[rtl] The FRANK example

[Nejib Ben Hadj-Alouane]

Hello,   My question is in reference to the frank_app.c file in the frank example RTL2.pre3.   There are macros FD_ZERO, FD_SET, FD_ISSET that I am unfamiliar with. I could guess what that part of the code is doing, but I need the details. Could someone give me some pointers (which .h files to see...)   Thanks.