device_create()-
>device_register()
->device_add()
函数如下所示:
int class_device_add(struct class_device *class_dev)
{
 ... ... kobject_uevent(&class_dev->kobj, KOBJ_ADD);
 // KOBJ_ADD是一个枚举值 
//调用了kobject_uevent_env(kobj, action, NULL); 
// action=KOBJ_ADD
}

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,char *envp_ext[])
{
  char **envp;
  char *buffer;
  char *scratch;
  int i = 0;
  ... ...
  /* 通过KOBJ_ADD获取字符串"add",所以action_string="add" */
  action_string = action_to_string(action);    // action=KOBJ_ADD
              
  /* environment index */
  envp = kzalloc(NUM_ENVP * sizeof (char *), GFP_KERNEL);  //分配一个环境变量索引值
  /* environment values */
　　　　buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);  //分配一个环境变量缓冲值  
/* event environemnt for helper process only */
/*设置环境变量*/
  envp[i++] = "HOME=/";
  envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
  scratch = buffer;
  envp [i++] = scratch;
  scratch += sprintf(scratch, "ACTION=%s", action_string) + 1; //"ACTION= add"
  envp [i++] = scratch;
  scratch += sprintf (scratch, "DEVPATH=%s", devpath) + 1;
  envp [i++] = scratch;
  scratch += sprintf(scratch, "SUBSYSTEM=%s", subsystem) + 1;
  ... ...
  /*调用应用程序,比如mdev*/
  if (uevent_helper[0]) {
   char *argv [3];
    argv [0] = uevent_helper;  // uevent_helper[]= "/sbin/hotplug";
    argv [1] = (char *)subsystem;
    argv [2] = NULL;
    call_usermodehelper (argv[0], argv, envp, 0);  //调用应用程序,根据传入的环境变量参数来创建设备节点
  }
}

class_device: argv[0]=/sbin/mdev  　　　　　 //调用mdev
class_device: argv[1]=sixth_dev      //类名
class_device: envp[0]=HOME=/
class_device: envp[1]=PATH=/sbin:/bin:/usr/sbin:/usr/bin
class_device: envp[2]=ACTION=add  　　　　　　　　//add:表示添加设备节点,　　若=remove:表示卸载设备节点
class_device: envp[3]=DEVPATH=/class/sixth_dev/buttons 　　//设备的路径
class_device: envp[4]=SUBSYSTEM=sixth_dev   　　　　//类名
class_device: envp[5]=SEQNUM=745
class_device: envp[6]=MAJOR=252       //主设备号
class_device: envp[7]=MINOR=0

int mdev_main(int argc, char **argv)
{
... ...
action = getenv("ACTION");  　　 //获取传进来的执行参数,它等于“add”,则表示创建设备节点
env_path = getenv("DEVPATH"); 　　　　//获取设备的路径“/class/sixth_dev/buttons”
sprintf(temp, "/sys%s", env_path); //指定temp (真正设备路径)为“/sys/class/sixth_dev/buttons”
if (!strcmp(action, "remove"))   //卸载设备节点
     make_device(temp, 1);
else if (!strcmp(action, "add")) {  //创建设备节点
      make_device(temp, 0);
 ... ... 
}

static void make_device(char *path, int delete) //delete=0:创建, delete=1:卸载
{
  /*判断创建的设备节点是否是有效的设备*/
  if (!delete) {
    strcat(path, "/dev");
    len = open_read_close(path, temp + 1, 64);
    *temp++ = 0;
    if (len < 1) return;
  }
device_name = bb_basename(path); //通过设备路径,来获取要创建/卸载的设备节点名称
      //例: path =“/sys /class/sixth_dev/buttons”,那么device_name=“buttons”
type = path[5]=='c' ? S_IFCHR : S_IFBLK;  //判断如果是在/sys/class/目录下,那么就是字符设备
            //因为块设备,是存在/sys/block/目录下的
/* 如果配置了支持mdev.conf选项，那么就解析里边内容并执行 */
 if (ENABLE_FEATURE_MDEV_CONF) { 
  /* mmap the config file */
fd = open("/etc/mdev.conf", O_RDONLY);  //调用/etc/mdev.conf配置文件
  　　　　 ... ...   //开始操作 mdev.conf配置文件
}
  if (!delete) {     //如果是创建设备节点
    if (sscanf(temp, "%d:%d", &major, &minor) != 2) return; //获取主次设备号
　　　　　　　　/*调用mknod ()创建字符设备节点*/
if (mknod(device_name, mode | type, makedev(major, minor)) && errno != EEXIST)
      bb_perror_msg_and_die("mknod %s", device_name);
    if (major == root_major && minor == root_minor)
      symlink(device_name, "root");
    /*若配置了支持mdev.conf选项,则调用chown命令来改变属主,默认uid和gid=0 */
    if (ENABLE_FEATURE_MDEV_CONF) chown(device_name, uid, gid);
}
  if (delete) unlink(device_name);   //如果是卸载设备节点
}

<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]