Re: POSIX Message Queues [solved]

Re: POSIX Message Queues [solved]

[Ardhan Madras]

        I admit, this will be an embarrassing question, the both process need to know when to send the message (process A) and when to receive it (process B), I forget the classic UNIX's stuff called synchronisation tool like semaphore. I really forget this magic ;). But anyway, thanks for reading self-answered question.

        Ardhan


--- ajhwb@knac.com wrote:

From:	"Ardhan Madras" <ajhwb@knac.com>
To:	"Glynn Clements" <glynn@gclements.plus.com>
Cc:	<linux-c-programming@vger.kernel.org>
Subject: Re: POSIX Message Queues
Date:	Thu, 2 Sep 2010 02:51:56 -0700

Hi all,

        I use message queue to exchange data between 2 unrelated process, as shown below

Program A:
while (1) {
    mq_receive(); /* wait and receive request */
    /* process request */
    mq_send(); /* reply the request result */
}

Program B:
mq_send(); /* send request */
mq_receive(); /* process result */

        How it's possible to process A that call mq_send() receive it's *own* send data using mq_receive(), so process B that intended to read the message (mq_receive()) could not read it?, the problem is *sometime* happen, at most program A reply what program B requested. Did I miss something here?

        Ardhan



--- glynn@gclements.plus.com wrote:

From: Glynn Clements <glynn@gclements.plus.com>
To: <ajhwb@knac.com>
Cc: <linux-c-programming@vger.kernel.org>
Subject: Re: POSIX Message Queues
Date: Tue, 15 Jun 2010 16:33:25 +0100


Ardhan Madras wrote:

> It was mq_open(), sorry my bad ;p
> 
> I did umask(0001) before creating the queue, it's works now. 'Program
> A' that requested O_RDWR flag to the queue can send and receive
> message as 'Program B' did. I also try to demonstrating this problem
> with another machine (Linux of course), as result i can O_RDWR'ed
> program A queue flag without have to call umask() first. Is this
> behavior can be set through system configuration?

A process' umask is inherited from its parent. For interactive
sessions, you can set it in your ~/.profile or similar. For daemons
started by init, you can set it in the rc script which starts the
daemon.

> fchmod() also works here, sorry, but what do you mean with 'not
> portable'?, do you mean fchmod() intended to be used only for file, in
> other words "not every system support fchmod() with message queue
> descriptor" right?

Yes.

-- 
Glynn Clements <glynn@gclements.plus.com>




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