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From: Jeffrey Walton <noloader@gmail.com>
To: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: kernelnewbies <kernelnewbies@kernelnewbies.org>
Subject: Re: How to make /dev/ttyACM0 (and friends) exclusive?
Date: Sat, 2 Mar 2019 14:36:12 -0500	[thread overview]
Message-ID: <CAH8yC8kkwZLD+get-V-kp1jYMmG7ovYTnQVJdis-DdCyVGTzdw@mail.gmail.com> (raw)
In-Reply-To: <3824.1551552333@turing-police>

On Sat, Mar 2, 2019 at 1:45 PM <valdis.kletnieks@vt.edu> wrote:
>
> On Fri, 01 Mar 2019 23:29:53 -0500, Jeffrey Walton said:
>
> > I cannot seem to open the device in exclusive mode. The current open
> > is (I also tried with O_EXCL):
> >
> >     int modem = open(device_path, O_RDWR | O_NOCTTY | O_SYNC);
>
> So what happens?  Does the open fail? If so, what does perror() say about it?
> Or does the open work but the flag is ignored?

Yes, both are allowed to open the device.

> Have you tried making the *other* program open it with O_EXCL so your
> caller-id program can't mess with its state?

Yes, I used O_EXCL . I have no control over other programs (this is
the reason I can't use something like a named semaphore or lock file).

I can say when I write a second test program that uses O_EXCL, it is
also allowed to open the device even when the device was already
opened O_EXCL.

I feel like I am missing something... Does Linux consider the modem a
shared resource instead of an exclusive resource? What use cases
support two different programs sending commands to the modem at the
same time?

Jeff

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  reply	other threads:[~2019-03-02 19:37 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 [this message]
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
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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