Hi Tim, the other day you mention that you played with attaching datatypes (specifically static data types) as keyvalues. This is very interesting. The first thing I notice is that these datatypes will not be global, which is good. but it also means that they are attached to communicators, not the MPI environment. Am I correct? This would solve the problem of freeing datatypes on program exit. This is different from the idea of attaching it to environment that I was mentioning, but it is possible that attaching it to the communicator is actually a better idea. At the very least, because MPI has a mechanism to “attach” stuff to communicators, but not to the environment.