DLL (Dynamic Link Library) ?

Andrew

What is DLL and Why do we need to use .dll files?

8 Replies

Know the answer? Post it — somebody with the same question will find it here.

Sam Hobbs15y ago
Well guys, I am not sure how the reply from Ananth prasath got here but I suspect it came from some other universe; it seems entirely out of place for our universe.
Vilas Gite15y ago
Hi Sam Sir,
Thanking you for crucial guidance.
-------------------------------------
Ananth prasath15y ago
Hi

Steps to perform:
- Create in Microsoft Visual C++ 6.0 an MFC application (.dll or .exe);
- Go to the menu View, ClassWizard.
- Select an option Add Class, from a type library.
- Browse to the location of your *.dll.
- A window, which displays the content of your *.dll, will appear. Select all classes you want to include in your project (Ctrl+A select all content, or Ctrl + mouse click for selecting a specific set of classes).
- Push the button Open. Header file *.h and *.cpp implementation file will be generated.
- Close the ClassWizard.
Now, all services you selected for use in you project are available!

With Best Regards
Natarajan R (UI Designer)
KAPSYSTEM (www.kapsystem.com)
Sam Hobbs15y ago
Yes, Vilas I know that the other site(s) say that a DLL is a collection of programs. I considered whether I should say that that is what the other site(s) say. At least now we have it clarified. I know you said it because that is what the other site(s) say.

Based on that one page in the searchwinit.techtarget.com web site, my impression is that it is not the best site to rely on. That one article also says that a DLL is not loaded into memory unless the main program actually uses it, but I think that is not something different between a Dll and an exe file. Portions of exe files are also not loaded unless they are actually used so the article needs to explain more and include the part about exe files not being loaded fully or it should just not say that about DLL files.

So I will get a little advanced here. The following will probably not be understood by anyone unfamiliar with Windows and if you don't understand then you can ignore this if you want.

An exe file gets mapped into its process's virtual memory address space in the same manner that a file can be mapped into a process's address space. The virtual memory pages are not brought into real memory unless the memory is accessed, which results in a page fault. The page fault results in a page worth of data being read into real memory. In this manner, large resources such as images can be included in a exe or DLL file but never read into memory unless they are actually used. I am not sure of the details of how it is done and I am sure it is done more efficiently than I make it sound.
Vilas Gite15y ago
Hi Sam Sir,

I had read information about DLL from site : http://searchwinit.techtarget.com/definition/dynamic-link-library that DLL is collection of small programs and its used in application when its required.

Also I read about from site : how to dll files used in program : http://msdn.microsoft.com/en-us/library/3707x96z(v=vs.80).aspx (in example : Add.cs and Multi.cs are used in Testcode.cs)

Sam Sir, I am not misleading here….i am here for learn more by experts and learn by problem (fourms)

----------------------------------------------------------------------------------------------------

If this reply hepls your post…then "MARK AS ANSWER"!
Sam Hobbs15y ago
I think that saying that a DLL is a collection of small programs is misleading. I think it is more clear to call a DLL a collection of functions.

Also note that there are very many types of DLLs. It is often confusing to use the term DLL since that does not make it clear what type of DLL it is. For C# a "Class Library" project creates a DLL and if you have a DLL that is a class library then it is much better to call it a class library instead of DLL. Also, most COM objects exist in DLLs (I think they can also exist in EXE files) and it is better to call them COM objects instead of DLL. Note that most of the Windows APIs are DLLs.
Amit Choudhary15y ago
Well, There are two type of file which are used to run a program/Application. one is self executed i.e. your .exe files and another is using some running application to load itself in a process i.e. your DLL's.
The main diff is DLL does not run by itself it needs a running process to load into memory for execution.
A DLL does not have any Main() function.
A DLL is basically a library that we keep separate from the main running application just to load then when required.
A DLL can also contains your resources like images, icon etc.

These are basic ideas for DLL.

Thanks.
Vilas Gite15y ago
Hi Andrew!
DLL stands for : Dynamic Link Library. DLL is a collection of small programs and which is used in Big Application.

for more details in brief with coding please refer

Create and Use of dll : http://msdn.microsoft.com/en-us/library/3707x96z(v=vs.80).aspx

http://searchwinit.techtarget.com/definition/dynamic-link-library

http://support.microsoft.com/kb/815065

----------------------------------------------------------------------------------------------------

If this reply hepls your post…then "MARK AS ANSWER"!

Yes, Vilas I know that the other site(s) say that a DLL is a collection of programs. I considered whether I should say that that is what the other site(s) say. At least now we have it clarified. I know you said it because that is what the other site(s) say.

Based on that one page in the searchwinit.techtarget.com web site, my impression is that it is not the best site to rely on. That one article also says that a DLL is not loaded into memory unless the main program actually uses it, but I think that is not something different between a Dll and an exe file. Portions of exe files are also not loaded unless they are actually used so the article needs to explain more and include the part about exe files not being loaded fully or it should just not say that about DLL files.

So I will get a little advanced here. The following will probably not be understood by anyone unfamiliar with Windows and if you don't understand then you can ignore this if you want.

An exe file gets mapped into its process's virtual memory address space in the same manner that a file can be mapped into a process's address space. The virtual memory pages are not brought into real memory unless the memory is accessed, which results in a page fault. The page fault results in a page worth of data being read into real memory. In this manner, large resources such as images can be included in a exe or DLL file but never read into memory unless they are actually used. I am not sure of the details of how it is done and I am sure it is done more efficiently than I make it sound.