API Compatibility Macros in HDF5

Audience

Compatibility Issues

This document, “API Compatibility Macros in HDF5,” introduces the approach taken by The HDF Group in HDF Release 1.8.0 to help existing users of HDF5 address compatibility issues in the HDF5 API. The companion document, New Features in HDF5 Release 1.8.0 and Format Compatibility Considerations, discusses features introduced in HDF5 Release 1.8.0, the HDF5 API calls associated with those features, and the potential file format compatibility issues that may result if the new features are used.

Summary and Motivation

HDF5 Release 1.8.0 includes a number of new features that will offer many users of HDF5 substantial performance improvements and expanded capabilities. Many of these features can only be accessed via revised API calls. Given the scope of the changes, 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 either 1.6.x or 1.8.x (currently 1.8.0) functions. We refer to these as the 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:

• Source code is not available - only binaries are available; updating the application is not feasible.
• Source code is available, but there are no resources to update it.
• 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.
• 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.

Understanding and Using the Macros

Concretely, consider the function H5Acreate in HDF5 releases prior to 1.8.0:

Original function name and signature, in releases prior to 1.8.0:
hid_t H5Acreate( hid_t loc_id, const char *attr_name, hid_t type_id, hid_t space_id, hid_t acpl_id )

Updated function and signature, introduced in release 1.8.0:
hid_t H5Acreate2( hid_t loc_id, const char *attr_name, hid_t type_id, hid_t space_id, hid_t acpl_id, hid_t aapl_id )

Original function and signature, renamed in release 1.8.0:
hid_t H5Acreate1( hid_t loc_id, const char *attr_name, hid_t type_id, hid_t space_id, hid_t acpl_id )

API compatibility macro, introduced in release 1.8.0:
H5Acreate

The function names ending in "1" or "2" are referred to as version-numbered names, and the corresponding functions are referred to as version-numbered functions. For new code development, The HDF Group recommends use of the compatibility macro mapped to the latest version of the function. The original versions of these functions, with names ending in "1", should be considered deprecated and, in general, should not be used when developing new code.

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 compatibility macro mapping options, is considered in the following sequence:

1. What compatibility macro configuration option was used to build the HDF5 Library? We refer to this selection as the library mapping.

2. Was a compatibility macro global compile-time option specified when the application was built? We refer to this (optional) selection as the application mapping. If an application mapping exists, it overrides the library mapping.

3. Were any compatibility macro function-level compile-time options specified when the application was built? We refer to these (optional) selections as function mappings. If function mappings exist, they override library and application mappings for the relevant API compatibility macros.

The tables that follow summarize the macro mapping behaviors, and the configuration and compile-time options that control the mappings. The macro H5Gcreate is used to demonstrate mapping behavior.

Regardless of the macro mapping options used, the 1.8.x functions will always be available by explicitly calling the version-numbered functions by their version-numbered names. For example, H5Gcreate2. Through the compatibility macro mapping options provided, it is possible to disallow calls to the deprecated 1.6.x functions, such as H5Gcreate1. This capability can be used to guarantee only the most recent versions of the functions are being called.

Library Mapping Options

When the HDF5 Library is built, 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.

Refer to the file libhdf5.settings in the directory where the HDF5 library is installed to determine the configure flags used to build the library. In particular, look for the two lines shown here:

Default Version of Public Symbols: v18
With Deprecated Public Symbols: Yes

Application Mapping Options

When an application using HDF5 APIs is built and linked with the HDF5 Library, compile-time options to 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 --disable-deprecated-symbols flag, then the deprecated functions will not be available, regardless of the application mapping 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 --disable-deprecated-symbols flag, or -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 version-numbered function. For example, the H5Gcreate can be mapped to either H5Gcreate1 or H5Gcreate2. When used, the value of the H5Gcreate_vers compile-time version flag determines which function will be called:

• When H5Gcreate_vers is set to 1, the macro H5Gcreate will be mapped to H5Gcreate1.
       h5cc ... -DH5Gcreate_vers=1 ...

• When H5Gcreate_vers is set to 2, the macro H5Gcreate will be mapped to H5Gcreate2.
       h5cc ... -DH5Gcreate_vers=2 ...

• When H5Gcreate_vers is not set, the macro H5Gcreate will be mapped to either H5Gcreate1 or H5Gcreate2, based on the application mapping, if one was specified, or on the library mapping.
       h5cc ...

As of Release 1.8.0, the API compatibility macros, the function mapping compile-time version flags and values, and the corresponding version-numbered functions are as indicated:

See the HDF5 Reference Manual  for complete descriptions of all API compatibility macros and version-numbered functions shown in Table 3.

It is possible to specify multiple function mappings for a single application build:

h5cc ... -DH5Gcreate_vers=1 -DH5Dcreate_vers=2...

The function mappings can be used to guarantee that a given API compatibility macro will be mapped to the desired underlying function version regardless of the library or application mappings. In cases where an application may benefit greatly from features offered by some of the later APIs, or must continue to use some earlier API versions for compatibility reasons, this fine-grained control may be very important.

As noted earlier, the function mappings can only reference version-numbered functions that are included in the HDF5 library, as determined by the configure flag used to build the library. For example, if the HDF5 library being linked with the application was built with the --disable-deprecated-symbols option, version 1 of the underlying functions would not be available, and the example above that defined H5Gcreate_ver=1 would not be supported.

The function mappings do not negate any available functions. If H5Gcreate1 is available in the installed version of the HDF5 Library, and the application was not compiled with the -DH5_NO_DEPRECATED_SYMBOLS flag, the function H5Gcreate1 will remain available to the application through its version-numbered name. Similarly, H5Gcreate2 will remain available to the application as H5Gcreate2. The function mapping version flag H5Gcreate_vers only controls the mapping of the API compatibility macro H5GCreate to one of the two available functions.

Compatibility Macros in HDF5 1.6.8 and Later

This allows users to write code that can be used with any version of the library since 1.6.8 and any library compilation options except H5_NO_DEPRECATED_SYMBOLS, by always using the "1" version of versioned functions and types. For example, H5Gcreate1 will always be interpreted in exactly the same manner by any version of the library since 1.6.8.

This can be especially useful in any case where the programmer does not have direct control over global macro definitions, such as when writing code meant to be copied to multiple applications or when writing code in a header file.

Common Use Case

1. Build the HDF5 library without specifying any library mapping configure flag. In this default mode, both 1.6.x and 1.8.x versions of the underlying functions are available, and the API compatibility macros will be mapped to the 1.8.x version-numbered functions. For example, H5Gcreate will be mapped to H5Gcreate2.

2. Compile the application with the -DH5_USE_16_API application mapping option, and link with the HDF5 library built in step 1. No changes should be required to build the application. The API compatibility macros, for example H5Gcreate, replace the actual function names that were used in versions of the library prior to 1.8.0. Because the application mapping overrides the library mapping, the macros will all be mapped to the 1.6.x versions of the functions.

3. Remap one API compatibility macro at a time (or sets of macros), to use the 1.8.x versions. At each stage, use the function mappings to map the macros being worked on to the 1.8.x versions. For example, use the -DH5Gcreate_vers=2 version flag setting to remap the H5Gcreate macro to H5Gcreate2, the 1.8.x version. During this step, the application code will need to be modified to change the calling parameters used with the API compatibility macros to match the number and type of the 1.8.x version-numbered functions. The macro name, for example H5Gcreate, should continue to be used in the code, to allow for possible re-mappings to later version-numbered functions in a future release.

4. After all macros have been migrated to the 1.8.x version-numbered functions in step 3, compile the application without any application or function mappings. This build uses the library mappings set in step 1, and maps API compatibility macros to the 1.8.x versions.

5. Finally, compile the application with the application mapping -DH5_NO_DEPRECATED_SYMBOLS, and address any failures to complete the application migration process.

The HDF Group Help Desk:
Describes HDF5 Release 1.8.7, targeted for May 2011.