Use this mode if you do not run an OS on your system. #define NO_SYS to 1. Feed incoming packets to netif->input(pbuf, netif) function from mainloop, not from interrupt context. You can allocate a pbuf in interrupt context and put them into a queue which is processed from mainloop.
Call sys_check_timeouts() periodically in the mainloop.
Porting: implement all functions in sys_time, sys_prot and Compiler/platform abstraction.
You can only use "raw" APIs in this mode.
Sample code: