Merge branch 'feat/p2-vl-strings' into feat/p2-perf-coverage

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -54,6 +54,91 @@
   whole-row hyperslabs, points, empty selections; every type, both byte
   orders, ranks 1-4).
 
+### Variable-length data (2026-09-26)
+- **VL values in files with 4-byte offsets** (`sizeof_addr = 4`). A VL
+  string attribute came back as `AttrValue::Raw`, a VL member of a compound
+  failed with `GlobalHeapObjectNotFound`, and VL datasets failed with a
+  size mismatch. Two causes: `Datatype::type_size()` reported 16 for every
+  VL type (the element is 4 + offset size + 4 bytes: 12 here), and the
+  global heap was parsed without the padding libhdf5 puts after its
+  collection and object headers (`H5HG_SIZEOF_HDR`/`H5HG_SIZEOF_OBJHDR`
+  round up to 8), so with 4-byte lengths every object was looked up 4
+  bytes early. `Datatype::VariableLength` now carries the element `size`
+  stored in the datatype message (**breaking** for code that builds or
+  exhaustively destructures that variant; patterns with `..` are
+  unaffected), and it is written back as stored. Tested against h5py
+  (`crates/clawhdf5/tests/vl_offset4_interop.rs`).
+- **Wrong data: VL strings with an embedded NUL, and VL elements whose heap
+  object has the wrong size.** libhdf5 hands VL strings over as C strings,
+  so h5py reads `"a\0b"` as `"a"`; `read_vl_strings` returned the NUL and
+  what followed. An element whose heap object is not exactly
+  `length × base size` bytes is refused by libhdf5 ("Expected global heap
+  object size does not match"); we returned the object cut or padded to
+  the length. Both now behave as libhdf5, and a heap address of 0 is a null
+  element (empty) whatever its length. The new
+  `clawhdf5_format::vl_data::VlResolver` does this and parses each global
+  heap collection once per read: `read_vl_strings` parsed the whole
+  collection again for every element. `vl_data::check_element_size` refuses
+  a VL datatype whose stored size is not 4 + offset size + 4 (libhdf5
+  ignores the stored size). The conformance probe resolves VL elements
+  with `VlResolver` too; conformance unchanged at 575 of 697.
+- **VL data through the facade.** VL-string datasets (h5py's default `str`
+  dtype) failed `read_string` with "type mismatch: expected String, got
+  VariableLength". `Dataset::read_string` now reads fixed- and
+  variable-length strings; new `read_string_bytes` (a VL string's exact
+  bytes, as h5py's `Dataset[()]` returns them), `read_string_selection`,
+  `read_vlen::<T>()` / `read_vlen_selection::<T>()` for VL sequences of
+  numbers (`T` = `f64`, `f32`, `i64`, `i32`, `u64`; converted like the
+  other typed readers), and `File::decode_strings` / `decode_string_bytes`
+  / `decode_vlen` for VL values in compound fields and `AttrValue::Raw`
+  attributes. `MmapDataset` and `LazyDataset` gain `read_string` for VL
+  strings, `read_string_bytes` and `read_vlen`. Checked against h5py with
+  8- and 4-byte offsets: scalar and 1-/2-D, ASCII and UTF-8, empty strings,
+  contiguous, compact, chunked with gzip/shuffle, never-written and
+  partly written chunks, hyperslab selections, VL members of compound
+  datasets and attributes (`crates/clawhdf5/tests/vl_data_interop.rs`).
+  NetCDF-4 `string` variables now read through
+  `clawhdf5_netcdf4::Variable::read_string` (checked against netCDF4-python
+  in `crates/clawhdf5-netcdf4/tests/interop_tests.rs`).
+
+- **Crafted global heaps could exhaust memory.** `VlResolver` kept an owned
+  copy of every object of every heap collection it read, so collections
+  nested inside each other's object data made a 744 KB file take 1.58 GB
+  (and `read_vl_strings` before it did the same). The cache now records
+  where objects lie instead of copying them, is dropped past a 32 MiB
+  budget, and a collection overlapping one already read is an error
+  (libhdf5 never writes one). New `GlobalHeapCollection::parse_index`
+  locates a collection's objects without copying them; `parse` and
+  `parse_index` refuse a collection running past the end of the file or an
+  object running past its collection. Conformance unchanged at 575 of 697
+  (`crates/clawhdf5-format/tests/vl_heap_bounds.rs`).
+
+- **Every reader resolves VL data the same way.** `h5rs` (`dump`, `ls`,
+  `diff`, `check --data`) had its own lenient VL decoder: a heap object
+  longer than the element's length was cut to it (h5py refuses it), a null
+  string printed `""` where h5dump prints `NULL`, the stored element size
+  was trusted, and each heap collection was kept as a copy for the whole
+  run. It now resolves through `VlResolver`, so `dump` matches h5dump byte
+  for byte on VL strings (`"a\0b"` as `"a"`, null as `NULL`), VL sequences
+  and 4-byte-offset files, `dump --json` gives h5py's values, and
+  `check --data` reports any heap object whose size is not exactly the
+  element's length × base size. `clawhdf5-wasm` already resolved VL strings
+  with `read_vl_strings`; it now uses `VlResolver` and refuses a VL type
+  whose stored element size disagrees with the file, as `File` does
+  (`crates/clawhdf5-tools/tests/h5rs_interop.rs`,
+  `crates/clawhdf5-wasm/tests/vl_strings.rs`). New
+  `VlResolver::element` / `string_element` resolve one element in place.
+
+- **A VL element at the undefined heap address is an error**, as in
+  libhdf5 ("addr undefined"). One of length 0 read as `""` in every reader
+  (`File`, `h5rs`, `clawhdf5-wasm`, `read_vl_strings`, `read_vl_bytes`).
+  libhdf5 writes a null element with heap address 0, which still reads as
+  empty, and h5py writes `""` as a zero-size heap object at a real address,
+  so no file libhdf5 or h5py writes is affected
+  (`a_vl_element_at_the_undefined_heap_address_fails_like_h5py` in
+  `crates/clawhdf5/tests/vl_data_interop.rs`). `read_vl_bytes` now also
+  treats address 0 as null whatever the length, as `VlResolver` does.
+
 ### Plugin filters (2026-09-26)
 - **LZF, bitshuffle, bzip2 and Blosc read and write, in pure Rust.** Files
   written by h5py with `compression="lzf"`, or with hdf5plugin's
@@ -257,10 +342,10 @@
     printed with its address; exit 1 when there are any. libhdf5's h5check
     reads only the 1.8 format. On the conformance corpus it passes all 418
     files that both clawhdf5 and h5py read in full, and `check --data` flags
-    134 of the 150 CVE and fuzzer files of the `cve_hdf5` corpus (tank,
+    135 of the 150 CVE and fuzzer files of the `cve_hdf5` corpus (tank,
     2026-09-26). `--data` also follows variable-length data into its global
     heap collections and reports a damaged one at its address. It inherits
-    the library's tolerance, though: 9 of the 16 it passes are files h5dump
+    the library's tolerance, though: 8 of the 15 it passes are files h5dump
     1.14.6 rejects (see `docs/known-issues.md`, header checks).
   - Values over `--max-bytes` (default 1 GiB) are reported instead of read;
     a panic is caught and reported as an internal error (exit 3).

conformance/probe/src/main.rs

@@ -19,9 +19,8 @@
 //! with its message, location and the clawhdf5 frames of its backtrace.
 
 use std::cell::RefCell;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::panic::{self, AssertUnwindSafe};
-use std::rc::Rc;
 
 use clawhdf5_format::attribute::extract_attributes_full;
 use clawhdf5_format::data_layout::DataLayout;
@@ -29,7 +28,6 @@ use clawhdf5_format::data_read;
 use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
 use clawhdf5_format::datatype::{Datatype, DatatypeByteOrder};
 use clawhdf5_format::filter_pipeline::FilterPipeline;
-use clawhdf5_format::global_heap::GlobalHeapCollection;
 use clawhdf5_format::group_v1::{self, GroupEntry};
 use clawhdf5_format::group_v2;
 use clawhdf5_format::message_type::MessageType;
@@ -37,6 +35,7 @@ use clawhdf5_format::object_header::ObjectHeader;
 use clawhdf5_format::signature;
 use clawhdf5_format::superblock::Superblock;
 use clawhdf5_format::symbol_table::SymbolTableMessage;
+use clawhdf5_format::vl_data::{VlResolver, check_element_size};
 use serde_json::{Map, Value, json};
 use sha2::{Digest, Sha256};
 
@@ -111,7 +110,10 @@ struct Ctx<'a> {
     os: u8,
     ls: u8,
     base_dir: std::path::PathBuf,
-    heaps: RefCell<HashMap<u64, Result<Rc<GlobalHeapCollection>, String>>>,
+    /// Resolves variable-length elements as the library does (null
+    /// elements, strings cut at a NUL, heap objects of the wrong size
+    /// refused), caching each heap collection.
+    vl: RefCell<VlResolver<'a>>,
 }
 
 impl<'a> Ctx<'a> {
@@ -130,33 +132,6 @@ impl<'a> Ctx<'a> {
         }
     }
 
-    fn heap_obj(&self, addr: u64, idx: u32) -> Result<Vec<u8>, String> {
-        let coll = {
-            let mut cache = self.heaps.borrow_mut();
-            cache
-                .entry(addr)
-                .or_insert_with(|| {
-                    GlobalHeapCollection::parse(self.data, addr as usize, self.ls)
-                        .map(Rc::new)
-                        .map_err(e)
-                })
-                .clone()?
-        };
-        coll.get_object(idx as u16)
-            .map(|o| o.data.clone())
-            .ok_or_else(|| {
-                format!("GlobalHeapObjectNotFound {{ collection_address: {addr}, index: {idx} }}")
-            })
-    }
-
-    fn read_offset(&self, b: &[u8]) -> u64 {
-        let mut v = 0u64;
-        for (i, x) in b.iter().take(self.os as usize).enumerate() {
-            v |= (*x as u64) << (8 * i);
-        }
-        v
-    }
-
     fn canon(&self, dt: &Datatype, b: &[u8], out: &mut Vec<u8>) -> Result<(), String> {
         let size = dt.type_size() as usize;
         if b.len() < size {
@@ -204,41 +179,27 @@ impl<'a> Ctx<'a> {
                 }
             }
             Datatype::VariableLength {
+                size: vl_size,
                 is_string,
                 base_type,
                 ..
             } => {
-                let len = u32::from_le_bytes([b[0], b[1], b[2], b[3]]) as usize;
+                check_element_size(*vl_size, self.os).map_err(e)?;
-                let addr = self.read_offset(&b[4..]);
+                let el = &b[..size];
-                let idx_off = 4 + self.os as usize;
-                let idx = u32::from_le_bytes([
-                    b[idx_off],
-                    b[idx_off + 1],
-                    b[idx_off + 2],
-                    b[idx_off + 3],
-                ]);
-                let obj = if len == 0 || addr == 0 || addr == u64::MAX >> (64 - 8 * self.os as u32)
-                {
-                    Vec::new()
-                } else {
-                    self.heap_obj(addr, idx)?
-                };
                 if *is_string {
-                    let l = len.min(obj.len());
+                    let s = self.vl.borrow_mut().string_bytes(el).map_err(e)?;
-                    canon_str(&obj[..l], out);
+                    canon_str(&s[0], out);
                 } else {
                     let bs = base_type.type_size() as usize;
-                    if bs == 0 {
+                    // The borrow ends here: the base type may itself be
-                        return Err("canon: VL base size 0".into());
+                    // variable-length.
-                    }
+                    let seq = self.vl.borrow_mut().sequences(el, bs).map_err(e)?;
-                    let need = len.checked_mul(bs).ok_or("canon: VL overflow")?;
+                    let seq = &seq[0];
-                    if len > 0 && obj.len() < need {
+                    let len = seq.len() / bs;
-                        return Err(format!("canon: VL object {} < {need}", obj.len()));
-                    }
                     out.push(b'V');
                     out.extend_from_slice(&(len as u32).to_le_bytes());
                     for i in 0..len {
-                        self.canon(base_type, &obj[i * bs..], out)?;
+                        self.canon(base_type, &seq[i * bs..], out)?;
                     }
                 }
             }
@@ -744,7 +705,7 @@ fn main() {
             .parent()
             .map(|p| p.to_path_buf())
             .unwrap_or_default(),
-        heaps: RefCell::new(HashMap::new()),
+        vl: RefCell::new(VlResolver::new(hdf5, sb.offset_size, sb.length_size)),
     };
     let mut objects: Vec<Value> = Vec::new();
     let mut visited = HashSet::new();

crates/clawhdf5-format/src/datatype.rs

@@ -125,6 +125,11 @@ pub enum Datatype {
     },
     /// Class 9: Variable-length type.
     VariableLength {
+        /// Size of one element as stored in the file: a sequence length (4
+        /// bytes), a global heap collection address (the file's
+        /// `offset_size`) and an object index (4 bytes) — 16 in a file with
+        /// 8-byte offsets, 12 with 4-byte offsets.
+        size: u32,
         is_string: bool,
         padding: Option<StringPadding>,
         charset: Option<CharacterSet>,
@@ -771,6 +776,7 @@ impl Datatype {
                 pos += consumed;
                 Ok((
                     Datatype::VariableLength {
+                        size,
                         is_string,
                         padding,
                         charset,
@@ -1017,6 +1023,7 @@ impl Datatype {
                 Self::build_header(3, 1, [bf0, 0, 0], *size)
             }
             Datatype::VariableLength {
+                size,
                 is_string,
                 padding,
                 charset,
@@ -1039,7 +1046,7 @@ impl Datatype {
                 } else {
                     0
                 };
-                let mut buf = Self::build_header(9, 1, [bf0, bf1, 0], 16);
+                let mut buf = Self::build_header(9, 1, [bf0, bf1, 0], *size);
                 buf.extend_from_slice(&base_type.serialize());
                 buf
             }
@@ -1208,7 +1215,7 @@ impl Datatype {
             Datatype::Compound { size, .. } => *size,
             Datatype::Reference { size, .. } => *size,
             Datatype::Enumeration { size, .. } => *size,
-            Datatype::VariableLength { .. } => 16, // typically pointer + length
+            Datatype::VariableLength { size, .. } => *size,
             Datatype::Array {
                 base_type,
                 dimensions,
@@ -1889,11 +1896,13 @@ mod tests {
         let (dt, _) = Datatype::parse(&buf).unwrap();
         match dt {
             Datatype::VariableLength {
+                size,
                 is_string,
                 padding,
                 charset,
                 base_type,
             } => {
+                assert_eq!(size, 16);
                 assert!(is_string);
                 assert_eq!(padding, Some(StringPadding::NullTerminate));
                 assert_eq!(charset, Some(CharacterSet::Utf8));
@@ -1914,11 +1923,13 @@ mod tests {
         let (dt, _) = Datatype::parse(&buf).unwrap();
         match dt {
             Datatype::VariableLength {
+                size,
                 is_string,
                 padding,
                 charset,
                 base_type,
             } => {
+                assert_eq!(size, 16);
                 assert!(!is_string);
                 assert_eq!(padding, None);
                 assert_eq!(charset, None);
@@ -1928,6 +1939,19 @@ mod tests {
         }
     }
 
+    #[test]
+    fn variable_length_size_is_the_stored_size() {
+        // A file with 4-byte offsets stores 12-byte VL elements (length 4 +
+        // address 4 + index 4); the type used to report 16 regardless, so
+        // every read laid the elements out 16 bytes apart.
+        let mut buf = build_dt_header(9, 1, [0x01, 0x00, 0], 12);
+        buf.extend_from_slice(&build_fixed_point(1, false, false, 0, 8));
+        let (dt, _) = Datatype::parse(&buf).unwrap();
+        assert_eq!(dt.type_size(), 12);
+        // And it is written back as stored.
+        assert_eq!(dt.serialize()[4..8], 12u32.to_le_bytes());
+    }
+
     #[test]
     fn test_array_2d() {
         // Array [3][4] of i32 LE, version 3

crates/clawhdf5-format/src/global_heap.rs

@@ -1,7 +1,7 @@
 //! HDF5 Global Heap collection parsing.
 
 #[cfg(not(feature = "std"))]
-use alloc::vec::Vec;
+use alloc::{format, string::String, vec::Vec};
 
 use crate::error::FormatError;
 
@@ -52,11 +52,42 @@ fn read_length(data: &[u8], offset: usize, length_size: u8) -> Result<u64, Forma
     })
 }
 
+fn object_overrun_msg(index: u16, size: usize, collection_size: u64) -> String {
+    format!(
+        "global heap object {index} ({size} bytes) runs past the end of its \
+         {collection_size}-byte collection"
+    )
+}
+
 /// Round up to next multiple of 8.
 fn pad8(x: usize) -> usize {
     (x + 7) & !7
 }
 
+/// Where one object of a global heap collection lies in the file, without
+/// its data: see [`GlobalHeapCollection::parse_index`].
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct GlobalHeapObjectRef {
+    /// Object index (1-based; 0 is the free space marker).
+    pub index: u16,
+    /// Reference count.
+    pub reference_count: u16,
+    /// Offset of the object's data in the file data the collection was
+    /// parsed from.
+    pub offset: usize,
+    /// Size of the object's data in bytes.
+    pub size: usize,
+}
+
+/// A global heap collection's objects, located but not copied.
+#[derive(Debug, Clone)]
+pub struct GlobalHeapIndex {
+    /// Total size of this collection including header.
+    pub collection_size: u64,
+    /// The objects, in file order.
+    pub objects: Vec<GlobalHeapObjectRef>,
+}
+
 impl GlobalHeapCollection {
     /// Parse a global heap collection at the given offset in the file data.
     pub fn parse(
@@ -64,8 +95,38 @@ impl GlobalHeapCollection {
         offset: usize,
         length_size: u8,
     ) -> Result<GlobalHeapCollection, FormatError> {
-        // signature(4) + version(1) + reserved(3) + collection_size(length_size)
+        let index = Self::parse_index(file_data, offset, length_size)?;
-        let header_size = 8 + length_size as usize;
+        Ok(GlobalHeapCollection {
+            collection_size: index.collection_size,
+            objects: index
+                .objects
+                .iter()
+                .map(|o| GlobalHeapObject {
+                    index: o.index,
+                    reference_count: o.reference_count,
+                    data: file_data[o.offset..o.offset + o.size].to_vec(),
+                })
+                .collect(),
+        })
+    }
+
+    /// Locate the objects of the global heap collection at `offset` without
+    /// copying their data, so a caller can keep many collections indexed
+    /// for the cost of their object headers.
+    ///
+    /// The collection must lie inside `file_data`, and every object inside
+    /// the collection, as libhdf5 lays them out; an object that runs past
+    /// its collection is an error.
+    pub fn parse_index(
+        file_data: &[u8],
+        offset: usize,
+        length_size: u8,
+    ) -> Result<GlobalHeapIndex, FormatError> {
+        // signature(4) + version(1) + reserved(3) + collection_size(length_size),
+        // padded to a multiple of 8 as libhdf5 lays it out (`H5HG_SIZEOF_HDR`).
+        // With 8-byte lengths the padding is 0; with 4-byte lengths it is 4,
+        // and reading without it put every object 4 bytes early.
+        let header_size = pad8(8 + length_size as usize);
         ensure_len(file_data, offset, header_size)?;
 
         if file_data[offset..offset + 4] != GCOL_SIGNATURE {
@@ -78,25 +139,25 @@ impl GlobalHeapCollection {
         }
 
         let collection_size = read_length(file_data, offset + 8, length_size)?;
-        let collection_size_usize =
+        let collection_end = usize::try_from(collection_size)
-            usize::try_from(collection_size).map_err(|_| FormatError::UnexpectedEof {
+            .ok()
-                expected: u64::MAX as usize,
+            .and_then(|size| offset.checked_add(size))
-                available: file_data.len(),
-            })?;
-        let collection_end =
-            offset
-                .checked_add(collection_size_usize)
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
                 available: file_data.len(),
             })?;
+        if collection_end > file_data.len() {
+            return Err(FormatError::UnexpectedEof {
+                expected: collection_end,
+                available: file_data.len(),
+            });
+        }
 
         let mut pos = offset + header_size;
         let mut objects = Vec::new();
 
         // Parse objects until we hit index 0 (free space) or run out of space
         while pos + 2 <= collection_end {
-            ensure_len(file_data, pos, 2)?;
             let object_index = u16::from_le_bytes([file_data[pos], file_data[pos + 1]]);
 
             if object_index == 0 {
@@ -104,28 +165,39 @@ impl GlobalHeapCollection {
                 break;
             }
 
-            // object_index(2) + reference_count(2) + reserved(4) + object_size(length_size)
+            // object_index(2) + reference_count(2) + reserved(4) +
-            let obj_header_size = 8 + length_size as usize;
+            // object_size(length_size), padded to 8 (`H5HG_SIZEOF_OBJHDR`).
-            ensure_len(file_data, pos, obj_header_size)?;
+            let obj_header_size = pad8(8 + length_size as usize);
+            ensure_len(&file_data[..collection_end], pos, obj_header_size)?;
 
             let reference_count = u16::from_le_bytes([file_data[pos + 2], file_data[pos + 3]]);
-            let object_size = read_length(file_data, pos + 8, length_size)? as usize;
+            let object_size = usize::try_from(read_length(file_data, pos + 8, length_size)?)
+                .map_err(|_| FormatError::Overflow("global heap object size".into()))?;
 
             pos += obj_header_size;
-            ensure_len(file_data, pos, object_size)?;
+            if pos
-            let data = file_data[pos..pos + object_size].to_vec();
+                .checked_add(object_size)
+                .is_none_or(|end| end > collection_end)
+            {
+                return Err(FormatError::VlDataError(object_overrun_msg(
+                    object_index,
+                    object_size,
+                    collection_size,
+                )));
+            }
 
-            objects.push(GlobalHeapObject {
+            objects.push(GlobalHeapObjectRef {
                 index: object_index,
                 reference_count,
-                data,
+                offset: pos,
+                size: object_size,
             });
 
             // Advance past data + padding to 8-byte boundary
-            pos += pad8(object_size);
+            pos = pos.saturating_add(pad8(object_size));
         }
 
-        Ok(GlobalHeapCollection {
+        Ok(GlobalHeapIndex {
             collection_size,
             objects,
         })
@@ -149,10 +221,11 @@ mod tests {
         let ls = length_size as usize;
 
         // Calculate total size
-        let header_size = 8 + ls;
+        // libhdf5 pads both headers to a multiple of 8.
+        let header_size = pad8(8 + ls);
         let mut obj_size_total = 0usize;
         for (_, _, data) in objects {
-            let obj_header = 8 + ls;
+            let obj_header = pad8(8 + ls);
             obj_size_total += obj_header + pad8(data.len());
         }
         // Free space marker (2 bytes for index 0)
@@ -170,6 +243,7 @@ mod tests {
             8 => buf.extend_from_slice(&(collection_size as u64).to_le_bytes()),
             _ => panic!("unsupported length_size"),
         }
+        buf.resize(header_size, 0);
 
         // Objects
         for (index, ref_count, data) in objects {
@@ -181,6 +255,7 @@ mod tests {
                 8 => buf.extend_from_slice(&(data.len() as u64).to_le_bytes()),
                 _ => panic!("unsupported"),
             }
+            buf.resize(buf.len() + (pad8(8 + ls) - (8 + ls)), 0);
             buf.extend_from_slice(data);
             // Pad to 8 bytes
             let padded = pad8(data.len());

crates/clawhdf5-format/src/vl_data.rs

@@ -5,10 +5,12 @@
 //! `sequence_length(4 LE) + collection_address(offset_size LE) + object_index(4 LE)`.
 
 #[cfg(not(feature = "std"))]
-use alloc::{string::String, vec::Vec};
+use alloc::{collections::BTreeMap, format, string::String, vec, vec::Vec};
+#[cfg(feature = "std")]
+use std::collections::BTreeMap;
 
 use crate::error::FormatError;
-use crate::global_heap::GlobalHeapCollection;
+use crate::global_heap::{GlobalHeapCollection, GlobalHeapIndex};
 
 /// A parsed variable-length element reference (global heap ID).
 #[derive(Debug, Clone)]
@@ -109,7 +111,218 @@ fn is_undefined_address(addr: u64, offset_size: u8) -> bool {
     }
 }
 
+/// The size of one variable-length element in a file with `offset_size`-byte
+/// addresses: a sequence length (4), a global heap collection address and an
+/// object index (4). libhdf5 computes it this way rather than trusting the
+/// datatype message (`H5T_set_loc`).
+pub fn element_size(offset_size: u8) -> usize {
+    4 + offset_size as usize + 4
+}
+
+/// Refuse a variable-length datatype whose stored element size is not the
+/// one this file's offset size implies. Its elements would be laid out with
+/// a stride libhdf5 does not use, so every value after the first would be
+/// read from the wrong place.
+pub fn check_element_size(stored_size: u32, offset_size: u8) -> Result<(), FormatError> {
+    let expected = element_size(offset_size);
+    if stored_size as usize != expected {
+        return Err(FormatError::VlDataError(format!(
+            "variable-length datatype stores {stored_size}-byte elements; a file with \
+             {offset_size}-byte offsets uses {expected}"
+        )));
+    }
+    Ok(())
+}
+
+/// A collection's objects, located in the file data but not copied:
+/// `(index, offset, size)` of the first object with each index, sorted by
+/// index.
+struct CachedCollection {
+    objects: Vec<(u16, usize, usize)>,
+}
+
+impl CachedCollection {
+    fn new(index: GlobalHeapIndex) -> Self {
+        let mut objects: Vec<(u16, usize, usize)> = index
+            .objects
+            .iter()
+            .map(|o| (o.index, o.offset, o.size))
+            .collect();
+        // Stable, so the first object with a repeated index is kept.
+        objects.sort_by_key(|o| o.0);
+        objects.dedup_by_key(|o| o.0);
+        Self { objects }
+    }
+
+    /// What this entry costs to keep, in bytes (roughly).
+    fn cost(&self) -> usize {
+        64 + self.objects.len() * core::mem::size_of::<(u16, usize, usize)>()
+    }
+
+    fn get(&self, index: u32) -> Option<(usize, usize)> {
+        let index = u16::try_from(index).ok()?;
+        let i = self.objects.binary_search_by_key(&index, |o| o.0).ok()?;
+        Some((self.objects[i].1, self.objects[i].2))
+    }
+}
+
+/// How many bytes of collection indexes a [`VlResolver`] keeps before it
+/// drops them and starts again. Values are never copied into the cache, so
+/// this bounds what a read retains however many collections it visits.
+const CACHE_BUDGET: usize = 32 << 20;
+
+/// Resolves variable-length elements against a file's global heap, parsing
+/// each heap collection once however many elements point into it.
+///
+/// Values follow libhdf5: an element whose heap address is 0 is null (an
+/// empty string or sequence), and an element whose heap object is not
+/// exactly `length × base size` bytes is an error ("Expected global heap
+/// object size does not match"), not a truncated or padded value.
+///
+/// Memory stays bounded on hostile files: the cache holds where each
+/// object lies, not a copy of it, up to a fixed budget; and collections
+/// that overlap one another are refused (libhdf5 never writes them), so a
+/// file cannot make the resolver parse the same bytes as the objects of
+/// many collections.
+pub struct VlResolver<'a> {
+    file_data: &'a [u8],
+    offset_size: u8,
+    length_size: u8,
+    cache: BTreeMap<u64, CachedCollection>,
+    cached_bytes: usize,
+    budget: usize,
+    /// Start → end of every collection parsed so far (kept when the cache
+    /// is dropped, to check overlaps).
+    extents: BTreeMap<usize, usize>,
+}
+
+impl<'a> VlResolver<'a> {
+    /// A resolver over `file_data` (the file from its superblock on), with
+    /// the superblock's offset and length sizes.
+    pub fn new(file_data: &'a [u8], offset_size: u8, length_size: u8) -> Self {
+        Self {
+            file_data,
+            offset_size,
+            length_size,
+            cache: BTreeMap::new(),
+            cached_bytes: 0,
+            budget: CACHE_BUDGET,
+            extents: BTreeMap::new(),
+        }
+    }
+
+    /// The size of one element in this file (see [`element_size`]).
+    pub fn element_size(&self) -> usize {
+        element_size(self.offset_size)
+    }
+
+    /// Split `raw` into elements; its length must be a whole number of them.
+    fn elements(&self, raw: &[u8]) -> Result<Vec<VlElement>, FormatError> {
+        let size = self.element_size();
+        if !raw.len().is_multiple_of(size) {
+            return Err(FormatError::VlDataError(format!(
+                "{} bytes is not a whole number of {size}-byte variable-length elements",
+                raw.len()
+            )));
+        }
+        parse_vl_references(raw, (raw.len() / size) as u64, self.offset_size)
+    }
+
+    /// The bytes of one element: `length × base_size` bytes from the heap,
+    /// or `None` for a null element.
+    fn resolve(
+        &mut self,
+        vl: &VlElement,
+        base_size: usize,
+    ) -> Result<Option<&'a [u8]>, FormatError> {
+        let addr = vl.collection_address;
+        if addr == 0 {
+            return Ok(None);
+        }
+        let data = self.object(vl)?;
+        let expected = (vl.length as usize)
+            .checked_mul(base_size)
+            .ok_or_else(|| FormatError::Overflow("variable-length element size".into()))?;
+        if data.len() != expected {
+            return Err(FormatError::VlDataError(format!(
+                "global heap object {} in the collection at {addr} holds {} bytes; the element \
+                 says {} × {base_size}",
+                vl.object_index,
+                data.len(),
+                vl.length
+            )));
+        }
+        Ok(Some(data))
+    }
+
+    /// One element (the first [`element_size`](Self::element_size) bytes of
+    /// `elem`) of a variable-length sequence whose base type is `base_size`
+    /// bytes: its `length × base_size` bytes, or `None` for a null element
+    /// (heap address 0).
+    pub fn element(
+        &mut self,
+        elem: &[u8],
+        base_size: usize,
+    ) -> Result<Option<&'a [u8]>, FormatError> {
+        let vl = parse_vl_references(elem, 1, self.offset_size)?;
+        self.resolve(&vl[0], base_size)
+    }
+
+    /// One variable-length string element: its bytes up to the first NUL,
+    /// or `None` for a null element (h5dump prints it as `NULL`, h5py
+    /// returns it as empty).
+    pub fn string_element(&mut self, elem: &[u8]) -> Result<Option<&'a [u8]>, FormatError> {
+        Ok(self.element(elem, 1)?.map(cut_at_nul))
+    }
+
+    /// The strings of the variable-length string elements in `raw`, as
+    /// bytes. A string ends at its first NUL, as libhdf5 returns it (it
+    /// converts each to a C string); a null element is empty.
+    pub fn string_bytes(&mut self, raw: &[u8]) -> Result<Vec<Vec<u8>>, FormatError> {
+        self.elements(raw)?
+            .iter()
+            .map(|vl| Ok(self.resolve(vl, 1)?.map(cut_at_nul).unwrap_or(&[]).to_vec()))
+            .collect()
+    }
+
+    /// The strings of the variable-length string elements in `raw`, decoded
+    /// as UTF-8 with invalid sequences replaced by U+FFFD (see
+    /// [`string_bytes`](Self::string_bytes) for the exact bytes).
+    pub fn strings(&mut self, raw: &[u8]) -> Result<Vec<String>, FormatError> {
+        Ok(self
+            .string_bytes(raw)?
+            .into_iter()
+            .map(|b| match String::from_utf8(b) {
+                Ok(s) => s,
+                Err(e) => String::from_utf8_lossy(e.as_bytes()).into_owned(),
+            })
+            .collect())
+    }
+
+    /// The sequences of the variable-length sequence elements in `raw`, each
+    /// as its `length × base_size` bytes in the base type's encoding.
+    pub fn sequences(&mut self, raw: &[u8], base_size: usize) -> Result<Vec<Vec<u8>>, FormatError> {
+        if base_size == 0 {
+            return Err(FormatError::VlDataError(
+                "variable-length sequence of a zero-size base type".into(),
+            ));
+        }
+        self.elements(raw)?
+            .iter()
+            .map(|vl| Ok(self.resolve(vl, base_size)?.unwrap_or(&[]).to_vec()))
+            .collect()
+    }
+}
+
+/// A string's bytes up to its first NUL.
+fn cut_at_nul(s: &[u8]) -> &[u8] {
+    &s[..s.iter().position(|&b| b == 0).unwrap_or(s.len())]
+}
+
 /// Resolve VL strings from raw data by looking up each element in the global heap.
+///
+/// Reads the first `num_elements` elements of `raw`. Strings end at their
+/// first NUL and invalid UTF-8 is replaced, as in [`VlResolver::strings`].
 pub fn read_vl_strings(
     file_data: &[u8],
     raw_data: &[u8],
@@ -117,35 +330,23 @@ pub fn read_vl_strings(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<String>, FormatError> {
-    let refs = parse_vl_references(raw_data, num_elements, offset_size)?;
+    let raw = first_elements(raw_data, num_elements, offset_size)?;
-    let mut result = Vec::with_capacity(refs.len());
+    VlResolver::new(file_data, offset_size, length_size).strings(raw)
+}
 
-    for vl in &refs {
+/// The first `num_elements` elements of `raw`, or an error if it is shorter.
-        if vl.length == 0 && is_undefined_address(vl.collection_address, offset_size) {
+fn first_elements(raw: &[u8], num_elements: u64, offset_size: u8) -> Result<&[u8], FormatError> {
-            result.push(String::new());
+    let total = usize::try_from(num_elements)
-            continue;
+        .ok()
-        }
+        .and_then(|n| n.checked_mul(element_size(offset_size)))
-        if vl.length == 0 && vl.collection_address == 0 {
+        .ok_or(FormatError::UnexpectedEof {
-            result.push(String::new());
+            expected: usize::MAX,
-            continue;
+            available: raw.len(),
-        }
+        })?;
-
+    raw.get(..total).ok_or(FormatError::UnexpectedEof {
-        let coll =
+        expected: total,
-            GlobalHeapCollection::parse(file_data, vl.collection_address as usize, length_size)?;
+        available: raw.len(),
-        let obj = coll.get_object(vl.object_index as u16).ok_or(
+    })
-            FormatError::GlobalHeapObjectNotFound {
-                collection_address: vl.collection_address,
-                index: vl.object_index as u16,
-            },
-        )?;
-
-        // The object data is the raw string bytes
-        let len = (vl.length as usize).min(obj.data.len());
-        let s = String::from_utf8_lossy(&obj.data[..len]).into_owned();
-        result.push(s);
-    }
-
-    Ok(result)
 }
 
 /// Resolve VL sequences from raw data, returning each element's bytes.
@@ -153,7 +354,9 @@ pub fn read_vl_strings(
 /// Each element is the sequence's full encoding — element count × base type
 /// size bytes, in the base type's byte order — so a sequence of `i32` yields
 /// four bytes per value. Decode it with the base type (e.g.
-/// [`crate::data_read::read_as_i64`]).
+/// [`crate::data_read::read_as_i64`]). This does not know the base type, so
+/// it returns each heap object whole; [`VlResolver::sequences`] also checks
+/// the object's size against the element's length.
 pub fn read_vl_bytes(
     file_data: &[u8],
     raw_data: &[u8],
@@ -162,35 +365,97 @@ pub fn read_vl_bytes(
     length_size: u8,
 ) -> Result<Vec<Vec<u8>>, FormatError> {
     let refs = parse_vl_references(raw_data, num_elements, offset_size)?;
+    let mut resolver = VlResolver::new(file_data, offset_size, length_size);
     let mut result = Vec::with_capacity(refs.len());
 
     for vl in &refs {
-        if vl.length == 0
+        // A heap address of 0 is a null element, as in VlResolver.
-            && (is_undefined_address(vl.collection_address, offset_size)
+        if vl.collection_address == 0 {
-                || vl.collection_address == 0)
-        {
             result.push(Vec::new());
             continue;
         }
-
-        let coll =
-            GlobalHeapCollection::parse(file_data, vl.collection_address as usize, length_size)?;
-        let obj = coll.get_object(vl.object_index as u16).ok_or(
-            FormatError::GlobalHeapObjectNotFound {
-                collection_address: vl.collection_address,
-                index: vl.object_index as u16,
-            },
-        )?;
-
         // The heap object holds the whole sequence. `vl.length` counts
         // elements, not bytes, so it is only the byte length when the base
         // type is one byte wide.
-        result.push(obj.data.clone());
+        let obj = resolver.object(vl)?;
+        result.push(obj.to_vec());
     }
 
     Ok(result)
 }
 
+impl<'a> VlResolver<'a> {
+    /// The heap object `vl` points to, whatever its size; its collection is
+    /// parsed on first use.
+    fn object(&mut self, vl: &VlElement) -> Result<&'a [u8], FormatError> {
+        let addr = vl.collection_address;
+        // libhdf5 writes a null element with address 0, never the undefined
+        // address, and fails to read one ("addr undefined") even when its
+        // length is 0; we returned an empty value.
+        if is_undefined_address(addr, self.offset_size) {
+            return Err(FormatError::VlDataError(format!(
+                "variable-length element (length {}) has the undefined global heap address",
+                vl.length
+            )));
+        }
+        if !self.cache.contains_key(&addr) {
+            let offset = usize::try_from(addr).map_err(|_| FormatError::UnexpectedEof {
+                expected: usize::MAX,
+                available: self.file_data.len(),
+            })?;
+            let index =
+                GlobalHeapCollection::parse_index(self.file_data, offset, self.length_size)?;
+            // parse_index checked that the collection lies in the file.
+            let end = offset + index.collection_size as usize;
+            self.check_overlap(offset, end)?;
+            let coll = CachedCollection::new(index);
+            if self.cached_bytes.saturating_add(coll.cost()) > self.budget {
+                self.cache.clear();
+                self.cached_bytes = 0;
+            }
+            self.cached_bytes += coll.cost();
+            self.cache.insert(addr, coll);
+        }
+        let (start, size) = self.cache[&addr].get(vl.object_index).ok_or(
+            FormatError::GlobalHeapObjectNotFound {
+                collection_address: addr,
+                index: vl.object_index as u16,
+            },
+        )?;
+        Ok(&self.file_data[start..start + size])
+    }
+
+    /// Record the collection at `start..end`, refusing one that overlaps a
+    /// collection already read. libhdf5 allocates each collection its own
+    /// block; overlapping ones only come from a crafted file, where they let
+    /// every byte be parsed again as the objects of each collection.
+    fn check_overlap(&mut self, start: usize, end: usize) -> Result<(), FormatError> {
+        if let Some(&known) = self.extents.get(&start) {
+            return if known == end {
+                Ok(())
+            } else {
+                Err(FormatError::VlDataError(format!(
+                    "global heap collection at {start} changed size"
+                )))
+            };
+        }
+        let before = self.extents.range(..start).next_back();
+        let after = self.extents.range(start..).next();
+        let clash = match (before, after) {
+            (Some((&s, &e)), _) if e > start => Some(s),
+            (_, Some((&s, _))) if s < end => Some(s),
+            _ => None,
+        };
+        if let Some(other) = clash {
+            return Err(FormatError::VlDataError(format!(
+                "global heap collection at {start} overlaps the one at {other}"
+            )));
+        }
+        self.extents.insert(start, end);
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -285,16 +550,27 @@ mod tests {
     }
 
     #[test]
-    fn null_vl_element_empty_string() {
+    fn an_undefined_heap_address_is_an_error_even_at_length_0() {
-        // length=0, address=undefined
+        // libhdf5 fails the read ("addr undefined"); h5py and libhdf5 write
-        let mut raw = Vec::new();
+        // a null element with address 0. We returned "".
-        raw.extend_from_slice(&0u32.to_le_bytes()); // length=0
+        let mut file_data = vec![0u8; 256];
-        raw.extend_from_slice(&u64::MAX.to_le_bytes()); // undefined address
+        build_gcol_at(&mut file_data, 64, &[(1, b"x")]);
-        raw.extend_from_slice(&0u32.to_le_bytes()); // index
+        for (os, undef) in [(8u8, u64::MAX), (4, 0xFFFF_FFFF)] {
-
+            for length in [0, 1] {
-        let file_data = vec![0u8; 16];
+                let mut raw = element(1, 64, 1, os);
-        let strings = read_vl_strings(&file_data, &raw, 1, 8, 8).unwrap();
+                raw.extend(element(length, undef, 1, os));
-        assert_eq!(strings, vec![""]);
+                let mut r = VlResolver::new(&file_data, os, 8);
+                let e = r.string_bytes(&raw).unwrap_err().to_string();
+                assert!(e.contains("undefined"), "{e}");
+                assert!(r.sequences(&raw, 1).is_err());
+                assert!(r.string_element(&raw[raw.len() / 2..]).is_err());
+                let n = 2;
+                assert!(read_vl_strings(&file_data, &raw, n, os, 8).is_err());
+                assert!(read_vl_bytes(&file_data, &raw, n, os, 8).is_err());
+                // The defined element alone still reads.
+                assert_eq!(r.strings(&raw[..raw.len() / 2]).unwrap(), ["x"]);
+            }
+        }
     }
 
     #[test]
@@ -333,6 +609,126 @@ mod tests {
         assert_eq!(bytes, vec![vec![0xDE, 0xAD], vec![0xBE, 0xEF, 0xCA]]);
     }
 
+    fn element(length: u32, addr: u64, index: u32, offset_size: u8) -> Vec<u8> {
+        let mut raw = length.to_le_bytes().to_vec();
+        raw.extend_from_slice(&addr.to_le_bytes()[..offset_size as usize]);
+        raw.extend_from_slice(&index.to_le_bytes());
+        raw
+    }
+
+    #[test]
+    fn strings_end_at_the_first_nul() {
+        // libhdf5 hands each VL string over as a C string, so h5py sees
+        // "a\0b" as "a"; we used to return the NUL and what followed.
+        let mut file_data = vec![0u8; 512];
+        build_gcol_at(&mut file_data, 64, &[(1, b"a\0b"), (2, b"cd")]);
+        let mut raw = element(3, 64, 1, 8);
+        raw.extend(element(2, 64, 2, 8));
+        let mut r = VlResolver::new(&file_data, 8, 8);
+        assert_eq!(
+            r.string_bytes(&raw).unwrap(),
+            vec![b"a".to_vec(), b"cd".to_vec()]
+        );
+        assert_eq!(
+            read_vl_strings(&file_data, &raw, 2, 8, 8).unwrap(),
+            ["a", "cd"]
+        );
+    }
+
+    #[test]
+    fn a_heap_object_of_the_wrong_size_is_an_error() {
+        // libhdf5: "Expected global heap object size does not match". We
+        // used to return the object cut to the element's length.
+        let mut file_data = vec![0u8; 512];
+        build_gcol_at(&mut file_data, 64, &[(1, b"cdefgh"), (2, &[1, 0, 0, 0])]);
+        let mut r = VlResolver::new(&file_data, 8, 8);
+        assert!(r.string_bytes(&element(3, 64, 1, 8)).is_err());
+        assert!(r.string_bytes(&element(9, 64, 1, 8)).is_err());
+        assert!(read_vl_strings(&file_data, &element(3, 64, 1, 8), 1, 8, 8).is_err());
+        // A sequence of one i32 is 4 bytes; of two, 8.
+        assert_eq!(
+            r.sequences(&element(1, 64, 2, 8), 4).unwrap(),
+            vec![vec![1, 0, 0, 0]]
+        );
+        assert!(r.sequences(&element(2, 64, 2, 8), 4).is_err());
+        assert!(r.sequences(&element(1, 64, 2, 8), 0).is_err());
+    }
+
+    #[test]
+    fn address_zero_is_null_whatever_the_length() {
+        // libhdf5 treats a heap address of 0 as a null element.
+        let file_data = vec![0u8; 64];
+        let mut r = VlResolver::new(&file_data, 8, 8);
+        assert_eq!(
+            r.string_bytes(&element(5, 0, 1, 8)).unwrap(),
+            vec![Vec::<u8>::new()]
+        );
+        assert_eq!(
+            r.sequences(&element(5, 0, 1, 8), 4).unwrap(),
+            vec![Vec::<u8>::new()]
+        );
+    }
+
+    #[test]
+    fn four_byte_offsets_use_twelve_byte_elements() {
+        let mut file_data = vec![0u8; 512];
+        build_gcol_at(&mut file_data, 64, &[(1, b"one"), (2, b""), (3, b"three")]);
+        let mut raw = element(3, 64, 1, 4);
+        raw.extend(element(0, 64, 2, 4));
+        raw.extend(element(5, 64, 3, 4));
+        assert_eq!(raw.len(), 36);
+        let mut r = VlResolver::new(&file_data, 4, 8);
+        assert_eq!(r.element_size(), 12);
+        assert_eq!(r.strings(&raw).unwrap(), ["one", "", "three"]);
+        // Not a whole number of elements.
+        assert!(r.strings(&raw[..30]).is_err());
+    }
+
+    #[test]
+    fn the_cache_stays_within_its_budget_and_rereads_what_it_dropped() {
+        // Twenty collections of three objects each; a budget that holds
+        // about two of them. Reading every element twice must still return
+        // the right strings after the cache is dropped.
+        let mut file_data = vec![0u8; 64];
+        let mut raw = Vec::new();
+        for c in 0..20u64 {
+            let at = file_data.len();
+            let names: Vec<String> = (0..3).map(|i| format!("c{c}o{i}")).collect();
+            let objs: Vec<(u16, &[u8])> = names
+                .iter()
+                .enumerate()
+                .map(|(i, n)| (i as u16 + 1, n.as_bytes()))
+                .collect();
+            build_gcol_at(&mut file_data, at, &objs);
+            for (i, n) in names.iter().enumerate() {
+                raw.extend(element(n.len() as u32, at as u64, i as u32 + 1, 8));
+            }
+        }
+        raw.extend(raw.clone());
+        let mut r = VlResolver::new(&file_data, 8, 8);
+        let one = CachedCollection {
+            objects: vec![(0, 0, 0); 3],
+        }
+        .cost();
+        r.budget = 2 * one + 1;
+        let want: Vec<String> = (0..2)
+            .flat_map(|_| (0..20).flat_map(|c| (0..3).map(move |i| format!("c{c}o{i}"))))
+            .collect();
+        for (k, chunk) in raw.chunks(16).enumerate() {
+            assert_eq!(r.strings(chunk).unwrap(), [want[k].clone()]);
+            assert!(r.cached_bytes <= r.budget);
+            assert!(r.cache.len() <= 2);
+        }
+    }
+
+    #[test]
+    fn element_size_is_checked_against_the_offset_size() {
+        assert!(check_element_size(16, 8).is_ok());
+        assert!(check_element_size(12, 4).is_ok());
+        assert!(check_element_size(16, 4).is_err());
+        assert!(check_element_size(524_304, 8).is_err());
+    }
+
     #[test]
     fn parse_vl_references_truncated_error() {
         let raw = vec![0u8; 10]; // too short for 1 element with offset_size=8

crates/clawhdf5-format/tests/vl_heap_bounds.rs

@@ -0,0 +1,161 @@
+//! Crafted files cannot make variable-length reads retain memory, or take
+//! time, out of proportion to the file.
+//!
+//! `VlResolver` used to keep an owned copy of every object of every
+//! collection it parsed, for the whole read. A file whose global heap
+//! collections nest inside each other's object data — each element
+//! pointing at a different one — then made retained memory O(K × file
+//! size): a 744 KB file took 1.58 GB. The same nesting, with every
+//! collection's object chain jumping to one shared run of tiny objects,
+//! made the parse time O(K × M) as well. libhdf5 never writes overlapping
+//! collections; they are now refused, and the cache holds only where
+//! objects lie.
+//!
+//! Peak heap use is measured with a counting global allocator, so the
+//! cases run one after another in a single test.
+
+use std::alloc::{GlobalAlloc, Layout, System};
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::time::{Duration, Instant};
+
+use clawhdf5_format::vl_data::VlResolver;
+
+struct Counting;
+
+static CURRENT: AtomicUsize = AtomicUsize::new(0);
+static PEAK: AtomicUsize = AtomicUsize::new(0);
+
+unsafe impl GlobalAlloc for Counting {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        let p = unsafe { System.alloc(layout) };
+        if !p.is_null() {
+            let now = CURRENT.fetch_add(layout.size(), Ordering::Relaxed) + layout.size();
+            PEAK.fetch_max(now, Ordering::Relaxed);
+        }
+        p
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        unsafe { System.dealloc(ptr, layout) };
+        CURRENT.fetch_sub(layout.size(), Ordering::Relaxed);
+    }
+}
+
+#[global_allocator]
+static ALLOC: Counting = Counting;
+
+/// Bytes allocated at the peak of `f`, above what was live when it started.
+fn peak_during<T>(f: impl FnOnce() -> T) -> (T, usize) {
+    let base = CURRENT.load(Ordering::Relaxed);
+    PEAK.store(base, Ordering::Relaxed);
+    let out = f();
+    (out, PEAK.load(Ordering::Relaxed) - base)
+}
+
+fn put_header(file: &mut [u8], at: usize, size: u64) {
+    file[at..at + 4].copy_from_slice(b"GCOL");
+    file[at + 4] = 1;
+    file[at + 8..at + 16].copy_from_slice(&size.to_le_bytes());
+}
+
+fn put_object(file: &mut [u8], at: usize, index: u16, size: u64) {
+    file[at..at + 2].copy_from_slice(&index.to_le_bytes());
+    file[at + 2..at + 4].copy_from_slice(&1u16.to_le_bytes());
+    file[at + 8..at + 16].copy_from_slice(&size.to_le_bytes());
+}
+
+fn element(length: u32, addr: u64, index: u32) -> Vec<u8> {
+    let mut e = length.to_le_bytes().to_vec();
+    e.extend_from_slice(&addr.to_le_bytes());
+    e.extend_from_slice(&index.to_le_bytes());
+    e
+}
+
+/// K collections 32 bytes apart, each running to the end of the file with
+/// one object covering the rest of it (and so every later collection).
+/// Element i is that object of collection i.
+fn nested(k: usize) -> (Vec<u8>, Vec<u8>) {
+    let base = 64;
+    let end = base + 32 * k + 64;
+    let mut file = vec![0u8; end];
+    let mut raw = Vec::new();
+    for i in 0..k {
+        let at = base + 32 * i;
+        put_header(&mut file, at, (end - at) as u64);
+        let obj = (end - at - 32) as u64;
+        put_object(&mut file, at + 16, 1, obj);
+        raw.extend(element(obj as u32, at as u64, 1));
+    }
+    (file, raw)
+}
+
+/// K collections 32 bytes apart, each with a first object that jumps over
+/// the later collections to one shared run of M empty objects, so parsing
+/// every collection walks all M.
+fn shared_tail(k: usize, m: usize) -> (Vec<u8>, Vec<u8>) {
+    let base = 64;
+    let tail = base + 32 * k + 32;
+    let end = tail + 16 * m + 16;
+    let mut file = vec![0u8; end];
+    let mut raw = Vec::new();
+    for i in 0..k {
+        let at = base + 32 * i;
+        put_header(&mut file, at, (end - at) as u64);
+        let jump = (tail - at - 32) as u64;
+        put_object(&mut file, at + 16, 1, jump);
+        raw.extend(element(jump as u32, at as u64, 1));
+    }
+    for j in 0..m {
+        put_object(&mut file, tail + 16 * j, (j % 65_000 + 2) as u16, 0);
+    }
+    (file, raw)
+}
+
+#[test]
+fn overlapping_collections_are_refused_in_bounded_memory_and_time() {
+    for (name, (file, raw)) in [
+        ("nested", nested(2000)),
+        ("shared tail", shared_tail(500, 10_000)),
+    ] {
+        let start = Instant::now();
+        let (result, peak) = peak_during(|| {
+            let mut r = VlResolver::new(&file, 8, 8);
+            (r.string_bytes(&raw), r.sequences(&raw, 1).map(|s| s.len()))
+        });
+        let took = start.elapsed();
+        // libhdf5 never writes overlapping collections, and refuses these
+        // files; so do we, rather than returning what they claim.
+        let (strings, sequences) = result;
+        let e = strings.expect_err(name).to_string();
+        assert!(e.contains("overlaps"), "{name}: {e}");
+        assert!(sequences.is_err(), "{name}");
+        // Measured before the fix: 129 MB ("nested", 64 KB file) and 350 MB
+        // ("shared tail", 176 KB file) live at the peak; after, 97 KB and
+        // 0.9 MB.
+        assert!(
+            peak < 4 * file.len() + (1 << 20),
+            "{name}: peak {peak} bytes for a {}-byte file",
+            file.len()
+        );
+        assert!(took < Duration::from_secs(5), "{name}: took {took:?}");
+    }
+}
+
+/// Collections that do not overlap still read, however many elements point
+/// into them, and the first object of a collection is returned for its
+/// index (as before).
+#[test]
+fn separate_collections_still_read() {
+    let mut file = vec![0u8; 64 + 3 * 64];
+    let mut raw = Vec::new();
+    for i in 0..3usize {
+        let at = 64 + 64 * i;
+        put_header(&mut file, at, 64);
+        put_object(&mut file, at + 16, 1, 3);
+        file[at + 32..at + 35].copy_from_slice(format!("s{i}!").as_bytes());
+        raw.extend(element(3, at as u64, 1));
+    }
+    raw.extend(element(3, 64, 1));
+    let mut r = VlResolver::new(&file, 8, 8);
+    assert_eq!(r.strings(&raw).unwrap(), ["s0!", "s1!", "s2!", "s0!"]);
+}

crates/clawhdf5-netcdf4/tests/interop_tests.rs

@@ -350,3 +350,30 @@ ds.close()
     let press_vals = press_var.read_raw_f32().unwrap();
     assert_eq!(press_vals, vec![1000.0f32, 850.0, 500.0, 200.0]);
 }
+
+#[test]
+fn netcdf4_python_string_variable_clawhdf5_reads() {
+    // NC_STRING variables are HDF5 variable-length strings, which
+    // `read_string` refused ("expected String, got VariableLength") until
+    // 2026-09-26.
+    skip_if_no_netcdf4!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("strings.nc");
+    let path_str = path.display().to_string();
+    let script = format!(
+        r#"
+import netCDF4 as nc
+import numpy as np
+ds = nc.Dataset("{path_str}", "w", format="NETCDF4")
+ds.createDimension("station", 4)
+v = ds.createVariable("name", str, ("station",))
+v[:] = np.array(["Oslo", "", "São Paulo", "x"], dtype=object)
+ds.close()
+"#
+    );
+    run_python(&script);
+
+    let file = NetCDF4File::open(&path).unwrap();
+    let names = file.variable("name").unwrap().read_string().unwrap();
+    assert_eq!(names, vec!["Oslo", "", "São Paulo", "x"]);
+}

crates/clawhdf5-tools/README.md

@@ -76,8 +76,13 @@ print the same bytes as h5dump 1.14.6 and as Debian's h5dump 1.14.5 (the
 was run in that image on 2026-09-26) — `dump_matches_h5dump` in
 `tests/h5rs_interop.rs` checks this, and `dump_shows_nul_padding_in_nested_strings`
 that null-padded strings show their NULs (`"a\000b"`) at any depth, as
-h5dump's do. Not covered by those tests: references, opaque, bitfield,
+h5dump's do. `dump_prints_vl_data_like_h5dump` covers variable-length
-variable-length sequences and virtual datasets. Known differences from
+strings (one with an embedded NUL, which prints up to the NUL; empty; null,
+which prints `NULL`), variable-length sequences, a VL compound member and
+a VL attribute, with 8- and 4-byte offsets. Not covered by those tests:
+references, opaque, bitfield, non-ASCII UTF-8 (h5dump prints each byte
+above 0x7f as a sign-extended octal escape, h5rs the character) and
+virtual datasets. Known differences from
 h5dump:
 
 - Floats print at their own precision (a `float32` 0.1 prints as `0.1`),
@@ -233,9 +238,11 @@ extension) and checks:
 (which catches corrupt compressed data and Fletcher-32 mismatches), and
 follows every variable-length element (strings and sequences, also inside
 compounds and arrays) of every dataset and attribute into its global heap
-collection: a collection that does not parse, a missing heap object, or a
+collection: a collection that does not parse or overlaps another, a missing
-sequence longer than its heap object is a problem at the collection's
+heap object, or a heap object whose size is not exactly the element's
-address. Data the
+length times its base size (libhdf5 refuses such an element) is a problem
+at the collection's address. Variable-length elements are resolved by the
+library's `VlResolver`, as `clawhdf5::File` resolves them. Data the
 tool cannot decode (a filter it does not implement, such as szip, or a
 dataset over `--max-bytes`) is a `note:`, not a problem. Every problem is
 printed with the address of the structure involved; the exit status is 0
@@ -252,9 +259,10 @@ none at all without `--data`), and objects reachable only by external links. It
 clawhdf5's parsers, so it accepts what they accept: some header damage that
 libhdf5 refuses goes unreported. Of the 150 CVE and fuzzer files of the
 HDF Group's `cve_hdf5` corpus (`cvefiles/` and `fuzzerfiles/`),
-`check --data` passes 16, and h5dump 1.14.6 rejects 9 of those (tank,
+`check --data` passes 15, and h5dump 1.14.6 rejects 8 of those (tank,
 2026-09-26, `h5rs check --data F` and `h5dump F` per file; before the
-library's header checks it passed 28, of which h5dump rejects 21).
+library's header checks it passed 28, of which h5dump rejects 21, and 16
+and 9 before a VL type's stored element size was checked).
 
 ## Robustness
 

crates/clawhdf5-tools/src/check.rs

@@ -15,11 +15,13 @@ use clawhdf5_format::btree_v2::{BTreeV2Header, collect_btree_v2_records};
 use clawhdf5_format::data_layout::DataLayout;
 use clawhdf5_format::dataspace::{Dataspace, DataspaceType};
 use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::error::FormatError;
 use clawhdf5_format::group_info::GroupInfoMessage;
 use clawhdf5_format::link_info::LinkInfoMessage;
 use clawhdf5_format::message_type::MessageType;
 use clawhdf5_format::object_header::ObjectHeader;
 use clawhdf5_format::symbol_table::SymbolTableMessage;
+use clawhdf5_format::vl_data::{VlResolver, check_element_size, parse_vl_references};
 
 use crate::cli::{Args, Out};
 use crate::h5::{Error, ErrorKind, H5, Kind};
@@ -49,6 +51,15 @@ found, 3 internal error.";
 
 const MAX_CHUNKS_CHECKED: usize = 10_000_000;
 
+/// A variable-length element's problem, worded as `check` reports heap
+/// problems ("global heap ...").
+fn heap_problem(e: FormatError) -> String {
+    match e {
+        FormatError::VlDataError(m) if m.starts_with("global heap") => m,
+        e => format!("global heap: {e}"),
+    }
+}
+
 /// Whether values of `dt` hold variable-length data (in the global heap).
 fn has_vl(dt: &Datatype, depth: u32) -> bool {
     if depth > 32 {
@@ -104,6 +115,8 @@ struct Checker<'a> {
     btrees_seen: HashSet<u64>,
     /// Global heap collections already read (with --data).
     gcols_seen: HashSet<u64>,
+    /// Resolves variable-length elements (with --data), for the whole file.
+    vl: VlResolver<'a>,
     panicked: bool,
 }
 
@@ -157,6 +170,7 @@ pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
         heaps_seen: HashSet::new(),
         btrees_seen: HashSet::new(),
         gcols_seen: HashSet::new(),
+        vl: VlResolver::new(h5.data(), h5.os(), h5.ls()),
         panicked: false,
     };
     c.superblock();
@@ -707,62 +721,48 @@ impl Checker<'_> {
         }
         match dt {
             Datatype::VariableLength {
+                size,
                 is_string,
                 base_type,
                 ..
             } => {
-                let os = usize::from(self.h5.os());
+                // Resolved by the library's VlResolver, as every other
-                let (Some(lenb), Some(addrb), Some(idxb)) =
+                // reader resolves them (and as libhdf5 does): a heap object
-                    (b.get(..4), b.get(4..4 + os), b.get(4 + os..8 + os))
+                // whose size is not the element's length × base size, a
-                else {
+                // collection that overlaps another, or a missing object is
+                // a problem at the collection's address.
+                let Ok(vl) = parse_vl_references(b, 1, self.h5.os()) else {
                     return;
                 };
-                let le = |x: &[u8]| {
+                let gcol = vl[0].collection_address;
-                    x.iter()
+                if gcol == 0 || bad.contains_key(&gcol) {
-                        .enumerate()
-                        .fold(0u64, |a, (i, &v)| a | (u64::from(v) << (8 * i)))
-                };
-                let (len, gcol, idx) = (le(lenb), le(addrb), le(idxb));
-                let undef = if os >= 8 {
-                    u64::MAX
-                } else {
-                    (1u64 << (8 * os)) - 1
-                };
-                if len == 0 || gcol == 0 || gcol == undef || bad.contains_key(&gcol) {
                     return;
                 }
-                let obj = match self.h5.heap_object(gcol, idx as u32) {
+                if let Err(e) = check_element_size(*size, self.h5.os()) {
-                    Ok(o) => o,
+                    bad.insert(gcol, e.to_string());
+                    return;
+                }
+                let bs = if *is_string {
+                    1
+                } else {
+                    base_type.type_size() as usize
+                };
+                if bs == 0 {
+                    return;
+                }
+                let obj = match self.vl.element(b, bs) {
+                    Ok(o) => o.unwrap_or(&[]),
                     Err(e) => {
-                        bad.insert(e.addr.unwrap_or(gcol), e.msg);
+                        bad.insert(gcol, heap_problem(e));
                         return;
                     }
                 };
                 if self.gcols_seen.insert(gcol) {
                     self.counts.global_heaps += 1;
                 }
-                let bs = if *is_string {
+                if !*is_string && has_vl(base_type, depth + 1) {
-                    1
+                    for eb in obj.chunks_exact(bs) {
-                } else {
+                        self.vl_element(base_type, eb, depth + 1, bad);
-                    u64::from(base_type.type_size())
-                };
-                if len
-                    .checked_mul(bs)
-                    .is_none_or(|need| need > obj.len() as u64)
-                {
-                    bad.insert(
-                        gcol,
-                        format!(
-                            "global heap object {idx} holds {} bytes; the element needs {len} x {bs}",
-                            obj.len()
-                        ),
-                    );
-                    return;
-                }
-                if !*is_string && bs > 0 && has_vl(base_type, depth + 1) {
-                    let bs = bs as usize;
-                    for k in 0..len as usize {
-                        self.vl_element(base_type, &obj[k * bs..(k + 1) * bs], depth + 1, bad);
                     }
                 }
             }

crates/clawhdf5-tools/src/diff.rs

@@ -699,6 +699,9 @@ impl Diff {
         }
         match (x, y) {
             (Value::Str(p), Value::Str(q)) => p == q,
+            // h5diff compares a null VL string equal to an empty one.
+            (Value::NullStr, Value::NullStr) => true,
+            (Value::NullStr, Value::Str(s)) | (Value::Str(s), Value::NullStr) => s.is_empty(),
             (Value::Bytes(p), Value::Bytes(q)) | (Value::OtherRef(p), Value::OtherRef(q)) => p == q,
             (Value::Compound(p), Value::Compound(q)) => {
                 p.len() == q.len()

crates/clawhdf5-tools/src/h5.rs

@@ -8,7 +8,6 @@
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
-use std::rc::Rc;
 
 use clawhdf5::File;
 use clawhdf5_format::attribute::{AttributeMessage, extract_attributes_tolerant};
@@ -20,7 +19,6 @@ use clawhdf5_format::datatype::Datatype;
 use clawhdf5_format::error::FormatError;
 use clawhdf5_format::filter_pipeline::FilterPipeline;
 use clawhdf5_format::fractal_heap::FractalHeapHeader;
-use clawhdf5_format::global_heap::GlobalHeapCollection;
 use clawhdf5_format::group_v1;
 use clawhdf5_format::link_info::LinkInfoMessage;
 use clawhdf5_format::link_message::{LinkMessage, LinkTarget};
@@ -191,7 +189,6 @@ pub struct H5 {
     pub path: PathBuf,
     pub file: File,
     pub max_bytes: u64,
-    heaps: RefCell<HashMap<u64, std::result::Result<Rc<GlobalHeapCollection>, String>>>,
     /// Fractal heaps whose blocks were verified: `None` = sound.
     verified_heaps: RefCell<HashMap<u64, Option<Error>>>,
 }
@@ -211,7 +208,6 @@ impl H5 {
             path: path.to_path_buf(),
             file,
             max_bytes: DEFAULT_MAX_BYTES,
-            heaps: RefCell::new(HashMap::new()),
             verified_heaps: RefCell::new(HashMap::new()),
         })
     }
@@ -433,29 +429,6 @@ impl H5 {
         r.map_or(Ok(()), Err)
     }
 
-    /// The global heap object `idx` of the collection at `addr` (cached per
-    /// collection).
-    pub fn heap_object(&self, addr: u64, idx: u32) -> Result<Vec<u8>> {
-        let coll = {
-            let mut cache = self.heaps.borrow_mut();
-            cache
-                .entry(addr)
-                .or_insert_with(|| match usize::try_from(addr) {
-                    Ok(a) => GlobalHeapCollection::parse(self.data(), a, self.ls())
-                        .map(Rc::new)
-                        .map_err(|e| e.to_string()),
-                    Err(_) => Err("address out of range".into()),
-                })
-                .clone()
-                .map_err(|e| Error::at(addr, format!("global heap: {e}")))?
-        };
-        let idx16 = u16::try_from(idx)
-            .map_err(|_| Error::at(addr, format!("global heap object index {idx} out of range")))?;
-        coll.get_object(idx16)
-            .map(|o| o.data.clone())
-            .ok_or_else(|| Error::at(addr, format!("global heap has no object {idx}")))
-    }
-
     /// The dataspace of the dataset at `path` with a virtual dataset's
     /// extent resolved from its sources (as libhdf5 reports it) instead of
     /// the stored one.

crates/clawhdf5-tools/src/value.rs

@@ -3,7 +3,10 @@
 //! Decoding never panics: a short buffer, an unknown byte order or a
 //! dangling heap reference becomes [`Value::Error`].
 
+use std::cell::RefCell;
+
 use clawhdf5_format::datatype::{Datatype, DatatypeByteOrder, ReferenceType, StringPadding};
+use clawhdf5_format::vl_data::{VlResolver, check_element_size};
 use serde_json::Value as J;
 
 use crate::dtype;
@@ -16,6 +19,9 @@ pub enum Value {
     /// its own precision.
     Float(f64, u8),
     Str(String),
+    /// A null variable-length string (heap address 0): h5dump prints it as
+    /// `NULL`, h5py reads it as empty.
+    NullStr,
     /// Opaque, bitfield, time and oversized integers.
     Bytes(Vec<u8>),
     /// An enum member (name, when the value matches one) and its value.
@@ -129,10 +135,10 @@ fn decode_float(dt: &Datatype, b: &[u8]) -> Value {
     }
 }
 
-fn trim_string(b: &[u8], pad: Option<&StringPadding>) -> String {
+fn trim_string(b: &[u8], pad: &StringPadding) -> String {
     let cut = b.iter().position(|&c| c == 0).unwrap_or(b.len());
     let mut s = &b[..cut];
-    if matches!(pad, Some(StringPadding::SpacePad)) {
+    if matches!(pad, StringPadding::SpacePad) {
         while let [rest @ .., b' '] = s {
             s = rest;
         }
@@ -140,22 +146,20 @@ fn trim_string(b: &[u8], pad: Option<&StringPadding>) -> String {
     String::from_utf8_lossy(s).into_owned()
 }
 
-/// Little-endian unsigned integer of `b` (up to 8 bytes).
-fn le(b: &[u8]) -> u64 {
-    b.iter()
-        .take(8)
-        .enumerate()
-        .fold(0u64, |a, (i, &x)| a | (u64::from(x) << (8 * i)))
-}
-
 /// Decodes elements of one file.
 pub struct Decoder<'a> {
     pub h5: &'a H5,
+    /// Variable-length elements are resolved as the library resolves them
+    /// (so as libhdf5 does), not by a decoder of our own.
+    vl: RefCell<VlResolver<'a>>,
 }
 
 impl<'a> Decoder<'a> {
     pub fn new(h5: &'a H5) -> Self {
-        Self { h5 }
+        Self {
+            h5,
+            vl: RefCell::new(VlResolver::new(h5.data(), h5.os(), h5.ls())),
+        }
     }
 
     /// Decode element `i` of `raw`, an array of `dt` elements.
@@ -187,7 +191,7 @@ impl<'a> Decoder<'a> {
             Datatype::Time { .. } | Datatype::BitField { .. } | Datatype::Opaque { .. } => {
                 Value::Bytes(b.to_vec())
             }
-            Datatype::String { padding, .. } => Value::Str(trim_string(b, Some(padding))),
+            Datatype::String { padding, .. } => Value::Str(trim_string(b, padding)),
             Datatype::Compound { members, .. } => {
                 let mut out = Vec::with_capacity(members.len());
                 for m in members {
@@ -246,61 +250,43 @@ impl<'a> Decoder<'a> {
                 Value::Array(out)
             }
             Datatype::VariableLength {
+                size,
                 is_string,
-                padding,
                 base_type,
                 ..
-            } => self.decode_vlen(*is_string, padding.as_ref(), base_type, b, depth),
+            } => match check_element_size(*size, self.h5.os()) {
+                Ok(()) => self.decode_vlen(*is_string, base_type, b, depth),
+                Err(e) => Value::Error(e.to_string()),
+            },
         }
     }
 
-    fn decode_vlen(
+    /// A variable-length element, resolved by the library's
-        &self,
+    /// [`VlResolver`]: a string ends at its first NUL, a heap object whose
-        is_string: bool,
+    /// size is not the element's length × base size is an error, and a
-        padding: Option<&StringPadding>,
+    /// heap address of 0 is null — all as libhdf5 (and so h5dump and h5py)
-        base: &Datatype,
+    /// has it.
-        b: &[u8],
+    fn decode_vlen(&self, is_string: bool, base: &Datatype, b: &[u8], depth: u32) -> Value {
-        depth: u32,
-    ) -> Value {
-        let os = usize::from(self.h5.os());
-        let (Some(lenb), Some(addrb), Some(idxb)) =
-            (b.get(..4), b.get(4..4 + os), b.get(4 + os..8 + os))
-        else {
-            return Value::Error("short VL element".into());
-        };
-        let len = le(lenb) as usize;
-        let addr = le(addrb);
-        let idx = le(idxb) as u32;
-        let undef = if os >= 8 {
-            u64::MAX
-        } else {
-            (1u64 << (8 * os)) - 1
-        };
-        let obj = if len == 0 || addr == 0 || addr == undef {
-            Vec::new()
-        } else {
-            match self.h5.heap_object(addr, idx) {
-                Ok(o) => o,
-                Err(e) => return Value::Error(e.to_string()),
-            }
-        };
         if is_string {
-            let l = len.min(obj.len());
+            return match self.vl.borrow_mut().string_element(b) {
-            return Value::Str(trim_string(&obj[..l], padding));
+                Ok(Some(s)) => Value::Str(String::from_utf8_lossy(s).into_owned()),
+                Ok(None) => Value::NullStr,
+                Err(e) => Value::Error(e.to_string()),
+            };
         }
         let bs = base.type_size() as usize;
         if bs == 0 {
             return Value::Error("VL base type of size 0".into());
         }
-        match len.checked_mul(bs) {
+        let obj = match self.vl.borrow_mut().element(b, bs) {
-            Some(need) if need <= obj.len() => {}
+            Ok(o) => o.unwrap_or(&[]),
-            _ => return Value::Error("VL sequence longer than its heap object".into()),
+            Err(e) => return Value::Error(e.to_string()),
-        }
+        };
-        let mut out = Vec::with_capacity(len);
+        Value::Seq(
-        for k in 0..len {
+            obj.chunks_exact(bs)
-            out.push(self.decode(base, &obj[k * bs..], depth + 1));
+                .map(|e| self.decode(base, e, depth + 1))
-        }
+                .collect(),
-        Value::Seq(out)
+        )
     }
 }
 
@@ -351,6 +337,7 @@ pub fn text(v: &Value, h5paths: &dyn Fn(u64) -> Option<String>) -> String {
         Value::Int(i) => i.to_string(),
         Value::Float(f, w) => fmt_float(*f, *w),
         Value::Str(s) => format!("\"{}\"", escape(s)),
+        Value::NullStr => "NULL".into(),
         Value::Bytes(b) => hex(b),
         Value::Enum(Some(n), _) => n.clone(),
         Value::Enum(None, i) => i.to_string(),
@@ -411,6 +398,7 @@ pub fn to_json(v: &Value, h5paths: &dyn Fn(u64) -> Option<String>) -> J {
             }
         }
         Value::Str(s) => J::from(s.as_str()),
+        Value::NullStr => J::from(""),
         Value::Bytes(b) | Value::OtherRef(b) => J::from(hex(b)),
         Value::Enum(_, i) => to_json(&Value::Int(*i), h5paths),
         Value::Compound(ms) => J::Array(ms.iter().map(|(_, v)| to_json(v, h5paths)).collect()),

crates/clawhdf5-tools/tests/gen_vl_files.py

@@ -0,0 +1,128 @@
+"""Write the variable-length data files the h5rs VL tests run on.
+
+usage: gen_vl_files.py OUTDIR
+
+For 8-byte (`vl8`) and 4-byte (`vl4`) offsets, writes OUTDIR/vl8.h5 and
+OUTDIR/vl4.h5, which libhdf5 reads in full, and OUTDIR/bad8.h5 and
+OUTDIR/bad4.h5, whose `bad` and `badseq` elements 0 have a length that
+disagrees with their global heap object (libhdf5: "Expected global heap
+object size does not match"), and whose `undef` element 1 has length 0 and
+the undefined heap address (libhdf5: "addr undefined"). h5py cannot write a VL string with a NUL in
+it or a null element in a contiguous dataset, so those are patched in.
+
+Prints one JSON object: for each file, each dataset's values as h5py reads
+them one element at a time (strings as text, sequences as lists, a compound
+as a list of its fields), with null for an element h5py cannot read; the
+root attribute `va`; and the addresses of the `bad` elements' collections.
+"""
+
+import json
+import os
+import struct
+import sys
+
+import h5py
+import numpy as np
+
+out = sys.argv[1]
+S = h5py.string_dtype("utf-8")
+I4 = h5py.vlen_dtype(np.dtype("<i4"))
+
+
+def create(path, sizes):
+    if sizes is None:
+        return h5py.File(path, "w")
+    fcpl = h5py.h5p.create(h5py.h5p.FILE_CREATE)
+    fcpl.set_sizes(*sizes)
+    return h5py.File(h5py.h5f.create(path.encode(), h5py.h5f.ACC_TRUNC, fcpl=fcpl))
+
+
+def element(length, addr, index, os_):
+    return struct.pack("<I", length) + addr.to_bytes(os_, "little") + struct.pack("<I", index)
+
+
+def good(path, sizes):
+    os_ = 8 if sizes is None else sizes[0]
+    with create(path, sizes) as f:
+        f.create_dataset(
+            "d", data=np.array(["aXb", "", "ok", "zz", "hello"], dtype=object), dtype=S
+        )
+        u = f.create_dataset("u", shape=(4,), dtype=S, chunks=(1,))
+        u[1] = "w"
+        s = f.create_dataset("seq", shape=(3,), dtype=I4)
+        s[0] = [1, 2, 3]
+        s[1] = []
+        s[2] = [-5]
+        s = f.create_dataset("sequ", shape=(3,), dtype=I4, chunks=(1,))
+        s[0] = [7, 8]
+        ct = np.dtype([("id", "<i4"), ("name", S)])
+        arr = np.zeros(3, dtype=ct)
+        arr["id"] = [1, 2, 3]
+        arr["name"] = ["one", "", "three"]
+        f.create_dataset("cmp", data=arr)
+        f.attrs.create("va", np.array(["p", "", "q"], dtype=object), dtype=S)
+        off = f["d"].id.get_offset()
+    b = bytearray(open(path, "rb").read())
+    i = b.index(b"aXb")
+    b[i + 1] = 0  # "a\0b"
+    es = 8 + os_
+    b[off + 2 * es : off + 3 * es] = element(2, 0, 1, os_)  # "ok" -> null
+    open(path, "wb").write(bytes(b))
+
+
+def bad(path, sizes):
+    os_ = 8 if sizes is None else sizes[0]
+    with create(path, sizes) as f:
+        f.create_dataset("bad", data=np.array(["cdefgh", "ok"], dtype=object), dtype=S)
+        s = f.create_dataset("badseq", shape=(2,), dtype=I4)
+        s[0] = [1, 2, 3]
+        s[1] = [4]
+        f.create_dataset("undef", data=np.array(["x", "", "yz"], dtype=object), dtype=S)
+        off, soff = f["bad"].id.get_offset(), f["badseq"].id.get_offset()
+        uoff = f["undef"].id.get_offset()
+    b = bytearray(open(path, "rb").read())
+    gcol = int.from_bytes(b[off + 4 : off + 4 + os_], "little")
+    struct.pack_into("<I", b, off, 3)  # "cdefgh": length 6 -> 3
+    struct.pack_into("<I", b, soff, 2)  # [1, 2, 3]: length 3 -> 2
+    # "": length 0 at the undefined address (all 0xff), which libhdf5 fails
+    # to read ("addr undefined"); it writes a null element as address 0.
+    es = 8 + os_
+    b[uoff + es : uoff + 2 * es] = element(0, (1 << (8 * os_)) - 1, 1, os_)
+    open(path, "wb").write(bytes(b))
+    return gcol
+
+
+def value(v):
+    if isinstance(v, bytes):
+        return v.decode()
+    if isinstance(v, str):
+        return v
+    if isinstance(v, np.void):
+        return [value(x) for x in v]
+    if isinstance(v, np.ndarray):
+        return [value(x) for x in v]
+    return v.item() if hasattr(v, "item") else v
+
+
+def read(ds):
+    got = []
+    for i in range(ds.shape[0]):
+        try:
+            got.append(value(ds[i]))
+        except OSError:
+            got.append(None)
+    return got
+
+
+result = {}
+for tag, sizes in (("8", None), ("4", (4, 4))):
+    g, x = os.path.join(out, f"vl{tag}.h5"), os.path.join(out, f"bad{tag}.h5")
+    good(g, sizes)
+    gcol = bad(x, sizes)
+    with h5py.File(g, "r") as f:
+        result[f"vl{tag}"] = {n: read(f[n]) for n in ("d", "u", "seq", "sequ", "cmp")}
+        result[f"vl{tag}"]["va"] = [value(s) for s in f.attrs["va"]]
+    with h5py.File(x, "r") as f:
+        result[f"bad{tag}"] = {n: read(f[n]) for n in ("bad", "badseq", "undef")}
+        result[f"bad{tag}"]["gcol"] = gcol
+json.dump(result, sys.stdout)

crates/clawhdf5-tools/tests/h5rs_interop.rs

@@ -773,3 +773,138 @@ fn every_subcommand_rejects_a_non_hdf5_file_cleanly() {
     assert_eq!(code(&h5rs(&["ls"])), 2);
     assert_eq!(code(&h5rs(&["--help"])), 0);
 }
+
+// ---------------------------------------------------------------------------
+// variable-length data
+// ---------------------------------------------------------------------------
+
+/// Runs `tests/gen_vl_files.py`: VL strings (with an embedded NUL, empty
+/// and null elements), VL sequences, a VL compound member and a VL
+/// attribute, with 8- and 4-byte offsets, plus files whose heap objects
+/// disagree with their elements' lengths.
+fn generate_vl() -> Option<Files> {
+    if missing(python_available(), "python3 with h5py") {
+        return None;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let script = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/gen_vl_files.py");
+    let out = Command::new(python())
+        .arg(&script)
+        .arg(dir.path())
+        .output()
+        .expect("run gen_vl_files.py");
+    assert!(
+        out.status.success(),
+        "gen_vl_files.py failed:\n{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let values = serde_json::from_slice(&out.stdout).expect("gen_vl_files.py output");
+    Some(Files { dir, values })
+}
+
+/// `dump` resolves VL elements through the library's `VlResolver`, as
+/// libhdf5 does: "a\0b" prints as "a", a null string as NULL (it printed
+/// ""), and with 4-byte offsets too; the output is h5dump's byte for byte.
+#[test]
+fn dump_prints_vl_data_like_h5dump() {
+    let Some(f) = generate_vl() else { return };
+    for name in ["vl8.h5", "vl4.h5"] {
+        let p = f.p(name);
+        let ours = stdout(&h5rs(&["dump", &p]));
+        assert!(
+            ours.contains(r#"(0): "a", "", NULL, "zz", "hello""#),
+            "{name}:\n{ours}"
+        );
+        assert!(ours.contains(r#"(0): NULL, "w", NULL, NULL"#), "{name}");
+        assert!(ours.contains("(0): (1, 2, 3), (), (-5)"), "{name}");
+        if missing(tool_available("h5dump"), "h5dump") {
+            continue;
+        }
+        let reference = run("h5dump", &[&p]);
+        assert!(reference.status.success(), "{name}: {reference:?}");
+        assert_eq!(ours, stdout(&reference).replacen(&p, name, 1), "{name}");
+    }
+}
+
+/// `dump --json` gives the values h5py reads, element by element; and an
+/// element whose heap object is not its length × base size is an error, as
+/// in h5py, not a truncated value (it printed "cde" and (1, 2)); so is a
+/// length-0 element at the undefined heap address (it printed "").
+#[test]
+fn dump_json_vl_values_match_h5py() {
+    let Some(f) = generate_vl() else { return };
+    for tag in ["8", "4"] {
+        let (good, bad) = (format!("vl{tag}"), format!("bad{tag}"));
+        let o = h5rs(&["dump", "--json", &f.p(&format!("{good}.h5"))]);
+        assert!(o.status.success(), "{good}: {o:?}");
+        let doc: serde_json::Value = serde_json::from_slice(&o.stdout).unwrap();
+        let want = &f.values[&good];
+        for d in doc["datasets"].as_object().unwrap().values() {
+            let path = d["alias"][0].as_str().unwrap();
+            assert_eq!(d["value"], want[&path[1..]], "{good}: {path}");
+        }
+        let attrs = &doc["groups"][doc["root"].as_str().unwrap()]["attributes"];
+        assert_eq!(attrs[0]["name"], "va");
+        assert_eq!(attrs[0]["value"], want["va"], "{good}: va");
+
+        let o = h5rs(&["dump", "--json", &f.p(&format!("{bad}.h5"))]);
+        let doc: serde_json::Value = serde_json::from_slice(&o.stdout).unwrap();
+        let want = &f.values[&bad];
+        for d in doc["datasets"].as_object().unwrap().values() {
+            let path = d["alias"][0].as_str().unwrap();
+            let got = d["value"].as_array().unwrap();
+            let want = want[&path[1..]].as_array().unwrap();
+            assert_eq!(got.len(), want.len(), "{bad}: {path}");
+            for (g, w) in got.iter().zip(want) {
+                if w.is_null() {
+                    // h5py cannot read it: neither can we.
+                    let e = g["error"]
+                        .as_str()
+                        .unwrap_or_else(|| panic!("{bad}: {path}: {g}"));
+                    let why = if path == "/undef" {
+                        "undefined"
+                    } else {
+                        "holds"
+                    };
+                    assert!(e.contains(why), "{bad}: {path}: {e}");
+                } else {
+                    assert_eq!(g, w, "{bad}: {path}");
+                }
+            }
+        }
+    }
+}
+
+/// `check --data` holds VL elements to libhdf5's rule: a heap object whose
+/// size is not exactly the element's length × base size is a problem (it
+/// only caught objects shorter than the element), and so is an element at
+/// the undefined heap address.
+#[test]
+fn check_data_flags_mis_sized_vl_heap_objects() {
+    let Some(f) = generate_vl() else { return };
+    for tag in ["8", "4"] {
+        let o = h5rs(&["check", "--data", &f.p(&format!("vl{tag}.h5"))]);
+        let s = stdout(&o);
+        assert_eq!(code(&o), 0, "vl{tag}: {s}");
+        assert!(
+            s.contains("global heap collections read: 1"),
+            "vl{tag}: {s}"
+        );
+
+        let o = h5rs(&["check", "--data", &f.p(&format!("bad{tag}.h5"))]);
+        let s = stdout(&o);
+        assert_eq!(code(&o), 1, "bad{tag}: {s}");
+        let at = f.values[format!("bad{tag}")]["gcol"].as_u64().unwrap();
+        for (path, what) in [("/bad", "6 bytes"), ("/badseq", "12 bytes")] {
+            let want = format!("problem: {at:#x} {path}: variable-length data: global heap object");
+            assert!(s.contains(&want), "bad{tag}: no {want:?} in\n{s}");
+            assert!(s.contains(what), "bad{tag}: {s}");
+        }
+        // A length-0 element at the undefined heap address: libhdf5 fails
+        // to read it; check skipped it.
+        let undef: u64 = if tag == "8" { u64::MAX } else { 0xffff_ffff };
+        let want = format!("problem: {undef:#x} /undef: variable-length data: global heap:");
+        assert!(s.contains(&want), "bad{tag}: no {want:?} in\n{s}");
+        assert!(s.contains("undefined global heap address"), "bad{tag}: {s}");
+    }
+}

crates/clawhdf5-wasm/src/core.rs

@@ -9,6 +9,7 @@
 use clawhdf5::{AttrValue, File, Selection};
 use clawhdf5_format::data_read;
 use clawhdf5_format::datatype::{Datatype, DatatypeByteOrder};
+use clawhdf5_format::vl_data::{VlResolver, check_element_size};
 
 /// Errors are reported to JavaScript as messages.
 pub type Result<T> = std::result::Result<T, String>;
@@ -221,6 +222,12 @@ impl Reader {
             None => (Selection::All, shape.clone()),
             Some(h) => hyperslab_selection(h, &shape)?,
         };
+        // A VL type whose stored element size is not the one the file's
+        // offset size implies is refused before its data is read, as
+        // `File::read_string` refuses it.
+        if let Datatype::VariableLength { size, .. } = array_base(&dt) {
+            check_element_size(*size, self.file.superblock().offset_size).map_err(err)?;
+        }
         let raw = ds.read_selection(&selection).map_err(err)?;
         let data = self.decode(&raw, &dt)?;
         out_shape.extend(element_shape(&dt));
@@ -270,22 +277,13 @@ impl Reader {
             Datatype::VariableLength {
                 is_string: true, ..
             } if !is_array => {
-                let size = dt.type_size() as usize;
+                // The library's resolver, as File::read_string uses: a
-                if size == 0 || !raw.len().is_multiple_of(size) {
+                // string ends at its first NUL and a heap object of the
-                    return Err(format!(
+                // wrong size is an error, as in libhdf5 and h5py.
-                        "{} bytes is not a whole number of {size}-byte string references",
-                        raw.len()
-                    ));
-                }
                 let sb = self.file.superblock();
                 Data::Strings(
-                    clawhdf5_format::vl_data::read_vl_strings(
+                    VlResolver::new(self.file.as_bytes(), sb.offset_size, sb.length_size)
-                        self.file.as_bytes(),
+                        .strings(raw)
-                        raw,
-                        (raw.len() / size) as u64,
-                        sb.offset_size,
-                        sb.length_size,
-                    )
                         .map_err(err)?,
                 )
             }

crates/clawhdf5-wasm/tests/vl_strings.rs

@@ -0,0 +1,169 @@
+//! The wasm reader resolves VL strings with the library's `VlResolver`, so
+//! it returns what `File::read_string` and h5py return: a string ends at
+//! its first NUL, a null element is empty, a heap object of the wrong size
+//! is an error, an element at the undefined heap address is an error, and a
+//! VL datatype whose stored element size disagrees with the file's offset
+//! size is refused. Checked with 8- and 4-byte offsets.
+//!
+//! Skipped when python3 with h5py is missing, unless
+//! `CLAWHDF5_REQUIRE_INTEROP=1`. `CLAWHDF5_PYTHON` names the interpreter.
+
+use std::process::Command;
+
+use clawhdf5::File;
+use clawhdf5_wasm::core::{Data, Reader};
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn h5py_available() -> bool {
+    let ok = Command::new(python())
+        .args(["-c", "import h5py, numpy"])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false);
+    if !ok {
+        assert!(
+            std::env::var("CLAWHDF5_REQUIRE_INTEROP").as_deref() != Ok("1"),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but python with h5py is not available"
+        );
+        eprintln!("SKIP: python with h5py not available");
+    }
+    ok
+}
+
+/// For each offset size: `vl{8,4}.h5` with dataset `d` = "a\0b", "", null,
+/// "zz" (patched: h5py writes neither a NUL nor a null element);
+/// `bad{8,4}.h5` whose element 0 claims 3 bytes of a 6-byte heap object;
+/// `size{8,4}.h5` whose VL datatype message stores a 24-byte element; and
+/// `undef{8,4}.h5` whose element 1 has length 0 and the undefined heap
+/// address.
+/// Prints h5py's reading of each element as hex, or "error".
+const SCRIPT: &str = r#"
+import struct, sys, h5py, numpy as np
+out = sys.argv[1]
+S = h5py.string_dtype('utf-8')
+def create(path, os_):
+    if os_ == 8:
+        return h5py.File(path, 'w', libver='earliest')
+    # The earliest format, so the patched object header has no checksum.
+    fcpl = h5py.h5p.create(h5py.h5p.FILE_CREATE); fcpl.set_sizes(4, 4)
+    fapl = h5py.h5p.create(h5py.h5p.FILE_ACCESS)
+    fapl.set_libver_bounds(h5py.h5f.LIBVER_EARLIEST, h5py.h5f.LIBVER_V18)
+    return h5py.File(h5py.h5f.create(path.encode(), h5py.h5f.ACC_TRUNC, fcpl=fcpl, fapl=fapl))
+def elem(length, addr, index, os_):
+    return struct.pack('<I', length) + addr.to_bytes(os_, 'little') + struct.pack('<I', index)
+def make(path, os_, values):
+    with create(path, os_) as f:
+        f.create_dataset('d', data=np.array(values, dtype=object), dtype=S)
+        return f['d'].id.get_offset()
+for os_ in (8, 4):
+    es = 8 + os_
+    p = '%s/vl%d.h5' % (out, os_)
+    off = make(p, os_, ['aXb', '', 'ok', 'zz'])
+    b = bytearray(open(p, 'rb').read())
+    b[b.index(b'aXb') + 1] = 0
+    b[off + 2 * es:off + 3 * es] = elem(2, 0, 1, os_)
+    open(p, 'wb').write(bytes(b))
+    p = '%s/bad%d.h5' % (out, os_)
+    off = make(p, os_, ['cdefgh', 'ok'])
+    b = bytearray(open(p, 'rb').read())
+    struct.pack_into('<I', b, off, 3)
+    open(p, 'wb').write(bytes(b))
+    p = '%s/size%d.h5' % (out, os_)
+    make(p, os_, ['x', 'yy'])
+    b = bytearray(open(p, 'rb').read())
+    # datatype message: version 1, class 9 (VL); string, null-terminated, UTF-8
+    pat = bytes([0x19, 0x01, 0x01, 0x00]) + struct.pack('<I', es)
+    assert b.count(pat) == 1, b.count(pat)
+    i = b.index(pat)
+    struct.pack_into('<I', b, i + 4, 24)
+    open(p, 'wb').write(bytes(b))
+    p = '%s/undef%d.h5' % (out, os_)
+    off = make(p, os_, ['x', '', 'yz'])
+    b = bytearray(open(p, 'rb').read())
+    b[off + es:off + 2 * es] = elem(0, (1 << (8 * os_)) - 1, 1, os_)
+    open(p, 'wb').write(bytes(b))
+    for name in ('vl', 'bad', 'size', 'undef'):
+        with h5py.File('%s/%s%d.h5' % (out, name, os_), 'r') as f:
+            got = []
+            for i in range(f['d'].shape[0]):
+                try:
+                    got.append(f['d'][i].hex())
+                except OSError:
+                    got.append('error')
+            print('%s%d\t%s' % (name, os_, ','.join(got)))
+"#;
+
+#[test]
+fn vl_strings_read_like_file_and_h5py() {
+    if !h5py_available() {
+        return;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let out = Command::new(python())
+        .args(["-c", SCRIPT])
+        .arg(dir.path())
+        .output()
+        .expect("run python");
+    assert!(
+        out.status.success(),
+        "{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let h5py: std::collections::HashMap<String, String> = String::from_utf8(out.stdout)
+        .unwrap()
+        .lines()
+        .map(|l| {
+            let (k, v) = l.split_once('\t').unwrap();
+            (k.to_string(), v.to_string())
+        })
+        .collect();
+    let read = |name: &str| {
+        let bytes = std::fs::read(dir.path().join(format!("{name}.h5"))).unwrap();
+        let wasm = Reader::open(bytes).unwrap().read("/d", None);
+        let file = File::open(dir.path().join(format!("{name}.h5")))
+            .unwrap()
+            .dataset("d")
+            .unwrap()
+            .read_string();
+        (wasm, file)
+    };
+    for os in [8, 4] {
+        // h5py: "a\0b" is "a"; the null element (address 0) is empty.
+        assert_eq!(h5py[&format!("vl{os}")], "61,,,7a7a");
+        let (wasm, file) = read(&format!("vl{os}"));
+        let Data::Strings(wasm) = wasm.unwrap().data else {
+            panic!("vl{os}: not strings")
+        };
+        assert_eq!(wasm, ["a", "", "", "zz"], "vl{os}");
+        assert_eq!(wasm, file.unwrap(), "vl{os}");
+
+        // h5py refuses the mis-sized element; so do both readers.
+        assert_eq!(h5py[&format!("bad{os}")], "error,6f6b");
+        let (wasm, file) = read(&format!("bad{os}"));
+        assert!(wasm.unwrap_err().contains("holds 6 bytes"), "bad{os}");
+        assert!(file.is_err(), "bad{os}");
+
+        // libhdf5 ignores the stored element size and reads the values;
+        // File refuses the datatype rather than guess its layout, and the
+        // wasm reader now does the same (it read with the stored size).
+        assert_eq!(h5py[&format!("size{os}")], "78,7979");
+        let (wasm, file) = read(&format!("size{os}"));
+        let e = wasm.unwrap_err();
+        assert!(e.contains("stores 24-byte elements"), "size{os}: {e}");
+        assert!(file.is_err(), "size{os}");
+
+        // Length 0 at the undefined heap address: libhdf5 fails the read
+        // ("addr undefined"); both readers returned "".
+        assert_eq!(h5py[&format!("undef{os}")], "78,error,797a");
+        let (wasm, file) = read(&format!("undef{os}"));
+        let e = wasm.unwrap_err();
+        assert!(
+            e.contains("undefined global heap address"),
+            "undef{os}: {e}"
+        );
+        assert!(file.is_err(), "undef{os}");
+    }
+}

crates/clawhdf5/src/lazy.rs

@@ -422,11 +422,47 @@ impl<'f, R: HDF5Read> LazyDataset<'f, R> {
         Ok(data_read::read_as_u64(&raw, &dt)?)
     }
 
-    /// Read all data as `String` values.
+    /// Read all data as `String` values: fixed- or variable-length strings
+    /// (see [`Dataset::read_string`](crate::Dataset::read_string)).
     pub fn read_string(&self) -> Result<Vec<String>, Error> {
         let raw = self.read_raw()?;
         let dt = self.datatype()?;
-        Ok(data_read::read_as_strings(&raw, &dt)?)
+        crate::vlen::decode_strings(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
+    }
+
+    /// Read a variable-length string dataset as the exact bytes of each
+    /// string (see
+    /// [`Dataset::read_string_bytes`](crate::Dataset::read_string_bytes)).
+    pub fn read_string_bytes(&self) -> Result<Vec<Vec<u8>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        crate::vlen::decode_string_bytes(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
+    }
+
+    /// Read a variable-length sequence dataset as one `Vec<T>` per element
+    /// (see [`Dataset::read_vlen`](crate::Dataset::read_vlen)).
+    pub fn read_vlen<T: crate::vlen::VlenValue>(&self) -> Result<Vec<Vec<T>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        crate::vlen::decode_vlen(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
     }
 
     /// Read all attributes of this dataset.

crates/clawhdf5/src/lib.rs

@@ -30,6 +30,7 @@ pub mod lazy;
 pub mod mmap_file;
 pub mod reader;
 pub mod types;
+pub mod vlen;
 pub mod writer;
 
 pub use error::Error;
@@ -38,6 +39,7 @@ pub use lazy::{LazyDataset, LazyFile, LazyGroup};
 pub use mmap_file::{MmapDataset, MmapFile, MmapGroup};
 pub use reader::{Dataset, File, Group};
 pub use types::{AttrValue, DType};
+pub use vlen::VlenValue;
 pub use writer::FileBuilder;
 #[cfg(feature = "parallel")]
 pub use writer::{DatasetSpec, create_datasets_parallel};

crates/clawhdf5/src/mmap_file.rs

@@ -336,11 +336,47 @@ impl<'f> MmapDataset<'f> {
         Ok(data_read::read_as_u64(&raw, &dt)?)
     }
 
-    /// Read all data as `String` values.
+    /// Read all data as `String` values: fixed- or variable-length strings
+    /// (see [`Dataset::read_string`](crate::Dataset::read_string)).
     pub fn read_string(&self) -> Result<Vec<String>, Error> {
         let raw = self.read_raw()?;
         let dt = self.datatype()?;
-        Ok(data_read::read_as_strings(&raw, &dt)?)
+        crate::vlen::decode_strings(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
+    }
+
+    /// Read a variable-length string dataset as the exact bytes of each
+    /// string (see
+    /// [`Dataset::read_string_bytes`](crate::Dataset::read_string_bytes)).
+    pub fn read_string_bytes(&self) -> Result<Vec<Vec<u8>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        crate::vlen::decode_string_bytes(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
+    }
+
+    /// Read a variable-length sequence dataset as one `Vec<T>` per element
+    /// (see [`Dataset::read_vlen`](crate::Dataset::read_vlen)).
+    pub fn read_vlen<T: crate::vlen::VlenValue>(&self) -> Result<Vec<Vec<T>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        crate::vlen::decode_vlen(
+            self.file.hdf5_bytes(),
+            &dt,
+            &raw,
+            self.file.offset_size(),
+            self.file.length_size(),
+        )
     }
 
     /// For contiguous datasets, return a zero-copy slice into the mmap.

crates/clawhdf5/src/reader.rs

@@ -262,6 +262,56 @@ impl File {
         }
     }
 
+    /// Decode the strings in `raw`, a buffer of elements of `datatype` read
+    /// from this file — for instance a variable-length string field of a
+    /// compound ([`clawhdf5_format::data_read::read_compound_fields`]) or an
+    /// [`AttrValue::Raw`] attribute. Variable-length strings are resolved in
+    /// this file's global heap; see [`Dataset::read_string`] for the values.
+    pub fn decode_strings(&self, datatype: &Datatype, raw: &[u8]) -> Result<Vec<String>, Error> {
+        crate::vlen::decode_strings(
+            self.as_bytes(),
+            datatype,
+            raw,
+            self.offset_size(),
+            self.length_size(),
+        )
+    }
+
+    /// Like [`decode_strings`](Self::decode_strings) for variable-length
+    /// strings, returning each string's exact bytes (see
+    /// [`Dataset::read_string_bytes`]).
+    pub fn decode_string_bytes(
+        &self,
+        datatype: &Datatype,
+        raw: &[u8],
+    ) -> Result<Vec<Vec<u8>>, Error> {
+        crate::vlen::decode_string_bytes(
+            self.as_bytes(),
+            datatype,
+            raw,
+            self.offset_size(),
+            self.length_size(),
+        )
+    }
+
+    /// Decode the variable-length sequences in `raw`, a buffer of elements
+    /// of the sequence type `datatype` read from this file (a compound
+    /// field, an [`AttrValue::Raw`] attribute, ...). See
+    /// [`Dataset::read_vlen`].
+    pub fn decode_vlen<T: crate::vlen::VlenValue>(
+        &self,
+        datatype: &Datatype,
+        raw: &[u8],
+    ) -> Result<Vec<Vec<T>>, Error> {
+        crate::vlen::decode_vlen(
+            self.as_bytes(),
+            datatype,
+            raw,
+            self.offset_size(),
+            self.length_size(),
+        )
+    }
+
     fn parse_header(&self, address: u64) -> Result<ObjectHeader, FormatError> {
         ObjectHeader::parse(
             self.data.as_bytes(),
@@ -498,11 +548,58 @@ impl<'f> Dataset<'f> {
         Ok(data_read::read_as_u64(&raw, &dt)?)
     }
 
-    /// Read all data as `String` values.
+    /// Read all data as `String` values, in row-major order.
+    ///
+    /// Works for fixed-length and variable-length string datasets (h5py's
+    /// default `str` dtype). A variable-length string ends at its first NUL
+    /// and a null element (e.g. never written) is `""`, as h5py returns
+    /// them; bytes that are not valid UTF-8 are replaced with U+FFFD — use
+    /// [`read_string_bytes`](Self::read_string_bytes) for the exact bytes.
     pub fn read_string(&self) -> Result<Vec<String>, Error> {
         let raw = self.read_raw()?;
         let dt = self.datatype()?;
-        Ok(data_read::read_as_strings(&raw, &dt)?)
+        self.file.decode_strings(&dt, &raw)
+    }
+
+    /// Read a variable-length string dataset as the exact bytes of each
+    /// string (what h5py's `Dataset[()]` returns), in row-major order.
+    pub fn read_string_bytes(&self) -> Result<Vec<Vec<u8>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        self.file.decode_string_bytes(&dt, &raw)
+    }
+
+    /// Read the selected elements of a fixed- or variable-length string
+    /// dataset (see [`read_string`](Self::read_string)).
+    pub fn read_string_selection(
+        &self,
+        selection: &clawhdf5_format::selection::Selection,
+    ) -> Result<Vec<String>, Error> {
+        let raw = self.read_selection(selection)?;
+        let dt = self.datatype()?;
+        self.file.decode_strings(&dt, &raw)
+    }
+
+    /// Read a variable-length sequence dataset (h5py
+    /// `vlen_dtype(np.int32)`, ...) as one `Vec<T>` per element, in
+    /// row-major order. The base type must be an integer or float type; it
+    /// is converted to `T` as [`read_f64`](Self::read_f64) and the other
+    /// typed readers convert. A null element is an empty sequence.
+    pub fn read_vlen<T: crate::vlen::VlenValue>(&self) -> Result<Vec<Vec<T>>, Error> {
+        let raw = self.read_raw()?;
+        let dt = self.datatype()?;
+        self.file.decode_vlen(&dt, &raw)
+    }
+
+    /// Read the selected elements of a variable-length sequence dataset
+    /// (see [`read_vlen`](Self::read_vlen)).
+    pub fn read_vlen_selection<T: crate::vlen::VlenValue>(
+        &self,
+        selection: &clawhdf5_format::selection::Selection,
+    ) -> Result<Vec<Vec<T>>, Error> {
+        let raw = self.read_selection(selection)?;
+        let dt = self.datatype()?;
+        self.file.decode_vlen(&dt, &raw)
     }
 
     // ----- Selection-based read methods -----

crates/clawhdf5/src/vlen.rs

@@ -0,0 +1,143 @@
+//! Variable-length data: VL strings and VL sequences of numbers.
+//!
+//! A variable-length element stores a reference into the file's global heap;
+//! these helpers resolve the references in a buffer of raw elements (from a
+//! dataset read, a selection, a compound field or an [`AttrValue::Raw`]
+//! attribute) against the file they came from.
+//!
+//! Values match libhdf5 (and h5py): a string ends at its first NUL, a null
+//! element is an empty string or sequence, and a heap object whose size
+//! disagrees with its element is an error rather than a truncated value.
+//!
+//! [`AttrValue::Raw`]: crate::AttrValue::Raw
+
+use clawhdf5_format::data_read;
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::error::FormatError;
+use clawhdf5_format::vl_data::{VlResolver, check_element_size};
+
+use crate::error::Error;
+
+mod sealed {
+    pub trait Sealed {}
+}
+
+/// A number type that [`Dataset::read_vlen`](crate::Dataset::read_vlen) can
+/// return: the sequence's base type is converted to it as libhdf5 converts
+/// numbers (the same rules as `read_f64`, `read_i64`, ...).
+pub trait VlenValue: sealed::Sealed + Sized {
+    #[doc(hidden)]
+    fn decode(raw: &[u8], base: &Datatype) -> Result<Vec<Self>, FormatError>;
+}
+
+macro_rules! vlen_value {
+    ($t:ty, $f:path) => {
+        impl sealed::Sealed for $t {}
+        impl VlenValue for $t {
+            fn decode(raw: &[u8], base: &Datatype) -> Result<Vec<Self>, FormatError> {
+                $f(raw, base)
+            }
+        }
+    };
+}
+
+vlen_value!(f64, data_read::read_as_f64);
+vlen_value!(f32, data_read::read_as_f32);
+vlen_value!(i64, data_read::read_as_i64);
+vlen_value!(i32, data_read::read_as_i32);
+vlen_value!(u64, data_read::read_as_u64);
+
+fn class_name(dt: &Datatype) -> &'static str {
+    match dt {
+        Datatype::FixedPoint { .. } => "integer",
+        Datatype::FloatingPoint { .. } => "float",
+        Datatype::Time { .. } => "time",
+        Datatype::String { .. } => "fixed-length string",
+        Datatype::BitField { .. } => "bitfield",
+        Datatype::Opaque { .. } => "opaque",
+        Datatype::Compound { .. } => "compound",
+        Datatype::Reference { .. } => "reference",
+        Datatype::Enumeration { .. } => "enum",
+        Datatype::VariableLength {
+            is_string: true, ..
+        } => "variable-length string",
+        Datatype::VariableLength { .. } => "variable-length sequence",
+        Datatype::Array { .. } => "array",
+    }
+}
+
+/// The strings in `raw`, elements of `dt`: fixed-length strings decoded as
+/// `read_string` always has, variable-length strings resolved in the heap.
+pub(crate) fn decode_strings(
+    file_data: &[u8],
+    dt: &Datatype,
+    raw: &[u8],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<String>, Error> {
+    match dt {
+        Datatype::VariableLength {
+            size,
+            is_string: true,
+            ..
+        } => {
+            check_element_size(*size, offset_size)?;
+            Ok(VlResolver::new(file_data, offset_size, length_size).strings(raw)?)
+        }
+        _ => Ok(data_read::read_as_strings(raw, dt)?),
+    }
+}
+
+/// The exact bytes of the variable-length strings in `raw`.
+pub(crate) fn decode_string_bytes(
+    file_data: &[u8],
+    dt: &Datatype,
+    raw: &[u8],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<Vec<u8>>, Error> {
+    match dt {
+        Datatype::VariableLength {
+            size,
+            is_string: true,
+            ..
+        } => {
+            check_element_size(*size, offset_size)?;
+            Ok(VlResolver::new(file_data, offset_size, length_size).string_bytes(raw)?)
+        }
+        other => Err(Error::Format(FormatError::TypeMismatch {
+            expected: "variable-length string",
+            actual: class_name(other),
+        })),
+    }
+}
+
+/// The sequences in `raw`, elements of the variable-length sequence type
+/// `dt`, converted to `T`.
+pub(crate) fn decode_vlen<T: VlenValue>(
+    file_data: &[u8],
+    dt: &Datatype,
+    raw: &[u8],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<Vec<T>>, Error> {
+    let Datatype::VariableLength {
+        size,
+        is_string: false,
+        base_type,
+        ..
+    } = dt
+    else {
+        return Err(Error::Format(FormatError::TypeMismatch {
+            expected: "variable-length sequence",
+            actual: class_name(dt),
+        }));
+    };
+    check_element_size(*size, offset_size)?;
+    let base_size = base_type.type_size() as usize;
+    VlResolver::new(file_data, offset_size, length_size)
+        .sequences(raw, base_size)?
+        .iter()
+        .map(|bytes| Ok(T::decode(bytes, base_type)?))
+        .collect()
+}

crates/clawhdf5/tests/vl_data_interop.rs

@@ -0,0 +1,494 @@
+//! Variable-length data (VL strings and VL sequences) read through the
+//! facade, checked against h5py/libhdf5.
+//!
+//! h5py writes each file — once with the default 8-byte offsets and once
+//! with 4-byte offsets and lengths (`sizeof_addr = 4`) — and prints what
+//! libhdf5 reads back; `File`, `MmapFile` and `LazyFile` must return the same
+//! values. Skipped when python3 with h5py is unavailable, unless
+//! `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+// `Selection::slice(&[0..1])` is one range per dimension, not a Vec of a range.
+#![allow(clippy::single_range_in_vec_init)]
+
+use std::collections::HashMap;
+use std::path::Path;
+use std::process::Command;
+
+use clawhdf5::{AttrValue, File, LazyFile, MmapFile, Selection};
+
+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)
+}
+
+macro_rules! skip_if_no_python {
+    () => {
+        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;
+        }
+    };
+}
+
+/// Run `script` and return its stdout as `key -> value`, one
+/// `key<TAB>value` line per key.
+fn run_python(script: &str) -> HashMap<String, String> {
+    let output = Command::new(python())
+        .args(["-c", script])
+        .output()
+        .expect("failed to run python");
+    assert!(
+        output.status.success(),
+        "python failed:\n{}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    String::from_utf8_lossy(&output.stdout)
+        .lines()
+        .filter_map(|line| {
+            let (k, v) = line.split_once('\t')?;
+            Some((k.to_string(), v.to_string()))
+        })
+        .collect()
+}
+
+/// `hex,hex,...` -> the strings' bytes.
+fn parse_strings(v: &str) -> Vec<Vec<u8>> {
+    v.split(',')
+        .map(|h| {
+            (0..h.len())
+                .step_by(2)
+                .map(|i| u8::from_str_radix(&h[i..i + 2], 16).unwrap())
+                .collect()
+        })
+        .collect()
+}
+
+/// `1 2 3|| -5` -> sequences.
+fn parse_seqs(v: &str) -> Vec<Vec<f64>> {
+    v.split('|')
+        .map(|s| s.split_whitespace().map(|x| x.parse().unwrap()).collect())
+        .collect()
+}
+
+fn utf8(bytes: &[Vec<u8>]) -> Vec<String> {
+    bytes
+        .iter()
+        .map(|b| String::from_utf8(b.clone()).unwrap())
+        .collect()
+}
+
+/// Writes `vl8.h5` (8-byte offsets) and `vl4.h5` (4-byte offsets and
+/// lengths) into `dir` and prints h5py's reading of both.
+const SCRIPT: &str = r#"
+import sys, h5py, numpy as np
+d = sys.argv[1]
+S = h5py.string_dtype('utf-8'); A = h5py.string_dtype('ascii')
+def make(path, sizes):
+    if sizes:
+        fcpl = h5py.h5p.create(h5py.h5p.FILE_CREATE); fcpl.set_sizes(*sizes)
+        f = h5py.File(h5py.h5f.create(path.encode(), h5py.h5f.ACC_TRUNC, fcpl=fcpl))
+    else:
+        f = h5py.File(path, 'w')
+    f.create_dataset('scalar_utf8', data='héllo', dtype=S)
+    f.create_dataset('scalar_ascii', data=b'hello', dtype=A)
+    f.create_dataset('d1', data=np.array(['a', '', 'ccc', 'δδ'], dtype=object), dtype=S)
+    f.create_dataset('d2', data=np.array([['x', 'yy', 'zzz'], ['', 'w', 'vv']], dtype=object), dtype=S)
+    f.create_dataset('chunked', data=np.array(['s%d' % i * (i % 5) for i in range(100)], dtype=object),
+                     dtype=S, chunks=(7,), compression='gzip')
+    f.create_dataset('chunked2d', data=np.array([['r%dc%d' % (r, c) for c in range(9)] for r in range(11)], dtype=object),
+                     dtype=S, chunks=(4, 4), compression='gzip', shuffle=True)
+    f.create_dataset('unwritten', shape=(5,), dtype=S, chunks=(2,))
+    p = f.create_dataset('partial', shape=(6,), dtype=S, chunks=(2,)); p[0] = 'first'; p[5] = 'last'
+    f.create_dataset('contig_empty', shape=(3,), dtype=S)
+    dcpl = h5py.h5p.create(h5py.h5p.DATASET_CREATE); dcpl.set_layout(h5py.h5d.COMPACT)
+    f.create_dataset('compact', data=np.array(['c1', '', 'c3'], dtype=object), dtype=S, dcpl=dcpl)
+    assert f['compact'].id.get_create_plist().get_layout() == h5py.h5d.COMPACT
+    f.attrs['vlattr'] = 'attr-value'
+    f.attrs.create('vlattr_arr', np.array(['p', 'qq', ''], dtype=object), dtype=S)
+    ct = np.dtype([('id', '<i4'), ('name', S), ('v', '<f8')])
+    arr = np.zeros(3, dtype=ct); arr['id'] = [1, 2, 3]; arr['name'] = ['one', '', 'three']; arr['v'] = [.5, 1.5, 2.5]
+    f.create_dataset('compound', data=arr)
+    f.attrs.create('compound_attr', arr)
+    v = f.create_dataset('vlen_i4', shape=(3,), dtype=h5py.vlen_dtype(np.dtype('<i4')))
+    v[0] = [1, 2, 3]; v[1] = []; v[2] = [-5]
+    v = f.create_dataset('vlen_f8', shape=(2, 2), dtype=h5py.vlen_dtype(np.dtype('<f8')), chunks=(1, 2), compression='gzip')
+    v[0, 0] = [1.5]; v[0, 1] = [2.5, 3.5]; v[1, 1] = [9.0]
+    v = f.create_dataset('vlen_u2_be', shape=(2,), dtype=h5py.vlen_dtype(np.dtype('>u2')))
+    v[0] = [1, 65535]; v[1] = [300]
+    f.attrs.create('vlen_attr', np.array([np.array([1, 2], dtype='<i8'), np.array([3], dtype='<i8')], dtype=object),
+                   dtype=h5py.vlen_dtype(np.dtype('<i8')))
+    f.close()
+
+def hexes(a):
+    return ','.join(bytes(x).hex() for x in np.asarray(a, dtype=object).ravel())
+def seqs(a):
+    return '|'.join(' '.join(repr(float(x)) for x in s) for s in np.asarray(a, dtype=object).ravel())
+
+for tag, sizes in (('8', None), ('4', (4, 4))):
+    path = '%s/vl%s.h5' % (d, tag)
+    make(path, sizes)
+    with h5py.File(path, 'r') as f:
+        for name in ('compact', 'scalar_utf8', 'scalar_ascii', 'd1', 'd2', 'chunked', 'chunked2d', 'unwritten',
+                     'partial', 'contig_empty'):
+            v = f[name][()]
+            print('%s:%s\t%s' % (tag, name, hexes([v] if np.ndim(v) == 0 else v)))
+        print('%s:d2[1,1:3]\t%s' % (tag, hexes(f['d2'][1, 1:3])))
+        print('%s:chunked[5:60:3]\t%s' % (tag, hexes(f['chunked'][5:60:3])))
+        print('%s:chunked2d[2:9:2,3:8]\t%s' % (tag, hexes(f['chunked2d'][2:9:2, 3:8])))
+        print('%s:compound.name\t%s' % (tag, hexes(f['compound']['name'])))
+        print('%s:compound_attr.name\t%s' % (tag, hexes(f.attrs['compound_attr']['name'])))
+        print('%s:vlattr\t%s' % (tag, hexes([f.attrs['vlattr'].encode()])))
+        print('%s:vlattr_arr\t%s' % (tag, hexes([s.encode() for s in f.attrs['vlattr_arr']])))
+        for name in ('vlen_i4', 'vlen_f8'):
+            print('%s:%s\t%s' % (tag, name, seqs(f[name][()])))
+        print('%s:vlen_f8[1,:]\t%s' % (tag, seqs(f['vlen_f8'][1, :])))
+        print('%s:vlen_attr\t%s' % (tag, seqs(f.attrs['vlen_attr'])))
+"#;
+
+fn make_files(dir: &Path) -> HashMap<String, String> {
+    let script = format!(
+        "import sys; sys.argv = ['x', {:?}]\n{SCRIPT}",
+        dir.display().to_string()
+    );
+    run_python(&script)
+}
+
+const STRING_DATASETS: [&str; 10] = [
+    "compact",
+    "scalar_utf8",
+    "scalar_ascii",
+    "d1",
+    "d2",
+    "chunked",
+    "chunked2d",
+    "unwritten",
+    "partial",
+    "contig_empty",
+];
+
+#[test]
+fn vl_string_datasets_read_like_h5py() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let expected = make_files(dir.path());
+    for tag in ["8", "4"] {
+        let path = dir.path().join(format!("vl{tag}.h5"));
+        let file = File::open(&path).unwrap();
+        let mmap = MmapFile::open(&path).unwrap();
+        let lazy = LazyFile::open_mmap(&path).unwrap();
+        for name in STRING_DATASETS {
+            let want = parse_strings(&expected[&format!("{tag}:{name}")]);
+            let ctx = format!("vl{tag}.h5 {name}");
+            let ds = file.dataset(name).unwrap();
+            assert_eq!(ds.read_string_bytes().unwrap(), want, "{ctx}");
+            assert_eq!(ds.read_string().unwrap(), utf8(&want), "{ctx}");
+            let m = mmap.dataset(name).unwrap();
+            assert_eq!(m.read_string_bytes().unwrap(), want, "{ctx} (mmap)");
+            assert_eq!(m.read_string().unwrap(), utf8(&want), "{ctx} (mmap)");
+            let l = lazy.dataset(name).unwrap();
+            assert_eq!(l.read_string_bytes().unwrap(), want, "{ctx} (lazy)");
+            assert_eq!(l.read_string().unwrap(), utf8(&want), "{ctx} (lazy)");
+        }
+    }
+}
+
+#[test]
+fn vl_string_selections_read_like_h5py() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let expected = make_files(dir.path());
+    let hyperslab = |start: &[u64], stride: &[u64], count: &[u64]| Selection::Hyperslab {
+        start: start.to_vec(),
+        stride: stride.to_vec(),
+        count: count.to_vec(),
+        block: vec![1; start.len()],
+    };
+    let cases = [
+        ("d2", "d2[1,1:3]", Selection::slice(&[1..2, 1..3])),
+        ("chunked", "chunked[5:60:3]", hyperslab(&[5], &[3], &[19])),
+        (
+            "chunked2d",
+            "chunked2d[2:9:2,3:8]",
+            hyperslab(&[2, 3], &[2, 1], &[4, 5]),
+        ),
+    ];
+    for tag in ["8", "4"] {
+        let file = File::open(dir.path().join(format!("vl{tag}.h5"))).unwrap();
+        for (name, key, sel) in &cases {
+            let want = utf8(&parse_strings(&expected[&format!("{tag}:{key}")]));
+            let got = file
+                .dataset(name)
+                .unwrap()
+                .read_string_selection(sel)
+                .unwrap();
+            assert_eq!(got, want, "vl{tag}.h5 {key}");
+        }
+        // A selection of VL integers is not strings.
+        assert!(
+            file.dataset("vlen_i4")
+                .unwrap()
+                .read_string_selection(&Selection::slice(&[0..1]))
+                .is_err()
+        );
+    }
+}
+
+#[test]
+fn vl_values_in_compounds_and_attributes_read_like_h5py() {
+    // With 4-byte offsets these failed with GlobalHeapObjectNotFound or came
+    // back as `AttrValue::Raw`: the VL type claimed 16-byte elements and the
+    // global heap was read without the padding libhdf5 puts after its
+    // headers.
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let expected = make_files(dir.path());
+    for tag in ["8", "4"] {
+        let file = File::open(dir.path().join(format!("vl{tag}.h5"))).unwrap();
+        let want = |key: &str| utf8(&parse_strings(&expected[&format!("{tag}:{key}")]));
+
+        let attrs = file.root().attrs().unwrap();
+        match &attrs["vlattr"] {
+            AttrValue::String(s) => assert_eq!(*s, want("vlattr")[0], "vl{tag}.h5"),
+            other => panic!("vl{tag}.h5 vlattr: {other:?}"),
+        }
+        match &attrs["vlattr_arr"] {
+            AttrValue::StringArray(s) => assert_eq!(*s, want("vlattr_arr"), "vl{tag}.h5"),
+            other => panic!("vl{tag}.h5 vlattr_arr: {other:?}"),
+        }
+
+        // Compound with a VL string member: dataset and attribute.
+        let ds = file.dataset("compound").unwrap();
+        let dt = ds.raw_datatype().unwrap();
+        let raw = ds.read_selection(&Selection::All).unwrap();
+        let fields = clawhdf5_format::data_read::read_compound_fields(&raw, &dt).unwrap();
+        let name = fields.iter().find(|f| f.name == "name").unwrap();
+        assert_eq!(
+            file.decode_strings(&name.datatype, &name.raw_data).unwrap(),
+            want("compound.name"),
+            "vl{tag}.h5 compound"
+        );
+        let id = fields.iter().find(|f| f.name == "id").unwrap();
+        assert_eq!(
+            clawhdf5_format::data_read::read_as_i64(&id.raw_data, &id.datatype).unwrap(),
+            vec![1, 2, 3]
+        );
+        let v = fields.iter().find(|f| f.name == "v").unwrap();
+        assert_eq!(
+            clawhdf5_format::data_read::read_as_f64(&v.raw_data, &v.datatype).unwrap(),
+            vec![0.5, 1.5, 2.5]
+        );
+
+        let AttrValue::Raw { datatype, data, .. } = &attrs["compound_attr"] else {
+            panic!("compound attribute is Raw");
+        };
+        let fields = clawhdf5_format::data_read::read_compound_fields(data, datatype).unwrap();
+        let name = fields.iter().find(|f| f.name == "name").unwrap();
+        assert_eq!(
+            file.decode_strings(&name.datatype, &name.raw_data).unwrap(),
+            want("compound_attr.name"),
+            "vl{tag}.h5 compound attribute"
+        );
+
+        // A VL sequence attribute.
+        let AttrValue::Raw { datatype, data, .. } = &attrs["vlen_attr"] else {
+            panic!("vlen attribute is Raw");
+        };
+        let got: Vec<Vec<i64>> = file.decode_vlen(datatype, data).unwrap();
+        let want_seqs = parse_seqs(&expected[&format!("{tag}:vlen_attr")]);
+        assert_eq!(
+            got,
+            want_seqs
+                .iter()
+                .map(|s| s.iter().map(|&x| x as i64).collect::<Vec<_>>())
+                .collect::<Vec<_>>(),
+            "vl{tag}.h5 vlen_attr"
+        );
+    }
+}
+
+#[test]
+fn vl_sequence_datasets_read_like_h5py() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let expected = make_files(dir.path());
+    for tag in ["8", "4"] {
+        let path = dir.path().join(format!("vl{tag}.h5"));
+        let file = File::open(&path).unwrap();
+        let seqs = |key: &str| parse_seqs(&expected[&format!("{tag}:{key}")]);
+
+        let i4 = file.dataset("vlen_i4").unwrap();
+        let want: Vec<Vec<i32>> = seqs("vlen_i4")
+            .iter()
+            .map(|s| s.iter().map(|&x| x as i32).collect())
+            .collect();
+        assert_eq!(i4.read_vlen::<i32>().unwrap(), want, "vl{tag}.h5 vlen_i4");
+        let as_f64: Vec<Vec<f64>> = i4.read_vlen().unwrap();
+        assert_eq!(as_f64, seqs("vlen_i4"));
+
+        let f8 = file.dataset("vlen_f8").unwrap();
+        assert_eq!(
+            f8.read_vlen::<f64>().unwrap(),
+            seqs("vlen_f8"),
+            "vl{tag}.h5"
+        );
+        assert_eq!(
+            f8.read_vlen_selection::<f64>(&Selection::slice(&[1..2, 0..2]))
+                .unwrap(),
+            seqs("vlen_f8[1,:]"),
+            "vl{tag}.h5 vlen_f8[1,:]"
+        );
+
+        // h5py returns big-endian VL elements byte-swapped (an h5py bug, see
+        // CONFORMANCE.md); the values written are [1, 65535] and [300].
+        assert_eq!(
+            file.dataset("vlen_u2_be")
+                .unwrap()
+                .read_vlen::<u64>()
+                .unwrap(),
+            vec![vec![1, 65535], vec![300]]
+        );
+
+        let mmap = MmapFile::open(&path).unwrap();
+        assert_eq!(
+            mmap.dataset("vlen_f8").unwrap().read_vlen::<f64>().unwrap(),
+            seqs("vlen_f8")
+        );
+        let lazy = LazyFile::open_mmap(&path).unwrap();
+        assert_eq!(
+            lazy.dataset("vlen_f8").unwrap().read_vlen::<f64>().unwrap(),
+            seqs("vlen_f8")
+        );
+
+        // Wrong kind of data is an error, not a value.
+        assert!(i4.read_string().is_err());
+        assert!(i4.read_string_bytes().is_err());
+        assert!(file.dataset("d1").unwrap().read_vlen::<f64>().is_err());
+    }
+}
+
+#[test]
+fn vl_strings_end_at_nul_and_mis_sized_elements_fail_like_h5py() {
+    // h5py cannot write a VL string with a NUL in it, so the file is patched:
+    // one string gets an embedded NUL, and two elements get a length that
+    // disagrees with their heap object. libhdf5 returns the string up to the
+    // NUL and refuses the others ("Expected global heap object size does
+    // not match"); we used to return the NUL and a truncated string.
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("patched.h5");
+    let script = format!(
+        r#"
+import struct, h5py, numpy as np
+path = {path:?}
+with h5py.File(path, 'w') as f:
+    f.create_dataset('d', data=np.array(['aXb', 'cdefgh', 'ij', 'ok'], dtype=object),
+                     dtype=h5py.string_dtype())
+    s = f.create_dataset('seq', shape=(2,), dtype=h5py.vlen_dtype(np.dtype('<i4')))
+    s[0] = np.array([1, 2, 3], dtype='<i4'); s[1] = np.array([4], dtype='<i4')
+    off = f['d'].id.get_offset(); soff = f['seq'].id.get_offset()
+b = bytearray(open(path, 'rb').read())
+i = b.index(b'aXb'); b[i + 1] = 0
+struct.pack_into('<I', b, off + 16, 3)    # 'cdefgh': length 6 -> 3
+struct.pack_into('<I', b, off + 32, 9)    # 'ij': length 2 -> 9
+struct.pack_into('<I', b, soff, 2)        # [1, 2, 3]: length 3 -> 2
+open(path, 'wb').write(bytes(b))
+with h5py.File(path, 'r') as f:
+    for i in range(4):
+        try:
+            print('d%d\t%s' % (i, f['d'][i].hex()))
+        except OSError as e:
+            print('d%d\terror' % i)
+    for i in range(2):
+        try:
+            print('seq%d\t%s' % (i, ' '.join(str(x) for x in f['seq'][i])))
+        except OSError as e:
+            print('seq%d\terror' % i)
+"#,
+        path = path.display().to_string()
+    );
+    let expected = run_python(&script);
+    assert_eq!(expected["d0"], "61", "h5py cuts 'a\\0b' at the NUL");
+    assert_eq!(expected["d1"], "error");
+    assert_eq!(expected["d2"], "error");
+    assert_eq!(expected["d3"], "6f6b");
+    assert_eq!(expected["seq0"], "error");
+    assert_eq!(expected["seq1"], "4");
+
+    let file = File::open(&path).unwrap();
+    let d = file.dataset("d").unwrap();
+    let one = |i: u64| d.read_string_selection(&Selection::slice(&[i..i + 1]));
+    assert_eq!(one(0).unwrap(), vec!["a"]);
+    assert!(one(1).is_err());
+    assert!(one(2).is_err());
+    assert_eq!(one(3).unwrap(), vec!["ok"]);
+    assert!(d.read_string().is_err());
+    let seq = file.dataset("seq").unwrap();
+    let one = |i: u64| seq.read_vlen_selection::<i32>(&Selection::slice(&[i..i + 1]));
+    assert!(one(0).is_err());
+    assert_eq!(one(1).unwrap(), vec![vec![4]]);
+}
+
+#[test]
+fn a_vl_element_at_the_undefined_heap_address_fails_like_h5py() {
+    // libhdf5 writes a null element with heap address 0 (h5py reads it as
+    // b''), and an empty string as a real zero-size heap object; neither
+    // uses the undefined address. An element of length 0 at the undefined
+    // address fails in libhdf5 ("addr undefined"); we returned "".
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("undef.h5");
+    let script = format!(
+        r#"
+import struct, h5py, numpy as np
+path = {path:?}
+with h5py.File(path, 'w') as f:
+    f.create_dataset('d', data=np.array(['x', '', 'yz', ''], dtype=object), dtype=h5py.string_dtype())
+    off = f['d'].id.get_offset()
+b = bytearray(open(path, 'rb').read())
+# h5py's '' (element 3): length 0 at a real heap address, not 0 or all 0xff.
+length, addr, _ = struct.unpack_from('<IQI', b, off + 48)
+print('empty\t%d %d' % (length, addr not in (0, 2**64 - 1)))
+struct.pack_into('<IQI', b, off + 16, 0, 2**64 - 1, 1)
+open(path, 'wb').write(bytes(b))
+with h5py.File(path, 'r') as f:
+    for i in range(4):
+        try:
+            print('d%d\t%s' % (i, f['d'][i].hex()))
+        except OSError as e:
+            print('d%d\terror %s' % (i, 'addr undefined' in str(e)))
+"#,
+        path = path.display().to_string()
+    );
+    let expected = run_python(&script);
+    assert_eq!(expected["empty"], "0 1", "h5py writes '' at a real address");
+    assert_eq!(expected["d0"], "78");
+    assert_eq!(expected["d1"], "error True");
+    assert_eq!(expected["d2"], "797a");
+    assert_eq!(expected["d3"], "");
+
+    let file = File::open(&path).unwrap();
+    let d = file.dataset("d").unwrap();
+    let one = |i: u64| d.read_string_selection(&Selection::slice(&[i..i + 1]));
+    assert_eq!(one(0).unwrap(), vec!["x"]);
+    let e = one(1).unwrap_err().to_string();
+    assert!(e.contains("undefined global heap address"), "{e}");
+    assert_eq!(one(2).unwrap(), vec!["yz"]);
+    assert_eq!(one(3).unwrap(), vec![""]);
+    assert!(d.read_string().is_err());
+    assert!(d.read_string_bytes().is_err());
+}

crates/clawhdf5/tests/vl_offset4_interop.rs

@@ -0,0 +1,126 @@
+//! Variable-length values in files with 4-byte offsets and lengths
+//! (`sizeof_addr = 4`), checked against h5py/libhdf5 through the
+//! `clawhdf5_format` decoders.
+//!
+//! These failed with `GlobalHeapObjectNotFound` or came back as
+//! `AttrValue::Raw`: the VL datatype claimed 16-byte elements whatever the
+//! file's offset size, and the global heap was read without the padding
+//! libhdf5 puts after its collection and object headers. Skipped when
+//! python3 with h5py is unavailable, unless `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+use std::process::Command;
+
+use clawhdf5::{AttrValue, File, Selection};
+use clawhdf5_format::data_read::{read_as_i64, read_compound_fields};
+use clawhdf5_format::vl_data::{read_vl_bytes, read_vl_strings};
+
+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)
+}
+
+#[test]
+fn vl_values_in_a_file_with_4_byte_offsets_read_like_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;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("offset4.h5");
+    let script = format!(
+        r#"
+import h5py, numpy as np
+S = h5py.string_dtype()
+fcpl = h5py.h5p.create(h5py.h5p.FILE_CREATE); fcpl.set_sizes(4, 4)
+with h5py.File(h5py.h5f.create({path:?}.encode(), h5py.h5f.ACC_TRUNC, fcpl=fcpl)) as f:
+    f.attrs['vlattr'] = 'attr-value'
+    f.attrs.create('vlattr_arr', np.array(['p', 'qq', ''], dtype=object), dtype=S)
+    ct = np.dtype([('id', '<i4'), ('name', S), ('v', '<f8')])
+    arr = np.zeros(3, dtype=ct); arr['id'] = [1, 2, 3]; arr['name'] = ['one', '', 'three']
+    f.create_dataset('compound', data=arr)
+    f.attrs.create('vlen_attr', np.array([np.array([1, 2], dtype='<i8'), np.array([3], dtype='<i8')],
+                   dtype=object), dtype=h5py.vlen_dtype(np.dtype('<i8')))
+with h5py.File({path:?}, 'r') as f:
+    assert f.id.get_create_plist().get_sizes() == (4, 4)
+    print(f.attrs['vlattr'])
+    print(','.join(f.attrs['vlattr_arr']))
+    print(','.join(s.decode() for s in f['compound']['name']))
+    print(';'.join(' '.join(str(x) for x in s) for s in f.attrs['vlen_attr']))
+"#,
+        path = path.display().to_string()
+    );
+    let output = Command::new(python())
+        .args(["-c", &script])
+        .output()
+        .expect("failed to run python");
+    assert!(
+        output.status.success(),
+        "python failed:\n{}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let lines: Vec<&str> = stdout.lines().collect();
+    let (vlattr, vlattr_arr, names, seqs) = (lines[0], lines[1], lines[2], lines[3]);
+
+    let file = File::open(&path).unwrap();
+    let sb = file.superblock();
+    assert_eq!((sb.offset_size, sb.length_size), (4, 4));
+
+    let attrs = file.root().attrs().unwrap();
+    match &attrs["vlattr"] {
+        AttrValue::String(s) => assert_eq!(s, vlattr),
+        other => panic!("vlattr: {other:?}"),
+    }
+    match &attrs["vlattr_arr"] {
+        AttrValue::StringArray(s) => assert_eq!(s.join(","), vlattr_arr),
+        other => panic!("vlattr_arr: {other:?}"),
+    }
+
+    // The compound's VL string member.
+    let ds = file.dataset("compound").unwrap();
+    let dt = ds.raw_datatype().unwrap();
+    assert_eq!(dt.type_size(), 24, "4 + 12-byte VL element + 8");
+    let raw = ds.read_selection(&Selection::All).unwrap();
+    let fields = read_compound_fields(&raw, &dt).unwrap();
+    let name = fields.iter().find(|f| f.name == "name").unwrap();
+    assert_eq!(name.datatype.type_size(), 12);
+    let got = read_vl_strings(file.as_bytes(), &name.raw_data, 3, 4, 4).unwrap();
+    assert_eq!(got.join(","), names);
+    let id = fields.iter().find(|f| f.name == "id").unwrap();
+    assert_eq!(
+        read_as_i64(&id.raw_data, &id.datatype).unwrap(),
+        vec![1, 2, 3]
+    );
+
+    // A VL sequence attribute.
+    let AttrValue::Raw { datatype, data, .. } = &attrs["vlen_attr"] else {
+        panic!("vlen_attr is Raw");
+    };
+    let clawhdf5_format::datatype::Datatype::VariableLength { base_type, .. } = datatype else {
+        panic!("vlen_attr is VL");
+    };
+    let got: Vec<String> = read_vl_bytes(file.as_bytes(), data, 2, 4, 4)
+        .unwrap()
+        .iter()
+        .map(|b| {
+            let v = read_as_i64(b, base_type).unwrap();
+            v.iter().map(i64::to_string).collect::<Vec<_>>().join(" ")
+        })
+        .collect();
+    assert_eq!(got.join(";"), seqs);
+}

docs/known-issues.md

@@ -164,13 +164,24 @@ fill-value item that did is fixed).
   is left out of `attrs()` (reported by `attrs_with_errors()`) instead of
   failing the others.
 - **Other readers:**
-  - VL-string datasets are not readable through `File`.
+  - VL-string datasets are not readable through `File`. **Fixed
+    2026-09-26:** `read_string` reads them (also `read_string_bytes`,
+    `read_string_selection`, and on `MmapFile`/`LazyFile`), with h5py's
+    values: strings end at a NUL, null elements (heap address 0) are `""`,
+    and an element at the undefined heap address is an error as in libhdf5
+    (it read as `""` until 2026-09-26); VL sequences of
+    numbers read with `read_vlen::<T>()`, and VL values inside compounds or
+    `AttrValue::Raw` attributes decode with `File::decode_strings` /
+    `File::decode_vlen` (`crates/clawhdf5/tests/vl_data_interop.rs`).
   - Variable-length values inside a compound (and VL-string attributes) in
     a file with 4-byte offsets (`sizeof_addr = 4`) fail with
     `GlobalHeapObjectNotFound` or come back as `Raw`: these paths assume
     the 16-byte element of an 8-byte-offset file. The datatype itself reads
     (it was refused as "member overlaps with previous member" until
-    2026-09-26).
+    2026-09-26). **Fixed 2026-09-26:** a VL type's element size is the one
+    its datatype message stores (12 with 4-byte offsets), and the global
+    heap is read with libhdf5's header padding
+    (`crates/clawhdf5/tests/vl_offset4_interop.rs`).
   - Metadata cache images are not supported.
   - x87 long double and binary128 are refused.
   - N-Bit on 64-bit scale-offset data and some N-Bit parameter layouts fail.
@@ -222,9 +233,10 @@ fill-value item that did is fixed).
   - (`cve-2024-32616` `/group1/dset3` and `cve-2025-2309`'s `Comp_OBJREF`
     attribute are h5py/numpy type-mapping failures, not libhdf5 refusals.)
   - `h5rs check` validates with the library's parsers, so it inherits what
-    they accept: of the 150 CVE and fuzzer files, `check --data` passes 16,
+    they accept: of the 150 CVE and fuzzer files, `check --data` passes 15,
-    and h5dump 1.14.6 rejects 9 of those (tank, 2026-09-26; 28 and 21
+    and h5dump 1.14.6 rejects 8 of those (tank, 2026-09-26; 28 and 21
-    before these checks).
+    before these checks, 16 and 9 before a VL type's stored element size
+    was checked, which flags `cve-2024-32608`).
 - **Writer:**
   - Nested groups beyond one level: path-like names are now refused, not
     created.
@@ -424,6 +436,32 @@ has produced more records than the file could physically hold.
 
 ---
 
+## Crafted global heaps exhaust the variable-length reader's memory
+
+**Status:** fixed on `feat/p2-vl-strings` (2026-09-26). Not a regression of
+that branch: every earlier release is affected through `read_vl_strings`.
+
+Reading variable-length values kept an owned copy of every object of every
+global heap collection visited, for the whole read. A file whose collections
+nest inside one another's object data (32 bytes apart, each element pointing
+at a different one) made retained memory O(elements × file size): a 744 KB
+file reached 1.58 GB. Letting every collection's object chain jump to one
+shared run of tiny objects made the parse time O(elements × objects) too.
+libhdf5 refuses such files.
+
+Now `VlResolver` caches where each object lies instead of a copy, drops its
+cache past a 32 MiB budget, and refuses a collection that overlaps one it
+has already read (libhdf5 gives each collection its own block, so only a
+crafted file has them). `GlobalHeapCollection::parse` (and the new
+`parse_index`) also refuse a collection that runs past the end of the file,
+or an object that runs past the end of its collection. Guarded by
+`crates/clawhdf5-format/tests/vl_heap_bounds.rs`, which measures peak heap
+use with a counting allocator. Still open: a file may point many elements
+at one large heap object, and a VL-*sequence* read then returns that
+object once per element, as h5py would.
+
+---
+
 ## Extensible Array chunk indexes read back wrong data past the inline elements
 
 **Status:** fixed on `main` (2026-09-20), after v2.6.0. **Every release up to
@@ -526,7 +564,9 @@ which is what libhdf5 itself writes.
   followed (no file system).
 - Variable-length string datasets are read by decoding `read_selection`'s
   bytes with `clawhdf5_format::vl_data` in the wasm crate; `File` itself still
-  cannot (see the audit gaps above).
+  cannot (see the audit gaps above). (`File` can since 2026-09-26. Since
+  2026-09-26 the wasm crate resolves them with the same `VlResolver` as
+  `File` and `h5rs`, so all three return h5py's values.)
 
 ## The Node.js package (`packages/clawhdf5-node`) does not work