install_int_ex - Adds or modifies a timer. Allegro game programming

NAME

       install_int_ex  -  Adds  or  modifies a timer. Allegro game programming
       library.

SYNOPSIS

       #include <allegro.h>

       int install_int_ex(void (*proc)(), int speed);

DESCRIPTION

       Adds a function to the list of user timer handlers or, if it is already
       installed,  retroactively  adjusts  its  speed (i.e makes as though the
       speed change occurred precisely at the last tick). The speed  is  given
       in  hardware  clock ticks, of which there are 1193181 a second. You can
       convert from other time  formats  to  hardware  clock  ticks  with  the
       macros:

          SECS_TO_TIMER(secs)  - give the number of seconds between
                                 each tick
          MSEC_TO_TIMER(msec)  - give the number of milliseconds
                                 between ticks
          BPS_TO_TIMER(bps)    - give the number of ticks each second
          BPM_TO_TIMER(bpm)    - give the number of ticks per minute

       There can only be sixteen timers in use at a time, and some other parts
       of Allegro (the GUI code, the mouse pointer display  routines,  rest(),
       the  FLI player, and the MIDI player) need to install handlers of their
       own, so you should avoid using too many at the same time. If  you  call
       this   routine   without  having  first  installed  the  timer  module,
       install_timer() will be called automatically.

       Your function will be called by the Allegro interrupt handler  and  not
       directly  by  the  processor, so it can be a normal C function and does
       not need a special wrapper. You should be aware, however, that it  will
       be  called in an interrupt context, which imposes a lot of restrictions
       on what you can do in it. It should not use large amounts of stack,  it
       must  not  make  any  calls  to  the  operating  system,  use C library
       functions, or contain any floating point code, and it must execute very
       quickly.  Don't  try to do lots of complicated code in a timer handler:
       as a general rule you should just set some flags and respond  to  these
       later in your main control loop.

       In  a  DOS protected mode environment like DJGPP, memory is virtualised
       and can be swapped to disk. Due to the non-reentrancy of DOS, if a disk
       swap  occurs  inside an interrupt handler the system will die a painful
       death, so you need to make sure you lock all the memory (both code  and
       data)  that  is  touched  inside  timer  routines.  Allegro  will  lock
       everything it uses, but you are responsible for  locking  your  handler
       functions.   The   macros   LOCK_VARIABLE  (variable),  END_OF_FUNCTION
       (function_name),    END_OF_STATIC_FUNCTION     (function_name),     and
       LOCK_FUNCTION  (function_name)  can  be used to simplify this task. For
       example, if you want an interrupt handler  that  increments  a  counter
       variable, you should write:

          volatile int counter;

          void my_timer_handler()
          {
             counter++;
          }

          END_OF_FUNCTION(my_timer_handler)

       and in your initialisation code you should lock the memory:

          LOCK_VARIABLE(counter);
          LOCK_FUNCTION(my_timer_handler);

       Obviously  this  can get awkward if you use complicated data structures
       and call other functions from within your handler, so you should try to
       keep your interrupt routines as simple as possible.

RETURN VALUE

       Returns  zero  on  success, or a negative number if there is no room to
       add a new user timer.

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

SEE ALSO