[lwip-devel] [bug #59638] Calling tcp_write() without TCP_WRITE_FLAG_COPY does in fact copy data

[Oleg Kobrin]

URL:
  <https://savannah.nongnu.org/bugs/?59638>

                 Summary: Calling tcp_write() without TCP_WRITE_FLAG_COPY does
in fact copy data
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: kiabrin
            Submitted on: Вт 08 дек 2020 07:27:57
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

Hello!
Debugging my code on TivaC MCU, NO_OS configuration, I realized after call to
tcp_write() with no TCP_WRITE_FLAG_COPY flag that pcb->snd_buf was decreased
and real pcb->unsent->p->payload points not to my data, but somewhere inside
lwip memory pool.
This is not good, as at high output traffic I experience tcp send buffer
overflows, causing data loss.

Previous ticket was incorrectly marked as invalid, quote:
"So where's the bug?
pcb->unsent->p->payload points to a small pbuf containing the packet headers.
This needs to be allocated for every packet, regardless of whether the
packet's payload is copied or not."

But it is not correct.
pcb->unsent->tcphdr points to header,
pcb->unsent->p->payload points to contents.

In my case: I checked again, writing a packet of 12 bytes without
TCP_WRITE_FLAG_COPY set decreases pcb->snd_buf by 12.

Another try:
calling err_t Err = tcp_write(Connects[Sock].pcb, Data, Len, 0); 
Len = 2404, pcb->snd_buf = 23722.
After call:
pcb->snd_buf = 21318.
23722 - 21318 = 2404.





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