Re: [lwip-users] tcp_output() when processing "tcp_input_pcb"

[Noam weissman]

Hi,

I am not sure I understand your problem but if I did understand, you please see 
below.

In RAW mode you have just one task that runs all your TCP/IP connections and 
servers.
One task means that every time you do something related to TCP only this part 
is running
And nothing else is working.

LwIP has its own house housekeeping operations (ACK, SYN, etc.....).

When you get a packet dedicated to your server, your own call back is 
triggered. You process
the data and call tcp_write

tcp_write adds a buffer to the list of buffers to be sent. But and here comes 
the BIG but. The code
that is sending the data (inside LwIP) will never be called if you do not exit 
from your own function 
and let the stack do its housekeeping tasks.

You cannot expect the data to be sent at zero delay after you put it to be sent.

I hope now it is clear.

BR,
Noam.

-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Valery Ushakov
Sent: ב 12 אוגוסט 2013 22:40
To: address@hidden
Subject: Re: [lwip-users] tcp_output() when processing "tcp_input_pcb"

Numb_Faith <address@hidden> wrote:

> When I use lwip1.4.1 and RAW API, I meet a problem. I use my 
> borad(STM32) as a server. When server receive a packet from client, I 
> hope it can send a packet immediately. So I call the function below:
> 
> /tcp_write(tpcb, DataTest, sizeof(DataTest), 0x01); tcp_output(tpcb);/
> 
> But when server receive a packet, the stack will call tcp_input(), and 
> tcp_input_pcb = pcb (the connection), so tcp_output(tpcb) would return.
> 
> When I comment the code below(in function tcp_output), it can got what 
> I want. But I'm still confused about why "If we are invoked by the TCP 
> input processing code, we do not output anything".
> 
> /* First, check if we are invoked by the TCP input processing code. If 
> so, we do not output anything.
>    Instead, we rely on the input processing code to call us when input 
> processing is done with. */ //  if (tcp_input_pcb == pcb) { //  return 
> ERR_OK; //  }

Input processing code _does_ call tcp_output() at:

http://git.savannah.gnu.org/cgit/lwip.git/tree/src/core/tcp_in.c?h=DEVEL-1_4_1#n381

        tcp_input_pcb = NULL;
        /* Try to send something out. */
        tcp_output(pcb);

At that point input processing is complete and pcb is in consistent state, etc, 
etc, so tcp_output() doesn't have to be concerned with possible iteractions 
with tcp_input().

-uwe


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