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)

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

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¶

OUTPUT PARAMETERS¶

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 return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

MPI applications can also implement their own error handlers by calling:

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

SEE ALSO:

COPYRIGHT¶

2003-2024, The Open MPI Community