cdm/src/main/java/ucar/nc2/NetcdfFileWriter.java
public Attribute addGroupAttribute(Group g, Attribute att) {
    if (!defineMode) throw new UnsupportedOperationException("not in define mode");

    if (!isValidObjectName(att.getShortName())) {
      String attName = createValidObjectName(att.getShortName());
      log.warn("illegal attribute name= " + att.getShortName() + " change to " + attName);
      att = new Attribute(attName, att.getValues());
    }

    return ncfile.addAttribute(g, att);
  }
public Attribute addGroupAttribute(Group g, Attribute att) {
    if (!defineMode) throw new UnsupportedOperationException("not in define mode");

    if (!isValidObjectName(att.getShortName())) {
      String attName = createValidObjectName(att.getShortName());
      log.warn("illegal attribute name= " + att.getShortName() + " change to " + attName);
      att = new Attribute(attName, att.getValues());
    }

    return ncfile.addAttribute(g, att);
  }
Add a Global attribute to the file. Must be in define mode.

@param g   the group to add to. if null, use root group
@param att the attribute.
@return the created attribute