Registering a Character Device
Before a character device can interact with the Linux kernel, you must register it so the kernel knows how to communicate with your driver. Registering a character device involves reserving a range of device numbers (major and minor), initializing a character device structure, and associating your driver's file operations with that device. This step is essential for allowing user-space programs to access your device through a node in the /dev directory.
char_reg_example.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647#include <linux/module.h> #include <linux/init.h> #include <linux/kernel.h> #include <linux/fs.h> #include <linux/cdev.h> #define DEVICE_NAME "mychardev" static dev_t dev_num; // will hold the major and minor number static struct cdev my_cdev; static int __init mychar_init(void) { int result; // Allocate a range of char device numbers result = alloc_chrdev_region(&dev_num, 0, 1, DEVICE_NAME); if (result < 0) { pr_err("Failed to allocate char device region\n"); return result; } // Initialize cdev structure and add it to kernel cdev_init(&my_cdev, NULL); // NULL for file_operations as placeholder my_cdev.owner = THIS_MODULE; result = cdev_add(&my_cdev, dev_num, 1); if (result < 0) { pr_err("Failed to add cdev\n"); unregister_chrdev_region(dev_num, 1); return result; } pr_info("Char device registered: major=%d minor=%d\n", MAJOR(dev_num), MINOR(dev_num)); return 0; } static void __exit mychar_exit(void) { cdev_del(&my_cdev); unregister_chrdev_region(dev_num, 1); pr_info("Char device unregistered\n"); } module_init(mychar_init); module_exit(mychar_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Your Name"); MODULE_DESCRIPTION("Simple Character Device Registration Example");
When registering a character device, you must assign it a unique identifier so the kernel and user-space applications can reference it. This identifier consists of a major number, which identifies the driver associated with the device, and a minor number, which distinguishes between different devices or instances managed by the same driver. In the example above, the alloc_chrdev_region function dynamically allocates a major number and a range of minor numbers. The cdev_init function prepares the character device structure, and cdev_add registers it with the kernel. You can retrieve the assigned major and minor numbers using the MAJOR() and MINOR() macros, which are printed for reference.
char_unreg_example.c
1234567891011121314#include <linux/module.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/cdev.h> static dev_t dev_num; static struct cdev my_cdev; static void __exit mychar_exit(void) { cdev_del(&my_cdev); unregister_chrdev_region(dev_num, 1); pr_info("Char device unregistered\n"); } module_exit(mychar_exit);
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Before a character device can interact with the Linux kernel, you must register it so the kernel knows how to communicate with your driver. Registering a character device involves reserving a range of device numbers (major and minor), initializing a character device structure, and associating your driver's file operations with that device. This step is essential for allowing user-space programs to access your device through a node in the /dev directory.
char_reg_example.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647#include <linux/module.h> #include <linux/init.h> #include <linux/kernel.h> #include <linux/fs.h> #include <linux/cdev.h> #define DEVICE_NAME "mychardev" static dev_t dev_num; // will hold the major and minor number static struct cdev my_cdev; static int __init mychar_init(void) { int result; // Allocate a range of char device numbers result = alloc_chrdev_region(&dev_num, 0, 1, DEVICE_NAME); if (result < 0) { pr_err("Failed to allocate char device region\n"); return result; } // Initialize cdev structure and add it to kernel cdev_init(&my_cdev, NULL); // NULL for file_operations as placeholder my_cdev.owner = THIS_MODULE; result = cdev_add(&my_cdev, dev_num, 1); if (result < 0) { pr_err("Failed to add cdev\n"); unregister_chrdev_region(dev_num, 1); return result; } pr_info("Char device registered: major=%d minor=%d\n", MAJOR(dev_num), MINOR(dev_num)); return 0; } static void __exit mychar_exit(void) { cdev_del(&my_cdev); unregister_chrdev_region(dev_num, 1); pr_info("Char device unregistered\n"); } module_init(mychar_init); module_exit(mychar_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Your Name"); MODULE_DESCRIPTION("Simple Character Device Registration Example");
When registering a character device, you must assign it a unique identifier so the kernel and user-space applications can reference it. This identifier consists of a major number, which identifies the driver associated with the device, and a minor number, which distinguishes between different devices or instances managed by the same driver. In the example above, the alloc_chrdev_region function dynamically allocates a major number and a range of minor numbers. The cdev_init function prepares the character device structure, and cdev_add registers it with the kernel. You can retrieve the assigned major and minor numbers using the MAJOR() and MINOR() macros, which are printed for reference.
char_unreg_example.c
1234567891011121314#include <linux/module.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/cdev.h> static dev_t dev_num; static struct cdev my_cdev; static void __exit mychar_exit(void) { cdev_del(&my_cdev); unregister_chrdev_region(dev_num, 1); pr_info("Char device unregistered\n"); } module_exit(mychar_exit);
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