## 8.8.1. Communicators

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The new functions that are replacements for the MPI-1 functions for caching on communicators are:

MPI_COMM_CREATE_KEYVAL(comm_copy_attr_fn, comm_delete_attr_fn, comm_keyval, extra_state)
IN comm_copy_attr_fn | copy callback function for comm_keyval (function)
IN comm_delete_attr_fn | delete callback function for comm_keyval (function)
OUT comm_keyval | key value for future access (integer)
IN extra_state | extra state for callback functions

`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)
`

`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
`

`static int MPI::Comm::Create_keyval(MPI::Comm::Copy_attr_function* comm_copy_attr_fn, MPI::Comm::Delete_attr_function* comm_delete_attr_fn, void* extra_state)
`

This function replaces MPI_KEYVAL_CREATE, whose use 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 either C, C++, or Fortran. MPI_COMM_NULL_COPY_FN is a function that does nothing other 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, whose use is deprecated.

The argument comm_delete_attr_fn may be specified as MPI_COMM_NULL_DELETE_FN from either C, C++, or Fortran. MPI_COMM_NULL_DELETE_FN is a function that does nothing, other than returning MPI_SUCCESS. MPI_COMM_NULL_DELETE_FN replaces MPI_NULL_DELETE_FN, whose use is deprecated.

The 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);
`

and

`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 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
`

and

`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
`

The C++ callbacks are:

`typedef int MPI::Comm::Copy_attr_function(const MPI::Comm& oldcomm, int comm_keyval, void* extra_state, void* attribute_val_in, void* attribute_val_out, bool& flag);
`

and

`typedef int MPI::Comm::Delete_attr_function(MPI::Comm& comm, int comm_keyval, void* attribute_val, void* extra_state);
`

MPI_COMM_FREE_KEYVAL(comm_keyval)
INOUT comm_keyval | key value (integer)

`int MPI_Comm_free_keyval(int *comm_keyval)
`

`MPI_COMM_FREE_KEYVAL(COMM_KEYVAL, IERROR)
INTEGER COMM_KEYVAL, IERROR
`

`static void MPI::Comm::Free_keyval(int& comm_keyval)
`

This call is identical to the MPI-1 call MPI_KEYVAL_FREE but is needed to match the new communicator-specific creation function. The use of MPI_KEYVAL_FREE is deprecated.

MPI_COMM_SET_ATTR(comm, comm_keyval, attribute_val)
INOUT comm | communicator from which attribute will be attached (handle)
IN comm_keyval | key value (integer)
IN attribute_val | attribute value

`int MPI_Comm_set_attr(MPI_Comm comm, int comm_keyval, void *attribute_val)
`

`MPI_COMM_SET_ATTR(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, IERROR)
INTEGER COMM, COMM_KEYVAL, IERROR
INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL
`

`void MPI::Comm::Set_attr(int comm_keyval, const void* attribute_val) const
`

This function replaces MPI_ATTR_PUT, whose use is deprecated. The C binding is identical. The Fortran binding differs in that attribute_val is an address-sized integer.

MPI_COMM_GET_ATTR(comm, comm_keyval, attribute_val, flag)
IN comm | communicator to which the attribute is attached (handle)
IN comm_keyval | key value (integer)
OUT attribute_val | attribute value, unless flag = false
OUT flag | false if no attribute is associated with the key (logical)

`int MPI_Comm_get_attr(MPI_Comm comm, int comm_keyval, void *attribute_val, int *flag)
`

`MPI_COMM_GET_ATTR(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, FLAG, IERROR)
INTEGER COMM, COMM_KEYVAL, IERROR
INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL
LOGICAL FLAG
`

`bool MPI::Comm::Get_attr(int comm_keyval, void* attribute_val) const
`

This function replaces MPI_ATTR_GET, whose use is deprecated. The C binding is identical. The Fortran binding differs in that attribute_val is an address-sized integer.

MPI_COMM_DELETE_ATTR(comm, comm_keyval)
INOUT comm | communicator from which the attribute is deleted (handle)
IN comm_keyval | key value (integer)

`int MPI_Comm_delete_attr(MPI_Comm comm, int comm_keyval)
`

`MPI_COMM_DELETE_ATTR(COMM, COMM_KEYVAL, IERROR)
INTEGER COMM, COMM_KEYVAL, IERROR
`

`void MPI::Comm::Delete_attr(int comm_keyval)
`

This function is the same as MPI_ATTR_DELETE but is needed to match the new communicator specific functions. The use of MPI_ATTR_DELETE is deprecated.

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