MPI_Alltoallw - All processes send data of different types to, and
receive data of different types from, all processes
int MPI_Alltoallw(void *sendbuf, int *sendcounts,
int *sdispls, MPI_Datatype *sendtypes,
void *recvbuf, int *recvcounts,
int *rdispls, MPI_Datatype *recvtypes, MPI_Comm comm)
MPI_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)
INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)
INTEGER COMM, IERROR
void MPI::Comm::Alltoallw(const void* sendbuf,
const int sendcounts, const int sdispls,
const MPI::Datatype sendtypes, void* recvbuf,
const int recvcounts, const int rdispls,
const MPI::Datatype recvtypes)
sendbuf Starting address of send buffer.
sendcounts Integer array, where entry i specifies the number of
elements to send to rank i.
sdispls Integer array, where entry i specifies the displacement (in
bytes, offset from sendbuf) from which to send data to rank
sendtypes Datatype array, where entry i specifies the datatype to use
when sending data to rank i.
recvcounts Integer array, where entry j specifies the number of
elements to receive from rank j.
rdispls Integer array, where entry j specifies the displacement (in
bytes, offset from recvbuf) to which data from rank j
should be written.
recvtypes Datatype array, where entry j specifies the datatype to use
when receiving data from rank j.
comm Communicator over which data is to be exchanged.
recvbuf Address of receive buffer.
IERROR Fortran only: Error status.
MPI_Alltoallw is a generalized collective operation in which all
processes send data to and receive data from all other processes. It
adds flexibility to MPI_Alltoallv by allowing the user to specify the
datatype of individual data blocks (in addition to displacement and
element count). Its operation can be thought of in the following way,
where each process performs 2n (n being the number of processes in
communicator comm) independent point-to-point communications (including
communication with itself).
for (i = 0, i < n; i++)
MPI_Send(sendbuf + sdispls[i], sendcounts[i],
sendtypes[i], i, ..., comm);
for (i = 0, i < n; i++)
MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
recvtypes[i], i, ..., comm);
Process j sends the k-th block of its local sendbuf to process k, which
places the data in the j-th block of its local recvbuf.
When a pair of processes exchanges data, each may pass different
element count and datatype arguments so long as the sender specifies
the same amount of data to send (in bytes) as the receiver expects to
Note that process i may send a different amount of data to process j
than it receives from process j. Also, a process may send entirely
different amounts and types of data to different processes in the
WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
When the communicator is an inter-communicator, the gather operation
occurs in two phases. The data is gathered from all the members of the
first group and received by all the members of the second group. Then
the data is gathered from all the members of the second group and
received by all the members of the first. The operation exhibits a
symmetric, full-duplex behavior.
The first group defines the root process. The root process uses
MPI_ROOT as the value of root. All other processes in the first group
use MPI_PROC_NULL as the value of root. All processes in the second
group use the rank of the root process in the first group as the value
When the communicator is an intra-communicator, these groups are the
same, and the operation occurs in a single phase.
The MPI_IN_PLACE option is not available for any form of all-to-all
The specification of counts, types, and displacements should not cause
any location to be written more than once.
All arguments on all processes are significant. The comm argument, in
particular, must describe the same communicator on all processes.
The offsets of sdispls and rdispls are measured in bytes. Compare this
to MPI_Alltoallv, where these offsets are measured in units of sendtype
and recvtype, respectively.
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++
functions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI:Exception object.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; 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.