Re: posix message queues, was Re: 2.6.4-rc1-mm1

[Krzysztof Benedyczak]

bert hubert wrote:

> On Sun, Feb 29, 2004 at 02:06:17PM -0800, Andrew Morton wrote:
>
>  
>
> > - Added the POSIX message queue implementation.  We're still stitching
> >  together a decent description of all of this.  Reference information is at
> > 	http://www.mat.uni.torun.pl/~wrona/posix_ipc/
> > and
> > 	http://www.opengroup.org/onlinepubs/007904975/basedefs/mqueue.h.html
> >    
>
> I can confirm that basic functionality is there. Both blocking and
> nonblocking operations work as advertised. Queue properly blocks when full
> or empty.
>
> mq_timedsend does not wait, it immediately returns ETIMEOUT with the queue
> is full:
>
>         struct timespec ts;
>         ts.tv_sec=1;
>    	 ts.tv_nsec=0;
>         sprintf(msgptr,"%05d %s",c,stime);
> 	 
>         if ( mq_timedsend(mqd,msgptr,msglen,msg_prio, &ts) )
> 	 {
> 	    perror("mq_send()");
> 	      break;
> 	 }
>
> results in:
>
> $ ./mqreceive /Q32x128
> 1: priority 0  len 64 text 00001 Mon Mar  1 12:20:11 2004  
> $ ./mqreceive /Q32x128
> 1: priority 0  len 64 text 00001 Mon Mar  1 12:20:11 2004  
> $ ./mqsend  /Q32x128
> $ ./mqsend  /Q32x128
> $ ./mqsend  /Q32x128
> mq_send(): Connection timed out  <- immediately
>  
It _should_ return immediately. mq_timedsend timeout must be given as 
absolute value, so if you want to have one second timeout from now,
you must do:

	ts.tv_sec = time(NULL) + 1;
   	ts.tv_nsec = 0;

And I don't have idea why POSIX defines timeout as absolute not relative 
;-).

> I would very much advise Michal and Krzysztof to add some basic examples to
> their page. I had to scour the internet to find some working code.
>  
Thats right. The above problem also shows that such examples can be 
usefull. We are just now updating man pages for the library (there are 
some obsolete informations) and I will prepare some well commented programs.

> Mounting the queuefs works but the fs, confusingly, is called mqueue and not
> mqueuefs.
>  
As proc or msdos. Of course it can be changed if more people will want 
'fs' suffix. Menuconfig help gives proper name.

Regards,
Krzysiek

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