Assuming you are using lwIP, do you really mean the function "udp_recv" which is only binding a udp receive (recv) function to a certain pcb = setting udp callback function?
In my lwIP application input data is polled through MX_LWIP_Process() in the main while loop, which again calls ... many functions:
I once tracked that down for IP4 to get a better overview for the "processing path":
Network Interface Layer:
main.c: poll: MX_LWIP_Process()
-> lwip.c: MX_LWIP_Process() -> ethernetif_input(&netif)
-> ethernetif.c ethernetif_input(&netif) -> p = low_level_input(netif)
-> ethernetif.c low_level_input(netif) -> netif->input(p, netif)
-> ethernet.c netif->input(p, netif) = ethernet_input(p, netif)
-> ethernet.c ethernet_input(p, netif) -> check: header, length, address (ARP), type -> ip4_input(p, netif)
Internet Layer:
-> ip4.c ip4_input(p, netif) -> address / broadcast? type? -> RAW, ICMP, IGMP, UDP, TCP ?
-> ICMP: icmp_input(p, netif)
-> IGMP: igmp_input(p, netif)
Transport Layer:
-> UDP: udp_input(p, netif) <<== so this might be of interest to you?
-> TCP: tcp_input(p, netif)
