As -[NSURL getResourceValue:forKey:error:] is documented, "If this method
returns YES and the value is populated with nil, it means that the resource
property is not available for the specified resource, and that no errors
occurred when determining that the resource property was unavailable." So YES
and nil is a valid response.
The NSURLVolumeMaximumFileSizeKey property value comes from two sources:
pathconf() with _PC_FILESIZEBITS, or from getattrlist() from the
ATTR_VOL_CAPABILITIES attribute and the VOL_CAP_FMT_2TB_FILESIZE capability.
Here's code (not the real code but the exact same algorithm) that shows how the
value is calculated:
int getMaxFileSize(const char *path, int64_t *maxFileSize)
{
int result;
struct statfs statfsBuf;
// get the file system's mount point path for the input path
result = statfs(path, &statfsBuf);
if ( result == 0 ) {
long fileSizeBits = pathconf(statfsBuf.f_mntonname, _PC_FILESIZEBITS);
if (fileSizeBits == -1) {
// if _PC_FILESIZEBITS isn't supported, check for
VOL_CAP_FMT_2TB_FILESIZE
bool fileSystemSupports2TBFileSize = false;
// get the supported capabilities
struct attrlist attrList;
struct volCapabilitiesBuf {
u_int32_t length;
vol_capabilities_attr_t capabilities;
} __attribute__((aligned(4), packed));
struct volCapabilitiesBuf volCaps;
memset(&attrList, 0, sizeof(attrList));
attrList.bitmapcount = ATTR_BIT_MAP_COUNT;
attrList.volattr = ATTR_VOL_INFO | ATTR_VOL_CAPABILITIES;
result = getattrlist(statfsBuf.f_mntonname, &attrList, &volCaps,
sizeof(volCaps), 0);
if ( result == 0 ) {
fileSystemSupports2TBFileSize =
((volCaps.capabilities.capabilities[VOL_CAPABILITIES_FORMAT] &
VOL_CAP_FMT_2TB_FILESIZE) &&
(volCaps.capabilities.valid[VOL_CAPABILITIES_FORMAT] &
VOL_CAP_FMT_2TB_FILESIZE));
}
if ( fileSystemSupports2TBFileSize ) {
// use Supports2TBFileSize
*maxFileSize = LONG_LONG_MAX;
}
else {
// otherwise, we don't know
*maxFileSize = -1LL;
}
}
else if ( fileSizeBits > 64 ) {
// off_t is signed long long, so it cannot be more than
LONG_LONG_MAX
*maxFileSize = LONG_LONG_MAX;
}
else if ( fileSizeBits < 32 ) {
// POSIX spec says 'Minimum Acceptable Value: 32' for FILESIZEBITS
*maxFileSize = INT_MAX;
}
else {
// 32...64 bits: shift off the bits we don't need
*maxFileSize = (int64_t)((u_int64_t)0xffffffffffffffffLL >>
(u_int64_t)(65 - fileSizeBits));
}
}
return result;
}
if NSURLVolumeMaximumFileSizeKey is returning nil with no errors, maxFileSize
is -1LL, and the only way maxFileSize will be -1LL is if pathconf() returned an
error, and the file system is saying VOL_CAP_FMT_2TB_FILESIZE is not valid and
set (capabilities).
Hope that helps™
- Jim
> On Mar 31, 2017, at 2:33 PM, Scott Talbert <s...@techie.net> wrote:
>
> Hello,
>
> Can anyone tell me why calling NSURL -getResourceValue:forKey:error: with key
> NSURLVolumeMaximumFileSizeKey would return a nil resource value for a given
> volume? The function returns YES and error is nil also.
>
> I can see from running dtruss that it in turn calls getattrlist(). If I call
> getattrlist() myself on the same volume, I can see the bits I would think it
> is looking at are set correctly - VOL_CAP_FMT_2TB_FILESIZE.
>
> Thanks,
> Scott
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