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MPI_Allgather_init(3) man page (version 5.0.0rc2)

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Name

MPI_Allgather, MPI_Iallgather, MPI_Allgather_init - Gathers data

from all processes and distributes it to all processes

Syntax

C Syntax


#include <mpi.h>
int MPI_Allgather(const void *sendbuf, int  sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Iallgather(const void *sendbuf, int  sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)
int MPI_Allgather_init(const void *sendbuf, int  sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

Fortran Syntax


USE MPI
! or the older form: INCLUDE ’mpif.h’
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
        RECVTYPE, COMM, IERROR)
    <type>    SENDBUF (*), RECVBUF (*)
    INTEGER    SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
    INTEGER    IERROR
MPI_IALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
        RECVTYPE, COMM, REQUEST, IERROR)
    <type>    SENDBUF(*), RECVBUF (*)
    INTEGER    SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
    INTEGER    REQUEST, IERROR
MPI_ALLGATHER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
            RECVTYPE, COMM, INFO, REQUEST, IERROR)
    <type>    SENDBUF(*), RECVBUF (*)
    INTEGER    SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
    INTEGER    INFO, REQUEST, IERROR

Fortran 2008 Syntax


USE mpi_f08
MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
        comm, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
    TYPE(*), DIMENSION(..) :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcount
    TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
        comm, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcount
    TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Request), INTENT(OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Allgather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
            comm, info, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcount
    TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Info), INTENT(IN) :: info
    TYPE(MPI_Request), INTENT(OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

Input Parameters

sendbuf
Starting address of send buffer (choice).
sendcount
Number of elements in send buffer (integer).
sendtype
Datatype of send buffer elements (handle).
recvbuf
Starting address of recv buffer (choice).
recvcount
Number of elements received from any process (integer).
recvtype
Datatype of receive buffer elements (handle).
comm
Communicator (handle).
info
Info (handle, persistent only).

Output Parameters

recvbuf
Address of receive buffer (choice).
request
Request (handle, non-blocking only).
IERROR
Fortran only: Error status (integer).

Description

MPI_Allgather is similar to MPI_Gather, except that all processes receive the result, instead of just the root. In other words, all processes contribute to the result, and all processes receive the result.

The type signature associated with sendcount, sendtype at a process must be equal to the type signature associated with recvcount, recvtype at any other process.

The outcome of a call to MPI_Allgather(...) is as if all processes executed n calls to


  MPI_Gather(sendbuf,sendcount,sendtype,recvbuf,recvcount,
             recvtype,root,comm),

for root = 0 , ..., n-1. The rules for correct usage of MPI_Allgather are easily found from the corresponding rules for MPI_Gather.

Example: The all-gather version of Example 1 in MPI_Gather. Using MPI_Allgather, we will gather 100 ints from every process in the group to every process.


MPI_Comm comm;
    int gsize,sendarray[100];
    int *rbuf;
    ...
    MPI_Comm_size( comm, &gsize);
    rbuf = (int *)malloc(gsize*100*sizeof(int));
    MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);

After the call, every process has the group-wide concatenation of the sets of data.

Use of In-place Option

When the communicator is an intracommunicator, you can perform an all-gather operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf. In this case, sendcount and sendtype are ignored. The input data of each process is assumed to be in the area where that process would receive its own contribution to the receive buffer. Specifically, the outcome of a call to MPI_Allgather that used the in-place option is identical to the case in which all processes executed n calls to


   MPI_ALLGATHER ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
   recvcount, recvtype, root, comm )
for root =0, ... , n-1.

Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT.

When Communicator is an Inter-communicator

When the communicator is an inter-communicator, the gather operation occurs in two phases. The data is gathered from all the members of the first group and received by all the members of the second group. Then the data is gathered from all the members of the second group and received by all the members of the first. The operation, however, need not be symmetric. The number of items sent by the processes in first group need not be equal to the number of items sent by the the processes in the second group. You can move data in only one direction by giving sendcount a value of 0 for communication in the reverse direction.

Errors

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

See Also

MPI_Allgatherv

MPI_Gather


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