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Technical Note FL34
VCBs and Drive Numbers : The Real Story
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The top of page IV-178 in The File Manager chapter of Inside Macintosh
in attempts to explain the behavior of two fields in a volume control block
when the corresponding disk is offline or ejected. Due to the fact that a
little bit is left unsaid, this paragraph is rather misleading. The two fields
in question are vcbDrvNum and vcbDRefNum (referred to as
ioVDrvInfo and ioVDRefNum in C and Pascal).
PBHGetVInfo can be used to access these fields.
[Jan 01 1987]
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Offline
When a mounted volume is placed offline, vcbDrvNum is cleared
and vcbDRefNum is set to the two's complement of the drive
number. Since drive numbers are assigned positive values (starting with one),
this will be a negative number. If vcbDrvNum is zero and
vcbDRefNum is negative, you know that the volume is offline.
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Ejected
When a volume is ejected, vcbDrvNum is cleared and vcbDRefNum
is set to the positive drive number. If vcbDrvNum is zero and vcbDRefNum
is positive, you know that the volume is ejected. Ejection implies being offline. There is
no such thing as "premature ejection".
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Summary
online offline ejected
vcbDrvNum >0 (DrvNum) 0 0
vcbDRefNum <0 (DRefNum) <0 (-DrvNum) >0 (DrvNum)
Please refrain from assuming anything about a VCB queue element beyond what is
documented in Inside Macintosh, and don't expect it to always be 178 bytes
in size. It grew when we went from MFS to HFS, and it may grow again. It's safest
to use calls like PBHGetVInfo to get the information that you need.
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References
The File Manager
Technical Note M.DV.DrvQEl -- Drive Queue Element Format
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