Object-oriented Fortran HDF5 interface

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Object-oriented Fortran HDF5 interface

Simple, robust, thin HDF5 polymorphic Fortran read/write interface. Reading or writing {real64,real32,int32,int64} from scalar to 7d is as simple as

use h5fortran

call h5write('my.h5', '/x', x)

call h5read('my.h5', '/y', y)

For NetCDF4 see nc4fortran. h5fortran is designed for "serial" HDF5 read/write. We don't yet implement the interface for "parallel" HDF5.

h5fortran is designed for easy use using static or shared linking from your project via:

  • cmake --install
  • CMake ExternalProject
  • CMake FetchContent
  • CMake + Git submodule
  • Meson subproject

Uses Fortran submodule for clean template structure. This easy-to-use, thin object-oriented modern Fortran library abstracts away the messy parts of HDF5 so that you can read / write various types/ranks of data with a single command. In distinction from other high-level HDF5 interfaces, h5fortran works to deduplicate code, using polymorphism wherever feasible and extensive test suite.

Polymorphic API with read/write for types int32, real32, real64 with rank scalar (0-D) through 7-D. 64-bit integers int64 are read/write from scalar through 3-D.

as well as character (string). If you need int64, we have a working example for that: src/concepts/int64.f90 that can easily be put into the h5fortran API--just make a GitHub Issue.

  • HDF5 attributes are also supported for read/write with type character, int32, real32, real64.
  • Array slicing on read and write is supported, that is, reading or writing part of a disk HDF5 array into a variable matching the slice shape.
  • Mismatched datatypes are coerced as per standard Fortran rules. For example, reading a float HDF5 variable into an integer Fortran variable: 42.3 => 42
  • Zlib (deflate) compression / decompression -- h5fortran will work without Zlib, but will save/load uncompressed data only.
  • create HDF5 soft link variables--arbitrarily many soft-linked variable names can point to an actual variable, which need not yet exist.

Tested on systems with HDF5 1.8, 1.10 and 1.12 including:

  • MacOS (homebrew)
  • Linux (Ubuntu, CentOS)
  • Windows Subsystem for Linux
  • Windows MSYS2
  • IBM Power with Gfortran

Compilers known to work include:

  • GCC (gfortran) ≥ 7
  • Intel oneAPI HPC compiler ≥ 2021

We welcome contributions. In general we hold to the geospace-code code of conduct.


Using CMake:

git clone https://github.com/geospace-code/h5fortran.git

cd h5fortran

cmake -B build

cmake --build build

for more details see Install.md.

For general use with non-CMake build systems, "h5fortran.pc" pkg-config file is also generated / installed.

Autobuild HDF5

h5fortran will automatically build the HDF5 and ZLIB libraries if needed. This is useful as many HPC have broken or ABI-incompatible HDF5 libraries installed. Building HDF5 and ZLIB takes about a minute on a typical laptop. To disable this autobuild behavior, use option:

cmake -B build -Dautobuild=off

To force building the HDF5 and ZLIB libraries, to gain better performance via optimizing for your system's CPU:

cmake -Dhdf5_external=on

cmake --build build

NOTE: If using Intel oneAPI on Windows, ensure that environment variable CC=icl as set manually in the command prompt:

set CC=icl
set FC=ifort

This is necessary to workaround techniques used by HDF5 CMake files that don't pickup the CMake set(ENV{CC}). Otherwise, HDF5 build failures may result due to defaulting to icl-clang.

By default we use Zlib 2.x a.k.a. zlib-ng. If you have a problem with Zlib-ng on your system, try the unmaintained Zlib 1.x by:

cmake -B build -Dzlib_legacy=on


The simplest example h5fortran usage is like:

use h5fortran

call h5write('golt.h5','/x', [1,2,3,4,5,6])


use h5fortran

real :: x2

if(.not. is_hdf5('golt.h5')) error stop 'golt.h5 is not an HDF5 file'

call h5read('golt.h5', '/x', x2)

For detailed examples see Examples.md.


  • The first character of the filename should be a character, NOT whitespace to avoid file open/creation errors.
  • Polymorphic array rank is implemented.

Getting HDF5 library

Instead of auto-building HDF5 via H5Fortran, one may build and install the HDF5 library by:

python3 scripts/build_hdf5.py

h5fortran: missing Fortran datatypes

  • arrays of rank > 7: prototyped in reader_nd.f90, writer_nd.f90. Fortran 2008 arrays are up to rank 15, but only recent compilers support.
  • complex64 / complex128: not natively handled in HDF5. There are performance impacts for compound datatypes. Many choose to write two datasets, one each for real and imaginary like A_real and A_imag
  • non-default character kind
  • logical / boolean: not supported natively by HDF5. h5py implements as enum.

Developer Info

Michael Hirsch, Ph.D.

Derived Types