Dynamic Link Library (DLL)

Dynamic Link Library (DLL)

DLL do not get loaded in RAM along with main program all the time. Memory does not get occupied unnecessarily. But if a DLL file is called by some exe then only it is loaded. The DLL files have the extension .DLL, .OCX, .DRV. Any DLL file can be called by an executable (.EXE) file only when it is required.

Advantages of DLL:

                               I.            Reduce uses of memory.

                             II.            DLL can be supported by multiple languages.

                          III.            If you want to change into function then there is no need to change into client program.

How the Client Program Finds a DLL:

If user link explicitly with LoadLibrary, user can specify the DLL’s full pathname. If user doesn’t specify the pathname, or if user link implicitly, Windows follows this search sequence to locate his DLL:

                                i.            The directory containing the EXE file

                              ii.            The process’s current directory

                            iii.            The Windows system directory

                           iv.            The Windows directory

                             v.            The directories listed in the Path environment variable

Difference between implicit and explicit linkage:

    Implicit Linkage:

a.     In implicit linkage the LIB file is used in which the name of corresponding DLL name is stored.

b.     All the DLL file get loaded that are associated with particular LIB file.

    Explicit Linkage:

a.     In explicit linkage LoadLibrary() function is used in which the path of DLL file is explicitly mentioned.

b.     We are explicitly specifying the name of required DLL in the LoadLibrary() function.

MFC DLL s- extension Vs Regular:

1.     Extension DLL:

It supports a C++ interface so it can export whole classes and user can instantiate those classes and user can also inherit classes exported from extension DLL. These types of DLLs are dynamically linked to the code in the DLL version of the MFC library and the client program must be dynamically linked to MFC library. Extension DLLs are quite small in size therefore it loads quickly.

2.     Regular DLL:

It supports only C style function. C++ classes cannot be exported by regular DLLs but C++ classes can be used inside the regular DLLs. It is also supported by WIN32 programming environment. It can be linked statically or dynamically. It is not necessary for the client program to be dynamically linked to MFC library.