The GDB in Emacs can't display source correctly

[wmaple]

Hello!

My Emacs runs under windows, and I use it for coding and debugging.(I have 
already create a development
environment with Cygwin.) When debugging under the Emacs, it is the common way 
that there are two
buffers, one accepts commands of gdb, the other displays the source code. But, 
when I start the program
to be debugged, there is no source code displayed in the source buffer. How can 
I do?