H5Tunregister removes a conversion function matching criteria such as soft or hard conversion, source and destination types, and the conversion function.
If a user is trying to remove a conversion function he registered, all parameters can be used. If he is trying to remove a library’s default conversion function, there is no guarantee the name and func parameters will match the user’s chosen values. Passing in some values may cause this function to fail. A good practice is to pass in NULL as their values. All parameters are optional. The missing parameters will be used to generalize the search criteria. The conversion function pointer type declaration is described in H5Tregister. | 
