GEOS SDK TechDocs
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Libraries
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2 Library Basics
GEOS libraries are designed to make code-sharing simple and efficient. They allow several different applications to make use of the same routines and classes while using the minimum amount of space.
Conventional libraries are fully included in each application. The usual technique is to write a header file which contains the code for the library's routines. Any application which needs to use the routines can then include this library. This has one main advantage: The code can be written and tested once, and applications can then rely on it to work. However, there is a severe drawback to this approach: every application which uses the library will contain identical code. This is not a problem for non-multitasking environments; if only a single application can run at a time, then there will be only a single copy of each library. In a multithreaded environment like GEOS, however, this is a very inefficient use of resources.
GEOS libraries solve this problem. Each geode specifies which libraries it will use, either at compile-time (in the
.gp
file) or at run-time (with
GeodeUseLibrary()
). The kernel will see to it that the library is loaded when necessary. This means that if a dozen applications are all using the same library, the code needs only be loaded once.
Conventional libraries contain only routines. GEOS libraries, on the other hand, may contain both routines and object classes. There are several advantages to defining a class in a library instead of in an application. First, there is the same code-sharing benefit that routines have. If a class is defined in a library, the heap will contain at most one copy of each of the class's methods, no matter how many applications use objects from that class. There is another advantage as well; all applications which use that class can be sure that they are using objects whose definitions are identical. This makes it possible for applications to send messages to objects owned by other geodes.
Writing a library is very much like writing an application. There are only a few differences, which are covered in this appendix. You should already be familiar with writing applications before you try to write a library.
GEOS SDK TechDocs
|
| Libraries
| 2 Library Basics