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[lwip-users] lwIP tcp_write fails: specifically tcp_sndbuf returning 0

From: justin . rice
Subject: [lwip-users] lwIP tcp_write fails: specifically tcp_sndbuf returning 0
Date: Thu, 25 Apr 2013 08:34:58 -0400

I have seen MANY MANY posts about this topic, but none are quite the same
situation as mine, and thus do not pose any solutions that work for me.

What I have:  I have a basic echo server example running on a Stellaris
LM3S6611.  I wrote the example based on code I found here
http://www.ultimaserial.com/avr_lwip_tcp.html .  I used the 'Raw API
Programming example'.  That code works fine.  I can connect to my board
using telnet, from a linux laptop, and send (I defined it this way) capital
'T' characters to toggle an LED on my board, while ALSO echoing back the
typed in data (T in this case).

What I want:  I want to extend this example to send a message over THE SAME
socket, when a button is pressed.

The problem:  I moved my pcb to global scope.  I added a button handler
with the following code:

void API_pushButtonState(void)
        int len;
        strncpy(buf, "button", 6);
        len = tcp_sndbuf(pcb);

        tcp_write(pcb, buf, len, TCP_WRITE_FLAG_COPY);
        tcp_sent(pcb, NULL);

From my understanding, and the LOADS of documentation I have read, this
should send my message out.  This will not work.  I have checked with
Wireshark.  Nothing is being sent out the wire.  I can see both messages of
the echo request (the msg from the computer to the board, and the echo
response from the board to the computer), but nothing when I press my

I have debugged the code to verify that when I press the button I am indeed
entering the function above.  I'm assuming my misunderstanding is with
tcp_sndbuf.  It always returns 0 when called from this function.  I can't
decide if the debugger is just lying to me, or if it actually is 0.  If I
call tcp_sndbuf at startup, I get some massive number (like 26285 or
something).  If I check the disassembly window DURING the call to this
function, it looks to me like when the tcp_sndbuf function gets called, its
literally two assembly instructions that move a constant value of 0 into a
register used for the length variable.

Im baffled.  Do I have something set up improperly with lwipopts.h?  What
am I missing?

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