CHANGELOG.md

@@ -253,6 +253,12 @@
   - A pipeline with Fletcher32 ahead of the compressor (h5py
     `set_fletcher32()` then `set_deflate()`) no longer fails with "deflate:
     output exceeds size limit".
+- `clawhdf5-format`: **HDF5 1.4/1.6-era files are readable.** Data Layout
+  message versions 1 and 2 (compact, contiguous, and chunked through the
+  version-1 B-tree) failed with `InvalidLayoutVersion` — 84 of the 686 files in
+  the 2026-09-25 audit sweep, 205 datasets. They now read as libhdf5 does;
+  checked byte for byte against h5py on HDF5's own test files
+  (`tests/legacy_format_interop.rs`).
 
 ### Storage
 - `clawhdf5-format`: **half-precision datasets.**
@@ -311,6 +317,19 @@
   - Two threads reading two chunked datasets through one `File` could get each
     other's chunks (the shared chunk cache was switched between datasets
     across separate lock acquisitions). The cache is now keyed by dataset.
+  - Compound datatype version 1 members with legacy array dimensions (HDF5
+    before 1.4, which had no array class) were read as a single scalar at
+    the member's offset; they are now array members, as in libhdf5
+    (`tarrold.h5`, `tcompound.h5`). Only reachable once layout versions 1/2
+    were readable, since the files that use it are that old.
+- `clawhdf5-format` reader — errors on valid files: a version-1 shared
+  message (a committed datatype in HDF5 1.4/1.6-era files) was read as if the
+  object header address followed the reserved bytes; it follows a link-name
+  offset (the reference is an old-style symbol table entry), so the reader
+  followed the name offset and failed with `InvalidObjectHeaderVersion`
+  (`tcompound.h5`). New `shared_message::parse_shared_ref_sized` takes the
+  superblock's length size; `parse_shared_ref` assumes it equals the offset
+  size.
 - `clawhdf5-format` reader — errors on valid files: enum and bool datasets
   through the numeric readers; the "don't filter partial edge chunks" layout
   flag; Fletcher32 ahead of deflate (NetCDF-4's order). Unknown-message flags

crates/clawhdf5-format/src/attribute.rs

@@ -97,7 +97,7 @@ impl AttributeMessage {
             return Ok(Cow::Borrowed(bytes));
         }
         let (file_data, offset_size) = file.ok_or(FormatError::UnresolvedSharedMessage)?;
-        let shared_ref = shared_message::parse_shared_ref(bytes, offset_size)?;
+        let shared_ref = shared_message::parse_shared_ref_sized(bytes, offset_size, length_size)?;
         shared_message::resolve_shared_message(
             file_data,
             &shared_ref,
@@ -407,7 +407,8 @@ pub fn extract_attributes_full(
         if msg.msg_type == MessageType::Attribute {
             if shared_message::is_shared(msg.flags) {
                 // Shared attribute: resolve the reference to get actual attribute data
-                let shared_ref = shared_message::parse_shared_ref(&msg.data, offset_size)?;
+                let shared_ref =
+                    shared_message::parse_shared_ref_sized(&msg.data, offset_size, length_size)?;
                 let resolved_data = shared_message::resolve_shared_message(
                     file_data,
                     &shared_ref,

crates/clawhdf5-format/src/data_layout.rs

@@ -1,7 +1,7 @@
 //! HDF5 Data Layout message parsing (message type 0x0008).
 
 #[cfg(not(feature = "std"))]
-use alloc::{string::String, vec::Vec};
+use alloc::{format, string::String, vec::Vec};
 
 #[cfg(feature = "std")]
 use std::string::String;
@@ -45,7 +45,9 @@ pub enum DataLayout {
         chunk_dimensions: Vec<u32>,
         /// B-tree address, or `None` if undefined.
         btree_address: Option<u64>,
-        /// Layout version (3 or 4).
+        /// Layout version (3 or 4). Version 1/2 messages (HDF5 1.4/1.6-era)
+        /// use the same version-1 B-tree chunk index as version 3 and are
+        /// reported as 3.
         version: u8,
         /// Chunk index type (v4 only).
         chunk_index_type: Option<u8>,
@@ -261,6 +263,7 @@ impl DataLayout {
         let layout_class = data[1];
 
         match version {
+            1 | 2 => Self::parse_v1_v2(data, offset_size),
             3 => Self::parse_v3(data, layout_class, offset_size, length_size),
             // v5 (emitted by HDF5 1.14+/2.0 with `libver=latest`) uses the same
             // message structure as v4 — only the version number was bumped.
@@ -269,6 +272,87 @@ impl DataLayout {
         }
     }
 
+    /// Layout message versions 1 and 2 (HDF5 before 1.6.3):
+    ///
+    /// ```text
+    /// version(1) · dimensionality(1) · layout class(1) · reserved(5)
+    /// · address(offset_size)      — contiguous and chunked only
+    /// · dimension sizes(4 × dimensionality)
+    /// · compact data size(4) · compact raw data — compact only
+    /// ```
+    ///
+    /// The dimension sizes are the dataset's (contiguous/compact) or the
+    /// chunk's (chunked) extent plus a trailing element-size dimension, as in
+    /// version 3's chunked form. libhdf5 ignores them for contiguous storage
+    /// and sizes the data from the dataspace; the product of the stored
+    /// dimensions is that same size, and a disagreement (a dimension that was
+    /// truncated to 32 bits) is caught by the reader's size check rather than
+    /// returning wrong data.
+    fn parse_v1_v2(data: &[u8], offset_size: u8) -> Result<DataLayout, FormatError> {
+        ensure_len(data, 0, 8)?;
+        let dimensionality = data[1] as usize;
+        let layout_class = data[2];
+        // H5O_LAYOUT_NDIMS: 32 dataspace dimensions + the element-size one.
+        if dimensionality > 33 {
+            return Err(FormatError::Overflow(format!(
+                "data layout dimensionality {dimensionality} exceeds 33"
+            )));
+        }
+        let mut p = 8;
+        let os = offset_size as usize;
+        let address = match layout_class {
+            1 | 2 => {
+                ensure_len(data, p, os)?;
+                let a = if is_undefined(data, p, offset_size) {
+                    None
+                } else {
+                    Some(read_offset(data, p, offset_size)?)
+                };
+                p += os;
+                a
+            }
+            0 => None,
+            _ => return Err(FormatError::InvalidLayoutClass(layout_class)),
+        };
+        ensure_len(data, p, dimensionality * 4)?;
+        let dims: Vec<u32> = data[p..p + dimensionality * 4]
+            .as_chunks::<4>()
+            .0
+            .iter()
+            .map(|c| u32::from_le_bytes(*c))
+            .collect();
+        p += dimensionality * 4;
+        match layout_class {
+            0 => {
+                ensure_len(data, p, 4)?;
+                let size =
+                    u32::from_le_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]]) as usize;
+                ensure_len(data, p + 4, size)?;
+                Ok(DataLayout::Compact {
+                    data: data[p + 4..p + 4 + size].to_vec(),
+                })
+            }
+            1 => {
+                let size = dims
+                    .iter()
+                    .try_fold(1u64, |acc, &d| acc.checked_mul(d as u64))
+                    .ok_or_else(|| {
+                        FormatError::Overflow(format!("contiguous layout size {dims:?}"))
+                    })?;
+                Ok(DataLayout::Contiguous { address, size })
+            }
+            _ => Ok(DataLayout::Chunked {
+                chunk_dimensions: dims,
+                btree_address: address,
+                version: 3,
+                chunk_index_type: None,
+                single_chunk_filtered_size: None,
+                single_chunk_filter_mask: None,
+                dont_filter_partial_edge_chunks: false,
+            }),
+        }
+    }
+
     fn parse_v3(
         data: &[u8],
         layout_class: u8,
@@ -546,6 +630,108 @@ impl DataLayout {
 mod tests {
     use super::*;
 
+    /// Version 1/2 header: version, dimensionality, class, reserved(5).
+    fn v1v2_header(version: u8, ndims: u8, class: u8) -> Vec<u8> {
+        vec![version, ndims, class, 0, 0, 0, 0, 0]
+    }
+
+    #[test]
+    fn v2_compact() {
+        let mut buf = v1v2_header(2, 2, 0);
+        // dims (3 elements of 2 bytes) — no address for compact
+        buf.extend_from_slice(&3u32.to_le_bytes());
+        buf.extend_from_slice(&2u32.to_le_bytes());
+        buf.extend_from_slice(&6u32.to_le_bytes()); // compact size (u32 in v1/v2)
+        buf.extend_from_slice(&[1, 0, 2, 0, 3, 0]);
+        assert_eq!(
+            DataLayout::parse(&buf, 8, 8).unwrap(),
+            DataLayout::Compact {
+                data: vec![1, 0, 2, 0, 3, 0]
+            }
+        );
+    }
+
+    #[test]
+    fn v1_contiguous_size_from_dimensions() {
+        let mut buf = v1v2_header(1, 3, 1);
+        buf.extend_from_slice(&0x800u32.to_le_bytes()); // 4-byte address
+        for d in [10u32, 20, 4] {
+            buf.extend_from_slice(&d.to_le_bytes());
+        }
+        assert_eq!(
+            DataLayout::parse(&buf, 4, 4).unwrap(),
+            DataLayout::Contiguous {
+                address: Some(0x800),
+                size: 800,
+            }
+        );
+    }
+
+    #[test]
+    fn v1_contiguous_undefined_address() {
+        let mut buf = v1v2_header(1, 2, 1);
+        buf.extend_from_slice(&[0xFF; 8]);
+        buf.extend_from_slice(&5u32.to_le_bytes());
+        buf.extend_from_slice(&8u32.to_le_bytes());
+        assert_eq!(
+            DataLayout::parse(&buf, 8, 8).unwrap(),
+            DataLayout::Contiguous {
+                address: None,
+                size: 40,
+            }
+        );
+    }
+
+    #[test]
+    fn v1_chunked_maps_to_btree_v1_index() {
+        let mut buf = v1v2_header(1, 3, 2);
+        buf.extend_from_slice(&0x1234u64.to_le_bytes());
+        for d in [50u32, 50, 4] {
+            buf.extend_from_slice(&d.to_le_bytes());
+        }
+        assert_eq!(
+            DataLayout::parse(&buf, 8, 8).unwrap(),
+            DataLayout::Chunked {
+                chunk_dimensions: vec![50, 50, 4],
+                btree_address: Some(0x1234),
+                version: 3,
+                chunk_index_type: None,
+                single_chunk_filtered_size: None,
+                single_chunk_filter_mask: None,
+                dont_filter_partial_edge_chunks: false,
+            }
+        );
+    }
+
+    #[test]
+    fn v1v2_rejects_bad_class_dimensionality_and_truncation() {
+        assert_eq!(
+            DataLayout::parse(&v1v2_header(1, 1, 3), 8, 8).unwrap_err(),
+            FormatError::InvalidLayoutClass(3)
+        );
+        assert!(matches!(
+            DataLayout::parse(&v1v2_header(2, 34, 1), 8, 8).unwrap_err(),
+            FormatError::Overflow(_)
+        ));
+        // Chunked, dims cut short.
+        let mut buf = v1v2_header(1, 2, 2);
+        buf.extend_from_slice(&0x10u64.to_le_bytes());
+        buf.extend_from_slice(&7u32.to_le_bytes());
+        assert!(matches!(
+            DataLayout::parse(&buf, 8, 8).unwrap_err(),
+            FormatError::UnexpectedEof { .. }
+        ));
+        // Compact, raw data shorter than its declared size.
+        let mut buf = v1v2_header(2, 1, 0);
+        buf.extend_from_slice(&4u32.to_le_bytes());
+        buf.extend_from_slice(&100u32.to_le_bytes());
+        buf.extend_from_slice(&[0; 4]);
+        assert!(matches!(
+            DataLayout::parse(&buf, 8, 8).unwrap_err(),
+            FormatError::UnexpectedEof { .. }
+        ));
+    }
+
     #[test]
     fn v3_compact() {
         let mut buf = vec![3u8, 0]; // version=3, class=0 (compact)

crates/clawhdf5-format/src/datatype.rs

@@ -423,13 +423,36 @@ impl Datatype {
                         ensure_len(data, pos, 4)?;
                         let byte_offset = LittleEndian::read_u32(&data[pos..pos + 4]) as u64;
                         pos += 4;
+                        // v1 members can be fixed-size arrays of their
+                        // datatype (HDF5 before 1.4 had no array class):
+                        // libhdf5 wraps such a member in an array type of the
+                        // first `ndims` of the four stored dimensions and
+                        // ignores the permutation.
+                        let mut legacy_dims = Vec::new();
                         if version == 1 {
                             ensure_len(data, pos, 28)?;
+                            let ndims = data[pos] as usize;
+                            if ndims > 4 {
+                                return Err(FormatError::InvalidDatatypeVersion {
+                                    class: class_id,
+                                    version,
+                                });
+                            }
+                            for i in 0..ndims {
+                                let at = pos + 12 + 4 * i;
+                                legacy_dims.push(LittleEndian::read_u32(&data[at..at + 4]));
+                            }
                             pos += 28;
                         }
-                        let (member_dt, consumed) =
+                        let (mut member_dt, consumed) =
                             Self::parse_with_depth(&data[pos..], depth + 1)?;
                         pos += consumed;
+                        if !legacy_dims.is_empty() {
+                            member_dt = Datatype::Array {
+                                base_type: Box::new(member_dt),
+                                dimensions: legacy_dims,
+                            };
+                        }
                         members.push(CompoundMember {
                             name,
                             byte_offset,
@@ -1318,6 +1341,66 @@ mod tests {
         assert_xyid_compound(dt);
     }
 
+    /// A v1 compound member with legacy array dimensions (HDF5 before 1.4,
+    /// e.g. `tarrold.h5`): `{ i: i16, f: f32[2][3] }`. The member must become
+    /// an array type, not a scalar at the member's offset.
+    #[test]
+    fn test_compound_v1_legacy_array_member() {
+        let i16le: [u8; 12] = [
+            0x10, 0x08, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00,
+        ];
+        let f32le: [u8; 20] = [
+            0x11, 0x20, 0x1f, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x00, 0x17, 0x08,
+            0x00, 0x17, 0x7f, 0x00, 0x00, 0x00,
+        ];
+        let mut b = vec![0x16, 0x02, 0x00, 0x00, 28, 0x00, 0x00, 0x00];
+        for (name, offset, ndims, dims, dt) in [
+            (&b"i"[..], 0u32, 0u8, [0u32; 4], &i16le[..]),
+            (&b"f"[..], 4, 2, [2, 3, 0, 0], &f32le[..]),
+        ] {
+            let mut padded = name.to_vec();
+            padded.resize((name.len() + 1 + 7) & !7, 0);
+            b.extend_from_slice(&padded);
+            b.extend_from_slice(&offset.to_le_bytes());
+            b.extend_from_slice(&[ndims, 0, 0, 0]);
+            b.extend_from_slice(&[0, 1, 2, 3]); // dimension permutation
+            b.extend_from_slice(&[0; 4]);
+            for d in dims {
+                b.extend_from_slice(&d.to_le_bytes());
+            }
+            b.extend_from_slice(dt);
+        }
+        let (dt, consumed) = Datatype::parse(&b).unwrap();
+        assert_eq!(consumed, b.len());
+        let Datatype::Compound { size, members } = dt else {
+            panic!("expected Compound, got {dt:?}");
+        };
+        assert_eq!(size, 28);
+        assert!(matches!(
+            members[0].datatype,
+            Datatype::FixedPoint { size: 2, .. }
+        ));
+        match &members[1].datatype {
+            Datatype::Array {
+                base_type,
+                dimensions,
+            } => {
+                assert_eq!(dimensions, &[2, 3]);
+                assert!(matches!(
+                    **base_type,
+                    Datatype::FloatingPoint { size: 4, .. }
+                ));
+            }
+            other => panic!("expected an array member, got {other:?}"),
+        }
+        assert_eq!(members[1].datatype.type_size(), 24);
+
+        // More than four legacy dimensions is not a valid message.
+        let mut bad = b.clone();
+        bad[8 + 8 + 4] = 5; // first member's dimensionality
+        assert!(Datatype::parse(&bad).is_err());
+    }
+
     #[test]
     fn test_compound_v2_padded_names_no_array_fields() {
         // v2 = v1 without the 28 bytes of per-member array fields; names are

crates/clawhdf5-format/src/shared_message.rs

@@ -154,13 +154,29 @@ pub fn is_shared(msg_flags: u8) -> bool {
 ///
 /// When the shared flag is set on a message, the data contains a reference
 /// instead of the actual message content.
+///
+/// Assumes the file's length size equals its offset size, which only matters
+/// for version-1 references; use [`parse_shared_ref_sized`] when the
+/// superblock's length size is known.
 pub fn parse_shared_ref(data: &[u8], offset_size: u8) -> Result<SharedMessageRef, FormatError> {
+    parse_shared_ref_sized(data, offset_size, offset_size)
+}
+
+/// [`parse_shared_ref`] with the superblock's length size, which locates the
+/// object header address in a version-1 reference.
+pub fn parse_shared_ref_sized(
+    data: &[u8],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<SharedMessageRef, FormatError> {
     ensure_len(data, 0, 2)?;
     let version = data[0];
     let ref_type = data[1];
 
     // Layouts (HDF5 spec IV.A.2 "Shared Message", and libhdf5's decoder):
-    //   v1: version, type, reserved(6), address     — always "committed"
+    //   v1: version, type, reserved(6), then an old-style symbol table
+    //       entry: link-name offset(length_size), object header address,
+    //       cache type(4), reserved(4), scratch(16)  — always "committed"
     //   v2: version, type, address                  — always "committed"
     //   v3: version, type, then a fractal-heap ID if type == SOHM, otherwise
     //       an address
@@ -177,7 +193,7 @@ pub fn parse_shared_ref(data: &[u8], offset_size: u8) -> Result<SharedMessageRef
         })
     };
     match version {
-        1 => address_at(2 + 6),
+        1 => address_at(2 + 6 + length_size as usize),
         2 => address_at(2),
         3 if ref_type == SHARE_TYPE_SOHM => {
             ensure_len(data, 2, FHEAP_ID_LEN)?;
@@ -434,7 +450,7 @@ pub fn message_data_with_sohm<'a>(
     if !is_shared(msg.flags) {
         return Ok(Cow::Borrowed(&msg.data));
     }
-    let shared_ref = parse_shared_ref(&msg.data, offset_size)?;
+    let shared_ref = parse_shared_ref_sized(&msg.data, offset_size, length_size)?;
     let table = if shared_ref.heap_id.is_some() {
         load_sohm_table(file_data, offset_size, length_size)?
     } else {
@@ -514,7 +530,7 @@ pub fn message_data<'a>(
     if !is_shared(msg.flags) {
         return Ok(Cow::Borrowed(&msg.data));
     }
-    let shared_ref = parse_shared_ref(&msg.data, offset_size)?;
+    let shared_ref = parse_shared_ref_sized(&msg.data, offset_size, length_size)?;
     resolve_shared_message(
         file_data,
         &shared_ref,
@@ -649,15 +665,26 @@ mod tests {
 
     #[test]
     fn parse_v1_ref() {
-        let mut data = Vec::new();
-        data.push(1); // version
-        data.push(0); // type
-        data.extend_from_slice(&[0u8; 6]); // reserved
-        data.extend_from_slice(&0x5678u64.to_le_bytes());
+        // Datatype message of `/group1/dset2` in HDF5's `tcompound.h5`
+        // (written in 2000): version 1, six reserved bytes, then an old-style
+        // symbol table entry — link-name offset 0x10, object header address
+        // 0x590 (the committed datatype `/type1`), cache type, reserved and
+        // scratch.
+        let mut data = vec![1, 0, 0, 0, 0, 0, 0, 0];
+        data.extend_from_slice(&0x10u64.to_le_bytes());
+        data.extend_from_slice(&0x590u64.to_le_bytes());
+        data.extend_from_slice(&[0; 24]);
 
-        let shared = parse_shared_ref(&data, 8).unwrap();
+        let shared = parse_shared_ref_sized(&data, 8, 8).unwrap();
         assert_eq!(shared.version, 1);
-        assert_eq!(shared.object_header_address, Some(0x5678));
+        assert_eq!(shared.object_header_address, Some(0x590));
+
+        // The name offset is a length: 4 bytes here, then an 8-byte address.
+        let mut data = vec![1, 0, 0, 0, 0, 0, 0, 0];
+        data.extend_from_slice(&0x10u32.to_le_bytes());
+        data.extend_from_slice(&0x590u64.to_le_bytes());
+        let shared = parse_shared_ref_sized(&data, 8, 4).unwrap();
+        assert_eq!(shared.object_header_address, Some(0x590));
     }
 
     #[test]

crates/clawhdf5-format/tests/fixtures/legacy/README.md

@@ -0,0 +1,13 @@
+# Legacy (HDF5 1.4/1.6-era) fixtures
+
+Unmodified copies of the HDF Group's own test files from
+https://github.com/HDFGroup/hdf5 at a3cf1ea82cc7a66e50029a688121e1b105a7ce88
+(BSD-style license, see that repository's `LICENSE`). Current libraries cannot
+write these structures, so they are kept as files.
+
+| File | Upstream path | Exercises |
+|---|---|---|
+| `deflate.h5` | `test/testfiles/deflate.h5` | Data Layout message v1, chunked + deflate (v1 B-tree index) |
+| `h5ex_g_iterate.h5` | `HDF5Examples/C/H5G/h5ex_g_iterate.h5` | Data Layout message v2, contiguous; an unallocated dataset |
+| `tarrold.h5` | `test/testfiles/tarrold.h5` | Compound datatype v1 members with legacy array dimensions |
+| `tcompound.h5` | `tools/test/testfiles/tcompound.h5` | Version-1 shared messages (committed datatypes); compound v1 array members with data |

crates/clawhdf5/tests/legacy_format_interop.rs

@@ -0,0 +1,220 @@
+//! Files written by HDF5 1.4/1.6-era libraries: Data Layout message versions
+//! 1 and 2, compound datatype version 1 array members, and version-1 shared
+//! message references. The fixtures are HDF5's own test files (see
+//! `clawhdf5-format/tests/fixtures/legacy/README.md`).
+//!
+//! The expected values were read with h5py 3.16 / HDF5 2.0; the interop test
+//! re-checks every dataset byte for byte against h5py, and is skipped when
+//! python3 with h5py is unavailable unless `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+use std::process::Command;
+
+use clawhdf5::{DType, File};
+use clawhdf5_format::selection::Selection;
+
+const FIXTURES: &str = concat!(
+    env!("CARGO_MANIFEST_DIR"),
+    "/../clawhdf5-format/tests/fixtures/legacy"
+);
+
+fn open(name: &str) -> File {
+    File::open(format!("{FIXTURES}/{name}")).unwrap()
+}
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+fn python_available() -> bool {
+    Command::new(python())
+        .args(["-c", "import h5py, numpy"])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+/// Layout v1, chunked (50x50 chunks of a 100x200 dataset), deflate: every
+/// read path goes through the version-1 B-tree chunk index.
+#[test]
+fn layout_v1_chunked_deflate() {
+    let file = open("deflate.h5");
+    let ds = file.dataset("Dataset1").unwrap();
+    assert_eq!(ds.shape().unwrap(), [100, 200]);
+    let expected: Vec<i32> = (0..100).flat_map(|_| (0..200).map(|j| j % 5)).collect();
+    assert_eq!(ds.read_i32().unwrap(), expected);
+
+    // A hyperslab that straddles four chunks.
+    let slab = Selection::Hyperslab {
+        start: vec![48, 48],
+        stride: vec![1, 1],
+        count: vec![4, 4],
+        block: vec![1, 1],
+    };
+    let raw = ds.read_selection(&slab).unwrap();
+    let got: Vec<i32> = raw
+        .as_chunks::<4>()
+        .0
+        .iter()
+        .map(|b| i32::from_le_bytes(*b))
+        .collect();
+    assert_eq!(got, [3, 4, 0, 1, 3, 4, 0, 1, 3, 4, 0, 1, 3, 4, 0, 1]);
+}
+
+/// Layout v2, contiguous: one dataset with storage, one never written (reads
+/// as its fill value, 0).
+#[test]
+fn layout_v2_contiguous() {
+    let file = open("h5ex_g_iterate.h5");
+    assert_eq!(file.dataset("G1/DS2").unwrap().read_i32().unwrap(), [1]);
+    assert_eq!(file.dataset("DS1").unwrap().read_i32().unwrap(), [0]);
+}
+
+/// Compound datatype version 1 members carrying legacy array dimensions
+/// (HDF5 before 1.4 had no array class). h5py: `[('i', '<i2'), ('f', '<f4',
+/// (4,)), ('l', '<i4', (4,)), ('d', '<f8')]`, itemsize 44.
+#[test]
+fn compound_v1_legacy_array_members() {
+    let file = open("tarrold.h5");
+    let ds = file.dataset("Dataset2").unwrap();
+    assert_eq!(
+        ds.dtype().unwrap(),
+        DType::Compound(vec![
+            ("i".into(), DType::I16),
+            ("f".into(), DType::Array(Box::new(DType::F32), vec![4])),
+            ("l".into(), DType::Array(Box::new(DType::I32), vec![4])),
+            ("d".into(), DType::F64),
+        ])
+    );
+    assert_eq!(ds.shape().unwrap(), [8, 9]);
+    assert_eq!(
+        ds.read_selection(&Selection::All).unwrap().len(),
+        8 * 9 * 44
+    );
+}
+
+/// Datasets whose committed datatype is referenced by a version-1 shared
+/// message, whose object header address follows a link-name offset. Values
+/// from h5py; the file is big-endian.
+#[test]
+fn shared_message_v1_committed_datatypes() {
+    let file = open("tcompound.h5");
+    let be_pairs = |name: &str| -> Vec<(i32, f32)> {
+        file.dataset(name)
+            .unwrap()
+            .read_selection(&Selection::All)
+            .unwrap()
+            .as_chunks::<8>()
+            .0
+            .iter()
+            .map(|b| {
+                (
+                    i32::from_be_bytes(b[..4].try_into().unwrap()),
+                    f32::from_be_bytes(b[4..].try_into().unwrap()),
+                )
+            })
+            .collect()
+    };
+    assert_eq!(
+        be_pairs("group1/dset2"),
+        [(0, 0.0), (1, 1.1), (2, 2.2), (3, 3.3), (4, 4.4)]
+    );
+    assert_eq!(
+        be_pairs("group2/dset5"),
+        [(0, 0.0), (1, 0.1), (2, 0.2), (3, 0.3), (4, 0.4)]
+    );
+
+    // `/type2`: { int_array: i32[4], float_array: f32[5][6] }, whose array
+    // members are compound v1 legacy dimensions.
+    let dset3 = file.dataset("group1/dset3").unwrap();
+    assert_eq!(
+        dset3.dtype().unwrap(),
+        DType::Compound(vec![
+            (
+                "int_array".into(),
+                DType::Array(Box::new(DType::I32), vec![4])
+            ),
+            (
+                "float_array".into(),
+                DType::Array(Box::new(DType::F32), vec![5, 6])
+            ),
+        ])
+    );
+    let raw = dset3.read_selection(&Selection::All).unwrap();
+    assert_eq!(raw.len(), 3 * 6 * (16 + 120));
+    assert_eq!(
+        &raw[..16],
+        &[0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3]
+    );
+    let first: Vec<f32> = raw[16..16 + 120]
+        .as_chunks::<4>()
+        .0
+        .iter()
+        .map(|b| f32::from_be_bytes(*b))
+        .collect();
+    let expected: Vec<f32> = (0..5)
+        .flat_map(|i| (0..6).map(move |j| (1 + i + j) as f32))
+        .collect();
+    assert_eq!(first, expected);
+}
+
+/// Every dataset in every fixture, byte for byte against h5py.
+#[test]
+fn legacy_fixtures_match_h5py() {
+    if !python_available() {
+        assert!(
+            !interop_required(),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but python3 with h5py is not available"
+        );
+        eprintln!("SKIP: python3 with h5py not available");
+        return;
+    }
+    for (name, datasets) in [
+        ("deflate.h5", &["Dataset1"][..]),
+        ("h5ex_g_iterate.h5", &["DS1", "G1/DS2"][..]),
+        ("tarrold.h5", &["Dataset1", "Dataset2"][..]),
+        (
+            "tcompound.h5",
+            &[
+                "dset1",
+                "group1/dset2",
+                "group1/dset3",
+                "group1/dset4",
+                "group2/dset5",
+            ][..],
+        ),
+    ] {
+        let path = format!("{FIXTURES}/{name}");
+        let script = format!(
+            r#"
+import h5py, numpy as np
+f = h5py.File({path:?}, "r")
+for n in {datasets:?}:
+    print(n, np.ascontiguousarray(f[n][()]).tobytes().hex())
+"#
+        );
+        let out = Command::new(python())
+            .args(["-c", &script])
+            .output()
+            .unwrap();
+        assert!(
+            out.status.success(),
+            "h5py: {}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+        let file = File::open(&path).unwrap();
+        for line in String::from_utf8(out.stdout).unwrap().lines() {
+            let (ds, hex) = line.split_once(' ').unwrap();
+            let ours = file
+                .dataset(ds)
+                .unwrap()
+                .read_selection(&Selection::All)
+                .unwrap();
+            let ours: String = ours.iter().map(|b| format!("{b:02x}")).collect();
+            assert_eq!(ours, hex, "{name}:{ds}");
+        }
+    }
+}

docs/known-issues.md

@@ -78,12 +78,20 @@ the VDS item, which is marked.
 
 - **Layout message versions 1 and 2** (HDF5 1.6-era files): 84 of the 686
   sweep files, `InvalidLayoutVersion`. This is the largest single gap.
+  **Fixed 2026-09-25:** versions 1 and 2 are parsed (compact, contiguous,
+  chunked via the v1 B-tree).
+- **Compound datatype version 1 array members** (found with the layout
+  fix; pre-1.4 files such as `tarrold.h5`): **wrong data** — the legacy
+  per-member dimensions were skipped, so an array member read as one scalar.
+  **Fixed 2026-09-25.**
 - **Virtual datasets:**
   - **Wrong data:** unmapped regions read as 0 instead of the fill value.
   - `%b` printf-style source names are not expanded.
   - Hyperslab selection versions 1 and 2 are refused.
 - **Files with a user block:** the base address is not applied.
 - **Old-style shared messages (version 1)** read the wrong address.
+  **Fixed 2026-09-25:** the address follows the link-name offset of the
+  embedded symbol table entry.
 - **Groups and links:**
   - Groups with a user-defined link type (e.g. 187) cannot be listed.
   - Dense groups with more than about 22 000 links cannot be listed.