How do you read the 128-bit NTFS FILE_ID for a directory and/or file?

A bit of searching took me to DeviceIoControl and there lies the answer to your question: FSCTL_GET_OBJECT_ID returns exactly the same IDs as in your output from fsutil.

Anyhow, the docs for BY_HANDLE_FILE_INFORMATION say that the 64-bit file ID already uniquely identifies a file on a given volume. According to Wikipedia, NTFS only supports a maximum of 2^32 files, so the 128-bit ID seems quite unnecessary.

Also please note that NOT every file does have a GUID. The GUID mechanisim is mostly used for .lnk files in order to keep the association when a the traget is moved. Only $Volume and the targets of link files have these GUIDs. Furthermore you can set them by hand.

Their advantage is that the GUID should not clash between volumes, while the file ID does. the FILE_ID is actually 48 bit of MFT_RECORD_NUMBER and 16 bits of MFT_SEQUENCE_ID