GetLogicalDrives() for loop

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 dri

Answer 1

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.
    }
}

Answer 2

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.