17.2.9. MPI_Allgather — Open MPI 5.0.3 documentation

# 17.2.9. MPI_Allgather

MPI_Allgather, MPI_Iallgather, MPI_Allgather_init - Gathers data from all processes and distributes it to all processes

## 17.2.9.1. SYNTAX

### 17.2.9.1.1. 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)

### 17.2.9.1.2. 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

### 17.2.9.1.3. 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

## 17.2.9.2. 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).

## 17.2.9.3. OUTPUT PARAMETERS

- `recvbuf`: Address of receive buffer (choice).

- `request`: Request (handle, non-blocking only).

- `ierror`: Fortran only: Error status (integer).

## 17.2.9.4. 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.

## 17.2.9.5. 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.

## 17.2.9.6. 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.

## 17.2.9.7. 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_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

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

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

- MPI_Allgatherv

- MPI_Gather