X-Recipient: archive-cygwin AT delorie DOT com
X-SWARE-Spam-Status: No, hits=-0.3 required=5.0	tests=AWL,BAYES_00,RP_MATCHES_RCVD,TW_FC
X-Spam-Check-By: sourceware.org
From: Manuel Wienand <Manuel DOT Wienand AT ubitronix DOT com>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Date: Mon, 24 Oct 2011 12:02:42 +0200
Subject: RE: 1.7.9: spawn brakes reopening of serial port
Message-ID: <0C11C5BF0B29FD43A8D0250F711D497F89DEDBFF17@ex01-ubitronix.ubitronix.local>
References: <0C11C5BF0B29FD43A8D0250F711D497F89DEDBFEE2 AT ex01-ubitronix DOT ubitronix DOT local> <20111021105003 DOT GB2979 AT calimero DOT vinschen DOT de>
In-Reply-To: <20111021105003.GB2979@calimero.vinschen.de>
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
X-IsSubscribed: yes
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
List-Id: <cygwin.cygwin.com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from base64 to 8bit by delorie.com id p9OA52Lq009820

Hallo Corinna,

thanks for the response. Yes, you are right, I missed the fact that the child process inherits the file descriptors.

I took a look at the documentation of exec and just to be clear on the descriptor/handler issue:
- file descriptor are passed down (basically always when calling the 'open' function)
- socket descriptors are *not* passed down (not sure, wasn't in the documentation)
- handles of semaphores, mutexes, memory maps, message queues are *not* passed down.
- signal mask, scheduling policies (at leat when using SCHED_FIFO, SCHED_RR or SCHED_SPORADIC) and directory streams are passed down
Is that correct?

Thanks

Manuel


> -----Original Message-----
> From: *** On Behalf Of Corinna Vinschen
> Sent: Friday, October 21, 2011 12:50 PM
> To: ***
> Subject: Re: 1.7.9: spawn brakes reopening of serial port
> 
> On Oct 20 10:19, Manuel Wienand wrote:
> > Hallo,
> >
> > it seems that spawning a process brakes the reopening of a serial
> port
> > (when done during the execution of that process). The result of the
> > open() function is "Permission denied (13)" in that case.
> 
> I had a look into your testcase.  What you see is expected behaviour.
> 
> On Windows, a serial port can only be opened once at a time, for
> exclusive access.  You could have tried that without starting a second
> process:
> 
>   open ("/dev/ttyS0", O_RDWR);  // succeeds
>   open ("/dev/ttyS0", O_RDWR);  // 2nd call fails with EACCES.
> 
> Now I hear you say that you close the descriptor before trying to open
> it the second time, but you're missing the fact that on spawn/exec the
> child process inherits the open file descriptors from the parent
> process.
> So, even if you close the descriptor in the parent, it's still open in
> the child.
> 
> Apparently you don't want the child process to inherit this descriptor,
> so you must set its "close-on-exec" flag.  There are basically two ways
> to do that
> 
>  - fd = open ("/dev/ttyS0", O_RDWR | O_CLOEXEC);
> 
>  - fd = open ("/dev/ttyS0", O_RDWR);
>    fcntl (fd, F_SETFD, fcntl (fd, F_GETFD) | FD_CLOEXEC);
> 
> I guess it goes without saying that the close-on-exec flag has to be
> set before calling spawn/exec.
> 
> And, btw., please use /dev/ttyS0 rather than /dev/com1, etc.  These are
> the "official" names for the serial ports, while the "dev/comX" syntax
> is only supported for backward compatibility.
> 
> 
> Corinna
> 
> --
> Corinna Vinschen                  Please, send mails regarding Cygwin
> to
> Cygwin Project Co-Leader          cygwin AT cygwin DOT com
> Red Hat
> 
> --
> Problem reports:       http://cygwin.com/problems.html
> FAQ:                   http://cygwin.com/faq/
> Documentation:         http://cygwin.com/docs.html
> Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple
> 
> 
> --
> This message has been scanned for viruses and
> dangerous content by MailScanner, and is
> believed to be clean.