Creating a Group

What is a Group?

An HDF5 group is a structure containing zero or more HDF5 objects. The two primary HDF5 objects are groups and datasets. To create a group, the calling program must:

Obtain the location identifier where the group is to be created.
Create the group.
Close the group.

To create a group, the calling program must contain the following calls:

  group_id = H5Gcreate (loc_id, name, size_hint);
  H5Gclose (group_id);

Programming Example

Description

The following example shows how to create and close a group. It creates a file called 'group.h5', creates a group called MyGroup in the root group, and then closes the group and file.
[ Download h5_crtgrp.c ]


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

main() {

   hid_t       file_id, group_id;  /* identifiers */
   herr_t      status;

   /* Create a new file using default properties. */
   file_id = H5Fcreate(FILE, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);

   /* Create a group named "/MyGroup" in the file. */
   group_id = H5Gcreate(file_id, "/MyGroup", 0);

   /* Close the group. */
   status = H5Gclose(group_id);

   /* Terminate access to the file. */
   status = H5Fclose(file_id);
}
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Remarks

H5Gcreate creates a new empty group and returns a group identifier.
```
  hid_t H5Gcreate (hid_t loc_id, const char *name, size_t size_hint) 
```
- The first parameter specifies the location to create the group.
- The second parameter specifies the name of the group to be created.
- The third parameter specifies how much file space to reserve to store the names that will appear in the group. If a non-positive value is supplied, then a default size is used. Passing a value of zero is usually adequate since the library is able to dynamically resize the name heap.
H5Gcreate creates a group named MyGroup in the root group of the specified file.
H5Gclose closes the group. This call is mandatory.
```
  herr_t H5Gclose (hid_t group_id) 
```

File Contents

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

Fig. 8.1 The Contents of 'group.h5'. Fig. 8.2 'group.h5' in DDL

HDF5 "group.h5" { GROUP "/" { GROUP "MyGroup" { } } }

The National Center for Supercomputing Applications
University of Illinois at Urbana-Champaign

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