Within the switch statement of the tcp_usrreq function, the case for PRU_LISTEN checks if the socket is bound to the port.

在tcp_usrreq函数的切换语句中，PRU_LISTEN的实例检查套接字是否绑定到端口。

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The switch case for PRU_SHUTDOWN calls the tcp_usrclosed function, which updates the state of the socket, depending on the current state.

PRU_SHUTDOWN的切换实例根据当前的状态调用tcp_usrclosed函数，以更新套接字的状态。

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The send socket call causes a context switch from user mode to kernel mode.

send套接字调用引发了从用户模式到内核模式的上下文切换。

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