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

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

       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

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.