MPI_Recv(3) 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.

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