Technical Note TB38

Strip With _OpenResFile and _OpenRFPerm

CONTENTS Introduction References Downloadables

This Technical Note discusses a bug in _OpenResFile and _OpenRFPerm which can cause system crashes and what you can do to avoid this problem. [Apr 01 1989]

Introduction

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.

The following code fragments give an example of this bug:

MPW Pascal

    VAR
        fileName : Str255;
        ref : INTEGER;

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

MPW C

    Str255        fileName;
    short int    ref;

    fileName = 'This File';

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))^);

MPW C

    Str255        fileName;
    short int    ref;

    fileName = 'This File';

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.

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References

M.OV.32BitClean

M.ME.StripAddress

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Downloadables