MPI_Allgather function

Gathers data from all members of a group and sends the data to all members of the group. The MPI_Allgather function is similar to the MPI_Gather function, except that it sends the data to all processes instead of only to the root. The usage rules for MPI_Allgather correspond to the rules for MPI_Gather.

Syntax

int MPIAPI MPI_Allgather(
  _In_  void         *sendbuf,
  _In_  int          sendcount,
  _In_  MPI_Datatype sendtype,
  _Out_ void         *recvbuf,
        int          recvcount,
        MPI_Datatype recvtype,
        MPI_Comm     comm
);

Parameters

• sendbuf [in]
  The pointer to the data to be sent to all processes in the group. The number and data type of the elements in the buffer are specified in the sendcount and sendtype parameters. Each element in the buffer corresponds to a process in the group.

  If the comm parameter references an intracommunicator, you can specify an in-place option by specifying MPI_IN_PLACE in all processes. The sendcount and sendtype parameters are ignored. Each process enters data in the corresponding receive buffer element. The nth process sends data to the nth element of the receive buffer.

• sendcount [in]
  The number of elements in the buffer that is specified in the sendbuf parameter. If sendcount is zero, the data part of the message is empty.

• sendtype [in]
  The MPI data type of the elements in the send buffer.

• recvbuf [out]
  The pointer to a buffer that contains the data that is received from each process. The number and data type of the elements in the buffer are specified in the recvcount and recvtype parameters.

• recvcount
  The number of elements in the receive buffer. If the count is zero, the data part of the message is empty.

• recvtype
  The MPI data type of the elements in the receive buffer.

• comm
  The MPI_Comm communicator handle.

Return value

Returns MPI_SUCCESS on success. Otherwise, the return value is an error code.

In Fortran, the return value is stored in the IERROR parameter.

Fortran

    MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, RECVTYPE, COMM, IERROR)
        <type> SENDBUF(*), RECVBUF(*)
        INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM, IERROR

Remarks

The type signature that is associated with the sendtype parameter on a process must be equal to the type signature that is associated with the recvtype parameter on any other process.

If the comm parameter references an intracommunicator, 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.

If the comm parameter references an intercommunicator, then each process of one group, for example, group A, contributes the number of data items that are specified in the sendcount parameter. This data is concatenated, and the result is stored at each process in the other group, group B. Conversely, the concatenation of the data of the processes in group B is stored at each process in group A. The send buffer parameters in group A must be consistent with the receive buffer parameters in group B, and vice versa.

The number of items, which are sent by processes in group A, do not have to equal the number of items that are sent by processes in group B. In particular, you can move data in only one direction by specifying sendcount == 0 for the communication in the reverse direction.

Requirements

See also

MPI Collective Functions

MPI_Datatype

MPI_Allgather