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I learned an example of usage of sockets. In this example a client sends a request to a server to open a socket and then the server (listening to a specific port) opens a socket and everything is fine, socket is "opened" from both sides (client and server).
But it is still not clear to me how flexible is this stuff. For example, is it possible for the client to close an opened (from both ends) socket and to reopen it again (under condition that the server keeps the socket opened).
Is it possible for the server to "know" that a socket was closed on the client side? Is it possible for the client to know that a socket was closed on the server side?
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One more important thing to me. What happens if a application (no mater server or client) crashes, abnormally terminated, killed? Will it close all sockets opened on the side of the application?
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What if an application on one side of the socket is switched off (killed, closed, terminated) and then it is switched on again (on the same IP address and the same port). Should we create a new socket between the two applications or we can use the old socket (created before the crash).
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Once a socket has been closed, it is not available for further networking use (i.e. can't be reconnected or rebound). A new socket needs to be created. Closing this socket will also close the socket's InputStream and OutputStream. If this socket has an associated channel then the channel is closed as well.
To enable a socket to remain open, an application should set the l_onoff member to a nonzero value and set the l_linger member to the desired timeout in seconds. To disable a socket from remaining open, an application only needs to set the l_onoff member of the linger structure to zero.
An application should always have a matching call to closesocket for each successful call to socket to return any socket resources to the system. The linger structure maintains information about a specific socket that specifies how that socket should behave when data is queued to be sent and the closesocket function is called on the socket.
The closesocket call will only block until all data has been delivered to the peer or the timeout expires. If the connection is reset because the timeout expires, then the socket will not go into TIME_WAIT state. If all data is sent within the timeout period, then the socket can go into TIME_WAIT state.
A socket can be used for a lot of things for which the answers to these questions would change, but I'll assume you're talking about TCP.
For example, is it possible for the client to close an opened (from both ends) socket and to reopen it again (under condition that the server keeps the socket opened).
No, because TCP will perform a goodbye and you can't pick up the connection from there again. You'd have to do the three-way handshake again, and that starts a brand new connection.
Is it possible for the server to "know" that a socket was closed on the client side? Is it possible for the client to know that a socket was closed on the server side?
Yes. TCP can send out a goodbye packet or one side can time out and it's entirely possible to detect these scenarios in both cases.
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