The send buffer specified by the MPI_SEND operation consists of count successive entries of the type indicated by datatype, starting with the entry at address buf. Note that we specify the message length in terms of number of elements, not number of bytes. The former is machine independent and closer to the application level.
The data part of the message consists of a sequence of count values, each of the type indicated by datatype. count may be zero, in which case the data part of the message is empty. The basic datatypes that can be specified for message data values correspond to the basic datatypes of the host language. Possible values of this argument for Fortran and the corresponding Fortran types are listed in Table 2 .
| MPI datatype | Fortran datatype |
| MPI_INTEGER | INTEGER |
| MPI_REAL | REAL |
| MPI_DOUBLE_PRECISION | DOUBLE PRECISION |
| MPI_COMPLEX | COMPLEX |
| MPI_LOGICAL | LOGICAL |
| MPI_CHARACTER | CHARACTER(1) |
| MPI_BYTE | |
| MPI_PACKED | |
Possible values for this argument for C and the corresponding C types are listed in Table 3 .
| MPI datatype | C datatype |
| MPI_CHAR | char |
| (treated as printable character) | |
| MPI_SHORT | signed short int |
| MPI_INT | signed int |
| MPI_LONG | signed long int |
| MPI_LONG_LONG_INT | signed long long int |
| MPI_LONG_LONG (as a synonym) | signed long long int |
| MPI_SIGNED_CHAR | signed char |
| (treated as integral value) | |
| MPI_UNSIGNED_CHAR | unsigned char |
| (treated as integral value) | |
| MPI_UNSIGNED_SHORT | unsigned short int |
| MPI_UNSIGNED | unsigned int |
| MPI_UNSIGNED_LONG | unsigned long int |
| MPI_UNSIGNED_LONG_LONG | unsigned long long int |
| MPI_FLOAT | float |
| MPI_DOUBLE | double |
| MPI_LONG_DOUBLE | long double |
| MPI_WCHAR | wchar_t |
| (defined in <stddef.h>) | |
| (treated as printable character) | |
| MPI_C_BOOL | _Bool |
| MPI_INT8_T | int8_t |
| MPI_INT16_T | int16_t |
| MPI_INT32_T | int32_t |
| MPI_INT64_T | int64_t |
| MPI_UINT8_T | uint8_t |
| MPI_UINT16_T | uint16_t |
| MPI_UINT32_T | uint32_t |
| MPI_UINT64_T | uint64_t |
| MPI_C_COMPLEX | float _Complex |
| MPI_C_FLOAT_COMPLEX (as a synonym) | float _Complex |
| MPI_C_DOUBLE_COMPLEX | double _Complex |
| MPI_C_LONG_DOUBLE_COMPLEX | long double _Complex |
| MPI_BYTE | |
| MPI_PACKED | |
| MPI datatype | C datatype | Fortran datatype |
| MPI_AINT | MPI_Aint | INTEGER (KIND=MPI_ADDRESS_KIND) |
| MPI_OFFSET | MPI_Offset | INTEGER (KIND=MPI_OFFSET_KIND) |
The datatypes MPI_BYTE and MPI_PACKED do not correspond to a
Fortran or C datatype. A value of type MPI_BYTE consists of a byte
(8 binary digits). A
byte is uninterpreted and is different from a character.
Different machines may have
different representations for characters, or may use more than one
byte to represent characters. On the other hand, a byte has the same
binary value on all machines.
The use of the type MPI_PACKED is explained in
Section Pack and Unpack
.
MPI requires support of
these datatypes,
which match the basic
datatypes of
Fortran and ISO C.
Additional MPI datatypes should be provided if the host language has
additional data types:
MPI_DOUBLE_COMPLEX for double precision complex in
Fortran declared to be of type DOUBLE COMPLEX;
MPI_REAL2,
MPI_REAL4 and MPI_REAL8 for Fortran reals, declared to be of
type REAL*2, REAL*4 and REAL*8, respectively;
MPI_INTEGER1 MPI_INTEGER2 and
MPI_INTEGER4 for Fortran integers, declared to be of type
INTEGER*1, INTEGER*2 and INTEGER*4, respectively; etc.
One goal of the design is to allow for MPI to be implemented as a library,
with no need for additional preprocessing or compilation.
Thus, one cannot assume that a communication call has information on the
datatype of variables in the communication buffer; this information must be
supplied by an explicit argument.
The need for such datatype information will become clear in
Section Data Conversion
.
( End of rationale.)
The datatypes MPI_C_BOOL, MPI_INT8_T, MPI_INT16_T,
MPI_INT32_T, MPI_UINT8_T, MPI_UINT16_T,
MPI_UINT32_T,
MPI_C_COMPLEX, MPI_C_FLOAT_COMPLEX,
MPI_C_DOUBLE_COMPLEX,
and MPI_C_LONG_DOUBLE_COMPLEX have no corresponding C++
bindings. This was intentionally done to avoid potential
collisions with the C preprocessor and namespaced C++ names.
C++ applications can use the C bindings with no loss of functionality.
( End of rationale.)
Rationale.
Rationale.
The datatypes MPI_AINT and MPI_OFFSET correspond to the
MPI-defined C types MPI_Aint and MPI_Offset and their Fortran
equivalents INTEGER (KIND= MPI_ADDRESS_KIND) and INTEGER
(KIND= MPI_OFFSET_KIND). This is described in Table 4
. See
Section Interlanguage Communication
for information on
interlanguage communication with these types.
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