Windows API

The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to a number of different platform implementations that are often referred to by their own names (for example, Win32 API); see the versions section. Almost all Windows programs interact with the Windows API; on the Windows NT line of operating systems, a small number (such as programs started early in the Windows startup process) use the Native API.[1]

Developer support is available in the form of the Windows Software Development Kit (SDK), providing documentation and tools necessary to build software based upon the Windows API and associated Windows interfaces.

The Windows API (Win32) is primarily focused on the C programming language[2] in that its exposed functions and data structures are described in that language in recent versions of its documentation. However, the API may be used by any programming language compiler or assembler capable of handling the (well defined) low level data structures along with the prescribed calling conventions for calls and callbacks. Similarly, the internal implementation of the API's functionality has been developed in several languages, historically.[3] Despite the fact that C lacks any notion of object-oriented programming, the Windows API as well as Windows itself has sometimes been described as object-oriented. There have also been many wrapper classes and extensions (from Microsoft or other sources) for object oriented languages that makes this object oriented structure more explicit (MFC, VCL, GDI+, etc.). For instance, Windows 8, while still providing the Windows API, also provides the WinRT API which is implemented in C++[4] and is object-oriented by its design.[4]

Overview

The functionality provided by the Windows API can be grouped into eight categories:[5]

Base Services
[6] Provide access to the fundamental resources available to a Windows system. Included are things like file systems, devices, processes, threads, and error handling. These functions reside in kernel.exe, krnl286.exe or krnl386.exe files on 16-bit Windows, and kernel32.dll on 32-bit Windows.
Advanced Services
Provide access to functionality additional to the kernel. Included are things like the Windows registry, shutdown/restart the system (or abort), start/stop/create a Windows service, manage user accounts. These functions reside in advapi32.dll on 32-bit Windows.
Graphics Device Interface
[7] Provides functionality for outputting graphical content to monitors, printers and other output devices. It resides in gdi.exe on 16-bit Windows, and gdi32.dll on 32-bit Windows in user-mode. Kernel-mode GDI support is provided by win32k.sys which communicates directly with the graphics driver.[8]
User Interface
[9] Provides the functionality to create and manage screen windows and most basic controls, such as buttons and scrollbars, receive mouse and keyboard input, and other functionality associated with the GUI part of Windows. This functional unit resides in user.exe on 16-bit Windows, and user32.dll on 32-bit Windows. Since Windows XP versions, the basic controls reside in comctl32.dll, together with the common controls (Common Control Library).
Common Dialog Box Library
[10] Provides applications the standard dialog boxes for opening and saving files, choosing color and font, etc. The library resides in a file called commdlg.dll on 16-bit Windows, and comdlg32.dll on 32-bit Windows. It is grouped under the User Interface category of the API.
Common Control Library
[11] Gives applications access to some advanced controls provided by the operating system. These include things like status bars, progress bars, toolbars and tabs. The library resides in a DLL file called commctrl.dll on 16-bit Windows, and comctl32.dll on 32-bit Windows. It is grouped under the User Interface category of the API.
Windows Shell
[12][13] Component of the Windows API allows applications to access the functionality provided by the operating system shell, as well as change and enhance it. The component resides in shell.dll on 16-bit Windows, and shell32.dll on 32-bit Windows. The Shell Lightweight Utility Functions are in shlwapi.dll. It is grouped under the User Interface category of the API.
Network Services
[14] Give access to the various networking capabilities of the operating system. Its sub-components include NetBIOS, Winsock, NetDDE, RPC and many others. This component resides in netapi32.dll on 32-bit Windows.

Web

The Internet Explorer web browser also exposes many APIs that are often used by applications, and as such could be considered a part of the Windows API. Internet Explorer has been included with the operating system since Windows 95 OSR2, and has provided web related services to applications since Windows 98.[15] Specifically, it is used to provide:

Multimedia

The classic windows Multimedia API is placed in winmm.dll and contains functions to play sound files, send and receive MIDI messages, access joysticks and other features of the so-called "MCI" subsystem of Windows (this stems from the "Multimedia Extensions" available for Windows 3.0 separately and as an integral part of the operating system since Windows 3.1, where they were located in mmsystem.dll).

Apart from that, Microsoft has provided the DirectX set of APIs as part of every Windows installation since Windows 95 OSR2. DirectX provides a loosely related set of graphics and gaming services, including:

Microsoft also provide several APIs for media encoding and playback:

Program interaction

The Windows API mostly concerns itself with the interaction between the operating system and an application. For communication between the different Windows applications among themselves, Microsoft has developed a series of technologies alongside the main Windows API. This started out with Dynamic Data Exchange (DDE), which was superseded by Object Linking and Embedding (OLE) and later by the Component Object Model (COM), Automation Objects, ActiveX controls, and the .NET Framework. There is not always a clear distinction between these technologies, and there is quite a lot of overlap.

The variety of terms is basically the result of grouping software mechanisms that relate to a particular aspect of software development. Automation specifically relates to exporting the functionality of an application or component (as an API) so that it can be controlled by another application instead of just by a human user, .NET is a self-contained general methodology and technology for developing Desktop and Web applications written in a variety of "Just in Time" compiled languages.

Windows.pas is a Pascal/Delphi unit which contains the Windows-specific API declarations. It is the Pascal equivalent to windows.h, used in C.[16]

Wrapper libraries

Various wrappers were developed by Microsoft that took over some of the more low level functions of the Windows API, and allowed applications to interact with the API in a more abstract manner. Microsoft Foundation Class Library (MFC) wrapped Windows API functionality in C++ classes, and thus allows a more object oriented way of interacting with the API. The Active Template Library (ATL) is a template oriented wrapper for COM. The Windows Template Library (WTL) was developed as an extension to ATL, and intended as a lightweight alternative to MFC.

Also notable are some of Borland's offerings. Object Windows Library (OWL) was released to compete with MFC, and offered a similar object-oriented wrapper. Borland later deprecated it in favour of the Visual Component Library (VCL), which is written in Object Pascal and available in both Delphi and C++ Builder.

Most application frameworks for Windows (at least partially) wrap the Windows API. Thus, the .NET Framework and Java, likewise any other programming languages under Windows, are (or contain) wrapper libraries.
Windows API Code Pack for Microsoft .NET Framework is a .Net wrapper library for Windows API.

History

The Windows API has always exposed a large part of the underlying structure of the Windows systems to the programmer. This has had the advantage of giving Windows programmers a great deal of flexibility and power over their applications. However, it also has given Windows applications a great deal of responsibility in handling various low-level, sometimes tedious, operations that are associated with a graphical user interface.

Charles Petzold, writer of several books about programming against the Windows API, has said: "The original hello world program in the Windows 1.0 SDK was a bit of a scandal. HELLO.C was about 150 lines long, and the HELLO.RC resource script had another 20 or so more lines. (...) Veteran programmers often curled up in horror or laughter when encountering the Windows hello-world program."[17]

Over the years, various changes and additions were made to the Windows Operating System, and the Windows API changed and grew to reflect this. The Windows API for Windows 1.0 supported fewer than 450 function calls, whereas modern versions of the Windows API support thousands. However, in general, the interface remained fairly consistent, and an old Windows 1.0 application will still look familiar to a programmer who is used to the modern Windows API.[18]

Microsoft has made an effort to maintain backwards compatibility. To achieve this, when developing new versions of Windows, Microsoft sometimes implemented workarounds to allow compatibility with third-party software that used the previous version in an undocumented or even inadvisable way. Raymond Chen, a Microsoft developer who works on the Windows API, has said: "I could probably write for months solely about bad things apps do and what we had to do to get them to work again (often in spite of themselves). Which is why I get particularly furious when people accuse Microsoft of maliciously breaking applications during OS upgrades. If any application failed to run on Windows 95, I took it as a personal failure."[19]

One of the largest changes to the Windows API was the transition from Win16 (shipped in Windows 3.1 and older) to Win32 (Windows NT and Windows 95 and up). While Win32 was originally introduced with Windows NT 3.1 and Win32s allowed usage of a Win32 subset before Windows 95, it was not until Windows 95 that widespread porting of applications to Win32 began. To ease the transition, in Windows 95, both for external developers and for Microsoft itself, a complex scheme of API thunks was used that could allow 32 bit code to call into 16 bit code (for the majority of Win16 APIs) and vice versa. Flat thunks allowed 32 bit code to call into 16 bit libraries, and the scheme was used extensively inside Windows 95's libraries to avoid porting the whole OS to Win32 itself in one chunk. In Windows NT, the OS was pure 32-bit (except the parts for compatibility with 16-bit applications), and only generic thunks were available to thunk from Win16 to Win32, as for Windows 95. The Platform SDK shipped with a compiler that could produce the code necessary for these thunks. The 64-bit version of Windows is also capable of running 32-bit applications. The SysWOW64 folder located in the Windows folder on the OS drive contains several tools to support 32-bit applications.

Versions

Almost every new version of Microsoft Windows has introduced its own additions and changes to the Windows API.[20] The name of the API, however, remained consistent between different Windows versions, and name changes were kept limited to major architectural and platform changes for Windows. Microsoft eventually changed the name of the then current Win32 API family into Windows API, and made it into a catch-all term for both past and future versions of the API.[5]

Other implementations

The Wine project ("Wine Is Not an Emulator") provides a Win32 API compatibility layer for Unix-like platforms; and is a compatibility layer between Linux kernel API and programs written for the Windows API.[25] ReactOS goes a step further and aims to provide an implementation of the entire Windows operating system, working closely with the Wine project to promote code re-use and compatibility. DosWin32 and HX DOS Extender are other projects which emulate the Windows API allowing the execution of simple Windows programs from a DOS command line. Odin is a project to emulate Win32 on top of OS/2, superseding the original Win-OS/2 emulation which was based on Microsoft code. Other minor implementations include the MEWEL and Zinc libraries which were intended to implement a subset of the Win16 API on top of DOS (see List of platform-independent GUI libraries).

Windows Interface Source Environment (or WISE) was a licensing program from Microsoft which allowed developers to recompile and run Windows-based applications on UNIX and Macintosh platforms. WISE SDKs were based on an emulation of the Windows API that could run on Unix and Macintosh platforms.[26]

Efforts toward standardization included Sun's Public Windows Interface (PWI) for Win16 (see also: Sun Windows Application Binary Interface (Wabi)), Willows Software's Application Programming Interface for Windows (APIW) for Win16 and Win32 (see also: Willows TWIN), and ECMA-234, which attempted to subject the Windows API to binding standardisation.

Compiler support

To develop software that utilizes the Windows API, a compiler must be able to use the Microsoft-specific DLLs listed above.[27] The compiler must either handle the header files that expose the interior API function names, or supply such files by itself.

For the C++ language, Zortech (later Symantec, then Digital Mars), Watcom and Borland have all produced well known commercial compilers that have been frequently used with both Win16, Win32s and Win32. Some of them supplied "memory extenders", allowing Win32 programs to run on Win16 with Microsoft's redistridutable Win32s DLL. The Zortech compiler was probably one of the first stable and usable C++ compilers for Windows programming, before Microsoft had a compiler for this language.

For certain classes of applications, the compiler system should also be able to handle IDL (interface definition language) files. Collectively, these prerequisites (compilers, tools, libraries, and headers) are known as the Microsoft Platform SDK. For a time, the Microsoft Visual Studio and Borland's integrated development system were the only IDEs that could provide this (although, the SDK itself is downloadable for free separately from the entire IDE suite, from Microsoft Windows SDK for Windows 7 and .NET Framework 4).

Nowadays, the MinGW and Cygwin projects also provide such an environment based on the GNU Compiler Collection, using a stand-alone header file collection to make linking against the Win32-specific DLLs simple. LCC-Win32 is a C compiler maintained by Jacob Navia, "free for non-commercial use". Pelles C is another freeware C compiler maintained by Pelle Orinius. Free Pascal is a free software Object Pascal compiler capable of writing software based on the Windows API. The MASM32 package is a mature project providing support for the Windows API under Microsoft's Macro Assembler by using custom made or converted headers and libraries from the Platform SDK. FASM allows to build Windows programs without the use of an external linker, even when running on Linux.

Windows specific compiler support is also required for the Structured Exception Handling feature (SEH). This system serves a dual purpose: it provides a substrate upon which language-specific exception handling can be implemented, and it is how the kernel notifies applications of exceptional conditions such as dereferencing an invalid pointer or stack overflow. The Microsoft/Borland C++ compilers had the ability to use this system as soon as it was introduced in Windows 95 and NT, however the actual implementation was undocumented and had to be reverse engineered for the Wine project and free compilers. SEH is based on pushing exception handler frames onto the stack, then adding them to a linked list stored in thread local storage (the first field of the thread environment block). When an exception is thrown, the kernel and base libraries unwind the stack running handlers and filters as they are encountered. Eventually, every exception unhandled by the application itself will be dealt with by the default backstop handler, which pops up the Windows common crash dialog.

See also

Notes

  1. ↑ Microsoft TechNet (November 2006). Inside Native Applications. Retrieved December 24, 2008.
  2. ↑ http://msdn.microsoft.com/en-us/library/bb384843.aspx
  3. ↑ Both the Pascal language and x86 assembly has been heavily used in earlier versions of the Windows API, before C became dominant. A reminiscence of this is that the API-functions still use the pascal calling convention to restore the stack from pushed parameters after a call (although they expect parameters pushed from right to left, as most C compilers do, by default).
  4. 1 2 Mayberry, Michael (2012). WinRT Revealed. New York City: Apress. p. 3. ISBN 978-1-4302-4585-8.
  5. 1 2 Microsoft Developer Network (July 2005). Overview of the Windows API. Retrieved August 28, 2005.
  6. ↑ Microsoft Developer Network (July 2005). Base Services. Retrieved August 28, 2005.
  7. ↑ Microsoft Developer Network (July 2005). Graphics Device Interface. Retrieved August 28, 2005.
  8. ↑ "G". Microsoft Developer Network. Retrieved 2009-01-28.
  9. ↑ Microsoft Developer Network (July 2005). User Interface. Retrieved August 28, 2005.
  10. ↑ Microsoft Developer Network (2005). Common Dialog Box Library. Retrieved September 22, 2005.
  11. ↑ Microsoft Developer Network (July 2005). Common Control Library. Retrieved August 28, 2005.
  12. ↑ Microsoft Developer Network (July 2005). Windows Shell. Retrieved August 28, 2005.
  13. ↑ Microsoft Developer Network (2005). Shell Programmer's Guide. Retrieved August 28, 2005.
  14. ↑ Microsoft Developer Network (July 2005). Network Services. Retrieved August 28, 2005.
  15. ↑ Microsoft Developer Network (January 2006); Programming and reusing the browser Retrieved January 22, 2006.
  16. ↑ Texeira, Steve and Xavier Pacheco (2002). Borland Delphi 6 Developer's Guide. Sams. p. 253. ISBN 0672321157.
  17. ↑ Charles Petzold (December 2001). Programming Microsoft Windows with C#. Microsoft Press. Beyond the Console, page 47.
  18. ↑ Charles Petzold (November 11, 1998). Programming Windows, Fifth Edition. Microsoft Press. APIs and Memory Models, page 9.
  19. ↑ Raymond Chen (October 15, 2003). What about BOZOSLIVEHERE and TABTHETEXTOUTFORWIMPS? Retrieved August 27, 2005.
  20. ↑ The Iseran Project (1996–2001). History of the Windows API. Retrieved October 7, 2005.
  21. ↑ Nomenclature of released 64-bit versions includes Windows XP Professional x64 Edition and x64 Editions of Windows Server 2003, Windows Vista and Windows Server 2008 on the AMD64 platform, and Windows 2000 Server Limited Edition, Windows XP 64-bit Edition, Windows Advanced Server 2003 for Itanium and Windows 2008 Advanced Server for Itanium on the IA-64 platform
  22. ↑ "Windows XP Professional x64 Edition home page". Microsoft.
  23. ↑ "Microsoft 64-bit Computing Overview". Microsoft.
  24. ↑ "MSDN: Getting Ready for 64-bit Windows". Microsoft.
  25. ↑ Softpedia. Softpedia http://www.softpedia.com/reviews/linux/Wine-Review-434892.shtml. Retrieved 15 October 2015. Missing or empty |title= (help)
  26. ↑ http://hyper.sunjapan.com.cn/~hz/win32/wise.htm
  27. ↑ COM-objects are outside Win32 and assumes a certain vtable layout.

References

  1. Windows application programming interface

External links

Wikibooks has a book on the topic of: Windows Programming
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