I don't think that's correct. While `malloc` uses `brk` syscall to allocate large memory areas, it uses non-trivial algorithms and data-structures to further divide that areas into smaller chunks which actually returned. Using syscall for every `malloc`/`free` is quite an overhead.
> fopen, FILE
> Wrapper around open, write, read, close.
They're not just wrappers. They implement internal buffering, some transformations (for example see "binary" mode, "text" mode.
> stdatomic implementations
> You can argue, these are wrappers around thread syscalls.
No, they're wrappers around compiler intrinsics which emit specific assembly instructions. At least for any sane architecture.
> I personally consider libc and the compiler (which both make a C implementation) to be part of the OS. I think this is both grounded in theory and in practice. Only in some weird middle ground between theory and practice you can consider them to not be.
C is used a lot in embedded projects. I even think that's the majority of C code nowadays. These projects usually don't use libc (as there's no operating system, so concept of file or process just doesn't make sense). So it's very important to separate C compiler and libc and C compiler must be able to emit code with zero dependencies.
> C is used a lot in embedded projects.
Sure that's a freestanding implementation, which primary distinction is that it doesn't rely on the libc. The notion of the libc being part of the OS in the wider sense, still holds water here, since here no OS corresponds to no libc.