Windows API


The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces available in the Microsoft Windows operating systems. The name Windows API collectively refers to several different platform implementations that are often referred to by their own names ; see the [|versions] section. Almost all Windows programs interact with the Windows API. On the Windows NT line of operating systems, a small number use the Native API.
Developer support is available in the form of a software development kit, Microsoft Windows SDK, providing documentation and tools needed to build software based on the Windows API and associated Windows interfaces.
The Windows API is focused mainly on the programming language C 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 able to handle the low-level data structures along with the prescribed calling conventions for calls and callbacks. Similarly, the internal implementation of the API's function has been developed in several languages, historically. Despite the fact that C is not an object-oriented programming language, the Windows API and Windows have both historically been described as object-oriented. There have also been many wrapper classes and extensions for object-oriented languages that make this object-oriented structure more explicit, Visual Component Library. For instance, Windows 8 provides the Windows API and the WinRT API, which is implemented in C++ and is object-oriented by design.

Overview

The functions provided by the Windows API can be grouped into eight categories:
; Base Services: Provide access to the basic 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 and KernelBase.dll on 32 and 64 bit Windows. These files reside in the folder \Windows\System32 on all versions of Windows.
; Advanced Services
; Graphics Device Interface: Provides functions to output graphics 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.
; User Interface: Provides the functions to create and manage screen windows and most basic controls, such as buttons and scrollbars, receive mouse and keyboard input, and other functions associated with the graphical user interface 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 Dialog Box Library: Provides applications the standard dialog boxes to open and save files, choose 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 dynamic-link library 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 functions provided by the operating system shell, and to 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 abilities of the operating system. Its subcomponents include NetBIOS, Winsock, NetDDE, remote procedure call and many more. 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. IE 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:
The classic windows Multimedia API is placed in winmm.dll and contains functions to play sound files, to send and receive MIDI messages, to access joysticks, and to facilitate all other features of the so-called MCI subsystem of Windows, which originates from the Multimedia Extensions available for Windows 3.0 separately and as an integral part of the operating system since Windows 3.1, at which time they were located in mmsystem.dll.
Apart from that, as part of every Windows version since Windows 95 OSR2, Microsoft has provided the DirectX APIs—a loosely related set of graphics and gaming services, which includes:
Microsoft also provides several APIs for media encoding and playback:
The Windows API is designed mostly for the interaction between the operating system and an application. For communication among different Windows applications, Microsoft has developed a series of technologies alongside the main Windows API. This started out with Dynamic Data Exchange, which was superseded by Object Linking and Embedding and later by the Component Object Model, 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 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 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.

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 wrapped Windows API functionality in C++ classes, and thus allows a more object-oriented way to interact with the API. The Active Template Library is a template oriented wrapper for COM. The Windows Template Library was developed as an extension to ATL, and intended as a smaller alternative to MFC.
Most application frameworks for Windows wrap the Windows API. Thus, the.NET Framework and Java, likewise any other programming languages under Windows, are 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 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:
#include
int main
The Windows version was still only one working line of code but it required many, many more lines of overhead. 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."
Over the years, various changes and additions were made to Windows systems, 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.
Microsoft has made an effort to maintain backward compatibility. To achieve this, when developing new versions of Windows, Microsoft sometimes implemented workarounds 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. 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 to the Windows API was the transition from Win16 to Win32. 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 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.

Versions

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 API versions.
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.
Windows Interface Source Environment 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.
Efforts toward standardization included Sun's Public Windows Interface for Win16, Willows Software's Application Programming Interface for Windows for Win16 and Win32, and ECMA-234, which attempted to standardize the Windows API bindingly.

Compiler support

To develop software that uses the Windows API, a compiler must be able to use the Microsoft-specific DLLs listed above. The compiler must either handle the header files that expose the interior API function names, or supply such files.
For the language C++, Zortech, 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 files. Collectively, these prerequisites 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 that could provide this.
, the MinGW and Cygwin projects also provide such an environment based on the GNU Compiler Collection, 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 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. 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. 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.