Available free space on HD

Hello guys, I have a doubt.

I need write a FTP like program (client and server) and I need to write a function that tells me the amount of free space on HD (to know if there is enough space to transfer the file).

So I informed myself and discovered that the "statvfs" function would resolve my problems. (I'm only using C language exclusively)

With "statvfs" you get the number of free blocks and the size (in bytes) of a block, so I think that the free space on disk (in bytes) is the following multiplication => "free blocks" x "size of a block"

Doing that I got "955344498688 bytes" ( I have an 1 TB HD), that's about 889.73 GB.

When I execute the "df -h" command on the terminal, it gives me this :

Filesystem Size Used Avail Use% Mounted on
/dev/sda1 915G 26G 844G 3% /

When I run "nautilus" (I'm on linux, of course), it says I have 905.4 GB of free space on the file system.

Can anyone explain me why do I have THREE different values for the free space on disk?? Which one should I believe?

Thank you all.

Here's someone with a similar question: file - Discrepancy between call to statvfs and df command - Stack Overflow