The send buffer specified by the MPI_SEND procedure 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. Possible values for this argument for C and the corresponding C types are listed in Table 3.
Table 2: Predefined MPI datatypes corresponding to Fortran datatypes
| 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 | |
| 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) |
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 datatypes*: MPI_DOUBLE_COMPLEX for double precision complex in Fortran declared to be of type DOUBLE COMPLEX; MPI_REAL2, MPI_REAL4, MPI_REAL8, and MPI_REAL16 for Fortran reals, declared to be of type REAL*2, REAL*4, REAL*8, and REAL*16, respectively; MPI_INTEGER1, MPI_INTEGER2, MPI_INTEGER4, and MPI_INTEGER8 for Fortran integers, declared to be of type INTEGER*1, INTEGER*2, INTEGER*4, and INTEGER*8, respectively; MPI_COMPLEX4, MPI_COMPLEX8, MPI_COMPLEX16, and MPI_COMPLEX32 for complex numbers in Fortran declared to be of type COMPLEX*4, COMPLEX*8, COMPLEX*16, and COMPLEX*32, 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 4.
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 5 are also supported in C and Fortran.
Table 5: Predefined MPI datatypes corresponding to C++ datatypes
| 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> |