Character Device Basics¶

A character device driver exposes a file-like interface to user space. User-space applications interact with the device through normal file operations such as open, read, write, poll, ioctl, and close.

What Problem It Solves¶

A character device is useful when a device or kernel service can be represented as a byte stream, command interface, or event source.

Typical examples in this learning repo include:

/dev/mygpio
/dev/mypoll
event-driven GPIO drivers
pollable timer/event drivers

Basic Registration Flow¶

alloc_chrdev_region()
        ↓
cdev_init()
        ↓
cdev_add()
        ↓
class_create()
        ↓
device_create()
        ↓
/dev/<device-name>

Key Concepts¶

Concept	Purpose
`dev_t`	Encodes major and minor numbers
`struct cdev`	Connects a character device to `file_operations`
`struct file_operations`	Defines driver callbacks for user-space operations
`device_create()`	Creates a device node under `/dev` through udev/devtmpfs

Minimal Driver Shape¶

static const struct file_operations mydev_fops = {
    .owner = THIS_MODULE,
    .open = mydev_open,
    .read = mydev_read,
    .write = mydev_write,
    .release = mydev_release,
};

Common Pitfalls¶

Warning

Creating a cdev does not automatically create /dev/<name>. The driver also needs a device class and device_create(), or the node must be created manually with mknod.

Note

DEVICE_NAME is the user-space device node name. It is separate from the Device Tree node name and the compatible string.