42. Message Data

Up: Blocking Send and Receive Operations Next: Message Envelope Previous: Blocking Send

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

Table 2: Predefined MPI datatypes corresponding to Fortran datatypes

Possible values for this argument for C and the corresponding C types are listed in Table 3 .

MPI datatype C datatype
MPI_CHAR signed 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_LONG_DOUBLE long double
MPI_WCHAR wchar_t
(defined in <stddef.h>)
(treated as printable character)

Table 3: Predefined MPI datatypes corresponding to C datatypes

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.)

Up: Blocking Send and Receive Operations Next: Message Envelope Previous: Blocking Send

Return to MPI-2.1 Standard Index
Return to MPI Forum Home Page

MPI-2.0 of July 1, 2008
HTML Generated on July 6, 2008