MPI_BSEND(3)

💌 MPI_BSEND(3) — okay bestie, this is the Open MPI one

So like… you opened the MPI_BSEND(3) page from Open MPI and now you're here 🫶. Don't worry, I got you. This is the whole thing, just with, like, actual personality.

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✨ SYNTAX ✨

💻 C Syntax

#include <mpi.h>

int MPI_Bsend(const void *buf, int count, MPI_Datatype datatype,
   int dest, int tag, MPI_Comm comm)

🧚 Fortran Syntax

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

MPI_BSEND(BUF, COUNT,DATATYPE, DEST, TAG, COMM, IERROR)
    <type>  BUF(*)
    INTEGER COUNT, DATATYPE, DEST, TAG, COMM, IERROR

🌸 Fortran 2008 Syntax

USE mpi_f08
MPI_Bsend(buf, count, datatype, dest, tag, comm, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN) :: buf
    INTEGER, INTENT(IN) :: count, dest, tag
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎀 INPUT PARAMETERS 🎀

• buf — the initial address of the send buffer, bestie. It's a choice type 💅
• count — how many entries are in the send buffer. Nonnegative integer, so no negative nonsense 🙈
• datatype — the datatype of each send buffer element (it's a handle ✨)
• dest — rank of the destination, an integer 👑
• tag — the message tag, also an integer 🎯
• comm — the communicator (handle, slay) 🌈

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💖 OUTPUT PARAMETERS 💖

• ierror — Fortran only: the error status, an integer 🍓

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🌟 DESCRIPTION 🌟

MPI_Bsend does a buffered-mode, blocking send. That's it. That's the vibe. Sending, but buffered, but blocking. Periodt 💅

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🔥 NOTES 🔥

This send is literally just a convenience function 😌 — it lets you send messages without worrying about where they get buffered, because you, yes you, had to provide that buffer space already with MPI_Buffer_attach. iykyk.

When you're deciding how much buffer space to allocate, remember bestie: that buffer space is not available for reuse by subsequent MPI_Bsends unless you are certain the message has been received. Not just "it should have been received" — actually received. No cap 🫶

⚠️ Like, for example, this code does not allocate enough buffer space:

MPI_Buffer_attach( b, n*sizeof(double) + MPI_BSEND_OVERHEAD );
for (i=0; i<m; i++) {
    MPI_Bsend( buf, n, MPI_DOUBLE, ... );
}

…because only enough buffer space is provided for a single send 😭, and the loop may start a second MPI_Bsend before the first is done making use of the buffer. Rude honestly.

In C, you can force the messages to be delivered by calling 💫:

MPI_Buffer_detach( &b, &n ); MPI_Buffer_attach( b, n );

MPI_Buffer_detach will not complete until all buffered messages are delivered, so like, it will wait for you. Iconic behaviour honestly 🧚

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💀 ERRORS 💀

Almost all MPI routines return an error value — C routines give it back as the return result of the function, and Fortran routines put it in the last argument. Both vibes, same info ✨

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. Okay that was a lot but we got through it 💜

Open MPI includes three predefined error handlers that can be used, and they are:

• 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, it 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, it 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

🫶 Just so you know: MPI does not guarantee that an MPI program can continue past an error. Like, at all. Be careful 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 ✨

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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Ok that's the whole page, you ate that 💅🦋💖 — source/version is Open MPI, and the date on this one is Sep 11, 2026, filed under MPI_BSEND(3). Go forth and send your buffers responsibly, bestie 🎀✨