fasync(在内核发生某个事情时向应用发信号， 应用需要把那个设备用fcntl改一下。)

fasync的解释

 fasync (fd，file，on)
　　当我们呼叫fcntl()系统呼叫，并使用F_SETFL命令来设定档案的参数时，VFS就会呼叫fasync ()这个函式，而当读写档案的动作完成时，行程会收到SIGIO的讯息。

=================================================================================

首先，打开目标设备。
第二，设置好目标设备的SIGIO信号处理程序。
第三，需要通过fcntl系统调用，使当前进程变成文件的主人。（这样才能使文件中的信号
发到当前进程）
第四，通过ioctl系统调用，将目标通道设置成异步操作模式。

在驱动程序的fops中，有一个函数
    int (*fasync)(int fd, struct file * file, int on);

在系统调用sys_ioctl的时候，会调用上面的fasync函数。

例如，鼠标器的
int fasync_aux(int fd, struct file * filp, int on)
{
    int retval;

    retval = fasync_helper(fd, filp, on, &queue->fasync);
    if (retval < 0)
        return retval;
    return 0;
}

这里的queue类似于(struct mydev *)filp->private_data，fasync则是里面的一个fasyn
c队列。
fasync_helper的作用是为当前进程创建一个fasync_struct数据结构，然后挂入目标设备
的fasync队列。

然后，在目标设备的常规驱动处理程序中，向该队列发送信号。

例如，鼠标驱动器的
void handle_mouse_event(unsigned char scancode)（这类似于usb驱动中的int urb，一
个回调函数）
{
    ......
    kill_fasync(&queue->fasync, SIGIO, POLL_IN);
    ......
}

即向每一个登记的进程发送SIGIO信号。

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/walkingman321/archive/2008/03/09/2161304.aspx

=================================================================================

一个简单例子：

#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>#include <asm/signal.h>#include <asm-generic/siginfo.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); printk("<0>""read buff is %s/n",buffer);                kill_fasync(&async, SIGIO, POLL_IN);        up(&sem); dump_stack();        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); printk("<0>""write buff is %s/n",buffer);        up(&sem);        if (async)        //        kill_fasync(&async, SIGIO, POLL_IN); dump_stack();        return count;err:        return -ERESTARTSYS;}static int chardev_fasync(int fd, struct file *filp, int mode){ //printk("<0>""/nin chardev_fasync/n"); dump_stack();        return fasync_helper(fd, filp, mode, &async);}static int chardev_release(struct inode *inodp, struct file *filp){ //printk("<0>""/nin chardev_release/n"); //printk("<0>""/nin chardev_release twice/n"); dump_stack();        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);

=================================================================================

原版例子，上面的改了东西，不对

=================================================================================

#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);

=================================================================================

测试程序

=================================================================================

/* * asynctest.c: use async notification to read stdin * * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet * Copyright (C) 2001 O'Reilly & Associates * * The source code in this file can be freely used, adapted, * and redistributed in source or binary form, so long as an * acknowledgment appears in derived source files.  The citation * should list that the code comes from the book "Linux Device * Drivers" by Alessandro Rubini and Jonathan Corbet, published * by O'Reilly & Associates.   No warranty is attached; * we cannot take responsibility for errors or fitness for use. */#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#include <signal.h>#include <fcntl.h>int gotdata=0;static int i = 0;void sighandler(int signo){    if (signo==SIGIO)        gotdata++;    printf("/nin sighandler %d/n",i);    return;}char buffer[4096];int main(int argc, char **argv){    int count;    struct sigaction action;    FILE *fp;    int fd = 0;    fd = open("/dev/test",O_RDWR);    memset(&action, 0, sizeof(action));    action.sa_handler = sighandler;    action.sa_flags = 0;    sigaction(SIGIO, &action, NULL);    fcntl(fd, F_SETOWN, getpid());    fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | FASYNC); ++i;    while(i != 2) {    //printf("/nin while %d/n",++i);        /* this only returns if a signal arrives */#if 0    fp = fopen("log2","aw+");    fprintf(fp,"/nin while %d/n",++i);    fclose(fp);#endif        sleep(2); /* one day */        if (!gotdata)            continue;        count=read(fd, buffer, 4);    printf("/nin while %d/n",++i);        /* buggy: if avail data is more than 4kbytes... */        write(fd,buffer,count);    //printf("/nin while %d/n",++i);        gotdata=0;    }}

=================================================================================

总结：

         fasync这个东西就是为了使驱动的读写和application的读写分开，使得application可以在驱动读写时去做别的事，通过kill_fasync

(kill_fasync(&async, SIGIO, POLL_IN);)发SIGIO信号给应用，应用通过fcntl把自己这个SIGIO的信号换成自己的响应函数，当驱动发

(kill_fasync(&async, SIGIO, POLL_IN);)给应用时应用就调用了自己的handler去处理。fasync_helper作用就是初始化fasync这个东西，包括分配内存和设置属性。最后记得在驱动的release里把fasync_helper初始化的东西free掉。

=================================================================================

POLL_IN POLL_OUT

=================================================================================

驱动程序向用户程序发信号
--------------------------------------------- 
    当设备有IO事件发生，就有机制保证向应用进程发送信号，显然设备驱动程序扮演重要角色，实际终端tty、网络socket等的标准实现已经包括了实时信号驱动的支持，所以，在Linux中它们可以如上直接使用。但有些设备的驱动程序还并没有支持，所以需要定制设备驱动程序。以下两个API应该是可以屏蔽所有相关琐碎操作(类似send_sig())的标准接口:
    int fasync_helper (int fd, struct file *filp, int mode, struct fasync_struct **fa);      
    void kill_fasync (struct fasync_struct **fa, int sig, int band);
    
    如果需要支持异步通知机制，如下设备结构中需要有异步事件通知队列(它应该与睡眠队列类似)，并且增加fasync()接口的实现(该函数将本进程登记到async_queue上去)。 当一个打开的文件FASYNC标志变化时(调用fcntl()函数，设置FASYNC文件标志时)，fasync()接口将被调用。 

    struct kpp_dev {
        struct cdev cdev;
        struct fasync_struct *async_queue;
    };
     
    static int kpp_fasync(int fd, struct file *filp, int mode)
    {
        struct kpp_dev *dev = filp->private_data; 
        return fasync_helper(fd, filp, mode, &dev->async_queue);
    }

    事件发生的时机，就是中断服务程序或相应的软中断中调用kill_fasync():
    if (dev->async_queue)
        kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
    如果是写操作，就是POLL_OUT。注意，无论用户进程设定了什么期望的信号，在这个环节，发送的一般就是SIGIO。注意在设备文件关闭(release方法)时，注意执行fasync()，使得本文件的操作从上述的设备异步事件等待链表中剥离。

    static int kpp_release(struct inode *inode, struct file *filp)
    {
        kpp_fasync(-1, filp, 0);
        return 0;
    }