MPI_BUFFER_ATTACH(3)

💖 MPI_BUFFER_ATTACH(3) 💖

okay bestie, so this is a page from the Open MPI manual, and we're doing MPI_Buffer_attach today ✨ — the glow-up your buffered sends have been waiting for. no cap, it's a short one but it matters.

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✨ NAME ✨

MPI_Buffer_attach — attaches a user-defined buffer for sending. yes literally that's the whole name section, she's concise 💅

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🦋 SYNTAX 🦋

C Syntax

#include <mpi.h>

int MPI_Buffer_attach(void *buf, int size)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_BUFFER_ATTACH(BUF, SIZE, IERROR)
    <type>  BUF(*)
    INTEGER SIZE, IERROR

Fortran 2008 Syntax

USE mpi_f08

MPI_Buffer_attach(buffer, size, ierror)
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buffer
    INTEGER, INTENT(IN) :: size
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🌸 INPUT PARAMETERS 🌸

• buf — the initial buffer address (it's a choice, which is MPI-speak for "we'll take whatever you've got, any type" 💁♀️).
• size — how big the buffer is, in bytes (integer).

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🎀 OUTPUT PARAMETER 🎀

• ierror — Fortran only, bestie: the error status (integer).

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💜 DESCRIPTION 💜

So what this does is hand MPI a buffer from your memory that it can use to buffer outgoing messages. that's the whole eras tour 🫶

key deets, no skipping:

• The buffer is only used by messages sent in buffered mode. she does not care about your regular sends, mind your business 💅
• Only one buffer can be attached to a process at a time. one. that's it. the limit does exist.

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⭐ NOTES ⭐

okay this is the part where people mess up, so listen up fr fr 🙈

The size you give should be the sum of the sizes of all the outstanding Bsends you intend to have, plus MPI_BSEND_OVERHEAD bytes for each Bsend that you do. for the purposes of calculating size, you should use MPI_Pack_size.

In other words, in the code:

MPI_Buffer_attach( buf, size )
MPI_Bsend( ..., count=20, datatype=type1, ... );
MPI_Bsend( ..., count=40, datatype=type2, ... );

the value of size in the MPI_Buffer_attach call should be greater than the value computed by:

MPI_Pack_size( 20, type1, comm, &s1 );
MPI_Pack_size( 40, type2, comm, &s2 );
size = s1 + s2 + 2 * MPI_BSEND_OVERHEAD;

MPI_BSEND_OVERHEAD gives the maximum amount of buffer space that may be used by the Bsend routines. this value lives in mpi.h for C and mpif.h for Fortran, iykyk 📌

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🌈 ERRORS 🌈

basically every MPI routine returns an error value: C routines as the return result of the function, and Fortran routines in the last argument. nothing new, but here's the full tea:

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. yeah. all of them. 💀
• 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. polite queen 👑

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. like, she might just leave the chat, no goodbye. plan accordingly 😭

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_Buffer_detach 👀

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👑 Copyright 👑

2003-2026, The Open MPI Community

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okay that's the whole page bestie, you ate that 🍓

💖 from your fave manual, Open MPI — dated Sep 10, 2026, filed under MPI_BUFFER_ATTACH(3). go attach a buffer and slay, periodt 💅✨🧚