Linux: socket close() vs shutdown()

Question

So on linux, shutdown() can take a parameter SHUT_RD, SHUT_WR or SHUT_RDWR to shutdown only part of the communication channel. But in terms of the TCP messages sending to the peer, how does it work?

In TCP state machine, the closing works in a 4-way handshake fashion,

(1)           (2)
FIN---------->

   <----------ACK
   <----------FIN

ACK----------->

So what messages dose it send when I do a shutdown(sock, SHUT_RD) or shutdown(sock, SHUT_WR)?

Answer 1

1. shutdown(sd, SHUT_WR) sends a FIN which the peer responds to with an ACK. Any further attempts to write to the socket will incur an error. However the peer can still continue to send data.

2. shutdown(sd, SHUT_RD) sends nothing on the network: it just conditions the local API to return EOS for any subsequent reads on the socket. The behaviour when receiving data on a socket that has been shutdown for read is system-dependent: Unix will ACK it and throw it away; Linux will ACK it and buffer it, which will eventually stall the sender; Windows will issue an RST, which the sender sees as 'connection reset by peer'.

Answer 2

The FIN packets don't have to be symmetric. Each end sends FIN when its local writer has closed the socket.