All processes in the group identified by the intracommunicator must call
the collective routine.
In many cases,
collective communication can occur ``in place''
with the output
buffer being identical to the input buffer. This is specified by
providing a special argument value, MPI_IN_PLACE, instead of the
send buffer or the receive buffer argument,
depending on the operation performed.
The ``in place'' operations are provided to reduce unnecessary memory motion by
both the MPI implementation and by the user. Note that while the simple check
of testing whether the send and receive buffers have the same address will
work for some cases (e.g., MPI_ALLREDUCE), they are inadequate in
others (e.g., MPI_GATHER, with root not equal to zero). Further,
Fortran explicitly prohibits aliasing of arguments; the approach of using a
special value to denote ``in place'' operation eliminates that difficulty.
( End of rationale.)
Advice to users.
By allowing the ``in place'' option, the receive buffer in many of the collective calls becomes a send-and-receive buffer. For this reason, a Fortran binding that includes INTENT must mark these as INOUT, not OUT.
Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use that MPI_BOTTOM has.
Some intracommunicator collective operations do not support the ``in place''
option (e.g., MPI_ALLTOALLV).
( End of advice to users.)