MPI_WAITANY(3)

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✨ MPI_WAITANY ✨

Okay bestie, welcome to the MPI_Waitany page from Open MPI — the "which one of y'all finished first?" of the MPI world 💅

🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_Waitany(int count, MPI_Request array_of_requests[],
     int *index, MPI_Status *status)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WAITANY(COUNT, ARRAY_OF_REQUESTS, INDEX, STATUS, IERROR)
     INTEGER COUNT, ARRAY_OF_REQUESTS(*), INDEX
     INTEGER STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Waitany(count, array_of_requests, index, status, ierror)
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Request), INTENT(INOUT) :: array_of_requests(count)
     INTEGER, INTENT(OUT) :: index
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

💖 INPUT PARAMETERS 💖

  • count — the list length, an integer. Like, how many requests are you rolling into the group chat 🫶
  • array_of_requests — the array of requests (array of handles). All your pending ops, lined up ✨

🎀 OUTPUT PARAMETERS 🎀

  • index — index of the handle for the operation that completed (integer). In the range 0 to count-1. In Fortran, the range is 1 to count. Note the glow-up of the indexing, bestie — C starts at 0, Fortran starts at 1 🙈
  • status — the status object (status).
  • ierror — Fortran only: error status (integer).

🦋 DESCRIPTION 🦋

So here's the vibe: a call to MPI_Waitany lets you wait for the completion of one out of several requests. It's giving "first one to finish wins" energy, and honestly? Iconic 👑

Your array_of_requests list is allowed to contain null or inactive handles — no judgment, they're just not the main character today. But heads up: if the list contains no active handles (like, the list has length zero or every single entry is null or inactive), then the call returns immediately with index = MPI_UNDEFINED, plus an empty status. Nothing happened, and it's not being shady about it, it just literally had nothing to wait for 😭

The execution of MPI_Waitany(count, array_of_requests, index, status) has the same effect as executing MPI_Wait(&array_of_requests[i], status), where i is the value returned by index — unless the value of index is MPI_UNDEFINED, obviously. And MPI_Waitany with an array containing one active entry is equivalent to MPI_Wait. Math is mathing ✅

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. Lowkey a resource-saving slay 💅

Example: Client-server code (starvation can occur).

⚠️ Yes, the original really does say starvation can occur — that's a real warning, not a quirky character trait. Don't ignore it.
CALL MPI_COMM_SIZE(comm, size, ierr)
CALL MPI_COMM_RANK(comm, rank, ierr)
IF(rank > 0) THEN         ! client code
    DO
       CALL MPI_ISEND(a, n, MPI_REAL, 0, tag, comm, request, ierr)
       CALL MPI_WAIT(request, status, ierr)
    END DO
ELSE         ! rank=0: server code
       DO i=1, size-1
          CALL MPI_IRECV(a(1,i), n, MPI_REAL, i tag, &
                   comm, request_list(i), ierr)
       END DO
       DO
          CALL MPI_WAITANY(size-1, request_list, index, status, ierr)
          CALL DO_SERVICE(a(1,index))  ! handle one message
          CALL MPI_IRECV(a(1, index), n, MPI_REAL, index, tag, &
                    comm, request_list(index), ierr)
       END DO
END IF

🔥 ERRORS 🔥

Almost all MPI routines return an error value, bestie — C routines hand it back as the return result of the function, and Fortran routines stick it in the last argument. That's just how they roll 🫶

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. That's a whole lot of chain-of-command, but okay, we respect the hierarchy 💜

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

  • MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. Yeah. Everyone. 💀
  • 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, it 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, it aborts only the local process. Context matters, bestie — scope that chaos correctly 🔥
  • MPI_ERRORS_RETURN — returns an error code to the application. The chill one. The "here, you deal with it" 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. No cap — this is the real deal, so build your error handling like you mean it 😤

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.

Also, per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, MPI errors on requests passed to MPI_Waitany 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_Waitany if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example. Important detail, don't skip it just 'cause it's long 🧚

🌈 SEE ALSO 🌈

  • MPI_Comm_set_errhandler
  • MPI_File_set_errhandler
  • MPI_Test
  • MPI_Testall
  • MPI_Testany
  • MPI_Testsome
  • MPI_Wait
  • MPI_Waitall
  • MPI_Waitsome
  • MPI_Win_set_errhandler

2003-2026, The Open MPI Community 💖

And that's the whole page, bestie — this one was authored by the Open MPI girlies, dated Sep 11, 2026, and it's MPI_WAITANY(3) all the way down. Go wait on something iconic. Periodt. 🦋✨

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