# MPI_Compare_and_swap

Perform one-sided atomic compare-and-swap.

## Synopsis

    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)

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.

## 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
  datatype 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 (non-negative integer)
win
  window object (handle)

## Notes

This operation is atomic with respect to other "accumulate" operations.

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 Section 5.9.2 on page 176. The origin and result buffers (origin_addr and result_addr) must be disjoint.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. The predefined error handler `MPI_ERRORS_RETURN` may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

MPI_ERR_COUNT
  Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_OP
  Invalid operation. MPI operations (objects of type `MPI_Op`) must either be one of the predefined operations (e.g., `MPI_SUM`) or created with `MPI_Op_create`.

MPI_ERR_RANK
  Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_SOURCE`.

MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.

MPI_ERR_WIN
  Invalid MPI window object