Win9x が多く存在していた頃のコードの MFC を使用しない方法での書き直し．
前のものは次の様になっている．

//	J048221	SDK32:GetDiskFreeSpace と GetDiskFreeSpaceEx について
typedef BOOL (WINAPI *P_GDFSE)(LPCTSTR, PULARGE_INTEGER,PULARGE_INTEGER, PULARGE_INTEGER);
// ..
	fResult = ::GetDiskFreeSpace(pszDrive,&SectorsPerCluster,&BytesPerSector,&FreeClusters,&TotalClusters) ;
	if (fResult) {
		TotalBytes    	= (__int64)TotalClusters * SectorsPerCluster * BytesPerSector ;
		TotalFreeBytes	= (__int64)FreeClusters  * SectorsPerCluster * BytesPerSector ;
		FreeBytesAvailable = TotalFreeBytes ;
		}
// ..
{
	P_GDFSE	pGetDiskFreeSpaceEx = NULL;
	#ifdef UNICODE
		pGetDiskFreeSpaceEx = (P_GDFSE)GetProcAddress(GetModuleHandle(_T("kernel32.dll")),"GetDiskFreeSpaceExW") ;
	#else
		pGetDiskFreeSpaceEx = (P_GDFSE)GetProcAddress(GetModuleHandle(_T("kernel32.dll")),"GetDiskFreeSpaceExA") ;
	#endif
	if (pGetDiskFreeSpaceEx) {
		fResult = pGetDiskFreeSpaceEx (pszDrive,
			(PULARGE_INTEGER)&FreeBytesAvailable,
			(PULARGE_INTEGER)&TotalBytes,
			(PULARGE_INTEGER)&TotalFreeBytes);
		}
	}

https://www.betaarchive.com/wiki/index.php?title=Microsoft_KB_Archive/231497

Windows 環境では次の様な感じ？

class	DiskFree	{
public:
	DiskFree ()	{	Free = Total = 0 ;	}
public:
	u_64	Free ;
	u_64	Total ;
	} ;

DiskFree	GetDiskFree	(LPCTSTR path)
{
	DiskFree	df ;
	u_64		freeC = 0 ;
	u_64		free  = 0 ;
	u_64		total = 0 ;
	if (::GetDiskFreeSpaceEx(path,(PULARGE_INTEGER)&freeC,(PULARGE_INTEGER)&total,(PULARGE_INTEGER)&free)) {
		df.Free = free ;
		df.Total= total;
		}
	return	df ;
	}

Linux 環境では ::statvfs が使えるみたいで，次の様なコードで取得してデバッガで確認．

#include    <iostream>
#include    <sys/statvfs.h>

int main()
{
	struct statvfs	vfs = { 0 } ;
	::statvfs(".", &vfs) ;
	return 0 ;
	}

vfs     	{...}           	statvfs
  f_bsize 	4096            	unsigned long
  f_frsize	4096            	unsigned long
  f_blocks	1452408524      	__fsblkcnt_t
  f_bfree 	200259802       	__fsblkcnt_t
  f_bavail	199469255       	__fsblkcnt_t
  f_files 	183001088       	__fsfilcnt_t
  f_ffree 	181248356       	__fsfilcnt_t
  f_favail	181248356       	__fsfilcnt_t
  f_fsid  	3941329918106335254	unsigned long
  f_flag  	4096            	unsigned long
  f_namemax	255             	unsigned long
  __f_spare	                 	int [6]
    [0]   	0               	int
    [1]   	0               	int
    [2]   	0               	int
    [3]   	0               	int
    [4]   	0               	int
    [5]   	0               	int

Iwao@AS5202T:/volume1/home/Iwao $ cd gcc_test/Test/t_linux/T_vfs/T_s_vfs/
Iwao@AS5202T:/volume1/home/Iwao/gcc_test/Test/t_linux/T_vfs/T_s_vfs $ ll
total 12
drwxrwxrwx    2 Iwao     users       4.0K Jul  1 22:01 ./
drwxrwxrwx    4 Iwao     users       4.0K Jul  1 22:00 ../
-rwxrwxrwx    1 Iwao     users        581 Jul  1 21:55 T_s_vfs.cpp*
Iwao@AS5202T:/volume1/home/Iwao/gcc_test/Test/t_linux/T_vfs/T_s_vfs $ cat T_s_vfs.cpp
#include    <iostream>
#include    <clocale>

#include    <sys/statvfs.h>

#include    "_tdefine.hxx"
#include    "t_tstrng.hxx"

int     _tmain  (int argc,TCHAR* argv[])
{
        _tsetlocale(LC_ALL,_T("")) ;
        {
                struct  statvfs vfs = { 0 } ;
                if (::statvfs(".", &vfs) == 0) {
                        u_64    free = vfs.f_bavail * vfs.f_frsize ;
                        u_64    total= vfs.f_blocks * vfs.f_frsize ;
                        std::tout << ::To_tstring_Ki(free) << _T(" / ") << ::To_tstring_Ki(total) << std::endl ;
                        std::tout << ::To_tstring_cs(free) << _T(" / ") << ::To_tstring_cs(total) << std::endl ;
                        }
                }
        return  0 ;
        }
Iwao@AS5202T:/volume1/home/Iwao/gcc_test/Test/t_linux/T_vfs/T_s_vfs $ g++ T_s_vfs.cpp -Wall
Iwao@AS5202T:/volume1/home/Iwao/gcc_test/Test/t_linux/T_vfs/T_s_vfs $ ./a.out
760.90 G /   5.41 T
817,005,420,544 / 5,949,065,314,304
Iwao@AS5202T:/volume1/home/Iwao/gcc_test/Test/t_linux/T_vfs/T_s_vfs $

diskfree.hxx