If creating that function for a standard compiler, you need to adhere to the C/C++ specification. The implementation is upto you. If you want to have a look at some of the implementations see this and this.

I wouldn't. It a C primitive function and is written to access the keyboard input buffer, which is generally beyond the compiler's capability.

It would have to be compiler and os specific which is why the C standards did not dictate how the function is to be implemented. You could use non-standard functions in conio.h like kbhit() and getch(), assuming your compiler supports those functions. For 32-bit programs that run under MS-Windows you could use the win32 api console functions. Don't know about MAC or *nix.

getc_unlocked() and getchar_unlocked() functions conform to IEEE Std 1003.1-2001
(POSIX.1). so every *nix (mac is *nix below the hood) should have them as part of the
base system. (in linux, it would be in glibc.)

"getc_unlocked() and getchar_unlocked()functions are equivalent to getc() & getchar()
respectively, except that the caller is responsible for locking the stream with
flockfile(3) before calling them. These functions may be used to avoid the
overhead of locking the stream for each character, and to avoid input being
dispersed among multiple threads reading from the same stream."
-- from freebsd man pages.
i guess most *nix implementations would implement getchar as
flockfile, getchar_unlocked, unflockfile