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From: Yann Droneaud <ydroneaud@opteya.com>
To: noloader@gmail.com, Greg KH <greg@kroah.com>
Cc: kernelnewbies <kernelnewbies@kernelnewbies.org>
Subject: Re: How to make /dev/ttyACM0 (and friends) exclusive?
Date: Mon, 04 Mar 2019 16:38:44 +0100	[thread overview]
Message-ID: <004695b63cb9510353902e0afef45ea8c57c8282.camel@opteya.com> (raw)
In-Reply-To: <CAH8yC8mDG-fzaM3LGMw-C=sPs04Xmfs_CW2mGDCu0W+8hDG=fQ@mail.gmail.com>

Hi,

Le lundi 04 mars 2019 à 08:04 -0500, Jeffrey Walton a écrit :
> On Sun, Mar 3, 2019 at 6:00 AM Greg KH <greg@kroah.com> wrote:
> > 
> > On Sun, Mar 03, 2019 at 03:55:44AM -0500, Jeffrey Walton wrote:
> > > ...
> > 
> > Again, go delete modem manager off of your system, it is the thing that
> > keeps opening the port up to see if you have made a valid connection on
> > the device or not.  If you write your own program to talk to the device,
> > modem manager is not needed at all, and is known to cause this problem.
> 
> Thanks Greg. I deleted modem manager, then ran two instances of my
> program. Both opened the device with O_EXCL, and both opens succeeded.
> They proceeded to much with one another's state.
> 

http://man7.org/linux/man-pages/man2/open.2.html

  "In general, the behavior  of  O_EXCL is undefined if it is used
   without O_CREAT. There is one exception: on Linux 2.6 and later,
   O_EXCL can be used without O_CREAT if pathname refers to a block
   device. If the block device is in use by the system (e.g., mounted),
   open() fails with the error EBUSY."

O_EXCL is intended to be used to prevent opening an existing file. Said
differently, it's used to ensure a new file is created, useful to
prevent race condition, where multiple processes compete to create a
file. For example think of temporary file created with random name.

Regards

-- 
Yann Droneaud
OPTEYA



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  parent reply	other threads:[~2019-03-04 15:39 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-02  4:29 How to make /dev/ttyACM0 (and friends) exclusive? Jeffrey Walton
2019-03-02  7:50 ` Greg KH
2019-03-02 18:45 ` valdis.kletnieks
2019-03-02 19:36   ` Jeffrey Walton
2019-03-03  5:55     ` valdis.kletnieks
2019-03-03  8:55       ` Jeffrey Walton
2019-03-03 11:00         ` Greg KH
2019-03-04 13:04           ` Jeffrey Walton
2019-03-04 13:20             ` Greg KH
2019-03-04 15:38             ` Yann Droneaud [this message]
2019-03-04 22:01               ` valdis.kletnieks
2020-10-06 20:41                 ` Daniel Santos
2020-10-11 14:52                   ` linux lover

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