A close of a connection means that both peers agree that they don't want to communicate any more with each other. If only one peer closes the socket it just communicates with the FIN that it will no longer send any data. It also communicates to the local OS that it is no longer willing to receive any data - here close(sock) differs from shutdown(sock,SHUT_WR).
A call to read in the server will return 0 if the client closed or shutdown the socket, since this meant that no more data are send from client to server. The server then might decide to also close or shutdown the socket. But might also decide to send more data to the client, since the socket is not closed yet in the server. If the server sends more data the client will respond with a RST (connection reset) since it is not expecting any more data. When receiving the RST the server side socket gets automatically closed too.
while ((len = read(sockfd, buf, 256)) > 0){
In most cases read will return 0 here if the client has closed the connection. It will return -1 in case an error on the socket occurred, notably Connection reset. This might happen if the server has written data to the client while the client has already closed the connection (i.e. race condition), in which case the client will return with a RST. This error will be delivered on the next syscall on the socket, i.e. the read.
Check the edit please. Does
read()returns -1 or 0?@milanHrabos: "Does read() returns -1 or 0?" - see updated answer.
@milanHrabos: "Isn't it that the other side has to somehow handle the RST response?" - there is no reply to a RST. The handling when receiving a RST on a socket is to close the socket since obviously the connection to the client is no longer working.
@milanHrabos:
readblocks only when there are currently no bytes to read but there might be in the future, i.e. when it make sense to wait for new data. If the client explicitly says that it will no longer send more data (i.e. FIN) thenreadwill no longer wait for more data either - i.e. return 0.@milanHrabos It blocks while the socket receive buffer in the kernel is empty and no FIN has been received and no error has occurred.