A Linux character driver connects a device number to a struct cdev and its file operations. A basic implementation reserves a device-number range, initializes and adds the cdev, and—if it should appear in the device model—creates a class and registers a corresponding struct device. These are separate steps: cdev_add() makes callbacks reachable immediately, while device_create() registers a device with sysfs; neither alone defines the driver’s read/write behavior or guarantees a particular /dev node policy.
The examples below describe the registration model documented for Linux 7.1. Check API signatures and surrounding details against the kernel version you are targeting; the kernel documentation describes itself as a work in progress.
Understand the character-device mapping
A character device is addressed through a device number, represented by dev_t. The number identifies the device endpoint; the cdev associates that number or range with the driver’s file_operations. Those operations implement the behavior userspace invokes through file descriptors, such as reading, writing, or issuing supported commands.
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- Number allocation: reserve a device-number range and retain the assigned
dev_t. - File-operation dispatch: initialize and add a
struct cdevfor that range. - Device-model visibility: optionally register a
struct deviceunder a class so it appears in sysfs.
The registration name passed to the number-allocation API is not itself a promise about the name or creation policy of a /dev node.
Choose how to reserve device numbers
Dynamic allocation for most standalone drivers
alloc_chrdev_region() dynamically reserves a range and returns the assigned device number through a dev_t output parameter. Check its return value before proceeding, and keep the resulting number for both cdev registration and any matching device-model registration.
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Fixed allocation only when there is a reason
register_chrdev_region() reserves a known range. Use it when a fixed number is justified by the interface or deployment requirements; otherwise, dynamic allocation avoids assuming that a particular range is available. Both approaches reserve numbers, but the chosen number must be used consistently by the cdev and the associated device object.
See the Linux kernel’s Char devices API reference for the versioned API descriptions.
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Initialize and activate the cdev
- Prepare the driver’s state and file operations. The callbacks and every object they access must be ready before the cdev becomes reachable.
- Initialize the cdev. Call
cdev_init()with the cdev and the driver’sfile_operations. - Add the cdev for the intended range. Call
cdev_add()with the cdev, the allocated device number, and the number of device numbers it covers; check the return code.
Activation is immediate: the kernel API says that cdev_add() “adds the device represented by p to the system, making it live immediately.” Treat a successful return as the point at which userspace can reach the file operations. Do not publish the cdev before callback dependencies are initialized, and make error handling account for any registrations already completed.
Add a device-model entry when needed
A cdev can provide file-operation dispatch without creating a class-based device entry. If the driver should participate in the driver model and expose a device through sysfs, create the class first, then call device_create() with that class and the same dev_t handled by the cdev. Check its returned pointer using the kernel’s error-pointer handling rather than treating every non-NULL pointer as success.
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device_create() registers a struct device under the class and exposes device information in sysfs, including a dev attribute. This is distinct from implementing file operations. Whether a corresponding /dev node is created, and its naming and permissions, depend on the userspace environment; the API documentation does not establish one universal node-creation policy. Consult the device drivers infrastructure documentation for the device-model helper details.
Choose a registration approach that fits object lifetime
| Approach | When it fits | Important consideration |
|---|---|---|
cdev_init() and cdev_add() |
Direct cdev management when the driver needs to control cdev registration separately. | Coordinate the cdev’s lifetime with the private state accessed by its callbacks. |
cdev_device_add() |
When the cdev and struct device share a lifetime-managed containing object. |
The API warns that opens may occur even if the combined add operation fails, so callbacks and state must already be safe. |
These are useful patterns, not an exhaustive list of Linux device interfaces. For real hardware, a subsystem-specific interface may be more suitable than a bespoke character device.
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Design teardown around open file descriptors
Unwind successful registrations and release the reserved device-number range during teardown. The order should reflect which steps succeeded: remove the device-model entry if it was created, prevent new cdev opens, and release the number range only when the driver’s registration work is complete.
Most importantly, removing a cdev does not make existing file descriptors disappear. The kernel documentation states that after cdev_del(), the cdev can no longer be opened, but already-open cdevs remain and their file operations can still be called. Keep the private state and any resources those callbacks use valid until existing users can no longer reach them. A driver therefore needs an explicit lifetime strategy—such as references tied to open/release activity or another managed ownership arrangement—rather than freeing callback state immediately after cdev_del().
Add ioctl only for a durable command interface
Use established file operations for simple byte-oriented behavior. Add ioctl commands only when the device needs operations that do not fit those interfaces. Once userspace depends on an ioctl command and payload layout, changing it can break compatibility.
For new commands, use the documented _IO, _IOR, _IOW, and _IOWR macros to encode the command’s direction and associated data type. Choose the command type, number, and payload layout deliberately, and plan for the userspace ABI to remain stable. The kernel ioctl interface documentation explains the macros and compatibility concerns.
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