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 (example)
• CMake FetchContent (example)
• CMake + Git submodule
• Fortran Package Manager (fpm)

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 dataset types int32, int64, real32, real64, character (string) with rank scalar (0-D) through 7-D. Variable-length datasets "H5S_UNLIMITED" can also be read.

HDF5 attributes are also supported for read/write types int32, int64, real32, real64, character (string) with rank scalar (0-D) through 7-D.

For character, datasets and attributes are supported for fixed length and variable length strings. Character padding uses spaces as this is the default for Fortran. Space-padded, null-terminated and null-padded strings are also supported on read. UTF8 characters are passed through on read and write using the Fortran default character kind.

• 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
• Cray (using GCC or Intel backend)

Compilers known to work (tested on CI) include:

• GCC (gfortran) ≥ 8
• Intel oneAPI ≥ 2021

Compilers not currently working:

• NVidia HPC SDK 24.9: missing "rank()" intrinsic
• AOCC missing "rank()" intrinsic and trouble with "select type()" in submodule

---

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

Build

h5fortran can be built with CMake or fpm.

CMake

Using CMake:

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

cd h5fortran

cmake -B build

cmake --build build

By default, h5fortran CMake doesn't specify optimization flags to help working with old compilers like GCC 7. To enable optimization, set:

cmake -Bbuild -DCMAKE_BUILD_TYPE=Release

For more details see Install.md.

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

To save time, if not intended to use self-tests, you can skip the build of the test suite:

cmake -B build -Dh5fortran_BUILD_TESTING=off

Fortran Package Manager (fpm)

fpm build
fpm test
fpm install

Build HDF5 standalone

To build the HDF5 and ZLIB libraries, use the scripts folder.

## Usage

The simplest [example](./example/) h5fortran usage is like:

```fortran
use h5fortran

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

or

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.

Notes

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

HPC

On HPC, we suggest using the HDF5 library provided by the HPC system for best performance and to avoid compiler ABI issues.

h5fortran: missing HDF5 features

• 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 -- we do pass-through UTF8
• logical / boolean: not supported natively by HDF5. h5py implements as enum.
• variable length dataset writing

We didn't use type(c_ptr) and c_loc() internally for datasets as we observed problems when the actual argument is sliced on read/write. The current h5fortran impementation (Fortran select type for H5Dwrite/H5Dread) does work with sliced actual arguments.

HDF5 Fortran 2003 features

Acknowledgements

h5fortran was funded in part by NASA NNH19ZDA001N-HDEE grant 80NSSC20K0176.