The Great and Terrible implementation of MPI-2

function index


Nonblocking test for a message
int MPI_Iprobe(
  int source,
  int tag,
  MPI_Comm comm,
  int *flag,
  MPI_Status *status


[in] source rank, or MPI_ANY_SOURCE (integer)
[in] tag value or MPI_ANY_TAG (integer)
[in] communicator (handle)
[out] True if a message with the specified source, tag, and communicator is available (logical)
[out] status object (Status)


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.

If MPI_IPROBE returns flag = true, then the content of the status object can be subsequently accessed to find the source, tag and length of the probed message.

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 multi-threaded, it is the user's responsibility to ensure that the last condition holds.

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.

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.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid tag argument. Tags must be non-negative; tags in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_TAG. The largest tag value is available through the the attribute MPI_TAG_UB.
Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_SOURCE.

Example Code

The following sample code illustrates MPI_Iprobe.

#include "mpi.h"
#include <stdio.h>
int main( int argc, char * argv[] )
int rank;
    int sendMsg = 123;
    int recvMsg = 0;
int flag = 0;
    int count;
    MPI_Status status;
    MPI_Request request;
    int errs = 0;
    MPI_Init( 0, 0 );
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if(rank == 0)
        MPI_Isend( &sendMsg, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &request );
            MPI_Iprobe( 0, 0, MPI_COMM_WORLD, &flag, &status );
        MPI_Get_count( &status, MPI_INT, &count );
        if(count != 1)
        MPI_Recv( &recvMsg, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &status );
if (recvMsg != 123)
        MPI_Wait( &request, &status );
return errs;