I think most people don't use vanilla MinGW because you'd have to run it on Linux and cross compile. The MinGW toolkit itself (e.g. GCC) isn't targetted at Windows so ironically can't be compiled with MinGW, not even cross compiled - it needs something like Cygwin as in MSYS2. But you're right that it does provide unistd.h and I'm surprised about that. To be honest I was going to say in my comment that it doesn't but I fact checked first! I'm not sure that the bits missing count as "fundamental", I think they're all quite new bits rather than core Win32 APIs. But maybe I'm just showing my age.
Anyway, I'm sorry I've distracted away from your core point: as you've said, despite its name, it seems MinC just provides a user-mode API layer, just like Cygwin does.
No you don't, MinGW(-w64) targets windows directly (with MinGW statically linked in). I've built a Windows->Linux cross-compiler that depends solely on DLLs built-in to Windows (kernel32.dll, MSVCRT.dll, and user32.dll).
Granted that was hundreds of hours, some patches (only 8 lines though), and probably a bit of masochism.
I did of course need MSYS2 command line utilities like make and bison to run the GCC configuration/make scripts.
Although we use the mingw32 version of make along with the cross-compiler which also has no other dependencies (it uses cmd.exe as a shell if you don't have a bash.exe in your PATH).
> Anyway, I'm sorry I've distracted away from your core point
Side-tracks and tangents are the powerhouse of HN comments ;)
As for MinGW and Cygwin (and all the other *nix-on-Windows stuff), I take a bit more lower-level view than what (developer or user) environments they provide, or what tools they use.
In a nutshell, C++ binaries compiled with MinGW are binary-incompatible with C++ binaries compiled with MSVC, because they are built against different binary interfaces: x86_64-pc-windows-gnu (or x86_64-w64-mingw32, which appears to be a synonym) and x86_64-pc-windows-msvc respectively. The former is the C++ Itanium ABI, the latter is a partially-documented MSVC ABI.
Cygwin is an 'ABI' of its own too, because everything needs to be linked against cygwin1.dll, and when it is missing there is a runtime error. WSL1 and WSL2 from an ABI standpoint are indistinguishable—they are x86_64-unknown-linux-gnu.
This is sort of why we can have LLVM-MinGW, which is the entire LLVM suite and only LLVM binutils (see my parent comment for a link), without nary a GNU binary. It's essentially Clang, built to target the MinGW ABI (this can be built on Windows with MSVC, oddly enough).
Speaking of Linux, every glibc upgrade is essentially a different ABI (for some functions) presented to the developer—so much for Linus' assertion of 'don't break userspace'. Even if the kernel doesn't do it, the entirely user-space C library will do it for him. Oh, and musl libc is yet another ABI.
I have delved too far into the weeds of cross-compilation, and as a result I can't help but absolutely despise everything about the C and C++ ecosystem.
Anyway, I'm sorry I've distracted away from your core point: as you've said, despite its name, it seems MinC just provides a user-mode API layer, just like Cygwin does.