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

驱动代码：

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>
#include <linux/slab.h>
#include <linux/ioctl.h>
#include <linux/mman.h>

#define DEV_NAME "mydev"

static int major;
static struct cdev my_dev;
static char *rwbuf;
static int rlen;
static int wlen;

struct mydev_ioctl_args {
    int mode;
};

#define MYDEV_IOCTL_CLEAR 0
#define MYDEV_IOCTL_READ_OLD 1
#define MYDEV_IOCTL_READ_NEW 2

static long mydev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    struct mydev_ioctl_args args;

    if (copy_from_user(&args, (void *)arg, sizeof(args))) {
        return -EFAULT;
    }

    switch (args.mode) {
    case MYDEV_IOCTL_CLEAR:
        memset(rwbuf, 0, rlen);
        wlen = 0;
        break;
    case MYDEV_IOCTL_READ_OLD:
        wlen = 0;
        break;
    case MYDEV_IOCTL_READ_NEW:
        break;
    default:
        return -EINVAL;
    }

    return 0;
}

static int mydev_mmap(struct file *filp, struct vm_area_struct *vma)
{
    unsigned long pfn_start, pfn_end;
    unsigned long start = vma->vm_start;
    unsigned long size = vma->vm_end - vma->vm_start;

    if (size > rlen) {
        return -EINVAL;
    }

    pfn_start = virt_to_phys((void *)rwbuf) >> PAGE_SHIFT;
    pfn_end = virt_to_phys((void *)(rwbuf + rlen - 1)) >> PAGE_SHIFT;

    while (size > 0) {
        if (remap_pfn_range(vma, start, pfn_start, PAGE_SIZE, vma->vm_page_prot)) {
            return -EAGAIN;
        }
        start += PAGE_SIZE;
        pfn_start++;
        size -= PAGE_SIZE;
    }

    return 0;
}

static ssize_t mydev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
    int len;

    if (*f_pos >= rlen) {
        return 0;
    }

    if (count > rlen - *f_pos) {
        count = rlen - *f_pos;
    }

    if (wlen == 0) {
        len = snprintf(rwbuf, rlen, "123");
        wlen = len;
    }

    if (copy_to_user(buf, rwbuf + *f_pos, count)) {
        return -EFAULT;
    }

    *f_pos += count;

    return count;
}

static ssize_t mydev_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos)
{
    int len;

    if (count > 1024) {
        count = 1024;
    }

    if (*f_pos + count > rlen) {
        return -EINVAL;
    }

    len = copy_from_user(rwbuf + *f_pos, buf, count);

    if (len > 0) {
        return -EFAULT;
    }

    *f_pos += count;

    if (*f_pos > wlen) {
        wlen = *f_pos;
    }

    return count;
}

static int mydev_open(struct inode *inode, struct file *filp)
{
    if (filp->f_flags & O_EXCL) {
        return -EBUSY;
    }

    return 0;
}

static int mydev_release(struct inode *inode, struct file *filp)
{
    return 0;
}

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

static int __init mydev_init(void)
{
    dev_t dev;
    int ret;

    ret = alloc_chrdev_region(&dev, 0, 1, DEV_NAME);
    if (ret < 0) {
        return ret;
    }

    major = MAJOR(dev);

    cdev_init(&my_dev, &mydev_fops);
    my_dev.owner = THIS_MODULE;

    ret = cdev_add(&my_dev, dev, 1);
    if (ret < 0) {
        unregister_chrdev_region(MKDEV(major, 0), 1);
        return ret;
    }

    rwbuf = kzalloc(1024, GFP_KERNEL);
    if (!rwbuf) {
        cdev_del(&my_dev);
        unregister_chrdev_region(MKDEV(major, 0), 1);
        return -ENOMEM;
    }
    rlen = 1024;

    return 0;
}

static void __exit mydev_exit(void)
{
    kfree(rwbuf);
    cdev_del(&my_dev);
    unregister_chrdev_region(MKDEV(major, 0), 1);
}

module_init(mydev_init);
module_exit(mydev_exit);

MODULE_LICENSE("GPL");

测试代码：

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

#define DEV_NAME "/dev/mydev"

#define MYDEV_IOCTL_CLEAR 0
#define MYDEV_IOCTL_READ_OLD 1
#define MYDEV_IOCTL_READ_NEW 2

int main()
{
    int fd;
    char buf[1024];
    int len;
    char *ptr;
    int i;

    fd = open(DEV_NAME, O_RDWR);
    if (fd < 0) {
        perror("open");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    len = write(fd, "hello", 5);
    if (len < 0) {
        perror("write");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    len = write(fd, "world", 5);
    if (len < 0) {
        perror("write");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    len = ioctl(fd, MYDEV_IOCTL_READ_OLD);
    if (len < 0) {
        perror("ioctl");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    ptr = mmap(NULL, 1024, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        return -1;
    }

    printf("mmap: %.*s\n", 3, ptr);

    memcpy(ptr, "abc", 3);

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    munmap(ptr, 1024);

    len = ioctl(fd, MYDEV_IOCTL_READ_NEW);
    if (len < 0) {
        perror("ioctl");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    len = ioctl(fd, MYDEV_IOCTL_CLEAR);
    if (len < 0) {
        perror("ioctl");
        return -1;
    }

    len = read(fd, buf, sizeof(buf));
    if (len < 0) {
        perror("read");
        return -1;
    }

    printf("read: %.*s\n", len, buf);

    close(fd);

    return 0;
}