Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Global Variables in a Library
Global variables declared in a shared library cannot be imported by a Delphi application.
A library can be used by several applications at once, but each application has a copy of the library in its own process
space with its own set of global variables. For multiple libraries - or multiple instances of a library - to share memory,
they must use memory-mapped files. Refer to the your system documentation for further information.
Libraries and System Variables
Several variables declared in the 
System
 unit are of special interest to those programming libraries. Use IsLibrary
to determine whether code is executing in an application or in a library; IsLibrary is always False in an application
and True in a library. During a library's lifetime, 
HInstance
 contains its instance handle. 
CmdLine
 is always nil in
a library.
The DLLProc variable allows a library to monitor calls that the operating system makes to the library entry point. This
feature is normally used only by libraries that support multithreading. DLLProc is available on both Windows and
Linux but its use differs on each. On Win32, 
DLLProc
 is used in multithreading applications.; on Linux, it is used to
determine when your library is being unloaded. You should use finalization sections, rather than exit procedures, for
all exit behavior.
To monitor operating-system calls, create a callback procedure that takes a single integer parameterfor example,
procedure DLLHandler(Reason: Integer);
and assign the address of the procedure to the 
DLLProc
 variable. When the procedure is called, it passes to it one
of the following values.
DLL_PROCESS_DETACH Indicates that the library is detaching from the address space of the calling process as a result
of a clean exit or a call to 
FreeLibrary
.
DLL_PROCESS_ATTACH Indicates that the library is attaching to the address space of the calling process as the result of
a call to 
LoadLibrary
.
DLL_THREAD_ATTACH
Indicates that the current process is creating a new thread.
DLL_THREAD_DETACH
Indicates that a thread is exiting cleanly.
In the body of the procedure, you can specify actions to take depending on which parameter is passed to the
procedure.