MPI\_SCAN(3) Open MPI MPI\_SCAN(3)

MPI\_Scan, MPI\_Iscan <#mpi-iscan>, MPI\_Scan\_init <#mpi-scan-init> - Computes an inclusive scan (partial reduction)

# [SYNTAX](#SYNTAX)

## [C 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](#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](#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](#INPUT_PARAMETERS)

- **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](#OUTPUT_PARAMETERS)

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

# [DESCRIPTION](#DESCRIPTION)

MPI\_Scan is used to perform an inclusive prefix reduction on data distributed across the calling processes. The operation returns, in the *recvbuf* of the process with rank i, the reduction (calculated according to the function *op*) of the values in the *sendbuf*s of processes with ranks 0, …, i (inclusive). The type of operations supported, their semantics, and the constraints on send and receive buffers are as for MPI\_Reduce &lt;#mpi-reduce&gt;.

# [EXAMPLE](#EXAMPLE)

This example uses a user-defined operation to produce a segmented scan. 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 &lt;= n &lt;= j, [logical(n)](/man/enterprise-packages/openmpi-cuda-12.9/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. 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](#USE_OF_IN-PLACE_OPTION)

When the communicator is an intracommunicator, you can perform a scanning operation in place (the output buffer is used as the input buffer). 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.

# [NOTES ON COLLECTIVE OPERATIONS](#NOTES_ON_COLLECTIVE_OPERATIONS)

The reduction functions of type MPI\_Op do not return an error value. As a result, if the functions detect an error, all they can do is either call MPI\_Abort &lt;#mpi-abort&gt; 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.

# [ERRORS](#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 &lt;#mpi-init&gt;/MPI\_Init\_thread &lt;#mpi-init-thread&gt;, after MPI\_Finalize &lt;#mpi-finalize&gt;, 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 &lt;#mpi-comm-set-errhandler&gt; 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 &lt;#mpi-comm-spawn&gt;/MPI\_Comm\_spawn\_multiple &lt;#mpi-comm-spawn-multiple&gt;. 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 &lt;#mpi-abort&gt; was called on that communicator. If called on a window or file, acts as if MPI\_Abort &lt;#mpi-abort&gt; 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 &lt;#mpi-comm-create-errhandler&gt; then MPI\_Comm\_set\_errhandler &lt;#mpi-comm-set-errhandler&gt;
- MPI\_File\_create\_errhandler &lt;#mpi-file-create-errhandler&gt; then MPI\_File\_set\_errhandler &lt;#mpi-file-set-errhandler&gt;
- MPI\_Session\_create\_errhandler &lt;#mpi-session-create-errhandler&gt; then MPI\_Session\_set\_errhandler &lt;#mpi-session-set-errhandler&gt; or at MPI\_Session\_init &lt;#mpi-session-init&gt;
- MPI\_Win\_create\_errhandler &lt;#mpi-win-create-errhandler&gt; then MPI\_Win\_set\_errhandler &lt;#mpi-win-set-errhandler&gt;

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page &lt;#open-mpi&gt; for a full list of MPI error codes &lt;#open-mpi-errors&gt;.

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

**See also:**

- MPI\_Exscan &lt;#mpi-exscan&gt;
- MPI\_Op\_create &lt;#mpi-op-create&gt;
- MPI\_Reduce &lt;#mpi-reduce&gt;

# [Copyright](#Copyright)

2003-2026, The Open MPI Community

September 23, 2026

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Document: MPI_Scan(3) from openmpi-cuda-12.9 5.0.11-r0 (Chainguard OS)
Source: https://github.com/open-mpi/ompi