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上一篇中globalmem仅仅作为使用private_data 的范例，直接访问全局变量globalmem_devp会更加结构清晰。如果globalmem不只包括一个设备，而是同时包括两个或两个以上的设备，采用private_data 的优势就会显现出来。
在不对上一篇中代码的globalmem_read()、globalmem_write()、 globalmem_ioctl()等重要函数及 globalmem_fops结构体等数据结构进行任何修改的前提下，只是简单地修改 globalmem_init()、globalmem_exit()和 globalmem_open()，就可以轻松地让globalmem驱动中包含两个同样的设备（次设备号分别为0和1） ，如下代码所示：
/*====================================================================== A globalmem driver as an example of char device drivers There are two same globalmems in this driver This example is to introduce the function of file->private_data The initial developer of the original code is Baohua Song . All Rights Reserved.======================================================================*/#include #include #include #include #include #include #include #include #include #include #include #define GLOBALMEM_SIZE 0x1000 /*全局内存最大4K字节*/#define MEM_CLEAR 0x1 /*清0全局内存*/#define GLOBALMEM_MAJOR 254 /*预设的globalmem的主设备号*/static globalmem_major = GLOBALMEM_MAJOR;/*globalmem设备结构体*/struct globalmem_dev { struct cdev cdev; /*cdev结构体*/ unsigned char mem[GLOBALMEM_SIZE]; /*全局内存*/ };struct globalmem_dev *globalmem_devp; /*设备结构体指针*//*文件打开函数*/int globalmem_open(struct inode *inode, struct file *filp){ /*将设备结构体指针赋值给文件私有数据指针*/ struct globalmem_dev *dev; dev = container_of(inode->i_cdev,struct globalmem_dev,cdev); filp->private_data = dev; return 0;}/*文件释放函数*/int globalmem_release(struct inode *inode, struct file *filp){ return 0;}/* ioctl设备控制函数 */static int globalmem_ioctl(struct inode *inodep, struct file *filp, unsigned int cmd, unsigned long arg){ struct globalmem_dev *dev = filp->private_data;/*获得设备结构体指针*/ switch (cmd) { case MEM_CLEAR: memset(dev->mem, 0, GLOBALMEM_SIZE); printk(KERN_INFO "globalmem is set to zero\n"); break; default: return - EINVAL; } return 0;}/*读函数*/static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos){ unsigned long p = *ppos; unsigned int count = size; int ret = 0; struct globalmem_dev *dev = filp->private_data; /*获得设备结构体指针*/ /*分析和获取有效的写长度*/ if (p >= GLOBALMEM_SIZE) return count ? - ENXIO: 0; if (count > GLOBALMEM_SIZE - p) count = GLOBALMEM_SIZE - p; /*内核空间->用户空间*/ if (copy_to_user(buf, (void*)(dev->mem + p), count)) { ret = - EFAULT; } else { *ppos += count; ret = count; printk(KERN_INFO "read %d bytes(s) from %d\n", count, p); } return ret;}/*写函数*/static ssize_t globalmem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos){ unsigned long p = *ppos; unsigned int count = size; int ret = 0; struct globalmem_dev *dev = filp->private_data; /*获得设备结构体指针*/ /*分析和获取有效的写长度*/ if (p >= GLOBALMEM_SIZE) return count ? - ENXIO: 0; if (count > GLOBALMEM_SIZE - p) count = GLOBALMEM_SIZE - p; /*用户空间->内核空间*/ if (copy_from_user(dev->mem + p, buf, count)) ret = - EFAULT; else { *ppos += count; ret = count; printk(KERN_INFO "written %d bytes(s) from %d\n", count, p); } return ret;}/* seek文件定位函数 */static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig){ loff_t ret = 0; switch (orig) { case 0: /*相对文件开始位置偏移*/ if (offset < 0) { ret = - EINVAL; break; } if ((unsigned int)offset > GLOBALMEM_SIZE) { ret = - EINVAL; break; } filp->f_pos = (unsigned int)offset; ret = filp->f_pos; break; case 1: /*相对文件当前位置偏移*/ if ((filp->f_pos + offset) > GLOBALMEM_SIZE) { ret = - EINVAL; break; } if ((filp->f_pos + offset) < 0) { ret = - EINVAL; break; } filp->f_pos += offset; ret = filp->f_pos; break; default: ret = - EINVAL; break; } return ret;}/*文件操作结构体*/static const struct file_operations globalmem_fops ={ .owner = THIS_MODULE, .llseek = globalmem_llseek, .read = globalmem_read, .write = globalmem_write, .ioctl = globalmem_ioctl, .open = globalmem_open, .release = globalmem_release,};/*初始化并注册cdev*/static void globalmem_setup_cdev(struct globalmem_dev *dev, int index){ int err, devno = MKDEV(globalmem_major, index); cdev_init(&dev->cdev, &globalmem_fops); dev->cdev.owner = THIS_MODULE; dev->cdev.ops = &globalmem_fops; err = cdev_add(&dev->cdev, devno, 1); if (err) printk(KERN_NOTICE "Error %d adding LED%d", err, index);}/*设备驱动模块加载函数*/int globalmem_init(void){ int result; dev_t devno = MKDEV(globalmem_major, 0); /* 申请设备号*/ if (globalmem_major) result = register_chrdev_region(devno, 2, "globalmem"); else /* 动态申请设备号 */ { result = alloc_chrdev_region(&devno, 0, 2, "globalmem"); globalmem_major = MAJOR(devno); } if (result < 0) return result; /* 动态申请2个设备结构体的内存*/ globalmem_devp = kmalloc(2*sizeof(struct globalmem_dev), GFP_KERNEL); if (!globalmem_devp) /*申请失败*/ { result = - ENOMEM; goto fail_malloc; } memset(globalmem_devp, 0, 2*sizeof(struct globalmem_dev)); globalmem_setup_cdev(&globalmem_devp[0], 0); globalmem_setup_cdev(&globalmem_devp[1], 1); return 0; fail_malloc: unregister_chrdev_region(devno, 1); return result;}/*模块卸载函数*/void globalmem_exit(void){ cdev_del(&(globalmem_devp[0].cdev)); cdev_del(&(globalmem_devp[1].cdev)); /*注销cdev*/ kfree(globalmem_devp); /*释放设备结构体内存*/ unregister_chrdev_region(MKDEV(globalmem_major, 0), 2); /*释放设备号*/}MODULE_AUTHOR("Song Baohua");MODULE_LICENSE("Dual BSD/GPL");module_param(globalmem_major, int, S_IRUGO);module_init(globalmem_init);module_exit(globalmem_exit);
container_of()的作用是通过结构体成员的指针找到对应结构体的指针，这个技巧在 Linux 内核编程中十分常用。在container_of（inode->i_cdev,structglobalmem_dev,cdev）语句中，传给container_of()的第1个参数是结构体成员的指针，第2 个参数为整个结构体的类型，第3 个参数为传入的第1 个参数即结构体成员的类型，container_of()返回值为整个结构体的指针。
//gmen_two_test.c#include #include #include #include #include #define MEM_CLEAR 0x1 /*清0全局内存*/ int main(int argc, char **argv){ int fd0 = 0; int fd1 = 0; int ret = 0; int length = 0; char buffer[1024]; fd0 = open("/dev/globalmem_two0",O_RDWR); //以读写的方式打开 if(fd0<0) { printf("Can not open /dev/leds\n"); close(fd0); return 0; } //写入字符 memset(buffer, 0, 1024); strcpy(buffer,"test globalmem\n"); length = strlen(buffer); printf("0 写入的字符length = %d, %s", length, buffer); ret = lseek(fd0, 0, SEEK_SET); //定位为相对文件开头0处 ret = write(fd0, buffer, length); //读取字符 memset(buffer, 0, 1024); ret = lseek(fd0, 0, SEEK_SET); //定位为相对文件开头0处 ret = read(fd0, buffer, length); if(ret>0) { printf("0 清除内存前读出的字符length = %d, %s", ret, buffer); } //清除字符 memset(buffer, 0, 1024); ret = lseek(fd0, 0, SEEK_SET); //定位为相对文件开头0处 ret = ioctl(fd0, MEM_CLEAR, 0); ret = read(fd0, buffer, length); if(ret>0) { printf("0 清除内存后读出的字符length = %d, %s", ret, buffer); } close(fd0); // fd1 = open("/dev/globalmem_two1",O_RDWR); //以读写的方式打开 if(fd1<0) { printf("Can not open /dev/leds\n"); close(fd1); return 0; } //写入字符 memset(buffer, 0, 1024); strcpy(buffer,"test globalmem\n"); length = strlen(buffer); printf("\n\n1 写入的字符length = %d, %s", length, buffer); ret = lseek(fd1, 0, SEEK_SET); //定位为相对文件开头0处 ret = write(fd1, buffer, length); //读取字符 memset(buffer, 0, 1024); ret = lseek(fd1, 0, SEEK_SET); //定位为相对文件开头0处 ret = read(fd1, buffer, length); if(ret>0) { printf("1 清除内存前读出的字符length = %d, %s", ret, buffer); } //清除字符 memset(buffer, 0, 1024); ret = lseek(fd1, 0, SEEK_SET); //定位为相对文件开头0处 ret = ioctl(fd1, MEM_CLEAR, 0); ret = read(fd1, buffer, length); if(ret>0) { printf("1 清除内存后读出的字符length = %d, %s", ret, buffer); } close(fd1); return 0;}
[root@localhostglobalmem_two]# make
[root@localhostglobalmem_two]# insmod globalmem_two.ko
[root@localhostglobalmem_two]# mknod /dev/globalmem_two0 c 247 0
[root@localhostglobalmem_two]# mknod /dev/globalmem_two1 c 247 1
[root@localhostglobalmem_two]# gcc -o gmen_two_test gmen_two_test.c
[root@localhostglobalmem_two]# ./gmen_two_test
0 写入的字符length = 15, test globalmem
0 清除内存前读出的字符length = 15, test globalmem
0 清除内存后读出的字符length = 15,
1 写入的字符length = 15, test globalmem
1 清除内存前读出的字符length = 15, test globalmem
1 清除内存后读出的字符length = 15, [root@localhost globalmem_two]#
[1] 宋宝华.Linux设备驱动开发详解
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