MPI_EXSCAN(3)

📦

version 5.0.10 · 7 versions

🌸 MPI_EXSCAN 🌸

okay bestie, this is the manual page for MPI_EXSCAN(3) from Open MPI, and it's about MPI_Exscan, MPI_Iexscan — Computes an exclusive scan (partial reduction) 💖 (fun fact: there's also an MPI_Exscan_init bopping around in the syntax below, so don't be confused when you see her 👑)

It's giving "group project where everyone adds up what came before them and literally nobody includes themselves" ✨

✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count,
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
     MPI_Request *request)

int MPI_Exscan_init(const void *sendbuf, void *recvbuf, int count,
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
     MPI_Info info, MPI_Request *request)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, IERROR

MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

MPI_EXSCAN_INIT(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR

🎀 Fortran 2008 Syntax

USE mpi_f08
MPI_Exscan(sendbuf, recvbuf, count, datatype, op, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iexscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Exscan_init(sendbuf, recvbuf, count, datatype, op, comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

💌 INPUT PARAMETERS 💌

sendbuf — the send buffer (it's a choice situation 💅).
count — how many elements live in the input buffer (an integer, bestie).
datatype — the data type of the elements in the input buffer (a handle).
op — the operation you wanna do (handle).
comm — the communicator (handle).

🫶 OUTPUT PARAMETERS 🫶

recvbuf — the receive buffer (choice).
request — the request (handle, and it's non-blocking only — noted 🔥).
ierror — Fortran only: the error status (integer).

👑 DESCRIPTION 👑

So MPI_Exscan does an exclusive prefix reduction on data that's spread out across all the processes calling it. The vibe: the process with rank i gets back, in its recvbuf, the reduction (calculated according to the function op) of the values sitting in the sendbufs of the processes with ranks 0, …, i-1. Yes, exclusive — she does NOT include herself, that's the whole point, no cap.

Compare her to MPI_Scan, who calculates over the range 0, …, i (inclusive). Different girl, different energy. The types of operations supported, their semantics, and all the constraints on send and receive buffers are exactly the same as for MPI_Reduce.

🚨 Warning bestie: the value in recvbuf on process 0 is undefined and unreliable, because recvbuf just isn't significant for process 0. She said "not my problem." But the value of recvbuf on process 1 is always the value in sendbuf on process 0. That one's solid, periodt. 💖

🌈 USE OF IN-PLACE OPTION 🌈

You get the in place option for intracommunicators by passing MPI_IN_PLACE in the sendbuf argument. When you do that, the input data is taken from the receive buffer, and then replaced by the output data. Recycling, but make it HPC ♻️✨

Heads up though: MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM. iykyk.

And because the in-place option turns the receive buffer into a send-and-receive buffer, any Fortran binding that includes INTENT has to mark these as INOUT, not OUT. Not optional, bestie, mark the INOUT 😭

💅 NOTES 💅

MPI does not specify which process computes which operation. Lowkey important: in particular, both processes 0 and 1 may participate in the computation even though the results for both processes' recvbuf are degenerate. So literally all processes, including 0 and 1, must provide the same op. Everyone's invited to the function, even the ones whose output is a mystery 🎀

It can be argued — from a mathematical perspective — that the definition of MPI_Exscan is unsatisfactory because the output at process 0 is undefined. Like, the "mathematically correct" output for process 0 would be the unit element of the reduction operation. Cute in theory! But such a definition of an exclusive scan would not work with user-defined op functions, because there's no way for MPI to "know" the unit value for those custom operations. She can't read minds, bestie 🧚

⭐ NOTES ON COLLECTIVE OPERATIONS ⭐

The reduction functions of type MPI_Op do not return an error value. So if those functions detect an error, all they can do is either call MPI_Abort or silently skip the problem 🙈

Which means: if the error handler is changed from MPI_ERRORS_ARE_FATAL to something else (e.g., MPI_ERRORS_RETURN), then no error may be indicated. Yeah. It's giving quiet quitting.

The reason for this is the performance problems in ensuring that all collective routines return the same error value.

🔥 ERRORS 🔥

Almost all MPI routines return an error value — C routines as the return result of the function, and Fortran routines in the last argument.

Before that error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. That initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. 💀
MPI_ERRORS_ABORT — An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.
MPI_ERRORS_RETURN — Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
MPI_File_create_errhandler then MPI_File_set_errhandler
MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error. So maybe don't count on it, bestie 😭

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also:

MPI_Op_create
MPI_Reduce
MPI_Scan

🎀 Copyright 🎀

2003-2026, The Open MPI Community

and that's the whole page, bestie 🦋 tysm for reading — this was MPI_EXSCAN(3) from Open MPI, dated Sep 10, 2026, copyright 2003-2026, The Open MPI Community. now go forth and scan exclusively, you absolute icon 💅✨🌸

📦 where this page comes from

ubuntu stonking: openmpi-doc 5.0.10-6 · other
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms