简单linux字符设备驱动程序

本文代码参考《LINUX设备驱动程序》第三章 字符设备驱动程序
本文中的“字符设备”是一段大小为PAGE_SIZE的内存空间
功能：向字符设备写入字符串；从字符设备读出字符串
代码：
1. scull.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/types.h>
#include <linux/cdev.h>
#include <asm/uaccess.h>

#include "scull.h"

int scull_major = SCULL_MAJOR;
int scull_minor = 0;

module_param(scull_major, int, S_IRUGO);
module_param(scull_minor, int, S_IRUGO);

struct scull_dev *scull_device;

int scull_trim(struct scull_dev *dev)
{
if (dev)
{
if (dev->data)
{
kfree(dev->data);
}
dev->data = NULL;
dev->size = 0;
}
return 0;
}

int scull_open(struct inode *inode, struct file *filp)
{
struct scull_dev *dev;

dev = container_of(inode->i_cdev, struct scull_dev, cdev);
filp->private_data = dev;

if ((filp->f_flags & O_ACCMODE) == O_WRONLY)
{
if (down_interruptible(&dev->sem))
{
return -ERESTARTSYS;
}
scull_trim(dev);
up(&dev->sem);
}

return 0;
}

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

ssize_t scull_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
struct scull_dev *dev = filp->private_data;
ssize_t retval = 0;

if (down_interruptible(&dev->sem))
{
return -ERESTARTSYS;
}
if (*f_pos >= dev->size)
{
goto out;
}
if (*f_pos + count > dev->size)
{
count = dev->size - *f_pos;
}

if (!dev->data)
{
goto out;
}

if (copy_to_user(buf, dev->data + *f_pos, count))
{
retval = -EFAULT;
goto out;
}

*f_pos += count;
retval = count;

out:
up(&dev->sem);
return retval;
}

ssize_t scull_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos)
{
struct scull_dev *dev = filp->private_data;
ssize_t retval = -ENOMEM;

if (down_interruptible(&dev->sem))
{
return -ERESTARTSYS;
}

if (!dev->data)
{
dev->data = kmalloc(SCULL_BUFFER_SIZE, GFP_KERNEL);
if (!dev->data)
{
goto out;
}
memset(dev->data, 0, SCULL_BUFFER_SIZE);
}

if (count > SCULL_BUFFER_SIZE - dev->size)
{
count = SCULL_BUFFER_SIZE - dev->size;
}

if (copy_from_user(dev->data + dev->size, buf, count))
{
retval = -EFAULT;
goto out;
}

dev->size += count;
retval = count;

out:
up(&dev->sem);
return retval;
}

loff_t scull_llseek(struct file *filp, loff_t off, int whence)
{
struct scull_dev *dev = filp->private_data;
loff_t newpos;

switch(whence)
{
case 0:
newpos = off;
break;
case 1:
newpos = filp->f_pos + off;
break;
case 2:
newpos = dev->size + off;
break;
default:
return -EINVAL;
}
if (newpos < 0)
{
return -EINVAL;
}
filp->f_pos = newpos;
return newpos;
}

struct file_operations scull_fops = {
.owner = THIS_MODULE,
.llseek = scull_llseek,
.read = scull_read,
.write = scull_write,
.open = scull_open,
.release = scull_release,
};

void  scull_cleanup_module(void)
{
dev_t devno = MKDEV(scull_major, scull_minor);

if (scull_device)
{
scull_trim(scull_device);
cdev_del(&scull_device->cdev);
kfree(scull_device);
}
unregister_chrdev_region(devno, 1);
}

static void scull_setup_cdev(struct scull_dev *dev)
{
int err, devno = MKDEV(scull_major, scull_minor);

cdev_init(&dev->cdev, &scull_fops);
dev->cdev.owner = THIS_MODULE;
dev->cdev.ops = &scull_fops;
err = cdev_add(&dev->cdev, devno, 1);

if (err)
{
printk(KERN_NOTICE "Error %d adding scull", err);
}
}

static int __init scull_init_module(void)
{
int result;
dev_t dev = 0;

if (scull_major)
{
dev = MKDEV(scull_major, scull_minor);
result = register_chrdev_region(dev, 1, "scull");
}
else
{
result = alloc_chrdev_region(&dev, scull_minor, 1, "scull");
scull_major = MAJOR(dev);
}
if (result < 0)
{
printk(KERN_WARNING "scull: can't get major %d\n", scull_major);
return result;
}

scull_device = kmalloc(sizeof(struct scull_dev), GFP_KERNEL);
if (!scull_device)
{
result = -ENOMEM;
goto fail;
}
memset(scull_device, 0, sizeof(struct scull_dev));

init_MUTEX(&scull_device->sem);

scull_setup_cdev(scull_device);

return 0;

fail:
scull_cleanup_module();
return result;
}

module_init(scull_init_module);
module_exit(scull_cleanup_module);

MODULE_LICENSE("GPL");

2. scull.h

#ifndef _SCULL_H
#define _SCULL_H

#define SCULL_MAJOR 0
#define SCULL_BUFFER_SIZE PAGE_SIZE

struct scull_dev {
char *data;
unsigned long size;
struct semaphore sem;
struct cdev cdev;
};

#endif

3. Makefile

obj-m += scull.o

CURRENT_PATH:=$(shell pwd)
LINUX_KERNEL:=$(shell uname -r)
LINUX_KERNEL_PATH:=/usr/src/kernels/$(LINUX_KERNEL)

all:
make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PATH) modules
clean:
make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PATH) clean

4. 验证效果

1) make

2) insmod scull.ko

3) cat /proc/devices | grep scull //获取字符设备scull的主设备号,我的是248

4) cd /dev/

5) mknod scull c 248 0

6) echo hello, world > scull //向字符设备写入“hello, world”

7) cat scull //读出字符设备中的内容，这里应该显示“hello, world”