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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
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 Can-
cel.")
A message can be received by a receive operation only if it is
addressed to the receiving process, and if its source, tag, and commu-
nicator (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 opera-
tion.
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 oper-
ation 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 mes-
sages 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++ func-
tions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
SEE ALSO
MPI_Irecv
MPI_Probe
Open MPI 1.2 March 2007 MPI_Recv(3OpenMPI)
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