Iterating over Group Members

How to Iterate Over Group Members

This section discusses how to find names and object types of HDF5 group members.

The HDF5 Group interface has a function, H5Giterate, to iterate over the group members.

Operations on each group member can be performed during the iteration process. The operator function and its data are passed to the iterator as parameters. There are no restrictions on what kind of operations can be performed on group members during the iteration procedure.

The following steps are involved:

Write an operator function which will be used during the iteration process. The HDF5 library defines the operator function signature and return values.
Open the group to iterate through.
Use H5Giterate to iterate through the group or just a few members of the group.

Programming Example

Description

In this example we iterate through the members of the root group. The operator function displays the members' names and their object types. The object type can be a group, dataset, or named datatype. [ Download h5_iterate.c ]


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

#include <hdf5.h>

#define FILE   "iterate.h5"
#define  FALSE 0

/* 1-D dataset with fixed dimensions */
#define SPACE1_NAME  "Space1"
#define SPACE1_RANK	1
#define SPACE1_DIM1	4

herr_t file_info(hid_t loc_id, const char *name, void *opdata);
                                     /* Operator function */
int 
main(void) {
    hid_t		file;		/* HDF5 File IDs		*/
    hid_t		dataset;	/* Dataset ID			*/
    hid_t		group;      /* Group ID             */
    hid_t		sid;       /* Dataspace ID			*/
    hid_t		tid;       /* Datatype ID			*/
    hsize_t		dims[] = {SPACE1_DIM1};
    herr_t		ret;		/* Generic return value		*/

/* Compound datatype */
typedef struct s1_t {
    unsigned int a;
    unsigned int b;
    float c;
} s1_t;

    /* Create file */
    file = H5Fcreate(FILE, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);

    /* Create dataspace for datasets */
    sid = H5Screate_simple(SPACE1_RANK, dims, NULL);

    /* Create a group */
    group=H5Gcreate(file,"Group1",-1);

    /* Close a group */
    ret = H5Gclose(group);

    /* Create a dataset  */
    dataset=H5Dcreate(file,"Dataset1",H5T_STD_U32LE,sid,H5P_DEFAULT);

    /* Close Dataset */
    ret = H5Dclose(dataset);

    /* Create a datatype */
    tid = H5Tcreate (H5T_COMPOUND, sizeof(s1_t));

    /* Insert fields */
    ret=H5Tinsert (tid, "a", HOFFSET(s1_t,a), H5T_NATIVE_INT);

    ret=H5Tinsert (tid, "b", HOFFSET(s1_t,b), H5T_NATIVE_INT);

    ret=H5Tinsert (tid, "c", HOFFSET(s1_t,c), H5T_NATIVE_FLOAT);

    /* Save datatype for later */
    ret=H5Tcommit (file, "Datatype1", tid);

    /* Close datatype */
    ret = H5Tclose(tid);

    /* Iterate through the file to see members of the root group */

    printf(" Objects in the root group are:\n");
    printf("\n");

    H5Giterate(file, "/", NULL, file_info, NULL);

    /* Close file */
    ret = H5Fclose(file);

    return 0;
}

/*
 * Operator function.
 */
herr_t file_info(hid_t loc_id, const char *name, void *opdata)
{
    H5G_stat_t statbuf;

    /*
     * Get type of the object and display its name and type.
     * The name of the object is passed to this function by 
     * the Library. Some magic :-)
     */
    H5Gget_objinfo(loc_id, name, FALSE, &statbuf);
    switch (statbuf.type) {
    case H5G_GROUP: 
         printf(" Object with name %s is a group \n", name);
         break;
    case H5G_DATASET: 
         printf(" Object with name %s is a dataset \n", name);
         break;
    case H5G_TYPE: 
         printf(" Object with name %s is a named datatype \n", name);
         break;
    default:
         printf(" Unable to identify an object ");
    }
    return 0;
 }

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

The output of this program is:

 Objects in the root group are:

 Object with name Dataset1 is a dataset 
 Object with name Datatype1 is a named datatype 
 Object with name Group1 is a group 
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Remarks

The operator function in this example is called file_info. The signature of the operator function is as follows:
```
  herr_t *(H5G_operator_t) (hid_group_id, const char* name, void *operator_data)
```
- The first parameter is a group identifier for the group being iterated over. It is passed to the operator by the iterator function, H5Giterate.
- The second parameter is the name of the current object. The name is passed to the operator function by the HDF5 library.
- The third parameter is the operator data. It is passed to and from the operator by the iterator, H5Giterate. The operator function in this example just prints the name and type of the current object and then exits. This information can also be used to open the object and perform different operations or queries. For example a named datatype object's name can be used to open the datatype and query its properties.
The operator return value defines the behavior of the iterator.
- A zero return value causes the iterator to continue, returning zero when all group members have been processed.
- A positive value causes the iterator to immediately return that value, indicating a short-circuit success. The iterator can be restarted at the next group member.
- A negative value causes the iterator to immediately return that value, indicating failure. The iterator can be restarted at the next group member.
In this example the operator function returns 0, which causes the iterator to continue and go through all group members.
The function H5Gget_objinfo is used to determine the type of the object. It also returns the modification time, number of hard links, and some other information.
The signature of this function is as follows:
```
     herr_t H5Gget_objinfo(hid_t loc_id, const char * name, hbool_t follow_link,
                            H5G_stat_t *statbuf)
```
- The first two arguments specify the object by its location and name. This example uses the group identifier and name relative to the group to specify the object.
- The third argument is a flag which indicates whether a symbolic link should be followed or not. A zero value indicates that information should be returned for the link itself, but not about the object it points to. The root group in this example does not have objects that are links, so this flag is not important for our example.
- The fourth argument is a buffer to return information in. Type information is returned into the field "type" of the H5G_stat_t data structure (statbuf.type). Possible values are: H5G_GROUP, H5G_DATASET, H5G_TYPE, and H5G_LINK.
The H5Giterate function has the following signature:
```
   int H5Giterate(hid_t loc_id, const char *name , idx,
                  H5G_operator_t operator, void * operator_data) 
```
- The first parameter is the group for the group being iterated over.
- The second parameter is the group name.
- The third parameter specifies that iteration begins with the idx object in the group and the next element to be processed is returned in the idx parameter. In our example NULL is used to start at the first group member. Since no stopping point is returned in this case, the iterator cannot be restarted if one of the calls to its operator returns a non-zero value.
- The fourth parameter is an operator function.
- The fifth argument is the operator data. We used NULL since no data was passed to and from the operator.

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

hdfhelp@ncsa.uiuc.edu

Iterating over Group Members

Contents:

How to Iterate Over Group Members

Programming Example

Description

Remarks

Last Modified: August 27, 1999