Linux game programming An introduction to the use of interval timers and asynchronous input notifications

Our prior animation demo We achieved the illusion of “smooth” and “flicker-free” animation, by synchronizing drawing-operations with Vertical Retrace But more is needed in a game that’s “fun” Some deficiencies in our ‘animate1’ demo: –Ball movements tied to Vertical Retrace –The viewer lacked any real-time control How can we overcome these limitations?

Decoupling ‘move’ from ‘draw’ Our program-loop had used logic like this: do { vsync();// delay until start of the next retrace hide_ball(); // “erase” the ball move_ball();// adjust its location show_ball();// “redraw” the ball --count;// decrement a counter } while ( count > 0 );

Linux provides ‘interval timers’ #include struct itimervalitval, itold; itval.it_value.tv_sec = 2; itval.it_value.tv_usec = 0; itval.it_interval.tv_sec = 0; itval.it_interval.tv_usec = 10000; setitimer( ITIMER_REAL, &itval, &itold ); (See the ‘man’ page for additional details)

SIGALRM When timer “expires” our application gets notified, by being sent a signal from Linux Normally an application gets terminated if the SIGALRM signal is delivered to it But we can alter that default behavior, by installing a ‘signal-handler’ that we design We can ‘move-the-ball’ when SIGALRM is received, regardless of Vertical Retrace

Our signal-handler void on_alarm( int signum ) { // modify these global variables ball_xcoordinate += xincrement; ball_ycoordinate += yincrement; } // The ‘signal()’ function “installs” our handler signal( SIGALRM, on_alarm);

Main program-loop “revised” We can now omit ball-movement in main loop: do { vsync();// delay until start of the next retrace hide_ball(); // “erase” the old ball oldx = newx; oldy = newy;// remember new position show_ball();// “redraw” the new ball --count;// decrement a counter } while ( count > 0 ); Ball-movement is managed by ‘signal-handler’

Giving the user control Linux supports “asynchronous” terminal i/o We can reprogram the terminal console so a SIGIO signal will be sent to our program whenever the user decides to press a key And we can install a ‘signal-handler’ of our own design that executes if SIGIO arrives This will allow a user to “control” our game

‘Noncanonical’ terminal i/o We already learned how to reprogram the terminal to allow “raw” keyboard input #include struct termiostty; tcgetattr( 0, &tty );// get tty settings tty.c_lflag &= ~( ICANON | ECHO | ISIG ); tty.c_cc[ VMIN ] = 1; tty.c_cc[ VTIME ] = 0; tcsetattr( 0, TCSAFLUSH, &tty );// install

Handling a key-press Here’s a ‘simple’ signal-handler that lets a user decide to terminate our program (by hitting the -key) instead of the program itself deciding to quit when a counter reaches zero void on_input( int signum ) { intinch = 0; read( 0, &inch, 4 ); if ( inch == ESCAPE_KEY ) done = 1; }

Enabling ‘asynchronous’ I/O Now we need to install our signal-handler, specify which program will receive SIGIO, then enable the delivery of these signals signal( SIGIO, on_input ); fcntl( 0, F_SETOWN, getpid() ); int flagval = fcntl( 0, F_GETFL, NULL ); flagval |= O_ASYNC;// turn on flag-bit fcntl( 0, F_SETFL, flagval );

Program-loop revised again done = 0; do{ oldx = xloc, oldy = yloc;// remember draw_ball();// use current location vsync();// await next retrace hide_ball();// erase previous ball } while ( !done ); // ‘xloc’, ‘yloc’, and ‘done’ get changed by handlers

Enhancment: more user-control In ‘pong’ game the user moves a ‘paddle’ The paddle can only be moved left or right Lower wall of the playing-court is removed Ball will “escape” unless it hits the paddle Keyboard has left and right “arrow keys” Our input signal-handler could “move” the paddle whenever a user hits an arrow-key

In-class exercise #1 Before you remove the game-court’s lower wall, see if you can implement the paddle- movements (left or right) in response to a user’s hitting the left-arrow or right-arrow You will need to investigate the numerical code-sequence that Linux generates when a user hits one of these arrow-keys Our ‘rawtty.cpp’ application may be useful!

In-class exercise #2 For brevity, our ‘animate2’ demo-program removed the capability of users controlling the speed of the ball-movements (with an argument supplied on the command-line) Can you devise a way for users to exert “real-time” control over the ball’s speed by pressing keys while the program is running (for example, the ‘+’ key and the ‘-’ key)?