#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;
| 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) | 
The MPI standard originally made vague statements about padding and alignment; this was intended to allow the simple definition of structures that couldbe sent with a count greater than one.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 );
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.
Location:type_struct.c