| Developer: | Microsoft |
| Replaces: | DOS API |
| Operating System: | Microsoft Windows, OS/2 |
| Genre: | API |
| License: | Proprietary |
| Website: | https://learn.microsoft.com/en-us/windows/apps/ |
The Windows API, informally WinAPI, is the foundational application programming interface (API) that allows a computer program to access the features of the Microsoft Windows operating system in which the program is running.
Each major version of the Windows API has a distinct name that identifies a compatibility aspect of that version. For example, Win32 is the major version of Windows API that runs on 32-bit systems. The name, Windows API, collectively refers to all versions of this capability of Windows.
Microsoft provides developer support via a software development kit, Microsoft Windows SDK, which includes documentation and tools for building software based on the Windows API.
The functions provided by the Windows API can be grouped into eight categories:[1]
win32k.sys which communicates directly with the graphics driver.[3] [4]
The Windows API is defined in the C programming language.[12] Its functions and data structures are defined in C syntax (see windows.h). However, the API may be consumed via any programming language that can inter-operate with the API data structures and calling conventions for function calls and callbacks.
Of note, the internal implementation of the API functions has been developed in several languages other than C.
Despite the fact that C is not an object-oriented programming (OOP) language, the Windows API is somewhat object-oriented due to its use of handles. Various other technologies from Microsoft and others make this object-oriented aspect more apparent by using an OOP language such as C++ -- see Microsoft Foundation Class Library (MFC), Visual Component Library (VCL), GDI+. Of note, Windows 8 provides the Windows API and the WinRT API, which is implemented in C++[13] and is object-oriented by design.[13]
Windows.pas is a Pascal/Delphi unit that exposes the features of Windows API. It is the Pascal equivalent to the C language windows.h.[14]
The Windows API is for the most part intended for a program to access operating system features. For communication among different Windows applications, Microsoft has developed a series of technologies alongside Windows API. This started with Dynamic Data Exchange (DDE), which was superseded by Object Linking and Embedding (OLE) and later by the Component Object Model (COM), Automation Objects, ActiveX controls, and the .NET Framework.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 much overlap.
The variety of terms is basically the result of grouping software mechanisms that relate to a given aspect of software development. Automation specifically relates to exporting the function of an application or component (as an application programming interface (API)) so that it can be controlled by other applications instead of by human users only, .NET is a self-contained general methodology and technology to develop desktop and web applications written in a variety of just-in-time (JIT) compiled languages.
Many Microsoft technologies use the Windows API -- as most software running on Windows does. As middle-ware between Windows API and an application, these technologies provide some access to Windows API. Some technologies are described as wrapping Windows API, but this is debatable since no other technology provides or exposes all of the capabilities of Windows API.
Although almost all Windows programs use the Windows API, on the Windows NT line of operating systems, programs that start early in the Windows startup process use the Native API instead.[15]
The Windows API has always exposed a large part of the underlying structure of the Windows systems to programmers. This had the advantage of giving them much flexibility and power over their applications, but also creates great responsibility in how applications handle various low-level, sometimes tedious, operations that are associated with a graphical user interface.
For example, a beginning C programmer will often write the simple "hello world" as their first assignment. The working part of the program is only a single printf line within the main subroutine. The overhead for linking to the standard I/O library is also only one line:
int main(void)
Charles Petzold, who wrote several books about programming for the Windows API, 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."[16] Petzold explains that while it was the first Windows sample programs developers were introduced to, it was quite "fancy" and more complex than needed. Tired of people ridiculing the length of the sample, he eventually reduced it to a simple MessageBox call.[17]
Over the years, various changes and additions were made to Windows systems, and the Windows API changed and grew to reflect this.[18] 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.[19]
Microsoft has made an effort to maintain backward compatibility. To achieve this, when developing new versions of Windows, Microsoft sometimes implemented workarounds[20] to allow compatibility with third-party software that used the prior 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."[21]
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 use 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, for developers outside and inside Microsoft, a complex scheme of API thunks was used that could allow 32-bit code to call into 16-bit code (for most 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 in one batch. In Windows NT, the OS was pure 32-bit, except 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 needed for these thunks. Versions of 64-bit Windows are also able to run 32-bit applications via WoW64. The SysWOW64 folder located in the Windows folder on the OS drive contains several tools to support 32-bit applications.[22]
Each version of Microsoft Windows contains a version of Windows API, and almost every new version of Microsoft Windows has introduced additions and changes to the Windows API.[23]
The name, Windows API, refers to essentially the same capability in each version of Windows, but there is another name for this capability that is based on major architectural aspects of the Windows version that contains it. When there was only one version, it was simply called Windows API. Then, when the first major update was made, Microsoft gave it the name Win32 and gave the first version the name Win16. The term Windows API refers to both versions and all subsequently developed major versions.[1]
The Wine project provides a Win32 API compatibility layer for Unix-like platforms, between Linux kernel API and programs written for the Windows API. ReactOS goes a step further and aims to implement the full 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 to allow executing simple Windows programs from a DOS command line. Odin is a project to emulate Win32 on 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 DOS (see List of platform-independent GUI libraries).
Windows Interface Source Environment (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 emulator of the Windows API that could run on those platforms.[27]
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 standardize the Windows API bindingly.
To develop software that uses the Windows API, a compiler must be able to use the Microsoft-specific DLLs listed above (COM-objects are outside Win32 and assume a certain vtable layout). The compiler must either handle the header files that expose the interior API function names, or supply such files.
For the language C++, Zortech (later Symantec, then Digital Mars), Watcom and Borland have all produced well known commercial compilers that have been used often with Win16, Win32s, and Win32. Some of them supplied memory extenders, allowing Win32 programs to run on Win16 with Microsoft's redistributable Win32s DLL. The Zortech compiler was probably one of the first stable and usable C++ compilers for Windows programming, before Microsoft had a C++ compiler.
For certain classes of applications, the compiler system should also be able to handle interface description language (IDL) 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 integrated development environments (IDEs) that could provide this (although, the SDK is downloadable for free separately from the entire IDE suite, from Microsoft Windows SDK for Windows 7 and .NET Framework 4).
, the MinGW and Cygwin projects also provide such an environment based on the GNU Compiler Collection (GCC), using a stand-alone header file set, to make linking against the Win32-specific DLLs simple. LCC-Win32 is a C compiler maintained by Jacob Navia, freeware for non-commercial use. Pelles C is a freeware C compiler maintained by Pelle Orinius. Free Pascal is a free software Object Pascal compiler that supports the Windows API. The MASM32 package is a mature project providing support for the Windows API under Microsoft Macro Assembler (MASM) by using custom made or converted headers and libraries from the Platform SDK. Flat assembler FASM allows building Windows programs without using an external linker, even when running on Linux.
Windows specific compiler support is also needed for Structured Exception Handling (SEH). This system serves two purposes: it provides a substrate on 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 will be dealt with by the default backstop handler, which pops up the Windows common crash dialog.