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
1 #include <linux/init.h>
2 #include <linux/module.h>
3 #include <linux/fs.h>
4 #include <linux/slab.h>
5 #include <linux/errno.h>
6 #include <linux/types.h>
7 #include <linux/cdev.h>
8 #include <asm/uaccess.h>
9
10 #include "scull.h"
11
12 int scull_major = SCULL_MAJOR;
13 int scull_minor = 0;
14
15 module_param(scull_major, int, S_IRUGO);
16 module_param(scull_minor, int, S_IRUGO);
17
18 struct scull_dev *scull_device;
19
20 int scull_trim(struct scull_dev *dev)
21 {
22 if (dev)
23 {
24 if (dev->data)
25 {
26 kfree(dev->data);
27 }
28 dev->data = NULL;
29 dev->size = 0;
30 }
31 return 0;
32 }
33
34 int scull_open(struct inode *inode, struct file *filp)
35 {
36 struct scull_dev *dev;
37
38 dev = container_of(inode->i_cdev, struct scull_dev, cdev);
39 filp->private_data = dev;
40
41 if ((filp->f_flags & O_ACCMODE) == O_WRONLY)
42 {
43 if (down_interruptible(&dev->sem))
44 {
45 return -ERESTARTSYS;
46 }
47 scull_trim(dev);
48 up(&dev->sem);
49 }
50
51 return 0;
52 }
53
54 int scull_release(struct inode *inode, struct file *filp)
55 {
56 return 0;
57 }
58
59 ssize_t scull_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
60 {
61 struct scull_dev *dev = filp->private_data;
62 ssize_t retval = 0;
63
64 if (down_interruptible(&dev->sem))
65 {
66 return -ERESTARTSYS;
67 }
68 if (*f_pos >= dev->size)
69 {
70 goto out;
71 }
72 if (*f_pos + count > dev->size)
73 {
74 count = dev->size - *f_pos;
75 }
76
77 if (!dev->data)
78 {
79 goto out;
80 }
81
82 if (copy_to_user(buf, dev->data + *f_pos, count))
83 {
84 retval = -EFAULT;
85 goto out;
86 }
87
88 *f_pos += count;
89 retval = count;
90
91 out:
92 up(&dev->sem);
93 return retval;
94 }
95
96 ssize_t scull_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos)
97 {
98 struct scull_dev *dev = filp->private_data;
99 ssize_t retval = -ENOMEM;
100
101 if (down_interruptible(&dev->sem))
102 {
103 return -ERESTARTSYS;
104 }
105
106 if (!dev->data)
107 {
108 dev->data = kmalloc(SCULL_BUFFER_SIZE, GFP_KERNEL);
109 if (!dev->data)
110 {
111 goto out;
112 }
113 memset(dev->data, 0, SCULL_BUFFER_SIZE);
114 }
115
116 if (count > SCULL_BUFFER_SIZE - dev->size)
117 {
118 count = SCULL_BUFFER_SIZE - dev->size;
119 }
120
121 if (copy_from_user(dev->data + dev->size, buf, count))
122 {
123 retval = -EFAULT;
124 goto out;
125 }
126
127 dev->size += count;
128 retval = count;
129
130 out:
131 up(&dev->sem);
132 return retval;
133 }
134
135 loff_t scull_llseek(struct file *filp, loff_t off, int whence)
136 {
137 struct scull_dev *dev = filp->private_data;
138 loff_t newpos;
139
140 switch(whence)
141 {
142 case 0:
143 newpos = off;
144 break;
145 case 1:
146 newpos = filp->f_pos + off;
147 break;
148 case 2:
149 newpos = dev->size + off;
150 break;
151 default:
152 return -EINVAL;
153 }
154 if (newpos < 0)
155 {
156 return -EINVAL;
157 }
158 filp->f_pos = newpos;
159 return newpos;
160 }
161
162 struct file_operations scull_fops = {
163 .owner = THIS_MODULE,
164 .llseek = scull_llseek,
165 .read = scull_read,
166 .write = scull_write,
167 .open = scull_open,
168 .release = scull_release,
169 };
170
171 void scull_cleanup_module(void)
172 {
173 dev_t devno = MKDEV(scull_major, scull_minor);
174
175 if (scull_device)
176 {
177 scull_trim(scull_device);
178 cdev_del(&scull_device->cdev);
179 kfree(scull_device);
180 }
181 unregister_chrdev_region(devno, 1);
182 }
183
184 static void scull_setup_cdev(struct scull_dev *dev)
185 {
186 int err, devno = MKDEV(scull_major, scull_minor);
187
188 cdev_init(&dev->cdev, &scull_fops);
189 dev->cdev.owner = THIS_MODULE;
190 dev->cdev.ops = &scull_fops;
191 err = cdev_add(&dev->cdev, devno, 1);
192
193 if (err)
194 {
195 printk(KERN_NOTICE "Error %d adding scull", err);
196 }
197 }
198
199 static int __init scull_init_module(void)
200 {
201 int result;
202 dev_t dev = 0;
203
204 if (scull_major)
205 {
206 dev = MKDEV(scull_major, scull_minor);
207 result = register_chrdev_region(dev, 1, "scull");
208 }
209 else
210 {
211 result = alloc_chrdev_region(&dev, scull_minor, 1, "scull");
212 scull_major = MAJOR(dev);
213 }
214 if (result < 0)
215 {
216 printk(KERN_WARNING "scull: can't get major %d
", scull_major);
217 return result;
218 }
219
220 scull_device = kmalloc(sizeof(struct scull_dev), GFP_KERNEL);
221 if (!scull_device)
222 {
223 result = -ENOMEM;
224 goto fail;
225 }
226 memset(scull_device, 0, sizeof(struct scull_dev));
227
228 init_MUTEX(&scull_device->sem);
229
230 scull_setup_cdev(scull_device);
231
232 return 0;
233
234 fail:
235 scull_cleanup_module();
236 return result;
237 }
238
239 module_init(scull_init_module);
240 module_exit(scull_cleanup_module);
241
242 MODULE_LICENSE("GPL");
2. scull.h
1 #ifndef _SCULL_H
2 #define _SCULL_H
3
4 #define SCULL_MAJOR 0
5 #define SCULL_BUFFER_SIZE PAGE_SIZE
6
7 struct scull_dev {
8 char *data;
9 unsigned long size;
10 struct semaphore sem;
11 struct cdev cdev;
12 };
13
14 #endif
3. Makefile
1 obj-m += scull.o
2
3 CURRENT_PATH:=$(shell pwd)
4 LINUX_KERNEL:=$(shell uname -r)
5 LINUX_KERNEL_PATH:=/usr/src/kernels/$(LINUX_KERNEL)
6
7 all:
8 make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PATH) modules
9 clean:
10 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”