MPI_KEYVAL_CREATE(3)				     Open MPI				      MPI_KEYVAL_CREATE(3)

MPI_Keyval_create  — Generates a new attribute key — this routine is deprecated in favor of MPI_Comm_create_keyval
<#mpi-comm-create-keyval>.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Keyval_create(MPI_Copy_function *copy_fn,
	       MPI_Delete_function *delete_fn, int *keyval, void *extra_state)

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

	  MPI_KEYVAL_CREATE(COPY_FN, DELETE_FN, KEYVAL, EXTRA_STATE, IERROR)
	       EXTERNAL	       COPY_FN, DELETE_FN
	       INTEGER KEYVAL, EXTRA_STATE, IERROR

INPUT PARAMETERS
       • copy_fn: Copy callback function for keyval.

       • delete_fn: Delete callback function for keyval.

       • extra_state: Extra state for callback functions.

OUTPUT PARAMETERS
       • keyval: Key value for future access (integer).

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

DESCRIPTION
       Note  that  use	of  this  routine  is  deprecated  as  of  MPI-2.  Please  use	MPI_Comm_create_keyval	<#
       mpi-comm-create-keyval> instead.

       Generates a new attribute key. Keys are locally unique in a process and opaque to the user, though they are
       explicitly stored in integers. Once allocated, the key value can be used to associate attributes and access
       them on any locally defined communicator.

       The  copy_fn function is invoked when a communicator is duplicated by MPI_Comm_dup <#mpi-comm-dup>. copy_fn
       should be of type MPI_Copy_function, which is defined as follows:

	  typedef int MPI_Copy_function(MPI_Comm oldcomm, int keyval,
					void *extra_state, void *attribute_val_in,
					void *attribute_val_out, int *flag)

       A Fortran declaration for such a function is as follows:

	  SUBROUTINE COPY_FUNCTION(OLDCOMM, KEYVAL, EXTRA_STATE, ATTRIBUTE_VAL_IN,
		      ATTRIBUTE_VAL_OUT, FLAG, IERR)
	  INTEGER OLDCOMM, KEYVAL, EXTRA_STATE,
	  ATTRIBUTE_VAL_IN, ATTRIBUTE_VAL_OUT, IERR
	  LOGICAL FLAG

       The copy callback function is invoked for each key value in oldcomm in arbitrary order. Each  call  to  the
       copy  callback  is  made with a key value and its corresponding attribute. If it returns flag = 0, then the
       attribute is deleted in the duplicated communicator. Otherwise ( flag = 1), the new attribute value is  set
       to  the	value returned in attribute_val_out. The function returns MPI_SUCCESS on success and an error code
       on failure (in which case MPI_Comm_dup <#mpi-comm-dup> will fail).

       copy_fn may be specified as MPI_NULL_COPY_FN or MPI_DUP_FN from either C or Fortran; MPI_NULL_COPY_FN is	 a
       function	 that does nothing other than return flag = 0, and MPI_SUCCESS. MPI_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.

NOTES
       Key values are global (available for any and all communicators).

       There  are  subtle  differences	between C and Fortran that require that the copy_fn be written in the same
       language that MPI_Keyval_create is called from.	This  should  not  be  a  problem  for	most  users;  only
       programmers using both Fortran and C in the same program need to be sure that they follow this rule.

       Even  though  both  formal  arguments attribute_val_in and attribute_val_out are of type void*, their usage
       differs. The C copy function is passed by MPI in attribute_val_in  the  value  of  the  attribute,  and	in
       attribute_val_out  the  address of the attribute, so as to allow the function to return the (new) attribute
       value. The use of type void * for both is to avoid messy type casts.

       A valid copy function is one that completely duplicates the information by making a full duplicate copy	of
       the  data  structures  implied  by  an  attribute;  another  might just make another reference to that data
       structure, while using a reference-count mechanism. Other types of attributes might not copy at	all  (they
       might be specific to oldcomm only).

       Analogous to copy_fn is a callback deletion function, defined as follows. The delete_fn function is invoked
       when  a	communicator  is  deleted  by  MPI_Comm_free <#mpi-comm-free> or when a call is made explicitly to
       MPI_Attr_delete <#mpi-attr-delete>. delete_fn should be of type MPI_Delete_function, which  is  defined	as
       follows:

	  typedef int MPI_Delete_function(MPI_Comm comm, int keyval,
	      void *attribute_val, void *extra_state);

       A Fortran declaration for such a function is as follows:

	  SUBROUTINE DELETE_FUNCTION(COMM, KEYVAL,ATTRIBUTE_VAL, EXTRA_STATE, IERR)
	      INTEGER COMM, KEYVAL, ATTRIBUTE_VAL, EXTRA_STATE, IERR

       This  function  is  called  by  MPI_Comm_free  <#mpi-comm-free>,	 MPI_Attr_delete  <#mpi-attr-delete>,  and
       MPI_Attr_put <#mpi-attr-put> to do whatever  is	needed	to  remove  an	attribute.  The	 function  returns
       MPI_SUCCESS  on	success	 and  an  error code on failure (in which case MPI_Comm_free <#mpi-comm-free> will
       fail).

       delete_fn may be specified as MPI_NULL_DELETE_FN from either C or FORTRAN; MPI_NULL_DELETE_FN is a function
       that does nothing, other than returning MPI_SUCCESS.

       The special key value MPI_KEYVAL_INVALID is never returned by MPI_Keyval_create. Therefore, it can be  used
       for static initialization of key values.

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_Keyval_free <#mpi-keyval-free>

	  • MPI_Comm_create_keyval <#mpi-comm-create-keyval>

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

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				      MPI_KEYVAL_CREATE(3)
