Runtime exceptions

Revision as of 05:23, 21 September 2020

When a error is detected while executing a program, like a division by zero, access to a invalid memory location, etc, most languages provide a way define code to try to handle them.
Default action is usually to issue a message and kill the program.
.Net (the enviroment OpenSimulator runs on) creates a block of data (object) called Exception, with information about what happened and where.
The runtime control code aborts current execution and looks if the code path been executed is inside a try block. If there are catch blocks after if, the runtime calls them with that Exception as argument, so their code can try to handle it, preventing the program to just die.
There can be derivatives types of Exception, to make more clean the problem, and even include more specific information.
A catch can be made only sensitive to that type by just declaring it using that type name as argument:

catch(exceptionTypeName ex)

Using this mechanism, programs can also create their own types and issue (throw) them when there is a need to abort execution, and go back to a relevant catch{} block. This is for example how llResetScript stops the script execution and goes back to control, that then does the reset.

On YEngine lower case in used for exception names, and only knows 2 types, for now.

• exception ( ie .net Exception) means any kind of Exception a script can be allowed to intercept.
• scriptexception (.net ScriptException) a exception issued by the scripts supporting code.

most relevant cases for scripts will be type scriptexception.
note that scriptexception and any other exception types are also exception (they derive from it), so a catch(exception ex) will be triggered by it.

This limited set is because the scripts support code was made for XEngine, and needs to support it. For example a division by zero could just be the .net DivideByZeroException, instead it is converted into scriptexception. so we need to look to the exception message to identify what they really are. Note that this limited set also makes life easier for YEngine scripts error handling, so not really a XEngine limitation.