MPI_PROBE(3)					     Open MPI					      MPI_PROBE(3)

MPI_Probe — Blocking test for a message.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Probe(int source, int tag, MPI_Comm comm, MPI_Status *status)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_PROBE(SOURCE, TAG, COMM, STATUS, IERROR)
	       INTEGER SOURCE, TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Probe(source, tag, comm, status, ierror)
	       INTEGER, INTENT(IN) :: source, tag
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Status) :: status
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • source: Source rank or MPI_ANY_SOURCE (integer).

       • tag: Tag value or MPI_ANY_TAG (integer).

       • comm: Communicator (handle).

OUTPUT PARAMETERS
       • status: Status object (status).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       The  MPI_Probe  and MPI_Iprobe <#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  in  the status variable. For example, the user may allocate memory for the receive buffer, according
       to the length of the probed message.

       MPI_Probe behaves like MPI_Iprobe <#mpi-iprobe> except that it is a blocking call that returns only after a
       matching message has been found.

       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.

       The semantics of MPI_Probe and MPI_Iprobe <#mpi-iprobe> guarantee progress: If a call to MPI_Probe has been
       issued by a process, and a send that matches the probe has been initiated by some process, then the call to
       MPI_Probe  will	return,	 unless	 the  message is received by another concurrent receive operation (that is
       executed by another thread at the probing process). Similarly, if a process busy waits with  MPI_Iprobe	<#
       mpi-iprobe>  and	 a  matching  message  has  been  issued,  then	 the call to MPI_Iprobe <#mpi-iprobe> will
       eventually return flag = true unless the message is received by another concurrent receive operation.

       Example 1: Use blocking probe to wait for an incoming message.

	  CALL MPI_COMM_RANK(comm, rank, ierr)
	  IF (rank == 0) THEN
	     CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
	  ELSE IF(rank == 1) THEN
	     CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
	  ELSE	 ! rank == 2
	     DO i=1, 2
		CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
			       comm, status, ierr)
		IF (status(MPI_SOURCE) = 0) THEN
		   CALL MPI_RECV(i, 1, MPI_INTEGER, 0, 0, status, ierr)
		ELSE
		   CALL MPI_RECV(x, 1, MPI_REAL, 1, 0, status, ierr)
		END IF
	     END DO
	  END IF

       Each message is received with the right type.

       Example 2: A program similar to the previous example, but with a problem.

	  CALL MPI_COMM_RANK(comm, rank, ierr)
	  IF (rank == 0) THEN
	     CALL MPI_SEND(i, 1, MPI_INTEGER, 2, 0, comm, ierr)
	  ELSE IF(rank == 1) THEN
	     CALL MPI_SEND(x, 1, MPI_REAL, 2, 0, comm, ierr)
	  ELSE
	     DO i=1, 2
		CALL MPI_PROBE(MPI_ANY_SOURCE, 0,
			       comm, status, ierr)
		IF (status(MPI_SOURCE) == 0) THEN
		   CALL MPI_RECV(i, 1, MPI_INTEGER, MPI_ANY_SOURCE, &
				 0, status, ierr)
		ELSE
		   CALL MPI_RECV(x, 1, MPI_REAL, MPI_ANY_SOURCE, &
				 0, status, ierr)
		END IF
	     END DO
	  END IF

       We slightly modified Example 2, using MPI_ANY_SOURCE as the source argument in the  two	receive	 calls	in
       statements  labeled  100 and 200. The program is now incorrect: The receive operation may receive a message
       that is distinct from the message probed by the preceding call to MPI_Probe.

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard
       <https://www.mpi-forum.org/docs/>,   MPI	  errors   on  messages	 queried  by  MPI_Probe	 do  not  set  the
       status.MPI_ERROR field in the returned status.  The error code is  always  passed  to  the  back-end  error
       handler	and  may  be passed back to the caller through the return value of MPI_Probe if the back-end error
       handler returns it.  The pre-defined MPI	 error	handler	 MPI_ERRORS_RETURN  exhibits  this  behavior,  for
       example.

       See also:

	  • MPI_Iprobe <#mpi-iprobe>

	  • MPI_Cancel <#mpi-cancel>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					      MPI_PROBE(3)
