ReactOS is a free and open-source operating system for amd64/i686 personal computers intended to be binary-compatible with computer programs and device drivers developed for Windows Server 2003 and later versions of Microsoft Windows. ReactOS has been noted as a potential open-source drop-in replacement for Windows and for its information on undocumented Windows APIs.
ReactOS has been in development since 1996. , it is still considered feature-incomplete alpha software, and is therefore recommended by the developers only for evaluation and testing purposes. However, many Windows applications are working, such as Adobe Reader 9.3, GIMP 2.6, and LibreOffice 5.4.
ReactOS is primarily written in C, with some elements, such as ReactOS , written in C++. The project partially implements Windows API functionality and has been ported to the AMD64 processor architecture. ReactOS, as part of the FOSS ecosystem, re-uses and collaborates with many other FOSS projects, most notably the Wine project, which presents a Windows compatibility layer for Unix-like operating systems.
Around 1996, a group of free and open-source software developers started a project called FreeWin95 to implement a clone of Windows 95. The project stalled in discussions of the design of the system.
While FreeWin95 had started out with high expectations, there still had not been any builds released to the public by the end of 1997. As a result, the project members, led by then coordinator Jason Filby, joined together to revive the project. The revived project sought to duplicate the functionality of Windows NT. In creating the new project, a new name, ReactOS, was chosen. The project began development in February 1998 by creating the basis for a new NT kernel and basic drivers. The name ReactOS was coined during an IRC chat. While the term "OS" stood for operating system, the term "react" referred to the group's dissatisfaction with – and reaction to – Microsoft's monopolistic position.