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Technical Q&A

ME07 - Finding the VM Backing Store
(6-February-98)


Q I'm writing a backup program and I'd like to find the VM Storage file, so as to avoid backing it up. How do I do this?

A The best way of finding the current VM Storage file is to use a previously undocumented Gestalt selector: 
enum {
    gestaltVMBackingStoreFileRefNum = 'vmbs'
};

The result of this selector is an file reference number to the active "VM Storage" file. You can convert that reference number to an FSSpec by calling PBGetFCBInfoSync, as shown below:

static OSErr FindVMStorage(FSSpec *fss)
{
    OSErr err;
    long gestaltResult;
    FCBPBRec fcbPB;

    err = Gestalt(gestaltVMBackingStoreFileRefNum, &gestaltResult);
    if (err == noErr) {
        fcbPB.ioNamePtr = fss->name;
        fcbPB.ioVRefNum = 0;
        fcbPB.ioRefNum = gestaltResult;
        fcbPB.ioFCBIndx = 0;
        err = PBGetFCBInfoSync(&fcbPB);
    }
    if (err == noErr) {
        fss->vRefNum = fcbPB.ioFCBVRefNum;
        fss->parID = fcbPB.ioFCBParID;
    }
    return err;
}

-- Quinn "The Eskimo!"
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