Gathers data from all tasks and distribute it to all


#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

address of receive buffer (choice)


The MPI standard (1.0 and 1.1) says that

The jth block of data sent fromeach proess is received by every process and placed in the jth block of thebuffer recvbuf.

This is misleading; a better description is

The block of data sent from the jth process is received by everyprocess 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) havean additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routinein C. In Fortran, MPI routines are subroutines, and are invoked with thecall statement.

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


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 lastargument. Before the value is returned, the current MPI error handler iscalled. By default, this error handler aborts the MPI job. The error handlermay be changed with MPI_Errhandler_set; the predefined error handlerMPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue pastan error.

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid buffer pointer. Usually a null buffer where one is not valid.