Re: [lwip-users] UDP send example

[Patrick Klos]

On 7/10/2020 6:44 AM, Jon Bean wrote:
> Hi
>
> I am trying to setup an application on a Texas Instruments TIVA micro 
> controller. What I am trying to do is first receive a UDP packet from 
> a pc to the board. I then want to be able to send packets back when I 
> have data to send. I have managed to do this using the UDP echo 
> example. But this send a packet back when it gets a response. I tried 
> calling my send function from a loop in main. But it just crashed. I 
> asked on the TI form and was told I need to call the udp_sendto 
> function from an interrupt. I cahnged the code so that it is aclled 
> from the LWIP timer handler. But its still crashing. Can anyone see an 
> issue with the code below or have a working example? Thanks
>
> The udp_addr and udp_port vars are what I set when I get the receive 
> packet from the host. The new_udp_pcb is created in my setup function.
>
> void eth_udp_tx()
> {
>     struct pbuf *p;
>     uint8_t buf[4];
>
>     p = pbuf_alloc(PBUF_TRANSPORT, 4, PBUF_POOL);
>     p->len = 4;
>     p->tot_len = 4;
>     p->payload = buf;
>
>     if (p)
>     {
>         udp_sendto(new_udp_pcb, p, udp_addr, udp_port);
>         pbuf_free(p);
>     }
> }

I'm pretty sure it's a bad idea to replace the point in p->payload with 
your own pointer.  That might be your entire problem?  Copy your data to 
the payload buffer provided and try that.

Also note, you don't have to set p->len or p->totlen - LwIP handles that 
for you in most or all cases.

If that doesn't work, here is the code I use to send a simple UDP 
packet: (udp_data is an array of bytes)

            p = pbuf_alloc(PBUF_TRANSPORT, sizeof(udp_data), PBUF_RAM);
            if (p)
            {
                memcpy(p->payload, udp_data, sizeof(udp_data));
                i = udp_sendto(udp_pcb, p, &broadcast_address, my_ip_port);
                if (i != ERR_OK)
                {
                    printf("Got error %d when sending UDP packet!\n", i);
                }
                else
                {
                    if (debug)
                        printf("Sent (%d.%03d microseconds [%d])\n", 
subtick/120, ((subtick%120)*1000)/120, subtick);
                }
                pbuf_free(p);
            }
            else
            {
                printf("Couldn't allocate a pbuf!!\n");
            }

Good luck!

Patrick Klos
Klos Technologies, Inc.