字符设备是3大类设备（字符设备、块设备和网络设备）中较简单的一类设备，其驱动程序中完成的主要工作是初始化、添加和删除cdev结构体，申请和释放设备号，以及填充 file_operations结构体中的操作函数，实现file_operations结构体中的read()、write()和ioctl()等函数是驱动设计的主体工作。
参考例程源码
/* * 虚拟字符设备globalmem实例: * 在globalmem字符设备驱动中会分配一片大小为 GLOBALMEM_SIZE（4KB） * 的内存空间，并在驱动中提供针对该片内存的读写、控制和定位函数，以供用户空间的进程能通过 * Linux系统调用访问这片内存。 */#include #include #include #include #include #include #include #include #include #include #include #define DEV_NAME "globalmem" /* /dev中显示的设备名 */#define DEV_MAJOR 0 /* 指定主设备号，为0则动态获取 *//* ioctl用的控制字 */#define GLOBALMEM_MAGIC 'M'#define MEM_CLEAR _IO(GLOBALMEM_MAGIC, 0)/*--------------------------------------------------------------------- local vars *//*globalmem设备结构体*/typedef struct { struct cdev cdev; /* 字符设备cdev结构体*/#define MEM_SIZE 0x1000 /*全局内存最大4K字节*/ unsigned char mem[MEM_SIZE]; /*全局内存*/ struct semaphore sem; /*并发控制用的信号量*/} globalmem_dev_t;static int globalmem_major = DEV_MAJOR;static globalmem_dev_t *globalmem_devp; /*设备结构体指针*//*--------------------------------------------------------------------- file operations *//*文件打开函数*/static int globalmem_open(struct inode *inodep, struct file *filep){ /* 获取dev指针 */ globalmem_dev_t *dev = container_of(inodep->i_cdev, globalmem_dev_t, cdev); filep->private_data = dev; return 0;}/*文件释放函数*/static int globalmem_release(struct inode *inodep, struct file *filep){ return 0;}/*读函数*/static ssize_t globalmem_read(struct file *filep, char __user *buf, size_t len, loff_t *ppos){ globalmem_dev_t *dev = filep->private_data; unsigned long p = *ppos; int ret = 0; /*分析和获取有效的长度*/ if (p > MEM_SIZE) { printk(KERN_EMERG "%s: overflow!\n", __func__); return - ENOMEM; } if (len > MEM_SIZE - p) { len = MEM_SIZE - p; } if (down_interruptible(&dev->sem)) /* 获得信号量*/ return - ERESTARTSYS; /*内核空间->用户空间*/ if (copy_to_user(buf, (void*)(dev->mem + p), len)) { ret = - EFAULT; }else{ *ppos += len; printk(KERN_INFO "%s: read %d bytes from %d\n", DEV_NAME, (int)len, (int)p); ret = len; } up(&dev->sem); /* 释放信号量*/ return ret;}/*写函数*/static ssize_t globalmem_write(struct file *filep, const char __user *buf, size_t len, loff_t *ppos){ globalmem_dev_t *dev = filep->private_data; int ret = 0; unsigned long p = *ppos; if (p > MEM_SIZE) { printk(KERN_EMERG "%s: overflow!\n", __func__); return - ENOMEM; } if (len > MEM_SIZE - p) { len = MEM_SIZE - p; } if (down_interruptible(&dev->sem)) /* 获得信号量*/ return - ERESTARTSYS; /*用户空间->内核空间*/ if (copy_from_user(dev->mem + p, buf, len)) { ret = - EFAULT; }else{ *ppos += len; printk(KERN_INFO "%s: written %d bytes from %d\n", DEV_NAME, (int)len, (int)p); ret = len; } up(&dev->sem); /* 释放信号量*/ return ret;}/* seek文件定位函数 */static loff_t globalmem_llseek(struct file *filep, loff_t offset, int start){ globalmem_dev_t *dev = filep->private_data; int ret = 0; if (down_interruptible(&dev->sem)) /* 获得信号量*/ return - ERESTARTSYS; switch (start) { case SEEK_SET: if (offset < 0 || offset > MEM_SIZE) { printk(KERN_EMERG "%s: overflow!\n", __func__); return - ENOMEM; } ret = filep->f_pos = offset; break; case SEEK_CUR: if ((filep->f_pos + offset) < 0 || (filep->f_pos + offset) > MEM_SIZE) { printk(KERN_EMERG "%s: overflow!\n", __func__); return - ENOMEM; } ret = filep->f_pos += offset; break; default: return - EINVAL; break; } up(&dev->sem); /* 释放信号量*/ printk(KERN_INFO "%s: set cur to %d.\n", DEV_NAME, ret); return ret;}/* ioctl设备控制函数 */static long globalmem_ioctl(struct file *filep, unsigned int cmd, unsigned long arg){ globalmem_dev_t *dev = filep->private_data; switch (cmd) { case MEM_CLEAR: if (down_interruptible(&dev->sem)) /* 获得信号量*/ return - ERESTARTSYS; memset(dev->mem, 0, MEM_SIZE); up(&dev->sem); /* 释放信号量*/ printk(KERN_INFO "%s: clear.\n", DEV_NAME); break; default: return - EINVAL; } return 0;}/*文件操作结构体*/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, .compat_ioctl = globalmem_ioctl};/*---------------------------------------------------------------------*//*初始化并注册cdev*/static int globalmem_setup(globalmem_dev_t *dev, int minor){ int ret = 0; dev_t devno = MKDEV(globalmem_major, minor); cdev_init(&dev->cdev, &globalmem_fops); dev->cdev.owner = THIS_MODULE; ret = cdev_add(&dev->cdev, devno, 1); if (ret) { printk(KERN_NOTICE "%s: Error %d dev %d.\n", DEV_NAME, ret, minor); } return ret;}/*设备驱动模块加载函数*/static int __init globalmem_init(void){ int ret = 0; dev_t devno; /* 申请设备号*/ if(globalmem_major){ devno = MKDEV(globalmem_major, 0); ret = register_chrdev_region(devno, 2, DEV_NAME); }else{ /* 动态申请设备号 */ ret = alloc_chrdev_region(&devno, 0, 2, DEV_NAME); globalmem_major = MAJOR(devno); } if (ret < 0) { return ret; } /* 动态申请设备结构体的内存，创建两个设备 */ globalmem_devp = kmalloc(2*sizeof(globalmem_dev_t), GFP_KERNEL); if (!globalmem_devp) { unregister_chrdev_region(devno, 2); return - ENOMEM; } ret |= globalmem_setup(&globalmem_devp[0], 0); /* globalmem0 */ ret |= globalmem_setup(&globalmem_devp[1], 1); /* globalmem1 */ if(ret) return ret; init_MUTEX(&globalmem_devp[0].sem); /*初始化信号量*/ init_MUTEX(&globalmem_devp[1].sem); printk(KERN_INFO "globalmem: up %d,%d.\n", MAJOR(devno), MINOR(devno)); return 0;}/*模块卸载函数*/static void __exit globalmem_exit(void){ cdev_del(&globalmem_devp[0].cdev); cdev_del(&globalmem_devp[1].cdev); kfree(globalmem_devp); unregister_chrdev_region(MKDEV(globalmem_major, 0), 2); printk(KERN_INFO "globalmem: down.\n");}/* 定义参数 */module_param(globalmem_major, int, S_IRUGO);module_init(globalmem_init);module_exit(globalmem_exit);/* 模块描述及声明 */MODULE_AUTHOR("Archie Xie ");MODULE_LICENSE("Dual BSD/GPL");MODULE_DESCRIPTION("A char device module just for demo.");MODULE_ALIAS("cdev gmem");MODULE_VERSION("1.0");
用户空间验证
切换到root用户插入模块
insmod globalmem.ko
创建设备节点（后续例程会展示自动创建节点的方法）
cat /proc/devices 找到主设备号majormknod /dev/globalmem0 c major 0 和 /dev/globalmem1 c major 1
读写测试
echo "hello world" > /dev/globalmemcat /dev/globalmem
