USB设备/ dev / sdb的opendir

Question

I am trying to implement in c program a way to detect if usb is connected or not. I noticed that when the usb is connected, then the following command from shell will result as following:

root:~# ls /dev/sdb
/dev/sdb

If usb is disconnected then I get

root:~# ls /dev/sdb
ls: /dev/sdb: No such file or directory

I therefore assumed that best way to detect usb connected from c program shall be by doing opendir("/dev/sdb"), but the open call is always failed.

Do you have any idea what's best methd to do this detection ?

The main goal, is knowing in run-time program where the udev mounted the harddisk, and where the usb flash drive (if plugged in). Thanks, Ran

Answer 1

A) Why your opendir call failed? It's easy to understand. Because /dev/sdb is not a directory. For me it's hard to understand what you expect, when you try to open device as a directory. If you want to get file list from you usb drive, you need to mount the partition ( /dev/sdb1 , /dev/sdb2 , etc. not the /dev/sdb ) to some mount point ( directory ). And if you mount it successfully, then you can open the directory with your call. Many linux distributions mount usb drive automatically. You can look to your distribution documentation to get the information about automatical mounting of the usb drives.

B) I think it's easy to understand why detecting the usb device this way is bad idea.

=> Different computers have different amount of drives. So on many computers /dev/sdb - is a hard drive.

=> Asuming you know that there is only one hard drive, the disk naming still can change after computer reboot, so it's possible that after reboot the usb drive will become sda and hard drive will become sdb (but usually this doesn't happen).

=> Generally, it's not possible to predict a new letter ( sdb or sdc or sde ).

=> You can't access to the usb devices, that are not usb drives.

=> There are tonns of other problems with such solution.

C) I didn't understand your goal completely, but if you want your program to be make some action if the usb device is plugged in, that the best solution you can do is read about udev and the event system of your distribution. If you want to make with your usb device some low level operations, you can read about libusb . You can get the general information about usb devices with lsusb command which is usually a part of a distributions. You can google for some other infermer commands.

Answer 2

The device /dev/sdb is not a directory, it's just a device, so you should use a system call like stat that will tell you whether a file exists instead of trying to calling opendir on it.
Please note that /dev/sdb could be any kind of hard drive depending on the setup of your specific system; it is not necessarily a USB drive.

Alternatively, you could execute the lsusb utility and parse its output.