Unspecified Behavior.

MPI does not specify behavior of some aspects of MPI implementations. These include

• Buffering. MPI does not specify how much buffering must be provided by an implementation.
• Standard I/O. MPI does not require that standard language I/O facilities be provided.
• Process startup and management. MPI does not specify how processes are started up or shut down. In particular, the state of a program before MPI_Init and after MPI_Finalize is not specified.

It is where MPI does not specify behavior that implementations differ. Applications that do not make assumptions about these features are portable. Portability issues are discussed in more detail in Section 7.