UNBUFFERED INPUT
You normally can't read input a character at a time from the terminal.  By
default, the terminal is configured to be in line-by-line mode, so that
the editing characters (like the backspace key) can be used by the user
when entering a line.
   
The man page for tty in section 4 of the manual discusses how to change the
terminal settings to do things like put the terminal in character-by-character
mode so that you can read each character as it is typed.  To view this man
page, type "man 4 tty".
   
Below is an example of how to use the functions described in that man page.
It is a function called "keypressed", which turns on character-by-character
mode, then uses another special function call (the FIONREAD ioctl call) to ask
the system how many characters are waiting to be read, then turns off
character-by-character mode (or, actually, puts it back to whatever it was
before the function started); the net result is a deter- mination of whether
or not a key has been pressed.  Note that this function would not work if it
did not turn on character-by-character mode, because FIONREAD would continue
to return 0 until the user hit return.
   
/* Procedure to check for waiting input on the tty.  Does not */
/* actually read the input.                                   */
   
#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <stdio.h>
   
int
keypressed()
{  
     /* These are for ioctl */
     struct sgttyb tty, ntty;
     int ttyset, stat, arg;
     
     ttyset = 0;
   
     /*
      * The TIOCGETP ioctl call gets the tty information structure.
      * See tty(4) for details about the contents of that structure.
      */
     stat = ioctl(0, TIOCGETP, &tty);
     if (stat == -1) {
          perror("ioctl");
          return(-1);
     }
     /* FIONREAD returns the number of characters of waiting input */ 
     stat = ioctl(0, FIONREAD, &arg); 
     if (stat == -1) {
          perror("ioctl");
          return(-1);
     } 

     if (ttyset) {
          /* put the tty characteristics back to their original form */
          stat = ioctl(0, TIOCSETN, &tty);
          if (stat == -1) {
               perror("ioctl");
               return(-1);
          }
     }

     return(arg); 
}