Get Hard Disk Size dynamically in Cocoa

I'm trying to get the the size of the main HD. Here's the code I'm using:

NSDictionary *attributes = [[NSFileManager defaultManager] attributesOfFileSystemForPath:@"/" error:nil];

But this code doesn't give me the correct size of the HD. If my HD 开发者_高级运维has a capacity of 320gb that will return 290GB. I think there are other volumes but I don't know how to calculate them.

What I want to know is a way to get the total size of my hard drive.

It sounds like you're getting the correct answer, actually. People who sell hard drives like to use the power-of-10 gigabyte to describe the capacity of the disk, while the computer itself cares about how many multiples of 2 are available, and displays the gibibytes. A gibibyte is bigger than a gigabyte. The SI prefix "giga-" means 10⁹; the so-called binary prefix "gibi-" was chosen to be the closest power of two, 2³⁰, which is 1,073,741,824.

The true number of bytes is the same in either case, it's just reported differently depending on whether you divide by 1,000,000,000 or 1,073,741,824. For example, 320,000,000,000 ÷ 1,073,741,824 = 298.023, and you might lose a bit more to area the disk reserves for itself.