LFN

LFN stands for Long File Name, a feature of the FAT family of file systems that enables file and directory names longer than the legacy 8.3 convention (eight characters for the base name and three for the extension). Long names are implemented using additional directory entries, known as LFN entries, that precede the standard 8.3 entry. The Long File Name feature was introduced with the VFAT extension in Windows 95 OSR2 and has since become common in FAT12, FAT16, and FAT32 volumes. In practice, most modern operating systems (Windows, Linux, macOS) support LFNs on FAT volumes, though older DOS environments lacking VFAT support cannot read them.

How it works: When a file or directory has a long name, the filesystem stores the long name across one or more 32-byte LFN directory entries. Each LFN entry has the attribute 0x0F and contains a portion of the name in Unicode (UTF-16LE). A long name can be split into chunks that fit three fields in the entry: Name1 (five UTF-16 characters), Name2 (six characters), and Name3 (two characters), allowing up to 13 Unicode characters per LFN entry. The final entry in the sequence is followed by a regular 32-byte directory entry that stores the actual short name (the 8.3 SFN). A sequence number in the first byte (with a high-bit indicating the last LFN entry) and a checksum tied to the SFN help maintain integrity and link the long name to its short counterpart.

Limitations and notes: The SFN remains the canonical name for compatibility with older software. LFNs support up to a total length of about 255 characters and rely on Unicode, which can lead to normalization and case-handling differences across platforms. Some embedded or restricted environments may disable or limit LFN usage.