LinuxQuestions.org - help : mount of root /

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I run a little script that prints the available space for file system:

Code:

#!/usr/bin/perl5

sub CheckDiskSpace

{

        use Filesys::DiskFree;

        my $handle = new Filesys::DiskFree;

        $handle->df();



$loc1 = '/dev';

$loc2 = '/var/run';

$loc3 = '/boot';

print "... $loc1, $loc2, $loc3, '/' \n";



        foreach my $file ($loc1,$loc2,$loc3,'/' )

        {

                my $bytesavailable=$handle->avail($file);

                print "at $file, available bits = $bytesavailable \n";

        }

}



CheckDiskSpace();

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The result is:
... /dev,/var/run,/boot,'/'
at /dev, available bits = 1028820992
at /var/run, available bits = 1028730880
at /boot, available bits =
at /, available bits =
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I then type command "df -a" and get:

Code:

Filesystem          1K-blocks      Used Available Use% Mounted on

proc                        0        0        0  -  /proc

/sys                        0        0        0  -  /sys

varrun                1004736      116  1004620  1% /var/run

varlock                1004736        0  1004736  0% /var/lock

udev                  1004736        28  1004708  1% /dev

devshm                1004736        12  1004724  1% /dev/shm

devpts                      0        0        0  -  /dev/pts

tmpfs                  1004736        12  1004724  1% /dev/shm

securityfs                  0        0        0  -  /sys/kernel/security

gvfs-fuse-daemon    620395268  10647256 609748012  2% /root/.gvfs

/dev/sda1              497829    139305    332822  30% /media/_boot

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I then run "gparted" and get the picture as attached. It shows /dev/sda3 is mounted to '/'.

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The thing I do not understand is:
Why cannot perl script tell the available size of "/"?
Why doesnot df command show "/" as a mount point while gparted correctly shows that?

I tried to mount /dev/sda3 to '/' but it says it already exists.
My system is ubuntu server.

Can someone help me? Thanks.