Question

对于家庭作业，我写了一个角色设备驱动程序。它似乎工作正常。我可以读写它。问题是，当我读取设备时，它会无休止地循环，一遍又一遍地打印出消息缓冲区的内容。

这似乎应该是相当直接的。只需使用copy_to_user（），但事实证明这是非常有问题的。

无论如何，这是代码。我认为问题出在gdev_read（）函数中。 printk用作调试和谈话点，因为我必须在课堂上展示这个项目。

/*
 * Implement a generic character pseudo-device driver
 */

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

/* you need these, or the kernel will be tainted */
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("A simple sample character device driver");

/*
 * function prototypes
 */
int init_module(void);
void cleanup_module(void);
static ssize_t gdev_read(struct file *, char *, size_t, loff_t *);
static ssize_t gdev_write(struct file *, const char *, size_t, loff_t *);
static int gdev_open(struct inode *, struct file *);
static int gdev_release(struct inode *, struct file *);

/* macros */
#define TRUE 1
#define FALSE 0
#define MAX_MSG_LEN 64

/*
 * global variables
 */
static dev_t dev_num;   /* device number, for new device */
static char *mesg;  /* buffer for message */


/* file operations structure, so my device knows how to act */
static struct file_operations fops = {
    .owner =    THIS_MODULE,
    .read =     gdev_read,
    .write =    gdev_write,
    .open =     gdev_open,
    .release =  gdev_release,
};

/* character device struct.  Declaired here, but initialized elsewhere */
struct cdev *gdev;

int init_module(void)
{   
    int err;
    printk(KERN_ALERT "in init_module\n");

    if(alloc_chrdev_region(&dev_num, 0, 1, "/dev/gdev")){
        printk(KERN_INFO "Could not allocate device numbers\n");
        printk(KERN_INFO "Module gdev not loaded\n");
        return -1;
    }

    /* now I need to make the device and register it */
    gdev = cdev_alloc();
    gdev->owner = THIS_MODULE;
    gdev->ops = &fops;
    err = cdev_add(gdev, dev_num, 1);
    if(err){
        printk(KERN_NOTICE "Error %d adding gdev", err);
        return err;
    }

    mesg = (char *)vmalloc(MAX_MSG_LEN);

    printk(KERN_INFO "Module gdev successfully loaded.\n");
    printk(KERN_INFO "gdev Major Number: %d\n", MAJOR(dev_num));

    return 0;
}


void cleanup_module(void)
{
    printk(KERN_ALERT "in cleanup_module\n");
    unregister_chrdev_region(dev_num, 3);
    vfree(mesg);
    cdev_del( gdev );
    printk(KERN_INFO "Module gdev unregistered\n");
}

static ssize_t gdev_read(struct file *filp, char *page, 
            size_t len, loff_t *offset)
{
    ssize_t bytes = len < MAX_MSG_LEN ? len : MAX_MSG_LEN;
    printk(KERN_ALERT "in gdev_read\n");
    if(copy_to_user(page, mesg, bytes)){
        return -EFAULT;
    }
    return bytes;
}

static ssize_t gdev_write(struct file *filp, const char *page, 
            size_t len, loff_t *offset)
{
    ssize_t bytes = len < MAX_MSG_LEN ? len : MAX_MSG_LEN; 
    printk(KERN_ALERT "in gdev_write\n");
    if(copy_from_user(mesg, page, bytes)){
        return -EFAULT;
    }

    return bytes;
}

static int gdev_open(struct inode *inode, struct file *filp)
{
    printk(KERN_ALERT "in gdev_open\n");
    return 0;
}

static int gdev_release(struct inode *inode, struct file *filp)
{
    printk(KERN_ALERT "in gdev_release\n");
    /* doesn't do anything because it doesn't need too */
    return 0;
}

Answer 1

如果未从read()返回零（在您的情况下为gdev_read()），则将再次调用读取函数。要停止此操作，请使用loff_t *offset参数。在(*offset) += bytes;之后使用copy_to_user()读取的字节数增加。下次调用read()时，offset将是您将其递增到的值。现在只需检查您之前发送的字节数，并仅发送剩余的内容。你的功能应如下所示：

static ssize_t gdev_read(struct file *filp, char *page, 
            size_t len, loff_t *offset)
{
    ssize_t bytes = len < (MAX_MSG_LEN-(*offset)) ? len : (MAX_MSG_LEN-(*offset));
    printk(KERN_ALERT "in gdev_read\n");
    if(copy_to_user(page, mesg, bytes)){
        return -EFAULT;
    }
    (*offset) += bytes;
    return bytes;
}

Answer 2

您可以使用'linux / fs.h'中的'simple_read_from_buffer'函数：

static ssize_t gdev_read(struct file *filep, char __user *buff, size_t count, loff_t *offp)
{
    return simple_read_from_buffer(buff, count, offp, my_buffer, buffer_len);
}

您的模块中定义了

' my_buffer '和' buffer_len '。

从字符设备读取时无休止地循环

2 个答案: