Character driver example

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. See Chapter 10, Drivers for Character Devices for more information. Byte-Stream I/O.  · 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.  · 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.

char_device_id= MAJOR (mydev);//Get the major no. llk_cdev= cdev_alloc (); // Get an allocated cdev structure. /*Register the character Device*/. veda_cdev-owner=THIS_MODULE; veda_cdev-ops= char_device_file_ops; /* Add this to the kernel */. ret=cdev_add (llk_cdev,devicenums,1); if (ret < 0) {. static struct cdev c_dev; // Global variable for the character device structure: static struct class *cl; // Global variable for the device class: static int my_open (struct inode *i, struct file *f) {printk (KERN_INFO " Driver: open() "); return 0;} static int my_close (struct inode *i, struct file *f) {printk (KERN_INFO " Driver: close() "); return 0;}. As can be seen from the example above, device-type information can be found using the ls command. The special character files are identified by the c character in the first column of the command output, and the block type by the character b. In columns 5 and 6 of the result you can see the major, respectively the minor for each device.

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