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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.^[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.^[ref]

An original allocation example requests four minor values starting at 20. If the returned major is $M$, the reserved range in this example isIt 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.^[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.^[ref]

Unwinding a failed character-device registration releases resources acquired before an initialization failure while preserving the lifetime of anything that concurrent users can still access. In Linux, reserving device numbers, publishing the character-device object and creating userspace-visible names are distinct steps. The cleanup path must know which steps actually succeeded.

Consider an original abstract initialization sequence: reserve a number range, initialize a character-device object, publish it, then finish a later setup step. The following table tracks successful acquisitions; it is not a complete driver implementation.

alloc_chrdev_region reserves a range and reports failure through its return value. cdev_init initializes the object with its operations; cdev_add publishes it and makes it live immediately. unregister_chrdev_region releases a successful number reservation. Pair each release with the acquisition that actually succeeded, using the same first number and count.^[ref]

For a concrete bookkeeping example, suppose a successful reservation contains four interfaces starting at returned pair $(M,20)$. Failure before publication requires releasing that four-number range. Releasing only three leaves part of the reservation behind; releasing a guessed starting major or an unacquired range is a different error. The returned major from dynamic character-device number allocation must be retained rather than reconstructed from a pathname.

Publication changes the lifetime problem. cdev_del prevents new opens, but existing open file descriptors can still invoke their file operations. Therefore removing the device from lookup is not a proof that its backing state can immediately be freed. Character-device lifetime after deletion must account for the references and callbacks already in progress.^[ref]

The combined cdev_device_add helper has an especially important failure contract: callers must assume userspace could open the device and call operations even if the helper returns failure. A blanket error handler that immediately frees all backing state would violate that contract.^[ref] Design ownership and reference handling around the documented publication boundary, then release each owned resource once. The general lesson is that reverse acquisition order must be combined with concurrency-aware lifetime management.