Overview

hdf5 wraps libHDF5 — the standard storage format for scientific and engineering data used by NASA, CERN, financial institutions, and ML research. HDF5 files are self-describing hierarchical containers: each file is a filesystem of typed N-dimensional arrays with metadata attributes.

Quick Start

uses hdf5;

var f := OpenFile('/data/experiment.h5', 'w');

// Write datasets
WriteFloat64(f, '/signals/temperature', [22.1, 22.5, 23.0, 22.8]);
WriteInt32(f,   '/counts/events',       [10, 14, 9, 17, 12]);

// Metadata attributes
SetAttrString(f, '/signals/temperature', 'units', 'Celsius');
SetAttrFloat(f,  '/signals/temperature', 'sample_rate', 1.0);

CloseFile(f);
WriteLn('Version: ', HDF5Version);

Read Data

var f := OpenFile('/data/experiment.h5', 'r');

var temps := ReadFloat64(f, '/signals/temperature');
for I := 0 to Length(temps) - 1 do
  WriteLn('T[', I, '] = ', temps[I]:0:2);

var units := GetAttrString(f, '/signals/temperature', 'units');
WriteLn('Units: ', units);

CloseFile(f);

2-D Matrices

// Store a 100×256 matrix (e.g. 100 feature vectors of dim 256)
var matrix: array of Double;
SetLength(matrix, 100 * 256);
// ... fill matrix ...

var f := OpenFile('/data/embeddings.h5', 'w');
WriteMatrix(f, '/embeddings', matrix, 100, 256);

// Read back
var rows, cols: Integer;
var data := ReadMatrix(f, '/embeddings', rows, cols);
WriteLn(rows, 'x', cols, ' matrix loaded');
CloseFile(f);

Groups and Navigation

var f := OpenFile('/data/runs.h5', 'w');

CreateGroup(f, '/run1');
CreateGroup(f, '/run2');
WriteFloat64(f, '/run1/voltage', voltageData);
WriteFloat64(f, '/run2/voltage', voltageData2);
SetAttrString(f, '/run1', 'date', '2026-03-13');

// List contents
var items := ListGroup(f, '/');
for I := 0 to Length(items) - 1 do
  WriteLn(items[I]);  // "run1", "run2"

// Check existence
if Exists(f, '/run1/voltage') then
  WriteLn('Dataset found');

CloseFile(f);

Compressed Storage

// Store large dataset with GZIP compression (level 6) and chunking
var f := OpenFile('/data/large.h5', 'w');
WriteCompressed(f, '/data/signals', bigArray,
  Rows    := 10000,
  Cols    := 1024,
  CompressLevel := 6,
  ChunkSize     := 1024);
CloseFile(f);

var info := DatasetInfo(OpenFile('/data/large.h5','r'), '/data/signals');
WriteLn('Compressed: ', info.Compressed);
WriteLn('Elements:   ', info.Size);