The first MPI call in a program must be MPI_INIT to
initialize the environment. This is usually followed by a call to
MPI_COMM_SIZE to determine the number of processes taking part in
the computation (the size of the ``virtual machine''), and a call to
MPI_COMM_RANK to find out the rank of the calling process
within the ``virtual machine''. Following are more explicit details of
the initial MPI function calls, and associated parameters and syntax, in
C and Fortran.