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
- 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.
SEE ALSO¶
MPI_Irecv
MPI_Probe