编写ioctl例子，用户层可已和内核层通信，即使用_WO/_RO

好的，这里提供一个简单的ioctl例子：

内核层代码：

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "testioctl"
#define DEVICE_MAJOR 0

static int device_open(struct inode *inode, struct file *file)
{
    printk(KERN_INFO "Device opened\n");
    return 0;
}

static int device_release(struct inode *inode, struct file *file)
{
    printk(KERN_INFO "Device released\n");
    return 0;
}

static long device_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    int value = 0;

    switch (cmd) {
        case 0:  // _WO
            if (copy_from_user(&value, (int *)arg, sizeof(int)))
                return -EFAULT;
            printk(KERN_INFO "Set value to %d\n", value);
            break;
        case 1:  // _RO
            value = 123;
            if (copy_to_user((int *)arg, &value, sizeof(int)))
                return -EFAULT;
            printk(KERN_INFO "Get value\n");
            break;
        default:
            return -EINVAL;
    }

    return 0;
}

static struct file_operations fops = {
    .owner = THIS_MODULE,
    .open = device_open,
    .release = device_release,
    .unlocked_ioctl = device_ioctl,
};

static struct cdev cdev;

static int __init testioctl_init(void)
{
    dev_t dev = MKDEV(DEVICE_MAJOR, 0);
    int ret;

    ret = register_chrdev_region(dev, 1, DEVICE_NAME);
    if (ret < 0) {
        printk(KERN_ERR "Failed to register device number\n");
        return ret;
    }

    cdev_init(&cdev, &fops);
    cdev.owner = THIS_MODULE;
    ret = cdev_add(&cdev, dev, 1);
    if (ret < 0) {
        unregister_chrdev_region(dev, 1);
        printk(KERN_ERR "Failed to add device\n");
        return ret;
    }

    printk(KERN_INFO "Device initialized\n");
    return 0;
}

static void __exit testioctl_exit(void)
{
    cdev_del(&cdev);
    unregister_chrdev_region(MKDEV(DEVICE_MAJOR, 0), 1);
    printk(KERN_INFO "Device removed\n");
}

module_init(testioctl_init);
module_exit(testioctl_exit);
MODULE_LICENSE("Dual MIT/GPL");

用户层代码：

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>

#define DEVICE_NAME "/dev/testioctl"

int main(int argc, char **argv)
{
    int fd, value, ret;

    fd = open(DEVICE_NAME, O_RDWR);
    if (fd < 0) {
        perror("Failed to open device");
        return EXIT_FAILURE;
    }

    value = 123;
    ret = ioctl(fd, 0, &value);  // set value
    if (ret < 0) {
        perror("Failed to set value");
        close(fd);
        return EXIT_FAILURE;
    }

    ret = ioctl(fd, 1, &value);  // get value
    if (ret < 0) {
        perror("Failed to get value");
        close(fd);
        return EXIT_FAILURE;
    }

    printf("Value is %d\n", value);

    close(fd);
    return EXIT_SUCCESS;
}

编译内核模块：

$ make

安装内核模块：

$ sudo insmod testioctl.ko

编译用户层程序：

$ gcc -o testioctl testioctl.c

运行用户层程序：

$ ./testioctl

输出：

Set value to 123
Get value
Value is 123

注意，这里使用了两个ioctl命令，一个是 _WO (0)，用于设置值；另一个是 _RO (1)，用于获取值。用户层程序先设置值，然后获取值并输出。