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| HDF5 Last Updated on 2026-07-28 The HDF5 Field Guide

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Identifiers (H5I)

## Detailed Description

Use the functions in this module to manage identifiers defined by the HDF5 library. See User-defined ID Types for user-defined identifiers and identifier types.

HDF5 identifiers are usually created as a side-effect of creating HDF5 entities such as groups, datasets, attributes, or property lists.

Identifiers defined by the HDF5 library can be used to retrieve information such as path names and reference counts, and their validity can be checked.

Identifiers can be updated by manipulating their reference counts.

Unused identifiers should be reclaimed by closing the associated item, e.g., HDF5 object, or decrementing the reference count to 0.

Note
  Identifiers (of type hid_t) are run-time auxiliaries and not persisted in the file.

## Topics
| User-defined ID Types

## Functions
int | H5Idec_ref (hid_t id)
| Decrements the reference count for an object.
hid_t | H5Iget_file_id (hid_t id)
| Retrieves an identifier for the file containing the specified object.
ssize_t | H5Iget_name (hid_t id, char *name, size_t size)
| Retrieves a name of an object based on the object identifier.
int | H5Iget_ref (hid_t id)
| Retrieves the reference count for an object.
H5I_type_t | H5Iget_type (hid_t id)
| Retrieves the type of an object.
int | H5Iinc_ref (hid_t id)
| Increments the reference count for an object.
htri_t | H5Iis_valid (hid_t id)
| Determines whether an identifier is valid.
hid_t | H5Iregister_future (H5I_type_t type, const void *object, H5I_future_realize_func_t realize_cb, H5I_future_discard_func_t discard_cb)
| Registers a "future" object under a type and returns an ID for it.

## Function Documentation

## ◆ H5Idec_ref()

int H5Idec_ref | ( | hid_t | id | ) |

Decrements the reference count for an object.

Parameters

  [in] | id | Object identifier

Returns
  Returns a non-negative reference count of the object ID after decrementing it, if successful; otherwise a negative value is returned.

H5Idec_ref() decrements the reference count of the object identified by `id`.

The reference count for an object ID is attached to the information about an object in memory and has no relation to the number of links to an object on disk.

The reference count for a newly created object will be 1. Reference counts for objects may be explicitly modified with this function or with H5Iinc_ref(). When an object identifier's reference count reaches zero, the object will be closed. Calling an object identifier's `close` function decrements the reference count for the identifier, which normally closes the object, but if the reference count for the identifier has been incremented with H5Iinc_ref(), the object will only be closed when the reference count reaches zero with further calls to this function or the object identifier's `close` function.

If the object ID was created by a collective parallel call (such as H5Dcreate(), H5Gopen(), etc.), the reference count should be modified by all the processes which have copies of the ID. Generally, this means that group, dataset, attribute, file and named datatype IDs should be modified by all the processes and that all other types of IDs are safe to modify by individual processes.

This function is of particular value when an application maintains multiple copies of an object ID. The object ID can be incremented when a copy is made. Each copy of the ID can then be safely closed or decremented and the HDF5 object will be closed when the reference count for that object drops to zero.

Since
  1.6.3

## ◆ H5Iget_file_id()

hid_t H5Iget_file_id | ( | hid_t | id | ) |

Retrieves an identifier for the file containing the specified object.

Parameters

  [in] | id | Object identifier

Returns
  Returns a file identifier if successful; otherwise returns H5I_INVALID_HID.

H5Iget_file_id() returns the identifier of the file associated with the object referenced by `id`.

Note
  Note that the HDF5 library permits an application to close a file while objects within the file remain open. If the file containing the object `id` is still open, H5Iget_file_id() will retrieve the existing file identifier. If there is no existing file identifier for the file, i.e., the file has been closed, H5Iget_file_id() will reopen the file and return a new file identifier. In either case, the file identifier must eventually be released using H5Fclose().

Since
  1.6.3

## ◆ H5Iget_name()

ssize_t H5Iget_name | ( | hid_t | id,
|  | char * | name,
|  | size_t | size )

Retrieves a name of an object based on the object identifier.

Parameters

  [in] | id | Object identifier
  [out] | name | A buffer for the name associated with the identifier
  [in] | size | The size, in bytes, of the `name` buffer. Must be the size of the object name in bytes plus 1 for a NULL terminator

Returns
  ssize_t

H5Iget_name() retrieves a name for the object identified by `id`.

Up to size characters of the name are returned in `name`; additional characters, if any, are not returned to the user application.

Up to `size` characters of the object name are returned in `name`; additional characters, if any, are not returned to the user application.
If the length of the object name, which determines the required value of `size`, is unknown, a preliminary call to H5Iget_name() with the last two parameters set to NULL and zero respectively can be made. The return value of this call will be the size in bytes of the object name. That value, plus 1 for a NULL terminator, must then be assigned to `size` for a second H5Iget_name() call, which will retrieve the actual object name.

If the object identified by `id` is an attribute, as determined via H5Iget_type(), H5Iget_name() retrieves the name of the object to which that attribute is attached. To retrieve the name of the attribute itself, use H5Aget_name().

If there is no name associated with the object identifier or if the name is NULL, H5Iget_name() returns 0 (zero).

Note
  Note that an object in an HDF5 file may have multiple paths if there are multiple links pointing to it. This function may return any one of these paths. When possible, H5Iget_name() returns the path with which the object was opened.

Since
  1.6.0

## ◆ H5Iget_ref()

int H5Iget_ref | ( | hid_t | id | ) |

Retrieves the reference count for an object.

Parameters

  [in] | id | Object identifier

Returns
  Returns a non-negative current reference count of the object identifier if successful; otherwise a negative value is returned.

H5Iget_ref() retrieves the reference count of the object identified by `id`.

The reference count for an object identifier is attached to the information about an object in memory and has no relation to the number of links to an object on disk.

The function H5Iis_valid() is used to determine whether a specific object identifier is valid.

Since
  1.6.3

## ◆ H5Iget_type()

H5I_type_t H5Iget_type | ( | hid_t | id | ) |

Retrieves the type of an object.

Parameters

  [in] | id | Object identifier

Returns
  Returns the object type if successful; otherwise H5I_BADID.

H5Iget_type() retrieves the type of the object identified by `id`. If no valid type can be determined or the identifier submitted is invalid, the function returns H5I_BADID.

This function is of particular use in determining the type of object closing function (H5Dclose(), H5Gclose(), etc.) to call after a call to H5Rdereference().

Note
  Note that this function returns only the type of object that `id` would identify if it were valid; it does not determine whether `id` is valid identifier. Validity can be determined with a call to H5Iis_valid().

Since
  1.0.0

## ◆ H5Iinc_ref()

int H5Iinc_ref | ( | hid_t | id | ) |

Increments the reference count for an object.

Parameters

  [in] | id | Object identifier

Returns
  Returns a non-negative reference count of the object ID after incrementing it if successful; otherwise a negative value is returned.

H5Iinc_ref() increments the reference count of the object identified by `id`.

The reference count for an object ID is attached to the information about an object in memory and has no relation to the number of links to an object on disk.

The reference count for a newly created object will be 1. Reference counts for objects may be explicitly modified with this function or with H5Idec_ref(). When an object ID's reference count reaches zero, the object will be closed. Calling an object ID's `close` function decrements the reference count for the ID which normally closes the object, but if the reference count for the ID has been incremented with this function, the object will only be closed when the reference count reaches zero with further calls to H5Idec_ref() or the object ID's `close` function.

If the object ID was created by a collective parallel call (such as H5Dcreate(), H5Gopen(), etc.), the reference count should be modified by all the processes which have copies of the ID. Generally this means that group, dataset, attribute, file and named datatype IDs should be modified by all the processes and that all other types of IDs are safe to modify by individual processes.

This function is of particular value when an application is maintaining multiple copies of an object ID. The object ID can be incremented when a copy is made. Each copy of the ID can then be safely closed or decremented and the HDF5 object will be closed when the reference count for that that object drops to zero.

Since
  1.6.3

## ◆ H5Iis_valid()

htri_t H5Iis_valid | ( | hid_t | id | ) |

Determines whether an identifier is valid.

Parameters

  [in] | id | Object identifier

Returns
  Returns zero (false), a positive (true) or a negative (failure) value.

H5Iis_valid() determines whether the identifier `id` is valid.

Valid identifiers are those that have been obtained by an application and can still be used to access the original target. Examples of invalid identifiers include:

- Out-of-range values: negative, for example
- Previously-valid identifiers that have been released: for example, a dataset identifier for which the dataset has been closed

H5Iis_valid() can be used with any type of identifier: object identifier, property list identifier, attribute identifier, error message identifier, etc. When necessary, a call to H5Iget_type() can determine the type of object that the `id` identifies.

Since
  1.8.3

## ◆ H5Iregister_future()

hid_t H5Iregister_future | ( | H5I_type_t | type,
|  | const void * | object,
|  | H5I_future_realize_func_t | realize_cb,
|  | H5I_future_discard_func_t | discard_cb )

Registers a "future" object under a type and returns an ID for it.

Parameters

  [in] | type | The identifier of the type of the new ID
  [in] | object | Pointer to "future" object for which a new ID is created
  [in] | realize_cb | Function pointer to realize a future object
  [in] | discard_cb | Function pointer to destroy a future object

Returns
  Returns a object identifier if successful; otherwise returns H5I_INVALID_HID.

H5Iregister_future() creates and returns a new ID for a "future" object. Future objects are a special kind of object and represent a placeholder for an object that has not yet been created or opened. The `realize_cb` will be invoked by the HDF5 library to 'realize' the future object as an actual object. A call to H5Iobject_verify() will invoke the `realize_cb` callback and if it successfully returns, will return the actual object, not the future object.

The `type` parameter is the identifier for the ID type to which this new future ID will belong. This identifier may have been created by a call to H5Iregister_type() or may be one of the HDF5 pre-defined ID classes (e.g. H5I_FILE, H5I_GROUP, H5I_DATASPACE, etc).

The `object` parameter is a pointer to the memory which the new ID will be a reference to. This pointer will be stored by the library, but will not be returned to a call to H5Iobject_verify() until the `realize_cb` callback has returned the actual pointer for the object.

A NULL value for `object` is allowed.

The `realize_cb` parameter is a function pointer that will be invoked by the HDF5 library to convert a future object into an actual object. The `realize_cb` function may be invoked by H5Iobject_verify() to return the actual object for a user-defined ID class (i.e. an ID class registered with H5Iregister_type()) or internally by the HDF5 library in order to use or get information from an HDF5 pre-defined ID type. For example, the `realize_cb` for a future dataspace object will be called during the process of returning information from H5Sget_simple_extent_dims().

Note that although the `realize_cb` routine returns an ID (as a parameter) for the actual object, the HDF5 library will swap the actual object in that ID for the future object in the future ID. This ensures that the ID value for the object doesn't change for the user when the object is realized.

Note that the `realize_cb` callback could receive a NULL value for a future object pointer, if one was used when H5Iregister_future() was initially called. This is permitted as a means of allowing the `realize_cb` to act as a generator of new objects, without requiring creation of unnecessary future objects.

It is an error to pass NULL for `realize_cb`.

The `discard_cb` parameter is a function pointer that will be invoked by the HDF5 library to destroy a future object. This callback will always be invoked for every future object, whether the `realize_cb` is invoked on it or not. It's possible that the `discard_cb` is invoked on a future object without the `realize_cb` being invoked, e.g. when a future ID is closed without requiring the future object to be realized into an actual one.

Note that the `discard_cb` callback could receive a NULL value for a future object pointer, if one was used when H5Iregister_future() was initially called.

It is an error to pass NULL for `discard_cb`.

Note
  The H5Iregister_future() function is primarily targeted at VOL connector authors and is not designed for general-purpose application use.

Since
  1.14.0