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.TH "MPI_COMPARE_AND_SWAP" "3" "May 30, 2025" "" "Open MPI"
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.sp
\fI\%MPI_Compare_and_swap\fP — Perform RMA compare\-and\-swap
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#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)
.ft P
.fi
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.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
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
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.fi
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.SS Fortran 2008 Syntax
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.INDENT 3.5
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.nf
.ft C
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
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.fi
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.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBorigin_addr\fP: Initial address of buffer (choice).
.IP \(bu 2
\fBcompare_addr\fP: Initial address of compare buffer (choice).
.IP \(bu 2
\fBresult_addr\fP: Initial address of result buffer (choice).
.IP \(bu 2
\fBdatatype\fP: Data type of the entry in origin, result, and target buffers (handle).
.IP \(bu 2
\fBtarget_rank\fP: Rank of target (nonnegative integer).
.IP \(bu 2
\fBtarget_disp\fP: Displacement from start of window to beginning of target buffer (nonnegative integer).
.IP \(bu 2
\fBwin\fP: Window object (handle).
.UNINDENT
.SH OUTPUT PARAMETER
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.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
This function compares one element of type \fIdatatype\fP in the compare
buffer \fIcompare_addr\fP with the buffer at offset \fItarget_disp\fP in the
target window specified by \fItarget_rank\fP and \fIwin\fP and replaces the
value at the target with the value in the origin buffer \fIorigin_addr\fP if
the compare buffer and the target buffer are identical. The original
value at the target is returned in the buffer \fIresult_addr\fP\&. The
parameter \fIdatatype\fP 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.
.sp
The origin and result buffers (\fIorigin_addr\fP and \fIresult_addr\fP) must be
disjoint.
.SH NOTES
.sp
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 \fI\%MPI_Compare_and_swap\fP function.
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
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.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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.sp
MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.SH COPYRIGHT
2003-2025, The Open MPI Community
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