MPI_Alltoall(3)

NAME
MPI_Alltoall - Sends data from all to all processes

SYNOPSIS
#include <mpi.h>
int MPI_Alltoall(void *sbuf, int scount, MPI_Datatype sdtype,
                void* rbuf, int rcount, MPI_Datatype rdtype,
                MPI_Comm comm)

INPUT PARAMETERS
sbuf
    - starting address of send buffer (choice)
scount
    - number of elements to send to each process (integer)
sdtype
    - data type of send buffer elements (handle)
rcount
    - number of elements received from any process (integer)
rdtype
    - data type of receive buffer elements (handle)
comm
    - communicator (handle)

OUTPUT PARAMETER
rbuf
    - address of receive buffer (choice)

NOTES
The rcount parameter is the value to be expected from each process, not the total that is expected from all processes. The receive buffer rbuf , however, needs to be large enough to hold the data that will be received from all processes, however. For example, scount and sdtype and typically the same as rcount and rdtype .

USAGE WITH IMPI EXTENSIONS
LAM/MPI does not yet support invoking this function on a communicator that contains ranks that are non-local IMPI procs.

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.

ERRORS
If an error occurs in an MPI function, the current MPI error handler is called to handle it. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Errhandler_set ; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned (in C and Fortran; this error handler is less useful in with the C++ MPI bindings. The predefined error handler MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value needs to be recovered). Note that MPI does not guarantee that an MPI program can continue past an error. 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. The C++ bindings for MPI do not return error values; instead, error values are communicated by throwing exceptions of type MPI::Exception (but not by default). Exceptions are only thrown if the error value is not MPI::SUCCESS . Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error, there will be no way to recover what the actual error value was.
MPI_ERR_COMM
    - Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank ).
MPI_ERR_OTHER
    - A collective implementation was not able to be located at run-time for this communicator.
MPI_ERR_OTHER
    - A communicator that contains some non-local IMPI procs was used for some function which has not yet had the IMPI extensions implemented yet. For example, most collectives on IMPI communicators have not been implemented yet.
MPI_ERR_COUNT
    - Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
MPI_ERR_TYPE
    - Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit ).
MPI_ERR_BUFFER
    - Invalid buffer pointer. Usually a null buffer where one is not valid.

MORE INFORMATION
For more information, please see the official MPI Forum web site, which contains the text of both the MPI-1 and MPI-2 standards. These documents contain detailed information about each MPI function (most of which is not duplicated in these man pages). http://www.mpi-forum.org/

ACKNOWLEDGEMENTS
The LAM Team would like the thank the MPICH Team for the handy program to generate man pages ("doctext" from ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial formatting, and some initial text for most of the MPI-1 man pages.

LOCATION
alltoall.c