int register_chrdev_region(dev_t first, unsigned int count,char *name);

register_chrdev_region works well if you know ahead of time exactly which device numbers you want. Often, however, you will not know which major numbers your device will use; to move over to the use of dynamicly-allocated device numbers on the fly, use the following:

int alloc_chrdev_region(dev_t *dev, unsigned int firstminor,unsigned int count, char *name);

cdev_alloc:
struct cdev * cdev_alloc (void);
Allocates and returns a cdev structure, or NULL on failure.

cdev_init:
void cdev_init (struct cdev *cdev,const struct file_operations *fops);
Initializes cdev, remembering fops, making it ready to add to the system with cdev_add.

cdev_add
int cdev_add (struct cdev *p,dev_t dev,unsigned count);
cdev_add adds the device represented by p to the system, making it live immediately, and can be used by the user.

cdev_del
void cdev_del ( struct cdev *p);
cdev_del removes p from the system, possibly freeing the structure itself.

Hope it will clear the doubts.