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：
《Linux设备驱动开发详解：基于最新的Linux 4.0内核》