c++ - GetDiskFreeSpaceEx reports wrong number of free bytes

Question

__int64 i64FreeBytes
unsigned __int64 lpFreeBytesAvailableToCaller,
                 lpTotalNumberOfBytes,
                 lpTotalNumberOfFreeBytes; // variables used to obtain 
                                           // the free space on the  drive

GetDiskFreeSpaceEx (Manager.capDir,
(PULARGE_INTEGER)&lpFreeBytesAvailableToCaller,
(PULARGE_INTEGER)&lpTotalNumberOfBytes,
(PULARGE_INTEGER)&lpTotalNumberOfFreeBytes);

i64FreeBytes = lpTotalNumberOfFreeBytes;
_tprintf(_T ("Number of bytes free on the drive:%I64u 
"),
     lpTotalNumberOfFreeBytes);

I am working on a data management routine which is a Windows CE command line application. The above code shows how I get the number of free bytes on a particular drive which contains the folder Manager.capdir (it is the variable containing the full path name of the directory).

My question is, the number of free bytes reported by the above code (the _tprintf statement) doesn't match with the number of free bytes of the drive (which i check by doing a right click on the drive).

I wish to know if the reason for this difference?

Answer 1

Quoting (with editing) the documentation for GetDiskFreeSpaceEx, emphasis mine:

lpFreeBytesAvailable [out, optional]-

A pointer to a variable that receives the total number of free bytes on a disk that are available to the user who is associated with the calling thread.

This parameter can be NULL.

If per-user quotas are being used, this value may be less than the total number of free bytes on a disk.

lpTotalNumberOfBytes [out, optional]-

A pointer to a variable that receives the total number of bytes on a disk that are available to the user who is associated with the calling thread.

This parameter can be NULL.

If per-user quotas are being used, this value may be less than the total number of bytes on a disk.

To determine the total number of bytes on a disk or volume, use IOCTL_DISK_GET_LENGTH_INFO.

In other words, this number depends on the user, and if you want to match the value returned by Explorer, use lpFreeBytesAvailable.