ReactOS is primarily written in C, with some elements, such as ReactOS Explorer, written in C++. The project is ported to the ARM and AMD64 processor architectures, and partially implements Windows API functionality. The latter is assisted by including parts from the Wine compatibility layer for Unix-like operating systems, but other functionality is implemented by the developers themselves. However, progress has been hampered by a lack of developers with the relevant skill-sets.
An extensive code audit is in place to protect against legal problems, such that implementation of the Windows API is only done by means of a complete clean room reverse engineering process. This has been in place following claims made in 2006 by a former developer and a third party in separate incidents that the project has either contained disassembled assembly code from Windows, or files directly originating from Microsoft. Both allegations have had no adverse legal consequences to the project, and development continues to this day.
ReactOS has been noted for its information on undocumented Windows APIs and more generally as an open-source drop-in replacement for Windows. The project aim, as cited from itself, is to allow users of Windows to completely renounce use of proprietary commercial software without having to switch to an operating system that is not binary-compatible with Windows, such as Linux. However, a lack of corporate backers and dedicated developers have limited its efforts to realize this.
The name ReactOS was coined by project founder Jason Filby, an Oracle developer from Durban, South Africa. 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.
ReactOS is primarily written in C, with some elements, such as ReactOS Explorer, written in C++. The project relies on MinGW for compilation, and contributes to its development through the submission of patches to its components.
The developers aim to make the kernel more compatible with Windows NT version 5.2 (Windows Server 2003), the usermode APIs with Windows NT 6 (Windows Vista), and to add support for more applications and hardware. DirectX support is undertaken through ReactX, an in-house implementation. 2D hardware-accelerated rendering is done natively, while other drawing functionality is redirected to OpenGL as a stopgap solution.
Development is limited by a lack of people with relevant experience. As of 24 March 2010 (2010 -03-24)[update], in the ReactOS entry in Ohloh, the page followed through the "Very large, active development team" link lists 37 developers who have contributed over a 12-month period and a cumulative total of 97 present and former users who have contributed code to the project via Subversion since its inception. By contrast, there are 1000 or so developers who worked on Windows 7 alone, organized into 25 teams, with each team averaging 40 developers. In addition, in his presentation at Hackmeeting 2009 in Milan, ReactOS developer Michele C. noted that most of the developers learn about Windows architecture while working on ReactOS and have no prior knowledge.
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