observing on-off socket problem

recently I encounter a problem. I am using two programs A and B, developed by someone else, which use TCP sockets to communicate each other, A is server, B is client. That is what I observed: when I start both A and B, they run and communicate with each other, if I first kill A, then restart A again, now by checking the processes, A is successfully launched, but cannot be connected by B, no matter I restart B. however, If I continue to kill this non-detectable A and start A again, it can be detected by B.

At the same time, if I close B's sock开发者_JAVA百科et before kill A, then when I start A and B, they work very well.

what the problem might be and is there some way to see the opened sockets when I kill A?

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It depends on the OS you are using.
lsof -p <pid> is quite common on UNIX and lets you list all file descriptors used by a process. netstat is probably available and will also list opened ports.

This is probably due to the TIME_WAIT state. When you kill A, the server port is still allocated by the OS and can be reused only if A sets a specific flag when opening server port to be able to reuse this port (SO_REUSEADDR). Otherwise, A won't be able to reuse the server port until it is closed by the OS (can take a few minutes, reason why when you continue to kill A, at some point, the port is available again). I don't know what A is doing if it cannot open the server port because of that.