portable alternative to kbhit() and getch() and system("cls")

Question

I need a way to use kbhit and getch functionality in a portable way. I'm currently developing a simple ascii game and I need to detect if a key is pressed. If it is I need to read it and if it isn't I need to continue without waiting for input. I would prefer not to echo it, but I won't be to picky about that. I think kbhit and getch would be great for this, BUT I'm only allowed to use fully portable code(well at least code for linux, mac and PC, not a lot of other OSes come to mind though). As I understand it the termios, curses, and conio libraries aren't fully implemented on all three OSes I need. I'm at a loss. Every solution I have found uses non-portable code. Is there someway I'm able to write portable functions for this myself? I'm currently including stdio.h, stdlib.h, and time.h. I also need a portable way to clear the screen as I'm currently using system("cls") and system("clear") which must also be changed every time I change the OS, or is the a way I could do an if-else and detect the OS the code is running on to switch between these two statements. Here is a segment of code that has these functions:

    char key = ' ';
    while(1)
    {
        system("cls");
        if (_kbhit())
        {
            key =_getch();
            printf("output: %c", key);
        }    
        else
            printf("output:");
    }

This is essentially what functionality I need in my code, but I can't figure out a portable way to do it, and my teacher requires the code to work on Linux, mac, and pc using standard c and standard libraries. Please help! And no c++ please, we are using c.

EDIT: I don't think ncurses wasn't quite what I was looking for. Someone recommended I use #ifdef to implement these at compile time. I like this solution, but I need some help understanding how to do this on linux and mac as I can only test on windows with my current setup. hopefully I will soon have linux running on my other machine for testing, but OSX has a big price tag with it, so I would appreciate the help. Here's the current code:

 //libraries
#include <stdio.h>      //used for i/o
#include <stdlib.h>     //used for clearing the screen
#include <time.h>       //used to get time for random number generator

//check OS and include necessary libraries
#ifdef _WIN32
    //code for Windows (32-bit and 64-bit, this part is common)
    #include <conio.h>
    #define CLEARSCREEN system("cls")
    #define CHECKKEY _kbhit()
    #define NBGETCHAR getch()

#elif __APPLE__
    //code for mac
    #define CLEARSCREEN system("clear")
    #define CHECKKEY 
    #define NBGETCHAR 

#elif __linux__
    //code for linux
    #define CLEARSCREEN system("clear")
    #define CHECKKEY 
    #define NBGETCHAR 

#else
#   error "Unknown compiler"
#endif

int main()
{
    char key = ' ';
    while(1)
    {
        CLEARSCREEN;
        if (CHECKKEY)
        {
            key=NBGETCHAR;
            printf("output: %c", key);
        }    
        else
            printf("output:");
    }
}

Answer 1

You should look into the portable ncurses library. In addition to many other tools for drawing in the terminal, it provides a keyboard interface which includes a getch() function.