Re: Wireshark and serial ports

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On Wed, Aug 23, 2023 at 6:20 PM Jeffrey Walton <noloader@xxxxxxxxx> wrote:
> On Wed, Aug 23, 2023 at 5:21 PM Steve Underwood <coppice12@xxxxxxxxx> wrote:
> >
> > Has anyone used wireshark while using USB serial ports with Fedora 37?
> > As soon as I start wireshark the communication on my serial ports
> > breaks. Thinking this might be something to do with wireshark scanning
> > for available interfaces, I tried starting wireshark with "wireshark -i
> > enp3c0", but it still seems to list all the interfaces, and it still
> > messes up my serial communication. I'm sure I was doing things like this
> > in 2022 without any trouble.
> I have not found myself in your position, so I can't comment on
> Wireshark and serial ports.
> But I have been in the position of NetworkManager messing with my
> serial ports. To work around it in my code, I made the file descriptor
> exclusive so NetworkManager could not open it:
>     /* Open the modem. It will be configured for exclusive use below. */
>     s_modem = open(device_path, O_RDWR | O_NOCTTY | O_SYNC);
>     ret = errno;
>     if (s_modem == -1) {
>         log_error("Failed to open device %s: %s\n", device_path,
> strerror(errno));
>         s_modem = 0; errno = ret;
>         goto finished;
>     }
>     /* Verify the modem is a terminal we can configure and control */
>     if (!isatty(s_modem))
>     {
>         log_error("Device %s is not a tty\n", device_path);
>         close(s_modem); s_modem = 0;
>         ret = errno = ENOTTY;
>         goto finished;
>     }
>     /* Get exclusive access to the modem. */
>     /* Don't allow NetworkManager and ModemManager access */
>     if (ioctl(s_modem, TIOCEXCL, NULL) == -1) {
>         log_warn("Failed to set TIOCEXCL on device: %s\n", strerror(errno));
>     }

Oh, and I forgot to mention... if I recall correctly (going back
several years)... open() is supposed to use O_EXCL. But I seem to
recall it is silently ignored in this case. Hence the need for the
ioct() and TIOCEXCL to set an exclusive mode.

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