# MPI_Waitany

Waits for any specified MPI Request to complete

## Synopsis

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

## Input Parameters

count
  list length (integer)
array_of_requests
  array of requests (array of handles)

## Output Parameters

indx
  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). May be `MPI_STATUS_IGNORE`.

## Notes

If all of the requests are `MPI_REQUEST_NULL`, then `indx` is returned as `MPI_UNDEFINED`, and `status` is returned as an empty status.

While it is possible to list a request handle more than once in the array_of_requests, such an action is considered erroneous and may cause the program to unexecpectedly terminate or produce incorrect results.

## Notes on the MPI_Status argument

The `MPI_ERROR` field of the status return is only set if the return from the MPI routine is `MPI_ERR_IN_STATUS`. That error class is only returned by the routines that take an array of status arguments (`MPI_Testall`, `MPI_Testsome`, `MPI_Waitall`, and `MPI_Waitsome`). In all other cases, the value of the `MPI_ERROR` field in the status is unchanged. See section 3.2.5 in the MPI-1.1 specification for the exact text.

For send operations, the only use of status is for `MPI_Test_cancelled` or in the case that there is an error in one of the four routines that may return the error class `MPI_ERR_IN_STATUS`, in which case the `MPI_ERROR` field of status will be set. In that case, the value will be set to `MPI_SUCCESS` for any send or receive operation that completed successfully, or `MPI_ERR_PENDING` for any operation which has neither failed nor completed.

## 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_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_REQUEST
  Invalid `MPI_Request`. Either null or, in the case of a `MPI_Start` or `MPI_Startall`, not a persistent request.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).