Implement FAT12 Filesystem Writer: Create Files and Update FAT

Mutating FAT Tables

Writing FAT12 means updating directory entries, allocating FAT clusters, and preserving boot sector invariants. Cluster chains are singly linked through 12-bit entries packed two per byte triplet in the FAT.

Allocate and Chain

Create file workflow:

• Find free cluster from FAT scan
• Write directory entry with starting cluster
• Extend chain with 0xFF8.. 0xFFF EOC markers

For example, a 3-cluster file might use clusters 2 → 3 → 4 with FAT entries linking forward.

Why This Matters in Production

Every field you pack here , the space-padded 8.3 filename, the 0xFFF end-of-chain marker , matches exactly what tools like mkfs.fat and fsck.fat validate, and what real DOS and early Windows machines wrote to floppies for two decades. Forgetting to write the FAT back to disk after updating it in memory is a classic bug that silently corrupts the whole filesystem on the next mount.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will implement create, write, and close that updates FAT and directory on disk image. The task asks you to verify round-trip by re-reading with your FAT12 reader.