MPI_REQUEST_FREE(3)

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version 5.0.10 · 7 versions
, in ubuntu stonking

💖 MPI_Request_free(3) — bestie edition 💖

okay so. this is MPI_Request_free, from the Open MPI manual, section 3. she's the one who lets your request object go without making you sit there and wait for the actual communication to finish. it's giving "you're free now, go live your life, i'll handle the rest" ✨

MPI_Request_free — Frees a communication request object.

🌸 SYNOPSIS 🌸

C Syntax

#include <mpi.h>

int MPI_Request_free(MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REQUEST_FREE(REQUEST, IERROR)
     INTEGER REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Request_free(request, ierror)
     TYPE(MPI_Request), INTENT(INOUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🎀 INPUT/OUTPUT PARAMETER 🎀

  • request — Communication request (handle). (that's the thing you're setting free, bestie 🕊️)

💅 DESCRIPTION 💅

So what this does: it lets a request object get deallocated without you waiting around for the associated communication to complete. no blocking, no Ctrl+C-ing your whole vibe out of impatience. 💀

MPI_Request_free marks that request object for deallocation and sets request to MPI_REQUEST_NULL. Any ongoing communication tied to that request? it's still allowed to complete. the request itself only actually gets deallocated after its completion. so like, it's not a cancellation, it's a "you go, i'm not watching" 💖

✨ NOTES ✨

Once a request is freed by a call to MPI_Request_free, you literally cannot check whether the associated communication succeeded using MPI_Wait or MPI_Test. nope. and if an error happens later during the communication, an error code cannot be returned to you — that kind of error has to be treated as fatal. 😭

So naturally everyone's like "okay but how do i know when the operations completed??" valid question, bestie. and the answer is: depending on the program logic, there may be other ways the program knows certain operations have completed — and that's what makes using MPI_Request_free practical at all.

Example time (they gave us one, and she ate): an active send request could be freed when the program's logic guarantees the receiver sends a reply to the message that was sent — the arrival of that reply tells the sender the send has completed and the send buffer can be reused. clean. iconic. 👑

An active receive request, though? should never be freed, because the receiver will have no way to verify that the receive completed and that the receive buffer can be reused. do not do the thing. periodt. 🚫

Example:

CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
IF(rank == 0) THEN
    DO i=1, n
      CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
      CALL MPI_REQUEST_FREE(req, ierr)
      CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
      CALL MPI_WAIT(req, status, ierr)
    END DO
ELSE IF (rank == 1) THEN
    CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
    CALL MPI_WAIT(req, status)
    DO I=1, n-1
       CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
       CALL MPI_REQUEST_FREE(req, ierr)
       CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
       CALL MPI_WAIT(req, status, ierr)
    END DO
    CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
    CALL MPI_WAIT(req, status)
END IF

This routine is normally used to free persistent requests created with either MPI_Recv_init or MPI_Send_init and friends. but like, it can also be used to free a request created with MPI_Irecv or MPI_Isend and friends — in that case, the use (yes, the manpage says "use", we're keeping it, it's part of the lore) can not use the test/wait routines on the request.

It is permitted to free an active request. but once freed, you can not use the request in a wait or test routine (e.g., MPI_Wait). no take-backs, no second chances, she's gone 💨

🌈 ERRORS 🌈

Almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines tuck it into 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 counts as 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. everyone's going down together, bestie 💀
  • 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. 🫠

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.

See also:

  • MPI_Isend
  • MPI_Irecv
  • MPI_Issend
  • MPI_Ibsend
  • MPI_Irsend
  • MPI_Recv_init
  • MPI_Send_init
  • MPI_Ssend_init
  • MPI_Rsend_init
  • MPI_Test
  • MPI_Wait
  • MPI_Waitall
  • MPI_Waitany
  • MPI_Waitsome
  • MPI_Testall
  • MPI_Testany
  • MPI_Testsome

2003-2026, The Open MPI Community

and that's the whole page, bestie 💜 this was brought to you by the Open MPI girlies, version-tagged Sep 10, 2026, from section 3 of the manual. go free those requests responsibly, mwah 💅✨

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