often it means the necessary header file isn't included or the function/variable isn't available in the files included.

Usually when I've recieved this error, I'ved noticed my function prototype's formal parameters don't match my function's actual parameters.

Or you've just completely forgotten to code the function...

Anyway, that's a good reason why to code each function separately, and not try to code 3-4 of them before testing out the program.

no.no.. I just used the keyword extern, it is giving the error

no.no.. I just used the keyword extern, it is giving the error

Maybe your extern variable is referring to nothing or can't be found. Be sure to have that variable declared globally in other source file.

another possibility is that the variable or
function is defined in more than one file.
in this case your error message would look
like
'multiple definition of <identifier>'
or
'one or more multiply defined symbols'

if you tried to declare a variable in another file

extern int first = 0 ; // definition - multiply defined
  extern int second ; // declaration - ok