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.
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 functionality provided by the Windows API can be grouped into eight categories:
- Base Services
-  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
-  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.syswhich communicates directly with the graphics driver.
- User Interface
-  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
-  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
-  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
-  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
-  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.
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. Specifically, it is used to provide:
- An embeddable web browser control, contained in shdocvw.dll and mshtml.dll.
- The URL moniker service, held in urlmon.dll, which provides COM objects to applications for resolving URLs. Applications can also provide their own URL handlers for others to use.
- A library for assisting with multi-language and international text support (mlang.dll).
- DirectX Transforms, a set of image filter components.
- XML support (the MSXML components, held in msxml*.dll).
- Access to the Windows Address Book.
- Direct3D for hardware accelerated 3D graphics.
- Direct2D for hardware accelerated 2D vector graphics.
- DirectSound for low level hardware accelerated sound card access.
- DirectInput for communication with input devices such as joysticks and gamepads.
- DirectPlay as a multiplayer gaming infrastructure. This component has been deprecated as of DirectX 9 and Microsoft no longer recommends its use for game development.
Microsoft also provide several APIs for media encoding and playback:
- DirectShow which builds and runs generic multimedia pipelines. It is comparable to the GStreamer framework and is often used to render in-game videos and build media players (Windows Media Player is based upon it). DirectShow is no longer recommended for game development.
- Media Foundation a newer digital media API that is intended to replace DirectShow.
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.
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.
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 C programmers often curled up in horror or laughter when encountering the Windows hello-world program."
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, where in modern versions of the Windows API there are 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.
An emphasis has been made by Microsoft on maintaining software backwards compatibility. To achieve this, when developing a new version of Windows, Microsoft sometimes had to implement workarounds to allow compatibility with third-party software that used the previous version's API in an undocumented or even (programmatically) illegal 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."
One of the largest changes the Windows API underwent 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 and (for the majority of Win16 APIs) 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.
Almost every new version of Microsoft Windows has introduced its own additions and changes to the Windows API. 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.
- Win16 is the API for the first, 16-bit versions of Microsoft Windows. These were initially referred to as simply the "Windows API", but were later renamed to Win16 in an effort to distinguish them from the newer, 32-bit version of the Windows API. The functions of Win16 API mainly reside in the core files of the OS: kernel.exe (or krnl286.exe or krnl386.exe), user.exe and gdi.exe. Despite the file extension of exe, these actually are dynamically linked libraries.
- Win32 is the 32-bit API for modern versions of Windows. The API consists of functions implemented, as with Win16, in system DLLs. The core DLLs of Win32 are kernel32.dll, user32.dll, and gdi32.dll. Win32 was introduced with Windows NT. The version of Win32 shipped with Windows 95 was initially referred to as Win32c, with the "c" standing for "compatibility", but this term was later abandoned by Microsoft in favour of Win32.
- Win32s is an extension for the Windows 3.1x family of Microsoft Windows that implemented a subset of the Win32 API for these systems. The "s" stands for "subset".
- Win64 is the variant of the API implemented on 64-bit platforms of the Windows architecture (as of 2011[update] AMD64 and IA-64.) Both 32-bit and 64-bit versions of an application can be still compiled from a single codebase, although some older APIs have been deprecated, and some of the APIs that were already deprecated in Win32 were entirely removed. All memory pointers are 64-bit by default (the LLP64 model), so the source code has to be checked for compatibility with 64-bit pointer arithmetic and rewritten as necessary.
- WinCE is the implementation of the Windows API for the Windows CE operating system.
The Wine project is an attempt to provide a Win32 API compatibility layer for Unix-like platforms. 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.
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.
Other minor implementations and/or standardisation efforts 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), and ECMA-234 which attempted to subject the Windows API to binding standardisation.
To develop software that utilizes the Windows API, a compiler must be able to handle and import the Microsoft-specific DLLs and COM-objects. The compiler must either handle the header files that expose the interior API function names, or supply such files by itself. 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 long time, the Microsoft Visual Studio family of compilers and tools and Borland's compilers were the only tools that could provide this (although at least in the case of Windows, 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 Microsoft DLLs possible. LCC-Win32 is a "free for non-commercial use" C compiler maintained by Jacob Navia. 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.
- Object Windows Library
- .NET Framework
- Microsoft Foundation Class Library
- Microsoft Windows library files
- Windows legacy audio components
- Linux kernel API
- Wine – a compatibility layer between Linux kernel API and programs written for the Windows API
- Microsoft TechNet (November 2006). Inside Native Applications. Retrieved December 24, 2008.
- Microsoft Developer Network (July 2005). Overview of the Windows API. Retrieved August 28, 2005.
- Microsoft Developer Network (July 2005). Base Services. Retrieved August 28, 2005.
- Microsoft Developer Network (July 2005). Graphics Device Interface. Retrieved August 28, 2005.
- "G". Microsoft Developer Network. Retrieved 2009-01-28.
- Microsoft Developer Network (July 2005). User Interface. Retrieved August 28, 2005.
- Microsoft Developer Network (2005). Common Dialog Box Library. Retrieved September 22, 2005.
- Microsoft Developer Network (July 2005). Common Control Library. Retrieved August 28, 2005.
- Microsoft Developer Network (July 2005). Windows Shell. Retrieved August 28, 2005.
- Microsoft Developer Network (2005). Shell Programmer's Guide. Retrieved August 28, 2005.
- Microsoft Developer Network (July 2005). Network Services. Retrieved August 28, 2005.
- Microsoft Developer Network (January 2006); Programming and reusing the browser Retrieved January 22, 2006.
- Charles Petzold (December 2001). Programming Microsoft Windows with C#. Microsoft Press. Beyond the Console, page 47.
- Charles Petzold (November 11, 1998). Programming Windows, Fifth Edition. Microsoft Press. APIs and Memory Models, page 9.
- Raymond Chen (October 15, 2003). What about BOZOSLIVEHERE and TABTHETEXTOUTFORWIMPS? Retrieved August 27, 2005.
- The Iseran Project (1996–2001). History of the Windows API. Retrieved October 7, 2005.
- 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
- "Windows XP Professional x64 Edition home page". Microsoft.
- "Microsoft 64-bit Computing Overview". Microsoft.
- "MSDN: Getting Ready for 64-bit Windows". Microsoft.
- Windows application programming interface
- Diomidis Spinellis. A critique of the Windows application programming interface. Computer Standards & Interfaces, 20(1):1–8, November 1998. doi:10.1016/S0920-5489(98)00012-9.
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- MSDN Windows API
- C++ Win32 API Tutorial
- Straight C Win32 API Tutorial no MFC
- Advanced Win32 api newsgroup
- French Win32 api newsgroup
- Windows API Code Pack for Microsoft .NET Framework
- ECMA-234 – ECMA standard for a subset of the Windows API
- Windows API – Developers discussion forum – Win32 API discussion forum.
- winapiexec – a tool to run WinAPI functions from command line.