fasync方法事例

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                #include linux/module.h>
#include linux/init.h>
#include linux/fs.h>
#include linux/kdev_t.h>
#include linux/cdev.h>
#include linux/kernel.h>
#include linux/string.h>
#include asm/uaccess.h>
#define DEVICE_NAME     "chardev"
static int chardev_open(struct inode *inodp, struct file *filp);
static ssize_t chardev_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos);
static ssize_t chardev_write(struct file *filp, const char __user *buff, size_t count, loff_t *f_pos);
static int chardev_fasync(int fd, struct file *filp, int mode);
static int chardev_release(struct inode *inodp, struct file *filp);
static struct file_operations chardev_fops = {
        .open   = chardev_open,
        .read   = chardev_read,
        .write  = chardev_write,
        .fasync = chardev_fasync,
        .release = chardev_release,
};
static struct fasync_struct *async = NULL;
static struct semaphore sem;
static struct cdev *cdevp;
static dev_t devno;
static char buffer[8192];
static int chardev_open(struct inode *inodp, struct file *filp)
{
        return 0;
}
static ssize_t chardev_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos)
{
        if (down_interruptible(&sem))
                goto err;
        count = strlen(buffer);
        copy_to_user(buff, buffer, count);
        up(&sem);
        return count;
err:
        return -ERESTARTSYS;
}
static ssize_t chardev_write(struct file *filp, const char __user *buff, size_t count, loff_t *f_pos)
{
        if (down_interruptible(&sem))
                goto err;
        memset(buffer, '\0', sizeof(buffer));
        copy_from_user(buffer, buff, count);
        up(&sem);
        if (async)
                kill_fasync(&async, SIGIO, POLL_IN);
        return count;
err:
        return -ERESTARTSYS;
}
static int chardev_fasync(int fd, struct file *filp, int mode)
{
        return fasync_helper(fd, filp, mode, &async);
}
static int chardev_release(struct inode *inodp, struct file *filp)
{
        return chardev_fasync(-1, filp, 0);
}
static int __init chardev_init(void)
{
        int ret;
        ret = alloc_chrdev_region(&devno, 0, 1, DEVICE_NAME);
        if (ret  0)
                goto out;
        init_MUTEX(&sem);
        cdevp = cdev_alloc();
        if (!cdevp)
                goto alloc_err;
        cdev_init(cdevp, &chardev_fops);
        ret = cdev_add(cdevp, devno, 1);
        if (!ret)
                goto out;
alloc_err:
        unregister_chrdev_region(devno, 1);
out:
        return ret;
}
static void __exit chardev_exit(void)
{
        cdev_del(cdevp);
        unregister_chrdev_region(devno, 1);
}
MODULE_LICENSE("GPL");
module_init(chardev_init);
module_exit(chardev_exit);


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