GEOS SDK TechDocs
|
| 
| 
1 Parse Library Behavior
| 
3 Coding with the Parse Library
The Parse library provides many built-in functions. Furthermore, each application can define its own functions. Every function is associated with a function ID number. Built-in functions have ID numbers assigned to them in the library code; application-defined functions are given ID numbers by the application. ID numbers are word-sized unsigned integers. All built-in ("internal") functions have ID numbers which are less than the constant FUNCTION_ID_FIRST_EXTERNAL_FUNCTION_BASE; all application-defined ("external") functions have ID numbers which are greater than this constant.
When the Parser reads an identifier token whose position indicates that it is a function, it converts the identifier to a function token (containing a function ID). The parser first checks to see if the identifier is the name of a built-in function. If so, it looks up the function's ID number and stores it in the function token.
If the identifier is not the name of a built-in function, the Parser calls the application's callback routine to get the function's ID number. The application must assign each function a word-sized ID which is greater than or equal to the constant FUNCTION_ID_FIRST_EXTERNAL_FUNCTION_BASE. This constant is defined as 0x8000.
When the Evaluator needs to evaluate a function, it checks to see if the function is external or internal. If the function is internal, it looks up the functions address and calls it. If the function is external, it calls the application's callback routine and passes the function ID. In either case, it passes a pointer to the argument stack and the number of arguments. The function is responsible for popping all the arguments off the stack and pushing the result. It can also push an error message on the stack. All of this is discussed at length in the advanced section.
GEOS SDK TechDocs
|
|
| 1 Parse Library Behavior
| 3 Coding with the Parse Library