ReactOS

ReactOS has been in development since 1996. As of August 2026, it is still considered to be feature-incomplete alpha software. Therefore, it is recommended by the developers to be used 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 written in C++, such as the ReactOS File Explorer. The project partially implements Windows API functionality and has been ported to the AMD64 processor architecture. ReactOS is part of the FOSS ecosystem so it re-uses and collaborates with many other FOSS projects, most notably the Wine project that presents a Windows compatibility layer for Unix-like operating systems.

History

Early development

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 was founded and coordinated by Yannick Majoros. The project stalled in discussions on 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 inspired reaction to Microsoft's monopolistic position.

In 2002, the ReactOS Foundation was established in Moscow with Maxim Osowski and Aleksey Bragin as executive officers and Vladimir Bragin, Saveliy Tretiakov, and Alexey Ivanov on the board of directors. In 2015, the foundation was liquidated.

Internal audit

In order to avoid copyright prosecution, ReactOS had to be expressly completely distinct and non-derivative from Windows, a goal that needed very careful work. A claim was made on 17 January 2006 by developer Hartmut Birr on the ReactOS developers mailing list (ros-dev) that ReactOS contained code derived from disassembling Microsoft Windows. The code that Birr disputed involved the function BadStack in syscall.S, as well as other unspecified items. Comparing this function to disassembled binaries from Windows XP, Birr argued that the BadStack function was simply copy-pasted from Windows XP, given that they were identical. Alex Ionescu, the author of the code, asserted that while the Windows XP binary in question was indeed disassembled and studied, the code was not merely copy-pasted, but reimplemented; the reason why the functions were identical, Ionescu claimed, was because there was only one possible way to implement the function.

On 27 January 2006, the developers responsible for maintaining the ReactOS code repository disabled access after a meeting was held to discuss the allegations. When approached by NewsForge, Microsoft declined to comment on the incident. Since ReactOS is a free and open-source software development project, the claim triggered a negative reaction from the free software community: in particular, Wine barred several inactive developers from providing contributions and formal high-level cooperation between the two projects remained difficult as of 2006.

In a statement on its website, ReactOS cited differing legal definitions of what constitutes clean-room reverse engineering as a cause for the conflict. To avoid potential litigation, companies sometimes enact a policy where reimplementation based on disassembled code must be written by someone other than the person having disassembled and examined the original code. ReactOS clarified its Intellectual Property Policy Statement requirements on clean room reverse engineering to avoid potential infringement of United States law. An internal source code audit was conducted to ensure that only clean room reverse engineering was used, and all developers were made to sign an agreement committing them to comply with the project's policies on reverse engineering. Contributors to its development were not affected by these events and all access to the software development tools was restored shortly afterward. In September 2007, with the audit nearing completion, the audit status was removed from the ReactOS homepage. Though the audit was completed, specific details were not made public, as it was only an internal effort to ensure compliance with the project's own policies.

Also, the 2004 leaked Windows source code was not seen as legal risk for ReactOS, as the trade secret was considered indefensible in court due to broad spread.

Axel Rietschin, who is a kernel engineer at Microsoft, claimed that he recognized some specific bits in the ReactOS kernel that are unlikely to result from a clean room reimplementation, such as internal data structures and variable names having the exact same name in both ReactOS and the research kernel. Rietschin suggests that the project took source code from the Windows Research Kernel, which was licensed to universities and has been leaked multiple times.

Google Summer of Code participation

Starting in 2006, the ReactOS project participated in several Google Summers of Code. For example, in the GSoC 2011, ReactOS mentored a student project which integrated lwIP into the network stack.

Lobbying for support in Russia

Between 2007 and 2015, Russian ReactOS contributors representing the ReactOS Foundation made efforts at lobbying the Russian federal government. As of April 2019, these efforts have not yielded government support for ReactOS, and the Russia-based ReactOS Foundation has been dissolved since November 2015.

• In 2007, then State Duma member and chairman of Russian Center of Free Technologies, Viktor Alksnis met with project coordinator Aleksey Bragin, who gave a presentation and demonstration of the project, showing ReactOS running with Total Commander and Mozilla Firefox.
• In 2011, a demonstration was given to then president Dmitry Medvedev during a visit to a high school in Verhnerusskoe, Stavropol, attended by one of the development team members.
• On 31 July 2012, Vladimir Putin was also given a demonstration during his visit as President of Russia to Seliger Youth Forum, attended by Marat Karatov, one of the development team members.
• On 1 April 2015, Bragin attended an expert meeting of the Russian Ministry of Communications on reducing Russia's dependency from proprietary software imports. Its minutes name the "creation of an open source operating system based on ReactOS for PCs, laptops and other mobile devices", as well as the "creation of an open source operating system for servers based on ReactOS", placing it second among three named proposals.

Funding campaigns

On 1 May 2012, a €30,000 funding campaign was started to finance additional development projects. At the end of the year, approximately 50% of the funding goal was achieved and it was decided to continue the funding campaign without deadlines. The money went to ReactOS Deutschland e. V.. As the tax law in Germany for this form of a registered voluntary association (Eingetragener Verein) makes it problematic to pay developers directly; indirect possibilities like stipends were evaluated.

Thorium Core Cloud Desktop project

When ReactOS was awarded as Project of the Month on SourceForge in June 2013, a crowdfunding campaign on Kickstarter was announced in an interview with the project's coordinator, Aleksey Bragin. On 23 December 2013 the announced project was revealed as a Kickstarter campaign with the goal of US$120,000 was started. The Thorium Core Cloud Desktop dubbed cloud computing service would use ReactOS as core and could allow the use of Windows compatible applications from mobile devices (like smartphones, tablets), workstations, or any other connected device. On 21 February 2014, fundraising ended short of the target amount, with $48,965 of $120,000 raised, resulting in no transferred money.

ReactOS Community Edition

In April 2014, the ReactOS project announced an Indiegogo campaign to launch ReactOS Community Edition, a version of ReactOS based on the 0.4 release. The flexible funding campaign had a goal of $50,000 with additional stretch goals beyond that. Development of ReactOS Community Edition would be community-centric, with ReactOS users voting and funding to decide which software and hardware drivers the project will aim to support. On 1 June 2014, the flexible crowdfunding campaign on Indiegogo ended, raising $25,141 for the development of the community edition, and the voting process to support hardware and software was started shortly thereafter.

ReactOS Hackfest

The ReactOS project organized a hackfest from 7 to 12 August 2015, in the German city of Aachen. The Hackfest resulted in many features being added to ReactOS.

The second hackfest was organized by the ReactOS project from 14 to 18 August 2017, in the German city of Cologne.

The third and fourth hackfests were organized by the ReactOS project from 16 to 21 August 2018 and from 15 to 20 August 2019 in Berlin respectively.