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Hi, recently ive been trying to find out the differences between the windows api (the dll files) and the windows.h header file, i found that the windows.h file contains the declarations and the dll files contain the definitions.
Could you explain to me why this is done in two different files, why not just have the definition and declaration in one file, wouldnt it be less hassle?
they could then Microsoft would have to reveal the source code , which they do not want to do think of it as driving a car , you interact with it , via steering , brakes , gears however common people have no idea what is going on inside , same with API , headers files allow you to use given set of libraries without knowing anything about them ,
It helps developers in what the want to do rather than how they will do it. Perfect example is DirectX or OpenGL , developers want to devepol games they are not interested in how it interacts with underlying video cards.
Thanks for the reply :)
Ive also been trying to find out what actually is the Windows API, is it the windows.h file or the dll files, or a compilation of both which form it?
API
Viggy
Already seen that :)
You have to distinguish between (Windows) API and (Windows) SDK.
By short:
- Windows API (application programming interface) is the set of functions exposed by Windows system libraries (kernel32.dll, user32.dll, etc). API allows other applications to interact with the Windows system.
- Windows SDK (software development kit) contains source header files, static libraries, tools, documentation. SDK helps developers to write applications which are using Windows API to interact with the Windows system.
Windows.h is a source header file shipped with Windows SDK.
Ok, so the dll files form the windows api, and the windows.h file is a part of the windows sdk.
Is the windows sdk automatically provided for us by the IDE when we choose to create a windows application?
It depends on the IDE.Quote:
Originally Posted by TpOreilly
Ok, im guessing mine does, I use MS Visual Studio.
Carrying on from this, couldnt they have both the declarations and the definitions in the dll files?Quote:
Originally Posted by aamir121a
If declarations is in dll files how do you know the function names ?.Quote:
declarations and the definitions in the dll files.
dll files are just like compiled source file, the form is .dll instead of .cpp
First you need to understand why dll files are being written and also find out the answer of function declaration and definition.
I would compare a DLL with an EXE (or LIB), not a CPP file. A DLL is compiled code, all ready to run.Quote:
Originally Posted by hypheni
Viggy
yes you are right Viggy. dll is compiled code ready to run by an exe.
by calling the function names from exe a dll can execute its task, may be missed some term to explain OP exactly.
Ok :) so to summarise and closeee this thread:
The windows.h file contains the declarations and the dll files contain the definitions of the functions within the windows API.
They seperate the declarations from the definitions because they dont want people to know how the functions actually work.
And the dll files are the windows api, and the windows.h file is a part of the windows sdk.
Correct me if im wrong.
a dll is like an exe, but it is not a stand alone application and is meant to be loaded inside a process. It runs inside a thread attached to your process. It is to provide implementations dynamically vs statically. Instead of being compiled together, they link together during runtime. a dll is a format that the os uses to provide dynamic linking. I suggest you read http://en.wikipedia.org/wiki/Dynamic...on_and_binding.
It is not to hide code. You don't need the c/c++ source, but only a compiled( assembly machine code ) version in a specific format that the os can use. This is why dlls are needed. It has to do with loading up dependencies at run-time or during exe loading.
thanks
The dll files contain binary implementation of the functions declared in h files and compiled from definitions contained in source code at MS.Quote:
Originally Posted by TpOreilly
They separate declaration from implementation because historically linking with Windows APIQuote:
They seperate the declarations from the definitions because they dont want people to know how the functions actually work.
And the dll files are the windows api, and the windows.h file is a part of the windows sdk.
Correct me if im wrong.
was designed as native C linkage which never had any mechanism for compiling type declarations into binary format. And still does not. :)
Please make sure you understand how declaration, definition and implementation differ from each other. Also please note, not only h files comprise SDK, but import libraries (lib) make another important part of it. :)
Yeah, I understand that a dll file is not source code, but actually source code already compiled into a language which the computer understands. And i think...
declaration = predefining a function, declaring it.
definition = the code inside a function which describes it behavior.
implementation = Is implementation when we use the function, when it is turned into code which the computer understands?
Okay, as long as we're clear with this, let's get back to your original question. What would be a reason for placing declaration to dll? And what bad in having it in h files?
Let's take COM model which really has type declarations embedded in dll server. To use interface function you need to extract the declaration and ultimately turn it to h file. Oops!
Let's take .NET model which also has embedded assembly metadata in executables. To use it you need a special runtime, CLR. Oops!
Every time you want type declaration supplied along with implementation you need a special infrastructure to deal with the one. While basic Platform SDK approach requires nothing special but the SDK installed only on developer's system, and no precious memory and disk space get spent for unnecessary type info on user's side. Well, as I said, the reason mainly is in Windows history and backward compatibility, amazing compatibility I would say. And having separate h files is a very, very moderate price for this benefit.
Ok. i appreciate your replies very much so :) So, from this forum ive learned that:
The windows.h file contains the declarations and the dll files contain the definitions (already compiled in computer language) of the functions within the windows API.
And the dll files are the windows api, and the windows.h file is a part of the windows sdk.
This is all i want to know at this stage. Am i right in saying this. Please answer with a simple yes or no, because at this stage more computer talk confuses me :D
It is yes about h files and declarations. And no, dll files do not contain definitions, as the term 'definition' (as well as declaration) is applicable only to source code and at compile time, but not an executable and runtime. It's really important to distinguish between these two and follow the terminology.
Ohhh I get you :)
They are actually the definitions, but when we use the word 'definition' we are talking about a definition in source code, when we refer to the definition in the dll file it is actually called an implementation because it have been compiled into a language the computer understands :D
Not quite.Quote:
Originally Posted by TpOreilly
The terms "definition" (and "declaration") have a very specific meaning in C/C++. They relate to how language constructs are physically organized in source code. To know the details you must learn the language.
Most Windows technologies, such as DLL and COM, are language independent. But if you're a C/C++ programmer and want to use any of these technologies you hopefully will be presented with some C/C++ source code to simplify your task. And this is where "definition" and "declaration" in the C/C++ sense enters the picture.