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Everything is a File

One of the unique things about Unix as an operating system is that regards everything as a file. Files can be divided into three categories; ordinary or plain files, directories, and special or device files.

Directories in Unix are properly known as directory files. They contain information such as owner, permissions, and size for a set of files.

Ordinary or plain files in Unix are not all text files. They may also contain ASCII text, binary data, and program input or output. Executable binaries (programs) are also files, as are commands. When a user enters a command, the associated file is retrieved and executed. This is an important feature and contributes to the flexibility of Unix.

Special files are also known as device files. In Unix all physical devices are accessed via device files; they are what programs use to communicate with hardware. Files hold information on location, type, and access mode for a specific device. There are two types of device files; character and block, as well as two modes of access.

Block device files are used to access block device I/O. Block devices do buffered I/O, meaning that the the data is collected in a buffer until a full block can be transfered.

Character device files are associated with character or raw device access. They are used for unbuffered data transfers to and from a device. Rather than transferring data in blocks the data is transfered character by character. One transfer can consist of multiple characters.

4. So what about a device that could be accessed in character or block mode? How many device files would it have?

Some devices, such as disk partitions, may be accessed in block or character mode. Because each device file corresponds to a single access mode, physical devices that have more than one access mode will have more than one device file.

Device files are found in the /dev directory. Each device is assigned a major and minor device number. The major device number identifies the type of device, i.e. all SCSI devices would have the same number as would all the keyboards. The minor device number identifies a specific device, i.e. the keyboard attached to this workstation.

Device files are created using the mknod command. The form for this command is:

mknod device-name type major minor

The major and minor device numbers are indexed to device switches. There are two types of device switches; cdevsw for character devices and bdevsw for block devices. These switches are kernel structures that hold the names of all the control routines for a device and tell the kernel which driver module to execute. Device switches are actually tables that look something like this:

Using the ls command in the /dev directory will show entries that look like:

brw-r----- 1 root sys 1, 0 Aug 31 16:01 /dev/sd1a

The "b" before the permissions indicates that this is a block device file. When a user enters /dev/sd1a the kernel sees the file opening, realizes that it's major device number 1, and calls up the SCSIbus function to handle it.

Terms used: SCSI, tty, ASCII, I/O.

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