The Great and Terrible implementation of MPI-2

function index


Get the true lower bound and extent for a datatype
int MPI_Type_get_true_extent(
  MPI_Datatype datatype,
  MPI_Aint *true_lb,
  MPI_Aint *true_extent


[in] datatype to get information on (handle)
[out] true lower bound of datatype (integer)
[out] true size of datatype (integer)


Suppose we implement gather as a spanning tree implemented on top of point-to-point routines. Since the receive buffer is only valid on the root process, one will need to allocate some temporary space for receiving data on intermediate nodes. However, the datatype extent cannot be used as an estimate of the amount of space that needs to be allocated, if the user has modified the extent using the MPI_UB and MPI_LB values. A new function is provided which returns the true extent of the datatype.

true_lb returns the offset of the lowest unit of store which is addressed by the datatype, i.e., the lower bound of the corresponding typemap, ignoring MPI_LB markers. true_extent returns the true size of the datatype, i.e., the extent of the corresponding typemap, ignoring MPI_LB and MPI_UB markers, and performing no rounding for alignment. If the typemap associated with datatype is

Typemap = { (type0, disp0), ... , (typen-1, dispn-1)}




true_extent (Typemap) = true_ub(Typemap) - true_lb(typemap).

(Readers should compare this with the definitions in Section 3.12.3 of the MPI-1 standard, which describes the function MPI_TYPE_EXTENT.)

The true_extent is the minimum number of bytes of memory necessary to hold a datatype, uncompressed.

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

Notes for Fortran

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

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 guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Type_get_true_extent.

Insert code here.