--------------------------------------------------------------------------------

Symbian OS SDK v8.1a >> Symbian OS guide >>  About Symbian OS development >> Subsystems and APIs

--------------------------------------------------------------------------------

Subsystems and APIs

--------------------------------------------------------------------------------

Overview
Symbian OS is a large system, containing hundreds of classes and thousands of member functions. Like most complex systems, it is easiest to approach by dividing it into large areas, and gradually narrowing your focus down to the areas that are most important to your tasks. The largest divisions that Symbian has found useful are called subsystems. For example, the Application Engines subsystem contains all the APIs that can be used to manipulate data for the standard applications such as Contacts.

Subsystems can be thought of as conceptual areas that group related functionality together: they aren't binary files such as DLLs, or source files such as headers. The system documentation in this Developer Library uses them consistently to divide up functionality.

The documentation for subsystems is in turn divided up into APIs: groups of classes identified by descriptive English names. These too don't necessarily have a one-to-one relationship with binary libraries or headers, though there is very often a close correspondence. This is just because the requirements for those who need to understand and use the system are sometimes a little different from those of the system implementers. But the detail of libraries and headers are fully given in the API Reference documentation.

--------------------------------------------------------------------------------

Note
If you have used versions of Symbian OS previous to v6.0, you will probably be familiar with module names such as E32 and BAFL, which derived from directory source code names. This edition of the documentation usually avoids this type of terminology in favour of more comprehensible English subsystem and API names. This change may initially make the system stranger than it is. In fact, there is a very large degree of carry over from previous releases, and, for example, the Descriptors API that is described under Buffers and Strings as part of the subsystem System, is much as it was when referred to as just part of the large User library (or EUSER). We do hope though that new users, and eventually you as well, will find the new divisions, as consistently used throughout Symbian OS documentation, simpler to use and remember.

The "System" subsystem, which existed in v6.0, was split at v6.1 into two subsystems, "Base" and "Graphics".

--------------------------------------------------------------------------------

The subsystems
We can now look at particulars: the following figure shows the subsystems, followed by a brief description of each:

Subsystems

(Note that the placing of the blocks on the diagram roughly reflects whether the subsystem has a higher or lower level, but is not intended to provide a map of the subsystem dependencies).

--------------------------------------------------------------------------------

Base
The Base group of APIs cannot be ignored by any developer, whatever the type of program that is being written. An important fact to realise for the developer coming to Symbian OS for the first time is that certain fundamentals of programming are handled through class libraries specifically designed and optimised for small-memory devices, rather than through the standard C function library, or the standard C++ library (or that part of it known as the STL). This means that the Base APIs contain the classes that handle strings, arrays, lists, error handling, and even the basic types used for integers and real numbers. As the name implies, it also contains the APIs for genuine system programmers, who need to manipulate in detail system fundamentals such as threads, processes, and memory.

A good place to start to understand the Base APIs is Essential Idioms, which concentrates on the object oriented nature of the operating system, and the surprises that it might hold for those coming from other platforms.

--------------------------------------------------------------------------------

Security
The Security subsystem contains the components that provide cryptographic algorithms, certificate management, and

[1] [2] [3] 下一页