# MPI_Waitsome

Waits for some given MPI Requests to complete

## Synopsis

    int MPI_Waitsome(int incount, MPI_Request array_of_requests[],
                     int *outcount, int array_of_indices[], MPI_Status array_of_statuses[])

## Input Parameters

incount
  length of array_of_requests (integer)
array_of_requests
  array of requests (array of handles)

## Output Parameters

outcount
  number of completed requests (integer)
array_of_indices
  array of indices of operations that completed (array of integers)
array_of_statuses
  array of status objects for operations that completed (array of Status). May be `MPI_STATUSES_IGNORE`.

## Notes

The array of indicies are in the range `0` to `incount - 1` for C and in the range `1` to `incount` for Fortran.

Null requests are ignored; if all requests are null, then the routine returns with `outcount` set to `MPI_UNDEFINED`.

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.

`MPI_Waitsome` provides an interface much like the Unix `select` or `poll` calls and, in a high qualilty implementation, indicates all of the requests that have completed when `MPI_Waitsome` is called. However, `MPI_Waitsome` only guarantees that at least one request has completed; there is no guarantee that all completed requests will be returned, or that the entries in `array_of_indices` will be in increasing order. Also, requests that are completed while `MPI_Waitsome` is executing may or may not be returned, depending on the timing of the completion of the message.

## 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 guarantee 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`).

MPI_ERR_IN_STATUS
  The actual error value is in the `MPI_Status` argument. This error class is returned only from the multiple-completion routines (`MPI_Testall`, `MPI_Testany`, `MPI_Testsome`, `MPI_Waitall`, `MPI_Waitany`, and `MPI_Waitsome`). The field `MPI_ERROR` in the status argument contains the error value or `MPI_SUCCESS` (no error and complete) or `MPI_ERR_PENDING` to indicate that the request has not completed.

The MPI Standard does not specify what the result of the multiple completion routines is when an error occurs. For example, in an `MPI_WAITALL`, does the routine wait for all requests to either fail or complete, or does it return immediately (with the MPI definition of immediately, which means independent of actions of other MPI processes)? MPICH has chosen to make the return immediate (alternately, local in MPI terms), and to use the error class `MPI_ERR_PENDING` (introduced in MPI 1.1) to indicate which requests have not completed. In most cases, only one request with an error will be detected in each call to an MPI routine that tests multiple requests. The requests that have not been processed (because an error occured in one of the requests) will have their `MPI_ERROR` field marked with `MPI_ERR_PENDING`.