字符设备主次设备号的作用

     One of the basic features of the Linux kernel is that it abstracts the handling of devices. All hardware devices look like regular files; they can be opened, closed, read and written using the same, standard, system calls that are used to manipulate files. Every device in the system is represented by a file. For block (disk) and character devices, these device files are created by the mknod command and they describe the device using major and minor device numbers. Network devices are also represented by device special files but they are created by Linux as it finds and initializes the network controllers in the system.

    To UNIX, everything is a file. To write to the hard disk, you write to a file. To read from the keyboard is to read from a file. To store backups on a tape device is to write to a file. Even to read from memory is to read from a file. If the file from which you are trying to read or to which you are trying to write is a "normal" file, the process is fairly easy to understand: the file is opened and you read or write data. If, however, the device you want to access is a special device file (also referred to as a device node), a fair bit of work needs to be done before the read or write operation can begin.

    One key aspect of understanding device files lies in the fact that different devices behave and react differently. There are no keys on a hard disk and no sectors on a keyboard, though you can read from both. The system, therefore, needs a mechanism whereby it can distinguish between the various types of devices and behave accordingly.

    To access a device accordingly, the operating system must be told what to do. Obviously, the manner in which the kernel accesses a hard disk will be different from the way it accesses a terminal. Both can be read from and written to, but that's about where the similarities end. To access each of these totally different kinds of devices, the kernel needs to know that they are, in fact, different.

    Inside the kernel are functions for each of the devices the kernel is going to access. All the routines for a specific device are jointly referred to as the device driver. Each device on the system has its own device driver. Within each device driver are the functions that are used to access the device. For devices such as a hard disk or terminal, the system needs to be able to (among other things) open the device, write to the device, read from the device, and close the device. Therefore, the respective drivers will contain the routines needed to open, write to, read from, and close (among other things) those devices.

    The kernel needs to be told how to access the device. Not only does the kernel need to be told what kind of device is being accessed but also any special information, such as the partition number if it's a hard disk or density if it's a floppy, for example. This is accomplished by the major number and minor number of that device.

    All devices controlled by the same device driver have a common major device number. The minor device numbers are used to distinguish between different devices and their controllers. Linux maps the device special file passed in system calls (say to mount a file system on a block device) to the device's device driver using the major device number and a number of system tables, for example the character device table, chrdevs. The major number is actually the offset into the kernel's device driver table, which tells the kernel what kind of device it is (whether it is a hard disk or a serial terminal). The minor number tells the kernel special characteristics of the device to be accessed. For example, the second hard disk has a different minor number than the first. The COM1 port has a different minor number than the COM2 port, each partition on the primary IDE disk has a different minor device number, and so forth. So, for example, /dev/hda2, the second partition of the primary IDE disk has a major number of 3 and a minor number of 2.

    It is through this table that the routines are accessed that, in turn, access the physical hardware. Once the kernel has determined what kind of device to which it is talking, it determines the specific device, the specific location, or other characteristics of the device by means of the minor number.