Re: [Xenomai-help] writing from user space through fifo problems

[Philippe Gerum <rpm@xenomai.org>]

NZG wrote:
> On Thursday 21 February 2008 10:23:15 am Jan Kiszka wrote:
>> That's good. This test will also allow you to check it against
>>  - more recent Xenomai (please check the Changelog for pipe-related
>>    changes since 2.3.1, I bet there are a few)
>>  - some other arch, specifically x86 (IIRC, there is still some Linux
>>    scheduling artifact left in the I-pipe patch for blackfin, but it
>>    shouldn't make a difference here - well, who knows...)
> Well, in the process of writing a simple test I kindof found the problem.
> My user space program is writing and then immediatly closing, which appears to 
> destroy the data if it hasn't been read by then. Inserting a sleep between 
> write and close causes it to work.
>

Yes, releasing a fildes flushes the input queue from lingering data
before closing the user-space side.

> I noticed this because my test program was working because it just so happened 
> to pause until the real time task ended before closing the fd.
> 
> I remember doing this before, now that it's figured out, but it's still kind 
> of problematic because it doesn't appear to be possible to block on user 
> space writes.
> 
> Opening with O_SYNC does not cause it to block, and fsync(fd) doesn't seem to 
> do anything.

O_SYNC support has to be implemented by the driver, and Xenomai
implements this for the pipe driver since 2.4-rc1. Therefore, passing
O_SYNC to open() using 2.3.1 will just lead to a nop.

 anybody have any suggestions on how to know data has been
> received before closing a pipe after a write? Surely somebody's done this 
> before.
> 

The only way I see with 2.3.1 would be to make a non-realtime shadow
from your user-space thread (rt_task_shadow() with priority 0), then
have it pend on a Xenomai semaphore signaled by your real-time task when
it has consumed the packet.

> thanks,
> NZG
> 
> 
> 
> 
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-- 
Philippe.