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## 2. What have you got and what to do with it?

This chapter describes some rules you ought to obey, and how to use GEOSLib.

## 2.1 Usage

Apart from this file, which merely describes only standard GEOS library functions, you should read `grc` (GEOS resource compiler) documentation. There are informations about necessary resource files (each GEOS application needs at least one) and the building process - what should be done and in what order. Please also read cc65's documentation on how to compile C, assembler and link everything together.

All in all, you just need to place

> `

>     #include <geos.h>

> `

on top of your source.

As a general rule read the sources of example programs and read the headers. These are the most reliable sources of knowledge ;). You will also find there many C macros representing various arguments passed to functions. Please use them. You will find your sources easier to understand, and it will be easier to find bugs.

All types used in GEOSLib are `unsigned`.

Screen coordinates are given in pixels unless stated differently.

## 2.2 Notes on style

Contrary to typical GEOS assembly program which has a main function called after loading that setups the screen, menus, icons etc. exiting from `main` function in C is equivalent to calling `exit()`. These two are the only safe methods of terminating applications. DO NOT USE `EnterDeskTop`! Your data may be lost as library destructors and functions registered with `atexit` will not be called.

For GEOS GUI applications the recommended program structure is to have everything initialized in `main` function and at the end of it a call to `MainLoop()` function. WARNING! This function never returns, any code between `MainLoop();` and the end of `main` will not be executed. You have to call `exit()` explicitly somewhere in your code (e.g. in a menu handler or via DialogBox action).

Whenever possible use definitions from `gsym.h`. The resulting code is translated by cc65 into series of `lda` and `sta`, so you can't do it better :-).

Don't hesitate to use library functions. Everything was written with size and speed in mind. In fact many calls are just redirections to GEOS kernal which results in simple `jsr`.

The `main` function receives the standard `argc` and `argv` parameters. There are always either 1 or 3 parameters. DOS application name is always set as `argv[0]`. If present, `argv[1]` and `argv[2]` will be set to data filename and data diskname (it only works if user double-clicks on data file associated with your application). Note that it is up to your application to determine which of the available (up to four) disk drives has the disk with given diskname inside. If this fails your program should ask to insert the proper disk into one of available drives.

You might wonder why I have chosen sometimes weird order of arguments in functions. I just wanted to avoid unnecessary pushing and popping arguments from stack because cc65 can pass single `unsigned int` through CPU registers.

Do not try to compile in strict ANSI mode. Library uses cc65 extensions which are not available in ANSI.

It is possible to use dynamically loaded modules, three such modules are provided: GEOS TGI driver, GEOS EMD driver (for VDC extended memory) and GEOS JOY driver. Just make sure that their filenames appear UPPERCASE in DeskTop. There are no more special recommendations, read cc65 documentation about modules and demo programs source code.

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