Example character device driver code

 · 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”).Estimated Reading Time: 9 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. 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 *, size_t, loff_t *); ssize_t (*write) (struct file .

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”). Major = register_chrdev (0, DEVICE_NAME, fops); if (Major device failed with %d ", Major); return Major;} printk (KERN_INFO " I was assigned major number %d. To talk to ", Major); printk (KERN_INFO " the driver, create a dev file with "); printk (KERN_INFO " 'mknod /dev/ %s c %d 0'. ", DEVICE_NAME, Major);. In the UNIX world there are two categories of device files and thus device drivers: character and block. This division is done by the speed, volume and way of organizing the data to be transferred from the device to the system and vice versa. In the first category, there are slow devices, which manage a small amount of data, and access to data does not require frequent seek queries. Examples are devices such as keyboard, mouse, serial ports, sound card, joystick.

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