Traditional sockets implementations have an irritating limitation, atleast for TCP sockets: it is impossible to gracefully and cleanly shutdown listening (server) sockets.The problem is that kernels accept new TCP connections for listeningsockets without asking your server; accept() doesn't actually accept the connection, it just gives you the next one that the kernel hasalready accepted on your behalf.

This means that to shut down cleanly,you need a two-stage process: first you tell the kernel to stop acceptingnew connections