CUED C Frequently Asked Questions

• I've compiled my primes.c program, but when I run it by typing "primes" nothing happens. - There's a system program called "primes" which by default is being called in preference to your own program. Either rename your program or type "./primes".
• I read that curly brackets are used to make statements into blocks so I did "for(i=0;i<100;{array[i]=i; i++});" to initialise an array - This won't work. "for(i=0;i<100;array[i]=i,i++);" will, but you may as well make "array[i]=i" the body of the loop.
• How can I read characters from the keyboard without waiting if no key have been pressed? This is O/S dependent. Here's a demo of a solution that should work under most versions of Unix

  #include <stdio.h>
  #include <time.h>
  #include <signal.h>
  #include <sys/types.h>
  
  
  #ifndef hpux		/* BSD style */
  #   include <sgtty.h>
  #else hpux		/* HP-UX style (and other System V systems) */
  #   include <termio.h>
  #endif hpux
  
  
  static int tty_mode = 0;
  
  #ifdef TCGETA
      static struct termio orig_tty;
      static struct termio new_tty;
  #else
      static struct sgttyb orig_tty;
      static struct sgttyb new_tty;
  #endif
  
  
  cbreak()
  {
      if (tty_mode == 0)
      {
  #ifdef TCGETA
  	ioctl(0, TCGETA, &orig_tty);
  #else
  	ioctl(0, TIOCGETP, &orig_tty);
  #endif
  	tty_mode = 1;
  	new_tty = orig_tty;
      }
  
  #ifdef ICANON
      new_tty.c_lflag &= ~(ICANON | ECHO);
      new_tty.c_cc[VMIN] = 1;
      new_tty.c_cc[VTIME] = 0;
      ioctl(0, TCSETA, &new_tty);
  #else
      new_tty.sg_flags |= CBREAK;
      new_tty.sg_flags &= ~ECHO;
      ioctl(0, TIOCSETN, &new_tty);
  #endif
  }
  
  
  normal()
  {
      if (tty_mode == 1)
      {
  #ifdef TCSETA
  	ioctl(0, TCSETA, &orig_tty);
  #else
  	ioctl(0, TIOCSETN, &orig_tty);
  #endif
  	new_tty = orig_tty;
      }
  }
  
  
  int key_pressed();
  
  main()
  {
  	int x=0;
  	long int loop=1;
  
  	/* Setup the terminal.  (This is just for a quick example) */
  	cbreak();
  
  	while(loop++) {
  /*
   * Loop is just to let you know that it's still alive.
   */
  		if ( (loop % 1234) == 0)
  			printf("At %ld\n", loop);
  
  		if ( (x=key_pressed()) != -1) {
  			printf("Got one, kp=%c\n",x);
  			fflush(stdout);
  		}
  /*
   * Press 'q' to exit.
   */
  		if ( x == 'q') {
  			normal();
  			exit(0);
  		}
  	}
  }
  
  /*
   *	This function will return -1 if no key is available, or the key
   *	that was pressed by the user.  It is checking stdin, without blocking.
   *
   */
  int key_pressed()
  {
  	int mask=1;
  	static char keypressed;
  	struct timeval waittime;
  	int num_chars_read;
  
  	waittime.tv_sec=0;
  	waittime.tv_usec=0;
  	if (select(1,&mask,0,0,&waittime)) {
  		num_chars_read=read(0,&keypressed,1);
  		if (num_chars_read == 1)
  			return((int)keypressed);
  	}
  	return(-1);
  }