Linux Kernel Architecture

Mauerer app03.tex V1 - 09/04/2008 6:11pm Page 1209

Appendix C: Notes on C

printf("one\n"); break;

case 2:

printf("two\n"); break;

case 3:

printf("three\n");

default:

printf("default\n");

}

The code generates the following output:

wolfgang@meitner>./switch

three

default

Because thecasestatement for 3 doesnotinclude abreakstatement, code flow descends to thedefault

label, which under normal circumstances would not have been selected. Generally,switchstatements

canbeexitedonlybymeansofbreakstatements (or at the end of the statement itself). Once a suitable

statement is found, the code descends until it reaches a corresponding statement — regardless of whether

it comes across further labels or not.

C.2.7 Doubly Linked Lists

Doubly linked lists appear in practically every larger data structure of the kernel. A number of general

functions and structures are therefore provided to implement such lists for a wide range of purposes.

Chapter 1 discussed the API needed to work with lists. This section describes its implementation, which

involves some interesting aspects of generic programming in C.

The starting point for linked lists is the following data structure that can be embedded in other data

structures:

<list.h>

struct list_head {

struct list_head *next, *prev;

};

The meaning of the elements is clear.nextpoints to the next element, andprevpoints to the previous

element. The list is also organized cyclically — in other words, the predecessor of the first list element is

the last entry, and the successor of thelast list element is the first entry.

Implementation of list functions is made more difficult by the following conditions:

❑ The list elements need not be at the beginning ofa structure, but may be located anywhere in

the structure. Because list processing is supposed to function with any data types, this causes

problems if a selected element is to be typecast into the target data type.

❑ Several list elements can be used jointly in a structure so that they can be held in various lists.