MorphOS API and Its Organization

Grzegorz Kraszewski

An Application Programmer Interface of an operating system consists of thousands of functions usually. MorphOS is no exception here. Its kernel is not monolithic however. The API is functionally (and physically) divided into libraries. Only a few largest libraries contain more than 50 functions. A core set of most important libraries is contained in the system boot image. The rest is placed on the system partition in MOSSYS:Libs (libraries delivered with the system) and SYS:Libs (third party libraries) directories. Disk based libraries are loaded on demand. All these libraries are shared, it means all processes using a library execute the same code loaded to memory once.

Libraries Overview

MorphOS comes with over 100 different libraries. Not all of them are listed below, just the most common ones. Browse the system autodocs in the SDK for more.

• exec.library, the master one. This is the system core, responsible for processes scheduling, control and creation, communication between processes, memory management, managing other libraries and overall system control. This is the only library which is always opened and cannot be closed.

• dos.library, responsible for file and console input/output. Provides interface to advanced filesystem functions (like scanning directories for example). Cooperates with exec.library in process creation. Delivers basic system time services.

• graphics.library, is responsible for low-level graphics functions like drawing pixels and other primitives, copying rectangular blocks of display, scrolling etc. Many programs do not use it directly.

• intuition.library, delivers an intermediate level graphics interface objects like screens and windows. Interfaces to user input devices (mouse and keyboard to name a few). Provides very basic user controls (gadgets). Provides also BOOPSI (Basic Object Oriented System for Intuition), a language independent object oriented programming framework, used commonly by other components.

• muimaster.library, the main interface to MUI (Magic User Interface), which is the MorphOS high level GUI toolkit. Provides complete, object oriented framework (based on BOOPSI) for GUI driven applications and rich set of GUI objects.

• locale.library, is responsible for system and applications internationalization. This simple yet powerful subsystem allows for supporting multiple language versions of program with single executable, also provides localization data such as date format, local currency, timezone, number grouping and more.

• bsdsocket.library is an interface for TCP/IP networking, compatible with BSD sockets. What is unusal with this library, it is neither built into kernel, nor placed on disk. The TCP/IP stack creates it in memory dynamically.

How to Use a Library in an Application

In typical cases it is pretty automatic. The only thing which has to be done is including the main library header file, which is <proto/[libname].h>, for example <proto/exec.h>, <proto/muimaster.h> and so on. Library opening and closing is handled automatically by the startup code provided by either libnix or ixemul.library. Then one can just use functions from the library.

A few big libraries have separate subdirectories in the system include tree. Examples of such libraries are exec.library, dos.library or graphics.library. Header files in these directories contain definitions of constants, data structures, attributes etc. used by the library, divided by functionality. Including of these headers depends on functions used in the application, for example to use exec.library memory allocation functions one has to include <exec/memory.h> header.

Some libraries prefer to have one single header file in libraries directory. Examples are <libraries/locale.h> or <libraries/mui.h> (the latest is some deviation from the naming rule). This single header may be automatically included from proto file or not.

There are a few cases, where automatic library handling does not work, or cannot be used.

• Third party libraries. Most of them are not included in the autoopen feature.
• Custom startup code (linking with −nostartfiles).
• Opening libraries in a subprocess.
• Dynamic on-demand library opening.
• The library base has been defined in the application code. Autoopening for this library is automatically disabled then.

In all these cases libraries have to be handled manually.

Manual Library Opening and Closing

While not as comfortable as automatic handling, manual opening and closing of libraries is not very complicated. A library base variable has to be defined, then two functions from exec.library: OpenLibrary() and CloseLibrary() have to be used.

The library base is defined in its proto file (the main header) as a global variable. It is a pointer to Library structure, which should be treated as an opaque pointer. A result returned by OpenLibrary() should be placed in the base before any function of the library is called. Then, when the library is no longer needed, it should be closed with CloseLibrary() using the base as argument. Then the layout of usig hypothetical foobar.library looks as follows:

/* inside <proto/foobar.h> */

struct Library *FoobarBase;

/* inside application */

#include <proto/foobar.h>

if (FoobarBase = OpenLibrary((STRPTR)"foobar.library", 7))

{

  /* use library functions here */

  CloseLibrary(FoobarBase);

}

The OpenLibrary() call takes two parameters. The first one is just the name of the library to be opened. It is only the name, without path. MorphOS searches a few locations for the library in the following order:

• MOSSYS:Libs/
• LIBS:
• current directory of application
• PROGDIR:Libs/

In the fourth path PROGDIR: is an automatic assign pointing to the directory containing application executable.

The second parameter of OpenLibrary() is the minimum required version. Zero here opens any version, any positive number means "this version or higher". There is no straightforward way for requesting particular version of a library. It can be noticed that this approach works if newer versions are always backward compatible with older ones. On the other hand it avoids having multiple versions of the same library in the system, which is a common problem with Linux shared objects.

The value returned by OpenLibrary() should be always checked against NULL. Even a library built into the boot image may fail to open (because of memory shortage for example, or having too low version). Printing some error message in case of fail is definitely a good idea.

Every succesfull OpenLibrary() call must be matched with CloseLibrary(). Resource leak is created otherwise.

There are two special cases for manual library opening and closing: exec.library and dos.library. The first one is always opened and cannot be closed, as stated above. The library base for it (named SysBase) is defined and initialized in the startup code. If declared manually in application, it should be declared as extern. The dos.library is opened and closed as any other library, but because the startup code needs it, the DOSBase is defined and initialized there. As a result the application does not need to open dos.library before using it. If declared in application, DOSBase should be extern too.

As the Amiga historical heritage, some most important library bases (SysBase, DOSBase, IntuitionBase, GfxBase and a few more) are not defined as struct Library* but as pointers to library specific structures. Direct poking of these structures was unavoidable in early AmigaOS versions. In MorphOS it is neither needed nor recommended. One can avoid the above definitions (which forces unnecessary typecasting in OpenLibrary() and CloseLibrary()) by #defining __NOLIBBASE__ symbol before including proto files. This disables library bases definitions. All bases can (and must) be then explicitly defined in the code as pointers to struct Library.

Also for some traditional reasons, names of some library bases do not follow [Libname]Base scheme. Most important deviations are: SysBase for exec.library, DOSBase for dos.library (capitalization), GfxBase for graphics.library, MUIMasterBase for muimaster.library (capitalization), CyberGfxBase for cybergraphics.library. In any case base name can be checked looking at library proto header.

Using the proper base name is very important, as it is used as an implicit argument in all the calls of library functions.