[OMPI users] Open MPI error to define MPI_Datatype in header file

[Jack Bryan]

Hi,
I need to define a (Open MPI) MPI_Datatype in a header file so that all other 
files that include it can find it. 
I also try to use extern to do decleration in .h file and then define them in 
.cpp file. 
But, I always get error: 
undefined reference 
It is not allowed in Open MPI ? 
Why ? 
Any help is really appreciated. 
Thanks