Technotes

Strip With _OpenResFile and _OpenRFPerm

Technote TB 38 April 1989

Written by: Byron Han April 1989

This Technical Note discusses a bug in _OpenResFile and _OpenRFPerm which can cause system crashes and what you can do to avoid this problem.

The traps _OpenResFile and _OpenRFPerm call some common code in 128K and later ROMs which was affected by some system patches for early print drivers. The problem is that the common code checks an attribute bit in the pointer to the string name to see if it is a dereferenced handle. If the pointer has the resource attribute bit set, the Resource Manager assumes that it is a dereferenced handle and calls _RecoverHandle. This usually works, but when the string is embedded in a code resource, the Resource Manager calls _RecoverHandle with an invalid master pointer.

Note: In MPW C, this bug is not a problem, unless you use either the -b, -b2, or -b3 options, which embed string constants in the code segment. If you use these options, you must deal with this bug.

The following code fragments give an example of this bug:

MPW Pascal

	VAR
	    fileName : Str255;
	    ref : INTEGER;

	BEGIN
	    fileName := 'This File';
	    ref := OpenResFile(fileName);
	END

MPW C

	Str255		fileName;
	short int	ref;

	fileName = 'This File';
	ref = OpenResFile(fileName);

Calling _StripAddress on the pointer to the filename prior to calling _OpenResFile or _OpenRFPerm solves the problem:

MPW Pascal

	VAR
	    fileName : Str255;
	    ref : INTEGER;

	BEGIN
	    fileName := 'This File';
	    ref := OpenResFile(StringPtr(StripAddress(@fileName))^);
	END;

MPW C

	Str255		fileName;
	short int	ref;

	fileName = 'This File';
	ref = OpenResFile((StringPtr)StripAddress((Ptr)fileName));

By always calling _StripAddress before calling _OpenResFile or _OpenRFPerm, you will not have to deal with this problem, life will be good, and you will be able to rest a bit easier.

Further Reference:

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