MPI_IPROBE(3)

💅 MPI_Iprobe — Nonblocking test for a message 🦋

okay so this is MPI_IPROBE(3), straight out of the Open MPI manual, and bestie, it's literally the "hey is anyone there??" function of MPI. no commitment, just vibes — you peek at your mail, you don't open it. ✨

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🌸 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

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🎀 INPUT PARAMETERS 🎀

• source — the source rank, or MPI_ANY_SOURCE if you're being chill about who talks to you (integer).
• tag — a tag value, or MPI_ANY_TAG if labels are so not the point rn (integer).
• comm — the communicator, aka the group chat this is happening in (handle).

💖 OUTPUT PARAMETERS 💖

• flag — the message-waiting flag, your little "yes queen there's mail" boolean (logical).
• status — the status object, it's giving receipts (status).
• ierror — Fortran only: error status (integer). 🫶

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🌈 DESCRIPTION 🌈

so MPI_Probe and MPI_Iprobe let you check on incoming messages without actually receiving them. like reading the notification preview and deciding if you're gonna open the text. 💅 based on what the probe hands back (basically, the info in status), you get to pick how you want to receive the thing — a classic move is allocating your receive buffer to exactly the length of the probed message, which is honestly so considerate.

MPI_Iprobe(source, tag, comm, flag, status) returns flag = true if there's a message that can be received and that matches the pattern you specified with source, tag, and comm. lowkey the important part: the call matches the same message that a call to MPI_Recv(..., source, tag, comm, status) at the same point in the program would have received, and it returns in status the same value MPI_Recv would have returned. otherwise, the call returns flag = false and leaves status undefined (yes, undefined — don't go reading it, bestie 🙈).

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. chef's kiss, all the deets.

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 last condition holds. no cap, that's on you. 😭

the source argument of MPI_Probe can be MPI_ANY_SOURCE, and the tag argument can be MPI_ANY_TAG, so you can probe for messages from an arbitrary source and/or with an arbitrary tag. but a specific communication context must be provided with the comm argument — that one is not optional, periodt.

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. efficiency icon behavior. 👑

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. probe away, she's unbothered. 💅

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⭐ NOTE ⭐

multi-threaded application developers should remember that two threads calling MPI_Iprobe may return true for the same message in both threads. yes both. simultaneously. it's giving chaos but it's documented chaos. 💀

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🔥 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. dramatic, but make it official.
• 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. the polite one. 🫶

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. like, no promises, bestie. 😭

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

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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that's the whole page, bestie — written by Open MPI, dated Sep 11, 2026, and yeah we read the entire thing together, you ate that 💜🧚‍♀️✨