高级I/O操作-非阻塞型I/O

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1 参考书籍
参考书籍 《嵌入式Linux驱动开发教程》 华清远见嵌入式学院
2 简介
如果应用程序以非阻塞的方式打开设备文件，当资源不可用时，驱动就应该立刻返回并用一个错误码EAGAIN来通知应用程序此时资源不可用。
3 范例代码

/*非阻塞型I/O*/
/*vser.c*/
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>

#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/kfifo.h>

#include <linux/ioctl.h>
#include <linux/uaccess.h>

#include "vser.h"

#define VSER_MAJOR  256
#define VSER_MINOR  0
#define VSER_DEV_CNT    1
#define VSER_DEV_NAME   "vser"

struct vser_dev {
    unsigned int baud; /*波特率*/
    struct option opt; /*帧格式信息*/
    struct cdev cdev;
};

DEFINE_KFIFO(vsfifo, char, 32);
static struct vser_dev vsdev;


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

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

static ssize_t vser_read(struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
    int ret;
    unsigned int copied = 0;

    if( kfifo_is_empty(&vsfifo)) /*判断fifo是否为空*/
    {
        /*判断设备文件是否是以非阻塞的方式打开*/
        if(filp->f_flags & O_NONBLOCK)
        {
            return -EAGAIN;
        }
    }
    ret = kfifo_to_user(&vsfifo, buf, count, &copied);

    return ret == 0 ? copied : ret;
}

static ssize_t vser_write(struct file *filp, const char __user *buf, size_t count, loff_t *pos)
{
    int ret;
    unsigned int copied = 0;

    if(kfifo_is_full(&vsfifo))
    {
        if(filp->f_flags & O_NONBLOCK)
            return -EAGAIN;
    }
    ret = kfifo_from_user(&vsfifo, buf, count, &copied);

    return ret == 0 ? copied : ret;
}

static long vser_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
     if(_IOC_TYPE(cmd) != VS_MAGIC)
     {
         return -ENOTTY;
     }

     switch(cmd)
     {
         case VS_SET_BAUD:
            vsdev.baud=arg;
            break;
         case VS_GET_BAUD:
            arg=vsdev.baud;
            break;
         case VS_SET_FFMT:
            if( copy_from_user(&vsdev.opt,(struct option __user*)arg,sizeof(struct option)) )
                return -EFAULT;
               break;
         case VS_GET_FFMT:
            if( copy_to_user((struct option __user*)arg,&vsdev.opt,sizeof(struct option)) )
                return -EFAULT;
            break;
         default:
            return -ENOTTY;
     }

    return 0;

}

static struct file_operations vser_ops = {
    .owner = THIS_MODULE,
    .open = vser_open,
    .release = vser_release,
    .read = vser_read,
    .write = vser_write,
    .unlocked_ioctl = vser_ioctl,
};


static int __init vser_init(void)
{
    int ret;
    dev_t dev;  /*设备号*/

    dev = MKDEV(VSER_MAJOR, VSER_MINOR);

    /*注册字符设备*/
    ret = register_chrdev_region(dev, VSER_DEV_CNT, VSER_DEV_NAME);
    if (ret)
        goto reg_err;

    cdev_init(&vsdev.cdev, &vser_ops);

    vsdev.cdev.owner = THIS_MODULE;
    vsdev.baud = 115200;
    vsdev.opt.datab = 8;
    vsdev.opt.parity = 0; /*奇偶位*/
    vsdev.opt.stopb = 1;
    /*cdev对象初始化以后，就要添加到内核中的cdev_map散列表中 */
    ret = cdev_add(&vsdev.cdev, dev, VSER_DEV_CNT);

    if (ret)
        goto add_err;

    return 0;

add_err:
    unregister_chrdev_region(dev, VSER_DEV_CNT);
reg_err:
    return ret;
}


/*卸载函数*/
static void __exit vser_exit(void)
{

    dev_t dev;

    dev = MKDEV(VSER_MAJOR, VSER_MINOR);

    cdev_del(&vsdev.cdev);
    unregister_chrdev_region(dev, VSER_DEV_CNT);
}



module_init(vser_init);
module_exit(vser_exit);

测试文件：

/*test.c*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>

#include "vser.h"

int main()
{
    int fd;
    int ret;
    unsigned int baud;
    char rbuf[32]={0};
    char wbuf[32]="XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";

    fd=open("/dev/vser0",O_RDWR|O_NONBLOCK);
    if(fd == -1)
    {
        goto fail;
    }

    ret=read(fd,rbuf,sizeof(rbuf));
    if(ret < 0)
    {
        perror("read");     
    }

    ret=write(fd,wbuf,sizeof(wbuf));
    if(ret < 0)
    {
        perror("first write\n");        
    }

    ret=write(fd,wbuf,sizeof(wbuf));
        if(ret < 0)
    {
        perror("second write");     
    }
    close(fd);
    exit(EXIT_SUCCESS);


fail:
    perror("ioctl test");
    exit(EXIT_FAILURE);

}