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MPI_Bcast.3.rst

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MPI_Bcast

:ref:`MPI_Bcast`, :ref:`MPI_Ibcast` - Broadcasts a message from the process with rank root to all other processes of the group.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Bcast(void *buffer, int count, MPI_Datatype datatype,
    int root, MPI_Comm comm)

int MPI_Ibcast(void *buffer, int count, MPI_Datatype datatype,
    int root, MPI_Comm comm, MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_BCAST(BUFFER, COUNT, DATATYPE, ROOT, COMM, IERROR)
    <type>  BUFFER(*)
    INTEGER COUNT, DATATYPE, ROOT, COMM, IERROR

MPI_IBCAST(BUFFER, COUNT, DATATYPE, ROOT, COMM, REQUEST, IERROR)
    <type>  BUFFER(*)
    INTEGER COUNT, DATATYPE, ROOT, COMM, REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Bcast(buffer, count, datatype, root, comm, ierror)
    TYPE(*), DIMENSION(..) :: buffer
    INTEGER, INTENT(IN) :: count, root
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ibcast(buffer, count, datatype, root, comm, request, ierror)
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buffer
    INTEGER, INTENT(IN) :: count, root
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Request), INTENT(OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT/OUTPUT PARAMETERS

  • buffer: Starting address of buffer (choice).
  • count: Number of entries in buffer (integer).
  • datatype: Data type of buffer (handle).
  • root: Rank of broadcast root (integer).
  • comm: Communicator (handle).

OUTPUT PARAMETERS

  • request: Request (handle, non-blocking only).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

:ref:`MPI_Bcast` broadcasts a message from the process with rank root to all processes of the group, itself included. It is called by all members of group using the same arguments for comm, root. On return, the contents of root's communication buffer has been copied to all processes.

General, derived datatypes are allowed for datatype. The type signature of count, datatype on any process must be equal to the type signature o f count, datatype at the root. This implies that the amount of data sent must be equal to the amount received, pairwise between each process and the root. :ref:`MPI_Bcast` and all other data-movement collective routines make this restriction. Distinct type maps between sender and receiver are still allowed.

Example: Broadcast 100 ints from process 0 to every process in the group.

MPI_Comm comm;
int array[100];
int root=0;
//...
MPI_Bcast( array, 100, MPI_INT, root, comm);

As in many of our sample code fragments, we assume that some of the variables (such as comm in the example above) have been assigned appropriate values.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

When the communicator is an inter-communicator, the root process in the first group broadcasts data to all the 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 arguments of the processes in the second group must be consistent with the send buffer argument of the root process in the first group.

NOTES

This function does not support the in-place option.

ERRORS