Use of count variable in register_chrdev_region

Question

As of I understand, the signature of the register_chrdev_region is described as follows

extern int register_chrdev_region(dev_t firstmajor,unsigned int count,const char*dev_name);
//firstmajor: The major number requested for reservation of the dirver
//dev_name: Name of the device associated with the major number(for procfs and sysfs)
//count: Total number of contagious device numbes that is requested??

I don't get the usage of count arguement in the function (in alloc_chrdev_region as well). Please explain a simple use case of reserving contagious device numbers for the driver

Reference 3.2.2 in http://www.makelinux.net/ldd3/chp-3-sect-2

Answer 1

The comment said:

/**     
 * alloc_chrdev_region() - register a range of char device numbers
 * @dev: output parameter for first assigned number
 * @baseminor: first of the requested range of minor numbers
 * @count: the number of minor numbers required
 * @name: the name of the associated device or driver
 *      
 * Allocates a range of char device numbers.  The major number will be
 * chosen dynamically, and returned (along with the first minor number)
 * in @dev.  Returns zero or a negative error code.
 */

and you can find an example in fs/fuse/cuse.c:

/* determine and reserve devt */
devt = MKDEV(arg->dev_major, arg->dev_minor);
if (!MAJOR(devt))
        rc = alloc_chrdev_region(&devt, MINOR(devt), 1, devinfo.name);
else
        rc = register_chrdev_region(devt, 1, devinfo.name);
if (rc) {
        printk(KERN_ERR "CUSE: failed to register chrdev region\n");
        goto err;
}