1. Linux Character Device Driver
2. Linux Character Device Driver For Mac

Character Device Driver The kernel offers several subroutines or functions in user space, which allow the end user application programmer to interact with the hardware. Usually, in UNIX or Linux systems, this dialogue is performed through functions or subroutines in order to read and write files.

• Linux Device Drivers, 2nd Edition. There's no need to include the header explicitly in the drivers, however, because linux/fs.h> does it. Registers a character.
• The Device Driver Source Code. The source code for the ebbchar device driver is provided in Listing 2. Similar to the code in the first article in this series, there is an init() function and an exit() function.
• The starting ‘c’ means its a character device, 1 is the major number and 8 is the minor number. A Linux driver is a Linux module which can be loaded and linked to the kernel at runtime.
• LINUX CHARACTER DEVICE DRIVERS DEVELOPMENT. This intensive course transforms an IT-Professional or a Student into a Linux Device Driver & Kernel Developer for Character Storage devices, Block Storage devices & Network Storage devices and for those Programmers who already working on Linux, but looking for a methodical and concept clearing professional training on Linux system and kernel.

I know the definition

Linux Character Device Driver

A character device driver is one that transfers data directly to and from a user process.

But can some one explain this in a more intutive way? First of all there should be a device. What is the device in the above definition?

If you say it can be a file, then can we say file reading and putting the data on the console an example of character driver?

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4 Answers

What exactly is a character driver ?

Device driver is Integration of two pieces of code. First piece of code is how the driver services are made available to the application.(user space)

Second piece of code is the hardware access part. Instructions to carry out physical operation on target hardware.

Based on first piece of code we have three models. Character model, Block model, Network model. we can say character driver, block driver, Network driver.

First piece of code is totally kernel specific. what Interface kernel provides to access the hardware. Implementing first piece of code on Linux,windows on other os may be different. we should know what inteface it offers to provide services to application .

In linux From user perspective everyhardware is a file. Why means at boot time all the hardware devices present are detected and added to device tree. Based on hardware corresponding device node gets created automatically in /dev directory. As mentioned above if it is char device---> character device node. Block device---> block device node gets created.

To write character driver. we create a device node in /dev directory , assigning a major no and application usually performing read/write operations on device file.

We implement driver operations and assign driver operations to file structure (fops) pointer.

Device file request received by VFS. VFS turns device file operations into your corresponding driver operations.

APP--->dev file----> fops--->driver---->device.

dev file ---> interface between application and driver.

driver interacts ----> device

You can refer http://lwn.net/images/pdf/LDD3/ch03.pdf.

According to my knowledge,

The device can be your own private structure or system object.

The driver is said to be a char driver because the data read and write is in byte range

If you are writing your char driver you can use char buffer or kfifo to read and write into the device.

you can create your device file in procfs,and can read/write as u wish and this is accomplished though your char driver

I hope that my answer helps you

Think of device driver as abstraction of various hardware that the kernel has to deal with. The device driver knows the details of hardware it is communicating with. So the kernel reads and writes data as it would do with any other filesystem file.

If you can make some reading on Device Drivers, you will find out that the system calls of fopen, read, write, fclose are all mapped to driver specific function calls.

There is 2 types of drivers: block driver and char driver. The difference is that first one handles with blocks of data and second one receives/transmits data byte by byte.

Ok, for example you have a device and you want to talk to it, how to do it?

You have a driver, simply a number of functions, that will be called by the kernel at the right time. Driver itself can't do anything (neither send data, nor receive data), you need to say your driver what it should do. As you should know so far, you can't discretely communicate with a kernel from user space. There are some tricks that you can use. What you are talking about named as device file. Your driver generates a file for your device and when you write something to this file from user space, driver is notified that it is some information to handle, it takes this information and provide this data to write function, that will transmit data to your device(in this case byte by bite).

Hopefully, this what you wanted to know.

Not the answer you're looking for? Browse other questions tagged driver or ask your own question.

In Unix-likeoperating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in MS-DOS, OS/2, and Microsoft Windows. These special files allow an application program to interact with a device by using its device driver via standard input/outputsystem calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.

Device files usually provide simple interfaces to standard devices (such as printers and serial ports), but can also be used to access specific unique resources on those devices, such as disk partitions. Additionally, device files are useful for accessing system resources that have no connection with any actual device such as data sinks and random number generators.

There are two general kinds of device files in Unix-like operating systems, known as character special files and block special files. The difference between them lies in how much data is read and written by the operating system and hardware. These together can be called device special files in contrast to named pipes, which are not connected to a device but are not ordinary files either.

MS-DOS borrowed the concept of special files from Unix but renamed them devices.^[1] Because early versions of MS-DOS did not support a directory hierarchy, devices were distinguished from regular files by making their names reserved words, chosen for a degree of compatibility with CP/M.

In some Unix-like systems, most device files are managed as part of a virtual file system traditionally mounted at /dev, possibly associated with a controlling daemon, which monitors hardware addition and removal at run time, making corresponding changes to the device file system if that's not automatically done by the kernel, and possibly invoking scripts in system or user space to handle special device needs. The FreeBSD and DragonFly BSD implementations have named the virtual device file system devfs and the associated daemondevd. Linux primarily uses a user space implementation known as udev, but there are many variants. Darwin, and operating systems such as macOS based on it, have a purely kernel-based device file system.

In Unix systems which support chroot process isolation, such as Solaris Containers, typically each chroot environment needs its own /dev; these mount points will be visible on the host OS at various nodes in the global file system tree. By restricting the device nodes populated into chroot instances of /dev, hardware isolation can be enforced by the chroot environment (a program can not meddle with hardware that it can neither see nor name—an even stronger form of access control than Unix file system permissions).

MS-DOS managed hardware device contention (see TSR) by making each device file exclusive open. An application attempting to access a device already in use would discover itself unable to open the device file node. A variety of device driver semantics are implemented in Unix and Linux concerning concurrent access.^[2]

• 1Unix and Unix-like systems
  • 1.6devfs

Unix and Unix-like systems[edit]

Device nodes correspond to resources that an operating system's kernel has already allocated. Unix identifies those resources by a major number and a minor number,^[3] both stored as part of the structure of a node. The assignment of these numbers occurs uniquely in different operating systems and on different computer platforms. Generally, the major number identifies the device driver and the minor number identifies a particular device (possibly out of many) that the driver controls:^[4] in this case, the system may pass the minor number to a driver. However, in the presence of dynamic number allocation, this may not be the case (e.g. on FreeBSD 5 and up).

As with other special file types, the computer system accesses device nodes using standard system calls and treats them like regular computer files. Two standard types of device files exist; unfortunately their names are rather counter-intuitive for historical reasons, and explanations of the difference between the two are often incorrect as a result.

Character devices[edit]

Character special files or character devices provide unbuffered, direct access to the hardware device. They do not necessarily allow programs to read or write single characters at a time; that is up to the device in question. The character device for a hard disk, for example, will normally require that all reads and writes are aligned to block boundaries and most certainly will not allow reading a single byte.

Character devices are sometimes known as raw devices to avoid the confusion surrounding the fact that a character device for a piece of block-based hardware will typically require programs to read and write aligned blocks.

Block devices[edit]

Block special files or block devices provide buffered access to hardware devices, and provide some abstraction from their specifics.^[5] Unlike character devices, block devices will always allow the programmer to read or write a block of any size (including single characters/bytes) and any alignment. The downside is that because block devices are buffered, the programmer does not know how long it will take before written data is passed from the kernel's buffers to the actual device, or indeed in what order two separate writes will arrive at the physical device. Additionally, if the same hardware exposes both character and block devices, there is a risk of data corruption due to clients using the character device being unaware of changes made in the buffers of the block device.

Most systems create both block and character devices to represent hardware like hard disks. FreeBSD and Linux notably do not; the former has removed support for block devices,^[6] while the latter creates only block devices. In Linux, to get a character device for a disk one must use the 'raw' driver, though one can get the same effect as opening a character device by opening the block device with the Linux-specific O_DIRECT flag.

Pseudo-devices[edit]

Device nodes on Unix-like systems do not necessarily have to correspond to physical devices. Nodes that lack this correspondence form the group of pseudo-devices. They provide various functions handled by the operating system. Some of the most commonly used (character-based) pseudo-devices include:

• /dev/null – accepts and discards all input written to it; provides an end-of-file indication when read from
• /dev/zero – accepts and discards all input written to it; produces a continuous stream of Null characters (zero-value bytes) as output when read from
• /dev/full – produces a continuous stream of Null characters (zero-value bytes) as output when read from, and generates a 'disk full' error when attempting to write to it
• /dev/random – when read, it produces as output a variable-length stream of numbers from the kernel's cryptographically secure pseudorandom number generator. Its exact behavior varies by implementation, and sometimes variants such as /dev/urandom or /dev/arandom are also provided.

Additionally, BSD-specific pseudo-devices with an ioctl interface may also include:

• /dev/pf – allows userland processes to control of the PF (firewall) through an ioctl interface
• /dev/bio – provides ioctl access to devices otherwise not found as /dev nodes, used by bioctl to implement RAID management in OpenBSD and NetBSD
• /dev/sysmon – is used by NetBSD's sysmon_envsys framework for hardware monitoring, accessed in the userland through proplib(3) by the envstat utility^[7]

Node creation[edit]

Nodes are created by the mknodsystem call. The command-line program for creating nodes is also called mknod. Nodes can be moved or deleted by the usual filesystem system calls (rename, unlink) and commands (mv, rm).

Some Unix versions include a script named makedev or MAKEDEV to create all necessary devices in the directory /dev. It only makes sense on systems whose devices are statically assigned major numbers (e.g. by means of hardcoding it in their kernel module).

Naming conventions[edit]

Linux Character Device Driver For Mac

The following prefixes are used for the names of some devices in the /dev hierarchy, to identify the type of device:

• lp: line printers (compare lp)
• pt: pseudo-terminals (virtual terminals)
• tty: terminals

Some additional prefixes have come into common use in some operating systems:

• fb: frame buffer
• fd: (platform) floppy disks, though this same abbreviation is also commonly used to refer to file descriptor
• hd: (“classic”) IDE driver (previously used for ATA hard disk drive, ATAPI optical disc drives, etc.)
  • hda: the master device on the first ATA channel (usually identified by major number 3 and minor number 0)
  • hdb: the slave device on the first ATA channel
  • hdc: the master device on the second ATA channel
    • hdc1: first primary partition on this disk (example)
    • hdc5: first logical drive in the extended partition (example)
  • hdd: the slave device on the second ATA channel
• parport, pp: parallel ports
• NVMe driver
  • nvme0: first registered device's device controller (character device)
  • nvme0n1: first registered device's first namespace (block device)
  • nvme0n1p1: first registered device's first namespace's first partition (block device)
• SCSI driver, also used by libATA (modern PATA/SATA driver), USB, IEEE 1394, etc.
  • sd: mass-storage driver
    • sda: first registered device
      • sda4: last partition on this disk (example)
      • sda6: second logical drive in the extended partition (example)
    • sdb, sdc, etc.: second, third, etc. registered devices
  • ses: Enclosure driver
  • sg: generic SCSI layer
  • sr: “ROM” driver (data-oriented optical disc drives; scd is just a secondary alias)
  • st: magnetic tape driver
• tty: terminals
  • ttyS: (platform) serial port driver
  • ttyUSB: USB serial converters, modems, etc.

The canonical list of the prefixes used in Linux can be found in the Linux Device List, the official registry of allocated device numbers and /dev directory nodes for the Linux operating system.^[8]

For most devices, this prefix is followed by a number uniquely identifying the particular device. For hard drives, a letter is used to identify devices and is followed by a number to identify partitions. Thus a file system may 'know' an area on a disk as /dev/sda3, for example, or 'see' a networked terminal session as associated with /dev/pts/14.

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On disks using the typical PC master boot record, the device numbers of primary and the optional extended partition are numbered 1 through 4, while the indexes of any logical partitions are 5 and onwards, regardless of the layout of the former partitions (their parent extended partition does not need to be the fourth partition on the disk, nor do all four primary partitions have to exist).

Device names are usually not portable between different Unix-like system variants, for example, on some BSD systems, the IDE devices are named /dev/wd0, /dev/wd1, etc.

devfs[edit]

devfs is a specific implementation of a device file system on Unix-like operating systems, used for presenting device files. The underlying mechanism of implementation may vary, depending on the OS.

Maintaining these special files on a physically implemented file system (i.e. harddrive) is inconvenient, and as it needs kernel assistance anyway, the idea arose of a special-purpose logical file system that is not physically stored.

Also defining when devices are ready to appear is not entirely trivial. The 'devfs' approach is for the device driver to request creation and deletion of 'devfs' entries related to the devices it enables and disables.

Implementations[edit]

MS-DOS[edit]

MS-DOS borrowed the concept of special files from Unix but renamed them devices.^[1] Because early versions of MS-DOS did not support a directory hierarchy, devices were distinguished from regular files by making their names reserved words. This means that certain file names were reserved for devices, and should not be used to name new files or directories.^[13]The reserved names themselves were chosen to be compatible with 'special files' handling of PIP command in CP/M. There were two kinds of devices in MS-DOS: Block Devices (used for disk drives) and Character Devices (generally all other devices, including COM and PRN devices).^[14] PIPE, MAILSLOT, and MUP are other standard Windows devices.^[15]

DOS, TOS, OS/2, and Windows[edit]

A device file is a reserved keyword used in MS/PC DOS, Atari TOS, OS/2, and Microsoft Windows systems to allow access to certain ports and devices.

DOS uses device files for accessing printers and ports. Most versions of Windows also contain this support, which can cause confusion when trying to make files and folders of certain names, as they cannot have these names.^[16] Versions 2.x of MS-DOS provide the AVAILDEVCONFIG.SYS parameter that, if set to FALSE, makes these special names only active if prefixed with DEV, thus allowing ordinary files to be created with these names.^[17]

GEMDOS, the DOS-like part of Atari TOS, supported similar device names to DOS, but unlike DOS it required a trailing ':' character (on DOS, this is optional) to identify them as devices as opposed to normal filenames (thus 'CON:' would work on both DOS and TOS, but 'CON' would name an ordinary file on TOS but the console device on DOS). In MiNT and MagiC, a special UNIX-like unified filesystem view accessed via the 'U:' drive letter also placed device files in 'U:DEV'.

Using shell redirection and pipes, data can be sent to or received from a device. For example, typing the following will send the file c:data.txt to the printer:

IOCS[edit]

The 8-bit operating system of Sharppocket computers like the PC-E500, PC-E500S etc. consists of a BASIC interpreter, a DOS 2-like File Control System (FCS) implementing a rudimentary 12-bit FAT-like filesystem, and a BIOS-like Input/Output Control System (IOCS) implementing a number of standard character and block device drivers as well as special file devices including STDO:/SCRN: (display), STDI:/KYBD: (keyboard), COM: (serial I/O), STDL:/PRN: (printer), CAS: (cassette tape), E:/F:/G: (memory file), S1:/S2:/S3: (memory card), X:/Y: (floppy), SYSTM: (system), and NIL: (function).^[23]

See also[edit]

References[edit]

1. ^ ^ab'Windows for Workgroups: How VSHARE.386 Manages File Sharing'. Support.microsoft.com. 1999-09-22. Retrieved 2014-01-22.Cite web requires website= (help)
2. ^Corbet, Jonathan; Kroah-Hartman, Greg; Rubini, Alessandro (2005). 'Access Control on a Device File'. Linux Device Drivers, 3rd Edition. O'Reilly. Retrieved 2017-04-28. The next step beyond a single-open device is to let a single user open a device in multiple processes but allow only one user to have the device open at a time.
3. ^Kernighan, Brian W.; Pike, Rob (1984). The UNIX Programming Environment. Prentice-Hall. p. 66. ISBN0-13-937681-X.
4. ^Neil Brown (2010-10-27). 'Ghosts of Unix Past: a historical search for design patterns'. LWN.net. Retrieved 2014-03-30.
5. ^'IEEE Std 1003.1, 2013 Edition'. Retrieved 2014-04-24.Cite web requires website= (help)
6. ^'FreeBSD Architecture Handbook'. Retrieved 2013-03-07.Cite web requires website= (help)
7. ^'usr.sbin/envstat/envstat.c'. BSD Cross Reference. NetBSD. Lay summary.
8. ^Linux Assigned Names and Numbers Authority (2009-04-06). 'Linux allocated devices (2.6+ version)'. Linux kernel (Documentation/devices.txt). Archived from the original on 2016-04-24. Retrieved 2013-06-08.Cite uses deprecated parameter dead-url= (help)
9. ^'Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev'. LWN. Retrieved 2009-08-10.Cite web requires website= (help)
10. ^'Project Black change log'. Retrieved 2016-05-15.Cite web requires website= (help)
11. ^'The drive U: in MagiC'. 2016-03-28. Archived from the original on 2017-01-15. Retrieved 2017-01-09.Cite uses deprecated parameter dead-url= (help); Cite web requires website= (help)
12. ^'FreeMiNT-Portal - mint.doc'. 2000-04-27. Archived from the original on 2017-01-15. Retrieved 2017-01-09.Cite uses deprecated parameter dead-url= (help); Cite web requires website= (help)
13. ^'Avoid Creating Macintosh Filenames that are NT Device Names'. Support.microsoft.com. 2006-11-01. Retrieved 2014-01-22.Cite web requires website= (help)
14. ^'device attributes'. Stanislavs.org. Retrieved 2014-01-22.Cite web requires website= (help)
15. ^'REG: CurrentControlSet Entries PART 2: SessionManager'. Support.microsoft.com. 2006-11-01. Retrieved 2014-01-22.Cite web requires website= (help)
16. ^ ^ab'MS-DOS Device Driver Names Cannot be Used As File Names'. Revision 2.0. Microsoft. 2003-05-12. KB74496, Q74496. Archived from the original on 2012-07-21.Cite uses deprecated parameter dead-url= (help); Cite web requires website= (help)
17. ^'Undocumented Commands'. 4dos.info. Kevtronics. 2002-04-12. Retrieved 2014-05-16.
18. ^ ^abcdefIBM Operating System/2 Technical Reference - Programming Family(PDF). 1 (1st ed.). IBM. September 1987 [1986]. Archived(PDF) from the original on 2017-01-03.Cite uses deprecated parameter dead-url= (help)
19. ^ ^abcdefghiHewlett-Packard - Technical Reference Manual - Portable PLUS (1 ed.). Corvallis, OR, USA: Hewlett-Packard Company, Portable Computer Division. August 1985. 45559-90001. Retrieved 2016-11-27.Cite uses deprecated parameter dead-url= (help)
20. ^ ^abcdefghiHewlett-Packard - Technical Reference Manual - Portable PLUS(PDF) (2 ed.). Portable Computer Division, Corvallis, OR, USA: Hewlett-Packard Company. December 1986 [August 1985]. 45559-90006. Archived(PDF) from the original on 2016-11-28. Retrieved 2016-11-27.Cite uses deprecated parameter dead-url= (help)
21. ^ ^abcPaul, Matthias R. (1997-10-02). 'Caldera OpenDOS 7.01/7.02 Update Alpha 3 IBMBIO.COM README.TXT'. Archived from the original on 2003-10-04. Retrieved 2009-03-29.Cite uses deprecated parameter dead-url= (help); Cite web requires website= (help)[1]
22. ^Paterson, Tim; Microsoft (2013-12-19) [1983]. 'Microsoft DOS V1.1 and V2.0: /msdos/v20source/SKELIO.TXT, /msdos/v20source/HRDDRV.ASM'. Computer History Museum, Microsoft. Retrieved 2014-03-25.Cite web requires website= (help) (Note: While the publishers claim this would be MS-DOS 1.1 and 2.0, it actually is SCP MS-DOS 1.25 and a mixture of Altos MS-DOS 2.11 and TeleVideo PC DOS 2.11.)
23. ^Technical Reference Manual PC-E500(PDF). Sharp Corporation, Information Systems Group, Personal Equipment Division. March 1990. p. 17. Archived(PDF) from the original on 2017-03-14. Retrieved 2017-03-14.Cite uses deprecated parameter dead-url= (help)

Further reading[edit]

• Philip Streck (2002-09-24). 'devfs for Management and Administration'. Linux Journal.
• Daniel Robbins (2001-10-01). 'Part 4: Introduction to devfs'. Common threads: Advanced filesystem implementor's guide. IBM.
• Daniel Robbins (2001-10-01). 'Part 5: Setting up devfs'. Common threads: Advanced filesystem implementor's guide. IBM.
• Daniel Robbins (2001-10-01). 'Part 6: Implementing devfs (using the init wrapper)'. Common threads: Advanced filesystem implementor's guide. IBM.
• Doug Gilbert (2001-01-22). 'DEVFS and SCSI'.Cite web requires website= (help)
• '3.3. Device Names in devfs'. The Linux 2.4 SCSI subsystem HOWTO: Chapter 3. Names and Addresses. Linux Documentation Project.
• 'Device File System Guide'. Gentoo Linux Documentation. Gentoo Foundation, Inc. Archived from the original on 2008-05-21. Retrieved 2008-06-19.Cite uses deprecated parameter dead-url= (help)
• Mark Ellis (2003-09-23). 'How to use kernel module autoloading with devfs and devfsd'. Linux From Scratch Hints.
• Martial Daumas (2003-09-18). 'How to create a basic mk_initrd command that works nice with LFS and devfs'. Linux From Scratch Hints.
• Jeroen Coumans (2003-04-19). 'How to setup devfs with your current LFS-configuration using devfsd. How you can use devfs from scratch'. Linux From Scratch Hints.
• Tushar Teredesai (2003-03-05). 'Using devfs and devfsd'. Linux From Scratch Hints.