I am using GetDiskFreeSpaceExW

Question

I am using

 if( !GetDiskFreeSpaceEx(
        lpDirectoryName,                  // directory name
        &m_uliFreeBytesAvailable,         // bytes available to caller
        &m_uliTotalNumberOfBytes,         // bytes on disk
        &m_uliTotalNumberOfFreeBytes) )   // free bytes on disk
        return false;
else
        diskFreeSpaceInKB = static_cast<long int>(m_uliTotalNumberOfFreeBytes.QuadPart/ONE_KB_IN_BYTES)  ;

That returns negative value for disk that have a capacity greater than 1TB.
I need the accurate value representing the available disk space so that the user can record a movie if there is enough space on that disk.

Answer 1

I see you typed diskFreeSpaceInKB = static_cast<long int>..., so I assume that diskFreeSpaceInKB is a long int variable.

First, let's have a look on the GetDiskFreeSpaceEx prototype.

BOOL WINAPI GetDiskFreeSpaceEx(
  _In_opt_  LPCTSTR         lpDirectoryName,
  _Out_opt_ PULARGE_INTEGER lpFreeBytesAvailable,
  _Out_opt_ PULARGE_INTEGER lpTotalNumberOfBytes,
  _Out_opt_ PULARGE_INTEGER lpTotalNumberOfFreeBytes
);

PULARGE_INTEGER = Pointer to ULARGE_INTEGER

ULARGE_INTERGER = An union that stores a 64-bit unsigned integer

Now let's see what you did.

diskFreeSpaceInKB = static_cast<long int>(m_uliTotalNumberOfFreeBytes.QuadPart/ONE_KB_IN_BYTES)

In Windows, int/long/long int are always signed 32-bit regardless you're compiling 32-bit or 64-bit build because Windows uses LLP64 model. So basically you're casting a unsigned 64-bit integer divided by ONE_KB_IN_BYTES to a signed 32-bit integer.

So why I get negative number?

Because the value exceeds the maximum positive value of the signed data type.

How to fix it?

Declare diskFreeSpaceInKB as UINT64, and static_cast<UINT64>.