非阻塞 IO(NIO)

上一节中 https://blog.csdn.net/tyustli/article/details/135140523，使用等待队列头实现了阻塞 IO

程序使用时，阻塞 IO 和非阻塞 IO 的区别在于文件打开的时候是否使用了
O_NONBLOCK 标志位。

• O_RDWR 默认以阻塞的方式打开
• O_NONBLOCK 以非阻塞的方式打开

模型

驱动程序

文件打卡时，文件打开标志存放在文件结构体 struct file f_flags 字段
文件结构体原型在 linux/fs.h 文件中

struct file {
	union {
		struct llist_node	f_llist;
		struct rcu_head 	f_rcuhead;
		unsigned int 		f_iocb_flags;
	};

	/*
	 * Protects f_ep, f_flags.
	 * Must not be taken from IRQ context.
	 */
	spinlock_t		f_lock;
	fmode_t			f_mode;
	atomic_long_t		f_count;
	struct mutex		f_pos_lock;
	loff_t			f_pos;
	unsigned int		f_flags;
	struct fown_struct	f_owner;
	const struct cred	*f_cred;
	struct file_ra_state	f_ra;
	struct path		f_path;
	struct inode		*f_inode;	/* cached value */
	const struct file_operations	*f_op;

	u64			f_version;
#ifdef CONFIG_SECURITY
	void			*f_security;
#endif
	/* needed for tty driver, and maybe others */
	void			*private_data;

#ifdef CONFIG_EPOLL
	/* Used by fs/eventpoll.c to link all the hooks to this file */
	struct hlist_head	*f_ep;
#endif /* #ifdef CONFIG_EPOLL */
	struct address_space	*f_mapping;
	errseq_t		f_wb_err;
	errseq_t		f_sb_err; /* for syncfs */
} __randomize_layout
  __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */

所以驱动中需要判断文件打开标志是否支持非阻塞方式

    new_chrdev_t *dev = (new_chrdev_t *)file->private_data;

    if (file->f_flags & O_NONBLOCK) {
        if (!dev->flag) {
            return -EAGAIN;
        }
    }

驱动程序源码

#include "asm-generic/errno-base.h"
#include "linux/device/class.h"
#include "linux/export.h"
#include "linux/uaccess.h"
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/cdev.h>
#include <linux/wait.h>

#define CHRDEVBASE_NAME "chrdevbase" /* 设备名 */
#define CHRDEVBASE_NUM 1             /* 设备数目 */

static char *string_test = "kernel data this tyustli test";

typedef struct {
    dev_t dev_id;          /* 设备号 */
    struct cdev c_dev;     /* cdev */
    struct class *class;   /* 类 */
    struct device *device; /* 设备 */
    int major;             /* 主设备号 */
    int minor;             /* 次设备号 */
    int flag;              /* read flag */
    char write_buf[100];
    char read_buf[100];
    wait_queue_head_t read_wait; /* 读等待队列头 */
} new_chrdev_t;

static new_chrdev_t new_chrdev1;

static int chrdevbase_open(struct inode *inode, struct file *file)
{
    file->private_data =
            container_of(inode->i_cdev, new_chrdev_t, c_dev); /* 设置私有数据 */

    printk("k: chrdevbase open\r\n");

    return 0;
}

static ssize_t chrdevbase_read(struct file *file, char __user *buf,
                               size_t count, loff_t *ppos)
{
    unsigned long ret = 0;
    new_chrdev_t *dev = (new_chrdev_t *)file->private_data;

    if (file->f_flags & O_NONBLOCK) {
        if (!dev->flag) {
            return -EAGAIN;
        }
    }

    wait_event_interruptible(dev->read_wait, dev->flag);
    dev->flag = 0;

    memcpy(dev->read_buf, string_test, strlen(string_test));

    ret = copy_to_user(buf, dev->read_buf, count);
    if (ret == 0) {
        printk("k: read data success\r\n");
    } else {
        printk("k: read data failed ret = %ld\r\n", ret);
    }

    return ret;
}

static ssize_t chrdevbase_write(struct file *file, const char __user *buf,
                                size_t count, loff_t *ppos)
{
    unsigned long ret = 0;

    new_chrdev_t *dev = (new_chrdev_t *)file->private_data;

    ret = copy_from_user(dev->write_buf, buf, count);
    if (ret == 0) {
        printk("k: write data success write data is: %s\r\n", dev->write_buf);
    } else {
        printk("k: write data failed ret = %ld\r\n", ret);
    }

    dev->flag = 1;
    wake_up_interruptible(&dev->read_wait);

    return count;
}

static int chrdevbase_release(struct inode *inode, struct file *file)
{
    printk("k: chrdevbase release\r\n");

    return 0;
}

static struct file_operations chrdevbase_fops = {
    .owner = THIS_MODULE,
    .open = chrdevbase_open,
    .read = chrdevbase_read,
    .write = chrdevbase_write,
    .release = chrdevbase_release,
};

static int __init chrdevbase_init(void)
{
    int err = 0;

    err = alloc_chrdev_region(&new_chrdev1.dev_id, 0, CHRDEVBASE_NUM,
                              CHRDEVBASE_NAME);
    if (err < 0) {
        printk("k: alloc chrdev region failed err = %d\r\n", err);
        goto err_chrdev;
    }

    /* get major 1 and minor 1 */
    new_chrdev1.major = MAJOR(new_chrdev1.dev_id);
    new_chrdev1.minor = MINOR(new_chrdev1.dev_id);
    printk("k: newcheled major=%d,minor=%d\r\n", new_chrdev1.major,
           new_chrdev1.minor);

    new_chrdev1.c_dev.owner = THIS_MODULE;
    cdev_init(&new_chrdev1.c_dev, &chrdevbase_fops);
    err = cdev_add(&new_chrdev1.c_dev, new_chrdev1.dev_id, 1);
    if (err < 0) {
        printk("k: cdev add failed err = %d\r\n", err);
        goto err_cdev_add;
    }
    new_chrdev1.class = class_create("chr_test1");
    if (IS_ERR(new_chrdev1.class)) {
        err = PTR_ERR(new_chrdev1.class);
        goto err_class_create;
    }

    new_chrdev1.device = device_create(new_chrdev1.class, NULL,
                                       new_chrdev1.dev_id, NULL, "chr_test1");
    if (IS_ERR(new_chrdev1.device)) {
        err = PTR_ERR(new_chrdev1.device);
        goto err_device_create;
    }

    /* 初始化等待队列头 */
    init_waitqueue_head(&new_chrdev1.read_wait);

    new_chrdev1.flag = 0;

    printk("k: base module init\r\n");

    return 0;

err_device_create:
    class_destroy(new_chrdev1.class);
err_class_create:
    cdev_del(&new_chrdev1.c_dev);
err_cdev_add:
    unregister_chrdev_region(new_chrdev1.dev_id, CHRDEVBASE_NUM);
err_chrdev:
    return err;
}

static void __exit chrdevbase_exit(void)
{
    device_destroy(new_chrdev1.class, new_chrdev1.dev_id);
    class_destroy(new_chrdev1.class);
    cdev_del(&new_chrdev1.c_dev);

    unregister_chrdev_region(new_chrdev1.dev_id, CHRDEVBASE_NUM);

    printk("k: base module exit!\r\n");
}

module_init(chrdevbase_init);
module_exit(chrdevbase_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("tyustli");
MODULE_INFO(intree, "Y"); /* loading out-of-tree module taints kernel */

应用程序

打开文件时使用 O_NONBLOCK 非阻塞方式打开文件

fd = open(filename, O_RDWR | O_NONBLOCK);

如果文件读取失败，循环读取

        retvalue = -1;
        while (retvalue < 0) {
            retvalue = read(fd, readbuf, 50);
            if (retvalue < 0) {
                printf("u: read file %s failed!\r\n", filename);
                sleep(3);
            } else {
                /*  读取成功，打印出读取成功的数据 */
                printf("u: read data:%s\r\n", readbuf);
            }
        }

完整应用程序代码

#include "stdio.h"
#include "unistd.h"
#include "sys/types.h"
#include "sys/stat.h"
#include "fcntl.h"
#include "stdlib.h"
#include "string.h"

static char usrdata[] = { "user data!" };

int main(int argc, char *argv[])
{
    int fd, retvalue;
    char *filename;
    char readbuf[100], writebuf[100];

    if (argc != 3) {
        printf("u: error Usage!\r\n");
        return -1;
    }

    filename = argv[1];

    /* 打开驱动文件 */
    fd = open(filename, O_RDWR | O_NONBLOCK);
    if (fd < 0) {
        printf("u: can't open file %s\r\n", filename);
        return -1;
    }

    /* 从驱动 文件读取数据 */
    if (atoi(argv[2]) == 1) {
        retvalue = -1;
        while (retvalue < 0) {
            retvalue = read(fd, readbuf, 50);
            if (retvalue < 0) {
                printf("u: read file %s failed!\r\n", filename);
                sleep(3);
            } else {
                /*  读取成功，打印出读取成功的数据 */
                printf("u: read data:%s\r\n", readbuf);
            }
        }
    }

    /* 向设备驱动写数据 */
    if (atoi(argv[2]) == 2) {
        memcpy(writebuf, usrdata, sizeof(usrdata));
        retvalue = write(fd, writebuf, 50);
        if (retvalue < 0) {
            printf("u: write file %s failed!\r\n", filename);
        }
    }

    /* 关闭设备 */
    retvalue = close(fd);
    if (retvalue < 0) {
        printf("u: can't close file %s\r\n", filename);
        return -1;
    }

    return 0;
}

模块使用

模块安装

modprobe my_module

查看设备节点

ls /dev

~ # ls /dev/
chr_test1        ptypc            tty32            tty7
console          ptypd            tty33            tty8
cpu_dma_latency  ptype            tty34            tty9
full             ptypf            tty35            ttyAMA0
gpiochip0        random           tty36            ttyAMA1
gpiochip1        root             tty37            ttyAMA2
gpiochip2        rtc0             tty38            ttyAMA3
gpiochip3        snd              tty39            ttyp0
hwrng            tty              tty4             ttyp1

模块使用

~ # /lib/modules/6.5.7+/my_app /dev/chr_test1 1 &
~ # k: chrdevbase open
u: read file /dev/chr_test1 failed!
u: read file /dev/chr_test1 failed!
~ # /lib/modules/6.5.7+/my_app /dev/chr_test1 2
k: chrdevbase open
k: write data success write data is: user data!
k: chrdevbase release
~ # k: read data success
k: chrdevbase release
u: read data:kernel data this tyustli test

[1]+  Done                       /lib/modules/6.5.7+/my_app /dev/chr_test1 1
~ #