NAME¶
MPI_Scatter - Sends data from one task to all tasks in a group.
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Scatter(void * sendbuf, int sendcount, MPI_Datatype sendtype,
void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
MPI_Comm comm)
Fortran Syntax¶
INCLUDE 'mpif.h'
MPI_SCATTER( SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
RECVTYPE, ROOT, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
INTEGER COMM, IERROR
C++ Syntax¶
#include <mpi.h>
void MPI::Comm::Scatter(const void* sendbuf, int sendcount,
const MPI::Datatype& sendtype, void* recvbuf,
int recvcount, const MPI::Datatype& recvtype,
int root) const
- sendbuf
- Address of send buffer (choice, significant only at
root).
- sendcount
- Number of elements sent to each process (integer,
significant only at root).
- sendtype
- Datatype of send buffer elements (handle, significant only
at root).
- recvcount
- Number of elements in receive buffer (integer).
- recvtype
- Datatype of receive buffer elements (handle).
- root
- Rank of sending process (integer).
- comm
- Communicator (handle).
OUTPUT PARAMETERS¶
- recvbuf
- Address of receive buffer (choice).
- IERROR
- Fortran only: Error status (integer).
DESCRIPTION¶
MPI_Scatter is the inverse operation to MPI_Gather.
The outcome is as if the root executed n send operations,
MPI_Send(sendbuf + i * sendcount * extent(sendtype), sendcount,
sendtype, i, ...)
and each process executed a receive,
MPI_Recv(recvbuf, recvcount, recvtype, i, ...).
An alternative description is that the root sends a message with MPI_Send(
sendbuf,
sendcount *
n,
sendtype, ...). This
message is split into
n equal segments, the ith segment is sent to the
ith process in the group, and each process receives this message as above.
The send buffer is ignored for all nonroot processes.
The type signature associated with
sendcount,
sendtype at the root
must be equal to the type signature associated with
recvcount,
recvtype at all processes (however, the type maps may be different).
This implies that the amount of data sent must be equal to the amount of data
received, pairwise between each process and the root. Distinct type maps
between sender and receiver are still allowed.
All arguments to the function are significant on process
root, while on
other processes, only arguments
recvbuf,
recvcount,
recvtype,
root,
comm are significant. The arguments
root and
comm must have identical values on all processes.
The specification of counts and types should not cause any location on the root
to be read more than once.
Rationale: Though not needed, the last restriction is imposed so as to
achieve symmetry with MPI_Gather, where the corresponding restriction (a
multiple-write restriction) is necessary.
Example: The reverse of Example 1 in the MPI_Gather manpage. Scatter sets
of 100 ints from the root to each process in the group.
MPI_Comm comm;
int gsize,*sendbuf;
int root, rbuf[100];
...
MPI_Comm_size(comm, &gsize);
sendbuf = (int *)malloc(gsize*100*sizeof(int));
...
MPI_Scatter(sendbuf, 100, MPI_INT, rbuf, 100,
MPI_INT, root, comm);
USE OF IN-PLACE OPTION¶
When the communicator is an intracommunicator, you can perform a gather
operation in-place (the output buffer is used as the input buffer). Use the
variable MPI_IN_PLACE as the value of the root process
recvbuf. In this
case,
recvcount and
recvtype are ignored, and the root process
sends no data to itself.
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 root process in the first
group sends data to all processes in the second group. The first group defines
the root process. That process uses MPI_ROOT as the value of its
root
argument. The remaining processes use MPI_PROC_NULL as the value of their
root argument. All processes in the second group use the rank of that
root process in the first group as the value of their
root argument.
The receive buffer argument of the root process in the first group must be
consistent with the receive buffer argument of the processes in the second
group.
ERRORS¶
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.
SEE ALSO¶
MPI_Scatterv
MPI_Gather
MPI_Gatherv