FTP Server busy waiting when file descriptors are exhausted

Chris Johns chrisj at rtems.org
Tue Apr 4 13:08:41 UTC 2006

Thomas Rauscher wrote:
> there has been little discussion so far. I still cannot see a clean way 
> to remove pending connections in the listen socket (besides closing it).

I asked a question about the networking stack and the case of no 
available fd's when in an accept call. I have found the code for RTEMS 
in cpukit/libnetworking/rtems/rtems_syscall.c:

	fd = rtems_bsdnet_makeFdForSocket (so, &socket_handlers);
	if (fd < 0) {
		TAILQ_INSERT_HEAD(&head->so_comp, so, so_list);
		soconnwakeup (head);
		rtems_bsdnet_semaphore_release ();
		return -1;

If there is no fd available the socket is placed back on the listen list.

Is this the best solution for RTEMS ?

I checked NetBSD's kernel and it will return an error from accept if the 
number of fd's for the process has reached its limit. A log message is 
generated. If there is no space the fd allocator blocks waiting. It 
looks like FreeBSD is the same.

If you limit the number of fd's as task has (ulimit), use up the fd's in 
a similar way as the test case then try and accept a connection the same 
error will appear. How-ever on Unix the machine should not have locked 
up just the task.

Possible solutions are:

1) The proposed patch.
2) Drop the connection if no fd is present (close the socket placed on 
the listen queue).
3) Block in the fd allocator (rtems_bsdnet_makeFdForSocket) until an fd 
appears. This could be for-ever or just a period of time.
4) Other suggestions.


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