Udp recv in locator.c

Hi,

I just wondering what is exactly the arg on the LocatorReceive function which is being called back by udp_recv?

The complete code is as follow   (as can be found on locator.c ) :

//*****************************************************************************
//
// This function is called by the lwIP TCP/IP stack when it receives a UDP
// packet from the discovery port.  It produces the response packet, which is
// sent back to the querying client.
//
//*****************************************************************************
static void
LocatorReceive(void *arg, struct udp_pcb *pcb, struct pbuf *p,
               struct ip_addr *addr, u16_t port)
{
    uint8_t *pui8Data;
    uint32_t ui32Idx;

    //
    // Validate the contents of the datagram.
    //
    pui8Data = p->payload;
    if((p->len != 4) || (pui8Data[0] != TAG_CMD) || (pui8Data[1] != 4) ||
       (pui8Data[2] != CMD_DISCOVER_TARGET) ||
       (pui8Data[3] != ((0 - TAG_CMD - 4 - CMD_DISCOVER_TARGET) & 0xff)))
    {
        pbuf_free(p);
        return;
    }

    //
    // The incoming pbuf is no longer needed, so free it.
    //
    pbuf_free(p);

    //
    // Allocate a new pbuf for sending the response.
    //
    p = pbuf_alloc(PBUF_TRANSPORT, sizeof(g_pui8LocatorData), PBUF_RAM);
    if(p == NULL)
    {
        return;
    }

    //
    // Calculate and fill in the checksum on the response packet.
    //
    for(ui32Idx = 0, g_pui8LocatorData[sizeof(g_pui8LocatorData) - 1] = 0;
        ui32Idx < (sizeof(g_pui8LocatorData) - 1); ui32Idx++)
    {
        g_pui8LocatorData[sizeof(g_pui8LocatorData) - 1] -=
            g_pui8LocatorData[ui32Idx];
    }

    //
    // Copy the response packet data into the pbuf.
    //
    pui8Data = p->payload;
    for(ui32Idx = 0; ui32Idx < sizeof(g_pui8LocatorData); ui32Idx++)
    {
        pui8Data[ui32Idx] = g_pui8LocatorData[ui32Idx];
    }

    //
    // Send the response.
    //
    udp_sendto(pcb, p, addr, port);

    //
    // Free the pbuf.
    //
    pbuf_free(p);
}

Thank you