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 1 .
| 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 2 .
| 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) |
| MPI_COUNT | MPI_Count | INTEGER (KIND=MPI_COUNT_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.
Rationale.
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_AINT,
MPI_OFFSET, and MPI_COUNT
correspond to the
MPI-defined C types MPI_Aint,
MPI_Offset, and MPI_Count
and their Fortran
equivalents INTEGER (KIND= MPI_ADDRESS_KIND),
INTEGER (KIND= MPI_OFFSET_KIND),
and INTEGER (KIND= MPI_COUNT_KIND).
This is described in Table 3
.
All predefined datatype handles are available in all language bindings.
See
Sections MPI Opaque Objects
and Interlanguage Communication
on page MPI Opaque Objects
and Interlanguage Communication
for information on interlanguage communication with these types.
If there is an accompanying C++ compiler then the datatypes in Table Message Data are also supported in C and Fortran.
| MPI datatype | C++ datatype |
| MPI_CXX_BOOL | bool |
| MPI_CXX_FLOAT_COMPLEX | std::complex<float> |
| MPI_CXX_DOUBLE_COMPLEX | std::complex<double> |
| MPI_CXX_LONG_DOUBLE_COMPLEX | std::complex<long double> |