Technotes

Handles VS Pointers--Identity Crisis

Technote ME 02 September 1987

Revised by: March 1988
Written by: Jim Friedlander September 1987

A handle is a handle and a pointer is a pointer. Applications should avoid embedding non-relocatable objects (that the system assumes will never move) in handles.

In order to avoid fragmentation, some applications embed pointers (non-relocatable memory manager objects) in handles, so that the handles can be moved around as needed. This can cause several problems, especially with the Macintosh II, and should be avoided.

For example, use of a handle to store a GrafPort can be particularly dangerous. A GrafPort must not move between the time that it is opened (OpenPort, NewWindow, NewDialog, etc.) and the time that it is closed (ClosePort, DisposeWindow, DisposDialog, etc.). Color QuickDraw keeps a list of open ports and pointers to them, so, if you create a GrafPort and it moves while still open, Color QuickDraw will (unknowingly) have a pointer to outer space instead of a pointer to a GrafPort. When it needs to use that pointer, it will get hopelessly confused and probably issue a system error to let you know.

As an aside, if you open a port by calling OpenPort or OpenCPort, you should always close the port by calling ClosePort or CloseCPort before calling DisposPtr on the port or you will orphan handles (visRgn, clipRgn and more).

Further Reference:

* QuickDraw
* The Memory Manager

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