An Application Programmer Interface of an operating system usually consists of thousands of functions. MorphOS is no exception here. Its kernel is not monolithic however. The API is functionally (and physically) divided into libraries. Only a few of the largest libraries contain more than 50 functions. A core set of the most important libraries is contained in the system boot image. The rest are 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, which means all processes using a library execute the same code loaded to memory once.
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.
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 and 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 which functions are used in the application, for example to use exec.library memory allocation functions one has to include the <exec/memory.h> header.
Other libraries have a single header file in the 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 the proto file or not.
There are a few cases, where automatic library handling does not work, or cannot be used.
In all of these cases, libraries have to be handled manually.
While not as convenient 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 a Library structure, which should be treated as an opaque pointer. The result returned by OpenLibrary() should be placed in the library base before any function of the library is called. Then, when the library is no longer needed, it should be closed with CloseLibrary() using its base as the argument. The layout for using the hypothetical foobar.library is as follows:
The OpenLibrary() call takes two arguments. 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:
In the fourth path PROGDIR: is an automatic assign pointing to the directory containing the 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 a particular version of a library. It can be noticed that this approach only works if newer versions of a library 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 a memory shortage for example, or having too low a version). Printing some error message in case of a fail is definitely a good idea. Every successful OpenLibrary() call must be matched with CloseLibrary(). Resource leak is created otherwise. It also makes it impossible to flush unused library from memory.
There are two special cases for manual library opening and closing: exec.library and dos.library. The first one is always open 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 an application's source code, it should be declared as an extern. The dos.library is opened and closed as with any other library, but because the startup code needs it, the DOSBase is already defined and initialized there. As a result the application does not need to open dos.library before using it. If declared in an application, DOSBase should be an extern too.