MPI_COMM_SET_ATTR(3)

💅 MPI_COMM_SET_ATTR 💅

okay so bestie, welcome to the Open MPI manual page for MPI_Comm_set_attr — it does exactly one thing and it does it flawlessly: it stores an attribute value next to a key. that's it. that's the whole vibe. 🎀

---

✨ NAME ✨

MPI_Comm_set_attr — Stores attribute value associated with a key.

---

🫶 SYNTAX 🫶

🌸 C Syntax 🌸

#include <mpi.h>

int MPI_Comm_set_attr(MPI_Comm comm, int comm_keyval, void *attribute_val)

🦋 Fortran Syntax 🦋

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_SET_ATTR(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, IERROR)
     INTEGER COMM, COMM_KEYVAL, IERROR
     INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL

🧚 Fortran 2008 Syntax 🧚

USE mpi_f08
MPI_Comm_set_attr(comm, comm_keyval, attribute_val, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(IN) :: comm_keyval
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: attribute_val
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

---

💖 INPUT/OUTPUT PARAMETER 💖

• comm — Communicator from which attribute will be attached (handle). it's giving "the group chat we're decorating" energy 🫶

🌈 INPUT PARAMETERS 🌈

• comm_keyval — Key value (integer). this is the lil name-tag for your attribute, slay 💅
• attribute_val — Attribute value. the actual thing you're attaching, iconic ✨

⭐ OUTPUT PARAMETER ⭐

• ierror — Fortran only: Error status (integer). yes bestie, only the Fortran girlies get this one 💀

---

👑 DESCRIPTION 👑

MPI_Comm_set_attr stores the stipulated attribute value attribute_val for subsequent retrieval by MPI_Comm_get_attr. literally it's the "put it in the drawer so you can find it later" function, no cap 🎀

if the value is already present, then the outcome is as if MPI_Comm_delete_attr was first called to delete the previous value (and the callback function delete_fn was executed), and a new value was next stored. so it's lowkey a glow-up: out with the old, in with the new ✨

the call is erroneous if there is no key with value comm_keyval; in particular MPI_KEYVAL_INVALID is an erroneous key value. bestie do NOT do that. 😭

the call will fail if the delete_fn function returned an error code other than MPI_SUCCESS. so yeah, if the cleanup girlie trips on her way out, the whole thing falls over 💀

this function replaces MPI_Attr_put, the use of which is deprecated — old news, we don't do that anymore, it's giving 2003 flip phone 📵. the C binding is identical. the Fortran binding differs in that attribute_val is an address-sized integer. iykyk 🦋

---

💜 NOTES 💜

values of the permanent attributes MPI_TAG_UB, MPI_HOST, MPI_IO, and MPI_WTIME_IS_GLOBAL may not be changed. those four are the main characters and they're not taking edits, periodt 👑

the type of the attribute value depends on whether C or Fortran is being used. in C, an attribute value is a pointer (void *); in Fortran, it is a single, address-size integer system for which a pointer does not fit in an integer. so like, different languages, different love languages, same key 🫶

if an attribute is already present, the delete function (specified when the corresponding keyval was created) will be called. she cleans up after herself, we stan 🌸

---

😭 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. so check your return values bestie, don't just vibe past them 🙈

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? yes. effective? also yes 💅
• 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 chill one, she just hands you the receipt 🧾

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. so like, an error is not always a "just keep swimming" moment, 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. 📚

---

🎀 Copyright 🎀

2003-2026, The Open MPI Community

---

okay that's the whole page bestie, you ate that 💅✨ this was brought to you by Open MPI, dated Sep 10, 2026, page on deck: MPI_COMM_SET_ATTR(3) — and honestly? iconic. periodt 🦋💖