MPI_Exscan is an exclusive scan: it performs a prefix reduction across all MPI processes in the given communicator, excluding the calling MPI process. In other words, each MPI process receives the result of the reduction operation on the values passed by the MPI processes with a lower rank. It implies that the result on the first MPI process is undefined, and that the buffer passed by the last MPI process for sending is not significant. MPI_Exscan is a collective operation; it must be called by all MPI processes in the communicator concerned. The variant of MPI_Exscan is the inclusive version MPI_Scan.