# MPI_Waitsome

Waits for some given communications to complete

## Synopsis

    #include "mpi.h"
    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)

## 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`.

## Note on status for send operations

For send operations, the only use of status is for MPI_Test_cancelled or in the case that there is an error, in which case the MPI_ERROR field of status will be set.

## 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_Errhandler_set`; 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.

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`.

Location:waitsome.c