Creating an Attribute

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 and read/write an attribute.

Creating an attribute

Creating an attribute is similar to the creation of a dataset. To create an attribute the application must specify the object which the attribute is attached to, the data type and space of the attribute data and the creation properties.

The steps to create an attribute are as follows:

Obtain the object identifier that the attribute is to be attached to.
Define the characteristics of the attribute and specify creation properties.
- Define the data type.
- Define the dataspace.
- Specify the creation properties.
Create the attribute.
Close the attribute and data type, dataspace, and creation property list if necessary.

To create an attribute, the calling program must contain the following calls:

     attr_id = H5Acreate(loc_id, attr_name, type_id, space_id, create_plist);
     H5Aclose(attr_id);

Reading/Writing an attribute

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

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

Obtain the attribute identifier.
Specify the attribute's memory data type.
Perform the desired operation.
Close the memory data type if necessary.

To read/write an attribute, the calling program must contain the following calls:

    status = H5Aread(attr_id, mem_type_id, buf);

    status = H5Awrite(attr_id, mem_type_id, buf);

Programming Example

Description

This example shows how to create and write a dataset attribute. It opens an existing file 'dset.h5', obtains the id of the dataset "/dset1", defines the attribute's dataspace, creates the dataset attribute, writes the attribute, and then closes the attribute's dataspace, attribute, dataset, and file.
[ Download h5_crtatt.c ]

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#include <hdf5.h>
#define FILE "dset.h5"

main() {

   hid_t       file_id, dataset_id, attribute_id, dataspace_id;  /* identifiers
*/
   hsize_t     dims;
   int         attr_data[2];
   herr_t      status;

   /* Initialize the attribute data. */
   attr_data[0] = 100;
   attr_data[1] = 200;

   /* Open an existing file. */
   file_id = H5Fopen(FILE, H5F_ACC_RDWR, H5P_DEFAULT);

   /* Open an existing dataset. */
   dataset_id = H5Dopen(file_id, "/dset");

   /* Create the data space for the attribute. */
   dims = 2;
   dataspace_id = H5Screate_simple(1, &dims, NULL);

   /* Create a dataset attribute. */
   attribute_id = H5Acreate(dataset_id, "attr", H5T_STD_I32BE, dataspace_id, H5P_DEFAULT);

   /* Write the attribute data. */
   status = H5Awrite(attribute_id, H5T_NATIVE_INT, attr_data);

   /* Close the attribute. */
   status = H5Aclose(attribute_id);

   /* Close the dataspace. */
   status = H5Sclose(dataspace_id);

   /* Close to the dataset. */
   status = H5Dclose(dataset_id);

   /* Close the file. */
   status = H5Fclose(file_id);
}
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Remarks

H5Acreate creates an attribute which is attached to the object specified by the first parameter, and returns an identifier.
```
  hid_t H5Acreate (hid_t loc_id, const char *name, hid_t type_id, 
                   hid_t space_id, hid_t create_plist) 
```
- The first parameter is the identifier of the object which the attribute is attached to.
- The second parameter is the name of the attribute to create.
- The third parameter is the identifier of the attribute's datatype.
- The fourth parameter is the identifier of the attribute's dataspace.
- The last parameter is the identifier of the creation property list. H5P_DEFAULT specifies the default creation property list.
H5Awrite writes the entire attribute, and returns the status of the write.
```
  herr_t H5Awrite (hid_t attr_id, hid_t mem_type_id, void *buf) 
```
- The first parameter is the identifier of the attribute to write.
- The second parameter is the identifier of the attribute's memory datatype.
- The last parameter is the data buffer.
When an attribute is no longer accessed by a program, H5Aclose must be called to release the attribute from use. This call is mandatory.
```
  herr_t H5Aclose (hid_t attr_id) 
```

File Contents

The contents of 'dset.h5' and the attribute definition are given in the following:

Fig. 7.1 '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
               }
            }
         }
      }
      }

Attribute Definition in DDL

Fig. 7.2 HDF5 Attribute Definition


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

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University of Illinois at Urbana-Champaign

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Creating an Attribute

Contents:

What is an Attribute?