MPI_WAITANY(3)

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💖 MPI_Waitany — Open MPI 💖

okay bestie, gather round, because this one is the multitasking queen of MPI and she deserves her moment. this is the MPI_Waitany page from the Open MPI manual, and by the end of this you're gonna be waiting on a whole gaggle of requests like it's nothing. periodt. ✨

🌸 NAME 🌸

MPI_Waitany — Waits for any specified send or receive to complete. yeah. any. not all, not some. literally just one, whoever finishes first, and she'll tell you which. 💅

✨ 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 — List length (integer). how many requests are we looking at, bestie? 👀
  • array_of_requests — Array of requests (array of handles). the whole little lineup of hopes and dreams u handed to MPI.

🦋 OUTPUT PARAMETERS 🦋

  • index — Index of handle for operation that completed (integer). In the range 0 to count-1. In Fortran, the range is 1 to count. (yes the Fortran girlies count from one, we love that for them 🍓)
  • status — Status object (status).
  • ierror — Fortran only: Error status (integer).

💜 DESCRIPTION 💜

A call to MPI_Waitany can be used to wait for the completion of one out of several requests. it's giving ✨*first come, first served*✨.

The array_of_requests list may contain null or inactive handles. If the list contains no active handles (list has length zero or all entries are null or inactive), then the call returns immediately with index = MPI_UNDEFINED, and an empty status. no drama, no waiting, she just says "nothing to see here" and moves on. 🙈

The execution of MPI_Waitany(count, array_of_requests, index, status) has the same effect as the execution of MPI_Wait(&array_of_requests[i], status), where i is the value returned by index (unless the value of index is MPI_UNDEFINED). MPI_Waitany with an array containing one active entry is equivalent to MPI_Wait <#mpi-wait>. iconic behavior honestly, one entry and she's basically just MPI_Wait in a cute outfit. 👑

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. free real estate. 💸

Example: Client-server code (starvation can occur). ⚠️ (yeah bestie, starvation. that's real, we'll talk about 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; 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. 💀 yeah she does NOT play.
  • 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. the polite one. 🎀

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. like... it might just be over. sorry bestie. 😭

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 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. (translation: don't go looking for the error in status.MPI_ERROR, it is NOT there, she sends it down the back-end pipeline instead 💅)

See also:

  • MPI_Comm_set_errhandler <#mpi-comm-set-errhandler>
  • MPI_File_set_errhandler <#mpi-file-set-errhandler>
  • MPI_Test <#mpi-test>
  • MPI_Testall <#mpi-testall>
  • MPI_Testany <#mpi-testany>
  • MPI_Testsome <#mpi-testsome>
  • MPI_Wait <#mpi-wait>
  • MPI_Waitall <#mpi-waitall>
  • MPI_Waitsome <#mpi-waitsome>
  • MPI_Win_set_errhandler <#mpi-win-set-errhandler>

2003-2026, The Open MPI Community

and that's the whole page bestie!! 🫶 thank u for reading, u ate that fr fr. this was the MPI_WAITANY(3) page from Open MPI, dated Sep 10, 2026. go wait on literally any request u want now, u got this. mwah. 💖🎀🦋✨

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