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MPI_Compare_and_swap

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Perform one-sided atomic compare-and-swap. 
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Synopsis

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int MPI_Compare_and_swap(const void *origin_addr, const void *compare_addr,
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                         void *result_addr, MPI_Datatype datatype, int target_rank,
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                         MPI_Aint target_disp, MPI_Win win)
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This function compares one element of type datatype in the compare buffer
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compare_addr with the buffer at offset target_disp in the target window
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specified by target_rank and win and replaces the value at the target with the
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value in the origin buffer origin_addr if the compare buffer and the target
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buffer are identical. The original value at the target is returned in the
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buffer result_addr.
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Input Parameters

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origin_addr
initial address of buffer (choice)
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compare_addr
initial address of compare buffer (choice)
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result_addr
initial address of result buffer (choice)
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datatype
datatype of the entry in origin, result, and target buffers (handle)
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target_rank
rank of target (nonnegative integer)
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target_disp
displacement from start of window to beginning of target buffer (non-negative integer)
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win
window object (handle)
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Notes

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This operation is atomic with respect to other "accumulate" operations.
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The parameter datatype must belong to one of the following categories of
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predefined datatypes: C integer, Fortran integer, Logical, Multi-language
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types, or Byte as specified in Section 5.9.2 on page 176. The origin and result
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buffers (origin_addr and result_addr) must be disjoint.
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Notes for Fortran

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All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
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an additional argument <tt>ierr</tt> at the end of the argument list.  <tt>ierr
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</tt>is an integer and has the same meaning as the return value of the routine
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in C.  In Fortran, MPI routines are subroutines, and are invoked with the
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<tt>call</tt> statement.
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All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
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</tt>in Fortran.
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Errors

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All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) return an error value;
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C routines as the value of the function and Fortran routines in the last
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argument.  Before the value is returned, the current MPI error handler is
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called.  By default, this error handler aborts the MPI job.  The error handler
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may be changed with <tt>MPI_Comm_set_errhandler</tt> (for communicators),
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<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
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RMA windows).  The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
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its use is deprecated.  The predefined error handler
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<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
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Note that MPI does not guarentee that an MPI program can continue past
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an error; however, MPI implementations will attempt to continue whenever
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possible.
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MPI_SUCCESS
No error; MPI routine completed successfully.
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MPI_ERR_ARG
Invalid argument. Some argument is invalid and is not
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identified by a specific error class (e.g., <tt>MPI_ERR_RANK</tt>).
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MPI_ERR_COUNT
Invalid count argument. Count arguments must be
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non-negative; a count of zero is often valid.
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MPI_ERR_OP
Invalid operation. MPI operations (objects of type <tt>MPI_Op</tt>)
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must either be one of the predefined operations (e.g., <tt>MPI_SUM</tt>) or
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created with <tt>MPI_Op_create</tt>.
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MPI_ERR_RANK
Invalid source or destination rank. Ranks must be between
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zero and the size of the communicator minus one; ranks in a receive
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(<tt>MPI_Recv</tt>, <tt>MPI_Irecv</tt>, <tt>MPI_Sendrecv</tt>, etc.) may also be <tt>MPI_ANY_SOURCE</tt>.
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MPI_ERR_TYPE
Invalid datatype argument. Additionally, this error can
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occur if an uncommitted MPI_Datatype (see <tt>MPI_Type_commit</tt>) is used
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in a communication call.
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MPI_ERR_WIN
Invalid MPI window object
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