GetLogicalDrives() for loop - c++

Question

I am new to the win32 api and need help trying to understand how the GetLogicalDrives() function works. I am trying to populate a cbs_dropdownlist with all the available drives that are not in use. here is what I have so far. I would appreciate any help.
void FillListBox(HWND hWndDropMenu)
{
DWORD drives = GetLogicalDrives();
for (int i=0; i<26; i++)
{
SendMessage(hWndDropMenu, CB_ADDSTRING, 0, (LPARAM)drives);
}
}

Answer 1

The function GetLogicalDrives returns a bitmask of the logical drives available. Here is how you would do it:
DWORD drives = GetLogicalDrives();
for (int i=0; i<26; i++)
{
if( !( drives & ( 1 << i ) ) )
{
TCHAR driveName[] = { TEXT('A') + i, TEXT(':'), TEXT('\\'), TEXT('\0') };
SendMessage(hWndDropMenu, CB_ADDSTRING, 0, (LPARAM)driveName);
}
}
The code checks whether the i-th bit in the bitmask is not set to 1 or true.

Answer 2

GetLogicalDrives returns a bitmask and to inspect it you need to use bitwise operators. To see if drive A is in use:
GetLogicalDrives() & 1 == 1
If drive A is unavailable, GetLogicalDrives() & 1 would yield 0 and the condition would fail.
To check the next drive you'll need to use the next multiple of 2, GetLogicalDrives() & 2, GetLogicalDrives() & 4 and so on.
You could use GetLogicalDriveStrings but this returns the inverse of what you want, all the used logical drives.
I would build a table instead, and index into that:
const char *drive_names[] =
{
"A:",
"B:",
...
"Z:"
};
Then your loop could be:
DWORD drives_bitmask = GetLogicalDrives();
for (DWORD i < 0; i < 32; i++)
{
// Shift 1 to a multiple of 2. 1 << 0 = 1 (0000 0001), 1 << 1 = 2 etc.
DWORD mask_index = 1 << i;
if (drives_bitmask & i == 0)
{
// Drive unavailable, add it to list.
const char *name = drive_names[i];
// ... do GUI work.
}
}