path: root/doxygen/dox/api-compat-macros.dox

/** \page api-compat-macros API Compatibility Macros
  \tableofcontents

  \section audience Audience
  The target audience for this document has existing applications that use the
  HDF5 library, and is considering moving to the latest HDF5 release to take
  advantage of the latest library features and enhancements.

  \section compat-issues Compatibility Issues
  With each major release of HDF5, such as 1.12 or 1.10, certain compatibility
  issues must be considered when migrating applications from an earlier major
  release.

  This document describes the approach taken by The HDF Group to help existing
  users of HDF5 address compatibility issues in the HDF5 API.

  \section summary Summary and Motivation
  In response to new and evolving requirements for the library and data format,
  several basic functions have changed since HDF5 was first released. To allow
  existing applications to continue to compile and run properly, all versions
  of these functions have been retained in the later releases of the HDF5 library.

  Given the scope of changes available with each major release of HDF5, and
  recognizing the potentially time-consuming task of editing all the affected
  calls in user applications, The HDF Group has created a set of macros that
  can be used to flexibly and easily map existing API calls to previous release
  functions. We refer to these as the \Emph{API compatibility macros}.

  The HDF Group generally encourages users to update applications to work with
  the latest HDF5 library release so that all new features and enhancements are
  available to them. At the same time, The HDF Group understands that, under
  some circumstances, updating applications may not be feasible or necessary.
  The API compatibility macros, described in this document, provide a bridge
  from old APIs to new and can be particularly helpful in situations such as these:
  <ul>
  <li>Source code is not available - only binaries are available; updating the
      application is not feasible.</li>
  <li>Source code is available, but there are no resources to update it.</li>
  <li>Source code is available, as are resources to update it, but the old
      version works quite well so updates are not a priority. At the same time,
      it is desirable to take advantage of certain efficiencies in the newer HDF5
      release that do not require code changes.</li>
  <li>Source code is available, as are resources to update it, but the applications
      are large or complex, and must continue to run while the code updates are
      carried out.</li>
  </ul>

  \section using Understanding and Using the Macros
  As part of latest HDF5 release, several functions that existed in previous
  versions of the library were updated with new calling parameters and given
  new names. The updated versions of the functions have a number (for, e.g., '2')
  at the end of the original function name. The original versions of these
  functions were retained and renamed to have an earlier number (for, e.g., '1')
  at the end of the original function name.

  For example, consider the function \Code{H5Lvisit} in HDF5 release 1.10
  as compared with 1.12:

  <table>
  <tr>
  <th>Original function name and signature in 1.10.0</th>
  <td colspan="1">
  \Code{herr_t H5Lvisit(hid_t grp_id, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate_t op, void *op_data)}
  </td>
  </tr>
  <tr>
  <th>Updated function and signature, introduced in release 1.12.0</th>
  <td colspan="1">
  \Code{herr_t H5Lvisit2(hid_t group_id, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate2_t op, void *op_data)}
  </td>
  </tr>
  <tr><th>Original function and signature, renamed in release 1.12.0</th>
  <td colspan="1">
  \Code{herr_t H5Lvisit1(hid_t group_id, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate1_t op, void *op_data)}
  </td>
  </tr>
  <tr><th>API compatibility macro, introduced in release 1.12.0</th>
  <td colspan="1">
  \Code{H5Lvisit}
  <p>The macro, \Code{H5Lvisit}, will be mapped to either \Code{H5Lvisit1} or
     \Code{H5Lvisit2}. The mapping is determined by a combination of the
     configuration options use to build the HDF5 library and compile-time
     options used to build the application. The calling parameters used with the
     \Code{H5Lvisit} compatibility macro should match the number and type of the
     function the macros will be mapped to (\Code{H5Lvisit1} or \Code{H5Lvisit2}).
  </p>
  <p>The function names ending in '1' or '2' are referred to as \Emph{versioned names},
     and the corresponding functions are referred to as \Emph{versioned functions}.
     For new code development, The HDF Group recommends use of the compatibility macro
     mapped to the latest version of the function. The original version of the function
     should be considered deprecated and, in general, should not be used when developing
     new code.
  </p>
  </td>
  </tr>
  </table>

  \section options Compatibility Macro Mapping Options
  To determine the mapping for a given API compatibility macro in a given application,
  a combination of user-controlled selections, collectively referred to as the
  \Emph{compatibility macro mapping options}, is considered in the following sequence:

  <table border="0" style="width: 100.0%;">
  <tr>
  <td valign="top" width="50%">
  <ol>
  <li>What compatibility macro configuration option was used to build the HDF5 library?
      We refer to this selection as the \Emph{library mapping}.</li>
  <li>Was a compatibility macro global compile-time option specified when the application
      was built? We refer to this (optional) selection as the \Emph{application mapping}.
      If an application mapping exists, it overrides the library mapping.
      \Emph{(See adjacent notes.)}</li>
  <li>Were any compatibility macro function-level compile-time options specified when the
      application was built? We refer to these (optional) selections as \Emph{function mappings}.
      If function mappings exist, they override library and application mappings for the
      relevant API compatibility macros. \Emph{(See adjacent notes.)}</li>
  </ol>
  </td>
  <td valign="top" width="50%">
  <table border="1" cellpadding="10" style="width: 100.0%;">
  <tr><td>
  \Bold{Notes:} An application mapping can map APIs to the same version or to a version
  older than the configured library mapping. When the application attempts to map APIs
  to a newer version of the API than the library was configured with, it will fail to
  "upgrade" the mapping (and may fail silently).

  When it is necessary to "upgrade" the macro mappings from those set in the library mapping,
  it must be done at the per-function level, using the function-level mappings. As long as
  one does not try to map a function to a version that was compiled out in the library mapping,
  individual functions can be upgraded or downgraded freely.
  </td>
  </tr>
  </table>
  </td>
  </tr>
  </table>

  \subsection lib-options Library Mapping Options
  When the HDF5 library is built, \Code{configure} flags can be used to control the API
  compatibility macro mapping behavior exhibited by the library. This behavior can be
  overridden by application and function mappings. One configure flag excludes deprecated
  functions from the HDF5 library, making them unavailable to applications linked with the
  library.

  <div align="center">Table 1:  Library Mapping Options
  <table border="1" cellpadding="3">
  <tr>
  <th>\Code{configure} flag</th>
  <th>Macros map to release<br/>(versioned function; \Code{H5Lvisit} shown)</th>
  <th>Deprecated functions available? <br/>(\Code{H5Lvisit1})</th>
  </tr>
  <tr align="center">
  <td>\Code{--with-default-api-version=v112} <br/> (the default in 1.12)</td>
  <td>1.12.x (\Code{H5Lvisit2})</td>
  <td>yes</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{--with-default-api-version=v110}</td>
  <td>1.10.x (\Code{H5Lvisit1})</td>
  <td>yes</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{--with-default-api-version=v18}</td>
  <td>1.8.x (\Code{H5Lvisit1})</td>
  <td>yes</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{--with-default-api-version=v16}</td>
  <td>1.6.x (\Code{H5Lvisit1})</td>
  <td>yes</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{--disable-deprecated-symbols}</td>
  <td>1.12.x (\Code{H5Lvisit2})</td>
  <td>no</td>
  </tr>
  </table>
  </div>
  Refer to the file \Code{libhdf5.settings} in the directory where the HDF5 library is
  installed to determine the \Code{configure} flags used to build the library. In particular,
  look for the two lines shown here under \Emph{Features}:

  \Code{Default API mapping: v112}

  \Code{With deprecated public symbols: yes}

  \subsection app-options Application Mapping Options
  When an application using HDF5 APIs is built and linked with the HDF5 library,
  compile-time options to \Code{h5cc} can be used to control the API compatibility
  macro mapping behavior exhibited by the application. The application mapping
  overrides the behavior specified by the library mapping, and can be overridden
  on a function-by-function basis by the function mappings.

  If the HDF5 library was configured with the \Code{--disable-deprecated-symbols} flag, then
  the deprecated functions will not be available, regardless of the application mapping options.

  <div align="center">Table 2:  Application Mapping Options
  <table border="1" cellpadding="3">
  <tr>
  <th>\Code{h5cc} option</th>
  <th>Macros map to release <br/>(versioned function; \Code{H5Lvisit} shown)</th>
  <th>Deprecated functions available? <br/>(\Code{H5Lvisit1})</th>
  </tr>
  <tr align="center">
  <td align="left">\Code{-DH5_USE_112_API} <br/> \Emph{(Default behavior if no option specified.)}</td>
  <td>1.12.x (\Code{HLvisit2})</td>
  <td>yes* <br/> \Emph{*if available in library}</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{-DH5_USE_110_API}</td>
  <td>1.10.x (\Code{HLvisit1})</td>
  <td>yes* <br/> \Emph{*if available in library}</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{-DH5_USE_18_API}</td>
  <td>1.8.x (\Code{H5Lvisit1})</td>
  <td>yes* <br/> \Emph{*if available in library}</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{-DH5_USE_16_API}</td>
  <td>1.6.x (\Code{H5Lvisit1})</td>
  <td>yes* <br/> \Emph{*if available in library}</td>
  </tr>
  <tr align="center">
  <td align="left">\Code{-DH5_NO_DEPRECATED_SYMBOLS}</td>
  <td>1.10.x (\Code{H5Lvisit1})</td>
  <td>no</td>
  </tr>
  </table>
  </div>

  \subsection fun-options Function Mapping Options
  Function mappings are specified when the application is built. These mappings
  can be used to control the mapping of the API compatibility macros to
  underlying functions on a function-by-function basis. The function mappings
  override the library and application mappings discussed earlier.

  If the HDF5 library was configured with the \Code{--disable-deprecated-symbols}
  flag, or \Code{-DH5_NO_DEPRECATED_SYMBOLS} is used to compile the application,
  then the deprecated functions will not be available, regardless of the function
  mapping options.

  For every function with multiple available versions, a compile-time version flag
  can be defined to selectively map the function macro to the desired versioned
  function. The function mapping consists of the function name followed by
  &quot;\Code{_vers}&quot; which is mapped by number to a specific function or
  struct:
  <table>
  <tr>
  <th>Macro</th>
  <th>Function Mapping</th>
  <th>Mapped to function or struct</th>
  </tr>
  <tr><td>\Code{H5xxx}</td>
  <td>\Code{H5xxx_vers=1}</td>
  <td>\Code{H5xxx1}</td>
  </tr>
  <tr>
  <td> </td>
  <td>\Code{H5xxx_vers=2}</td>
  <td>\Code{H5xxx2}</td>
  </tr>
  </table>

  For example, in version 1.10 the \Code{H5Rreference} macro can be mapped to
  either \Code{H5Rreference1} or \Code{H5Rreference2}. When used, the value of
  the \Code{H5Rreference_vers} compile-time version flag determines which
  function will be called:

  <ul>
  <li>When \Code{H5Rreference_vers} is set to \Code{1}, the macro \Code{H5Rreference}
      will be mapped to \Code{H5Rreference1}. <br/>
      \Code{H5cc ... -DH5Rreference_vers=1 ...}</li>
  <li>When \Code{H5Rdereference_vers} is set to \Code{2}, the macro \Code{H5Rdereference}
      will be mapped to \Code{H5Rdereference2}. <br/>
      \Code{h5cc ... -DH5Rreference_vers=2 ...}</li>
  <li>When \Code{H5Rreference_vers} is not set, the macro \Code{H5Rreference} will be
      mapped to either \Code{H5Rreference1} or \Code{H5Rreference2}, based on the
      application mapping, if one was specified, or on the library mapping. <br/>
      \Code{h5cc ... }</li>
  </ul>

  \warning Please be aware that some function mappings use mapped structures, as
           well.  If compiling an application with a function mapping that uses
           a mapped structure, you must include each function and mapped structure
           plus EVERY function that uses the mapped structure, whether or not that
           function is used in the application. \Emph{In 1.12, mappings of structures
           are used by the H5L and H5O function mappings.}\n\n
           For example, an application only calls \Code{H5Lvisit}, \Code{H5Ovisit},
           and \Code{H5Oget_info_by_name}. To compile this application with 1.10 APIs
           in 1.12 with the function specific mappings, then not only must
           \Code{H5Lvisit_vers}, \Code{H5Ovisit_vers}, and \Code{H5Oget_info_by_name_vers}
           be specified on the command line, but the mapped structures and every
           function that uses the mapped structures must be included, as well.
           The full compile line is shown below:
           \code{.sh}
           h5cc -DH5Lvisit_vers=1 -DH5Ovisit_vers=1 -DH5Oget_info_by_name_vers=1 \
                -DH5Lvisit_by_name_vers=1 -DH5Literate_vers=1 \
                -DH5Literate_by_name_vers= -DH5O_info_t_vers=1 -DH5L_info_t_vers=1 \
                -DH5L_iterate_t_vers=1 -DH5Lget_info_by_idx_vers=1 \
                -DH5Lget_info_vers=1 application.c
           \endcode

  \subsubsection fun-options-112 Function Mapping Options in Releases 1.12.x
  \subsubsection fun-options-110 Function Mapping Options in Releases 1.10.x
  \subsubsection fun-options-18 Function Mapping Options in Releases 1.8.x
  \subsubsection further Further Information
  \section macros Compatibility Macros in HDF5 1.6.8 and Later
  \section use-case Common Use Case
*/