extern "C" pid_t cygwin_winpid_to_pid (int winpid);

Given a Windows process ID, winpid, converts to the corresponding Cygwin process ID, if any. Returns -1 if Windows process ID does not correspond to a Cygwin process ID.
 

extern "C" cygwin_winpid_to_pid (int winpid);

pid_t mypid;

mypid = cygwin_winpid_to_pid (windows_pid);