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NAME

MPI_Gather - Gather a buffer from all processes in a communicator at root

SYNOPSIS

#include <mpi.h> int MPI_Gather(void *sendbuf , int sendcount , MPI_Datatype sendtype , void *recvbuf , int recvcount , MPI_Datatype recvtype , int root , MPI_Comm comm );

#include <pmpi.h> int PMPI_Gather(void *sendbuf , int sendcount , MPI_Datatype sendtype , void *recvbuf , int recvcount , MPI_Datatype recvtype , int root , MPI_Comm comm );

#include <mpi.h> void MPI::Comm::Gather(const void *sendbuf , int sendcount , const MPI::Datatype &sendtype , void *recvbuf , int recvcount , const MPI::Datatype &recvtype , int root ) const;

INCLUDE 'mpif.h' MPI_GATHER(sendbuf , sendcount , sendtype , recvbuf , recvcount , recvtype , root , comm , ierr ) <type> sendbuf (*), recvbuf (*) INTEGER sendcount , recvcount , root , comm , ierr

INPUT PARAMETERS

sendbuf - send buffer (array)

sendcount - count of elements in send buffer (integer)

sendtype - type of elements in send buffer (handle)

recvcount - count of elements to receive per rank (integer)

recvtype - type of elements in receive buffer (handle)

root - root rank (integer)

comm - communicator for messages (handle)

OUTPUT PARAMETER

recvbuf - receive buffer (array)

DESCRIPTION

The MPI_Gather() function is used to send a message to the process with rank root in the communicator comm , from all processes in that communicator. The root process then concatenates the messages into the receive buffer, recvbuf in rank order.

The sends may (or may not) block depending on the amount of internal buffering done in the MPI implementation. They consist of sendcount elements of type sendtype in an array specified by sendbuf . At the root, if sendbuf is MPI_IN_PLACE then the source of the message is taken from root's corresponding offset in recvbuf . This corresponds to no self-send or copy. Otherwise, the root will perform a copy from its sendbuf to its requisite offset in recvbuf .

Note that each process should send the same amount of data to root . If that is not the case, then the messages will be silently truncated to the size expected there. The recvtype and recvcount determine exactly how the messages are interpreted at root . Note that for performance reasons this MPI library does not check that the sent and received datatypes match.

The conceptual inverse of the effect of this function is the operation performed by the MPI_Scatter() function. If differing amounts of data need to be transferred to root , use the MPI_Gatherv() function.

The communicator must be a valid one (not MPI_COMM_NULL). PMPI_Gather() is the profiling version of this function.

ERRORS

All MPI routines except for MPI_Wtime and MPI_Wtick return an error code. The the current MPI error handler is invoked if the return value is not MPI_SUCCESS. The default error handler aborts, but this may be changed with by using the MPI_Errhandler_set() function. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarentee that an MPI program can continue past an error. In this implementation, all errors except MPI_ERR_INTERN or MPI_ERR_OTHER should always be recoverable.

In C, the error code is passed as the return value. In FORTRAN, all functions have an parameter ierr which returns the error code. MPI C++ functions do not directly return an error code. However, C++ users may want to use the MPI::ERRORS_THROW_EXCEPTIONS handler. This will throw an MPI::Exception with the corresponding error code. To prevent exceptions from being raised from within C and Fortran code, they will see all error return values as MPI_ERR_PENDING when this handler is chosen. In this implementation, call MPI::throw_exception() to throw the correct exception if this occurs.

MPI_SUCCESS - No error;

MPI_ERR_PENDING - Pending exception;

MPI_ERR_COMM - Invalid communicator;

MPI_ERR_COUNT - Invalid element count;

MPI_ERR_BUFFER - Invalid buffer;

MPI_ERR_TYPE - Invalid data type;

MPI_ERR_ROOT - Invalid root;

MPI_ERR_INTERN - Out of Memory. This may be fatal.

SEE ALSO

MPI_Send (3) MPI_Recv (3) MPI_Barrier (3) MPI_Scatter (3) MPI_Gatherv (3) MPI_Allgather (3) MPI_Allgatherv (3) MPI_Alltoall (3)

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