MPI_EXSCAN(3)

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version 5.0.11 · 16 versions

💅 MPI_EXSCAN — Open MPI edition, bestie ✨

Okay so hi, this is the MPI_EXSCAN(3) page from the Open MPI manual, and we're gonna talk about MPI_Exscan, MPI_Iexscan, and MPI_Exscan_init — the exclusive scan / partial reduction girlies. 🎀

✨ 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, bestie. It's a choice. 🫶
count — how many elements are in the input buffer (an integer, no cap).
datatype — data type of the elements in the input buffer (a handle).
op — the operation, yup, that's a handle too (handle).
comm — the communicator (handle, slay).

📤 OUTPUT PARAMETERS 📤

recvbuf — the receive buffer, also a choice. 💖
request — the request handle — non-blocking only, iykyk.
ierror — Fortran only: the error status, an integer.

🌈 DESCRIPTION 🌈

Okay so here's the tea ☕ — MPI_Exscan does an exclusive prefix reduction on data that's spread out across all your calling processes. The way it works: in the recvbuf of the process with rank i, you get the reduction (calculated according to the function op) of the values in the sendbufs of processes with ranks 0, …, i-1.

Lowkey compare that to MPI_Scan, which calculates over the range 0, …, i — inclusive. That extra i is doing a lot of work, ok?

The types of operations supported, their semantics, and the constraints on send and receive buffers are all as for MPI_Reduce. Periodt.

Now the important bit, don't skip this:

The value in recvbuf on process 0 is undefined and unreliable — recvbuf is just not significant for process 0, like at all. 🙈

And process 1's recvbuf is always the value in sendbuf on process 0. Cute, right? 🎀

🧚 USE OF IN-PLACE OPTION 🧚

The ‘in place’ option for intracommunicators is specified by passing MPI_IN_PLACE in the sendbuf argument. In that case the input data is taken from the receive buffer, and then replaced by the output data. Swap queen behavior honestly.

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

And because the in-place option turns the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT. Do not come for me on this, it's literally in the spec. 💅

⭐ NOTES ⭐

MPI does not specify which process computes which operation. In particular, both processes 0 and 1 may participate in the computation even though the results for both processes’ recvbuf are degenerate. So therefore — and this is the takeaway — all processes, including 0 and 1, must provide the same op. No exceptions, fr fr.

Now, some philosophy for you, bestie: it can be argued, from a mathematical perspective, that the definition of MPI_Exscan is unsatisfactory because the output at process 0 is undefined. 😭 The “mathematically correct” output for process 0 would be the unit element of the reduction operation. But — and this is the iconique plot twist — 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 these custom operations. The math girlies are gonna have to cope. 🦋

💀 NOTES ON COLLECTIVE OPERATIONS 💀

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

Thus, 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. Yes, that's real. No cap.

The reason for this? Performance problems in ensuring that all collective routines return the same error value. It's giving engineering tradeoff.

🚨 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. Standard.

Before the 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. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or 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 — here they are, iconic lineup 🌟:

MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. Dramatic, but valid.
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, it 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, it aborts only the local process. Versatile!
MPI_ERRORS_RETURN — Returns an error code to the application. The chill one.

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. Like, at all. 🙈

See the MPI man page for a full list of MPI error codes. And see the Error Handling section of the MPI-3.1 standard for more information. Go read it, bestie, it's giving lore. 📚

👑 SEE ALSO 👑

MPI_Op_create
MPI_Reduce
MPI_Scan

🍓 COPYRIGHT 🍓

2003-2026, The Open MPI Community

That's the whole page, babes — MPI_EXSCAN(3) from the Open MPI manual, Sep 11, 2026, Open MPI version in the footer, copyright 2003-2026, The Open MPI Community. Go forth and exclusive-scan responsibly. 💖✨🎀

📦 where this page comes from

arch rolling: openmpi 5.0.11-1 · BSD-3-Clause AND LicenseRef-MPICH
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: a language model's rewrite of the original, under its terms