.. _mpi_testsome:


MPI_Testsome
============

.. include_body

:ref:`MPI_Testsome` |mdash| Tests for completion of one or more previously
initiated communications in a list.

.. The following file was automatically generated
.. include:: ./bindings/mpi_testsome.rst

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

DESCRIPTION
-----------

Behaves like :ref:`MPI_Waitsome`, except that it returns immediately.

Returns in outcount the number of requests from the list
array_of_requests that have completed. Returns in the first outcount
locations of the array array_of_indices the indices of these operations
(index within the array array_of_requests; the array is indexed from 0
in C and from 1 in Fortran). Returns in the first outcount locations of
the array array_of_status the status for these completed operations. If
a request that completed was allocated by a nonblocking communication
call, then it is deallocated, and the associated handle is set to
MPI_REQUEST_NULL.

If no operation has completed it returns outcount = 0. If there is no
active handle in the list, it returns outcount = MPI_UNDEFINED.

:ref:`MPI_Testsome` is a local operation, which returns immediately, whereas
:ref:`MPI_Waitsome` blocks until a communication completes, if it was passed a
list that contains at least one active handle. Both calls fulfill a
fairness requirement: If a request for a receive repeatedly appears in a
list of requests passed to :ref:`MPI_Waitsome` or :ref:`MPI_Testsome`, and a matching
send has been posted, then the receive will eventually succeed unless
the send is satisfied by another receive; send requests also fulfill
this fairness requirement.

Errors that occur during the execution of :ref:`MPI_Testsome` are handled as
for :ref:`MPI_Waitsome`.

If your application does not need to examine the *array_of_statuses*
field, you can save resources by using the predefined constant
MPI_STATUSES_IGNORE can be used as a special value for the
*array_of_statuses* argument.


NOTES
-----

The use of :ref:`MPI_Testsome` is likely to be more efficient than the use of
:ref:`MPI_Testany`. The former returns information on all completed
communications; with the latter, a new call is required for each
communication that completes.

A server with multiple clients can use :ref:`MPI_Waitsome` so as not to starve
any client. Clients send messages to the server with service requests.
The server calls :ref:`MPI_Waitsome` with one receive request for each client,
then handles all receives that have completed. If a call to :ref:`MPI_Waitany`
is used instead, then one client could starve while requests from
another client always sneak in first.


ERRORS
------

.. include:: ./ERRORS.rst

For each invocation of :ref:`MPI_Testsome`, if one or more requests
generate an MPI error, only the *first* MPI request that caused an
error will be passed to its corresponding error handler. No other
error handlers will be invoked (even if multiple requests generated
errors). However, *all* requests that generate an error will have a
relevant error code set in the corresponding ``status.MPI_ERROR``
field (unless ``MPI_STATUSES_IGNORE`` was used).

If the invoked error handler allows :ref:`MPI_Testsome` to return to
the caller, the value ``MPI_ERR_IN_STATUS`` will be returned in the C
and Fortran bindings.

.. seealso::
   * :ref:`MPI_Comm_set_errhandler`
   * :ref:`MPI_File_set_errhandler`
   * :ref:`MPI_Test`
   * :ref:`MPI_Testall`
   * :ref:`MPI_Testany`
   * :ref:`MPI_Wait`
   * :ref:`MPI_Waitall`
   * :ref:`MPI_Waitany`
   * :ref:`MPI_Waitsome`
   * :ref:`MPI_Win_set_errhandler`