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Sep 92 Tips
Volume Number:8
Issue Number:5
Column Tag:Tips & Tidbits

Tips & Tidbits

By Neil Ticktin, Editor

This column is your opportunity to spread the word about little bits of information that you find out about. These tidbits can be programming related or they can be user tips that are particularly useful to programmers.

MacTutor will credit, in subscriptions or orders, $25 for every tip used, and $50 for the Tip of the Month. To submit a tip, send in a letter to the magazine.

Remember, AppleLink is our preferred communication method, but feel free to send something via America Online, GEnie or US Mail. If you do send snail mail, enclose a printed copy and a disk copy of the letter so that it does not have to be retyped.

Let us know what you think. Remember, this is your magazine. Try ‘em out and let us know...

Tip of the Month

When you have several applications running at once, and you hit the interrupt button, you will break into the application that happens to be executing at that time, which is not necessarily the one you want to debug.

To choose which application you break into, make sure that application is frontmost. Now hold down the mouse button somewhere over the menu bar (an empty portion will do), and with your other hand, press the interrupt button. You will end up in the desired application, somewhere in the middle of a call to MenuSelect.

Lawrence D’Oliveiro, Computer Services Dept

University of Waikato, Hamilton, New Zealand


The Make tool in MPW differs from the UNIX original in that it only generates the build commands; it cannot execute them. Apple provides a script called BuildProgram, that invokes Make, captures the output in a temporary script file, and executes it. However, when you’re still at the stage of flushing out syntax errors from your code, it is quicker not to use BuildProgram. Instead, use Make, and capture its output in a temporary window, such as the Worksheet window. Select the generated commands and execute them.

When you hit a syntax error, find the compilation command where the error was encountered, and delete all the commands prior to this-that is, all the commands that were successfully completed. Find and fix the syntax errors in your source file. Now instead of reinvoking BuildProgram or Make, simply reselect the leftover build commands, and reexecute them. Continue in this fashion, deleting the completed commands each time you hit an error, until you’ve completed the build.

If you’ve got nothing else but build commands in the temporary window and you’re using MPW version 3.2 or later, you can quickly select the commands to be deleted, once you’ve found the beginning of the command where the error occurred, by typing command-option-shift-uparrow to select from the current position to the beginning of the file.

- Lawrence D’Oliveiro, New Zealand


Many Makefiles are written with an initial rule that looks like this:

Everything ƒ Prog1 Prog2 Prog3

where Prog1, Prog2 and so on are the actual programs comprising the project. This is done so that if you invoke Make without any arguments, it will look for the first target by default, and putting this rule first will cause it to build the entire project.

However, if your project grows with more and more components, this becomes another rule that you have to remember to maintain in your Makefile. A more convenient technique is to invoke Make with the following command:

Make `Make -r`

The inner Make -r command returns a list of the “root” targets (that is, no other targets depend on these); the outer Make command builds all these targets. Now you can get rid of the Everything rule, and just keep adding new rules for the new components of your project, without having to worry about updating any existing rules.

- Lawrence D’Oliveiro, New Zealand


MPW’s Backup tool is very useful for backing up your projects, but things can get slightly complicated if there are too many files to fit on one disk. Without the -a option, Backup will only back up changed files; it won’t copy any files that aren’t already on the current backup disk.

But if you’ve been adding lots of files to your project, deciding which files should go onto which disks-and making sure you’ve backed them all up-can be too much trouble. You can make things much easier if you group your files into subfolders; just make sure that the contents of each subfolder will fit on one backup disk. Then you can use the command form Backup -a -r -level 1; the -r option means to check subfolders (and subfolders of subfolders, and so on), and -level 1 restricts the effect of -a (backup new as well as changed files) so it only applies to subfolders on down, not to files at the top level. This allows you to keep a handful of central files, such as your Makefile, at the top level, and they will only be backed up once.

If you have to move files from one subfolder to another, add the -d option to the Backup command; this will delete the moved files from their old subfolders on the backup disks.

Now, if Duplicate would only preserve the window positions of the icons it copies, my happiness would be complete...

- Lawrence D’Oliveiro, New Zealand


The following pair of commands will do a backup from the current directory onto a folder with the same name on a floppy disk in drive 1:

Set CurFolder "`Directory -q | 
 StreamEdit -d -e 
'/( :)*([¬:]*:)®1 / print ®1'`"

Backup -a -r -level 1 -from 
 : -to `Volumes 1`"{CurFolder}"

This uses the StreamEdit tool which was introduced in MPW 3.2. Put it on a custom menu somewhere in your UserStartup script, then you’ll never have to type it again...

- Lawrence D’Oliveiro, New Zealand


The following line of HyperTalk, executed from HyperCard 2.1, will hide the frontmost application:

DoMenu line 1 of menu "Application"

This works even if executed from a script running in the background! You can try other items in the application menu as well, but this doesn’t work for the “System Help” menu.

The same thing should also work from one of your own applications: call SystemMenu, passing it a menu ID of

-16489 (the application menu) and the appropriate item ID (e.g., 1 to hide the frontmost application, 2 to hide all but the frontmost, and 3 to show all applications).

- Lawrence D’Oliveiro, New Zealand

A 68040 Bug

A few months ago a rather large application I wrote started "hanging" on a Quadra 700. After poking around I tracked the problem to a small assembly language routine which is used to allow main thread and interrupt code to access the same linked list of messages.

The problem arises when the CAS2 instruction is used in the following form:

CAS2.l  Dn:Dn,D1:D0,(A0):(A1)

When the comparison fails, Dn gets updated from the second memory operand, (A1), instead of the first memory operand. Motorola has just verified the bug. A portion of Motorola's response, as relayed to me through DTS, says...

"I am responding to your question on the CAS2 instruction. You have indeed found a bug. Your analysis is correct.

If the compare registers for CAS2 are the same and the comparisons with the memory operands fail, then the FIRST memory operand should be stored in the data register, however, the 68040 incorrectly stores the second memory operand in the data register."

Note that this problem does not exist on the 68020 or the 68030.

- Bob Peters, Fermilab

Omega SANE backpatching!

I spent one entire Sunday to catch the bomb in this code:

(1) <if condition then>
(2)   <push a selector on stack>
(3) <else>
(4)   <push another selector on stack>
(5) _FP68K

On the first call Omega SANE analyzes the code and finds that (4) and (5) constitutes a “normal” SANE call. So, in the name of optimization, those 6 bytes are replaced in runtime by a direct JSR to the actual SANE routine. Imagine the spectacular view on a later occasion when (2) and (5) are executed!

- Sigmund Tveit, Norway


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