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[lwip-users] UDP send example

From: Jon Bean
Subject: [lwip-users] UDP send example
Date: Fri, 10 Jul 2020 11:44:00 +0100
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0


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);

void eth_udp_rx(void *arg, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port)
    uint32_t len;
    char *p_payload;
    uint8_t buf[16];
    int i;

    if (p != NULL)
        p_payload = (char *)p->payload;
        len = p->tot_len;

        for (i=0;i<len;i++)
            buf[i] = p_payload[i];

        udp_rx_done = 1;
        udp_addr = addr;
        udp_port = port;

        /* send received packet back to sender */
        //udp_sendto(new_udp_pcb, p, addr, port);
        /* free the pbuf */

void eth_udp_init(void)
    //struct udp_pcb * pcb;

    /* get new pcb */
    new_udp_pcb = udp_new();
    if (new_udp_pcb == NULL) {
        LWIP_DEBUGF(UDP_DEBUG, ("udp_new failed!\n"));

    /* bind to any IP address on port 23 */
    if (udp_bind(new_udp_pcb, IP_ADDR_ANY, 23) != ERR_OK) {
        LWIP_DEBUGF(UDP_DEBUG, ("udp_bind failed!\n"));

    /* set udp_echo_recv() as callback function
       for received packets */
    udp_recv(new_udp_pcb, eth_udp_rx, NULL);

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