MPI_COMM_FREE_KEYVAL(3)

💖 MPI_COMM_FREE_KEYVAL(3) 💖

okay bestie, welcome to the Open MPI manual, and today's episode is all about saying goodbye to an attribute key. yes, it's giving "it's not you, it's me" but for communicator cache attributes. let's go 🎀

🌸 NAME 🌸

MPI_Comm_free_keyval — Frees attribute key for communicator cache attribute.

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✨ SYNTAX ✨

💅 C Syntax

#include <mpi.h>

int MPI_Comm_free_keyval(int *comm_keyval)

🦋 Fortran Syntax

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

⭐ Fortran 2008 Syntax

USE mpi_f08
MPI_Comm_free_keyval(comm_keyval, ierror)
     INTEGER, INTENT(INOUT) :: comm_keyval
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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👑 INPUT/OUTPUT PARAMETER 👑

• comm_keyval:

🎀 OUTPUT PARAMETER 🎀

• ierror: Fortran only: Error status (integer).

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💜 DESCRIPTION 💜

okay so MPI_Comm_free_keyval frees an extant attribute key. that's the whole vibe. it sets the value of keyval to MPI_KEYVAL_INVALID, so after this call that key is giving expired, deceased, no longer with us 💀

and here's the part that lowkey surprises everyone: it is not erroneous to free an attribute key that is still in use. nope! because the actual free does not transpire until after all references (in other communicators on the process) to the key have been freed. so it's not instant, it's more like a ✨pending✨ situation.

and those references? your program has to explicitly free them itself, either:

• via calls to MPI_Comm_delete_attr that free one attribute instance, or
• via calls to MPI_Comm_free that free all attribute instances associated with the freed communicator.

this call is literally identical to the call MPI_Keyval_free but it's needed to match the communicator-specific creation function introduced in the MPI-2 standard. and ngl, the use of MPI_Keyval_free is deprecated. so bestie, use this one 🫶

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🧚 NOTES 🧚

key values are global (they can be used with any and all communicators). they're for everybody, no exclusivity here 💅

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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. 💀
• 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. periodt.

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_Comm_create_keyval

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🌈 Copyright 🌈

2003-2026, The Open MPI Community

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and that's the whole page, bestie!! you understood it, you ate it, no crumbs left 💅✨ this was Open MPI's MPI_COMM_FREE_KEYVAL(3), from Sep 10, 2026 — mwah, ily, go free some keyvals 🎀🦋💖