Subject: Disabling or capturing the break key

1. *****
 Q: I don't want the Break key to be able to interrupt my TP
programs. How is this done?
 Q2: I want to be able to capture the Break key in my TP program.
How is this done?
 Q3: How do I detect if a certain key has been pressed? (Often, how
do I detect, for example, if the CursorUp key has been pressed?)
 Q4: How do I detect if a cursor key or a function key has been
pressed?

 A: This set of frequently asked questions is basically a case of
RTFM (read the f*ing manual). But this feature is, admittedly, not
very prominently displayed in the Turbo Pascal reference. (As a
general rule we should not use the newsgroups as a replacement for
our possibly missing manuals, but enough of this line.)
   I'll only explain Q and Q2. The other two, Q3 and Q4 should be
evident from the example code.
   There is a CheckBreak variable in the Crt unit, which is true by
default. To turn it off use
     uses Crt;
     :
     CheckBreak := false;
     :
Besides turning off break checking this enables you to capture the
pressing of the break key as you would capture pressing ctrl-c. In
other words you can use e.g.
     :
  procedure TEST;
  var key : char;
  begin
    repeat
      if KeyPressed then
        begin
          key := ReadKey;
          case key of
             #0 : begin
                    key := ReadKey;
                    case key of
                      #59 : write ('F1 ');
                      #72 : write ('CursUp ');   { Detecting these }
                      #75 : write ('CursLf ');   { is often asked! }
                      #77 : write ('CursRg ');
                      #80 : write ('CursDn ');
                      else write ('0 ', ord(key), ' ');
                    end; {case}
                  end;
             #3 : begin    {ctrl-c or break}
                    writeln ('Break');
                    halt(1);
                  end;     { Terminate the program, or whatever }
            #27 : begin
                    write ('<esc> ');
                    exit;  { Exit test, continue program }
                  end;
            else write (key, ' ');
          end; {case}
        end; {if}
    until false;
  end;  (* test *)
     :
IMPORTANT: Don't test the ctrl-break feature just from within the TP
IDE, because it has ctlr-break handler ("interceptor") of its own
and may confuse you into thinking that ctrl-break cannot be
circumvented by the method given above.
  The above example has a double purpose. It also shows the
rudiments how you can detect if a certain key has been pressed. This
enables you to give input without echoing it to the screen, which is
a later FAQ in this collection.
  This is, however, not all there can be to break checking, since
the capturing is possible only at input time. It is also possible to
write a break handler to interrupt a TP program at any time. For
more details see Ohlsen & Stoker, Turbo Pascal Advanced Techniques,
Chapter 7. (For the bibliography, see FAQPASB.TXT in this same FAQ
collection).

 A2: This frequent question also elicits one of the most frequent
false answers. It is often suggested erroneously that the relevant
code would be
  uses dos;
  SetCBreak(false);
This is not so. It confuses MS-DOS and TP break checking with each
other. SetCBreak(false) will _*NOT*_ disable the Ctrl-Break key for
your Turbo Pascal program. What it does is "Sets the state of
Ctrl-Break checking in DOS". SetCBreak sets the state of Ctrl+Break
checking in DOS. When off (False), DOS only checks for Ctrl+Break
during I/O to console, printer, or communication devices. When on
(True), checks are made at every system call."
   This item goes to shows how important it is carefully to check
one's code and facts before claiming something.

 A3: Using the "CheckBreak := false;" method is not the only
alternative, however. Here is an example code for disabling
Ctrl-Break and Ctrl-C with interrupts
  uses Dos;
  var OldIntr1B : pointer;  { Ctrl-Break address }
      OldIntr23 : pointer;  { Ctrl-C interrupt handler }
      answer    : string;   { For readln test }
  {$F+}
  procedure NewIntr1B (flags,cs,ip,ax,bx,cx,dx,si,di,ds,es,bp : word);
            Interrupt;
  {$F-} begin end;
  {$F+}
  procedure NewIntr23 (flags,cs,ip,ax,bx,cx,dx,si,di,ds,es,bp : word);
            Interrupt;
  {$F-} begin end;
  begin
    GetIntVec ($1B, OldIntr1B);
    SetIntVec ($1B, @NewIntr1B);   { Disable Ctrl-Break }
    GetIntVec ($23, OldIntr23);
    SetIntVec ($23, @NewIntr23);   { Disable Ctrl-C }
    writeln ('Try breaking, disabled');
    readln (answer);
    SetIntVec ($1B, OldIntr1B);    { Enable Ctrl-Break }
    SetIntVec ($23, OldIntr23);    { Enable Ctrl-C }
    writeln ('Try breaking, enabled');
    readln (answer);
    writeln ('Done');
  end.
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