 Device number

A device number identifies a device interface using a major device number and a minor device number. In the Linux interfaces discussed here, the combined value has type dev_t; makedev combines the two components and major and minor recover them. The documented interpretation associates the major value with a device class and the minor value with an instance within that class.^[ref]

A device node also has an inode number, and its containing file system has a device identity of its own. The fields st_ino, st_dev and st_rdev therefore answer different questions: which inode, which containing device, and which represented device for a device-special file.^[ref]

For an original metadata comparison, stipulate two character-special nodes in one file system with inode numbers 40 and 90, each representing major 240, minor 7. They are different named file-system objects representing the same interface in this example. A third node with major 240, minor 8 represents a different minor interface. The file type remains relevant: character and block device numbers are not equated solely by matching two integers.

Combining the components is not decimal concatenation. The pairs $(1,23)$ and $(12,3)$ both become the character string 123 if their decimal spellings are joined without a separator. The structured pairs are nevertheless different. The appropriate dev_t conversion preserves their distinct components rather than imposing that ambiguous textual encoding.

For valid supported component values, the intended round trips areUse the defined conversion interface rather than assuming a particular in-memory packing. The device number is an interface identifier; it is not automatically a serial number, a stable physical-device identity or the process-local file descriptor returned by opening the node.