MPI_Comm_flush_buffer(3)

💅 MPI_Comm_flush_buffer(3) — the MPI girlies' guide

okay bestie, so this is a page from the MPI manual (that's section 3, the library-calls section, iykyk) and it's about one very specific little function that does one very specific little thing. Let's get into it 💖

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✨ NAME ✨

MPI_Comm_flush_buffer — block until all messages currently in the communicator-specific buffer of the calling process have been transmitted

like. it literally just waits. that's the whole job. it does not send anything new, it does not vibe, it just sits there until everything already sitting in your process's communicator-specific buffer has actually gone out the door. 🦋

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🌸 SYNOPSIS 🌸

int MPI_Comm_flush_buffer(MPI_Comm comm)

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🎀 INPUT PARAMETERS 🎀

• comm — communicator (handle)

yep. that's the only thing you hand it. one (1) communicator. and it's a handle, so it's the little opaque token thing, not the whole communicator itself. 💅

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🧵 THREAD AND INTERRUPT SAFETY 🧵

okay so the tea: this routine is thread-safe. that means multiple threads can call it at the same time and you do not need to bring your own user-provided thread locks. it's got itself handled, no cap. 🫶

but. and it's a big but. it is not interrupt safe. typically that's because of memory allocation routines like malloc, or other non-MPICH runtime routines, and those are just not interrupt-safe themselves. so like, thread-safe: yes. interrupt-safe: absolutely not. don't do that. 😭

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🌈 NOTES FOR FORTRAN 🌈

• Every single MPI routine in Fortran has an extra argument ierr tacked onto the end of the argument list. the only exceptions are MPI_WTIME and MPI_WTICK.
• ierr is an integer and it means exactly the same thing as the C routine's return value. same energy, different syntax.
• In Fortran, MPI routines aren't functions, they're subroutines, and you invoke them with the call statement. call-core behavior tbh.
• All MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. all of them. every single one. 💀

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🚨 ERRORS 🚨

so basically all MPI routines return an error value — except MPI_Wtime and MPI_Wtick, the two little rebels. In C that's the function's return value; in Fortran it's that last ierr argument we talked about.

before the value even comes back to you, the current MPI error handler gets called. and mm, by default? that error handler aborts the MPI job. yes, the whole job. so if you're out here rawdogging error handling, pls know what you're doing. 🙈

You can swap the error handler out with:

• MPI_Comm_set_errhandler — for communicators
• MPI_File_set_errhandler — for files
• MPI_Win_set_errhandler — for RMA windows

and yeah, the MPI-1 routine MPI_Errhandler_set may be used, but its use is deprecated, so like… why would you. 👑

If you'd rather get error values handed back to you instead of having everything explode, use the predefined error handler MPI_ERRORS_RETURN.

important caveat bestie: MPI does not guarantee that an MPI program can continue past an error. however, MPI implementations will attempt to continue whenever possible. so it's giving "we'll try" not "we promise". 😭

and here are the error codes, babes:

• MPI_SUCCESS — no error; the MPI routine completed successfully. the dream. ✨
• MPI_ERR_COMM — invalid communicator. a common error is to use a null communicator in a call — which is not even allowed in MPI_Comm_rank, so like, it's really not allowed. 💀
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the catch-all. the "mystery flavor" of errors. 🔥

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👀 SEE ALSO 👀

• MPI_Buffer_flush
• MPI_Session_flush_buffer

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okay that's the whole page, you made it, I'm so proud 💖 this was MPI_Comm_flush_buffer(3) from the MPI manual — the MPI source/version, dated 4/10/2026. go flush some buffers, periodt. 👑🎀✨