Ubuntu Manpage: MPI_Recv - Performs a standard-mode blocking receive.

Provided by: openmpi-doc_4.1.4-3ubuntu2_all

NAME

       MPI_Recv - 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

C++ Syntax

       #include <mpi.h>
       void Comm::Recv(void* buf, int count, const Datatype& datatype,
            int source, int tag, Status& status) const

       void Comm::Recv(void* buf, int count, const Datatype& datatype,
            int source, int tag) const

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,  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 Section 3.4 of the MPI-1 Standard,
       "Communication Modes."

       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 MPI_Get_count function to
determine the actual number of entries within that message.

To receive messages of unknown length, use the MPI_Probe function. (For more information
about MPI_Probe and MPI_Cancel, see their respective man pages; also, see Section 3.8 of
the MPI-1 Standard, "Probe and Cancel.")

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 proceses 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 Section 3.5 of
the MPI-1 Standard, "Semantics of Point-to-Point Communication."

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

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran  routines in the last argument. C++ functions do not return errors. If the default
       error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then  on  error  the  C++  exception
       mechanism will be used to throw an MPI::Exception object.

       Before  the  error value is returned, the current MPI error handler is called. By default,
       this error handler aborts the MPI job, except for I/O function errors. The  error  handler
       may    be   changed   with   MPI_Comm_set_errhandler;   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.