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MPI_Recv.3.rst

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MPI_Recv

:ref:`MPI_Recv` |mdash| Performs a standard-mode blocking receive.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Recv(void *buf, int count, MPI_Datatype datatype,
     int source, int tag, MPI_Comm comm, MPI_Status *status)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_RECV(BUF, COUNT, DATATYPE, SOURCE, TAG, COMM, STATUS, IERROR)
     <type>  BUF(*)
     INTEGER COUNT, DATATYPE, SOURCE, TAG, COMM
     INTEGER STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Recv(buf, count, datatype, source, tag, comm, status, ierror)
     TYPE(*), DIMENSION(..) :: buf
     INTEGER, INTENT(IN) :: count, source, tag
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • count: Maximum number of elements to receive (integer).
  • datatype: Datatype of each receive buffer entry (handle).
  • source: Rank of source (integer).
  • tag: Message tag (integer).
  • comm: Communicator (handle).

OUTPUT PARAMETERS

  • buf: Initial address of receive buffer (choice).
  • status: Status object (status).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

This basic receive operation, :ref:`MPI_Recv`, is blocking: it returns only after the receive buffer contains the newly received message. A receive can complete before the matching send has completed (of course, it can complete only after the matching send has started).

The blocking semantics of this call are described in the "Communication Modes" section of the MPI Standard.

The receive buffer contains a number (defined by the value of count) of consecutive elements. The first element in the set of elements is located at address_buf. The type of each of these elements is specified by datatype.

The length of the received message must be less than or equal to the length of the receive buffer. An MPI_ERR_TRUNCATE is returned upon the overflow condition.

If a message that is shorter than the length of the receive buffer arrives, then only those locations corresponding to the (shorter) received message are modified.

NOTES

The count argument indicates the maximum number of entries of type datatype that can be received in a message. Once a message is received, use the :ref:`MPI_Get_count` function to determine the actual number of entries within that message.

To receive messages of unknown length, use the :ref:`MPI_Probe` function. For more information about :ref:`MPI_Probe` and :ref:`MPI_Cancel`, see their respective man pages and the "Probe and Cancel" section of the MPI Standard.

A message can be received by a receive operation only if it is addressed to the receiving process, and if its source, tag, and communicator (comm) values match the source, tag, and comm values specified by the receive operation. The receive operation may specify a wildcard value for source and/or tag, indicating that any source and/or tag are acceptable. The wildcard value for source is source = MPI_ANY_SOURCE. The wildcard value for tag is tag = MPI_ANY_TAG. There is no wildcard value for comm. The scope of these wildcards is limited to the processes in the group of the specified communicator.

The message tag is specified by the tag argument of the receive operation.

The argument source, if different from MPI_ANY_SOURCE, is specified as a rank within the process group associated with that same communicator (remote process group, for intercommunicators). Thus, the range of valid values for the source argument is {0,...,n-1} {MPI_ANY_SOURCE}, where n is the number of processes in this group.

Note the asymmetry between send and receive operations: A receive operation may accept messages from an arbitrary sender; on the other hand, a send operation must specify a unique receiver. This matches a "push" communication mechanism, where data transfer is effected by the sender (rather than a "pull" mechanism, where data transfer is effected by the receiver).

Source = destination is allowed, that is, a process can send a message to itself. However, it is not recommended for a process to send messages to itself using the blocking send and receive operations described above, since this may lead to deadlock. See the "Semantics of Point-to-Point Communication" of the MPI Standard for more details.

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.

ERRORS

Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, MPI errors on messages received by :ref:`MPI_Recv` 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 :ref:`MPI_Recv` if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example.

.. seealso::
   * :ref:`MPI_Irecv`
   * :ref:`MPI_Probe`