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Cygwin is a collection of free software tools originally developed by Cygnus Solutions to allow various versions of Microsoft Windows to act somewhat like a UNIX system. It aims mainly at porting software that runs on POSIX systems (such as GNU/Linux systems, BSD systems, and UNIX systems) to run on Windows with little more than a recompilation. Programs ported with Cygwin work best on Windows NT and Windows 2000, but some may run acceptably on Windows 95 and Windows 98. Cygwin is presently maintained by employees of Red Hat and others.

Cygwin consists of a library that implements the POSIX system call API in terms of Win32 system calls, a GNU development toolchain (such as GCC and GDB) to allow basic software development tasks, and some application programs equivalent to common programs on the UNIX system. It added a port of the XFree86 implementation of the X Window System in 2001.

The package also includes a library called MinGW that works with the native MSVCRT library included with Windows; MinGW has less RAM and disk overhead, operates under a permissive licence, and can link to any software, but it does not implement as much of the POSIX specification as the Cygwin library does.

Red Hat normally licenses the Cygwin library under the GNU General Public License with an exception to allow linking to any free software whose license conforms to the Open Source Definition. (Red Hat also makes available expensive licenses to redistribute programs that use the Cygwin library under proprietary terms.)


Cygwin began in 1995 as a project of Steve Chamberlain, a Cygnus engineer who observed that NT and 95 used COFF as their object file format, and that GNU already included support for x86 and COFF, and the C library newlib; so at least in theory it should not be difficult to retarget GCC and get a cross compiler producing executables that would run on Windows. This proved to be so in practice, and a prototype came up quickly.

The next step was to attempt to bootstrap the compiler on a Windows system, but this required enough emulation of Unix to let the GNU configure shell script run, which requires a shell like bash, which in turn requires fork and standard I/O. Windows includes similar functionality, so the Cygwin library proper just needs to translate calls and manage private versions of data, such as file descriptors.

By 1996, other engineers had joined in, since it was clear that cygwin would be a useful way to provide Cygnus' embedded tools hosted on Windows systems (the previous strategy had been to use DJGPP). It was especially attractive because it was possible to do a three-way cross-compile, for instance to use a hefty Sun workstation to build, say, a Windows-x-MIPS cross-compiler, which was faster than using the PC of the time. Starting around 1998, Cygnus also began offering the Cygwin package as a product of interest in its own right.

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