= Linux device-number allocation
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\b[Linux device-number allocation] assigns or reserves the numerical identifiers through which device-special interfaces are registered. The relevant identity includes device type together with <major device number> and <minor device number>. It is distinct from naming a <device node>, assigning its <inode> number, or obtaining a process-local <file descriptor>.
The kernel's allocated-device list records established assignments. For example, character major 1 includes minor 3 for `/dev/null` and minor 5 for `/dev/zero`. The same integer major 1 has a separate block-device allocation for RAM disks. These entries concern different interface namespaces, not interchangeable interpretations of a single untyped integer.<Linux allocated devices in the Linux kernel documentation>[Entry 1, character memory devices and block RAM disks]{ref}
For character devices, the kernel API separates requesting a particular range with `register_chrdev_region` from <dynamic character-device number allocation> with `alloc_chrdev_region`. The latter receives a first minor and count, chooses the major dynamically, and returns the first assigned combined <device number>. The caller must check whether reservation succeeded.<Character-device number registration in the Linux kernel documentation>[Char devices, register_chrdev_region and alloc_chrdev_region]{ref}
An original allocation example requests four minor values starting at 20. If the returned major is $M$, the reserved range in this example is
$$
(M,20),\ (M,21),\ (M,22),\ (M,23).
$$
It does not contain $(M,24)$. Code should use the returned $M$ rather than guess a major from an earlier machine or boot. A numerical allocation can serve software interfaces as well as physical hardware, so four minors do not necessarily imply four physical peripherals.
Reserving numbers is only one step. The character-device object must also be associated with its operations and added to the kernel; `cdev_add` makes it live. Separately, userspace needs an appropriate <device node> or other naming arrangement. The registration name used for bookkeeping is not itself a pathname under `/dev`.<Character-device number registration in the Linux kernel documentation>[Char devices, cdev_init, cdev_add and __register_chrdev name description]{ref}
This separation explains a common diagnostic trap. Creating a node containing a chosen major/minor pair does not reserve that range or supply a working <device driver>. Conversely, allocating a range does not make every invented pathname reach it. Inspect the actual registered interface, file type and stored <device number> together when identifying a device.
The allocation is also not a persistent hardware serial number. Dynamic assignment can differ between configurations or registration histories. Releasing a number range with `unregister_chrdev_region` returns the reservation; a complete driver teardown must also handle the device object's lifetime and existing users. The kernel documentation notes that deleting a character-device object prevents new opens while operations on already open descriptors can remain possible.<Character-device number registration in the Linux kernel documentation>[Char devices, unregister_chrdev_region and cdev_del lifetime note]{ref}
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