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July 90 - Macintosh Q & A

July 90 - Macintosh Q & A


Q How can I keep track of a file the next time my application is launched?

A Technical Note #238, Getting a Full Pathname, documents the recommended method for "remembering" a file's location.

. . . you should remember the DirID of the directory the file is in along with its name. This way, you will still be able to find your file even if the directory has been moved. Under System 7.0 or later, save the file's unique 32-bit ID number as well, so that you can also find the file even if its name has changed.

To remember a file's location, keep the volume name, DirID, and filename. This information is all you need to locate any file. Standard File returns the DirID of the file in CurDirStore or the wdRefNum in the vRefNum field of the reply.record. Note that Technical Note #238 mentions how to get a file's DirID while in Standard File. Given the working directory, you can find its vRefNum and DirID by calling _GetWDInfo. Refer to Inside Macintosh, volume IV. Volume references and working directories are dynamic; they change every time the system is booted, so you cannot use the vRefNum or wdRefNum. Typically, the volume name and filename are not changed. The DirID will not change unless the user deletes the folder. Renaming the folder does not change its DirID.

First ask the user to locate the file by calling SFGetFile. Keep the volume name, DirID, and filename for this file. The next time you want to locate the file, use this same information. If you do not find the file, then again call SFGetFile asking the user to locate it.

DTS has an example application, SC.018.StdFile, which you may find helpful. You can find this in the Sample Code folder on the enclosed Developer Essentials disc.

Q How can I determine the size of my application's MultiFinder partition?

A It's really difficult to find the exact size of the memory partition that the application is running under. If it can be determined, I doubt that the effort would be worth the trouble. I think the real concern you have is the size of the available stack and heap, but not the entire partition. Since there is little that an application can do to change its partition size (except to change the 'SIZE' resource and then force a relaunch), the real concern would be to find the size of the available stack and heap. Included in the application's partition are the application parameters, jump table, application globals, and QuickDraw globals. The size of the partition is not easily determined. The only portions of an application's memory use that are adjustable at run time are the stack and the heap.The stack and heap sizes are fixed within the boundaries of the entire application partition. Increasing one decreases the other. There are Memory Manager calls to change the size of the heap. To increase the stack size, you decrease the heap's size.

Q In earlier versions of the Chooser, there was a limit of 16 volumes per server for AppleShare servers. Has this limit changed in System 6.0.4?

A The limit of 16 volumes per server in the Chooser has not changed with System 6.0.4. We hope to have a new version of the Chooser for System 7.0.

Q How do I force the Finder to update its windows after my application has changed a file's FndrInfo?

A There is no direct way to tell the Finder to update the desktop. The Finder will synchronize the desktop file's appearance after it detects that the volume's modification date has changed. Whenever you create or delete a file, or move it to another folder, the hierarchical file system (HFS) will change the modification date of the volume and that folder. When the Finder has noticed the volume's modification date has changed, it begins scanning about once every 10 seconds for changes in all of the open folders. Changing the file's FndrInfo or renaming it is not going to change the modification date. As a suggestion for an installer program, you can initially create a temporary file. Once all the files are installed you can delete the temporary file. Deleting this temporary file as a last step will cause the Finder's window to be updated.

Q My little application has two handles in memory that have been allocated. I want to lock one handle high in memory and the other one low in memory. I noticed that the Mac toolbox has the functionality to lock a handle high (MoveHHi); however, I did not notice any routine that would move the block low in memory, before a lock. I'm looking for a MoveHLow routine. Does one exist? If not, how would I go about doing this?

A There is no similar functionality for locking a handle low. The best way to go about doing this is to use NewPtr, which automatically allocates the block as low as possible. Of course, it's not a handle, but it's still a locked block as low in the heap as possible.

Another way to do this is to use ResrvMem which, as Inside Macintosh, volume 2, page 39 says, "will try every available means to place the block as close as possible to the bottom of the zone, including moving other blocks upward, expanding the zone, or purging blocks from it." Then make your call to NewHandle with the same size as requested in ResrvMem. That'll allocate the handle as low as possible.

Q How can I support multiple HFS partitions on a SCSI device?

A If at all possible, avoid trying to support partitions. We'll warn you up front that an ejectable drive that contains multiple HFS partitions is not going to be anything less than difficult. You'll be betteroff not attempting to support multiple HFS partitions. It greatly complicates the code, and there are user interface problems too. What if the user ejects one of the partitions? What should happen? This is technically difficult for the driver to handle.

If the user ejects a partition, then the driver might eject the media and mark all of its remaining partitions as off-line. If the user drags a partition to the trash, this should unmount only that partition (but then how would the user unmount the entire media?). The remaining partitions should be marked off-line and the user will see them as gray icons on the desktop. If users want to access one of these partitions, they'll get the Disk Switch alert. They need to insert the proper cartridge and the device will then post a disk insert event for every partition (because it cannot determine exactly which partition is really needed). This will again bring all partitions back. The trap _Offline should take care of all this for you, but it cannot be called at interrupt time. Therefore, the driver will need to use accRun calls to use _OffLine.

Again, the system doesn't support multiple HFS partitioned drives. It only expects to find one HFS partition on a volume. The system will attempt to read from the first HFS partition and then stop. If the first one is not bootable, then that device cannot be a startup device. If you attempt to put more than one partition on a device, then you have to perform additional hacks to mount them. Be warned that hacking this feature into your drive involves a compatibility risk.

All the work will be up to the driver. It will have to find the extra partitions and mount them. Each partition will have a drive queue entry having each element reference the same driver. When your driver's open routine is called, you call _AddDrive for each partition. This calls _Enqueue and installs each element into the drive queue. Once the driver is closed, you should remove each of the queue elements with _Dequeue.

Q I would like to write James Brown in jail, but now that he is on work release, where do I write?

A You can write the Godfather of Soul at
Lower Savannah Work Center
Route 4, Box 50
Aiken, SC 29801

Brown is serving concurrent six-year and six-year-and-three month terms for his involvement in a wild, two-state car chase in September of 1988. He won't be eligible for parole until 1992.

These questions and answers are compiled by the Macintosh Developer Technical Support group. *


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