The Internal Representation of Device Numbers
Within the kernel, the dev_t type (defined in <linux/types.h>) is used to hold device
numbers—both the major and minor parts. As of Version 2.6.0 of the kernel, dev_t is
a 32-bit quantity with 12 bits set aside for the major number and 20 for the minor
number. Your code should, of course, never make any assumptions about the inter-
nal organization of device numbers; it should, instead, make use of a set of macros
found in <linux/kdev_t.h>. To obtain the major or minor parts of a dev_t, use:
MAJOR(dev_t dev);
MINOR(dev_t dev);
If, instead, you have the major and minor numbers and need to turn them into a dev_t,
use:
MKDEV(int major, int minor);
Note that the 2.6 kernel can accommodate a vast number of devices, while previous
kernel versions were limited to 255 major and 255 minor numbers. One assumes
