How to receive data over Ethernet using LWIP, UDP - udp

Question

I'm trying to send data to and from my computer and an STM32H745 over Ethernet using LwIP and UDP. I have successfully configured the card and right now I can send data from the card to a Python script running on the computer. However, I don't understand how udp_recv works <udp,lwip> or how to receive data with UDP on LwIP in general, and I can't find examples that do just that. Where is the data being received? Should I even use udp_recv?
In the main loop I run MX_LWIP_Process, which runs ethernetif_input which somehow handles the received data, but I don't understand where it puts it.
Below is the main code, just for reference.
const char* message = "a";
HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_1); // orange
ip_addr_t PC_IPADDR;
IP_ADDR4(&PC_IPADDR, 192, 168, 1, 200);
u16_t port = 8000;
struct udp_pcb* my_udp = udp_new();
struct pbuf* udp_buffer = NULL;
/* Infinite loop */
for (;; )
{
MX_LWIP_Process();
HAL_GPIO_TogglePin(GPIOE, GPIO_PIN_1); // orange
HAL_Delay(1000);
udp_buffer = pbuf_alloc(PBUF_TRANSPORT, strlen(message), PBUF_RAM);
if (udp_buffer != NULL)
{
memcpy(udp_buffer->payload, message, strlen(message));
udp_sendto(my_udp, udp_buffer,&PC_IPADDR, port);
pbuf_free(udp_buffer);
}
//udp_recv (struct udp_pcb *pcb, udp_recv_fn recv, void *recv_arg)
}

Answer 1

udp_recv() does not actually receive UDP datagrams (despite its name). It registers a callback function that will then be called by MX_LWIP_Process() when a datagram has been buffered. It would better be called udp_set_recv_callback(), but it is what it is.
To that end you should call it once before your executive loop:
udp_bind( my_udp, IP_ADDR_ANY, port ) ;
udp_recv( my_udp, udp_receive_callback, NULL ) ;
/* Infinite loop */
for (;; )
{
// Run the CubeMX LwIP stack
MX_LWIP_Process() ;
...
}
Where udp_receive_callback is a function that will be invoked on receipt of a datagram:
void udp_receive_callback( void* arg, // User argument - udp_recv `arg` parameter
struct udp_pcb* upcb, // Receiving Protocol Control Block
struct pbuf* p, // Pointer to Datagram
const ip_addr_t* addr, // Address of sender
u16_t port ) // Sender port
{
// Process datagram here (non-blocking code)
...
// Must free receive pbuf before return
pbuf_free(p);
}
Examples include:
https://gist.github.com/iwanbk/1399729
https://github.com/STMicroelectronics/STM32CubeF2/blob/master/Projects/STM322xG_EVAL/Applications/LwIP/LwIP_UDP_Echo_Client/Src/udp_echoclient.c
Documentation can be found at https://www.nongnu.org/lwip/2_0_x/group__udp__raw.html