A high-quality implementation will allow any process (including those not started with a ``parallel application'' mechanism) to become an MPI process by calling MPI_INIT. Such a process can then connect to other MPI processes using the MPI_COMM_ACCEPT and MPI_COMM_CONNECT routines, or spawn other MPI processes. MPI does not mandate this behavior, but strongly encourages it where technically feasible.
Advice to implementors.
To start an MPI-1 application with more than one process requires some special coordination. The processes must be started at the ``same'' time, they must have a mechanism to establish communication, etc. Either the user or the operating system must take special steps beyond simply starting processes.
When an application enters MPI_INIT, clearly it must be able to determine if these special steps were taken. MPI-1 does not say what happens if these special steps were not taken --- presumably this is treated as an error in starting the MPI application. MPI-2 recommends the following behavior.
If a process enters MPI_INIT and determines that no special steps were taken (i.e., it has not been given the information to form an MPI_COMM_WORLD with other processes) it succeeds and forms a singleton MPI program, that is, one in which MPI_COMM_WORLD has size 1.
In some implementations, MPI may not be able to function without an ``MPI environment.'' For example, MPI may require that daemons be running or MPI may not be able to work at all on the front-end of an MPP. In this case, an MPI implementation may either
( End of advice to implementors.)