Running Linux, 5th Edition - Matthias Kalle Dalheimer [198]
brw-rw---- 1 root disk 3, 0 2004-04-06 15:27 /dev/hda
This is /dev/hda, which corresponds to the first IDE drive. First of all, note that the first letter of the permissions field is b, which means this is a block device file. (Normal files have a - in this first column, directories a d, and so on; we'll talk more about this in the next chapter.) Device files are denoted either by b, for block devices, or c, for character devices. A block device is usually a peripheral such as a hard drive: data is read and written to the device as entire blocks (where the block size is determined by the device; it may not be 1024 bytes as we usually call "blocks" under Linux), and the device may be accessed randomly. In contrast, character devices are usually read or written sequentially, and I/O may be done as single bytes. An example of a character device is a serial port.
Also, note that the size field in the ls -l listing is replaced by two numbers, separated by a comma. The first value is the major device number and the second is the minor device number. When a device file is accessed by a program, the kernel receives the I/O request in terms of the major and minor numbers of the device. The major number generally specifies a particular driver within the kernel, and the minor number specifies a particular device handled by that driver. For example, all serial port devices have the same major number, but different minor numbers. The kernel uses the major number to redirect an I/O request to the appropriate driver, and the driver uses the minor number to figure out which specific device to access. In some cases, minor numbers can also be used for accessing specific functions of a device.
The naming convention used by files in /dev is, to put it bluntly, a complete mess. Because the kernel itself doesn't care what filenames are used in /dev (it cares only about the major and minor numbers), the distribution maintainers, applications programmers, and device driver writers are free to choose names for a device file. Often, the person writing a device driver will suggest a name for the device, and later the name will be changed to accommodate other, similar devices. This can cause confusion and inconsistency as the system develops; hopefully, you won't encounter this problem unless you're working with newer device drivers—those that are under testing. A project called udev should soon solve the problem of clashing device names.
At any rate, the device files included in your original distribution should be accurate for the kernel version and for device drivers included with that distribution. When you upgrade your kernel or add additional device drivers (see "Building a New Kernel" in Chapter 18), you may need to add a device file using the mknod command. The format of this command is:
mknod -m permissions name type major minor
where:
name is the full pathname of the device to create, such as /dev/rft0
type is either c for a character device or b for a block device
major is the major number of the device
minor is the minor number of the device
-mpermissions is an optional argument that sets the permission bits of the new device file to permissions
For example, let's say you're adding a new device driver to the kernel, and the documentation says that you need to create the block device /dev/bogus, major number 42, minor number 0. You would use the following command:
mknod /dev/bogus b 42 0
Making devices is even easier with the command /dev/MAKEDEV that comes with many distributions—you specify only the kind of device you want, and MAKEDEV finds out the major and minor numbers for you.
Getting back to the mknod command, if you don't specify the -m permissions argument, the new device is given the permissions for a newly created file, modified by your current umask—usually 0644. To set the permissions for /dev/bogus to 0660 instead, we use:
mknod -m 660 /dev/bogus b 42 0
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