Re: Keyspan USB Serial Device Behavior.
Greg KH <[email protected]>
| Newsgroups | gmane.linux.usb.devel |
|---|---|
| Message-ID | <[email protected]> |
On Thu, Jun 07, 2007 at 03:21:14PM -0700, Youngwhan Song wrote: > Hi all, > > I am using Keyspan USB Serial Device. I found the below fact recently, > and want to get verification from you if this behavior is right or to > be updated. > > I found that > > 1. When the Keyspan USB device is plugged out, the application does > not handle file descriptor correctly. What application are you talking about? > 2. It checks the connectivity of USB device using open(), but > "OPEN" method does not tell actual connectivity. In real, when it is > not connected to anything, it returns file descriptor as -1. However, > once it is connected and opened, and when the device is disconnected, > the opened file descriptor does not to be gone automatically in kernel > side unless it calls close(). Yes, this is true. > In this case, since the previous file descriptor is still in kernel > side, the device driver returns another file descriptor when it tries > to open() to check the connectivity even though actual device is not > connected. Ok, but calling open is not the way to check if the device is really there. > 3. So, when it is disconnected and connected again, then the device > is recognized as ttyUSB1 (or another number different with previous > one) not ttyUSB0 because the previous file descriptor has not been > gone. Yes. > More simple question is How application knows whether Keyspan USB > Serial Device is connected or not. Is there any nice way to detect the > connectivity of the USB device? Look in sysfs, or use HAL to get a list of all present usb to serial devices. That's the best way. thanks, greg k-h ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ [email protected] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel