MPI_Buffer_attach(3)

💖 MPI_Buffer_attach(3) 💖

okay so bestie, this is a page from the MPI manual (section 3, the library calls one, iykyk) and today's main character is MPI_Buffer_attach. let's get into it 🌸

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✨ NAME ✨

MPI_Buffer_attach — attaches a user-provided buffer for sending 🫶

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✨ SYNOPSIS ✨

int MPI_Buffer_attach(void *buffer, int size)

int MPI_Buffer_attach_c(void *buffer, MPI_Count size)

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✨ INPUT PARAMETERS ✨

• buffer — the initial buffer address (this one's a choice, you get to pick it 💅)
• size — buffer size, in bytes (a non-negative integer, so no negative nonsense pls 🙈)

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✨ NOTES ✨

okay this is the part where you actually have to do math and it's giving homework BUT it's important so listen up 🔥

The size you pass in should be the sum of the sizes of all the outstanding Bsends you intend to have, plus MPI_BSEND_OVERHEAD for each Bsend that you do. And for the purposes of calculating size, you should use MPI_Pack_size, no cap.

In other words, in the code

MPI_Buffer_attach(buffer, 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;

see? it's literally just addition with extra steps 🎀

The MPI_BSEND_OVERHEAD gives the maximum amount of space that may be used in the buffer for use by the BSEND routines in using the buffer. This value is in mpi.h (for C) and mpif.h (for Fortran). go find it, it's right there 👑

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✨ THREAD AND INTERRUPT SAFETY ✨

bestie, the user is responsible for ensuring that multiple threads do not try to update the same MPI object from different threads. and this routine should not be used from within a signal handler. periodt.

The MPI standard defined a thread-safe interface but this does not mean that all routines may be called without any thread locks. For example, two threads must not attempt to change the contents of the same MPI_Info object concurrently. The user is responsible in this case for using some mechanism, such as thread locks, to ensure that only one thread at a time makes use of this routine. Because the buffer for buffered sends (e.g., MPI_Bsend) is shared by all threads in a process, the user is responsible for ensuring that only one thread at a time calls this routine or MPI_Buffer_detach. 😭 so like, one thread at a time or it's chaos.

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✨ NOTES FOR FORTRAN ✨

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and they're invoked with the call statement. so cute, so retro 🌈

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. 💜

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✨ ERRORS ✨

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job 💀 (yeah, dramatic). The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated 🚫. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. they're trying their best, okay? 🥺

• MPI_SUCCESS — No error; MPI routine completed successfully. slay ✅
• MPI_ERR_ARG — Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). 🙈
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code. 🫠

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✨ SEE ALSO ✨

MPI_Buffer_detach, MPI_Bsend

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and that's the whole page, bestie 💖 ngl this one was written back on 4/10/2026, and yeah the source is the MPI manual, page MPI_Buffer_attach(3) — attache your buffer, detach your buffer, live your truth 👑✨🦋