MPI_COMPARE_AND_SWAP(3)				     Open MPI				   MPI_COMPARE_AND_SWAP(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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				   MPI_COMPARE_AND_SWAP(3)
