RE: [ECOS] POSIX Queue Suspension

["Ali, Khurram" <Khurram_Ali@mentor.com>]

FYI

I got the latest source using anonymous cvs. Used the default
configuration with POSIX support. I see the behavior which I was
expecting.

I now have another question: How do I add support for printf? Which
library is this part of? I am using an x86 PC as my target.

Thank you,
Khurram

-----Original Message-----
From: ecos-discuss-owner@ecos.sourceware.org
[mailto:ecos-discuss-owner@ecos.sourceware.org] On Behalf Of Ali,
Khurram
Sent: Monday, June 20, 2005 1:49 PM
To: Andrew Lunn
Cc: eCos Discussion
Subject: RE: [ECOS] POSIX Queue Suspension

> If more than one thread is waiting to receive a message when a message
> arrives at an empty queue and the Priority Scheduling option is
supported, 
> then the thread of highest priority that has been waiting the longest 
> shall be selected to receive the message.

Thread L is the highest priority thread that has been waiting the
longest. Shouldn't it be resumed first?

Agreed that other threads also have the same priority, but since there
is an additional restriction of "waiting the longest" I would assume
that we can have two or more threads at the same priority?

Am I missing something?

Khurram

-----Original Message-----
From: Andrew Lunn [mailto:andrew@lunn.ch] 
Sent: Monday, June 20, 2005 12:22 PM
To: Ali, Khurram
Cc: eCos Discussion
Subject: Re: [ECOS] POSIX Queue Suspension

On Mon, Jun 20, 2005 at 10:54:00AM -0500, Ali, Khurram wrote:
> Hello,
> 
> I have two POSIX threads that are waiting to receive on a POSIX queue.
> Thread L suspends on the queue first. Thread K then suspends on the
> queue. However, when Thread J sends an element to the queue. Thread K
is
> resumed first. 
> 
> All threads have the same priority. All queue accesses are also made
> with the same priority. Shouldn't Thread L be resumed first. 
> 
> What could I be doing wrong?

You don't seem to of read the POSIX standard. 

http://www.opengroup.org/onlinepubs/009695399/functions/mq_receive.html

If the specified message queue is empty and O_NONBLOCK is not set in
the message queue description associated with mqdes, mq_receive()
shall block until a message is enqueued on the message queue or until
mq_receive() is interrupted by a signal. If more than one thread is
waiting to receive a message when a message arrives at an empty queue
and the Priority Scheduling option is supported, then the thread of
highest priority that has been waiting the longest shall be selected
to receive the message. Otherwise, it is unspecified which waiting
thread receives the message. If the specified message queue is empty
and O_NONBLOCK is set in the message queue description associated with
mqdes, no message shall be removed from the queue, and mq_receive()
shall return an error.

So eCos behavious seems correct to me.

        Andrew

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