8.7.2. Communicators

Functions for caching on communicators are:

Generates a new attribute key. Keys are locally unique in an MPI process, and opaque to 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 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.

With the mpi_f08 module, the Fortran callback functions are:

ABSTRACT INTERFACE
    SUBROUTINE MPI_Comm_copy_attr_function(oldcomm, comm_keyval, extra_state, attribute_val_in, attribute_val_out, flag, ierror)

TYPE(MPI_Comm) :: oldcomm
INTEGER :: comm_keyval, ierror
INTEGER(KIND=MPI_ADDRESS_KIND) :: extra_state, attribute_val_in, attribute_val_out
LOGICAL :: flag

and

ABSTRACT INTERFACE
    SUBROUTINE MPI_Comm_delete_attr_function(comm, comm_keyval, attribute_val, extra_state, ierror)

TYPE(MPI_Comm) :: comm
INTEGER :: comm_keyval, ierror
INTEGER(KIND=MPI_ADDRESS_KIND) :: attribute_val, extra_state

With the mpi module and (deprecated) mpif.h include file, the Fortran callback functions are:

SUBROUTINE COMM_COPY_ATTR_FUNCTION(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_FUNCTION(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, EXTRA_STATE, IERROR)

INTEGER COMM, COMM_KEYVAL, IERROR
INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL, EXTRA_STATE

The comm_copy_attr_fn function is invoked when a communicator is duplicated by MPI_COMM_DUP, MPI_COMM_IDUP, MPI_COMM_DUP_WITH_INFO or MPI_COMM_IDUP_WITH_INFO. comm_copy_attr_fn should be of type MPI_Comm_copy_attr_function. 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 or .FALSE., then the attribute is deleted in the duplicated communicator. Otherwise ( flag = 1 or .TRUE.), 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 or MPI_COMM_IDUP will fail).

The argument comm_copy_attr_fn may be specified as MPI_COMM_NULL_COPY_FN or MPI_COMM_DUP_FN from either C or Fortran. MPI_COMM_NULL_COPY_FN is a function that does nothing other than returning flag = 0 or .FALSE. (depending on whether the keyval was created with a C or Fortran binding to MPI_COMM_CREATE_KEYVAL) and MPI_SUCCESS. MPI_COMM_DUP_FN is a simple copy function that sets flag = 1 or .TRUE., 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.

Advice to users.

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). ( End of advice to users.)

Advice to implementors.

A C interface should be assumed for copy and delete functions associated with key values created in C; a Fortran calling interface should be assumed for key values created in Fortran. ( End of advice to implementors.)
Analogous to comm_copy_attr_fn is a callback deletion function, defined as follows. The comm_delete_attr_fn function is invoked when a communicator is deleted by MPI_COMM_FREE, MPI_COMM_DISCONNECT or when a call is made explicitly to MPI_COMM_DELETE_ATTR. comm_delete_attr_fn should be of type MPI_Comm_delete_attr_function.

This function is called by MPI_COMM_FREE, MPI_COMM_DISCONNECT, MPI_COMM_DELETE_ATTR, and MPI_COMM_SET_ATTR 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 will fail).

The argument comm_delete_attr_fn may be specified as MPI_COMM_NULL_DELETE_FN from either 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.

If an attribute copy function or attribute delete function returns other than MPI_SUCCESS, then the call that caused it to be invoked (for example, MPI_COMM_FREE) is erroneous.

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

Advice to implementors.

The predefined Fortran functions MPI_COMM_NULL_COPY_FN, MPI_COMM_DUP_FN, and MPI_COMM_NULL_DELETE_FN are defined in the mpi module (and deprecated mpif.h) and the mpi_f08 module with the same name, but with different interfaces. Each function can coexist twice with the same name in the same MPI library, one routine as an implicit interface outside of the mpi module, i.e., declared as EXTERNAL, and the other routine within mpi_f08 declared with CONTAINS. These routines have different link names, which are also different to the link names used for the routines used in C. ( End of advice to implementors.)

Advice to users.

Callbacks, including the predefined Fortran functions MPI_COMM_NULL_COPY_FN, MPI_COMM_DUP_FN, and MPI_COMM_NULL_DELETE_FN should not be passed from one application routine that uses the mpi_f08 module to another application routine that uses the mpi module or (deprecated) mpif.h include file, and vice versa; see also the advice to users on page Callback Functions. ( End of advice to users.)

Frees an extant attribute key. This function sets the value of keyval to MPI_KEYVAL_INVALID. Note that it is not erroneous to free an attribute key that is in use, because the actual free does not transpire until after all references (in other communicators on the MPI process) to the key have been freed. These references need to be explictly freed by the program, either via calls to MPI_COMM_DELETE_ATTR that free one attribute instance, or by calls to MPI_COMM_FREE that free all attribute instances associated with the freed communicator.

This function stores the stipulated attribute value attribute_val for subsequent retrieval by MPI_COMM_GET_ATTR. If the value is already present, then the outcome is as if MPI_COMM_DELETE_ATTRwas first called to delete the previous value (and the callback function comm_delete_attr_fn was executed), and a new value was next stored. The call is erroneous if there is no key with value keyval; in particular MPI_KEYVAL_INVALID is an erroneous key value. The call will fail if the comm_delete_attr_fn function returned an error code other than MPI_SUCCESS.

Retrieves attribute value by key. The call is erroneous if there is no key with value keyval. On the other hand, the call is correct if the key value exists, but no attribute is attached on comm for that key; in such case, the call returns flag = false. In particular MPI_KEYVAL_INVALID is an erroneous key value.

Advice to users.

The call to MPI_Comm_set_attr passes in attribute_val the value of the attribute; the call to MPI_Comm_get_attr passes in attribute_val the address of the location where the attribute value is to be returned. Thus, if the attribute value itself is a pointer of type void*, then the actual attribute_val parameter to MPI_Comm_set_attr will be of type void* and the actual attribute_val parameter to MPI_Comm_get_attr will be of type void**. ( End of advice to users.)

Rationale.

The use of a formal parameter attribute_val of type void* (rather than void**) avoids the messy type casting that would be needed if the attribute value is declared with a type other than void*. ( End of rationale.)

Delete attribute from cache by key. This function invokes the attribute delete function comm_delete_attr_fn specified when the keyval was created. The call will fail if the comm_delete_attr_fn function returns an error code other than MPI_SUCCESS.

Whenever a communicator is replicated using the function MPI_COMM_DUP, MPI_COMM_IDUP, MPI_COMM_DUP_WITH_INFO or MPI_COMM_IDUP_WITH_INFO, all call-back copy functions for attributes that are currently set are invoked (in arbitrary order). Whenever a communicator is deleted using the function MPI_COMM_FREE all callback delete functions for attributes that are currently set are invoked.