Back when I first had the shock of discovering that VS only supported C89 I remember poking around to figure out why and finding a rant from a higher-up on the compiler team along the lines of "C is a dead language, just use C++." What happened? Did he leave? Did an order come down from on high? Did he mellow with age?
Our current statement of support is that we support enough of C99 to unblock commonly used libraries. We support a large swath of C99 but it is correct that our support isn't complete yet as we prioritized C++ conformance. If there is a C99 library that you need to use that we don't work with, please let me know.
(I'm the VC++ dev manager)
It seems that MSVC really has improved its support for C99, though, and I'm glad about that. What about C11? Does it support _Atomic/stdatomic.h? I can't tell from Googling, and I'm too tired to install the CTP tonight... If it does, it'd be ahead of OS X, hopefully not for long. The C11/C++11 memory model is powerful, way nicer to use than volatile plus OS-specific library functions, and, compared to other features, it's difficult to properly implement as a shim. It's too bad that only C++ can take advantage of it on some platforms.
Existing libraries are one thing, but when writing new code (and yes, I want to write new code in C), one of the big three compilers not supporting a feature is strong motivation not to use it... even if it would be really nice to have. Maybe in the coming years Clang will improve its Windows support enough that I can just tell everyone to swap it in for cl.exe, so I won't have to care about MSVC's standards support, but for now it and GCC are missing a lot of features on Windows.
The C11 Standard Library has not been incorporated into any C++ Standard (not even the C++17 Working Paper as of right now), so we have not attempted to support it. On the other hand, our C++11 <atomic> implementation has been significantly improved in VC 2015.
I believe the choice was to spend time on adding C99 support, vs spending more time on C++ support. C++ had more people asking for it, so it got priority over C99.
