TCP/IP thread

When interacting with the network stack using the sequential API, all operations are not handled by the calling thread, but instead are passed to the lwIP network processing (TCP/IP) thread. Inter-thread communication is used inside lwIP to ensure that at the point the API function returns, operation is either complete, or for asynchronous operations, under way.

For example, to register a timeout callback the tcpip_timeout() function can be used from client threads to cross the thread boundary into the sequential TCP/IP thread. However, since the actual timeout callback handler registered will be executed within the sequential TCP/IP thread context, it can subsequently directly call the lwIP internal sys_timeout() if it needs to re-schedule its callback.

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