基于STM32F4系列MCU利用LAN8720模块实现的DHCP、DNS以及网络授时功能（已验证）

Raw TCP/IP interface for lwIP Authors: Adam Dunkels, Leon Woestenberg, Christiaan Simons lwIP provides three Application Program's Interfaces (APIs) for programs to use for communication with the TCP/IP code: * low-level "core" / "callback" or "raw" API. * higher-level "sequential" API. * BSD-style socket API. The sequential API provides a way for ordinary, sequential, programs to use the lwIP stack. It is quite similar to the BSD socket API. The model of execution is based on the blocking open-read-write-close paradigm. Since the TCP/IP stack is event based by nature, the TCP/IP code and the application program must reside in different execution contexts (threads). The socket API is a compatibility API for existing applications, currently it is built on top of the sequential API. It is meant to provide all functions needed to run socket API applications running on other platforms (e.g. unix / windows etc.). However, due to limitations in the specification of this API, there might be incompatibilities that require small modifications of existing programs. ** Threading lwIP started targeting single-threaded environments. When adding multi- threading support, instead of making the core thread-safe, another approach was chosen: there is one main thread running the lwIP core (also known as the "tcpip_thread"). The raw API may only be used from this thread! Application threads using the sequential- or socket API communicate with this main thread through message passing. As such, the list of functions that may be called from other threads or an ISR is very limited! Only functions from these API header files are thread-safe: - api.h - netbuf.h - netdb.h - netifapi.h - sockets.h - sys.h Additionaly, memory (de-)allocation functions may be called from multiple threads (not ISR!) with NO_SYS=0 since they are protected by SYS_LIGHTWEIGHT_PROT and/or semaphores. Only since 1.3.0, if SYS_LIGHTWEIGHT_PROT is set to 1 and LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT is set to 1, pbuf_free() may also be called from another thread or an ISR (since only then, mem_free - for PBUF_RAM - may be called from an ISR: otherwise, the HEAP is only protected by semaphores). ** The remainder of this document discusses the "raw" API. ** The raw TCP/IP interface allows the application program to integrate better with the TCP/IP code. Program execution is event based by having callback functions being called from within the TCP/IP code. The TCP/IP code and the application program both run in the same thread. The sequential API has a much higher overhead and is not very well suited for small systems since it forces a multithreaded paradigm on the application. The raw TCP/IP interface is not only faster in terms of code execution time but is also less memory intensive. The drawback is that program development is somewhat harder and application programs written for the raw TCP/IP interface are more difficult to understand. Still, this is the preferred way of writing applications that should be small in code size and memory usage. Both APIs can be used simultaneously by different application programs. In fact, the sequential API is implemented as an application program using the raw TCP/IP interface. --- Callbacks Program execution is driven by callbacks. 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 function for setting the application connection state is: - void tcp_arg(struct tcp_pcb *pcb, void *arg) Specifies the program specific state that should be passed to all other callback functions. The "pcb" argument is the current TCP connection control block, and the "arg" argument is the argument that will be passed to the callbacks. --- TCP connection setup The functions used for setting up connections is similar to that of the sequential API and of the BSD socket API. A new TCP connection identifier (i.e., a protocol control block - PCB) is created with the tcp_new() function. This PCB can then be either set to listen for new incoming connections or be explicitly connected to another host. - struct tcp_pcb *tcp_new(void) Creates a new connection identifier (PCB). If memory is not available for creating the new pcb, NULL is returned. - err_t tcp_bind(struct tcp_pcb *pcb, ip_addr_t *ipaddr, u16_t port) Binds the pcb to a local IP address and port number. The IP address can be specified as IP_ADDR_ANY in order to bind the connection to all local IP addresses. If another connection is bound to the same port, the function will return ERR_USE, otherwise ERR_OK is returned. - struct tcp_pcb *tcp_listen(struct tcp_pcb *pcb) Commands a pcb to start listening for incoming connections. When an incoming connection is accepted, the function specified with the tcp_accept() function will be called. The pcb will have to be bound to a local port with the tcp_bind() function. The tcp_listen() function returns a new connection identifier, and the one passed as an argument to the function will be deallocated. The reason for this behavior is that less memory is needed for a connection that is listening, so tcp_listen() will reclaim the memory needed for the original connection and allocate a new smaller memory block for the listening connection. tcp_listen() may return NULL if no memory was available for the listening connection. If so, the memory associated with the pcb passed as an argument to tcp_listen() will not be deallocated. - struct tcp_pcb *tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog) Same as tcp_listen, but limits the number of outstanding connections in the listen queue to the value specified by the backlog argument. To use it, your need to set TCP_LISTEN_BACKLOG=1 in your lwipopts.h. - void tcp_accepted(struct tcp_pcb *pcb) Inform lwIP that an incoming connection has been accepted. This would usually be called from the accept callback. This allows lwIP to perform housekeeping tasks, such as allowing further incoming connections to be queued in the listen backlog. ATTENTION: the PCB passed in must be the listening pcb, not the pcb passed into the accept callback! - void tcp_accept(struct tcp_pcb *pcb, err_t (* accept)(void *arg, struct tcp_pcb *newpcb, err_t err)) Specified the callback function that should be called when a new connection arrives on a listening connection. - err_t tcp_connect(struct tcp_pcb *pcb, ip_addr_t *ipaddr, u16_t port, err_t (* connected)(void *arg, struct tcp_pcb *tpcb, err_t err)); Sets up the pcb to connect to the remote host and sends the initial SYN segment which opens the connection. The tcp_connect() function returns immediately; it does not wait for the connection to be properly setup. Instead, it will call the function specified as the fourth argument (the "connected" argument) when the connection is established. If the connection could not be properly established, either because the other host refused the connection or because the other host didn't answer, the "err" callback function of this pcb (registered with tcp_err, see below) will be called. The tcp_connect() function can return ERR_MEM if no memory is available for enqueueing the SYN segment. If the SYN indeed was enqueued successfully, the tcp_connect() function returns ERR_OK. --- Sending TCP data TCP data is sent by enqueueing the data with a call to