Linux Kernel Architecture

Chapter 6: Device Drivers

6.2 Access to Devices

Device special files(often referred to asdevice files) are used to access expansion devices. These files are not

associated with a data segment on hard disk or on any other storage medium but establish a link with a

device driver in order to support communication with the expansion device. As far as the application is

concerned, there is little difference between processing regular files and devicefiles. Both are handled by

exactly the same library functions. However, for more convenient handling, several additional commands

are provided to perform actions on device files that are not possible on regular files.

6.2.1 Device Files

Let us examine the approach adopted by reference to a modem attached to a serial interface. The name

of the corresponding device file is/dev/ttyS0. The device is identified not by means of its filename but

by means of the major and minor number of the file; these numbers are managed as special attributes in

the filesystem.

The same tools used to read and write regular files are employed to write data to or read results from a

device file. For instance,

wolfgang@meitner>echo "ATZ" > /dev/ttyS0

sends an initialization string to a modem connected to the first serial interface.

6.2.2 Character, Block, and Other Devices

The ways in which data are exchanged between peripherals and the system can be classified into several

categories. Some devices are better suited to character-oriented exchange because the volumes of data

transferred are low. Others are better able to handle data blocks with a fixed number of bytes. The kernel

makes a distinction between character and block devices. The former category includes serial interfaces

and text consoles, while the latter covers hard disks, CD-ROM devices, and the like.

IdentifyingDevice Files

Both the above types can be distinguished by reference to the properties of their device files. Let us look

at some members of the/devdirectory.

wolfgang@meitner>ls -l /dev/sd{a,b} /dev/ttyS{0,1}

brw-r----- 1 root disk 8, 0 2008-02-21 21:06 /dev/sda

brw-r----- 1 root disk 8, 16 2008-02-21 21:06 /dev/sdb

crw-rw---- 1 root uucp 4, 64 2007-09-21 21:12 ttyS0

crw-rw---- 1 root uucp 4, 65 2007-09-21 21:12 ttyS1

In many aspects, the above output is not different from that for regular files, particularly as concerns the

access permissions. However, there are two important differences.

❑ The letter preceding the access permissions is eitherborcto denoteblockandcharacterdevices,

respectively.

❑ Instead of the file size, two figures are given; these are themajor numberandminor number.

Together they form a unique number that allowsthe kernel to find the corresponding device

driver.