Q: I have an application that needs
to report a disk's size and total free space available. How
do I ensure that I get the correct information on all
systems?
A: The answer to getting the correct
volume size depends on what version of the Mac OS you are
running and what calls are implemented for finding the
volume size.
If all you want is a call that "does the right thing"
then look no further than MoreFiles'
XGetVInfo function. If, however, you would like
an explanation of what to do for all of the cases, then read
on.
There are three ways of getting this information:
- Call
PBXGetVolInfo;
- Walk the Volume Control Block (VCB) list; or
- Call
PBHGetVInfo.
You can determine the best method by calling
Gestalt with the gestaltFSAttr
selector, and checking the
gestaltFSSupports4GBVols and gestaltFSSupports2TBVols
bits.
PBXGetVolInfo
If the gestaltFSSupports2TBVols
bit is set, then you should call PBXGetVolInfo.
PBXGetVolInfo is backwardly
compatible with small volumes, and works for all volumes
sizes currently supported by the Mac OS.
If you are writing PowerPC code, Q&A
FL07 has the needed glue code to call PBXGetVolInfo
from PowerPC code.
The PBXGetVolInfo call works
almost exactly as PBHGetVInfo.
However, it uses a XVolumeParam
structure, rather than a HVolumeParam
structure. The two structures are the same, except that the
XVolumeParam structure has two
additional fields to contain the extra information needed to
return information about volumes larger than 2 GB, and it
uses a previously unused field in the
HVolumeParam structure. The
filler2 field in HVolumeParam is
the ioXVersion field in the XVolumeParam
structure (make sure you set it to 0 for this version
of PBXGetVolInfo). The extra fields in the
XVolumeParam structure are:
UnsignedWide ioVTotalBytes; /* total number of
bytes on volume */
UnsignedWide ioVFreeBytes; /* number of free bytes on volume
*/
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Note:
If you care about the number of allocation
blocks, you will have to calculate that value
yourself because ioVNmAlBlks
is still an unsigned short in the XVolumeParam
structure. An HFS+ volume can have a many more than
65535 allocation blocks, so this value can be wrong.
By using ioVTotalBytes
and ioVAlBlkSiz you can
calculate the correct number of allocation blocks
(ioVTotalBytes/ioVAlBlkSiz
= allocation block size).
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Volume Control Blocks
If the gestaltFSSupports2TBVols
bit is not set, but the
gestaltFSSupports4GBVols bit is set, then you
will want to walk the VCB list to get the correct
information.
System 7.5 introduced support for volumes larger than 2
GB, up to a maximum of 4 GB. However,
PBHGetVInfo does not report volume sizes larger
than 2 GB. For compatibility reasons, the System 7.5
PBHGetVInfo call was changed to pin the number
of allocation blocks and free allocation blocks reported so
as to always be 2 GB or less. This change was made because a
significant number of programs use signed math to determine
the volume size and free space. They do not work if the real
values are returned.
For example, if the allocation block size is
0xFE00 (the largest possible under System 7.5),
the total number of allocation blocks and number of free
allocation blocks is pinned to 0x8102.
The volume's VCB still contains the real values, so code
that needs the real values can still get to them via the VCB
directly.
See the DTS sample code MoreFiles
for a routine (HGetVInfo) that shows how to
access the VCB to get the actual number of allocation
blocks.
PBHGetVInfo
The last option should be used only if you are running on
a Macintosh where Gestalt returns that neither
the gestaltFSSupports4GBVols bit nor the
gestaltFSSupports2TBVols bit is set. In this
case, call PBHGetVInfo to get the volume
information. This is the most basic way of determining a
volume size, and this method works on all Macintoshes with HFS support
and on all volumes (though it does not return the correct
information for volumes greater than 2 GB in size).
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