DLL – is an operating module, which contains a program code and resources, which are used by other applied programs or DLLs. Conceptually the dynamically configured library is analogical to the module – it provides procedures and functioning for the programs. Nevertheless, between dlls and modules there are some significant differences. Particularly, modules are компонуются statically, and DLL - dynamically. When a program uses the procedure or a function from a module, a copy of this procedure or function is statically configured with the executed file of a program. If two programs are executed at the same time, and are using one and the same procedure or function of a module, then there will be two copies of this subprogram present in the system. It would be more effective to use one copy. Such possibility is provided by the Dll. Unlike modules, dll is not configured using the DLL program. Instead of it, code and resources of DLL are located in a separate executed file with file type *.Dll. This file has to be present when executing program-client. Started by the program, procedures and functions are dynamically configured with their points of entrance in the applied DLL program. Other difference between modules from DLL is that modules can export types, constants, data and objects, and DLL – only procedures and functions. In order to use it in program Borland Pascal, dll does not necessarily need to be written on Borland Pascal. Besides, programs, written on other languages, can use DLL, which is written on Borland Pascal. DLL, thus, is perfectly suitable for program projects, which are realized on multiple languages.

Now already DLL – is not just a dynamically loaded library. You, probably, have already heard a number of times about ActiveX components. They can also be executed as ocx or dll files. And this is very clear, ActiveX now uses a lot of space and weights several times more, that the biggest version of DLL library. So the only normal way out to save space on the Winchester and memory is to put ActiveX into a dynamically loaded library. And although it is no longer the same Dll, it will still work using the same principles.

Dynamic libraries have one single disadvantage – more time is spent on loading. But if the library is already loaded by another program, it will appear much faster. This is because, after exiting a program a DLL-file is not unloaded of the memory. This happens only when windows don’t have enough memory and none of the programs uses this library at the moment. When using dynamic libraries there is only the main data in the launch file, and additional options are loaded gradually from DLL-files according to necessity. Thus, total speed of loading is decreased, and very significantly. Now we will review what DLL (Dynamic Link Library) are needed for and how to create them. DLL – is an area of code, which is saved in a file with file type .dll. A code can be used by other programs, but the library itself is not a program. Generally speaking, dynamically configured libraries represent a set of compiled functions. But such libraries have their peculiarities, as, for example, if some two or more programs for Windows are executed at the same time and are using functions, which are located in one DLL, then only one library will be located in the memory constantly, thus providing economic use of memory. Loading of a library into the memory can static and dynamic. During static loading DLL is automatically loaded during the launch of according program. Such DLLs contains export functions, description of which is located in the import library file (.lib). In order to use static loading you should at the stage of configuration attach to a program a .lib file of your DLL. In C++ Builder all you have to do is include a .lib file into a project, using project manager. During dynamic loading you can load DLL when necessary and unload, when it’s not necessary. But working with such libraries is more complicated than with static. Let’s review development and usage of DLL static loading.