5 files changed, 43 insertions, 12 deletions
diff --git a/pkg/btrfs/btrfsitem/item_empty.go b/pkg/btrfs/btrfsitem/item_empty.go
index 74645cc..93e2f9e 100644
--- a/pkg/btrfs/btrfsitem/item_empty.go
+++ b/pkg/btrfs/btrfsitem/item_empty.go
@@ -4,6 +4,6 @@ import (
 	"lukeshu.com/btrfs-tools/pkg/binstruct"
 )
 
-type Empty struct { // UNTYPED=0 ORPHAN_ITEM=48 TREE_BLOCK_REF=176 SHARED_BLOCK_REF=182 FREE_SPACE_EXTENT=199 QGROUP_RELATION=246
+type Empty struct { // ORPHAN_ITEM=48 TREE_BLOCK_REF=176 SHARED_BLOCK_REF=182 FREE_SPACE_EXTENT=199 QGROUP_RELATION=246
 	binstruct.End `bin:"off=0"`
 }
diff --git a/pkg/btrfs/btrfsitem/item_untyped.go b/pkg/btrfs/btrfsitem/item_untyped.go
new file mode 100644
index 0000000..51d0676
--- /dev/null
+++ b/pkg/btrfs/btrfsitem/item_untyped.go
@@ -0,0 +1,14 @@
+package btrfsitem
+
+import (
+	"lukeshu.com/btrfs-tools/pkg/binstruct"
+	"lukeshu.com/btrfs-tools/pkg/btrfs/internal"
+)
+
+type FreeSpaceHeader struct { // UNTYPED=0:FREE_SPACE_OBJECTID
+	Location      internal.Key `bin:"off=0x00, siz=0x11"`
+	Generation    int64        `bin:"off=0x11, siz=0x8"`
+	NumEntries    int64        `bin:"off=0x19, siz=0x8"`
+	NumBitmaps    int64        `bin:"off=0x21, siz=0x8"`
+	binstruct.End `bin:"off=0x29"`
+}
diff --git a/pkg/btrfs/btrfsitem/items.go b/pkg/btrfs/btrfsitem/items.go
index 3c4392e..c4ad7e9 100644
--- a/pkg/btrfs/btrfsitem/items.go
+++ b/pkg/btrfs/btrfsitem/items.go
@@ -31,12 +31,26 @@ func (o *Error) UnmarshalBinary(dat []byte) (int, error) {
 }
 
 // Rather than returning a separate error  value, return an Error item.
-func UnmarshalItem(keytyp Type, dat []byte) Item {
-	gotyp, ok := keytype2gotype[keytyp]
-	if !ok {
-		return Error{
-			Dat: dat,
-			Err: fmt.Errorf("btrfsitem.UnmarshalItem(typ=%v, dat): unknown item type", keytyp),
+func UnmarshalItem(key internal.Key, dat []byte) Item {
+	var gotyp reflect.Type
+	if key.ItemType == UNTYPED_KEY {
+		var ok bool
+		gotyp, ok = untypedObjID2gotype[key.ObjectID]
+		if !ok {
+			return Error{
+				Dat: dat,
+				Err: fmt.Errorf("btrfsitem.UnmarshalItem({ItemType:%v, ObjectID:%v}, dat): unknown object ID for untyped item",
+					key.ItemType, key.ObjectID),
+			}
+		}
+	} else {
+		var ok bool
+		gotyp, ok = keytype2gotype[key.ItemType]
+		if !ok {
+			return Error{
+				Dat: dat,
+				Err: fmt.Errorf("btrfsitem.UnmarshalItem({ItemType:%v}, dat): unknown item type", key.ItemType),
+			}
 		}
 	}
 	retPtr := reflect.New(gotyp)
@@ -44,15 +58,15 @@ func UnmarshalItem(keytyp Type, dat []byte) Item {
 	if err != nil {
 		return Error{
 			Dat: dat,
-			Err: fmt.Errorf("btrfsitem.UnmarshalItem(typ=%v, dat): %w", keytyp, err),
+			Err: fmt.Errorf("btrfsitem.UnmarshalItem({ItemType:%v}, dat): %w", key.ItemType, err),
 		}
 
 	}
 	if n < len(dat) {
 		return Error{
 			Dat: dat,
-			Err: fmt.Errorf("btrfsitem.UnmarshalItem(typ=%v, dat): left over data: got %d bytes but only consumed %d",
-				keytyp, len(dat), n),
+			Err: fmt.Errorf("btrfsitem.UnmarshalItem({ItemType:%v}, dat): left over data: got %d bytes but only consumed %d",
+				key.ItemType, len(dat), n),
 		}
 	}
 	return retPtr.Elem().Interface().(Item)
diff --git a/pkg/btrfs/btrfsitem/items.txt b/pkg/btrfs/btrfsitem/items.txt
index 8320650..59e3e76 100644
--- a/pkg/btrfs/btrfsitem/items.txt
+++ b/pkg/btrfs/btrfsitem/items.txt
@@ -20,7 +20,7 @@ ROOT_ITEM=132 Root
 SHARED_BLOCK_REF=182 Empty
 SHARED_DATA_REF=184 SharedDataRef
 TREE_BLOCK_REF=176 Empty
-UNTYPED=0 Empty
+UNTYPED=0:FREE_SPACE_OBJECTID FreeSpaceHeader
 UUID_RECEIVED_SUBVOL=252 UUIDMap
 UUID_SUBVOL=251 UUIDMap
 XATTR_ITEM=24 DirList
diff --git a/pkg/btrfs/btrfsitem/items_gen.go b/pkg/btrfs/btrfsitem/items_gen.go
index 02e2915..e60c50e 100644
--- a/pkg/btrfs/btrfsitem/items_gen.go
+++ b/pkg/btrfs/btrfsitem/items_gen.go
@@ -60,11 +60,13 @@ var keytype2gotype = map[Type]reflect.Type{
 	SHARED_BLOCK_REF_KEY:     reflect.TypeOf(Empty{}),
 	SHARED_DATA_REF_KEY:      reflect.TypeOf(SharedDataRef{}),
 	TREE_BLOCK_REF_KEY:       reflect.TypeOf(Empty{}),
-	UNTYPED_KEY:              reflect.TypeOf(Empty{}),
 	UUID_RECEIVED_SUBVOL_KEY: reflect.TypeOf(UUIDMap{}),
 	UUID_SUBVOL_KEY:          reflect.TypeOf(UUIDMap{}),
 	XATTR_ITEM_KEY:           reflect.TypeOf(DirList{}),
 }
+var untypedObjID2gotype = map[internal.ObjID]reflect.Type{
+	internal.FREE_SPACE_OBJECTID: reflect.TypeOf(FreeSpaceHeader{}),
+}
 
 func (BlockGroup) isItem()      {}
 func (Chunk) isItem()           {}
@@ -77,6 +79,7 @@ func (Extent) isItem()          {}
 func (ExtentDataRef) isItem()   {}
 func (FileExtent) isItem()      {}
 func (FreeSpaceBitmap) isItem() {}
+func (FreeSpaceHeader) isItem() {}
 func (FreeSpaceInfo) isItem()   {}
 func (Inode) isItem()           {}
 func (InodeRefList) isItem()    {}