H5A: Attribute Interface

Attribute API Functions

The C Interfaces:

H5Acreate H5Awrite H5Aread H5Aclose

H5Aget_name H5Aopen_name H5Aopen_idx H5Aget_space

H5Aget_type H5Aget_num_attrs H5Aiterate H5Adelete

The FORTRAN90 Interfaces:
In general, each FORTRAN90 subroutine performs exactly the same task as the corresponding C function. The links below go to the C function descriptions, which serve as general descriptions for both. A button, under Non-C API(s) at the end of the C function description, opens an external browser window displaying the FORTRAN90-specific information. You will probably want to adjust the size and location of this external window so that both browser windows are visible and to facilitate moving easily between them.

h5acreate_f h5awrite_f h5aread_f h5aclose_f

h5aget_name_f h5aopen_name_f h5aopen_idx_f h5aget_space_f

h5aget_type_f h5aget_num_attrs_f h5adelete_f

The Attribute interface, H5A, is primarily designed to easily allow small datasets to be attached to primary datasets as metadata information. Additional goals for the H5A interface include keeping storage requirement for each attribute to a minimum and easily sharing attributes among datasets.

Because attributes are intended to be small objects, large datasets intended as additional information for a primary dataset should be stored as supplemental datasets in a group with the primary dataset. Attributes can then be attached to the group containing everything to indicate a particular type of dataset with supplemental datasets is located in the group. How small is "small" is not defined by the library and is up to the user's interpretation.

See Attributes in the HDF5 User's Guide for further information.

Name: H5Acreate

Signature:

hid_t H5Acreate(hid_t loc_id, const char *name, hid_t type_id, hid_t space_id, hid_t create_plist )

Purpose:

Creates a dataset as an attribute of another group, dataset, or named datatype.

Description:

H5Acreate creates an attribute named name and attached to the object specified with loc_id. loc_id is a group, dataset, or named datatype identifier.

The attribute name specified in name must be unique. Attempting to create an attribute with the same name as an already existing attribute will fail, leaving the pre-existing attribute in place. To overwrite an existing attribute with a new attribute of the same name, first call H5Adelete then recreate the attribute with H5Acreate.

The datatype and dataspace identifiers of the attribute, type_id and space_id, respectively, are created with the H5T and H5S interfaces, respectively.

Currently only simple dataspaces are allowed for attribute dataspaces.

The create_plist_id property list is currently unused; it will be used in the future for optional attribute properties.

The attribute identifier returned from this function must be released with H5Aclose or resource leaks will develop.

Parameters:

hid_t loc_id

IN: Object (dataset, group, or named datatype) to be attached to.

const char *name

IN: Name of attribute to create.

hid_t type_id

IN: Identifier of datatype for attribute.

hid_t space_id

IN: Identifier of dataspace for attribute.

hid_t create_plist

IN: Identifier of creation property list (currently not used).

Returns:

Returns an attribute identifier if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aopen_name

Signature:

hid_t H5Aopen_name(hid_t loc_id, const char *name )

Purpose:

Opens an attribute specified by name.

Description:

H5Aopen_name opens an attribute specified by its name, name, which is attached to the object specified with loc_id. The location object may be either a group, dataset, or named datatype, which may have any sort of attribute. The attribute identifier returned from this function must be released with H5Aclose or resource leaks will develop.

Parameters:

hid_t loc_id

IN: Identifier of a group, dataset, or named datatype atttribute to be attached to.

const char *name

IN: Attribute name.

Returns:

Returns attribute identifier if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aopen_idx

Signature:

hid_t H5Aopen_idx(hid_t loc_id, unsigned int idx )

Purpose:

Opens the attribute specified by its index.

Description:

H5Aopen_idx opens an attribute which is attached to the object specified with loc_id. The location object may be either a group, dataset, or named datatype, all of which may have any sort of attribute. The attribute specified by the index, idx, indicates the attribute to access. The value of idx is a 0-based, non-negative integer. The attribute identifier returned from this function must be released with H5Aclose or resource leaks will develop.

Parameters:

hid_t loc_id

IN: Identifier of the group, dataset, or named datatype attribute to be attached to.

unsigned int idx

IN: Index of the attribute to open.

Returns:

Returns attribute identifier if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Awrite

Signature:

herr_t H5Awrite(hid_t attr_id, hid_t mem_type_id, void *buf )

Purpose:

Writes data to an attribute.

Description:

H5Awrite writes an attribute, specified with attr_id. The attribute's memory datatype is specified with mem_type_id. The entire attribute is written from buf to the file.

Datatype conversion takes place at the time of a read or write and is automatic. See the Data Conversion section of The Data Type Interface (H5T) in the HDF5 User's Guide for a discussion of data conversion, including the range of conversions currently supported by the HDF5 libraries.

Parameters:

hid_t attr_id

IN: Identifier of an attribute to write.

hid_t mem_type_id

IN: Identifier of the attribute datatype (in memory).

void *buf

IN: Data to be written.

Returns:

Returns a non-negative value if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aread

Signature:

herr_t H5Aread(hid_t attr_id, hid_t mem_type_id, void *buf )

Purpose:

Reads an attribute.

Description:

H5Aread reads an attribute, specified with attr_id. The attribute's memory datatype is specified with mem_type_id. The entire attribute is read into buf from the file.

Datatype conversion takes place at the time of a read or write and is automatic. See the Data Conversion section of The Data Type Interface (H5T) in the HDF5 User's Guide for a discussion of data conversion, including the range of conversions currently supported by the HDF5 libraries.

Parameters:

hid_t attr_id

IN: Identifier of an attribute to read.

hid_t mem_type_id

IN: Identifier of the attribute datatype (in memory).

void *buf

OUT: Buffer for data to be read.

Returns:

Returns a non-negative value if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aget_space

Signature:

hid_t H5Aget_space(hid_t attr_id)

Purpose:

Gets a copy of the dataspace for an attribute.

Description:

H5Aget_space retrieves a copy of the dataspace for an attribute. The dataspace identifier returned from this function must be released with H5Sclose or resource leaks will develop.

Parameters:

hid_t attr_id

IN: Identifier of an attribute.

Returns:

Returns attribute dataspace identifier if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aget_type

Signature:

hid_t H5Aget_type(hid_t attr_id)

Purpose:

Gets an attribute datatype.

Description:

H5Aget_type retrieves a copy of the datatype for an attribute.

The datatype is reopened if it is a named type before returning it to the application. The datatypes returned by this function are always read-only. If an error occurs when atomizing the return datatype, then the datatype is closed.

The datatype identifier returned from this function must be released with H5Tclose or resource leaks will develop.

Parameters:

hid_t attr_id

IN: Identifier of an attribute.

Returns:

Returns a datatype identifier if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aget_name

Signature:

ssize_t H5Aget_name(hid_t attr_id, size_t buf_size, char *buf )

Purpose:

Gets an attribute name.

Description:

H5Aget_name retrieves the name of an attribute specified by the identifier, attr_id. Up to buf_size characters are stored in buf followed by a \0 string terminator. If the name of the attribute is longer than (buf_size -1), the string terminator is stored in the last position of the buffer to properly terminate the string.

Parameters:

hid_t attr_id

IN: Identifier of the attribute.

size_t buf_size

IN: The size of the buffer to store the name in.

char *buf

IN: Buffer to store name in.

Returns:

Returns the length of the attribute's name, which may be longer than buf_size, if successful. Otherwise returns a negative value.

Non-C API(s):

Name: H5Aget_num_attrs

Signature:

int H5Aget_num_attrs(hid_t loc_id)

Purpose:

Determines the number of attributes attached to an object.

Description:

H5Aget_num_attrs returns the number of attributes attached to the object specified by its identifier, loc_id. The object can be a group, dataset, or named datatype.

Parameters:

hid_t loc_id

IN: Identifier of a group, dataset, or named datatype.

Returns:

Returns the number of attributes if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aiterate

Signature:

herr_t H5Aiterate(hid_t loc_id, unsigned * idx, H5A_operator_t op, void *op_data )

Purpose:

Calls a user's function for each attribute on an object.

Description:

H5Aiterate iterates over the attributes of the object specified by its identifier, loc_id. The object can be a group, dataset, or named datatype. For each attribute of the object, the op_data and some additional information specified below are passed to the operator function op. The iteration begins with the attribute specified by its index, idx; the index for the next attribute to be processed by the operator, op, is returned in idx. If idx is the null pointer, then all attributes are processed.

The prototype for H5A_operator_t is:
typedef herr_t (*H5A_operator_t)(hid_t loc_id, const char *attr_name, void *operator_data);

The operation receives the identifier for the group, dataset or named datatype being iterated over, loc_id, the name of the current attribute about the object, attr_name, and the pointer to the operator data passed in to H5Aiterate, op_data. The return values from an operator are:

• Zero causes the iterator to continue, returning zero when all attributes have been processed.
• Positive causes the iterator to immediately return that positive value, indicating short-circuit success. The iterator can be restarted at the next attribute.
• Negative causes the iterator to immediately return that value, indicating failure. The iterator can be restarted at the next attribute.

Parameters:

hid_t loc_id

IN: Identifier of a group, dataset or named datatype.

unsigned * idx

IN/OUT: Starting (IN) and ending (OUT) attribute index.

H5A_operator_t op

IN: User's function to pass each attribute to

void *op_data

IN/OUT: User's data to pass through to iterator operator function

Returns:

If successful, returns the return value of the last operator if it was non-zero, or zero if all attributes were processed. Otherwise returns a negative value.

Name: H5Adelete

Signature:

herr_t H5Adelete(hid_t loc_id, const char *name )

Purpose:

Deletes an attribute from a location.

Description:

H5Adelete removes the attribute specified by its name, name, from a dataset, group, or named datatype. This function should not be used when attribute identifiers are open on loc_id as it may cause the internal indexes of the attributes to change and future writes to the open attributes to produce incorrect results.

Parameters:

hid_t loc_id

IN: Identifier of the dataset, group, or named datatype to have the attribute deleted from.

const char *name

IN: Name of the attribute to delete.

Returns:

Returns a non-negative value if successful; otherwise returns a negative value.

Non-C API(s):

Name: H5Aclose

Signature:

herr_t H5Aclose(hid_t attr_id)

Purpose:

Closes the specified attribute.

Description:

H5Aclose terminates access to the attribute specified by its identifier, attr_id. Further use of the attribute identifier will result in undefined behavior.

Parameters:

hid_t attr_id

IN: Attribute to release access to.

Returns:

Returns a non-negative value if successful; otherwise returns a negative value.

Non-C API(s):