# 17.2.426. MPI_Waitany

MPI_Waitany — Waits for any specified send or receive to complete.

## 17.2.426.1. SYNTAX

### 17.2.426.1.1. C Syntax

    #include <mpi.h>

    int MPI_Waitany(int count, MPI_Request array_of_requests[],
         int *index, MPI_Status *status)

### 17.2.426.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_WAITANY(COUNT, ARRAY_OF_REQUESTS, INDEX, STATUS, IERROR)
         INTEGER COUNT, ARRAY_OF_REQUESTS(*), INDEX
         INTEGER STATUS(MPI_STATUS_SIZE), IERROR

### 17.2.426.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Waitany(count, array_of_requests, index, status, ierror)
         INTEGER, INTENT(IN) :: count
         TYPE(MPI_Request), INTENT(INOUT) :: array_of_requests(count)
         INTEGER, INTENT(OUT) :: index
         TYPE(MPI_Status) :: status
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.426.2. INPUT PARAMETERS

- `count`: List length (integer).

- `array_of_requests`: Array of requests (array of handles).

## 17.2.426.3. OUTPUT PARAMETERS

- `index`: Index of handle for operation that completed (integer). In the range 0 to count-1. In Fortran, the range is 1 to count.

- `status`: Status object (status).

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

## 17.2.426.4. DESCRIPTION

A call to MPI_Waitany can be used to wait for the completion of one out of several requests.

The array_of_requests list may contain null or inactive handles. If the list contains no active handles (list has length zero or all entries are null or inactive), then the call returns immediately with index = `MPI_UNDEFINED`, and an empty status.

The execution of MPI_Waitany(count, array_of_requests, index, status) has the same effect as the execution of `MPI_Wait(&array_of_requests[i], status)`, where i is the value returned by index (unless the value of index is `MPI_UNDEFINED`). MPI_Waitany with an array containing one active entry is equivalent to MPI_Wait.

If your application does not need to examine the status field, you can save resources by using the predefined constant `MPI_STATUS_IGNORE` as a special value for the status argument.

Example: Client-server code (starvation can occur).

    CALL MPI_COMM_SIZE(comm, size, ierr)
    CALL MPI_COMM_RANK(comm, rank, ierr)
    IF(rank > 0) THEN         ! client code
        DO
           CALL MPI_ISEND(a, n, MPI_REAL, 0, tag, comm, request, ierr)
           CALL MPI_WAIT(request, status, ierr)
        END DO
    ELSE         ! rank=0: server code
           DO i=1, size-1
              CALL MPI_IRECV(a(1,i), n, MPI_REAL, i tag, &
                       comm, request_list(i), ierr)
           END DO
           DO
              CALL MPI_WAITANY(size-1, request_list, index, status, ierr)
              CALL DO_SERVICE(a(1,index))  ! handle one message
              CALL MPI_IRECV(a(1, index), n, MPI_REAL, index, tag, &
                        comm, request_list(index), ierr)
           END DO
    END IF

## 17.2.426.5. 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.

Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard, MPI errors on requests passed to MPI_Waitany do not set the `status.MPI_ERROR` field in the returned status. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of MPI_Waitany if the back-end error handler returns it. The pre-defined MPI error handler `MPI_ERRORS_RETURN` exhibits this behavior, for example.

See also

- MPI_Comm_set_errhandler

- MPI_File_set_errhandler

- MPI_Test

- MPI_Testall

- MPI_Testany

- MPI_Testsome

- MPI_Wait

- MPI_Waitall

- MPI_Waitsome

- MPI_Win_set_errhandler