MPI_BUFFER_DETACH(3)

💫 MPI_Buffer_detach — the Open MPI manual page, bestie edition 💫

okay so this is MPI_Buffer_detach from the Open MPI manual (section 3, we love a library call). It removes an existing buffer — the kind you'd use with MPI_Bsend and friends. ✨

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📜 SYNTAX

C Syntax

#include <mpi.h>

int MPI_Buffer_detach(void *buf, int *size)

Fortran Syntax

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

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

Fortran 2008 Syntax

USE mpi_f08

MPI_Buffer_detach(buffer_addr, size, ierror)
    USE, INTRINSIC :: ISO_C_BINDING, ONLY
    TYPE(C_PTR), INTENT(OUT) :: buffer_addr
    INTEGER, INTENT(OUT) :: size
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎁 OUTPUT PARAMETERS

• buf — the initial buffer address (it's a choice, yes, that's what the spec says 💅).
• size — buffer size, in bytes (integer).
• ierror — Fortran only: error status (integer).

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🌸 DESCRIPTION

Detach the buffer that's currently associated with MPI. The call hands you back the address and the size of the buffer it just detached. This operation will block until all messages currently sitting in the buffer have been transmitted. So like… it waits. Patiently. 🫶 Once it returns, you're free to reuse or deallocate the space that buffer was taking up. iconic behavior, honestly.

Example: Calls to attach and detach buffers.

#define BUFFSIZE 10000

int size char *buff;
MPI_Buffer_attach( malloc(BUFFSIZE), BUFFSIZE);

// a buffer of 10000 bytes can now be used by MPI_Bsend

MPI_Buffer_detach( &buff, &size); // Buffer size reduced to zero
MPI_Buffer_attach( buff, size); // Buffer of 10000 bytes available again

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🧠 NOTES

The whole reason MPI_Buffer_detach gives you back the address and size of the buffer it's detaching is so nested libraries can replace and restore the buffer. no cap. For example, consider:

int size, mysize, idummy;
void *ptr, *myptr, *dummy;
MPI_Buffer_detach( &ptr, &size );
MPI_Buffer_attach( myptr, mysize );

/* ... library code ... */

MPI_Buffer_detach( &dummy, &idummy );
MPI_Buffer_attach( ptr, size );

This is much like the action of the UNIX signal routine, and it has the same strengths (it's simple) and weaknesses (it only works for nested usages). it's giving "small but mighty, don't push it". 😭

For Fortran: the Fortran binding for this routine is different. Because Fortran does not have pointers, it is impossible to provide a way to use the output of this routine to exchange buffers. In this case, only the size field is set.

For C: even though the buf argument is declared as void, it is really the address of a void pointer. See Rationale, below, for more details.

Even though the C functions MPI_Buffer_attach and MPI_Buffer_detach both have a first argument of type void*, these arguments are used differently: a pointer to the buffer is passed to MPI_Buffer_attach; the address of the pointer is passed to MPI_Buffer_detach, so that this call can return the pointer value. yeah. read that again. it's the whole plot twist. 💀

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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. like… it might just be over. 😭

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_attach

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👑 Copyright

2003-2026, The Open MPI Community

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and that's the whole page bestie, you ate it up fr fr 🎀 this was Open MPI's MPI_BUFFER_DETACH(3), dated Sep 10, 2026 — go forth and detach those buffers responsibly, mwah 💖🦋