Provided by: openmpi-doc_5.0.7-1_all 
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 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. 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 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 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_thread, after
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 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_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 was called on that communicator. If called on a
window or file, acts as if 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 then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
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, 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_Cancel
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_IPROBE(3)