Linux字符设备驱动实例

1、globalmem虚拟设备实例

globalmem为“全局内存”的意思，在globalmem字符设备中会分配一片大小为GLOBALMEM_SIZE（4KB）的内存空间，并在驱动中提供对这片内存的读写、控制和定位函数，供用户空间的进程能通过Linux系统调用获取和设置这片内存。

（1）头文件、宏以及设备结构体

#include <linux/module.h>

#include <linux/init.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <linux/slab.h>

#include <linux/uaccess.h>

#define GLOBALMEM_SIZE  0x1000

#define MEM_CLEAR  0x1

#define GLOBALMEM_MAJOR  230

static int globalmem_major = GLOBALMEM_MAJOR;

module_param(globalmem_major, int, S_IRUGO);

struct globalmem_dev {

    struct cdev cdev;

    unsigned char mem[GLOBALMEM_SIZE];

};

struct globalmem_dev *globalmem_devp;

定义的globalmem_dev结构体中，包含了对应于globalmem字符设备的cdev，使用的内存mem[GLOBALMEM_SIZE]。

（2）globalmem设备驱动模块的加载和卸载函数

static void globalmem_setup_cdev(struct globalmem_dev *dev, int index)

{

    int err, devno = MKDEV(globalmem_major, index);

    cdev_init(&dev->cdev, &globalmem_fops);

    dev->cdev.owner = THIS_MODULE;

    err = cdev_add(&dev->cdev, devno, );

    if (err) {

        printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);

    }

}

static int __init globalmem_init(void)

{

    int ret;

    dev_t devno = MKDEV(globalmem_major, );

    if (globalmem_major) {

        ret = register_chrdev_region(devno, , "globalmem");

    } else {

        ret = alloc_chrdev_region(&devno, , , "globalmem");

        globalmem_major = MAJOR(devno);

    }

    if (ret < )

        return ret;

    globalmem_devp = kzalloc(sizeof(struct globalmem_dev), GFP_KERNEL);

    if (!globalmem_devp) {

        ret = -ENOMEM;

        goto fail_malloc;

    }

    globalmem_setup_cdev(globalmem_devp, );

    return ;

fail_malloc:

    unregister_chrdev_region(devno, );

    return ret;

}

static void __exit globalmem_exit(void)

{

    cdev_del(&globalmem_devp->cdev);

    kfree(globalmem_devp);

    unregister_chrdev_region(MKDEV(globalmem_major, ), );

}

globalmem_setup_cdev()函数完成cdev的初始化化和添加，kzalloc()申请了一份globalmem_dev结构体的内存，并将其清0，在cdev_init()函数中，与globalmem的cdev关联的file_operations结构体如下所示：

static const struct file_operations globalmem_fops = {

    .owner = THIS_MODULE,

    .open = globalmem_open,

    .release = globalmem_release,

    .read = globalmem_read,

    .write = globalmem_write,

    .llseek = globalmem_llseek,

    .unlocked_ioctl = globalmem_ioctl,

};

（3）读写函数的实现

首先是读函数，函数的实现如下所示：

static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)

{

    unsigned long p = *ppos;

    unsigned int count = size;

    int ret = ;

    struct globalmem_dev *dev = filp->private_data;

    if (p > GLOBALMEM_SIZE)

        return ;

    if (count > GLOBALMEM_SIZE - p)

        count = GLOBALMEM_SIZE - p;

    if (copy_to_user(buf, dev->mem + p, count)) {

        ret = -EFAULT;

    } else {

        *ppos += count;

        ret = count;

        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);

    }

    return ret;

}

其中*ppos是读的位置相对于文件开头的漂移，如果该漂移大于或等于GLOBALMEM_SIZE，表示文件已经到了末尾，返回0（EOF）。

写函数的实现如下所示：

static ssize_t globalmem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)

{

    unsigned long p = *ppos;

    unsigned int count = size;

    int ret = ;

    struct globalmem_dev *dev = filp->private_data;

    if (p > GLOBALMEM_SIZE)

        return ;

    if (count > GLOBALMEM_SIZE - p)

        count = GLOBALMEM_SIZE - p;

    if (copy_from_user(dev->mem + p, buf, count)) {

        return -EFAULT;

    } else {

        *ppos += count;

        ret = count;

        printk(KERN_INFO "written %u bytes(s) from %lu\n", count, p);

    }

    return ret;

}

（4）seek函数的实现

seek()函数对文件定位的起始地址可以是文件开头（SEEK_SET,0）、当前位置（SEEK_CUR,1）和文件末尾（SEEK_END,2），在定位的时候，要检查用户请求的合法性，若不合法，函数返回错误号，若合法，更新文件的当前位置，并返回新的位置，实现如下所示：

static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig)

{

    loff_t ret = ;

    switch (orig) {

    case :

        if (offset < ) {

            ret = -EINVAL;

            break;

        }

        if ((unsigned int)offset > GLOBALMEM_SIZE) {

            ret = -EINVAL;

            break;

        }

        filp->f_pos = (unsigned int)offset;

        ret = filp->f_pos;

        break;

    case :

        if ((filp->f_pos + offset) > GLOBALMEM_SIZE) {

            ret = -EINVAL;

            break;

        }

        if ((filp->f_pos + offset) < ) {

            ret = -EINVAL;

            break;

        }

        filp->f_pos += offset;

        ret = filp->f_pos;

        break;

    default:

        ret = -EINVAL;

        break;

    }

    return ret;

}

（5）ioctl函数实现

static long globalmem_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)

{

    struct globalmem_dev *dev = filp->private_data;

    switch (cmd) {

    case MEM_CLEAR:

        memset(dev->mem, , GLOBALMEM_SIZE);

        printk(KERN_INFO "globalmem is set to zero\n");

        break;

    default:

        return -EINVAL;

    }

    return ;

}

（6）使用文件的私有数据

将文件的私有数据private_data指向设备的结构体，然后使用read()、write()、ioctl()、llseek()等函数通过private_data访问设备结构体，如下所示：

static int globalmem_open(struct inode *inode, struct file *filp)

{

    filp->private_data = globalmem_devp;

    return ;

}

static int globalmem_release(struct inode *inode, struct file *filp)

{

    return ;

}

（7）完整的globalmem驱动代码

#include <linux/module.h>

#include <linux/init.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <linux/slab.h>

#include <linux/uaccess.h>

#define GLOBALMEM_SIZE  0x1000

#define MEM_CLEAR  0x1

#define GLOBALMEM_MAJOR  230

static int globalmem_major = GLOBALMEM_MAJOR;

module_param(globalmem_major, int, S_IRUGO);

struct globalmem_dev {

    struct cdev cdev;

    unsigned char mem[GLOBALMEM_SIZE];

};

struct globalmem_dev *globalmem_devp;

static int globalmem_open(struct inode *inode, struct file *filp)

{

    filp->private_data = globalmem_devp;

    return ;

}

static int globalmem_release(struct inode *inode, struct file *filp)

{

    return ;

}

static long globalmem_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)

{

    struct globalmem_dev *dev = filp->private_data;

    switch (cmd) {

    case MEM_CLEAR:

        memset(dev->mem, , GLOBALMEM_SIZE);

        printk(KERN_INFO "globalmem is set to zero\n");

        break;

    default:

        return -EINVAL;

    }

    return ;

}

static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)

{

    unsigned long p = *ppos;

    unsigned int count = size;

    int ret = ;

    struct globalmem_dev *dev = filp->private_data;

    if (p > GLOBALMEM_SIZE)

        return ;

    if (count > GLOBALMEM_SIZE - p)

        count = GLOBALMEM_SIZE - p;

    if (copy_to_user(buf, dev->mem + p, count)) {

        ret = -EFAULT;

    } else {

        *ppos += count;

        ret = count;

        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);

    }

    return ret;

}

static ssize_t globalmem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)

{

    unsigned long p = *ppos;

    unsigned int count = size;

    int ret = ;

    struct globalmem_dev *dev = filp->private_data;

    if (p > GLOBALMEM_SIZE)

        return ;

    if (count > GLOBALMEM_SIZE - p)

        count = GLOBALMEM_SIZE - p;

    if (copy_from_user(dev->mem + p, buf, count)) {

        return -EFAULT;

    } else {

        *ppos += count;

        ret = count;

        printk(KERN_INFO "written %u bytes(s) from %lu\n", count, p);

    }

    return ret;

}

static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig)

{

    loff_t ret = ;

    switch (orig) {

    case :

        if (offset < ) {

            ret = -EINVAL;

            break;

        }

        if ((unsigned int)offset > GLOBALMEM_SIZE) {

            ret = -EINVAL;

            break;

        }

        filp->f_pos = (unsigned int)offset;

        ret = filp->f_pos;

        break;

    case :

        if ((filp->f_pos + offset) > GLOBALMEM_SIZE) {

            ret = -EINVAL;

            break;

        }

        if ((filp->f_pos + offset) < ) {

            ret = -EINVAL;

            break;

        }

        filp->f_pos += offset;

        ret = filp->f_pos;

        break;

    default:

        ret = -EINVAL;

        break;

    }

    return ret;

}

static const struct file_operations globalmem_fops = {

    .owner = THIS_MODULE,

    .llseek = globalmem_llseek,

    .read = globalmem_read,

    .write = globalmem_write,

    .unlocked_ioctl = globalmem_ioctl,

    .open = globalmem_open,

    .release = globalmem_release,

};

static void globalmem_setup_cdev(struct globalmem_dev *dev, int index)

{

    int err, devno = MKDEV(globalmem_major, index);

    cdev_init(&dev->cdev, &globalmem_fops);

    dev->cdev.owner = THIS_MODULE;

    err = cdev_add(&dev->cdev, devno, );

    if (err) {

        printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);

    }

}

static int __init globalmem_init(void)

{

    int ret;

    dev_t devno = MKDEV(globalmem_major, );

    if (globalmem_major) {

        ret = register_chrdev_region(devno, , "globalmem");

    } else {

        ret = alloc_chrdev_region(&devno, , , "globalmem");

        globalmem_major = MAJOR(devno);

    }

    if (ret < )

        return ret;

    globalmem_devp = kzalloc(sizeof(struct globalmem_dev), GFP_KERNEL);

    if (!globalmem_devp) {

        ret = -ENOMEM;

        goto fail_malloc;

    }

    globalmem_setup_cdev(globalmem_devp, );

    return ;

fail_malloc:

    unregister_chrdev_region(devno, );

    return ret;

}

static void __exit globalmem_exit(void)

{

    cdev_del(&globalmem_devp->cdev);

    kfree(globalmem_devp);

    unregister_chrdev_region(MKDEV(globalmem_major, ), );

}

module_init(globalmem_init);

module_exit(globalmem_exit);

MODULE_AUTHOR("HLY");

MODULE_LICENSE("GPL");

（8）globalmem驱动验证

使用make命令将源文件编译出驱动模块globalmem.ko文件，编译需要的Makefile如下所示：

# Makefile for globalmem driver

obj-m += globalmem.o

all:

    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:

    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

然后使用驱动模块命令加载模块，如下：

$ sudo insmod globalmem.ko

$ lsmod

然后使用下面的命令查看globalmem虚拟设备的设备号：

$ cat /proc/devices

然后，使用mknod创建设备节点：

# mknod /dev/globalmem c

# ls -al /dev/globalmem

接下来使用命令对该文件进行读写以测试：

# echo “Hello World” > /dev/globalmem

# cat /dev/globalmem

也可以使用系统调用函数open、write和read进行该虚拟设备的测试，测试的app.c文件如下：

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <unistd.h>

#include <stdio.h>

#include <string.h>

#define LENGTH  100

int main(int argc, char *argv[])

{

    int fd,len;

    char str[LENGTH];

    fd = open("/dev/globalmem", O_RDWR);

    if (fd) {

        write(fd, "Hello World", strlen("Hello World"));

        close(fd);

    }

    fd = open("/dev/globalmem", O_RDWR);

    len = read(fd, str, LENGTH);

    str[len] = '\0';

    printf("str:%s\n", str);

    close(fd);

    return ;

}

编写该app.c的Makefile文件，如下：

# Makefile by HLY

all: myapp

# Which compiler

CC = gcc

# Where are include files

INCLUDE = .

# Options for development

CFLAGS = -g -Wall -ansi

myapp: app.o

    $(CC) -o myapp app.o

clean:

    rm -rf *.o myapp

使用make命令将app.c编译成可执行文件myapp，然后执行程序，即可完成globalmem虚拟设备/dev/globalmem的读写测试。

参考：

巴特西

Linux字符设备驱动实例—globalmem驱动

最新文章

热门文章