MPI_Allgather

Gathers data from all tasks and distribute it to all

Synopsis

#include "mpi.h"
int MPI_Allgather ( sendbuf, sendcount, sendtype,
                    recvbuf, recvcount, recvtype, comm )
void             *sendbuf;
int               sendcount;
MPI_Datatype      sendtype;
void             *recvbuf;
int               recvcount;
MPI_Datatype      recvtype;
MPI_Comm          comm;

Input Parameters

	sendbuf	starting address of send buffer (choice)
	sendcount	number of elements in send buffer (integer)
	sendtype	data type of send buffer elements (handle)
	recvcount	number of elements received from any process (integer)
	recvtype	data type of receive buffer elements (handle)
	comm	communicator (handle)

Output Parameter

recvbuf

address of receive buffer (choice)

Notes

The MPI standard (1.0 and 1.1) says that

The jth block of data sent from each proess is received by every process and placed in the jth block of the buffer recvbuf.

This is misleading; a better description is

The block of data sent from the jth process is received by every process and placed in the jth block of the buffer recvbuf.

This text was suggested by Rajeev Thakur.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Errhandler_set; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error.

MPI_ERR_COMM

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

MPI_ERR_COUNT

Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_TYPE

Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).

MPI_ERR_BUFFER

Invalid buffer pointer. Usually a null buffer where one is not valid.

Location:allgather.c