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Chapter 3. Kernel Facilities and Helper Functions

The kernel is a standalone piece of software, as you'll see in this chapter, that does not make use

of any C library. It implements any mechanism you may encounter in modern libraries, and even

more, such as compression, string functions, and so on. We will walk step by step through the

most important aspects of such capabilities.

In this chapter, we will cover the following topic:

Introducing the kernel container data structure

Dealing with the kernel sleeping mechanism

Using timers

Delving into the kernel locking mechanism (mutex, spnlock)

Deferring work using a kernel dedicated API

Using IRQs

[12]

Understanding container_of macro

When it comes to managing several data structures in the code, you'll almost always need to

embed one structure into another and retrieve them at any moment without being asked questions

about memory offset or boundaries. Let's say you have a struct person, as defined here:

struct person {

int age;

char *name;

} p;

By only having a pointer on age or name, one can retrieve the whole structure wrapping

(containing) that pointer. As the name says, container_of macro is used to find the container of

the given field of a structure. The macro is defined in include/linux/kernel.h and looks like:

#define container_of(ptr, type, member) ({ \

const typeof(((type *)0)->member) * __mptr = (ptr); \

(type *)((char *)__mptr - offsetof(type, member)); })

Don't be afraid by the pointers; just see it as:

container_of(pointer, container_type, container_field);

Here are the elements of the preceding code fragment:

pointer: This is the pointer to the field in the structure

container_type: This is the type of structure wrapping (containing) the pointer

container_field: This is the name of the field to which pointer points inside the structure

Let us consider the following container:

struct person {

int age;

char *name;

};

Now let us consider one of its instance, along with a pointer to the name member:

struct person somebody;

[...]

char *the_name_ptr = somebody.name;

Along with a pointer to the name member (the_name_ptr), you can use the container_of macro in

order to get a pointer to the whole structure (container) that wraps this member by using the

following:

struct person *the_person;

the_person = container_of(the_name_ptr, struct person, name);

container_of takes the offset of name at the beginning of the struct into account to get the correct

pointer location. If you subtract the offset of the field name from the pointer the_name_ptr, you will

get the correct location. It is what the macro's last line does:

(type *)( (char *)__mptr - offsetof(type,member) );

[13]

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