Linker error

"What is the reason for such error?. The help is not very clear to me"

In what way unclear?
Here is the description:

L127: "The specified external symbol, requested in the specified module, has no corresponding PUBLIC symbol in any of the input files."
http://www.keil.com/support/man/docs/l166/l166_l127.htm

In your case, "The specified external symbol" is Convol; is there a Public definition of Convol anywhere in your Project?

christober,

That's a pretty standard error you'll see when you make a mistake writing a program. You've got a variable (or maybe a function) named "Convol" that you've declared as "extern" in the module REAL_T_FFT. The compiler accepts this and compiles that file, but leave it to the linker to find out which OTHER module has the actual definition of that variable. What's happened here is that the linker says that the name "Convol" isn't actually defined in ANY module in your project and so it can't proceed.

This could be caused by a typo, by you failing to include a library with your project, etc.

Here's a thread that illustrates what's going on:

http://www.8052.com/forum/read.phtml?id=85503

(it's about an 8051, but the principle is exactly the same)