MPI_SCAN_INIT(3)

📦

version 5.0.10 · 4 versions

🫶 MPI_SCAN_INIT 🫶

okay bestie, welcome to the Open MPI section 3 manual page for MPI_SCAN_INIT(3) 💖 you did NOT come here by accident. sit down, sip ur drink, and let's talk about inclusive scans because this one actually eats 🔥

✨ NAME ✨

MPI_Scan, MPI_Iscan, MPI_Scan_init — Computes an inclusive scan (partial reduction) 💅

so like… it scans, it reduces, it's inclusive (which is a vibe, we love inclusivity) 🫶

📝 SYNTAX 📝

🍓 C Syntax

#include <mpi.h>

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

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

int MPI_Scan_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_SCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, IERROR

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

MPI_SCAN_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_Scan(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_Iscan(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_Scan_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 🌸

the stuff you gotta HAND IN bestie, no gatekeeping 💅

sendbuf — Send buffer (choice).
count — Number of elements in input buffer (integer).
datatype — Data type of elements of input buffer (handle).
op — Operation (handle).
comm — Communicator (handle).
info — Info (handle, persistent only)

🎀 OUTPUT PARAMETERS 🎀

the stuff u get BACK, and it's giving results ✨

recvbuf — Receive buffer (choice).
request — Request (handle, non-blocking only).
ierror — Fortran only: Error status (integer).

💅 DESCRIPTION 💅

okay so here's the tea 🍵 MPI_Scan is the move when you wanna do an inclusive prefix reduction on data that's spread out across all the calling processes. And when I say inclusive I mean inclusive-inclusive — nobody gets left out, not even rank 0.

Here's the vibe: for the process with rank i, the recvbuf gets the reduction (calculated according to the function op) of the values sitting in the sendbufs of processes with ranks 0, …, i — inclusive, periodt 💖 so rank 0's recvbuf = just its own value, rank 1's = rank0+rank1, rank 2's = rank0+rank1+rank2… u get it, it's a lil conga line of accumulation 🦋

The type of operations supported, their semantics, and the constraints on send and receive buffers are exactly as for MPI_Reduce — so if you know that one, you already know this one, iykyk.

🦋 EXAMPLE 🦋

this example uses a user-defined operation to produce a segmented scan. ngl it's kinda iconic. A segmented scan takes, as input, a set of values and a set of logicals, where the logicals delineate the various segments of the scan. For example,

   values     v1      v2      v3      v4      v5      v6      v7      v8
   logicals   0       0       1       1       1       0       0       1
   result     v1    v1+v2     v3    v3+v4  v3+v4+v5   v6    v6+v7     v8

The result for rank j is thus the sum v(i) + … + v(j), where i is the lowest rank such that for all ranks n, i <= n <= j, logical(n) = logical(j). The operator that produces this effect is

                [ u ]     [ v ]     [ w ]
                [   ]  o  [   ]  =  [   ]
                [ i ]     [ j ]     [ j ]

where

  ( u + v if i = j w = ( ( v if i != j

⚠️ note that this is a noncommutative operator. order matters bestie, don't flip it 💀

C code that implements it is given below.

typedef struct {
        double val;
        int log;
} SegScanPair;

/*
 * the user-defined function
 */
void segScan(SegScanPair *in, SegScanPair *inout, int *len,
        MPI_Datatype *dptr)
{
        int i;
        SegScanPair c;

        for (i = 0; i < *len; ++i) {
                if (in->log == inout->log)
                        c.val = in->val + inout->val;
                else
                        c.val = inout->val;

                c.log = inout->log;
                *inout = c;
                in++;
                inout++;
        }
}

Note that the inout argument to the user-defined function corresponds to the right-hand operand of the operator. When using this operator, we must be careful to specify that it is noncommutative, as in the following:

int                     i, base;
SeqScanPair     a, answer;
MPI_Op          myOp;
MPI_Datatype    type[2] = {MPI_DOUBLE, MPI_INT};
MPI_Aint                disp[2];
int                     blocklen[2] = {1, 1};
MPI_Datatype    sspair;

/*
 * explain to MPI how type SegScanPair is defined
 */
MPI_Get_address(a, disp);
MPI_Get_address(a.log, disp + 1);
base = disp[0];
for (i = 0; i < 2; ++i)
        disp[i] -= base;
MPI_Type_struct(2, blocklen, disp, type, &sspair);
MPI_Type_commit(&sspair);

/*
 * create the segmented-scan user-op
 * noncommutative - set commute (arg 2) to 0
 */
MPI_Op_create((MPI_User_function *)segScan, 0, &myOp);
...
MPI_Scan(a, answer, 1, sspair, myOp, comm);

🔥 USE OF IN-PLACE OPTION 🔥

When the communicator is an intracommunicator, you can perform a scanning operation in place — meaning the output buffer is used as the input buffer, two birds one stone, we love efficiency 🫶

Use the variable MPI_IN_PLACE as the value of the sendbuf argument. The input data is taken from the receive buffer and replaced by the output data. boom. done. 💅

⭐ NOTES ON COLLECTIVE OPERATIONS ⭐

The reduction functions of type MPI_Op do not return an error value. so like… if they 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.

The reason for this is the performance problems in ensuring that all collective routines return the same error value. (it's giving "we could've but it would've been slow" 💀)

💀 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 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 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. (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, 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. (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. fr fr, no promises 😭

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.

👑 Copyright 👑

2003-2026, The Open MPI Community

and that's the whole page bestie, you ATE that 💅✨ this was MPI_SCAN_INIT(3) from the Open MPI crew, last touched Sep 10, 2026, and the copyright sits with 2003-2026, The Open MPI Community — go forth and scan inclusively, you main character 👑💖🦋

📦 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