There are two situations that cannot be handled by the required representations:
| MPI_REGISTER_DATAREP(datarep, read_conversion_fn, write_conversion_fn, dtype_file_extent_fn, extra_state) | |
| IN datarep | data representation identifier (string) |
| IN read_conversion_fn | function invoked to convert from file representation to native representation (function) |
| IN write_conversion_fn | function invoked to convert from native representation to file representation (function) |
| IN dtype_file_extent_fn | function invoked to get the extent of a datatype as represented in the file (function) |
| IN extra_state | extra state |
The call associates read_conversion_fn, write_conversion_fn, and dtype_file_extent_fn with the data representation identifier datarep. datarep can then be used as an argument to MPI_FILE_SET_VIEW, causing subsequent data access operations to call the conversion functions to convert all data items accessed between file data representation and native representation. MPI_REGISTER_DATAREP is a local operation and only registers the data representation for the calling MPI process. If datarep is already defined, an error in the error class MPI_ERR_DUP_DATAREP is raised using the default file error handler (see Section I/O Error Handling). The length of a data representation string is limited to the value of MPI_MAX_DATAREP_STRING. MPI_MAX_DATAREP_STRING must have a value of at least 64. No routines are provided to delete data representations and free the associated resources; it is not expected that an application will generate them in significant numbers.
typedef int MPI_Datarep_extent_function(MPI_Datatype datatype, MPI_Aint *extent, void *extra_state);
ABSTRACT INTERFACE
SUBROUTINE MPI_Datarep_extent_function(datatype, extent, extra_state, ierror)
TYPE(MPI_Datatype) :: datatype
INTEGER(KIND=MPI_ADDRESS_KIND) :: extent, extra_state
INTEGER :: ierror
SUBROUTINE DATAREP_EXTENT_FUNCTION(DATATYPE, EXTENT, EXTRA_STATE, IERROR)
INTEGER DATATYPE, IERROR
INTEGER(KIND=MPI_ADDRESS_KIND) EXTENT, EXTRA_STATE
The function dtype_file_extent_fn must return, in file_extent, the number of bytes required to store datatype in the file representation. The function is passed, in extra_state, the argument that was passed to the MPI_REGISTER_DATAREP call. MPI will only call this routine with predefined datatypes employed by the user.
Rationale.
This callback does not have a large count variant because it is
anticipated that large counts will not be required to represent the
extent output value.
( End of rationale.)
MPI_Datarep_conversion_function
also supports large count types in separate additional
MPI procedures in C (suffixed with the ``_c'') and
multiple abstract interfaces in Fortran when using USE mpi_f08.
If the extent cannot be represented in extent, the callback function shall set extent to MPI_UNDEFINED. The MPI implementation will then raise an error of class MPI_ERR_VALUE_TOO_LARGE.
typedef int MPI_Datarep_conversion_function(void *userbuf, MPI_Datatype datatype, int count, void *filebuf, MPI_Offset position, void *extra_state);
typedef int MPI_Datarep_conversion_function_c(void *userbuf, MPI_Datatype datatype, MPI_Count count, void *filebuf, MPI_Offset position, void *extra_state);
ABSTRACT INTERFACE
SUBROUTINE MPI_Datarep_conversion_function(userbuf, datatype, count, filebuf, position, extra_state, ierror)
USE, INTRINSIC :: ISO_C_BINDING, ONLY : C_PTR
TYPE(C_PTR), VALUE :: userbuf, filebuf
TYPE(MPI_Datatype) :: datatype
INTEGER :: count, ierror
INTEGER(KIND=MPI_OFFSET_KIND) :: position
INTEGER(KIND=MPI_ADDRESS_KIND) :: extra_state
ABSTRACT INTERFACE
SUBROUTINE MPI_Datarep_conversion_function_c(userbuf, datatype, count, filebuf, position, extra_state, ierror) !(_c)
USE, INTRINSIC :: ISO_C_BINDING, ONLY : C_PTR
TYPE(C_PTR), VALUE :: userbuf, filebuf
TYPE(MPI_Datatype) :: datatype
INTEGER(KIND=MPI_COUNT_KIND) :: count
INTEGER(KIND=MPI_OFFSET_KIND) :: position
INTEGER(KIND=MPI_ADDRESS_KIND) :: extra_state
INTEGER :: ierror
SUBROUTINE DATAREP_CONVERSION_FUNCTION(USERBUF, DATATYPE, COUNT, FILEBUF, POSITION, EXTRA_STATE, IERROR)
<TYPE> USERBUF(*), FILEBUF(*)
INTEGER DATATYPE, COUNT, IERROR
INTEGER(KIND=MPI_OFFSET_KIND) POSITION
INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE
The function
read_conversion_fn
must convert from
file data representation to native representation.
Before calling this routine,
MPI allocates and fills filebuf with count
contiguous data items.
The type of each data item matches the
corresponding entry for the predefined datatype
in the type signature of datatype.
The function is passed, in extra_state,
the argument that was passed to the MPI_REGISTER_DATAREP call.
The function must copy all count data items from filebuf
to userbuf in the distribution described by datatype,
converting each data item
from file representation to native representation.
datatype will be equivalent to the datatype that the user
passed to the
read
function.
If the size of datatype is less than the size
of the count data items, the conversion function must treat
datatype as being contiguously tiled over the userbuf.
The conversion function must
begin storing converted data at the location in userbuf
specified by position into the (tiled) datatype.
Advice to users.
Although the conversion functions have similarities
to MPI_PACK and
MPI_UNPACK,
one should note the differences in the use
of the arguments count and position.
In the conversion functions,
count is a count of data items
(i.e., count of typemap entries of datatype),
and position is an index into this typemap.
In MPI_PACK,
incount refers to the number of whole datatypes,
and position is a number of bytes.
( End of advice to users.)
Advice
to implementors.
A converted read operation could be implemented as follows:
Rationale.
Passing the conversion function a position and one datatype for the transfer
allows the conversion function to decode the datatype only once and
cache an internal representation of it on the datatype.
Then on subsequent calls, the conversion function can use the position
to quickly find its place in the datatype and continue
storing converted data where it left off at the end of the previous call.
( End of rationale.)
Advice to users.
Although the conversion function may usefully cache an
internal representation on the datatype, it should not cache
any state information specific to an ongoing conversion
operation, since it is possible for the same datatype to
be used concurrently in multiple conversion operations.
( End of advice to users.)
The function write_conversion_fn must convert from
native representation to file data representation.
Before calling this routine,
MPI allocates filebuf of a size large enough to hold count
contiguous data items.
The type of each data item matches the
corresponding entry for the predefined datatype
in the type signature of datatype.
The function must copy count data items from
userbuf in the distribution described by
datatype,
to a contiguous distribution in filebuf, converting each data item
from native representation to file representation.
If the size of datatype is less than the size
of count
data items,
the conversion function must treat
datatype as being contiguously tiled over the
userbuf.
The function must begin copying at the location in userbuf specified by position into the (tiled) datatype. datatype will be equivalent to the datatype that the user passed to the write function. The function is passed, in extra_state, the argument that was passed to the MPI_REGISTER_DATAREP call.
The predefined constant MPI_CONVERSION_FN_NULL may be used as either write_conversion_fn or read_conversion_fn in bindings of MPI_REGISTER_DATAREP without large counts in these conversion callbacks, whereas the constant MPI_CONVERSION_FN_NULL_C can be used in the large count version (i.e., MPI_Register_datarep_c). In either of these cases, MPI will not attempt to invoke write_conversion_fn or read_conversion_fn, respectively, but will perform the requested data access using the native data representation.
An MPI implementation must ensure that all data accessed is converted, either by using a filebuf large enough to hold all the requested data items or else by making repeated calls to the conversion function with the same datatype argument and appropriate values for position.
An implementation will only invoke the callback routines in this section ( read_conversion_fn, write_conversion_fn, and dtype_file_extent_fn) when one of the read or write routines in Section Data Access, or MPI_FILE_GET_TYPE_EXTENT is called by the user. dtype_file_extent_fn will only be passed predefined datatypes employed by the user. The conversion functions will only be passed datatypes equivalent to those that the user has passed to one of the routines noted above.
The conversion functions must be reentrant. User defined data representations are restricted to use byte alignment for all types. Furthermore, it is erroneous for the conversion functions to call any collective routines or to free datatype.
The conversion functions should return an error code. If the returned error code has a value other than MPI_SUCCESS, the implementation will raise an error in the class MPI_ERR_CONVERSION.