kill_async与fasync_helper函数

fasync是为了使驱动的读写和应用程序的读写分开，使得应用程序可以在驱动读写的时候去做别的事。

应用程序通过fcntl给自己的SIGIO信号安装自己的响应函数，

驱动通过kill_fasync(&async, SIGIO, POLL_IN); 发SIGIO信号给应用程序，应用程序就调用自己安装的响应函数去处理。

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，最后在驱动的release里把fasync_helper初始化的东西free掉。

这里有一份自己写的完整的内核驱动发送SIGIO和用户空间接收SIGIO的代码，测试过ok：

http://download.csdn.net/detail/luckywang1103/9050619

有些系统没有mdev的需要自己手动创建设备节点：在/dev/目录下mknod fasync_dev c 250 0

下面是驱动程序和测试的应用程序，还没试过，先摘下来；

转自：http://blog.csdn.net/mirkerson/article/details/7443907

驱动程序：

#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);

应用程序：

/*
 * 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;
    }
}