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Calculating Free Space on macOS

12th May 2017 | Programming

As a set of interesting programming exercises, I've written up a number of examples to calculate the amount of free space on a given disk running on macOS. The first example is a command line solution, while the other five examples take more programatic approaches.


To easily display the amount of available disk space and how much is free, use the built in utility df which returns the statistics about the amount of free disk space on a specified filesystem. This is a great little utility to use from the terminal or within a script. To get the details on the root drive, run the command df -kP / which will return data formatted like the following:

	Filesystem 1024-blocks      Used  Available Capacity  Mounted on
	/dev/disk2  2018614912 698624560 1319734352    35%    /

If you want just the amount of free disk space (calculated in kilobytes), use one of these two options which calls df and then parses out the pertinent data.

	df -kP / | tail -1 | awk '{print $4}'
	df -kP / | awk '/[0-9]%/{print $(NF-2)}'


The following program freesize.m takes five different approaches to calculate the free space on a drive. The first two examples use Cocoa's Foundation framework. The first example uses NSFileManager and asks for the NSFileSystemFreeSize attribute. Simple and straight forward.

The second example uses NSURL with some more modern approaches. This example gets a URL's resources and requests the value for the key NSURLVolumeAvailableCapacityKey. This code also makes use of NSByteCountFormatter, which was introduced with OS X 10.8 Mountain Lion. This method is a little more useful than the original way, since this can display the total available free space, which tends to be slightly smaller than all of the free space, not all which is available to the user, since some space might be reserved for the system.


getattrlist is a method I had read about in the books Advanced Mac OS X Programming and Mac OS X Internals before, but the available examples for this tool are quite sparse and seem to be rarely used. I spent several days trying to get this example to work, but the data never seemed to line up properly with the other results I was obtaining via other methods. getattrlist behaves in a curious and obfuscated way, by returning some blob of data, but one needs to carefully construct a custom structure in which to line up against the data blob. The data I was getting often would come out like the following:

	Volume size: 16229794380578291739
	Free space:  15103894473735667713
	Avail size:  1

This was obviously incorrect, which makes me suspect that the vol_attr structure was not lining up properly with the returned data, and I might even need to add some sort of padding inside the structure. There are far more easier methods to obtain the free space size than by using getattrlist.

statfs + statvfs

The final two examples are similar UNIX system calls to get file system information and statistics. These methods statfs and statvfs are nearly identical in how a structure is passed into a system call and then the necessary information is parsed out. The one major difference I found between these two system calls was that the f_bsize returned different values. The statfs structure returns a value of 4096 for f_bsize, whereas statvfs returns a value of 1048576, a value 256 times larger than the other one. To properly calculate the available free space using the statvfs call, I needed to use f_frsize instead. Multiply f_frsize by f_bavail and that will result in the total number of available bytes. To calculate that number in kilobytes, divide that product by 1024.

freesize.m Gist