MPI_IPROBE(3)					     Open MPI					     MPI_IPROBE(3)

MPI_Iprobe — Nonblocking test for a message.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Iprobe(int source, int tag, MPI_Comm comm, int *flag,
	       MPI_Status *status)

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

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Iprobe(source, tag, comm, flag, status, ierror)
	       INTEGER, INTENT(IN) :: source, tag
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       LOGICAL, INTENT(OUT) :: flag
	       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
       • flag: Message-waiting flag (logical).

       • status: Status object (status).

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

DESCRIPTION
       The  MPI_Probe  <#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.

       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
       <#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 as
       described in the “Return Status” subsection of  the  “Point-to-Point  Communication”  chapter  in  the  MPI
       Standard <https://www.mpi-forum.org/docs/> 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 multithreaded, it is the user’s responsibility to ensure that
       the last condition holds.

       The source argument of  MPI_Probe  <#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.

       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.

       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.

NOTE
       Multi-threaded  application  developers should remember that two threads calling MPI_Iprobe may return true
       for the same message in both threads.

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_Iprobe	 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_Iprobe 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_Probe <#mpi-probe>

	  • MPI_Cancel <#mpi-cancel>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					     MPI_IPROBE(3)
