HDF5 version 1.12.1 released on 2021-07-01
================================================================================


INTRODUCTION
============

This document describes the new features introduced in the HDF5 1.12.1 release.
It contains information on the platforms tested and known problems in this
release. For more details check the HISTORY*.txt files in the HDF5 source.

Note that documentation in the links below will be updated at the time of the
release.

Links to HDF5 documentation can be found on The HDF5 web page:

     https://portal.hdfgroup.org/display/HDF5/HDF5

The official HDF5 releases can be obtained from:

     https://www.hdfgroup.org/downloads/hdf5/

More information about the new features can be found at:

     https://portal.hdfgroup.org/display/HDF5/New+Features+in+HDF5+Release+1.12

If you have any questions or comments, please send them to the HDF Help Desk:

     help@hdfgroup.org


CONTENTS
========

- New Features
- Support for new platforms and languages
- Bug Fixes since HDF5-1.12.0
- Supported Platforms
- Tested Configuration Features Summary
- More Tested Platforms
- Known Problems
- CMake vs. Autotools installations


New Features
============

    Configuration:
    -------------
   - Adds C++ Autotools configuration file for Intel

      * Checks for icpc as the compiler
      * Copies most non-warning flags from intel-flags

      (DER - 2021/06/02)

    - Adds C++ Autotools configuration file for PGI

      * Checks for pgc++ as the compiler name (was: pgCC)
      * Other options basically match new C options (below)

      (DER - 2021/06/02)

    - Updates PGI C options

      * -Minform set to warn (was: inform) to suppress spurious messages
      * Sets -gopt -O2 as debug options
      * Sets -O4 as 'high optimization' option
      * Sets -O0 as 'no optimization' option
      * Removes specific settings for PGI 9 and 10

      (DER - 2021/06/02)

    - Added an option to make the global thread-safe lock a recursive R/W lock

      Prior to this release, the HDF5 library supported multi-threaded
      applications by placing a recursive global lock on the entire library,
      thus allowing only one thread into the library at a time.

      While this is still the default, the library can now be built with the
      recursive global lock replaced with a recursive read / write (R/W) lock
      that allows recursive writer locks.

      Currently, this change results in no functional change in the HDF5
      library, as all threads will have to acquire a write lock on entry, and
      thus obtain exclusive access to the HDF5 library as before. However, the
      addition of the recursive R/W lock is a prerequisite for further work
      directed at allowing some subset of the HDF5 API calls to enter the
      library with read locks.

      CMake:      HDF5_USE_RECURSIVE_RW_LOCKS (default: OFF, advanced)

      Autotools:  --enable-recursive-rw-locks [default=no]

      This feature only works with Pthreads. Win32 threads are not supported.

      (DER - 2021/05/10)

    - CMake no longer builds the C++ library by default

      HDF5_BUILD_CPP_LIB now defaults to OFF, which is in line with the
      Autotools build defaults.

      (DER - 2021/04/20)

    - Removal of pre-VS2015 work-arounds

      HDF5 now requires Visual Studio 2015 or greater, so old work-around
      code and definitions have been removed, including:

      * <inttypes.h>
      * snprintf and vsnprintf
      * llround, llroundf, lround, lroundf, round, roundf
      * strtoll and strtoull
      * va_copy
      * struct timespec

      (DER - 2021/03/22)

    - Added CMake variable HDF5_LIB_INFIX

      This infix is added to all library names after 'hdf5'.
      e.g. the infix '_openmpi' results in the library name 'libhdf5_openmpi.so'
      This name is used in packages on debian based systems.
      (see https://packages.debian.org/jessie/amd64/libhdf5-openmpi-8/filelist)

      (RG - 2021/03/22)

    - On macOS, Universal Binaries can now be built, allowing native execution on
      both Intel and Apple Silicon (ARM) based Macs.

      To do so, set CMAKE_OSX_ARCHITECTURES="x86_64;arm64"

      (SAM - 2021/02/07, github-311)

    - Added a configure-time option to control certain compiler warnings
      diagnostics

      A new configure-time option was added that allows some compiler warnings
      diagnostics to have the default operation. This is mainly intended for
      library developers and currently only works for gcc 10 and above. The
      diagnostics flags apply to C, C++ and Fortran compilers and will appear
      in "H5 C Flags", H5 C++ Flags" and H5 Fortran Flags, respectively. They
      will NOT be exported to h5cc, etc.

      The default is OFF, which will disable the warnings URL and color attributes
      for the warnings output. ON will not add the flags and allow default behavior.

      Autotools:    --enable-diags

      CMake:        HDF5_ENABLE_BUILD_DIAGS

      (ADB - 2021/02/05, HDFFV-11213)

    - CMake option to build the HDF filter plugins project as an external project

      The HDF filter plugins project is a collection of registered compression
      filters that can be dynamically loaded when needed to access data stored
      in a hdf5 file. This CMake-only option allows the plugins to be built and
      distributed with the hdf5 library and tools. Like the options for szip and
      zlib, either a tgz file or a git repository can be specified for the source.

      The option was refactored to use the CMake FetchContent process. This allows
      more control over the filter targets, but required external project command
      options to be moved to a CMake include file, HDF5PluginCache.cmake. Also
      enabled the filter examples to be used as tests for operation of the
      filter plugins.

      (ADB - 2020/12/10, OESS-98)

    - FreeBSD Autotools configuration now defaults to 'cc' and 'c++' compilers

      On FreeBSD, the autotools defaulted to 'gcc' as the C compiler and did
      not process C++ options. Since FreeBSD 10, the default compiler has
      been clang (via 'cc').

      The default compilers have been set to 'cc' for C and 'c++' for C++,
      which will pick up clang and clang++ respectively on FreeBSD 10+.
      Additionally, clang options are now set correctly for both C and C++
      and g++ options will now be set if that compiler is being used (an
      omission from the former functionality).

      (DER - 2020/11/28, HDFFV-11193)

    - Fixed POSIX problems when building w/ gcc on Solaris

      When building on Solaris using gcc, the POSIX symbols were not
      being set correctly, which could lead to issues like clock_gettime()
      not being found.

      The standard is now set to gnu99 when building with gcc on Solaris,
      which allows POSIX things to be #defined and linked correctly. This
      differs slightly from the gcc norm, where we set the standard to c99
      and manually set POSIX #define symbols.

      (DER - 2020/11/25, HDFFV-11191)

    - Added a configure-time option to consider certain compiler warnings
      as errors

      A new configure-time option was added that converts some compiler warnings
      to errors. This is mainly intended for library developers and currently
      only works for gcc and clang. The warnings that are considered errors
      will appear in the generated libhdf5.settings file. These warnings apply
      to C and C++ code and will appear in "H5 C Flags" and H5 C++ Flags",
      respectively. They will NOT be exported to h5cc, etc.

      The default is OFF. Building with this option may fail when compiling
      on operating systems and with compiler versions not commonly used by
      the library developers. Compilation may also fail when headers not
      under the control of the library developers (e.g., mpi.h, hdfs.h) raise
      warnings.

      Autotools:    --enable-warnings-as-errors

      CMake:        HDF5_ENABLE_WARNINGS_AS_ERRORS

      (DER - 2020/11/23, HDFFV-11189)

    - Autotools and CMake target added to produce doxygen generated documentation

      The default is OFF or disabled.
      Autoconf option is '--enable-doxygen'
          autotools make target is 'doxygen' and will build all doxygen targets
          CMake configure option is 'HDF5_BUILD_DOC'.
          CMake target is 'doxygen' for all available doxygen targets
          CMake target is 'hdf5lib_doc' for the src subdirectory

        (ADB - 2020/11/13, HDFFV-11243)

    - CMake option to use MSVC naming conventions with MinGW

      HDF5_MSVC_NAMING_CONVENTION option enables the use of MSVC naming
      conventions when using a MinGW toolchain

      (xan - 2020/10/30)

    - CMake option to statically link gcc libs with MinGW

      HDF5_MINGW_STATIC_GCC_LIBS allows to statically link libg/libstdc++
      with the MinGW toolchain

      (xan - 2020/10/30)

    - CMake option to build the HDF filter plugins project as an external project

      The HDF filter plugins project is a collection of registered compression
      filters that can be dynamically loaded when needed to access data stored
      in a hdf5 file. This CMake-only option allows the plugins to be built and
      distributed with the hdf5 library and tools. Like the options for szip and
      zlib, either a tgz file or a git repository can be specified for the source.

      The necessary options are (see the INSTALL_CMake.txt file):
          HDF5_ENABLE_PLUGIN_SUPPORT
          PLUGIN_TGZ_NAME or PLUGIN_GIT_URL
      There are more options necessary for various filters and the plugin project
      documents should be referenced.

      (ADB - 2020/10/16, OESS-98)

    - Added CMake option to format source files

      HDF5_ENABLE_FORMATTERS option will enable creation of targets using the
      pattern - HDF5_*_SRC_FORMAT - where * corresponds to the source folder
      or tool folder. All sources can be formatted by executing the format target;
      make format

      (ADB - 2020/09/24)

    - Disable memory sanity checks in the Autotools in release branches

      The library can be configured to use internal memory sanity checking,
      which replaces C API calls like malloc(3) and free(3) with our own calls
      which add things like heap canaries. These canaries can cause problems
      when external filter plugins reallocate canary-marked buffers.

      For this reason, the default will be to not use the memory allocation
      sanity check feature in release branches (e.g., hdf5_1_10_7).
      Debug builds in development branches (e.g., develop, hdf5_1_10) will
      still use them by default.

      This change only affects Autotools debug builds. Non-debug autotools
      builds and all CMake builds do not enable this feature by default.

      (DER - 2020/08/19)

    - Add file locking configure and CMake options

      HDF5 1.10.0 introduced a file locking scheme, primarily to help
      enforce SWMR setup. Formerly, the only user-level control of the scheme
      was via the HDF5_USE_FILE_LOCKING environment variable.

      This change introduces configure-time options that control whether
      or not file locking will be used and whether or not the library
      ignores errors when locking has been disabled on the file system
      (useful on some HPC Lustre installations).

      In both the Autotools and CMake, the settings have the effect of changing
      the default property list settings (see the H5Pset/get_file_locking()
      entry, below).

      The yes/no/best-effort file locking configure setting has also been
      added to the libhdf5.settings file.

      Autotools:

        An --enable-file-locking=(yes|no|best-effort) option has been added.

        yes:          Use file locking.
        no:           Do not use file locking.
        best-effort:  Use file locking and ignore "disabled" errors.

      CMake:

        Two self-explanatory options have been added:

        HDF5_USE_FILE_LOCKING
        HDF5_IGNORE_DISABLED_FILE_LOCKS

        Setting both of these to ON is the equivalent to the Autotools'
        best-effort setting.

      NOTE:
      The precedence order of the various file locking control mechanisms is:

        1) HDF5_USE_FILE_LOCKING environment variable (highest)

        2) H5Pset_file_locking()

        3) configure/CMake options (which set the property list defaults)

        4) library defaults (currently best-effort)

      (DER - 2020/07/30, HDFFV-11092)

    - CMake option to link the generated Fortran MOD files into the include
      directory.

      The Fortran generation of MOD files by a Fortran compile can produce
      different binary files between SHARED and STATIC compiles with different
      compilers and/or different platforms. Note that it has been found that
      different versions of Fortran compilers will produce incompatible MOD
      files. Currently, CMake will locate these MOD files in subfolders of
      the include directory and add that path to the Fortran library target
      in the CMake config file, which can be used by the CMake find library
      process. For other build systems using the binary from a CMake install,
      a new CMake configuration can be used to copy the pre-chosen version
      of the Fortran MOD files into the install include directory.

      The default will depend on the configuration of
      BUILD_STATIC_LIBS and BUILD_SHARED_LIBS:
            YES                   YES         Default to SHARED
            YES                   NO          Default to STATIC
            NO                    YES         Default to SHARED
            NO                    NO          Default to SHARED
      The defaults can be overridden by setting the config option
            HDF5_INSTALL_MOD_FORTRAN to one of NO, SHARED, or STATIC

      (ADB - 2020/07/09, HDFFV-11116)

    - CMake option to use AEC (open source SZip) library instead of SZip

      The open source AEC library is a replacement library for SZip. In
      order to use it for hdf5 the libaec CMake source was changed to add
      "-fPIC" and exclude test files. Autotools does not build the
      compression libraries within hdf5 builds. New option USE_LIBAEC is
      required to compensate for the different files produced by AEC build.

      (ADB - 2020/04/22, OESS-65)

    - CMake ConfigureChecks.cmake file now uses CHECK_STRUCT_HAS_MEMBER

      Some handcrafted tests in HDFTests.c has been removed and the CMake
      CHECK_STRUCT_HAS_MEMBER module has been used.

      (ADB - 2020/03/24, TRILAB-24)

    - Both build systems use same set of warnings flags

      GNU C, C++ and gfortran warnings flags were moved to files in a config
      sub-folder named gnu-warnings. Flags that only are available for a specific
      version of the compiler are in files named with that version.
      Clang C warnings flags were moved to files in a config sub-folder
      named clang-warnings.
      Intel C, Fortran warnings flags were moved to files in a config sub-folder
      named intel-warnings.

      There are flags in named "error-xxx" files with warnings that may
      be promoted to errors. Some source files may still need fixes.

      There are also pairs of files named "developer-xxx" and "no-developer-xxx"
      that are chosen by the CMake option:HDF5_ENABLE_DEV_WARNINGS or the
      configure option:--enable-developer-warnings.

      In addition, CMake no longer applies these warnings for examples.

      (2020/03/24, TRILAB-192)


    Library:
    --------
    - H5Gcreate1() now rejects size_hint parameters larger than UINT32_MAX

      The size_hint value is ultimately stored in a uint32_t struct field,
      so specifying a value larger than this on a 64-bit machine can cause
      undefined behavior including crashing the system.

      The documentation for this API call was also incorrect, stating that
      passing a negative value would cause the library to use a default
      value. Instead, passing a "negative" value actually passes a very large
      value, which is probably not what the user intends and can cause
      crashes on 64-bit systems.

      The Doxygen documentation has been updated and passing values larger
      than UINT32_MAX for size_hint will now produce a normal HDF5 error.
      
      (DER - 2021/04/29, HDFFV-11241)


    - H5Pset_fapl_log() no longer crashes when passed an invalid fapl ID

      When passed an invalid fapl ID, H5Pset_fapl_log() would usually
      segfault when attempting to free an uninitialized pointer in the error
      handling code. This behavior is more common in release builds or
      when the memory sanitization checks were not selected as a build
      option.

      The pointer is now correctly initialized and the API call now
      produces a normal HDF5 error when fed an invalid fapl ID.

      (DER - 2021/04/28, HDFFV-11240)

    - Fixes a segfault when H5Pset_mdc_log_options() is called multiple times

      The call incorrectly attempts to free an internal copy of the previous
      log location string, which causes a segfault. This only happens
      when the call is invoked multiple times on the same property list.
      On the first call to a given fapl, the log location is set to NULL so
      the segfault does not occur.

      The string is now handled properly and the segfault no longer occurs.

      (DER - 2021/04/27, HDFFV-11239)
      
    - HSYS_GOTO_ERROR now emits the results of GetLastError() on Windows

      HSYS_GOTO_ERROR is an internal macro that is used to produce error
      messages when system calls fail. These strings include errno and the
      the associated strerror() value, which are not particularly useful
      when a Win32 API call fails.

      On Windows, this macro has been updated to include the result of
      GetLastError(). When a system call fails on Windows, usually only
      one of errno and GetLastError() will be useful, however we emit both
      for the user to parse. The Windows error message is not emitted as
      it would be awkward to free the FormatMessage() buffer given the
      existing HDF5 error framework. Users will have to look up the error
      codes in MSDN.

      The format string on Windows has been changed from:

          "%s, errno = %d, error message = '%s'"

      to:

          "%s, errno = %d, error message = '%s', Win32 GetLastError() = %"PRIu32""

      for those inclined to parse it for error values.

      (DER - 2021/03/21)

    - File locking now works on Windows

      Since version 1.10.0, the HDF5 library has used a file locking scheme
      to help enforce one reader at a time accessing an HDF5 file, which can
      be helpful when setting up readers and writers to use the single-
      writer/multiple-readers (SWMR) access pattern.

      In the past, this was only functional on POSIX systems where flock() or
      fcntl() were present. Windows used a no-op stub that always succeeded.

      HDF5 now uses LockFileEx() and UnlockFileEx() to lock the file using the
      same scheme as POSIX systems. We lock the entire file when we set up the
      locks (by passing DWORDMAX as both size parameters to LockFileEx()).

      (DER - 2021/03/19, HDFFV-10191)

    - H5Epush_ret() now requires a trailing semicolon

      H5Epush_ret() is a function-like macro that has been changed to
      contain a `do {} while(0)` loop. Consequently, a trailing semicolon
      is now required to end the `while` statement. Previously, a trailing
      semi would work, but was not mandatory. This change was made to allow
      clang-format to correctly format the source code.

      (SAM - 2021/03/03)

    - Improved performance of H5Sget_select_elem_pointlist

      Modified library to cache the point after the last block of points
      retrieved by H5Sget_select_elem_pointlist, so a subsequent call to the
      same function to retrieve the next block of points from the list can
      proceed immediately without needing to iterate over the point list.

      (NAF - 2021/01/19)

    - Added H5VL_VERSION macro that indicates the version of the VOL framework
      implemented by a version of the library.  Currently, compatibility
      checking enforces that the 'version' field in the H5VL_class_t for
      a VOL connector must match the version of the VOL framework for the
      library when it is registered or dynamically loaded.

      (QAK - 2020/12/10)

    - Add BEST_EFFORT value to HDF5_USE_FILE_LOCKING environment variable

      This change adds a BEST_EFFORT to the TRUE/FALSE, 1/0 settings that
      were previously accepted. This option turns on file locking but
      ignores locking errors when the library detects that file locking
      has been disabled on a file system (useful on some HPC Lustre
      installations).

      The capitalization of BEST_EFFORT is mandatory.

      See the configure option discussion for HDFFV-11092 (above) for more
      information on the file locking feature and how it's controlled.

      (DER - 2020/07/30, HDFFV-11092)


    - Add H5Pset/get_file_locking() API calls

      This change adds new API calls which can be used to set or get the
      file locking parameters. The single API call sets both the "use file
      locking" flag and the "ignore disabled file locking" flag.

      When opening a file multiple times without closing, the file MUST be
      opened with the same file locking settings. Opening a file with different
      file locking settings will fail (this is similar to the behavior of
      H5Pset_fclose_degree()).

      See the configure option discussion for HDFFV-11092 (above) for more
      information on the file locking feature and how it's controlled.

      (DER - 2020/07/30, HDFFV-11092)

    - Added two new API routines for tracking library memory use:
      H5get_alloc_stats() and H5get_free_list_sizes().

      (QAK - 2020/03/25)

    - Add Mirror VFD

      Use TCP/IP sockets to perform write-only (W/O) file I/O on a remote
      machine. Must be used in conjunction with the Splitter VFD.

      (JOS - 2020/03/13, TBD)

    - Add Splitter VFD

      Maintain separate R/W and W/O channels for "concurrent" file writes
      to two files using a single HDF5 file handle.

      (JOS - 2020/03/13, TBD)


    C++ Library:
    ------------
    - Add wrappers for H5Pset/get_file_locking() API calls

      FileAccPropList::setFileLocking()
      FileAccPropList::getFileLocking()

      See the configure option discussion for HDFFV-11092 (above) for more
      information on the file locking feature and how it's controlled.

      (DER - 2020/07/30, HDFFV-11092)

    Java Library:
    -------------
    - Added new H5S functions.

      H5Sselect_copy, H5Sselect_shape_same, H5Sselect_adjust,
      H5Sselect_intersect_block, H5Sselect_project_intersection,
      H5Scombine_hyperslab, H5Smodify_select, H5Scombine_select
      wrapper functions added.

      (ADB - 2020/10/27, HDFFV-10868)

    - Add wrappers for H5Pset/get_file_locking() API calls

      H5Pset_file_locking()
      H5Pget_use_file_locking()
      H5Pget_ignore_disabled_file_locking()

      Unlike the C++ and Fortran wrappers, there are separate getters for the
      two file locking settings, each of which returns a boolean value.

      See the configure option discussion for HDFFV-11092 (above) for more
      information on the file locking feature and how it's controlled.

      (ADB - 2020/07/30, HDFFV-11092)


    Tools:
    ------
    - h5repack added help text for user-defined filters.

      Added help text line that states the valid values of the filter flag
      for user-defined filters;
          filter_flag: 1 is OPTIONAL or 0 is MANDATORY

      (ADB - 2021/01/14, HDFFV-11099)

    - h5repack added options to control how external links are handled.

      Currently h5repack preserves external links and cannot copy and merge
      data from the external files. Two options, merge and prune, were added to
      control how to merge data from an external link into the resulting file.
            --merge             Follow external soft link recursively and merge data.
            --prune             Do not follow external soft links and remove link.
            --merge --prune     Follow external link, merge data and remove dangling link.

      (ADB - 2020/08/05, HDFFV-9984)


Support for new platforms, languages and compilers
==================================================

    - Added macOS 11.2 Big Sur


Bug Fixes since HDF5-1.12.0 release
===================================

    Library
    -------
    - Fixed CVE-2018-14460

      The tool h5repack produced a segfault when the rank in dataspace
      message was corrupted, causing invalid read while decoding the
      dimension sizes.

      The problem was fixed by ensuring that decoding the dimension sizes
      and max values will not go beyong the end of the buffer.

      (BMR - 2021/05/12, HDFFV-11223)

    - Fixed CVE-2019-9151

      h5repack generated a segfault when an invalid memory read occurred on
      a corrupted file.

      The issue is now fixed. h5repack produces an error instead of segfault.

      (BMR - 2021/05/04, HDFFV-10718)

    - Fixed CVE-2018-11206

      The tool h5dump produced a segfault when the size of a fill value
      message was corrupted and caused a buffer overflow.

      The problem was fixed by verifying the fill value's size
      against the buffer size before attempting to access the buffer.

      (BMR - 2021/03/15, HDFFV-10480)

    - Fixed CVE-2018-14033 (same issue as CVE-2020-10811)

      The tool h5dump produced a segfault when the storage size message
      was corrupted and caused a buffer overflow.

      The problem was fixed by verifying the storage size against the
      buffer size before attempting to access the buffer.

      (BMR - 2021/03/15, HDFFV-11159/HDFFV-11049)

    - Remove underscores on header file guards

      Header file guards used a variety of underscores at the beginning of the define.

      Removed all leading (some trailing) underscores from header file guards.

      (ADB - 2021/03/03, #361)

    - Fixed issue with MPI communicator and info object not being
      copied into new FAPL retrieved from H5F_get_access_plist

      Added logic to copy the MPI communicator and info object into
      the output FAPL. MPI communicator is retrieved from the VFD, while
      the MPI info object is retrieved from the file's original FAPL.

      (JTH - 2021/02/15, HDFFV-11109)

    - Fixed problems with vlens and refs inside compound using
      H5VLget_file_type()

      Modified library to properly ref count H5VL_object_t structs and only
      consider file vlen and reference types to be equal if their files are
      the same.

      (NAF - 2021/01/22)

    - Fix bug and simplify collective metadata write operation when some ranks
      have no entries to contribute.  This fixes parallel regression test
      failures with IBM SpectrumScale MPI on the Summit system at ORNL.

      (QAK - 2020/09/02)

    - Avoid setting up complex MPI types with 0-length vectors, which some
      MPI implementations don't handle well.  (In particular, IBM
      SpectrumScale MPI on the Summit system at ORNL)

      (QAK - 2020/08/21)

    - Fixed use-of-uninitialized-value error

      Appropriate initialization of local structs was added to remove the
      use-of-uninitialized-value errors reported by MemorySanitizer.

      (BMR - 2020/8/13, HDFFV-11101)

    - Creation of dataset with optional filter

      When the combination of type, space, etc doesn't work for filter
      and the filter is optional, it was supposed to be skipped but it was
      not skipped and the creation failed.

      A fix is applied to allow the creation of a dataset in such
      situation, as specified in the user documentation.

      (BMR - 2020/8/13, HDFFV-10933)

    - Explicitly declared dlopen to use RTLD_LOCAL

      dlopen documentation states that if neither RTLD_GLOBAL nor
      RTLD_LOCAL are specified, then the default behavior is unspecified.
      The default on linux is usually RTLD_LOCAL while macos will default
      to RTLD_GLOBAL.

      (ADB - 2020/08/12, HDFFV-11127)

    - Fixed issues CVE-2018-13870 and CVE-2018-13869

      When a buffer overflow occurred because a name length was corrupted
      and became very large, h5dump crashed on memory access violation.

      A check for reading past the end of the buffer was added to multiple
      locations to prevent the crashes and h5dump now simply fails with an
      error message when this error condition occurs.

      (BMR - 2020/7/31, HDFFV-11120 and HDFFV-11121)

    - H5Sset_extent_none() sets the dataspace class to H5S_NO_CLASS which
      causes asserts/errors when passed to other dataspace API calls.

      H5S_NO_CLASS is an internal class value that should not have been
      exposed via a public API call.

      In debug builds of the library, this can cause assert() function to 
      trip. In non-debug builds, it will produce normal library errors.

      The new library behavior is for H5Sset_extent_none() to convert
      the dataspace into one of type H5S_NULL, which is better handled
      by the library and easier for developers to reason about.

      (DER - 2020/07/27, HDFFV-11027)

    - Fixed the segmentation fault when reading attributes with multiple threads

      It was reported that the reading of attributes with variable length string
      datatype will crash with segmentation fault particularly when the number of
      threads is high (>16 threads).  The problem was due to the file pointer that
      was set in the variable length string datatype for the attribute.  That file
      pointer was already closed when the attribute was accessed.

      The problem was fixed by setting the file pointer to the current opened file pointer
      when the attribute was accessed.  Similar patch up was done before when reading
      dataset with variable length string datatype.

      (VC - 2020/07/13, HDFFV-11080)

    - Fixed CVE-2018-17435

      The tool h52gif produced a segfault when the size of an attribute message
      was corrupted and caused a buffer overflow.

      The problem was fixed by verifying the attribute message's size against the
      buffer size before accessing the buffer.  h52gif was also fixed to display
      the failure instead of silently exiting after the segfault was eliminated.

      (BMR - 2020/6/19, HDFFV-10591)

    - Reduce overhead for H5open(), which is involved in public symbols like
      H5T_NATIVE_INT, etc.

      (QAK - 2020/06/18)

    - Cache last ID looked up for an ID type (dataset, datatype, file, etc),
      improving performance when accessing the same ID repeatedly.

      (QAK - 2020/06/11)

    - Streamline I/O to a single element, improving performance for record
      appends to chunked datasets.

      (QAK - 2020/06/11)

    - Remove redundant tagging of metadata cache entries for some chunked
      dataset operations, slightly improving performance for chunked
      datasets.

      (QAK - 2020/06/10)

    - Better detect selections with the same shape, improving performance for
      some uses of H5DOappend (and other situations).

      (QAK - 2020/06/07)

    - Don't allocate an empty (0-dimensioned) chunked dataset's chunk
      index, until the dataset's dimensions are increased.

      (QAK - 2020/05/07)


    Java Library
    ------------
    - JNI utility function does not handle new references.

      The JNI utility function for converting reference data to string did
      not use the new APIs. In addition to fixing that function, added new
      java tests for using the new APIs.

      (ADB - 2021/02/16, HDFFV-11212)

    - The H5FArray.java class, in which virtually the entire execution time
      is spent using the HDFNativeData method that converts from an array
      of bytes to an array of the destination Java type.

        1. Convert the entire byte array into a 1-d array of the desired type,
           rather than performing 1 conversion per row;
        2. Use the Java Arrays method copyOfRange to grab the section of the
           array from (1) that is desired to be inserted into the destination array.

      (PGT,ADB - 2020/12/29, HDFFV-10865)


    Configuration
    -------------
    - Refactor CMake configure for Fortran

      The Fortran configure tests for KINDs reused a single output file that was
      read to form the Integer and Real Kinds defines. However, if config was run
      more then once, the CMake completed variable prevented the tests from executing
      again and the last value saved in the file was used to create the define.
      Creating separate files for each KIND solved the issue.

      In addition the test for H5_PAC_C_MAX_REAL_PRECISION was not pulling in
      defines for proper operation and did not define H5_PAC_C_MAX_REAL_PRECISION
      correctly for a zero value. This was fixed by supplying the required defines.
      In addition it was moved from the Fortran specific HDF5UseFortran.camke file
      to the C centric ConfigureChecks.cmake file.

      (ADB - 2021/06/03)

    - Move emscripten flag to compile flags

      The emscripten flag, -O0, was removed from target_link_libraries command
      to the correct target_compile_options command.

      (ADB - 2021/04/26 HDFFV-11083)

    - Remove arbitrary warning flag groups from CMake builds

      The arbitrary groups were created to reduce the quantity of warnings being
      reported that overwhelmed testing report systems. Considerable work has
      been accomplished to reduce the warning count and these arbitrary groups
      are no longer needed.
      Also the default for all warnings, HDF5_ENABLE_ALL_WARNINGS, is now ON.

      Visual Studio warnings C4100, C4706, and C4127 have been moved to
      developer warnings, HDF5_ENABLE_DEV_WARNINGS, and are disabled for normal builds.

      (ADB - 2021/03/22, HDFFV-11228)

    - Reclassify CMake messages, to allow new modes and --log-level option

      CMake message commands have a mode argument. By default, STATUS mode
      was chosen for any non-error message. CMake version 3.15 added additional
      modes, NOTICE, VERBOSE, DEBUG and TRACE. All message commands with a mode
      of STATUS were reviewed and most were reclassified as VERBOSE. The new
      mode was protected by a check for a CMake version of at least 3.15. If CMake
      version 3.17 or above is used, the user can use the command line option
      of "--log-level" to further restrict which message commands are displayed.

      (ADB - 2021/01/11, HDFFV-11144)

    - Fixes Autotools determination of the stat struct having an st_blocks field

      A missing parenthesis in an autoconf macro prevented building the test
      code used to determine if the stat struct contains the st_blocks field.
      Now that the test functions correctly, the H5_HAVE_STAT_ST_BLOCKS #define
      found in H5pubconf.h will be defined correctly on both the Autotools and
      CMake. This #define is only used in the tests and does not affect the
      HDF5 C library.

      (DER - 2021/01/07, HDFFV-11201)

    - Add missing ENV variable line to hdfoptions.cmake file

      Using the build options to use system SZIP/ZLIB libraries need to also
      specify the library root directory. Setting the {library}_ROOT ENV
      variable was added to the hdfoptions.cmake file.

      (ADB - 2020/10/19 HDFFV-11108)

    - Fixed removal of Java lib files by "make uninstall"

      "make uninstall" was not removing 5 Java files from the lib directory.

      (LRK - 2020/08/24, HDFFV-10811)

    - Stopped addition of szip header and include directory path for
      incompatible libsz

      szlib.h is the same for both 32-bit and 64-bit szip, and the header file
      and its path were added to the HDF5 binary even though the configure
      check of a function in libsz later failed and szip compression was not
      enabled.  The header file and include path are now added only when the
      libsz function passes the configure check.

      (LRK - 2020/08/17, HDFFV-10830)


    Tools
    -----
    - Changed how h5dump and h5ls identify long double.

      Long double support is not consistent across platforms. Tools will always
      identify long double as 128-bit [little/big]-endian float nn-bit precision.
      New test file created for datasets with attributes for float, double and
      long double. In addition any unknown integer or float datatype will now
      also show the number of bits for precision.
      These files are also used in the java tests.

      (ADB - 2021/03/24, HDFFV-11229,HDFFV-11113)

    - Fixed tools argument parsing.

      Tools parsing used the length of the option from the long array to match
      the option from the command line. This incorrectly matched a shorter long
      name option that happened to be a subset of another long option.
      Changed to match whole names.

      (ADB - 2021/01/19, HDFFV-11106)

    - Fixed a bug with h5repack ignoring the "-q" and "-z" command-line options

        h5repack uses the -q and -z command-line options to specify the indexing
        options for objects in the file when repacking. Previously, h5repack
        would simply ignore any setting from these options. This has been fixed
        so that h5repack will correctly interpret the following settings:

          -q "name"           : Sort objects by name order
          -q "creation_order" : Sort objects by link creation order
          -z "ascending"      : Sort objects by ascending name/link creation order
          -z "descending"     : Sort objects by descending name/link creation order

        (JTH - 2020/04/16, HDFFV-11030)

    - The tools library was updated by standardizing the error stack process.

        General sequence is:
          h5tools_setprogname(PROGRAMNAME);
          h5tools_setstatus(EXIT_SUCCESS);
          h5tools_init();
          ... process the command-line (check for error-stack enable) ...
          h5tools_error_report();
          ... (do work) ...
          h5diff_exit(ret);

        (ADB - 2020/07/20, HDFFV-11066)

    - h5diff fixed a command line parsing error.

        h5diff would ignore the argument to -d (delta) if it is smaller than DBL_EPSILON.
        The macro H5_DBL_ABS_EQUAL was removed and a direct value comparision was used.

        (ADB - 2020/07/20, HDFFV-10897)

    - h5diff added a command line option to ignore attributes.

        h5diff would ignore all objects with a supplied path if the exclude-path argument is used.
        Adding the exclude-attribute argument will only exclude attributes, with the supplied path,
        from comparison.

        (ADB - 2020/07/20, HDFFV-5935)

    - h5diff added another level to the verbose argument to print filenames.

        Added verbose level 3 that is level 2 plus the filenames. The levels are:
          0 : Identical to '-v' or '--verbose'
          1 : All level 0 information plus one-line attribute status summary
          2 : All level 1 information plus extended attribute status report
          3 : All level 2 information plus file names

        (ADB - 2020/07/20, HDFFV-1005)


    Fortran API
    -----------
    - Fixed configure issue when building HDF5 with NAG Fortran 7.0.

      HDF5 now accounts for the addition of half-precision floating-point
      in NAG 7.0 with a KIND=16.

      (MSB - 2020/02/28, HDFFV-11033)


    High-Level Library
    ------------------
    - Eliminated unnecessary code in H5DOappend(), improving its performance.

      (QAK - 2020/06/05)


    Fortran High-Level APIs
    -----------------------
    -


    Documentation
    -------------
    - Updated doxygen comments with changes for release

      (ADB - 2021/05/03)

    - Fixed the dimension size description in the layout message table of the
      file format specification section IV.A.2.i.

      The layout message table described the dimension sizes starting from
      Dimension 0 to Dimension #n.  This was incorrect because it would mean
      the rank is n+1.

      The dimension size entries were changed to starting from Dimension 1 to
      Dimension #n.

      (VC – 2020/06/09, HDFFV-11079)


    F90 APIs
    --------
    - Fixed parallel make build failure due to a missing H5forkit dependency.

      (MSB, 2021/03/31, HDFFV-11232)


    C++ APIs
    --------
    - Added DataSet::operator=

        Some compilers complain if the copy constructor is given explicitly
        but the assignment operator is implicitly set to default.

        (LN - 2021/05/19)


    Testing
    -------
    - Stopped java/test/junit.sh.in installing libs for testing under ${prefix}

      Lib files needed are now copied to a subdirectory in the java/test
      directory, and on Macs the loader path for libhdf5.xxxs.so is changed
      in the temporary copy of libhdf5_java.dylib.

      (LRK - 2020/07/02, HDFFV-11063)


Supported Platforms
===================

    Linux 3.10.0-327.10.1.el7   GNU C (gcc), Fortran (gfortran), C++ (g++)
    #1 SMP x86_64 GNU/Linux       compilers:
    (jelly/kituo/moohan)            Version 4.8.5 20150623 (Red Hat 4.8.5-4)
                                    Version 4.9.3, 5.2.0, 7.1.0
                                  Intel(R) C (icc), C++ (icpc), Fortran (icc)
                                  compilers:
                                     Version 17.0.0.098 Build 20160721
                                  MPICH 3.2, 3.3

    Windows 10 x64                Visual Studio 2015 w/ Intel Fortran 18 (cmake)
                                  Visual Studio 2017 w/ Intel Fortran 19 (cmake)
                                  Visual Studio 2019 w/ Intel Fortran 19 (cmake)
                                  Visual Studio 2019 w/ MSMPI 10.1 (cmake)

    MacOS High Sierra 10.13.6     Apple LLVM version 10.0.0 (clang-1000.10.44.4)
    64-bit                        gfortran GNU Fortran (GCC) 6.3.0
    (bear)                        Intel icc/icpc/ifort version 19.0.4

    MacOS Big Sur 11.2            Apple clang version 12.0.5 (clang-1205.0.22.9)
                                  gfortran GNU Fortran (Homebrew GCC 10.2.0_3) 10.2.0
                                  Intel icc/icpc (ICC) 2021.1 Beta 20201112

Tested Configuration Features Summary
=====================================

    In the tables below
          y   = tested
          n   = not tested in this release
          C   = Cluster
          W   = Workstation
          x   = not working in this release
          dna = does not apply
          ( ) = footnote appears below second table
          <blank> = testing incomplete on this feature or platform

Platform                              C         F90/   F90      C++  zlib  SZIP
                                      parallel  F2003  parallel
Windows 10                              y        y/y    n        y    y     y
Windows 10 x64                          y        y/y    n        y    y     y
MacOS High Sierra 10.13.6 64-bit        n        y/y    n        y    y     y
MacOS Big Sur 11.2 64-bit               n        y/y    n        y    y     y
CentOS 6.7 Linux 2.6.18 x86_64 GNU      n        y/y    n        y    y     y
CentOS 6.7 Linux 2.6.32 x86_64 PGI      n        y/y    n        y    y     y
CentOS 7.2 Linux 2.6.32 x86_64 GNU      y        y/y    y        y    y     y
CentOS 7.2 Linux 2.6.32 x86_64 Intel    n        y/y    n        y    y     y
Linux 3.10.0-1127.10.1.1.ch6.ppc64le    n        y/n    n        y    y     y


Platform                                 Shared  Shared    Shared    Thread-
                                         C libs  F90 libs  C++ libs  safe
Windows 10                                 y       y         y         y
Windows 10 x64                             y       y         y         y
MacOS High Sierra 10.13.6 64-bit           y       n         y         y
MacOS Big Sur 11.2 64-bit                  y       n         y         y
CentOS 6.7 Linux 2.6.18 x86_64 GNU         y       y         y         y
CentOS 6.7 Linux 2.6.32 x86_64 PGI         y       y         y         n
CentOS 7.2 Linux 2.6.32 x86_64 GNU         y       y         y         n
CentOS 7.2 Linux 2.6.32 x86_64 Intel       y       y         y         n
Linux 3.10.0-1127.10.1.1.ch6.ppc64le       y       y         y         n

Compiler versions for each platform are listed in the preceding
"Supported Platforms" table.


More Tested Platforms
=====================

The following platforms are not supported but have been tested for this release.

    Linux 2.6.32-573.22.1.el6    GNU C (gcc), Fortran (gfortran), C++ (g++)
    #1 SMP x86_64 GNU/Linux       compilers:
    (platypus)                 Version 4.4.7 20120313
                                     Version 4.9.3, 5.3.0, 6.2.0
                                  PGI C, Fortran, C++ for 64-bit target on
                                  x86-64;
                                      Version 19.10-0
                                  MPICH 3.1.4 compiled with GCC 4.9.3

    Linux 3.10.0-327.18.2.el7     GNU C (gcc) and C++ (g++) compilers
    #1 SMP x86_64 GNU/Linux          Version 4.8.5 20150623 (Red Hat 4.8.5-4)
    (jelly)                       with NAG Fortran Compiler Release 6.1(Tozai)
                                  GCC Version 7.1.0
                                  OpenMPI 3.0.0-GCC-7.2.0-2.29
                                  Intel(R) C (icc) and C++ (icpc) compilers
                                     Version 17.0.0.098 Build 20160721
                                  with NAG Fortran Compiler Release 6.1(Tozai)
                                  PGI C (pgcc), C++ (pgc++), Fortran (pgf90)
                                  compilers:
                                     Version 18.4, 19.4
                                  MPICH 3.3
                                  OpenMPI 2.1.5, 3.1.3, 4.0.0

    Linux 3.10.0-327.10.1.el7     MPICH 3.1.4 compiled with GCC 4.9.3
    #1 SMP x86_64 GNU/Linux
    (moohan)

    Linux-3.10.0-1127.0.0.1chaos  openmpi/4.0.0
    #1 SMP x86_64 GNU/Linux           clang/6.0.0, 11.0.1
    (quartz)                          gcc/7.3.0, 8.3.0
                                      intel/16.0.4, 18.0.2, 19.0.4

    Linux-4.14.0-115.10.1.1       spectrum-mpi/rolling-release
    #1 SMP ppc64le GNU/Linux          clang/7.1.0, ibm-11.0.1
    (lassen)                          gcc/4.9.3, 8.3.1
                                      xl/2019.02.07, 2021.03.11

    Linux-4.12.14-150.52-default  cray-mpich/7.7.10
    #1 SMP x86_64 GNU/Linux           gcc/8.3.0
    (cori)                            intel/19.0.3

    Linux-4.4.180-94.107-default  cray-mpich/7.7.16
    # 1SMP x86_64 GNU/Linux           gcc/7.3.0, 8.3.0, 10.2.0
    (mutrino)                         intel/17.0.4, 18.0.5, 19.1.3

    Linux-3.10.0-                 spectrum-mpi/rolling-release
    1127.10.1.1chaos.ch6.ppc64le       clang/7.1.0, 11.0.1
    #1 SMP ppc64le GNU/Linux           gcc/4.9.3,8.3.1
    (ray)                              xl/2019.08.20,2021.03.11

    Fedora33 5.11.18-200.fc33.x86_64
    #1 SMP x86_64  GNU/Linux         GNU gcc (GCC) 10.3.1 20210422 (Red Hat 10.3.1-1)
                                     GNU Fortran (GCC) 10.3.1 20210422 (Red Hat 10.3.1-1)
                                     clang version 11.0.0 (Fedora 11.0.0-2.fc33)
                                     (cmake and autotools)

    Ubuntu20.04 5.8.0-53-generic-x86_64
    #60~20.04-Ubuntu SMP x86_64  GNU/Linux GNU gcc (GCC) 9.3.0-17ubuntu1
                                     GNU Fortran (GCC) 9.3.0-17ubuntu1
                                     clang version 10.0.0-4ubuntu1
                                     (cmake and autotools)

    Ubuntu20.10 5.8.0-53-generic-x86_64
    #60-Ubuntu SMP x86_64  GNU/Linux GNU gcc (GCC) 10.2.0-13ubuntu1
                                     GNU Fortran (GCC) 10.2.0-13ubuntu1
                                     Ubuntu clang version 11.0.0-2
                                     (cmake and autotools)

    SUSE15sp2 5.3.18-22-default
    #1 SMP x86_64  GNU/Linux         GNU gcc (SUSE Linux) 7.5.0
                                     GNU Fortran (SUSE Linux) 7.5.0
                                     clang version 7.0.1 (tags/RELEASE_701/final 349238)
                                     (cmake and autotools)

    Mac OS X El Capitan 10.11.6   Apple clang/clang++ version 7.3.0 from Xcode 7.3
    64-bit                        gfortran GNU Fortran (GCC) 5.2.0
    (osx1011dev/osx1011test)      Intel icc/icpc/ifort version 16.0.2


Known Problems
==============

    testflushrefresh.sh will fail when run with "make check-passthrough-vol"
    on centos7, with 3 Errors/Segmentation faults.  These will not occur when
    run with "make check".  See https://github.com/HDFGroup/hdf5/issues/673
    for details.

    The t_bigio test fails on several HPC platforms, generally by timeout with 
    OpenMPI 4.0.0 or with this error from spectrum-mpi: 
        *** on communicator MPI_COMM_WORLD
        *** MPI_ERR_COUNT: invalid count argument

    CMake files do not behave correctly with paths containing spaces.
    Do not use spaces in paths because the required escaping for handling spaces
    results in very complex and fragile build files.
    ADB - 2019/05/07

    At present, metadata cache images may not be generated by parallel
    applications.  Parallel applications can read files with metadata cache
    images, but since this is a collective operation, a deadlock is possible
    if one or more processes do not participate.

    CPP ptable test fails on both VS2017 and VS2019 with Intel compiler, JIRA
    issue: HDFFV-10628.  This test will pass with VS2015 with Intel compiler.

    The subsetting option in ph5diff currently will fail and should be avoided.
    The subsetting option works correctly in serial h5diff.

    Known problems in previous releases can be found in the HISTORY*.txt files
    in the HDF5 source. Please report any new problems found to
    help@hdfgroup.org.


CMake vs. Autotools installations
=================================

While both build systems produce similar results, there are differences.
Each system produces the same set of folders on linux (only CMake works
on standard Windows); bin, include, lib and share. Autotools places the
COPYING and RELEASE.txt file in the root folder, CMake places them in
the share folder.

The bin folder contains the tools and the build scripts. Additionally, CMake
creates dynamic versions of the tools with the suffix "-shared". Autotools
installs one set of tools depending on the "--enable-shared" configuration
option.
  build scripts
  -------------
  Autotools: h5c++, h5cc, h5fc
  CMake: h5c++, h5cc, h5hlc++, h5hlcc

The include folder holds the header files and the fortran mod files. CMake
places the fortran mod files into separate shared and static subfolders,
while Autotools places one set of mod files into the include folder. Because
CMake produces a tools library, the header files for tools will appear in
the include folder.

The lib folder contains the library files, and CMake adds the pkgconfig
subfolder with the hdf5*.pc files used by the bin/build scripts created by
the CMake build. CMake separates the C interface code from the fortran code by
creating C-stub libraries for each Fortran library. In addition, only CMake
installs the tools library. The names of the szip libraries are different
between the build systems.

The share folder will have the most differences because CMake builds include
a number of CMake specific files for support of CMake's find_package and support
for the HDF5 Examples CMake project.

