Re: [lwip-users] lwip_close() doesn't work when lwip_write() hangs

[Joel Cunningham]

You can use SO_SNDTIMEOUT, which should work on LwIP 1.4.1.  I have used it in my port with LwIP 1.4.1, so possibly there's a problem with your port?

I've also written applications that used non-blocking sockets and select to achieve a similar behavior of having blocking I/O that can be canceled.  The trick here is adding a second socket to the read FD set in select and then set select to block until your write or read is ready.  This second socket is bound to the loopback address.  When you want to cancel the blocking select from another thread, simply send a datagram to the additional socket, which will return the select call.  Then you can detect that a cancel/wakeup happened because the second socket is marked as ready.

Joel

On Oct 12, 2015, at 12:45 PM, Alhad Palkar <address@hidden> wrote:

Thanks. Is there anyway around lwip_write blocking forever?

On Mon, Oct 12, 2015 at 7:44 AM, Joel Cunningham <address@hidden> wrote:

LwIP doesn't support this kind of threading model.  Multiple threads can not perform simultaneous operations (read+write, write+close, etc.) on the same socket.  The main limitation is that the netconn only has a single semaphore for blocking the calling thread when entering the core context.

On the master branch there is support for this model but the feature is in alpha state (see LWIP_NETCONN_FULLDUPLEX). In LwIP 1.4.1, this is not supported.

Joel

On Oct 10, 2015, at 12:29 AM, alhadpalkar <address@hidden> wrote:

I am using branch 1.4.1 of lwip. I have a thread that connects to a remote
server and writes data to it using lwip_write(). Sometimes this hangs
indefinitely. Looks like its waiting on the op->completed semaphore which
never gets signaled.

I tried using the SO_SNDTIMEO socket option, but that just causes panics in
my system. So I tried another approach where I use a watchdog that detects
this hang and calls lwip_close(). But it looks like LWIP doesn't like it. I
hit this assert

netconn_free(struct netconn *conn)
{
LWIP_ASSERT("PCB must be deallocated outside this function", conn->pcb.tcp
== NULL);
...
}

On debugging further it looks like we end up getting the ERR_INPROGRESS in
do_delconn().

do_delconn(struct api_msg_msg *msg)
{
/* @todo TCP: abort running write/connect? */
if ((msg->conn->state != NETCONN_NONE) &&
(msg->conn->state != NETCONN_LISTEN) &&
(msg->conn->state != NETCONN_CONNECT)) {
/* this only happens for TCP netconns */
LWIP_ASSERT("msg->conn->type == NETCONN_TCP", msg->conn->type ==
NETCONN_TCP);
km_printf("err in progress\n");
msg->err = ERR_INPROGRESS;
...
}

so we never end up cleanup up the pcbs which leads to this assert.

Is there a way around this?






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