Minor device number

A minor device number is the second component of a device number. In the Linux interface, minor(d) extracts it from a value of type dev_t; the matching major device number supplies its device-class context.^[ref]

A concrete Linux allocation distinguishes two character-special interfaces sharing major1: /dev/null has minor3, while /dev/zero has minor5.^[ref] The former supplies end-of-file when read, while the latter supplies zero-valued bytes. Their different minor values select different interface behavior within the same major allocation.^[ref]

For an original pair comparison, let two registered character-device interfaces have numbers $(240,7)$ and $(241,7)$. Both have minor7, but their complete device numbers differ. Conversely, $(240,7)$ and $(240,8)$ share a major while selecting different minor values within it. A minor value alone is therefore insufficient to identify the interface.

Minor numbers are interpreted in the relevant device allocation; there is no universal rule that every minor counts disks, partitions or physical pieces of hardware. The device may also be a purely virtual interface. The conversion functions preserve the structured components:for supported component values. Extracting the component with the documented interface avoids assuming an implementation-specific bit packing.