字符设备驱动读写操作实现

一、读操作实现

ssize_t xxx_read(struct file *filp, char __user *pbuf, size_t count, loff_t *ppos);
完成功能：读取设备产生的数据
参数：filp：指向open产生的struct file类型的对象，表示本次read对应的那次openpbuf：指向用户空间一块内存，用来保存读到的数据count：用户期望读取的字节数ppos：对于需要位置指示器控制的设备操作有用，用来指示读取的起始位置，读完后也需要变更位置指示器的指示位置返回值：本次成功读取的字节数，失败返回-1

put_user(x,ptr)

x：char、int类型的简单变量名

unsigned long copy_to_user (void __user * to, const void * from, unsigned long n)

成功为返回0，失败非0

二、写操作实现

ssize_t xxx_write (struct file *filp, const char __user *pbuf, size_t count, loff_t *ppos);  
完成功能：向设备写入数据
参数：filp：指向open产生的struct file类型的对象，表示本次write对应的那次openpbuf：指向用户空间一块内存，用来保存被写的数据count：用户期望写入的字节数ppos：对于需要位置指示器控制的设备操作有用，用来指示写入的起始位置，写完后也需要变更位置指示器的指示位置返回值：本次成功写入的字节数，失败返回-1

get_user(x,ptr)

x：char、int类型的简单变量名

unsigned long copy_from_user (void * to, const void __user * from, unsigned long n)

成功为返回0，失败非0

实现代码：

mychar.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <asm/uaccess.h>#define BUF_LEN 100int major = 11;					//主设备号
int minor = 0;					//次设备号
int char_num = 1;				//设备号数量struct cdev mydev;char mydev_buf[BUF_LEN];
int curlen = 0;int mychar_open (struct inode *pnode, struct file *pfile)//打开设备
{printk("open\n");return 0;
}int mychar_close(struct inode *pnode, struct file *pfile)//关闭设备
{printk("close\n");return 0;
}ssize_t mychar_read (struct file *pfile, char __user *puser, size_t count, loff_t *p_pos) {int size = 0;int ret = 0;// 确定要读取的数据长度，如果请求大于设备当前数据长度，则读取全部可用数据if (count > curlen) {size = curlen;}else {size = count;}// 将设备数据复制到用户空间缓冲区ret = copy_to_user(puser, mydev_buf, size);if(ret) {printk("copy_to_user failed\n");return -1;}// 移动设备内部缓冲区，去除已读取的数据memcpy(mydev_buf, mydev_buf + size, curlen - size);curlen = curlen - size;// 返回实际读取的字节数return size;
}ssize_t mychar_write (struct file *pfile, const char __user *puser, size_t count, loff_t *p_pos) {int size = 0;int ret = 0;// 确定要写入的数据长度，如果请求大于设备缓冲区剩余空间，则写入剩余空间大小if (count > BUF_LEN - curlen) {size = BUF_LEN - curlen;}else {size = count;}// 从用户空间复制数据到设备缓冲区ret = copy_from_user(mydev_buf + curlen, puser, size);if(ret) {printk("copy_from_user failed\n");return -1;}// 更新设备缓冲区中的数据长度curlen = curlen + size;// 返回实际写入的字节数return size;
}struct file_operations myops = {.owner = THIS_MODULE,.open = mychar_open,.read = mychar_read,.write = mychar_write,.release = mychar_close,
};int __init mychar_init(void) 
{int ret = 0;dev_t devno = MKDEV(major, minor);/* 手动申请设备号 */ret = register_chrdev_region(devno, char_num, "mychar");if (ret) {/* 动态申请设备号 */ret = alloc_chrdev_region(&devno, minor, char_num, "mychar");if(ret){printk("get devno failed\n");return -1;}/*申请成功 更新设备号*/major = MAJOR(devno);}cdev_init(&mydev, &myops);mydev.owner = THIS_MODULE;cdev_add(&mydev, devno, char_num);return 0;
}void __exit mychar_exit(void) 
{dev_t devno = MKDEV(major, minor);printk("exit %d\n", devno);/* 从内核中移除一个字符设备 */cdev_del(&mydev);/* 回收设备号 */unregister_chrdev_region(devno, char_num);}MODULE_LICENSE("GPL");
module_init(mychar_init);
module_exit(mychar_exit);

testmychar_app.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>int main(int argc, char *argv[])
{int fd = -1;char buf[32] = "";if(argc < 2) {printf("The argument is too few\n");return -1;}fd = open(argv[1], O_RDWR);if(fd < 0) {perror("open");return -1;}write(fd, "hello", 6);read(fd, buf, 32);printf("buf = %s\n", buf);close(fd);fd = -1;return 0;
}

Makefile

ifeq ($(KERNELRELEASE),)ifeq ($(ARCH),arm)
KERNELDIR ?= /home/myubuntu/Linux_4412/kernel/linux-3.14 
ROOTFS ?= /opt/4412/rootfs
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
endif
PWD := $(shell pwd)modules:$(MAKE) -C $(KERNELDIR) M=$(PWD) modulesmodules_install:$(MAKE) -C $(KERNELDIR) M=$(PWD) modules INSTALL_MOD_PATH=$(ROOTFS) modules_installclean:rm -rf  *.o  *.ko  .*.cmd  *.mod.*  modules.order  Module.symvers   .tmp_versionselse
# obj-m += myhello.o# obj-m += mytest.o
# mytest-objs = test.o func.o# obj-m += testparam.oobj-m += mychar.o
endif