All Databases MacTech Vol 11-1995

Jun 95 Tips

Volume Number: 11

Issue Number: 6

Column Tag: Tips & Tidbits

Tips & Tidbits

By Neil Ticktin, Editor-in-Chief with special thanks to Steve Sisak

Note: Source code files accompanying article are located on MacTech CD-ROM or

source code disks.

TIP OF THE MONTH

GetKeys From Within MacsBug

There is a routine in the Macintosh toolbox called GetKeys. This routine is great

for game programmers who want a reasonably fast way to read the keyboard, without

using (slower) events. The problem for C and C++ programmers using this routine is

that the KeyMap type is a Pascal packed array. Each bit of the packed array is

designated to a certain key, the bit is set to 1 if the key is down, and set to zero if the

key is up. This array takes up 16 bytes (128 bits). C cannot access the elements of the

packed array like a normal array, so the programmer has to mask out some bits to get

the result that he/she wants. There is a desk accessory named “GetKeys,” that is made

just for this case. The problem is, you might not be on a machine with that program on

it.

Good thing MacsBug is able to help us. Here is how you locate the bit for the letter

“M”: go into MacsBug and type in “dm KeyMap”, but don’t hit return just yet. Now

strike the escape key, this should swap the screen. Press and hold down the letter “M”

on your keyboard, this should swap the screen back. Now, while still holding down

“M”, press return. This is what you should see:

Displaying memory from 0174:

00000174 0000 0000 0040 0000 0000 0000 0000 0000

•••••@••••••••••

The number “00000174” is the address of the KeyMap global variable. The next set of

numbers “0000 0000” is the first element of the C version of the array, in other

words it’s: “keyMap[0]”. The next set of numbers “0040 0000” is the second

element of the array, keyMap[1]. The next group of 8 hexadecimal digits is the third

element (keyMap[2]), and the last group of 8 hexadecimal digits is the fourth element

(keyMap[3]). The series of bullets is what the array looks like in ASCII form. In the

second group (“0040 0000”) there is a 4 in the midst of all of those zeros. This is

the bit that is set to 1 whenever the “M” key is held down. So, to see if the “M” is

down from within C we will do this:

KeyMap keyMap;

GetKeys( keyMap );

if( keyMap[1] & 0x00400000 )

{  DoMKeyDown();
}

That’s all there is to it!

Macneil Shonle, Yarmouth Maine

MacneilS@aol.com

Almost, But Not Quite

In his tip (MacTech, March 1995 issue, page 67), Jeff Beeghly assumes items

#1 and #2 to be respectively the default and the cancel items. That is not always true:

in fact these may even change between subsequent calls to the same dialog/alert if the

programmer used “SetDialogDefaultItem” and “SetDialogCancelItem” before calling

ModalDialog or set the “ALRT stage info” in ResEdit for the alert box. The safest way to

find the default item is to look at the “aDefItem” field of the DialogRecord that is

passed to the the custom modal filter procedure.

Moreover, I think it is best to let the original filter procedure track standard

events we’re not interested in, by calling it via “StdFilterProc” from our custom

filter proc and then returning its return value to the application.

- Luigi Belverato

Referenced by (3):