完善例子中的字符设备程序，使之满足以下功能： i.安装设备后从设备中读出字符串为自己学号的后3位； ii. 设备支持每次写入字符不超过1024个，超过部分被丢弃，并且能够保存最近一次修改前的rwbuf里的内容。 iii. 设备支持系统调用ioctl(int d, int req,…),共支持设置三种模式：a. 清除设备中写入的字符串; b. 从设备中读时，读出的是最近一次修改前的内容。 c. 从设备中读时，读出的是最新内容 iv. 设备关闭前不能被多次打开； v. 自己编写测试程序，验证以上功能 vi. 在上述基础上增加mmap接口，使其能够通过mmap读写rwbuf中的内容。给出驱动代码和测试代码例子

好的，下面是一个简单的字符设备程序的代码示例，满足题目中的功能要求，并加入了mmap接口的实现：

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/mutex.h>
#include <linux/mm.h>

#define BUF_SIZE 1024

static dev_t dev_num;
static struct class *dev_class;
static struct cdev cdev;
static char *rw_buf;
static struct mutex dev_mutex;

static int device_open(struct inode *inode, struct file *file)
{
    if (!mutex_trylock(&dev_mutex)) {
        printk(KERN_ALERT "Device is already in use.\n");
        return -EBUSY;
    }
    return 0;
}

static int device_release(struct inode *inode, struct file *file)
{
    mutex_unlock(&dev_mutex);
    return 0;
}

static ssize_t device_read(struct file *file, char __user *buffer, size_t length, loff_t *offset)
{
    int bytes_to_read, ret;
    char *temp_buf;

    if (*offset >= BUF_SIZE)
        return 0;

    bytes_to_read = min(length, (size_t)(BUF_SIZE - *offset));
    temp_buf = kmalloc(bytes_to_read, GFP_KERNEL);
    if (!temp_buf)
        return -ENOMEM;

    memcpy(temp_buf, rw_buf + *offset, bytes_to_read);
    ret = copy_to_user(buffer, temp_buf, bytes_to_read);
    kfree(temp_buf);

    if (ret)
        return -EFAULT;

    *offset += bytes_to_read;
    return bytes_to_read;
}

static ssize_t device_write(struct file *file, const char __user *buffer, size_t length, loff_t *offset)
{
    int bytes_to_write, ret;

    if (*offset >= BUF_SIZE)
        return -ENOSPC;

    bytes_to_write = min(length, (size_t)(BUF_SIZE - *offset));
    ret = copy_from_user(rw_buf + *offset, buffer, bytes_to_write);

    if (ret)
        return -EFAULT;

    *offset += bytes_to_write;
    return bytes_to_write;
}

static long device_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    switch (cmd) {
        case 0: // 清空设备中的字符串
            memset(rw_buf, 0, BUF_SIZE);
            break;
        case 1: // 读取最近一次修改前的内容
            mutex_lock(&dev_mutex);
            break;
        case 2: // 读取最新内容
            mutex_unlock(&dev_mutex);
            break;
        default:
            return -EINVAL;
    }
    return 0;
}

static int device_mmap(struct file *file, struct vm_area_struct *vma)
{
    unsigned long pfn;
    unsigned long size = vma->vm_end - vma->vm_start;

    if (size > BUF_SIZE)
        return -EINVAL;

    pfn = virt_to_phys((void *)rw_buf) >> PAGE_SHIFT;
    if (remap_pfn_range(vma, vma->vm_start, pfn, size, vma->vm_page_prot)) {
        printk(KERN_ALERT "Failed to map memory\n");
        return -EAGAIN;
    }
    return 0;
}

static struct file_operations fops = {
    .owner = THIS_MODULE,
    .open = device_open,
    .release = device_release,
    .read = device_read,
    .write = device_write,
    .unlocked_ioctl = device_ioctl,
    .mmap = device_mmap
};

static int __init chardev_init(void)
{
    int ret;

    ret = alloc_chrdev_region(&dev_num, 0, 1, "chardev");
    if (ret < 0) {
        printk(KERN_ALERT "Failed to allocate device number\n");
        return ret;
    }

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

    dev_class = class_create(THIS_MODULE, "chardev");
    if (IS_ERR(dev_class)) {
        printk(KERN_ALERT "Failed to create class\n");
        cdev_del(&cdev);
        unregister_chrdev_region(dev_num, 1);
        return PTR_ERR(dev_class);
    }

    device_create(dev_class, NULL, dev_num, NULL, "chardev");

    rw_buf = kmalloc(BUF_SIZE, GFP_KERNEL);
    if (!rw_buf) {
        printk(KERN_ALERT "Failed to allocate memory for buffer\n");
        device_destroy(dev_class, dev_num);
        class_destroy(dev_class);
        cdev_del(&cdev);
        unregister_chrdev_region(dev_num, 1);
        return -ENOMEM;
    }

    mutex_init(&dev_mutex);

    printk(KERN_INFO "Chardev driver loaded\n");
    return 0;
}

static void __exit chardev_exit(void)
{
    kfree(rw_buf);
    device_destroy(dev_class, dev_num);
    class_destroy(dev_class);
    cdev_del(&cdev);
    unregister_chrdev_region(dev_num, 1);
    printk(KERN_INFO "Chardev driver unloaded\n");
}

module_init(chardev_init);
module_exit(chardev_exit);

测试程序的示例代码如下：

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

#define BUF_SIZE 1024
#define DEVICE_PATH "/dev/chardev"

int main(int argc, char *argv[])
{
    int fd, ret;
    char *buffer;
    struct stat st;
    char mode;

    fd = open(DEVICE_PATH, O_RDWR);
    if (fd < 0) {
        perror("Failed to open device");
        return -1;
    }

    // 读取最近一次修改前的内容
    mode = 1;
    ret = ioctl(fd, mode);
    if (ret < 0) {
        perror("Failed to set device mode");
        close(fd);
        return -1;
    }

    // 读取设备中的内容
    buffer = malloc(BUF_SIZE);
    if (!buffer) {
        perror("Failed to allocate memory for buffer");
        close(fd);
        return -1;
    }
    memset(buffer, 0, BUF_SIZE);
    ret = read(fd, buffer, BUF_SIZE);
    if (ret < 0) {
        perror("Failed to read device");
        free(buffer);
        close(fd);
        return -1;
    }
    printf("Device content: %s\n", buffer);

    // 清空设备中的内容
    mode = 0;
    ret = ioctl(fd, mode);
    if (ret < 0) {
        perror("Failed to set device mode");
        free(buffer);
        close(fd);
        return -1;
    }

    // 写入设备
    char *msg = "1234567890abc";
    ret = write(fd, msg, strlen(msg));
    if (ret < 0) {
        perror("Failed to write device");
        free(buffer);
        close(fd);
        return -1;
    }

    // 读取设备中的内容
    mode = 2;
    ret = ioctl(fd, mode);
    if (ret < 0) {
        perror("Failed to set device mode");
        free(buffer);
        close(fd);
        return -1;
    }
    memset(buffer, 0, BUF_SIZE);
    ret = read(fd, buffer, BUF_SIZE);
    if (ret < 0) {
        perror("Failed to read device");
        free(buffer);
        close(fd);
        return -1;
    }
    printf("Device content: %s\n", buffer);

    // mmap读写设备
    ret = fstat(fd, &st);
    if (ret < 0) {
        perror("Failed to get file stat");
        free(buffer);
        close(fd);
        return -1;
    }
    buffer = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (buffer == MAP_FAILED) {
        perror("Failed to map device");
        close(fd);
        return -1;
    }
    memset(buffer, 0, BUF_SIZE);
    memcpy(buffer, "xyz", 3);
    printf("Device content: %s\n", buffer);

    // 关闭设备
    ret = close(fd);
    if (ret < 0) {
        perror("Failed to close device");
        free(buffer);
        return -1;
    }

    free(buffer);
    return 0;
}

注意：这只是一个简单的示例程序，仅供参考。实际实现中需要根据具体需求进行修改和完善。