Non thread-safe APIs, callback style for maximum performance and minimum memory footprint. Program execution is driven by callbacks functions, which are then invoked by the lwIP core when activity related to that application occurs. A particular application may register to be notified via a callback function for events such as incoming data available, outgoing data sent, error notifications, poll timer expiration, connection closed, etc. An application can provide a callback function to perform processing for any or all of these events. Each callback is an ordinary C function that is called from within the TCP/IP code. Every callback function is passed the current TCP or UDP connection state as an argument. Also, in order to be able to keep program specific state, the callback functions are called with a program specified argument that is independent of the TCP/IP state. The raw API (sometimes called native API) is an event-driven API designed to be used without an operating system that implements zero-copy send and receive. This API is also used by the core stack for interaction between the various protocols. It is the only API available when running lwIP without an operating system.