openmpi/ompi/mpi/man/man3/MPI_Iprobe.3

.\"Copyright 2006, Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.TH MPI_Iprobe 3OpenMPI "September 2006" "Open MPI 1.2" " "
.SH NAME
\fBMPI_Iprobe\fP \- Nonblocking test for a message.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Iprobe(int \fIsource\fP, int\fI tag\fP, MPI_Comm\fI comm\fP, int\fI *flag\fP,
	MPI_Status\fI *status\fP)

.SH Fortran Syntax
.nf
INCLUDE 'mpif.h'
MPI_IPROBE(\fISOURCE, TAG, COMM, FLAG, STATUS, IERROR\fP)
	LOGICAL	\fIFLAG\fP
	INTEGER	\fISOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR\fP 

.SH C++ Syntax
.nf
#include <mpi.h>
bool Comm::Iprobe(int \fIsource\fP, int \fItag\fP, Status& \fIstatus\fP) const

bool Comm::Iprobe(int \fIsource\fP, int \fItag\fP) const

.SH INPUT PARAMETERS
.ft R
.TP 1i
source
Source rank or MPI_ANY_SOURCE (integer).
.TP 1i
tag
Tag value or MPI_ANY_TAG (integer).
.TP 1i
comm
Communicator (handle).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
flag
Message-waiting flag (logical).
.TP 1i
status
Status object (status).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer). 

.SH DESCRIPTION
.ft R
The MPI_Probe and MPI_Iprobe operations allow checking of incoming messages without actual receipt of them. The user can then decide how to receive them, based on the information returned by the probe (basically, the information returned by status). In particular, the user may allocate memory for the receive buffer, according to the length of the probed message. 
.sp
MPI_Iprobe(source, tag, comm, flag, status) returns flag = true if there is a message that can be received and that matches the pattern specified by the arguments source, tag, and comm. The call matches the same message that would have been received by a call to MPI_Recv(\&..., source, tag, comm, status) executed at the same point in the program, and returns in status the same value that would have been returned by MPI_Recv(). Otherwise, the call returns flag = false, and leaves status undefined. 
.sp
If MPI_Iprobe returns flag = true, then the content of the status object can be subsequently accessed as described in Section 3.2.5 of the MPI-1 Standard, "Return Status," to find the source, tag, and length of the probed message.
.sp
A subsequent receive executed with the same context, and the source and tag returned in status by MPI_Iprobe will receive the message that was matched by the probe if no other intervening receive occurs after the probe. If the receiving process is multithreaded, it is the user's responsibility to ensure that the last condition holds.
.sp
The source argument of MPI_Probe can be MPI_ANY_SOURCE, and the tag argument can be MPI_ANY_TAG, so that one can probe for messages from an arbitrary source and/or with an arbitrary tag. However, a specific communication context must be provided with the comm argument.
.sp
If your application does not need to examine the \fIstatus\fP field, you can save resources by using the predefined constant MPI_STATUS_IGNORE as a special value for the \fIstatus\fP argument. 
.sp
It is not necessary to receive a message immediately after it has been probed for, and the same message may be probed for several times before it is received. 
.sp
.SH NOTE
Users of libmpi-mt should remember that two threads may do an MPI_Iprobe that actually returns true for the same message for both threads. 

.SH 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.
.sp
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.  

.SH SEE ALSO
.ft R
.sp
MPI_Probe
.br
MPI_Cancel