Creates a struct datatype


#include "mpi.h"
int MPI_Type_struct( count, blocklens, indices, old_types, newtype )
int           count;
int        blocklens[];
MPI_Aint      indices[];      
MPI_Datatype  old_types[];
MPI_Datatype *newtype;

Input Parameters

count number of blocks (integer) -- also number of entries in arrays array_of_types , array_of_displacements and array_of_blocklengths
blocklens number of elements in each block (array)
indices byte displacement of each block (array)
old_types type of elements in each block (array of handles to datatype objects)

Output Parameter

new datatype (handle)


If an upperbound is set explicitly by using the MPI datatype MPI_UB, thecorresponding index must be positive.

The MPI standard originally made vague statements about padding and alignment; this was intended to allow the simple definition of structures that couldbe sent with a count greater than one. For example,

    struct { int a; char b; } foo;
may have sizeof(foo) > sizeof(int) + sizeof(char); for example, sizeof(foo) == 2*sizeof(int). The initial version of the MPI standarddefined the extent of a datatype as including an epsilon that would haveallowed an implementation to make the extent an MPI datatypefor this structure equal to 2*sizeof(int). However, since different systems might define different paddings, there wasmuch discussion by the MPI Forum about what was the correct value ofepsilon, and one suggestion was to define epsilon as zero. This would have been the best thing to do in MPI 1.0, particularly sincethe MPI_UB type allows the user to easily set the end of the structure. Unfortunately, this change did not make it into the final document. Currently, this routine does not add any padding, since the amount ofpadding needed is determined by the compiler that the user is using tobuild their code, not the compiler used to construct the MPI library. A later version of MPICH may provide for some natural choices of padding(e.g., multiple of the size of the largest basic member), but users areadvised to never depend on this, even with vendor MPI implementations. Instead, if you define a structure datatype and wish to send or receivemultiple items, you should explicitly include an MPI_UB entry as thelast member of the structure. For example, the following code can be usedfor the structure foo
    blen[0] = 1; indices[0] = 0; oldtypes[0] = MPI_INT;
    blen[1] = 1; indices[1] = &foo.b - &foo; oldtypes[1] = MPI_CHAR;
    blen[2] = 1; indices[2] = sizeof(foo); oldtypes[2] = MPI_UB;
    MPI_Type_struct( 3, blen, indices, oldtypes, &newtype );

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) havean additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routinein C. In Fortran, MPI routines are subroutines, and are invoked with thecall statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGERin Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the lastargument. Before the value is returned, the current MPI error handler iscalled. By default, this error handler aborts the MPI job. The error handlermay be changed with MPI_Errhandler_set; the predefined error handlerMPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue pastan error.

No error; MPI routine completed successfully.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
This error is returned when some part of the MPICH implementation is unable to acquire memory.