Linux驱动开发入门（二）
本文制作一个char设备。撰写chardev.c如下：/* * http://linux.die.net/lkmpg/x569.html *//* *chardev.c: Creates a read-only char device that says how many times *you've read from the dev file */#include <linux/kernel.h>#include <linux/module.h>#include <linux/fs.h>#include <asm/uaccess.h> /* for put_user *//* *Prototypes - this would normally go in a .h file */int init_module(void);void cleanup_module(void);static int device_open(struct inode *, struct file *);static int device_release(struct inode *, struct file *);static ssize_t device_read(struct file *, char *, size_t, loff_t *);static ssize_t device_write(struct file *, const char *, size_t, loff_t *);#define SUCCESS 0#define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices */#define BUF_LEN 80 /* Max length of the message from the device *//** Global variables are declared as static, so are global within the file.*/static int Major; /* Major number assigned to our device driver */static int Device_Open = 0; /* Is device open? * Used to prevent multiple access to device */static char msg; /* The msg the device will give when asked */static char *msg_Ptr;static struct file_operations fops = { .read = device_read, .write = device_write, .open = device_open, .release = device_release};/* * This function is called when the module is loaded */int init_module(void){ Major = register_chrdev(0, DEVICE_NAME, &fops); if (Major < 0) { printk(KERN_ALERT "Registering char device failed with %d\n", Major); return Major; } printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major); printk(KERN_INFO "the driver, create a dev file with\n"); printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major); printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n"); printk(KERN_INFO "the device file.\n"); printk(KERN_INFO "Remove the device file and module when done.\n"); return SUCCESS;}/* * This function is called when the module is unloaded */void cleanup_module(void){ /* * Unregister the device */ unregister_chrdev(Major, DEVICE_NAME);}/* * Methods *//** Called when a process tries to open the device file, like * "cat /dev/mycharfile" */static int device_open(struct inode *inode, struct file *file){ static int counter = 0; if (Device_Open) return -EBUSY; Device_Open++; sprintf(msg, "I already told you %d times Hello world!\n", counter++); msg_Ptr = msg; try_module_get(THIS_MODULE); return SUCCESS;}/** Called when a process closes the device file. */static int device_release(struct inode *inode, struct file *file){ Device_Open--; /* We're now ready for our next caller */ /* * Decrement the usage count, or else once you opened the file, you'll * never get get rid of the module. */ module_put(THIS_MODULE); return 0;}/** Called when a process, which already opened the dev file, attempts to * read from it. */static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ char *buffer, /* buffer to fill with data */ size_t length, /* length of the buffer */ loff_t * offset){ /* * Number of bytes actually written to the buffer */ int bytes_read = 0; /* * If we're at the end of the message, * return 0 signifying end of file */ if (*msg_Ptr == 0) return 0; /* * Actually put the data into the buffer */ while (length && *msg_Ptr) { /* * The buffer is in the user data segment, not the kernel * segment so "*" assignment won't work.We have to use * put_user which copies data from the kernel data segment to * the user data segment. */ put_user(*(msg_Ptr++), buffer++); length--; bytes_read++; } /* * Most read functions return the number of bytes put into the buffer */ return bytes_read;}/* * Called when a process writes to dev file: echo "hi" > /dev/hello*/static ssize_tdevice_write(struct file *filp, const char *buff, size_t len, loff_t * off){ printk(KERN_ALERT "Sorry, this operation isn't supported.\n"); return -EINVAL;}
然后撰写Makefile：
# Comment/uncomment the following line to disable/enable debugging#DEBUG = y# Add your debugging flag (or not) to CFLAGSifeq ($(DEBUG),y)DEBFLAGS = -O -g # "-O" is needed to expand inlineselseDEBFLAGS = -O2endifEXTRA_CFLAGS += $(DEBFLAGS) #-I$(LDDINCDIR)ifneq ($(KERNELRELEASE),)# call from kernel build systemobj-m := chardev.oelseKERNELDIR ?= /lib/modules/$(shell uname -r)/buildPWD := $(shell pwd)default: $(MAKE) -C $(KERNELDIR) M=$(PWD) modules #LDDINCDIR=$(PWD)/../include modulesendifclean: rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versionsdepend .depend dep: $(CC) $(CFLAGS) -M *.c > .dependifeq (.depend,$(wildcard .depend))include .dependendif
接下来是生成目标文件：
http://dl.iteye.com/upload/attachment/0069/3529/f0a20418-fd5b-31ff-8ee1-8fa1bc3cecb6.png
然后是安装驱动，并查看内核的日志输出：
http://dl.iteye.com/upload/attachment/0069/3533/6f4988f8-5123-34ff-80a6-3865c7c6661e.png
看到驱动已经成功加载了，此时查看设备列表：
http://dl.iteye.com/upload/attachment/0069/3535/d3e742df-6681-3521-acbc-925f9a345b86.png
可以看到最后一行，设备已经加载了。此时使用mknod在dev中安装这个chardev设备：
http://dl.iteye.com/upload/attachment/0069/3537/0cda9749-1eaa-3778-8285-fb8050cfb5f9.png
此时已经可以使用这个设备了：
http://dl.iteye.com/upload/attachment/0069/3539/f9f767e3-b514-3b45-8250-ea5156462b14.png
测试完成后，删除设备并卸载驱动：
http://dl.iteye.com/upload/attachment/0069/3541/e21b6aa8-7d70-310b-b11f-ac32a205008d.png
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