Linux Character driver - Creating multiple device nodes

In our previous post, we created a single device /dev/my_null, let's modify the code to create multiple device nodes for example: 5 device nodes : /dev/my_null0, /dev/my_null1, /dev/my_null2, /dev/my_null3, /dev/my_null4

While allocating the device numbers, we need to specify the number of minor devices in the count argument:

int alloc_chrdev_region (dev_t *  dev,  unsigned  baseminor, unsigned  count, const char * name);

We need to create an array of struct cdev, and register each of the cdev with one minor number.

Code:

	#include <linux/kernel.h>
	#include <linux/module.h>
	#include <linux/kdev_t.h>
	#include <linux/types.h>
	#include <linux/fs.h>
	#include <linux/cdev.h>
	#define MAX_DEVICES 5
	MODULE_LICENSE("GPL");
	dev_t device_number;
	bool dynamic = true;
	struct class *my_class;
	static struct cdev my_cdev[MAX_DEVICES];
	static int mydevice_open(struct inode *inode, struct file *file)
	{
	pr_info("%s\n", __func__);
	return 0;
	}
	static int mydevice_release(struct inode *inode, struct file *file)
	{
	pr_info("%s\n", __func__);
	return 0;
	}
	ssize_t mydevice_read(struct file *file, char __user *user_buffer,
	size_t count, loff_t *offset)
	{
	pr_info("%s\n", __func__);
	return 0;
	}
	ssize_t mydevice_write(struct file *file, const char __user *user_buffer,
	size_t count, loff_t *offset)
	{
	pr_info("%s\n", __func__);
	return count;
	}
	struct file_operations fops = {
	.owner = THIS_MODULE,
	.open = mydevice_open,
	.release = mydevice_release,
	.read = mydevice_read,
	.write = mydevice_write
	};
	static int mynull_device_init(void)
	{
	int retval;
	int i = 0;
	pr_info("%s: In init\n", __func__);
	if (dynamic) {
	retval = alloc_chrdev_region(&device_number, 0, MAX_DEVICES, "embedded");
	}
	else {
	device_number = MKDEV(180, 0);
	retval = register_chrdev_region(device_number, MAX_DEVICES, "embedded");
	}
	if (!retval) {
	int major = MAJOR(device_number);
	dev_t my_device;
	my_class = class_create(THIS_MODULE, "my_driver_class");
	for (i = 0; i < MAX_DEVICES; i++) {
	my_device = MKDEV(major, i);
	cdev_init(&my_cdev[i], &fops);
	retval = cdev_add(&my_cdev[i], my_device, 1);
	if (retval) {
	pr_info("%s: Failed in adding cdev to subsystem "
	"retval:%d\n", __func__, retval);
	}
	else {
	device_create(my_class, NULL, my_device, NULL, "my_null%d", i);
	}
	}
	}
	else
	pr_err("%s: Failed in allocating device number "
	"Error:%d\n", __func__, retval);
	return retval;
	}
	static void mynull_device_exit(void)
	{
	int i = 0;
	int major = MAJOR(device_number);
	dev_t my_device;
	for (i = 0; i < MAX_DEVICES; i++) {
	my_device = MKDEV(major, i);
	cdev_del(&my_cdev[i]);
	device_destroy(my_class, my_device);
	}
	class_destroy(my_class);
	unregister_chrdev_region(device_number, MAX_DEVICES);
	pr_info("%s: In exit\n", __func__);
	}
	module_init(mynull_device_init);
	module_exit(mynull_device_exit);

view raw null_device.c hosted with ❤ by GitHub

Output:

Notes:

Try to change the MAX_DEVICES macro to other number and check how many maximum devices you can create