H5I_ITERATE calls the callback function op for each member of the identifier type type. The callback function type for op, H5I_iterate_func_t, is defined in H5Ipublic.h as:

typedef herr_t (*H5I_iterate_func_t)(hid_t id, void *udata);

op takes as parameters the identifier and a pass through of op_data, and returns an herr_t.

A positive return from op will cause the iteration to stop and H5I_ITERATE will return the value returned by op.  A negative return from op will cause the iteration to stop and H5I_ITERATE will return failure.  A zero return from op will allow iteration to continue, as long as there are other identifiers remaining in type.

H5I_type_t is defined in H5Ipublic.h in the HDF5 source code.  Valid H5I_type_t values for use with H5I_ITERATE are:
    H5I_FILE        = 1,        /* type ID for File objects                     */
    H5I_GROUP,                  /* type ID for Group objects                    */
    H5I_DATATYPE,               /* type ID for Datatype objects                 */
    H5I_DATASPACE,              /* type ID for Dataspace objects                */
    H5I_DATASET,                /* type ID for Dataset objects                  */
    H5I_MAP,                    /* type ID for Map objects                      */
    H5I_ATTR,                   /* type ID for Attribute objects                */
    H5I_VFL,                    /* type ID for virtual file layer               */
    H5I_VOL,                    /* type ID for virtual object layer             */
    H5I_GENPROP_CLS,            /* type ID for generic property list classes    */
    H5I_GENPROP_LST,            /* type ID for generic property lists           */
    H5I_ERROR_CLASS,            /* type ID for error classes                    */
    H5I_ERROR_MSG,              /* type ID for error messages                   */
    H5I_ERROR_STACK,            /* type ID for error stacks                     */
    H5I_SPACE_SEL_ITER,         /* type ID for dataspace selection iterator     */

The last value returned by op

None