Cocoa apis reporting incorrect values for free space, what should I use?

Question

Does anyone know what apis Apple is using for it's Get Info panel to determine free space in Lion? All of the code I have tried to get the same Available Space that Apple is reporting is failing, even Quick Look isn't displaying the same space that Get Info shows. This seems to happen if I delete a bunch of files and attempt to read available space.

When I use NSFileManager -> NSFileSystemFreeSize I get 42918273024 bytes

When I use NSURL -> NSURLVolumeAvailableCapacityKey i get 42918273024 bytes

When I use statfs -> buffer.f_bsize * buffer.f_bfree i get 43180417024 bytes

statfs gets similar results to Quick Look, but how do I match Get Info?

Answer 1

You are probably seeing a result of local Time Machine snapshot backups. The following quotes are from the following Apple Support article - OS X Lion: About Time Machine's "local snapshots" on portable Macs:

Time Machine in OS X Lion includes a new feature called "local snapshots" that keeps copies of files you create, modify or delete on your internal disk. Local snapshots compliment regular Time Machine backups (that are stored on your external disk or Time Capsule) giving you a "safety net" for times when you might be away from your external backup disk or Time Capsule and accidentally delete a file.

The article finishes by saying:

Note: You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed.

It looks like all the programmatic methods of measuring available disk space that you have tried give the true free space value on the disk, not the space that can be made available by removing local Time Machine backups. I doubt command line tools like df have been made aware of local Time Machine backups either.