---
name: "MPI_Compare_and_swap.openmpi"
section: "3"
platform: "linux"
package: "openmpi-doc"
version: "5.0.7-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Compare_and_swap.openmpi"
---

# MPI\_Compare\_and\_swap.openmpi(3)

*MPI\_Compare\_and\_swap* — Perform RMA compare-and-swap

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Compare_and_swap(const void *origin_addr, const void *compare_addr,
     void *result_addr, MPI_Datatype datatype, int target_rank,
     MPI_Aint target_disp, MPI_Win win)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMPARE_AND_SWAP(ORIGIN_ADDR, COMPARE_ADDR, RESULT_ADDR, DATATYPE, TARGET_RANK,
     TARGET_DISP, WIN, IERROR)
     <type> ORIGIN_ADDR, COMPARE_ADDR, RESULT_ADDR(*)
     INTEGER(KIND=MPI_ADDRESS_KIND) TARGET_DISP
     INTEGER DATATYPE, TARGET_RANK, WIN, IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Compare_and_swap(origin_addr, compare_addr, result_addr, datatype,
             target_rank, target_disp, win, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: origin_addr, compare_addr
     TYPE(*), DIMENSION(..) :: result_addr
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, INTENT(IN) :: target_rank
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: target_disp
     TYPE(MPI_Win), INTENT(IN) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **origin\_addr**: Initial address of buffer (choice).

- •

  **compare\_addr**: Initial address of compare buffer (choice).

- •

  **result\_addr**: Initial address of result buffer (choice).

- •

  **datatype**: Data type of the entry in origin, result, and target buffers (handle).

- •

  **target\_rank**: Rank of target (nonnegative integer).

- •

  **target\_disp**: Displacement from start of window to beginning of target buffer (nonnegative integer).

- •

  **win**: Window object (handle).

## OUTPUT PARAMETER

- •

  **ierror**: Fortran only: Error status (integer).

## DESCRIPTION

This function compares one element of type *datatype* in the compare buffer *compare\_addr* with the buffer at offset *target\_disp* in the target window specified by *target\_rank* and *win* and replaces the value at the target with the value in the origin buffer *origin\_addr* if the compare buffer and the target buffer are identical. The original value at the target is returned in the buffer *result\_addr*. The parameter *datatype* must belong to one of the following categories of predefined datatypes: C integer, Fortran integer, Logical, Multi-language types, or Byte as specified in MPI-3 section 5.9.2 on page 176.

The origin and result buffers (*origin\_addr* and *result\_addr*) must be disjoint.

## NOTES

It is the user’s responsibility to guarantee that, when using the accumulate functions, the target displacement argument is such that accesses to the window are properly aligned according to the data type arguments in the call to the *MPI\_Compare\_and\_swap* function.

## 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.

- •

  **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.

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

2003-2025, The Open MPI Community