Why some of popular operating system are not support for posix compliant?

Answer 1

Without getting into too much detail, it's largely a choice made by developers of the system whether or not providing a POSIX-compliant or at least POSIX-compatible system is worth the effort.

The aim of POSIX is not to be a standard defining operating systems themselves so much as standard for a suitable environment for source compatibility between different operating systems, primarily those of a Unix-compliant or Unix-like design. I believe full POSIX compliance is a requirement of the Single Unix Specification in turn, the SUS being a standard that actually DOES define what an operating system should be (In this case, to be considered a real UNIX system. Needless to say Windows is nowhere in the neighborhood of SUS compliance.).

The idea behind POSIX is to make it much easier for Unix/Unix-like software developers to write software and have it available for those who provide selections of software for any given Unix platform. Program written for POSIX OS A can be built for POSIX OS B with little or no changes to the source code of the program, usually the only changes needed are bits closer to the metal or usage of libraries not concerned with POSIX compliance.

POSIX does not concern itself or define any standard for binary compatibility. This is not just a matter of scope but the fact that such an application of the standard would only be useful on OS-to-OS ports of a program being made on the same system architectute, which would make POSIX less useful for Unix systems, which are frequently deployed in mixed-architecture environments.

Windows has a POSIX layer available as a download, but generally it's a limited, only halfway-compliant POSIX system. Windows itself is not designed in a way that is generally POSIX-compatible, since it's software architecture is a fundamentally very different thing from Unix. For most Windows users and developers: POSIX compliance is of very little practical use, as Windows' design still requires some very POSIX-unfriendly additions to source code to build correctly on Windows. For Windows to become a POSIX system would require Windows itself to be completely remade in ways that breaks all backwards compatibility with previous versions of Windows, something most users would not like. When open source software is ported to Windows, one generally uses MinGW or tools like cmake that provide build scripts/files for POSIX and non-POSIX systems alike.