# 18.2.82. MPI_Compare_and_swap

MPI_Compare_and_swap — Perform RMA compare-and-swap

## 18.2.82.1. SYNTAX

### 18.2.82.1.1. 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)

### 18.2.82.1.2. 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

### 18.2.82.1.3. 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

## 18.2.82.2. 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).

## 18.2.82.3. OUTPUT PARAMETER

- `ierror`: Fortran only: Error status (integer).

## 18.2.82.4. 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.

## 18.2.82.5. 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.

## 18.2.82.6. 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.