Windows API

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The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. Windows API_sentence_0

The name Windows API collectively refers to several different platform implementations that are often referred to by their own names (for example, Win32 API); see the versions section. Windows API_sentence_1

Almost all Windows programs interact with the Windows API. Windows API_sentence_2

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. Windows API_sentence_3

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. Windows API_sentence_4

The Windows API (Win32) 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. Windows API_sentence_5

However, the API may be used by any programming language compiler or assembler able to handle the (well-defined) low-level data structures along with the prescribed calling conventions for calls and callbacks. Windows API_sentence_6

Similarly, the internal implementation of the API's function has been developed in several languages, historically. Windows API_sentence_7

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. Windows API_sentence_8

There have also been many wrapper classes and extensions (from Microsoft and others) for object-oriented languages that make this object-oriented structure more explicit (Microsoft Foundation Class Library (MFC), Visual Component Library (VCL), GDI+, etc.). Windows API_sentence_9

For instance, Windows 8 provides the Windows API and the WinRT API, which is implemented in C++ and is object-oriented by design. Windows API_sentence_10

Overview Windows API_section_0

The functions provided by the Windows API can be grouped into eight categories: Windows API_sentence_11

Windows API_description_list_0

  • Base Services: Provide access to the basic resources available to a Windows system. Included are things like , 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.Windows API_item_0_0
  • Advanced Services: Provide access to functions beyond 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 and advapires32.dll on 32-bit Windows.Windows API_item_0_1
  • 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.Windows API_item_0_2
  • 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 (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).Windows API_item_0_3
  • 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.Windows API_item_0_4
  • 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 (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 API_item_0_5
  • 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.Windows API_item_0_6
  • Network Services: Give access to the various networking abilities of the operating system. Its subcomponents include NetBIOS, Winsock, NetDDE, remote procedure call (RPC) and many more. This component resides in netapi32.dll on 32-bit Windows.Windows API_item_0_7

Web Windows API_section_1

The Internet Explorer (IE) web browser also exposes many APIs that are often used by applications, and as such could be considered a part of the Windows API. Windows API_sentence_12

IE has been included with the operating system since Windows 95 OSR2 and has provided web-related services to applications since Windows 98. Windows API_sentence_13

Specifically, it is used to provide: Windows API_sentence_14

Windows API_unordered_list_1

  • An embeddable web browser control, contained in shdocvw.dll and mshtml.dll.Windows API_item_1_8
  • 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.Windows API_item_1_9
  • An HTTP client library which also takes into account system-wide Proxy settings (wininet.dll); however, Microsoft has added another HTTP client library called winhttp.dll which is smaller and more suitable for some applications.Windows API_item_1_10
  • A library to assist multi-language and international text support (mlang.dll).Windows API_item_1_11
  • DirectX Transforms, a set of image filter components.Windows API_item_1_12
  • XML support (the MSXML components, held in msxml*.dll).Windows API_item_1_13
  • Access to the Windows Address Books.Windows API_item_1_14

Multimedia Windows API_section_2

Program interaction Windows API_section_3

The Windows API is designed mostly for the interaction between the operating system and an application. Windows API_sentence_15

For communication among different Windows applications, Microsoft has developed a series of technologies alongside the main Windows API. Windows API_sentence_16

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. Windows API_sentence_17

There is not always a clear distinction between these technologies, and there is much overlap. Windows API_sentence_18

The variety of terms is basically the result of grouping software mechanisms that relate to a given aspect of software development. Windows API_sentence_19

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. Windows API_sentence_20

Windows.pas is a Pascal/Delphi unit which contains the Windows-specific API declarations. Windows API_sentence_21

It is the Pascal equivalent to windows.h, used in C. Windows API_sentence_22

Wrapper libraries Windows API_section_4

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. Windows API_sentence_23

Microsoft Foundation Class Library (MFC) wrapped Windows API functionality in C++ classes, and thus allows a more object-oriented way to interact with the API. Windows API_sentence_24

The Active Template Library (ATL) is a template oriented wrapper for COM. Windows API_sentence_25

The Windows Template Library (WTL) was developed as an extension to ATL, and intended as a smaller alternative to MFC. Windows API_sentence_26

Most application frameworks for Windows (at least partly) wrap the Windows API. Windows API_sentence_27

Thus, the .NET Framework and Java, likewise any other programming languages under Windows, are (or contain) wrapper libraries. Windows API_sentence_28

History Windows API_section_5

The Windows API has always exposed a large part of the underlying structure of the Windows systems to programmers. Windows API_sentence_29

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. Windows API_sentence_30

For example, a beginning C programmer will often write the simple "hello world" as their first assignment. Windows API_sentence_31

The working part of the program is only a single printf line within the main subroutine. Windows API_sentence_32

The overhead for linking to the standard I/O library is also only one line: Windows API_sentence_33

The Windows version was still only one working line of code but it required many, many more lines of overhead. Windows API_sentence_34

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. Windows API_sentence_35

HELLO.C was about 150 lines long, and the HELLO.RC resource script had another 20 or so more lines. Windows API_sentence_36

(...) Veteran programmers often curled up in horror or laughter when encountering the Windows hello-world program." Windows API_sentence_37

Over the years, various changes and additions were made to Windows systems, and the Windows API changed and grew to reflect this. Windows API_sentence_38

The Windows API for Windows 1.0 supported fewer than 450 function calls, whereas modern versions of the Windows API support thousands. Windows API_sentence_39

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. Windows API_sentence_40

Microsoft has made an effort to maintain backward compatibility. Windows API_sentence_41

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. Windows API_sentence_42

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). Windows API_sentence_43

Which is why I get particularly furious when people accuse Microsoft of maliciously breaking applications during OS upgrades. Windows API_sentence_44

If any application failed to run on Windows 95, I took it as a personal failure." Windows API_sentence_45

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). Windows API_sentence_46

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. Windows API_sentence_47

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. Windows API_sentence_48

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. Windows API_sentence_49

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. Windows API_sentence_50

The Platform SDK shipped with a compiler that could produce the code needed for these thunks. Windows API_sentence_51

Versions of 64-bit Windows are also able to run 32-bit applications via WoW64. Windows API_sentence_52

The SysWOW64 folder located in the Windows folder on the OS drive contains several tools to support 32-bit applications. Windows API_sentence_53

Versions Windows API_section_6

Almost every new version of Microsoft Windows has introduced its own additions and changes to the Windows API. Windows API_sentence_54

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. Windows API_sentence_55

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. Windows API_sentence_56

Windows API_unordered_list_2

  • 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 reside in mainly the core files of the OS: kernel.exe (or krnl286.exe or krnl386.exe), user.exe and gdi.exe. Despite the of exe, these actually are dynamic-link libraries.Windows API_item_2_15
  • Win32 is the 32-bit application programming interface (API) for versions of Windows from 95 onwards. 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 c meaning compatibility. This term was later abandoned by Microsoft in favor of Win32.Windows API_item_2_16
  • 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".Windows API_item_2_17
  • Win64 is the variant of the API implemented on 64-bit platforms of the Windows architecture (as of 2011 x86-64 and IA-64). Both 32-bit and 64-bit versions of an application can be still compiled from one codebase, although some older APIs have been deprecated, and some of the APIs that were already deprecated in Win32 were removed. All memory pointers are 64-bit by default (the LLP64 model), so the source code must be checked for compatibility with 64-bit pointer arithmetic and rewritten as necessary.Windows API_item_2_18
  • WinCE is the implementation of the Windows API for the Windows CE operating system.Windows API_item_2_19

Other implementations Windows API_section_7

The Wine project provides a Win32 API compatibility layer for Unix-like platforms, between Linux kernel API and programs written for the Windows API. Windows API_sentence_57

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. Windows API_sentence_58

DosWin32 and HX DOS Extender are other projects which emulate the Windows API to allow executing simple Windows programs from a DOS command line. Windows API_sentence_59

Odin is a project to emulate Win32 on OS/2, superseding the original Win-OS/2 emulation which was based on Microsoft code. Windows API_sentence_60

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 API_sentence_61

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. Windows API_sentence_62

WISE SDKs were based on an emulator of the Windows API that could run on those platforms. Windows API_sentence_63

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. Windows API_sentence_64

Compiler support Windows API_section_8

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). Windows API_sentence_65

The compiler must either handle the header files that expose the interior API function names, or supply such files. Windows API_sentence_66

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. Windows API_sentence_67

Some of them supplied memory extenders, allowing Win32 programs to run on Win16 with Microsoft's redistributable Win32s DLL. Windows API_sentence_68

The Zortech compiler was probably one of the first stable and usable C++ compilers for Windows programming, before Microsoft had a C++ compiler. Windows API_sentence_69

For certain classes of applications, the compiler system should also be able to handle interface description language (IDL) files. Windows API_sentence_70

Collectively, these prerequisites (compilers, tools, libraries, and headers) are known as the Microsoft Platform SDK. Windows API_sentence_71

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 ). Windows API_sentence_72

As of 2016, 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. Windows API_sentence_73

LCC-Win32 is a C compiler maintained by Jacob Navia, freeware for non-commercial use. Windows API_sentence_74

Pelles C is a freeware C compiler maintained by Pelle Orinius. Windows API_sentence_75

Free Pascal is a free software Object Pascal compiler that supports the Windows API. Windows API_sentence_76

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. Windows API_sentence_77

Flat assembler FASM allows building Windows programs without using an external linker, even when running on Linux. Windows API_sentence_78

Windows specific compiler support is also needed for Structured Exception Handling (SEH). Windows API_sentence_79

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. Windows API_sentence_80

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. Windows API_sentence_81

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). Windows API_sentence_82

When an exception is thrown, the kernel and base libraries unwind the stack running handlers and filters as they are encountered. Windows API_sentence_83

Eventually, every exception unhandled by the application will be dealt with by the default backstop handler, which pops up the Windows common crash dialog. Windows API_sentence_84

See also Windows API_section_9

Windows API_unordered_list_3

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: API.