About DLL's


2.2.11. Dynamic-Link Libraries (DLL's)	Table of Contents	2.2.11.2. Using DLL's

- 2.2.11.1. - Table of Contents

2. TMT Pascal Language Description

2.2. Pascal Language Structure

2.2.11. Dynamic-Link Libraries (DLL's)

2.2.11.1. About DLL's

Targets: OS/2, Win32

In Microsoft^® Windows^® and IBM^© OS/2^© operating systems, dynamic-link libraries (DLL) are modules that contain functions and data. A DLL is loaded at runtime by its calling modules (.EXE or DLL). When a DLL is loaded, it is mapped into the address space of the calling process.

Dynamic linking has the following advantages over static linking:

Processes that load a DLL at the same base address can use a single DLL simultaneously, sharing a single copy of the DLL code in physical memory. Doing this saves memory and reduces swapping.
When the functions in a DLL change, the applications that use them do not need to be recompiled or relinked as long as the function arguments, calling conventions, and return values do not change. In contrast, statically linked object code requires that the application be relinked when the functions change.
A DLL can provide after-market support. For example, a display driver DLL can be modified to support a display that was not available when the application was initially shipped.
Programs written in different programming languages can call the same DLL function as long as the programs follow the same calling convention that the function uses. The calling convention (such as C, Pascal, or standard call) controls the order in whichthe calling function must push the arguments onto the stack, whether the function or the calling function is responsible for cleaning up the stack, and whether any arguments are passed in registers. For more information, see the documentation included with your compiler.

A potential disadvantage to using DLLs is that the application is not self-contained; it depends on the existence of a separate DLL module. The system terminates processes using load-time dynamic linking if they require a DLL that is not found at processstartup and gives an error message to the user. The system does not terminate a process using run-time dynamic linking in this situation, but functions exported by the DLL are not available to the program.

DLLs can define two kinds of functions: exported and internal. The exported functions can be called by other modules. Internal functions can only be called from within the DLL where they are defined. Although DLLs can export data, its data is usually only used by its functions.

DLLs provide a way to modularize applications so that functionality can be updated and reused more easilly. They also help reduce memory overhead when several applications use the same functionality at the same time, because although each application gets its own copy of the data, they can share the code.

TMT Pascal Multi-target support DLLs for Win32 and OS/2 compilation targets. DLLs are not supported for MS-DOS protected mode target.


2.2.11. Dynamic-Link Libraries (DLL's)	Table of Contents	2.2.11.2. Using DLL's

- 2.2.11.1. -