RE: [lwip-users] raw api recv callback - procedure to(temporarily)refuse data?

[Jan Ulvesten]

When you allocated a pbuf using PBUF_POOL, you are actually allocating a
chain og buffers linked together. The length of each is p->len(and then
p->next->len and so on). The total length is p->tot_len

memcpy(p->payload, SlipBuff, recved); 

The above line will fail if recved > p->len. You might do something like
this:

struct pbuf *make_pbuf (BYTE *packet, int packet_len)
{
    struct pbuf *p, *q;
    int left = packet_len;
    
    p = pbuf_alloc(PBUF_TRANSPORT, left, PBUF_POOL);
    if (!p) return NULL;
    
    q = p;

    while (left > 0)
    {
        int chunk_size = min (left, q->len);
        memcpy (q->payload, packet, chunk_size);
        
        packet += chunk_size;
        left -= chunk_size;
        q = q->next;
    }
    return p;
}    

Jan Ulvesten
Senior Software Engineer
SICOM  AS

-----Original Message-----
From: Karl Kobel [mailto:address@hidden 
Sent: 10. mai 2005 05:29
To: 'Mailing list for lwIP users'
Subject: RE: [lwip-users] raw api recv callback - procedure
to(temporarily)refuse data?

All,

I'm stuck. I'm nearly done with a single threaded version of slip. I
will add it to contrib when I complete it.

The problem stems most likely from my lack of understanding of pbufs.

The follow2ing is the code that executes when a complete packet is
received:

        p = pbuf_alloc(PBUF_RAW, recved, PBUF_POOL);
        if (p != NULL)
        {
                memcpy(p->payload, SlipBuff, recved);
                p->len = recved;
                LWIP_DEBUGF(SLIP_DEBUG, ("slipif: Got packet\n"));
                recved = 0;
                return p;
        }

SlipBuff has the data,
Recved has the length.

Small packets are handled correctly. However larger packets (around 300
bytes) the TCP checksum fails. The following is the portion of tcp_input
that fails.

  /* Verify TCP checksum. */
  if (inet_chksum_pseudo(p, (struct ip_addr *)&(iphdr->src),
      (struct ip_addr *)&(iphdr->dest),
      IP_PROTO_TCP, p->tot_len) != 0) {
      LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_input: packet discarded due to
failing checksum 0x%04x\n",
        inet_chksum_pseudo(p, (struct ip_addr *)&(iphdr->src), (struct
ip_addr *)&(iphdr->dest),
      IP_PROTO_TCP, p->tot_len)));

The pbuf that fails has the next pointer pointing to another pbuf (four
total in the one I trapped), but the payload and len are correct for the
entire packet in the first pbuf. Also if I jump past the code after the
first pbuf checksum is totaled, the packet checksum is correct and the
packet is received correctly.

This has to be a result of me doing something wrong. Do I need to do
something different when I allocate the pbuf in the slip driver? I tried
PBUF_LINK as the first argument with the same results. Do I have to set
the next pointer to NULL?

Thanks,
Karl



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