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. * two higher-level "sequential" APIs: * netconn API * socket API (targeted at compatibility to posix- / BSD-sockets) You have to be careful when mixing raw and sequential APIs: raw API functions really only may be called from the main tcpip_thread. Also, registering the callbacks (or initializing a pcb) must be done in that context (e.g. at startup time in tcpip_init_callback or at runtime using tcpip_callback).