MPI_COMM_CREATE_KEYVAL(3)			     Open MPI				 MPI_COMM_CREATE_KEYVAL(3)

MPI_Comm_create_keyval — Generates a new attribute key.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Comm_create_keyval(MPI_Comm_copy_attr_function
	      *comm_copy_attr_fn, MPI_Comm_delete_attr_function
	      *comm_delete_attr_fn, int *comm_keyval,
	      void *extra_state)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'

	  MPI_COMM_CREATE_KEYVAL(COMM_COPY_ATTR_FN, COMM_DELETE_ATTR_FN,
	      COMM_KEYVAL, EXTRA_STATE, IERROR)
	      EXTERNAL COMM_COPY_ATTR_FN, COMM_DELETE_ATTR_FN
	      INTEGER COMM_KEYVAL, IERROR

	      INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Comm_create_keyval(comm_copy_attr_fn, comm_delete_attr_fn, comm_keyval,
		  extra_state, ierror)
	      PROCEDURE(MPI_Comm_copy_attr_function) :: comm_copy_attr_fn
	      PROCEDURE(MPI_Comm_delete_attr_function) :: comm_delete_attr_fn
	      INTEGER, INTENT(OUT) :: comm_keyval
	      INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: extra_state
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       •

	 comm_copy_attr_fn (Copy callback function for comm_keyval)
		(function).

       •

	 comm_delete_attr_fn (Delete callback function for)
		comm_keyval (function).

       • extra_state : Extra state for callback functions.

OUTPUT PARAMETER
       • comm_keyval : Key value for future access (integer).

       • ierror : Fortran only: Error status (integer).

DESCRIPTION
       This  function  replaces	 MPI_Keyval_create  <#mpi-keyval-create>,  the	use  of which is deprecated. The C
       binding is identical. The Fortran binding differs in that extra_state is an  address-sized  integer.  Also,
       the  copy  and  delete  callback	 functions  have  Fortran  bindings that are consistent with address-sized
       attributes. The argument comm_copy_attr_fn may be specified  as	MPI_COMM_NULL_COPY_FN  or  MPI_COMM_DUP_FN
       from  C	or Fortran. MPI_COMM_NULL_COPY_FN is a function that does nothing more than returning flag = 0 and
       MPI_SUCCESS. MPI_COMM_DUP_FN is a simple-minded copy function that sets flag =  1,  returns  the	 value	of
       attribute_val_in	 in  attribute_val_out,	 and  returns  MPI_SUCCESS.  These  replace  the  MPI-1 predefined
       callbacks MPI_NULL_COPY_FN and MPI_DUP_FN, the use of which is deprecated.  The two  C  callback	 functions
       are:

	  typedef int MPI_Comm_copy_attr_function(MPI_Comm oldcomm, int comm_keyval,
		       void *extra_state, void *attribute_val_in,
		       void *attribute_val_out, int *flag);

	  typedef int MPI_Comm_delete_attr_function(MPI_Comm comm, int comm_keyval,
		       void *attribute_val, void *extra_state);

       which  are the same as the MPI-1.1 calls but with a new name. The old names are deprecated. The two Fortran
       callback functions are:

	  SUBROUTINE COMM_COPY_ATTR_FN(OLDCOMM, COMM_KEYVAL, EXTRA_STATE,
		       ATTRIBUTE_VAL_IN, ATTRIBUTE_VAL_OUT, FLAG, IERROR)
	      INTEGER OLDCOMM, COMM_KEYVAL, IERROR

	      INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE, ATTRIBUTE_VAL_IN,
		  ATTRIBUTE_VAL_OUT
	      LOGICAL FLAG

	  SUBROUTINE COMM_DELETE_ATTR_FN(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, EXTRA_STATE,
		       IERROR)
	      INTEGER COMM, COMM_KEYVAL, IERROR

	      INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL, EXTRA_STATE

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.

       See also:

	  • MPI_Comm_free_keyval <#mpi-comm-free-keyval>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				 MPI_COMM_CREATE_KEYVAL(3)
