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MPI_Allgather - Gathers data from all processes and distributes it to
int MPI_Allgather(void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf, int recvcount,
MPI_Datatype recvtype, MPI_Comm comm)
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, COMM, IERROR)
<type> SENDBUF (*), RECVBUF (*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
void MPI::Comm::Allgather(const void* sendbuf, int sendcount, const
MPI::Datatype& sendtype, void* recvbuf, int recvcount,
const MPI::Datatype& recvtype) const = 0
sendbuf Starting address of send buffer (choice).
sendcount Number of elements in send buffer (integer).
sendtype Datatype of send buffer elements (handle).
recvcount Number of elements received from any process (integer).
recvtype Datatype of receive buffer elements (handle).
comm Communicator (handle).
recvbuf Address of receive buffer (choice).
IERROR Fortran only: Error status (integer).
MPI_Allgather is similar to MPI_Gather, except that all processes
receive the result, instead of just the root. In other words, all pro-
cesses contribute to the result, and all processes receive the result.
The type signature associated with sendcount, sendtype at a process
must be equal to the type signature associated with recvcount, recvtype
at any other process.
Example: The all-gather version of Example 1 in MPI_Gather. Using
MPI_Allgather, we will gather 100 ints from every process in the group
to every process.
MPI_Comm_size( comm, &gsize);
rbuf = (int *)malloc(gsize*100*sizeof(int));
MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);
After the call, every process has the group-wide concatenation of the
sets of data.
USE OF IN-PLACE OPTION
When the communicator is an intracommunicator, you can perform an all-
gather operation in-place (the output buffer is used as the input
buffer). Use the variable MPI_IN_PLACE as the value of both sendbuf
and recvbuf. In this case, sendcount and sendtype are ignored. The
input data of each process is assumed to be in the area where that
process would receive its own contribution to the receive buffer.
Specifically, the outcome of a call to MPI_Allgather that used the in-
place option is identical to the case in which all processes executed n
MPI_GATHER ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
recvcount, recvtype, root, comm )
for root =0, ... , n-1.
Note that MPI_IN_PLACE is a special kind of value; it has the same
restrictions on its use as MPI_BOTTOM.
Because the in-place option converts the receive buffer into a send-
and-receive buffer, a Fortran binding that includes INTENT must mark
these as INOUT, not OUT.
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, however, need
not be symmetric. The number of items sent by the processes in first
group need not be equal to the number of items sent by the the pro-
cesses in the second group. You can move data in only one direction by
giving sendcount a value of 0 for communication in the reverse direc-
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
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++ func-
tions 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.
Open MPI 1.2 September 2006 MPI_Allgather(3OpenMPI)
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