MPI_Session_attach_buffer(3)

💖 MPI_Session_attach_buffer — bestie, it's the session version 💖

okay so this is a page from the MPI manual, section 3 (library calls, the fun ones). And the function we're hyperfixating on today is MPI_Session_attach_buffer. Yes it's basically MPI_Buffer_attach but for your session, and yes that distinction matters. Sit down, grab a strawberry 🍓, let's go.

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✨ NAME ✨

MPI_Session_attach_buffer — attaches a user-provided buffer for buffered send in the session.

like. you hand MPI a blob of memory and you're like "here, this is yours now, use it for buffered sends." that's the whole vibe. 💅

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✨ SYNOPSIS ✨

int MPI_Session_attach_buffer(MPI_Session session, void *buffer, int size)

int MPI_Session_attach_buffer_c(MPI_Session session, void *buffer,
MPI_Count size)

two flavours, both iconic. the _c one takes an MPI_Count for size instead of an int, so if your sizes are Big Big, that's your girl. 👑

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✨ INPUT PARAMETERS ✨

• session — the session (handle). your lil session ID. that's it, that's the whole thing.
• buffer — initial buffer address (choice). a "choice" type, meaning it's polymorphic-ish about what pointer type you pass. just give it the address.
• size — buffer size, in bytes (non-negative integer). plz make it non-negative. negative bytes are not a thing and MPI will not be vibing with it.

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🧚 NOTES FOR FORTRAN 🧚

okay Fortran besties, this part is for you, listen up 🫶

• 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. so like, C gets the error back as the return value, Fortran gets it in ierr at the end. same energy, different seat. 💜
• In Fortran, MPI routines are subroutines, and are invoked with the call statement. call it. don't try to use it like a function. it's not giving function. it's giving subroutine.
• All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. yes, even the fancy ones. INTEGER. no cap.

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😭 ERRORS 😭

so here's the tea on errors, and it's lowkey important:

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. 😳 yes, aborts. your whole job. gone. main character exit.

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 — so like, it still works but it's wearing last season's fit, iykyk. 🙈

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.

fr fr read that again: MPI does not guarantee your program survives an error. implementations try to keep going, but "try" is doing a lot of heavy lifting there. don't build your whole architecture on the assumption that error recovery just works. 💀

here's the error codes, one bullet each, no skipping:

• MPI_SUCCESS — No error; MPI routine completed successfully. slay. we love her. ✨
• MPI_ERR_ARG — Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). so like, something's wrong but MPI's not gonna tell you which something. rude but ok.
• MPI_ERR_SESSION — Invalid session handle. your session handle isn't a real one, bestie. 🫠
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code. the catch-all. go ask MPI_Error_string for the details.

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🌈 SEE ALSO 🌈

go read these next, they're the related lore:

• MPI_Buffer_attach
• MPI_Bsend

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and that's the page!! 🌸🦋 go forth and attach your buffers responsibly, queen. this has been the MPI manual, page MPI_Session_attach_buffer(3), dated 4/10/2026. mwah. 💖✨🎀