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HDF5 Virtual Object Layer (VOL)

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# The HDF5 Virtual Object Layer (VOL)

## Introduction

The virtual object layer is an abstraction layer in the HDF5 library that intercepts all API calls that could potentially access objects in an HDF5 container and forwards those calls to a VOL connector, which implements the storage. The user or application gets the benefit of using the familiar and widely-used HDF5 data model and API, but can map the physical storage of the HDF5 file and objects to storage that better meets the application's data needs.

## The VOL Abstraction Layer

The VOL lies just under the public API. When a storage-oriented public APIcall is made, the library performs a few sanity checks on the input parameters and then immediately invokes a VOL callback, which resolves to an implementation in the VOL connector that was selected when opening or creating the file. The VOL connector then performs whatever operations are needed before control returns to the library, where any final library operations such as assigning IDs for newly created/opened datasets are performed before returning. This means that, for calls that utilize the VOL, all of the functionality is deferred to the VOL connector and the HDF5 library does very little work. An important consideration of this is that most of the HDF5 caching layers (metadata and chunk caches, page buffering, etc.) will not be available as those are implemented in the HDF5 native VOL connector and cannot be easily reused by external connectors.

The VOL Architecture

Not all public HDF5 API calls pass through the VOL. Only calls which require manipulating storage go through the VOL and require a VOL connector author to implement the appropriate callback. Dataspace, property list, error stack, etc. calls have nothing to do with storage manipulation or querying and do not use the VOL. This may be confusing when it comes to property list calls, since many of those calls set properties for storage. Property lists are just collections of key-value pairs, though, so a particular VOL connector is not required to set or get properties.

Another thing to keep in mind is that not every VOL connector will implement the full HDF5 public API. In some cases, a particular feature like variable-length types may not have been developed yet or may not have an equivalent in the target storage system. Also, many HDF5 public API calls are specific to the native HDF5 file format and are unlikely to have any use in other VOL connectors. A feature/capabilities flag scheme is being developed to help navigate this.

For more information about which calls go through the VOL and the mechanism by which this is implemented, see the connector author and library internals documentation.

## VOL Connectors

A VOL connector can be implemented in several ways:

- as a shared or static library linked to an application
- as a dynamically loaded plugin, implemented as a shared library
- and even as an internal connector, built into the HDF5 library itself

This section mostly focuses on external connectors, both libraries and plugins, as those are expected to be much more common than internal implementations.

A list of VOL connectors can be found here: Registered VOL Connectors

This list is incomplete and only includes the VOL connectors that have been registered with The HDF Group.

Not every connector in this collection is actively maintained by The HDF Group. It simply serves as a single location where important VOL connectors can be found. See the documentation in a connector's repository to determine its development status and the parties responsible for it.

A VOL template that contains build scripts (Autotools and CMake) and an empty VOL connector "shell" which can be copied and used as a starting point for building new connectors is located here: VOL Connector Template

This template VOL connector is for use in constructing terminal VOL connectors that do not forward calls to an underlying connector. The external pass-through VOL connector listed on the registered connector page can be used as a starting point for pass-through connectors.

The only current (non-test) internal VOL connector distributed with the library is the native file format connector (the "native VOL connector") which contains the code that handles native HDF5 (*.h5/hdf5) files. In other words, even the canonical HDF5 file format is implemented via the VOL, making it a core part of the HDF5 library and not an optional component which could be disabled.

It has not been completely abstracted from the HDF5 library, though, and is treated as a special case. For example, it cannot be unloaded and is always present.

## Quickstart

The following steps summarize how one would go about using a VOL connector with an application. More information on particular steps can be found later on in this document.

### Read The Documentation For The New VOL Connector

Many VOL connectors will require specific setup and configuration of both the application and the storage. Specific permissions may have to be set, configuration files constructed, and connector-specific setup calls may need to be invoked in the application. In many cases, converting software to use a new VOL connector will be more than just a straightforward drop-in replacement done by specifying a name in the VOL plugin environment variable.

### Use A VOL-Enabled HDF5 Library

The virtual object layer was introduced in HDF5 1.12.0, however that version of the VOL is deprecated due to inadequate support for pass-through connectors. These deficiencies have been addressed in HDF5 1.14.0, so VOL users and connector authors should target the 1.14.0 VOL API.

On Windows, it's probably best to use the same debug vs release configuration for the application and all libraries in order to avoid C runtime (CRT) issues. Pre-2015 versions of Visual Studio are not supported.

### Determine How You Will Set The VOL Connector

Fundamentally, setting a VOL connector involves modifying the file access property list (fapl) that will be used to open or create the file.

There are essentially three ways to do this:

- Direct use of H5Pset_vol()
- Library-specific API calls that call H5Pset_vol() for you
- Use the VOL environment variable, which will also call H5Pset_vol() for you

Exactly how you go about setting a VOL connector in a fapl, will depend on the complexity of the VOL connector and how much control you have over the application's source code. Note that the environment variable method, though convenient, has some limitations in its implementation, which are discussed below.

### If Needed: Update Your Code To Load And Use A VOL Connector

There are two concerns when modifying the application:

- It may be convenient to add connector-specific setup calls to the application.
- You will also need to protect any API calls which are only implemented in the native VOL connector as those calls will fail when using a non-native VOL connector. See the section Adapting HDF5 Software to Use the VOL, below. A list of native VOL API calls has been included in List of HDF5 Native VOL API Calls.

In some cases, using the VOL environment variable will work well for setting the connector and any associated storage setup and the application will not use API calls that are not supported by the VOL connector. In this case, no application modification will be necessary.

### If Using A Plugin: Make Sure The VOL Connector Is In The Search

Path The default location for all HDF5 plugins is set at configure time when building the HDF5 library. This is true for both CMake and the Autotools. The default locations for the plugins on both Windows and POSIX systems is listed further on in this document.

### Optional: Set The VOL Connector Via The Environment Variable

In place of modifying the source code of your application, you may be able to simply set the HDF5_VOL_CONNECTOR environment variable (see below). This will automatically use the specified VOL in place of the native VOL connector.

## Connector Use

Before a VOL connector can be set in a fapl, it must be registered with the library (H5Pset_vol requires the connector's hid_t ID) and, if a plugin, it must be discoverable by the library at run time.

### Registration

Before a connector can be used, it must be registered. This loads the connector into the library and give it an HDF5 hid_t ID. The H5VLregister_connector API calls are used for this.

hid_t H5VLregister_connector_by_name(const char *connector_name, hid_t vipl_id)
hid_t H5VLregister_connector_by_value(H5VL_class_value_t connector_value, hid_t vipl_id)
hid_t
int64_t hid_t
Definition H5Ipublic.h:60
H5VL_class_value_t
int H5VL_class_value_t
VOL connector identifiers.
Definition H5VLpublic.h:175
H5VLregister_connector_by_value
hid_t H5VLregister_connector_by_value(H5VL_class_value_t connector_value, hid_t vipl_id)
Registers a new VOL connector by value.
H5VLregister_connector_by_name
hid_t H5VLregister_connector_by_name(const char *connector_name, hid_t vipl_id)
Registers a new VOL connector by name.

When used with a plugin, these functions will check to see if an appropriate plugin with a matching name, value, etc. is already loaded and check the plugin path (see above) for matching plugins if this is not true. The functions return H5I_INVALID_HID if they are unable to register the connector. Many VOL connectors will provide a connector-specific init call that will load and register the connector for you.

Note the two ways that a VOL connector can be identified: by a name or by a connector-specific numerical value (H5VL_class_value_t is typedef’d to an integer). The name and value for a connector can be found in the connector's documentation or public header file.

Each call also takes a VOL initialization property list (vipl). The library adds no properties to this list, so it is entirely for use by connector authors. Set this to H5P_DEFAULT unless instructed differently by the documentation for the VOL connector.

As far as the library is concerned, connectors do not need to be explicitly unregistered as the library will unload the plugin and close the ID when the library is closed. If you want to close a VOL connector ID, either H5VLunregister_connector() or H5VLclose() can be used (they have the same internal code path). The library maintains a reference count on all open IDs and will not do the actual work of closing an ID until its reference count drops to zero, so it's safe to close IDs anytime after they are used, even while an HDF5 file that was opened with that connector is still open.

Note that it's considered an error to unload the native VOL connector. The library will prevent this. This means that, for the time being, the native VOL connector will always be available. This may change in the future so that the memory footprint of the native VOL connector goes away when not in use.

### Connector Versioning

The VOL connector struct provides a conn_version field for versioning connectors. The library developers are working on some best practices for versioning connectors.

### Connector-Specific Registration Calls

Most connectors will provide a special API call which will set the connector in the fapl. These will often be in the form of H5Pset_fapl_<name>(). For example, the DAOS VOL connector provides a H5Pset_fapl_daos() API call which will take MPI parameters and make this call. See the connector's documentation or public header file(s) for more information.

### H5Pset_vol()

The is the main library API call for setting the VOL connector in a file access property list. Its signature is:

herr_t H5Pset_vol(hid_t plist_id, hid_t new_vol_id, const void new_vol_info)
herr_t
int herr_t
Definition H5public.h:252
H5Pset_vol
herr_t H5Pset_vol(hid_t plist_id, hid_t new_vol_id, const void *new_vol_info)
Set the file VOL connector for a file access property list.

It takes the ID of the file access property list, the ID of the registered VOL connector, and a pointer to whatever connector-specific data the connector is expecting. This will usually be a data struct specified in the connector's header or a NULL pointer if the connecter requires no special information (as in the native VOL connector).

As mentioned above, many connectors will provide their own replacement for this call. See the connector's documentation for more information.

### VOL Connector Search Path

Dynamically loaded VOL connector plugins are discovered and loaded by the library using the same mechanism as dataset/group filter plugins. The default locations are:

Default locations

POSIX systems: /usr/local/hdf5/lib/plugin
Windows: %ALLUSERSPROFILE%/hdf5/lib/plugin
hdf5
Definition HDF5.F90:26

These default locations can be overridden by setting the HDF5_PLUGIN_PATH environment variable. There are also public H5PL API calls which can be used to add, modify, and remove search paths. The library will only look for plugins in the specified plugin paths. By default, it will NOT find plugins that are simply located in the same directory as the executable.

### Parameter Strings

Each VOL connector is allowed to take in a parameter string which can be parsed via H5VLconnector_str_to_info() to get an info struct which can be passed to H5Pset_vol().

herr_t H5VLconnector_str_to_info(const char *str, hid_t connector_id, void **info)
H5VLconnector_str_to_info
herr_t H5VLconnector_str_to_info(const char *str, hid_t connector_id, void **info)

And the obtained info can be freed via:

herr_t H5VLfree_connector_info(hid_t connector_id, void *vol_info)
H5VLfree_connector_info
herr_t H5VLfree_connector_info(hid_t connector_id, void *vol_info)

Most users will not need this functionality as they will be using either connector- specific setup calls which will handle registering and configuring the connector for them or they will be using the environment variable (see below).

### Environment Variable

The HDF5 library allows specifying a default VOL connector via an environment variable: HDF5_VOL_CONNECTOR. The value of this environment variable should be set to ”vol connector name <parameters>”.

This will perform the equivalent of:

1. H5VLregister_connector_by_name() using the specified connector name
2. H5VLconnector_str_to_info() using the specified parameters. This will go through the connector we got from the previous step and should return a VOL info struct from the parameter string in the environment variable.
3. H5Pset_vol() on the default fapl using the obtained ID and info.

The environment variable is parsed once, at library startup. Since the environment variable scheme just changes the default connector, it can be overridden by subsequent calls to H5Pset_vol(). The <parameters> is optional, so for connectors which do not require any special configuration parameters you can just set the environment variable to the name.

NOTE: Implementing the environment variable in this way means that setting the native VOL connector becomes somewhat awkward as there is no explicit HDF5 API call to do this. Instead you will need to get the native VOL connector's ID via H5VLget_connector_id_by_value(H5_VOL_NATIVE) and set it manually in the fapl using H5Pset_vol().

## Adapting HDF5 Software to Use the VOL

The VOL was engineered to be as unobtrusive as possible and, when a connector which implements most/all of the data model functionality is in use, many applications will require little, if any, modification. As mentioned in the quick start section, most modifications will probably consist of connector setup code (which can usually be accomplished via the environment variable), adapting code to use the new token-based API calls, and protecting native-VOL-connector-specific functions.

### haddr_t → H5O_token_t

Some HDF5 API calls and data structures refer to addresses in the HDF5 using the haddr_t type. Unfortunately, the concept of an ”address” will make no sense for many connectors, though they may still have some sort of location key (e.g.: a key in a key-value pair store).

As a part of the VOL work, the HDF5 API was updated to replace the haddr_t type with a new H5O_token_t type that represents a more generic object location. These tokens appear as an opaque byte array of H5O_MAX_TOKEN_SIZE bytes that is only meaningful for a particular VOL connector. They are not intended for interpretation outside of a VOL connector, though a connector author may provide an API call to convert their tokens to something meaningful for the storage.

typedef struct H5O_token_t {
uint8_t __data[H5O_MAX_TOKEN_SIZE];
} H5O_token_t;
H5O_MAX_TOKEN_SIZE
#define H5O_MAX_TOKEN_SIZE
Definition H5public.h:428
H5O_token_t
Definition H5public.h:437
H5O_token_t::__data
uint8_t __data[(16)]
Definition H5public.h:438

As an example, in the native VOL connector, the token stores an haddr_t address and addresses can be converted to and from tokens using H5VLnative_addr_to_token() and H5VLnative_token_to_addr().

herr_t H5VLnative_addr_to_token(hid_t loc_id, haddr_t addr, H5O_token_t *token)
herr_t H5VLnative_token_to_addr(hid_t loc_id, H5O_token_t token, haddr_t *addr)
haddr_t
uint64_t haddr_t
Definition H5public.h:355
H5VLnative_addr_to_token
herr_t H5VLnative_addr_to_token(hid_t loc_id, haddr_t addr, H5O_token_t *token)
Convert a haddr_t address to a native VOL connector token.
H5VLnative_token_to_addr
herr_t H5VLnative_token_to_addr(hid_t loc_id, H5O_token_t token, haddr_t *addr)
Convert a native VOL connector token to a haddr_t address.

Several API calls have also been added to compare tokens and convert tokens to and from strings.

herr_t H5Otoken_cmp(hid_t loc_id, const H5O_token_t *token1, const H5O_token_t *token2,
int *cmp_value)
herr_t H5Otoken_to_str(hid_t loc_id, const H5O_token_t *token, char **token_str)
herr_t H5Otoken_from_str(hid_t loc_id, const char *token_str, H5O_token_t *token)
H5Otoken_to_str
herr_t H5Otoken_to_str(hid_t loc_id, const H5O_token_t *token, char **token_str)
Serializes a connector's object token into a string.
H5Otoken_from_str
herr_t H5Otoken_from_str(hid_t loc_id, const char *token_str, H5O_token_t *token)
Deserializes a string into a connector object token.
H5Otoken_cmp
herr_t H5Otoken_cmp(hid_t loc_id, const H5O_token_t *token1, const H5O_token_t *token2, int *cmp_value)
Compares two VOL connector object tokens.

### Specific API Call Substitutions

#### H5Fis_hdf5() → H5Fis_accessible()

H5Fis_hdf5() does not take a file access property list (fapl). As this is where the VOL connector is specified, this call cannot be used with arbitrary connectors. As a VOL-enabled replacement, H5Fis_accessible() has been added to the library. It has the same semantics as H5Fis_hdf5(), but takes a fapl so it can work with any VOL connector.

Note that, at this time, H5Fis_hdf5() always uses the native VOL connector, regardless of the settings of environment variables, etc.

htri_t H5Fis_accessible(const char *container_name, hid_t fapl_id)
htri_t
int htri_t
Definition H5public.h:282
H5Fis_accessible
htri_t H5Fis_accessible(const char *container_name, hid_t fapl_id)
Checks if a file can be opened with a given file access property list.

Note also that an error is only returned if it is not possible to determine if a file (or container) is accessible. It is not an error to return 'false' for whether the file is accessible.

#### H5Oget_info[1|2]() → H5Oget_info3() and H5Oget_native_info()

The H5Oget_info1() and H5Oget_info2() family of HDF5 API calls are often used by user code to obtain information about an object in the file, however these calls returned a struct which contained native information and are thus unsuitable for use with arbitrary VOL connectors.

A new H5Oget_info3() family of API calls has been added to the library which only return data model information via a new H5O_info2_t struct. This struct also returns H5O_token_t tokens in place of haddr_t addresses.

H5Oget_info3(hid_t loc_id, H5O_info2_t *oinfo, unsigned fields)
herr_t H5Oget_info_by_name3(hid_t loc_id, const char *name, H5O_info2_t *oinfo,
unsigned fields, hid_t lapl_id)
herr_t H5Oget_info_by_idx3(hid_t loc_id, const char *group_name, H5_index_t idx_type,
H5_iter_order_t order, hsize_t n, H5O_info2_t *oinfo,
unsigned fields, hid_t lapl_id)
H5_iter_order_t
H5_iter_order_t
Definition H5public.h:379
hsize_t
uint64_t hsize_t
Definition H5public.h:304
H5_index_t
H5_index_t
Definition H5public.h:402
H5Oget_info_by_name3
herr_t H5Oget_info_by_name3(hid_t loc_id, const char *name, H5O_info2_t *oinfo, unsigned fields, hid_t lapl_id)
Retrieves the metadata for an object, identifying the object by location and relative name.
H5Oget_info3
herr_t H5Oget_info3(hid_t loc_id, H5O_info2_t *oinfo, unsigned fields)
Retrieves the metadata for an object specified by an identifier.
H5Oget_info_by_idx3
herr_t H5Oget_info_by_idx3(hid_t loc_id, const char *group_name, H5_index_t idx_type, H5_iter_order_t order, hsize_t n, H5O_info2_t *oinfo, unsigned fields, hid_t lapl_id)
Retrieves the metadata for an object, identifying the object by an index position.
H5O_info2_t
Definition H5Opublic.h:145
typedef struct H5O_info2_t {
unsigned long fileno; // File number that object is located in
H5O_token_t token; // Token representing the object
H5O_type_t type; // Basic object type (group, dataset, etc.)
unsigned rc; // Reference count of object
time_t atime; // Access time
time_t mtime; // Modification time
time_t ctime; // Change time
time_t btime; // Birth time
hsize_t num_attrs; // # of attributes attached to object
} H5O_info2_t;
H5O_type_t
H5O_type_t
Definition H5Opublic.h:107
H5O_info2_t::rc
unsigned rc
Definition H5Opublic.h:150
H5O_info2_t::ctime
time_t ctime
Definition H5Opublic.h:153
H5O_info2_t::mtime
time_t mtime
Definition H5Opublic.h:152
H5O_info2_t::type
H5O_type_t type
Definition H5Opublic.h:149
H5O_info2_t::atime
time_t atime
Definition H5Opublic.h:151
H5O_info2_t::fileno
unsigned long fileno
Definition H5Opublic.h:147
H5O_info2_t::num_attrs
hsize_t num_attrs
Definition H5Opublic.h:155
H5O_info2_t::btime
time_t btime
Definition H5Opublic.h:154
H5O_info2_t::token
H5O_token_t token
Definition H5Opublic.h:148

To return the native file format information, H5Oget_native_info() calls have been added which can return such data separate from the data model data.

herr_t H5Oget_native_info(hid_t loc_id, H5O_native_info_t *oinfo, unsigned fields)
herr_t H5Oget_native_info_by_name(hid_t loc_id, const char *name, H5O_native_info_t *oinfo,
unsigned fields, hid_t lapl_id)
herr_t H5Oget_native_info_by_idx(hid_t loc_id, const char *group_name, H5_index_t idx_type,
H5_iter_order_t order, hsize_t n, H5O_native_info_t *oinfo,
unsigned fields, hid_t lapl_id)
H5Oget_native_info_by_name
herr_t H5Oget_native_info_by_name(hid_t loc_id, const char *name, H5O_native_info_t *oinfo, unsigned fields, hid_t lapl_id)
Retrieve native file format information about an object given its name.
H5Oget_native_info
herr_t H5Oget_native_info(hid_t loc_id, H5O_native_info_t *oinfo, unsigned fields)
Retrieve native file format information about an object.
H5Oget_native_info_by_idx
herr_t H5Oget_native_info_by_idx(hid_t loc_id, const char *group_name, H5_index_t idx_type, H5_iter_order_t order, hsize_t n, H5O_native_info_t *oinfo, unsigned fields, hid_t lapl_id)
Retrieve native file format information about an object according to the order of an index.
H5O_native_info_t
Definition H5Opublic.h:164
typedef struct H5O_native_info_t {
H5O_hdr_info_t hdr; // Object header information
// Extra metadata storage for obj & attributes
struct {
H5_ih_info_t obj; // v1/v2 B-tree & local/fractal heap for groups,
// B-tree for chunked datasets
H5_ih_info_t attr; // v2 B-tree & heap for attributes
} meta_size;
} H5O_native_info_t;
H5_ih_info_t
Definition H5public.h:414
H5O_hdr_info_t
Definition H5Opublic.h:122
H5O_native_info_t::hdr
H5O_hdr_info_t hdr
Definition H5Opublic.h:165
H5O_native_info_t::meta_size
struct H5O_native_info_t::@260162027145300125277002203123231341201164141000 meta_size
H5O_native_info_t::attr
H5_ih_info_t attr
Definition H5Opublic.h:168
H5O_native_info_t::obj
H5_ih_info_t obj
Definition H5Opublic.h:167

#### H5Ovisit[1|2]() → H5Ovisit3()

The callback used in the H5Ovisit() family of API calls took an H5O info t struct parameter. As in H5Oget_info(), this both commingled data model and native file format information and also used native HDF5 file addresses.

New H5Ovisit3() API calls have been created which use the token-based, data-model-only H5O_info_t struct in the callback.

herr_t H5Ovisit3(hid_t obj_id, H5_index_t idx_type, H5_iter_order_t order, H5O_iterate2_t op,
void *op_data, unsigned fields)
herr_t H5Ovisit_by_name3(hid_t loc_id, const char *obj_name, H5_index_t idx_type,
H5_iter_order_t order, H5O_iterate2_t op, void *op_data,
unsigned fields, hid_t lapl_id)
H5O_iterate2_t
herr_t(* H5O_iterate2_t)(hid_t obj, const char *name, const H5O_info2_t *info, void *op_data)
Definition H5Opublic.h:195
H5Ovisit_by_name3
herr_t H5Ovisit_by_name3(hid_t loc_id, const char *obj_name, H5_index_t idx_type, H5_iter_order_t order, H5O_iterate2_t op, void *op_data, unsigned fields, hid_t lapl_id)
Recursively visits all objects accessible from a specified object.
H5Ovisit3
herr_t H5Ovisit3(hid_t obj_id, H5_index_t idx_type, H5_iter_order_t order, H5O_iterate2_t op, void *op_data, unsigned fields)
Recursively visits all objects accessible from a specified object.
typedef herr_t (*H5O_iterate2_t)(hid_t obj, const char *name, const H5O_info2_t *info, void *op_data)

#### H5Lget_info() → H5Lget_info2()

The H5Lget_info() API calls were updated to use tokens instead of addresses in the H5L_info_t struct.

herr_t H5Lget_info2(hid_t loc_id, const char *name, H5L_info2_t *linfo, hid_t lapl_id)
herr_t H5Lget_info_by_idx2(hid_t loc_id, const char *group_name, H5_index_t idx_type,
H5_iter_order_t order, hsize_t n, H5L_info2_t *linfo, hid_t lapl_id)
H5Lget_info2
herr_t H5Lget_info2(hid_t loc_id, const char *name, H5L_info2_t *linfo, hid_t lapl_id)
Returns information about a link.
H5Lget_info_by_idx2
herr_t H5Lget_info_by_idx2(hid_t loc_id, const char *group_name, H5_index_t idx_type, H5_iter_order_t order, hsize_t n, H5L_info2_t *linfo, hid_t lapl_id)
Retrieves metadata for a link in a group, according to the order within a field or index.
H5L_info2_t
Information struct for links.
Definition H5Lpublic.h:76
typedef struct {
H5L_type_t type; // Type of link
bool corder_valid; // Indicate if creation order is valid
int64_t corder; // Creation order
H5T_cset_t cset; // Character set of link name
union {
H5O_token_t token; // Token of location that hard link points to
size_t val_size; // Size of a soft link or UD link value
} u;
} H5L_info2_t;
H5L_type_t
H5L_type_t
Link class types.
Definition H5Lpublic.h:57
H5T_cset_t
H5T_cset_t
Definition H5Tpublic.h:94

#### H5Literate() and H5Lvisit() → H5Literate2() and H5Lvisit2()

The callback used in these API calls used the old H5L_info_t struct, which used addresses instead of tokens. These callbacks were versioned in the C library and now take modified H5L_iterate2_t callbacks which use the new token-based info structs.

herr_t H5Literate2(hid_t grp_id, H5_index_t idx_type, H5_iter_order_t order, hsize_t *idx,
H5L_iterate2_t op, void *op_data)
herr_t H5Literate_by_name2(hid_t loc_id, const char *group_name, H5_index_t idx_type,
H5_iter_order_t order, hsize_t *idx, H5L_iterate2_t op,
void *op_data, hid_t lapl_id)
herr_t H5Lvisit2(hid_t grp_id, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate2_t op,
void *op_data)
herr_t H5Lvisit_by_name2(hid_t loc_id, const char *group_name, H5_index_t idx_type,
H5_iter_order_t order, H5L_iterate2_t op, void *op_data, hid_t lapl_id)
H5L_iterate2_t
herr_t(* H5L_iterate2_t)(hid_t group, const char *name, const H5L_info2_t *info, void *op_data)
Prototype for H5Literate2(), H5Literate_by_name2() operator.
Definition H5Lpublic.h:94
H5Literate_by_name2
herr_t H5Literate_by_name2(hid_t loc_id, const char *group_name, H5_index_t idx_type, H5_iter_order_t order, hsize_t *idx, H5L_iterate2_t op, void *op_data, hid_t lapl_id)
Iterates through links in a group.
H5Literate2
herr_t H5Literate2(hid_t grp_id, H5_index_t idx_type, H5_iter_order_t order, hsize_t *idx, H5L_iterate2_t op, void *op_data)
Iterates over links in a group, with user callback routine, according to the order within an index.
H5Lvisit2
herr_t H5Lvisit2(hid_t grp_id, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate2_t op, void *op_data)
Recursively visits all links starting from a specified group.
H5Lvisit_by_name2
herr_t H5Lvisit_by_name2(hid_t loc_id, const char *group_name, H5_index_t idx_type, H5_iter_order_t order, H5L_iterate2_t op, void *op_data, hid_t lapl_id)
Recursively visits all links starting from a specified group.
typedef herr_t (*H5L_iterate2_t)(hid_t group, const char *name, const H5L_info2_t *info,
void *op_data);

#### H5Oopen by addr() → H5Oopen by token()

The new H5Oopen_by_token() API call can be used to open objects by the tokens that are returned by the various ”get info”, et al. API calls.

hid_t H5Oopen_by_token(hid_t loc_id, H5O_token_t token)
H5Oopen_by_token
hid_t H5Oopen_by_token(hid_t loc_id, H5O_token_t token)
Opens an object in an HDF5 file using its VOL independent token.

### Protect Native-Only API Calls

In HDF5 1.14.0, a way to determine support for optional calls has been added.

herr_t H5VLquery_optional(hid_t obj_id, H5VL_subclass_t subcls, int opt_type, uint64_t *flags)
H5VL_subclass_t
H5VL_subclass_t
Definition H5VLpublic.h:183
H5VLquery_optional
herr_t H5VLquery_optional(hid_t obj_id, H5VL_subclass_t subcls, int opt_type, uint64_t *flags)
Determine if a VOL connector supports a particular optional callback operation.

The call takes an object that is VOL managed (i.e.; file, group, dataset, attribute, object, committed datatype), the VOL subclass (an enum documented in H5VLpublic.h), an operation ”type” (discussed below), and an out parameter for the bitwise capabilities flags (also discussed below). Code that needs to protect a VOL-specific API call can call the function to see if the API call is supported, which will be reported via the flags. Specifically, if the H5VL_OPT_QUERY_SUPPORTED bit is set, the feature is supported. The other flags are more useful for VOL connector authors than end users.

In the case of the native VOL connector, the opt type operations are documented in H5VLnative.h. The current list of native operations is given at the end of this document, along with a list of native-only connector calls.

## Language Wrappers

Due to the parameter type and callback changes that were required in the C library API regarding the update from haddr_t addresses to H5O_token_t tokens and the difficulty in versioning the wrapper APIs, it was decided to update all of the wrappers to use tokens instead of addresses. This will allow the language wrappers to make use of the VOL, but at the expense of backward compatibility.

Information on the C API changes can be found above.

Affected API calls, by language:

### C++

- The visit_operator_t callback now uses a H5O_info2_t parameter instead of H5O_info1_t so the callback can be passed to H5Ovisit3() internally. This affects the H5Object::visit() method.
- The H5Location::getObjinfo() methods now take H5O_info2_t parameters.
- The H5Location::getLinkInfo() methods now return H5L_info2_t structs.
- H5File::isHdf5 uses H5Fis_accessible(), though it always passes H5P_DEFAULT as the fapl. It will only work with arbitrary VOL connectors if the default VOL connector is changed via the environment variable.

The C++ wrappers do not allow opening HDF5 file objects by address or token.

The public H5VL API calls found in H5VLpublic.h were NOT added to the C++ API.

### Fortran

As in the C API, these API calls had their structs updated to the token version so the h5o_info_t, etc. structs no longer contain native file format information and the callbacks will need to match the non-deprecated, token-enabled versions.

- h5lget_info_f
- h5lget_info_by_idx_f
- h5literate_f
- h5literate_by_name_f
- h5oget_info_f
- h5oget_info_by_idx_f
- h5oget_info_by_name_f
- h5oopen_by_token_f
- h5ovisit_f
- h5ovisit_by_name_f

Additionally, h5fis_hdf5_f was updated to use H5Fis_accessible internally, though with the same caveat as the C++ implementation: the default fapl is always passed in so arbitrary VOL connectors will only work if the default VOL connector is changed via the environment variable.

The public H5VL API calls found in H5VLpublic.h were also added to the Fortran wrappers.

### Java/JNI

- H5Fis_hdf5 Will fail when the library is built without deprecated symbols.
- H5Fis_accessible is available and takes a fapl, allowing it to work with arbitrary VOL connectors.
- The H5(O|L)get_info, H5(O|L)visit, and H5Literate calls were updated as in the C library.
- H5Oget_native_info_by_name et al. were added and they work as in the C library (e.g.: essentially native VOL connector only).
- H5Oopen_by_addr was replaced with H5Oopen_by_token.
- The public API calls in H5VLpublic.h were added to the JNI.

## Using VOL Connectors With The HDF5 Command-Line Tools

The following command-line tools are VOL-aware and can be used with arbitrary VOL connectors:

- (p)h5diff
- h5dump
- h5ls
- h5mkgrp
- h5repack

The VOL connector can be set either using the HDF5_VOL_CONNECTOR environment variable (see above) or via the command line. Each of the above tools takes command-line options to set the VOL connector by name or value and the VOL connector string, usually in the form of

--vol-(name|value|info)

See the individual tool's help for the options specific to that tool.

## Compatibility

### List of HDF5 Native VOL API Calls

These API calls will probably fail when used with terminal VOL connectors other than the native HDF5 file format connector. Their use should be protected in code that uses arbitrary VOL connectors. Note that some connectors may, in fact, implement some of this functionality as it is possible to mimic the native HDF5 connector, however this will probably not be true for most non-native VOL connectors.

Alphabetical list of HDF5 API calls specific to the native VOL connector
API | Description
--- | ---
H5Aget_num_attrs | deprecated
H5Aiterate1 | deprecated
H5Dchunk_iter |
H5Ddebug | Internal API routines
H5Dformat_convert | Internal API routines
H5Dget_chunk_index_type | Internal API routines
H5Dget_chunk_info |
H5Dget_chunk_info_by_coord |
H5Dget_chunk_storage_size |
H5Dget_num_chunks |
H5Dget_offset |
H5Dread_chunk1 |
H5Dread_chunk2 |
H5Dwrite_chunk |
H5FD* |
H5Fclear_elink_file_cache |
H5Fformat_convert | Internal API routines
H5Fget_dset_no_attrs_hint |
H5Fget_eoa |
H5Fget_file_image |
H5Fget_filesize |
H5Fget_free_sections |
H5Fget_freespace |
H5Fget_info1 | deprecated
H5Fget_info2 |
H5Fget_mdc_config |
H5Fget_mdc_hit_rate |
H5Fget_mdc_image_info |
H5Fget_mdc_logging_status |
H5Fget_mdc_size |
H5Fget_metadata_read_retry_info |
H5Fget_mpi_atomicity |
H5Fget_page_buffering_stats |
H5Fget_vfd_handle |
H5Fincrement_filesize |
H5Fis_hdf5 | deprecated
H5Freset_mdc_hit_rate_stats |
H5Freset_page_buffering_stats |
H5Fset_dset_no_attrs_hint |
H5Fset_latest_format | deprecated
H5Fset_libver_bounds |
H5Fset_mdc_config |
H5Fset_mpi_atomicity |
H5Fstart_mdc_logging |
H5Fstart_swmr_write |
H5Fstop_mdc_logging |
H5Gget_comment | deprecated
H5Giterate | deprecated
H5Gget_info |
H5Gget_info_by_name |
H5Gget_info_by_idx |
H5Gget_objinfo | deprecated
H5Gget_objname_by_idx | deprecated
H5Gget_objtype_by_idx | deprecated
H5Gset_comment | deprecated
H5Lget_info1 | deprecated
H5Lget_info_by_idx1 | deprecated
H5Literate1 | deprecated
H5Literate_by_name1 | deprecated
H5Lvisit1 | deprecated
H5Lvisit_by_name1 | deprecated
H5Oare_mdc_flushes_disabled |
H5Odisable_mdc_flushes |
H5Oenable_mdc_flushes |
H5Oget_comment |
H5Oget_comment_by_name |
H5Oget_info_by_idx1 | deprecated
H5Oget_info_by_idx2 | deprecated
H5Oget_info_by_name1 | deprecated
H5Oget_info_by_name2 | deprecated
H5Oget_info1 | deprecated
H5Oget_info2 | deprecated
H5Oget_native_info |
H5Oget_native_info_by_idx |
H5Oget_native_info_by_name |
H5Oopen_by_addr | deprecated
H5Oset_comment |
H5Oset_comment_by_name |
H5Ovisit1 | deprecated
H5Ovisit_by_name1 | deprecated
H5Ovisit2 | deprecated
H5Ovisit_by_name2 | deprecated

### List of HDF5 VOL-Independent API Calls

These HDF5 API calls do not depend on a particular VOL connector being loaded.

Alphabetical list of VOL-independent HDF5 API calls
API | Description
--- | ---
H5* |
H5Dfill |
H5Dgather |
H5Diterate |
H5Dscatter |
H5Dvlen_reclaim | deprecated
H5Dvlen_get_buf_size |
H5E* |
H5I* |
H5Lis_registered |
H5Lregister |
H5Lunpack_elink_val |
H5Lunregister |
H5PL* |
H5P* |
H5S* |
H5T* | non-committed
H5VL* |
H5Z* |

### List of Native VOL Optional Operation Values By Subclass

These values can be passed to the opt type parameter of H5VLquery optional().

List of Native VOL Optional Operation Values By Subclass
Subclass | API Reference | Definition
--- | --- | ---
H5VL_SUBCLS_ATTR | H5Aiterate (deprecated routine) | H5VL_NATIVE_ATTR_ITERATE_OLD
H5VL_SUBCLS_DATASET | H5Dformat_convert (internal) | H5VL_NATIVE_DATASET_FORMAT_CONVERT
H5Dget_chunk_index_type (internal) | H5VL_NATIVE_DATASET_GET_CHUNK_INDEX_TYPE
H5Dget_chunk_storage_size | H5VL_NATIVE_DATASET_GET_CHUNK_STORAGE_SIZE
H5Dget_num_chunks | H5VL_NATIVE_DATASET_GET_NUM_CHUNKS
H5Dget_chunk_info | H5VL_NATIVE_DATASET_GET_CHUNK_INFO_BY_IDX
H5Dget_chunk_info_by_coord | H5VL_NATIVE_DATASET_GET_CHUNK_INFO_BY_COORD
H5Dread_chunk1 / H5Dread_chunk2 | H5VL_NATIVE_DATASET_CHUNK_READ
H5Dwrite_chunk | H5VL_NATIVE_DATASET_CHUNK_WRITE
H5Dvlen_get_buf_size | H5VL_NATIVE_DATASET_GET_VLEN_BUF_SIZE
H5Dget_offset | H5VL_NATIVE_DATASET_GET_OFFSET
H5Dget_offset | H5VL_NATIVE_DATASET_CHUNK_ITER
H5VL_SUBCLS_FILE | H5Fclear_elink_file_cache | H5VL_NATIVE_FILE_CLEAR_ELINK_CACHE
H5Fget_file_image | H5VL_NATIVE_FILE_GET_FILE_IMAGE
H5Fget_free_sections | H5VL_NATIVE_FILE_GET_FREE_SECTIONS
H5Fget_freespace | H5VL_NATIVE_FILE_GET_FREE_SPACE
H5Fget_info1 / H5Fget_info2 | H5VL_NATIVE_FILE_GET_INFO
H5Fget_mdc_config | H5VL_NATIVE_FILE_GET_MDC_CONF
H5Fget_mdc_hit_rate | H5VL_NATIVE_FILE_GET_MDC_HR
H5Fget_mdc_size | H5VL_NATIVE_FILE_GET_MDC_SIZE
H5Fget_filesize | H5VL_NATIVE_FILE_GET_SIZE
H5Fget_vfd_handle | H5VL_NATIVE_FILE_GET_VFD_HANDLE
H5Freset_mdc_hit_rate_stats | H5VL_NATIVE_FILE_RESET_MDC_HIT_RATE
H5Fset_mdc_config | H5VL_NATIVE_FILE_SET_MDC_CONFIG
H5Fget_metadata_read_retry_info | H5VL_NATIVE_FILE_GET_METADATA_READ_RETRY_INFO
H5Fstart_swmr_write | H5VL_NATIVE_FILE_START_SWMR_WRITE
H5Fstart_mdc_logging | H5VL_NATIVE_FILE_START_MDC_LOGGING
H5Fstop_mdc_logging | H5VL_NATIVE_FILE_STOP_MDC_LOGGING
H5Fget_mdc_logging_status | H5VL_NATIVE_FILE_GET_MDC_LOGGING_STATUS
H5Fformat_convert (internal) | H5VL_NATIVE_FILE_FORMAT_CONVERT
H5Freset_page_buffering_stats | H5VL_NATIVE_FILE_RESET_PAGE_BUFFERING_STATS
H5Fget_page_buffering_stats | H5VL_NATIVE_FILE_GET_PAGE_BUFFERING_STATS
H5Fget_mdc_image_info | H5VL_NATIVE_FILE_GET_MDC_IMAGE_INFO
H5Fget_eoa | H5VL_NATIVE_FILE_GET_EOA
H5Fincrement_filesize | H5VL_NATIVE_FILE_INCR_FILESIZE
H5Fset_latest_format/H5Fset_libver_bounds | H5VL_NATIVE_FILE_SET_LIBVER_BOUNDS
H5Fget_dset_no_attrs_hint | H5VL_NATIVE_FILE_GET_MIN_DSET_OHDR_FLAG
H5Fset_dset_no_attrs_hint | H5VL_NATIVE_FILE_SET_MIN_DSET_OHDR_FLAG
H5Fget_mpi_atomicity | H5VL_NATIVE_FILE_GET_MPI_ATOMICITY
H5Fset_mpi_atomicity | H5VL_NATIVE_FILE_SET_MPI_ATOMICITY
Adjust file after open, with wrapping context | H5VL_NATIVE_FILE_POST_OPEN
H5VL_SUBCLS_GROUP | H5Giterate (deprecated routine) | H5VL_NATIVE_GROUP_ITERATE_OLD
H5Gget_objinfo (deprecated routine) | H5VL_NATIVE_GROUP_GET_OBJINFO
H5VL_SUBCLS_OBJECT | H5Gget_comment, H5Oget_comment, H5Oget_comment_by_name | H5VL_NATIVE_OBJECT_GET_COMMENT
H5Gset_comment, H5Oset_comment, H5Oset_comment_by_name | H5VL_NATIVE_OBJECT_SET_COMMENT
H5Odisable_mdc_flushes | H5VL_NATIVE_OBJECT_DISABLE_MDC_FLUSHES
H5Oenable_mdc_flushes | H5VL_NATIVE_OBJECT_ENABLE_MDC_FLUSHES
H5Oare_mdc_flushes_disabled | H5VL_NATIVE_OBJECT_ARE_MDC_FLUSHES_DISABLED
H5Oget_native_info, H5Oget_native_info_by_idx, H5Oget_native_info_by_name | H5VL_NATIVE_OBJECT_GET_NATIVE_INFO/td>

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