NAME

MPI_Allgather - Gather a buffer from all processes in a communicator

SYNOPSIS

#include <mpi.h> 
int MPI_Allgather(void *sendbuf , int sendcount , MPI_Datatype sendtype , void *recvbuf ,
	 int recvcount , MPI_Datatype recvtype , MPI_Comm comm ); 
#include <pmpi.h> 
int PMPI_Allgather(void *sendbuf , int sendcount , MPI_Datatype sendtype , void *recvbuf ,
	 int recvcount , MPI_Datatype recvtype , MPI_Comm comm ); 
#include <mpi.h> 
void MPI::Comm::Allgather(const void *sendbuf , int sendcount , const MPI::Datatype &sendtype ,
	 void *recvbuf , int recvcount , const MPI::Datatype &recvtype ) const; 
INCLUDE 'mpif.h' 
MPI_ALLGATHER(sendbuf , sendcount , sendtype , recvbuf , recvcount , recvtype , comm , ierr ) 
<type>  sendbuf (*),  recvbuf (*) 
INTEGER  sendcount ,  sendtype ,  recvcount ,  recvtype ,  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)

comm - communicator for messages (handle)

OUTPUT PARAMETER

recvbuf - receive buffer (array)

DESCRIPTION

The MPI_Allgather() function is used to send messages to all processes in the communicator comm , from all processes in that communicator. The messages are then concatenated into the receive buffer, recvbuf in rank order. This is equivalent to calling MPI_Gather() followed by a MPI_Bcast() of the results.

The sends consist of sendcount elements of type sendtype in an array specified by sendbuf . If sendbuf is MPI_IN_PLACE then the source of the message is taken from the corresponding offset in recvbuf . This results in no self-send or copy. Otherwise, there will be a partial copy from sendbuf to recvbuf .

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

The communicator must be a valid one (not MPI_COMM_NULL). PMPI_Allgather() 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_INTERN - Out of Memory. This may be fatal.

SEE ALSO

MPI_Send (3) MPI_Recv (3) MPI_Bcast (3) MPI_Gather (3) MPI_Gatherv (3) MPI_Allgatherv (3) MPI_Alltoall (3)

• Lockless
• MPI Documentation
• MPI_Allgather