Merge pull request #657 from E3SM-Project/dqwu/align_hdf5_string_with_netcdf4

This PR updates SCORPIO’s HDF5 string type handling to match
NetCDF4 behavior. It addresses differences in STRSIZE and
empty string handling, ensuring consistent results in h5dump
(between NetCDF4 and HDF5 outputs) and improving overall
compatibility.

Fixes #656

Author: dqwu

src/clib/pioc_support.cpp

@@ -6628,12 +6628,13 @@ int spio_hdf5_create(iosystem_desc_t *ios, file_desc_t *file, const char *filena
                        filename);
     }
 
-    if (H5Tset_size(h5_string_type, asize + 1) < 0)
+    assert(asize > 0);
+    if (H5Tset_size(h5_string_type, asize) < 0)
     {
         return pio_err(ios, file, PIO_EHDF5ERR, __FILE__, __LINE__,
                        "Creating file (%s) using HDF5 iotype failed. "
                        "The low level (HDF5) I/O library call failed to set the total size (%ld bytes) for a derived C-style string datatype",
-                       filename, asize + 1);
+                       filename, asize);
     }
 
     if (H5Tset_strpad(h5_string_type, H5T_STR_NULLTERM) < 0)
@@ -7235,7 +7236,11 @@ int spio_hdf5_put_att(iosystem_desc_t *ios, file_desc_t *file, int varid, const
     if (atttype == NC_CHAR)
     {
         /* String type */
-        space_id = H5Screate(H5S_SCALAR);
+        if (asize == 0)
+            space_id = H5Screate(H5S_NULL);
+        else
+            space_id = H5Screate(H5S_SCALAR);
+
         if (space_id == H5I_INVALID_HID)
         {
             return pio_err(ios, file, PIO_EHDF5ERR, __FILE__, __LINE__,
@@ -7254,12 +7259,12 @@ int spio_hdf5_put_att(iosystem_desc_t *ios, file_desc_t *file, int varid, const
         }
 
         /* For empty strings, asize is 0, while H5Tset_size() requires that size must be positive */
-        if (H5Tset_size(h5_xtype, asize + 1) < 0)
+        if (H5Tset_size(h5_xtype, (asize == 0 ? 1 : asize)) < 0)
         {
             return pio_err(ios, file, PIO_EHDF5ERR, __FILE__, __LINE__,
                            "Writing attribute (%s) associated with variable (varid=%d) to file (%s, ncid=%d) using HDF5 iotype failed. "
                            "The low level (HDF5) I/O library call failed to set the total size (%ld bytes) for a derived C-style string datatype",
-                           name, varid, pio_get_fname_from_file(file), file->pio_ncid, asize + 1);
+                           name, varid, pio_get_fname_from_file(file), file->pio_ncid, (asize == 0 ? 1 : asize));
         }
 
         if (H5Tset_strpad(h5_xtype, H5T_STR_NULLTERM) < 0)