%Character Devices
%Thadeu Cascardo

# Introduction

Devices in POSIX Systems are files in /dev directory. As with any files
in POSIX, they may be opened, closed, read from, written to, seeked,
ioctl'd, and others.

Examples of device files:

* /dev/sda - A SCSI block device
* /dev/ttyS0 - A Serial terminal device

# POSIX I/O calls

POSIX systems have some standard calls for I/O. Since devices are files,
these same system calls are used to work with devices. We are gonna work
with the following calls:

* open
* read
* write
* close
* lseek
* ioctl

# Device types and numbers

Linux devices may be of different types, including character devices,
block devices or network devices. Both character and block devices have
identifying numbers, a major and a minor number.

# VFS

# Introduction

The virtual file system is the hub for almost all operations in a Linux-based
system. It allows IPC with pipes, access to devices, including storage through
regular files and organization with directories.

# Everything is a file

In Unix, there's a say: "everything is a file, if it's not a file, it's a
process". Well, most things are really files, and that's why the VFS is at the
center of the system, including for device drivers.

# Special files, procfs and others

When handling with special files (character and block device nodes), procfs
files and others, we'll use some common structures. These include the
*struct file\\_operations*, *struct file* and *struct inode*.

Do not forget to include linux/fs.h.

# File Operations

* owner
* open
* release
* flush
* read
* write
* ioctl
* unlocked\\_ioctl
* llseek
* poll
* mmap
* many others

# Opened File

* f\\_mode
* f\\_flags
* f\\_pos
* f\\_op
* private\\_data

# Filesystem File: inode

The inode is a representation of the file as in its filesystem, including its
major/minor numbers and pointers to the corresponding device representation.

# Character Devices

# Device Number Macros

Nowadays, major number is 12 bits and minor number is 20 bits. This may change
in the future, so macros should be used.

* include linux/kdev\\_t.h
* MAJOR
* MINOR
* MKDEV

# Character devices allocation

In Linux, major and minor numbers have to be requested or allocated. The
calls to do that for character devices are:

* int register\\_chrdev\\_region (dev\\_t first, unsigned int count, char
*name);
* int alloc\\_chrdev\\_region (dev\\_t *dev, unsigned int firstminor,
unsigned int count, char *name);
* void unregister\\_chrdev\\_region (dev\\_t dev, unsigned int count);

# cdev structure

Use *cdev\\_alloc* or *cdev\\_init* to allocate or initalize a cdev structure.

Use *cdev\\_add* and *cdev\\_del* to register and unregister it.

# Open/Release

Implementing open and release should be very simple for many devices. They
usually allocate data, and multiplex devices by minor number.

# Inode structure

Use *imajor* and *iminor* to get the major and minor number from an inode
structure.

# Read

Read may write to user space less bytes than requested.

# Write

Write is very similar to read. Only problem is that many applications misbehave
when driver writes less bytes than requested.