# MPI_Allgather

Gathers data from all tasks and distribute the combined data to all tasks

## Synopsis

    int MPI_Allgather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
                      void *recvbuf, int recvcount, MPI_Datatype recvtype,
                      MPI_Comm comm)

## Input Parameters

sendbuf
  starting address of send buffer (choice)
sendcount
  number of elements in send buffer (integer)
sendtype
  data type of send buffer elements (handle)
recvcount
  number of elements received from any process (integer)
recvtype
  data type of receive buffer elements (handle)
comm
  communicator (handle)

## Output Parameters

recvbuf
  address of receive buffer (choice)

## Notes

The MPI standard (1.0 and 1.1) says that

The jth block of data sent from each process is received by every process and placed in the jth block of the buffer `recvbuf`.

This is misleading; a better description is

The block of data sent from the jth process is received by every process and placed in the jth block of the buffer `recvbuf`.

This text was suggested by Rajeev Thakur and has been adopted as a clarification by the MPI Forum.

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. The predefined error handler `MPI_ERRORS_RETURN` may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_COUNT
  Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.

MPI_ERR_BUFFER
  Invalid buffer pointer. Usually a null buffer where one is not valid.