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[lwip-users] pbuf and recvevent/recv_avail out of sync

From: Wolfgang Pfab
Subject: [lwip-users] pbuf and recvevent/recv_avail out of sync
Date: Thu, 02 Feb 2006 14:00:01 +0100

Hi everybody, 

-- First a short explanation in wich case I encounter a problem: 
I am using lwip with uc/os ii in an embedded system runing on a nios 2 cpu. A 
simple tftp server was implemented with the socket API. 
After receiving the last package of a file, I am waiting for the system to 
store the file in flash memory before sending the ACK. 
Sending large files it might happen that I run out of pbufs waiting for the 
file to be stored, because the client resends the last block. So far this is 
not a big deal, because I am only handling one connection at a time and do not 
send any data while waiting. 
Before sending the last ACK I use lwip_ioctl() to check for pending data in the 
pbufs and recvfrom() in non blocking mode to read the data and release the 

-- Here it comes: 
When I ran out of pbufs and read/release them, recvfrom() (in non blocking 
mode) will block on the tenth package. Both lwip_select() and lwip_ioctl() 
indicate that there is more data left, what is absolutely understandable, 
because more than ten pbufs are specified. 
When recvfrom() blocks and I send another datapackage to the server this 
package will be received. 
The problem will not occur if I set the number of pbufs to 9 or less. 

It seems to me, that rcvevent and recv_avail are incremented but no pbuf is 
allocated after the ninth one. I am not that familiar with the structure of 
lwip, but I am sure there is some kind of error checking that would prevent 
this from happening. 
I will appreciate any hint that could bring some light to this issue! 

Thanks in advance, 

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