Character device driver examples

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Character device drivers normally perform I/O in a byte stream. Examples of devices using character drivers include tape drives and serial ports. Character device drivers can also provide additional interfaces not present in block drivers, such as I/O control (ioctl) commands, memory mapping, and device polling. For example, every character driver needs to define a function that reads from the device. The file_operations structure holds the address of the module's function that performs that operation. Here is what the definition looks like for kernel struct file_operations { struct module *owner; loff_t (*llseek) (struct file *, loff_t, int); ssize_t (*read) (struct file *, char __user *, size_t, loff_t . Exercises ¶. 1. Register/unregister ¶. The driver will control a single device with the MY_MAJOR major and MY_MINOR minor (the macros defined in the 2. Register an already registered major ¶. 3. Open and close ¶. 4. Access restriction ¶. 5. Read operation ¶. Estimated Reading Time: 7 mins.

Device drivers have an associated major and minor number. For example, /dev/ram0 and /dev/null are associated with a driver with major number 1, and /dev/tty0 and /dev/ttyS0 are associated with a driver with major number 4. The major number is used by the kernel to identify the correct device driver when the device is accessed. Linux Character Device Example. Raw. chardev.c. /*. * chardev.c: Creates a read-only char device that says how many times you've. * read from the dev file. *. * You can have some fun with this by removing the module_get/put calls, * allowing the module to be removed while the file is still open. In this post, we would be writing a Linux device driver for a hypothetical character device which reverses any string that is given to it. i.e. If we write any string to the device file represented by the device and then read that file, we get the string written earlier but reversed (for eg., myDev being our device, echo “hello” /dev/myDev ; cat /dev/ myDev would print “olleh”).

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