What is sysfs?

• sysfs is a filesystem for exporting kernel objects.
• sysfs is a ram-based filesystem initially based on ramfs. It provides a means to export kernel data structures, their attributes, and the linkages between them to userspace.
• For every kobject that is registered with the system, a directory is created for it in sysfs. That directory is created as a subdirectory of the kobject's parent, expressing internal object hierarchies to userspace. Top-level directories in sysfs represent the common ancestors of object hierarchies; i.e. the subsystems the objects belong to. 
• Sysfs internally stores a pointer to the kobject that implements a directory in the sysfs_dirent object associated with the directory.

An attribute definition is simply:

struct attribute {
        char                    * name;
        struct module  *owner;
        umode_t                 mode;
};


int sysfs_create_file(struct kobject * kobj, const struct attribute * attr);
void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr);

A bare attribute contains no means to read or write the value of the

attribute. Subsystems are encouraged to define their own attribute
structure and wrapper functions for adding and removing attributes for
a specific object type. 

For example, the driver model defines struct device_attribute like:

struct device_attribute {
 struct attribute attr;
 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
   char *buf);
 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
    const char *buf, size_t count);
};

int device_create_file(struct device *, const struct device_attribute *);
void device_remove_file(struct device *, const struct device_attribute *);

It also defines this helper for defining device attributes: 

#define DEVICE_ATTR(_name, _mode, _show, _store) \
struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)

For example, declaring

static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo);

is equivalent to doing:

static struct device_attribute dev_attr_foo = {
 .attr = {
  .name = "foo",
  .mode = S_IWUSR | S_IRUGO,
 },
 .show = show_foo,
 .store = store_foo,
};