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Creating an Attribute


Contents:


What is an Attribute?

Attributes are small datasets that can be used to describe the nature and/or the intended usage of the object they are attached to. In this section, we show how to create, read, and write an attribute.

Creating an attribute

Creating an attribute is similar to creating a dataset. To create an attribute, the application must specify the object which the attribute is attached to, the datatype and dataspace of the attribute data, and the attribute creation property list.

The steps to create an attribute are as follows:

  1. Obtain the object identifier that the attribute is to be attached to.
  2. Define the characteristics of the attribute and specify the attribute creation property list.
    • Define the datatype.
    • Define the dataspace.
    • Specify the attribute creation property list.
  3. Create the attribute.
  4. Close the attribute and datatype, dataspace, and attribute creation property list, if necessary.

To create and close an attribute, the calling program must use H5Acreate/h5acreate_f and H5Aclose/h5aclose_f. For example:

C:

     attr_id = H5Acreate (dset_id, attr_name, type_id, space_id, creation_prp);
     status = H5Aclose (attr_id);
FORTRAN:
     CALL h5acreate_f (dset_id, attr_nam, type_id, space_id, attr_id, &
                       hdferr, creation_prp=creat_plist_id)
          or
     CALL h5acreate_f (dset_id, attr_nam, type_id, space_id, attr_id, hdferr)

     CALL h5aclose_f (attr_id, hdferr)

Reading/Writing an attribute

Attributes may only be read or written as an entire object; no partial I/O is supported. Therefore, to perform I/O operations on an attribute, the application needs only to specify the attribute and the attribute's memory datatype.

The steps to read or write an attribute are as follows.

  1. Obtain the attribute identifier.
  2. Specify the attribute's memory datatype.
  3. Perform the desired operation.
  4. Close the memory datatype if necessary.

To read and/or write an attribute, the calling program must contain the H5Aread/h5aread_f and/or H5Awrite/h5awrite_f routines. For example:

C:

    status = H5Aread (attr_id, mem_type_id, buf);
    status = H5Awrite (attr_id, mem_type_id, buf);
FORTRAN:
    CALL h5awrite_f (attr_id, mem_type_id, buf, hdferr)  
    CALL h5aread_f (attr_id, mem_type_id, buf, hdferr)

Programming Example

Description

This example shows how to create and write a dataset attribute. It opens an existing file dset.h5 in C (dsetf.h5 in FORTRAN), obtains the identifier of the dataset /dset, defines the attribute's dataspace, creates the dataset attribute, writes the attribute, and then closes the attribute's dataspace, attribute, dataset, and file.
NOTE: To download a tar file of the examples, including a Makefile, please go to the References page.

Remarks

File Contents

The contents of dset.h5 (dsetf.h5 for FORTRAN) and the attribute definition are shown below:

Fig. 7.1a   dset.h5 in DDL

HDF5 "dset.h5" {
GROUP "/" {
   DATASET "dset" {
      DATATYPE { H5T_STD_I32BE }
      DATASPACE { SIMPLE ( 4, 6 ) / ( 4, 6 ) }
      DATA {
         1, 2, 3, 4, 5, 6,
         7, 8, 9, 10, 11, 12,
         13, 14, 15, 16, 17, 18,
         19, 20, 21, 22, 23, 24
      }
      ATTRIBUTE "attr" {
         DATATYPE { H5T_STD_I32BE }
         DATASPACE { SIMPLE ( 2 ) / ( 2 ) }
         DATA {
            100, 200
         }
      }
   }
}
}
Fig. 7.1b   dsetf.h5 in DDL
HDF5 "dsetf.h5" {
GROUP "/" {
   DATASET "dset" {
      DATATYPE { H5T_STD_I32BE }
      DATASPACE { SIMPLE ( 6, 4 ) / ( 6, 4 ) }
      DATA {
         1, 7, 13, 19,
         2, 8, 14, 20,
         3, 9, 15, 21,
         4, 10, 16, 22,
         5, 11, 17, 23,
         6, 12, 18, 24
      }
      ATTRIBUTE "attr" {
         DATATYPE { H5T_STD_I32BE }
         DATASPACE { SIMPLE ( 2 ) / ( 2 ) }
         DATA {
            100, 200
         }
      }
   }
}
}

Attribute Definition in DDL

Fig. 7.2   HDF5 Attribute Definition

     <attribute> ::= ATTRIBUTE "<attr_name>" { <datatype>
                                               <dataspace>
                                               <data>  }


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Last Modified: November 21, 2001