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Nov 95 Tips
Volume Number:11
Issue Number:11
Column Tag:Tips & Tidbits

Tips & Tidbits

By Steve Sisak, Contributing Editor

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


Two Screen MacsBug

One feature of MacsBug that I like is the "swap" command, which lets you leave a second monitor display the debugger (and punches it out of the desktop).

Then, if each DebugStr() ends in ";g" you will get an onscreen log of the last 10 or 15 breaks. I used this to track an elusive bug that would freeze my Mac and trash Macsbug memory... I could see which breakpoint went last.

Matt Slot

Informant in the Menu Bar

I do drivers, and you just plain can’t set a breakpoint in ADB completion routines (freezes the keyboard so MacsBug is worthless!).

So I throw one of the routines below into the routine to see when a piece of code gets executed.

What does it do? It "lights up" a bar (length dependant on screen resolution) in the menu bar. So if you DotToggle(300); you get a flashing short line in the menu bar.

[Leave this out of production code! Beware of using this on 24 bit color screens. Make sure that the value of where will not cause anything to be written into the alpha channel. Your best bet is to test with the screen that’s only 8 bits deep. -sgs]

void DotOn(long where) {
   long  *dot;
   dot = (long *)(LMGetScrnBase() + where);
   *dot |= -1;
void DotOff(long where) {
   long  *dot;
   dot = (long *)(LMGetScrnBase() + where);
   *dot &= 0;
void DotToggle(long where) {
   long  *dot;
   dot = (long *)(LMGetScrnBase() + where);
   *dot ^= -1;

Dave Fleck

Informative Cursors

One technique that I have used in the past where dropping into the debugger wasn't an option, and logging wasn't getting flushed in time/took too long, was to create a bunch of cursors numbering 00 - 99, and made a call to set the cursor and return the number of the previous cursor:

short oldCursor = setDebugCursor(15);
    (void) setDebugCursor(oldCursor);

This way when the machine froze, the cursor would tell me what routine it had frozen in.

Tom Kimpton

MPW Editor Primitives

The MPW Command Reference describes 32 editor primitives which may be attached to any key sequence using the SetKey command. While the SetKey command itself is useful, the list of editor primitives alone is useful to MPW script writers.

Though only documented for use with SetKey, editor primitives may be used like any other MPW command: in scripts, command aliases, or AddMenu items. There are a couple of advantages:

1) They only work on the active window and have sharply defined functions, you don’t have to remember any selection expressions or argument lists. Compare:



       Find • "{Active}"

2) They’re fast. I haven’t put a stopwatch to it, but you can see the difference immediately in running a script that uses the primitives and one that doesn’t.

The MPW Command reference lists these 32 primitives, the names of which should be self-explanatory:
































There are also at least five primitives that aren’t in the command reference:






I can understand why Apple might not want users to casually drop into MacsBug from an MPW script, but I wonder why they chose not to document the scrolling commands? Whatever the reason, you should obviously use caution when playing with undocumented features.

Lee David Rimar
Absoft Corporation

Once Bitten Twice Shy

HGetState does not return a valid handle state when you pass it an empty handle (one whose master pointer is NULL). Instead, it returns an error code, so before you call HGetState, be sure to check that the handle isn’t empty and execute an alternate code path if it is.

I was bitten by this because I was using HGetState to determine if a handle was to a resource, and the resourced had been purged, so HGetStaate returned an error code instead of the handle flags and I incorrectly thought the handle wasn’t to a resource.

This is documented in Inside Macintosh, Memory, p. 1-61, but it bears some repeating. Remember to check the error values returned by the toolbox calls.

Eric Schlegel


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