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  2. RPG400-L
  3. January 2006

Re: General socket question: Would server knows the connection hasbeen broken if client drop the connection without calling close()



Wouldn't the program receive an error indication if the other end went
away and it tried to do something? Especially if they are blocking on a
select or read?
No. If no packets arrive, it doesn't know whether that means that the conenction is dead, or whether that means that the remove computer has powered off. The only way it can know is if data is sent that says "that computer isn't available", which isn't possible if the network cable is cut, or if the device that would normally send the message has been powered off. 

So it waits on the recv() or select() forever.  (Or until your timeout...)
IF you send something, it would theoretically be able to tell that the data was not received on the other end (assuming that we're talking about a TCP connection). However, it would take awhile to come to that conclusion because it'd keep trying to resend the packet for awhile before it gave up and reported the error.
IP was designed as an "unreliable network". The idea is that packets can be discarded or lost at any time, and it's up to the application (and not the underlying protocol) to detect this and recover. Remember, TCP/IP was originally designed for use by the military under conditions such as an invasion or a nuclear war. It'd be impossible for the hardware and protocol to be able to handle every possible error -- such as a bomb hitting a station where the switching equipment is stored. Instead, the application needs to be vigilant. If you should have data from the remote side by now, but you haven't, you assume that the connection is dead and get on with your life.

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