int MPI_Type_struct(int count, int array_of_blocklengths[],
MPI_Aint array_of_displacements[],
MPI_Datatype array_of_types[], MPI_Datatype *newtype)
The MPI standard originally made vague statements about padding and alignment; this was intended to allow the simple definition of structures that could be sent with a count greater than one.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 standard
defined the extent of a datatype as including an epsilon that would have
allowed an implementation to make the extent an MPI datatype
for this structure equal to 2*sizeof(int).
However, since different systems might define different paddings, there was
much discussion by the MPI Forum about what was the correct value of
epsilon, and one suggestion was to define epsilon as zero.
This would have been the best thing to do in MPI 1.0, particularly since
the 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 of
padding needed is determined by the compiler that the user is using to
build 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 are
advised to never depend on this, even with vendor MPI implementations.
Instead, if you define a structure datatype and wish to send or receive
multiple items, you should explicitly include an MPI_UB entry as the
last member of the structure. For example, the following code can be used
for the structure foo
blen[0] = 1; array_of_displacements[0] = 0; oldtypes[0] = MPI_INT;
blen[1] = 1; array_of_displacements[1] = &foo.b - &foo; oldtypes[1] = MPI_CHAR;
blen[2] = 1; array_of_displacements[2] = sizeof(foo); oldtypes[2] = MPI_UB;
MPI_Type_struct(3, blen, array_of_displacements, oldtypes, &newtype);
This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in 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 last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.