Everything in Linux is a file. This is a common phrase related to Linux. As and when one gradually learns Linux, he/she gets to understand the fundamentals behind this phrase. In Linux even hardware devices are recognized as files. These are known as Linux device files or special files. Here in this article, we shall be compass the way Linux deals with Device files.
Linux Device files
For Linux, devices can be termed as special files that represent physical or virtual devices attached to Linux system. These special files are used by the Linux operating system to enable access to the physical or virtual devices. This was done in order to make these device accessible using standard I/O operations. Every such device would need to have its specific device driver, which is a Linux kernel module, leveraging the users and the user space programs to interact with the devices.
Here is an abstract illustration:
As can be seen in the figure, the actual hardware is recognized as a file or node at the OS level. The user space applications access the device driver of the device node or device file through standard system calls.
In Linux, every device needs to have device driver so that it can be made accessible by the OS to the users. The device driver actually provides the standard input/output calls to access the device and interact with it. Worth mentioning, all the device drivers are kernel modules in Linux.
All the devices on the Linux can be viewed through:
cat /proc/devices
All the devices will be listed, for example:
Character devices: 1 mem 4 /dev/vc/0 4 tty 4 ttyS 5 /dev/tty 5 /dev/console 5 /dev/ptmx 5 ttyprintk 6 lp 7 vcs 10 misc 13 input 21 sg 29 fb 99 ppdev 108 ppp 116 alsa 128 ptm 136 pts 180 usb 189 usb_device 226 drm 251 hidraw 252 usbmon 253 bsg 254 rtc Block devices: 1 ramdisk 2 fd 259 blkext 7 loop 8 sd 9 md 11 sr 65 sd 66 sd 67 sd 68 sd 69 sd 70 sd 71 sd 128 sd 129 sd 130 sd 131 sd 132 sd 133 sd 134 sd 135 sd 252 device-mapper 253 pktcdvd 254 mdp
We see quite a lot familiar devices here, like, memory, ramdisk, usb, etc. However, apart from the external physical devices, Linux also recognises virtual devices which can be created given a memory range, and accessed using its device drivers through the I/O methods. For example, note the device ttyl, which is the terminal. Physically, a Linux terminal is not a device, but it is treated as a device, and hence listed out here.
Hence, Linux device drivers provide the capability to create and use virtual devices also on Linux.
One can also find all the device nodes created at:
ls /dev/
Types of devices
There are generally three types of devices in Linux:
1. Character Devices
2. Block devices
3. Network Devices
The segregation is majorly in the way these devices interact and perform I/O with the operating system. Hence, for all these three types of devices, the device drivers are also classified with these three types. Therefore, a character device will have a character device driver module associated with it, and similarly for block devices, block module and network module for network devices. Lets comprehend each of the type:
1. Character Devices
Character Devices are devices where data is a stream of bytes i.e. the input/output happens byte by byte. Therefore, the character device drivers would be in a such a way to implement such kind of streamed data. The interactive operations (often called the file operations) which would be most vital to implement are:
- open()
- read()
- write()
- close()
Examples: A terminal, Keyboard, etc
2. Block Devices
Block Devices are devices, as the name suggests, deal with blocks of data. Therefore, block device drivers are implemented to read/write block data and also supports random access data. However, since data is to be handled in blocks, therefore its implementation involves an extra intermediate buffer to store data between reads and writes. It has block device operations such as
- open()
- release()
- direct_access()
- revalidate_disk()
etc.
Examples: A floppy, hard disks
3. Network devices
Network device are the ones which are used to transmit data from one machine to another. The network device drivers implementation is entirely different to those of char devices and block devices. Here, they are exchange data in the form of data packets with another remote machine, using a standard protocol.They also deal with setting up ip addresses, configurations and modifying transmission parameters, traffic, etc.
Their I/O operations are specific to the data packet transmission protocol, as in, TCP/UDP.
Examples: NIC
Some interesting facts related to Linux device files
- One can access the device filesystem through /dev/
- Note from the /proc/devices output, even the Linux is aware of what all are character devices and what all are block devices. Network device are completely different class.
- One device driver can cater to various devices depending upon the functionality, like a device driver for a xerox/scanner machine would implement for xerox as well as scanner.
