[lwip-devel] [bug #31084] socket API returns always EMSGSIZE on non-blocking sockets if data size > send buffers

[Martin Velek]

URL:
  <http://savannah.nongnu.org/bugs/?31084>

                 Summary: socket API returns always EMSGSIZE on non-blocking
sockets if data size > send buffers
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: allew
            Submitted on: Mon 20 Sep 2010 01:56:22 PM GMT
                Category: sockets
                Severity: 3 - Normal
              Item Group: Feature Request
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: CVS Head

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Details:

I would like to open discussion about sending data over LWIP socket
API(1.4.x) in the non-blocking or MSG_DONTWAIT mode.
The current implementation returns always EMSGSIZE if data size is more than
((size > TCP_SND_BUF) || ((size / TCP_MSS) >
TCP_SND_QUEUELEN)). I cannot find exact description of this situation but I
think that lwip should send as much as possible(and  returns less than
required) and if the output buffer is full it would return -1 with error
EAGAIN or EWOULDBLOCK.

I have discovered this behavior during playing with xavante http
server + luasockets and LWIP socket API. Every accepted connection has option
TCP_NODELAY and O_NONBLOCK. Every attempt to send data(larger html page)
failed with EMSGSIZE.

I have tried similar behavior with Ubuntu 10.04.1 LTS,
setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, (int *)&optval,
sizeof(optval));
fcntl(sockfd, F_SETFL,O_NONBLOCK);
perror("");
n = write(newsockfd, buff, x);
perror("");
Only with big x (e.g. 655360) there is
Success
Success
n = 212992

Martin




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