MPI_File_write_at_all_begin(3)

✨ NAME ✨

MPI_File_write_at_all_begin — ok so bestie, this is the one that kicks off a split collective write 💅

(and yes, if you're wondering, this is that page, MPI_File_write_at_all_begin(3), from the MPI manual — glad you found it, mwah 🫶)

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🌸 SYNOPSIS 🌸

int MPI_File_write_at_all_begin(MPI_File fh, MPI_Offset offset, const void *buf,
int count, MPI_Datatype datatype)

int MPI_File_write_at_all_begin_c(MPI_File fh, MPI_Offset offset,
const void *buf, MPI_Count count,
MPI_Datatype datatype)

Two versions, bestie — the classic one and the _c one for the big-count girlies 👑

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🎀 INPUT PARAMETERS 🎀

• fh — your file handle (a handle, iykyk)
• offset — where in the file we're writing (an integer, literally where you're at in life)
• buf — the initial address of the buffer (a choice — pick your fighter)
• count — how many elements are living in that buffer (an integer)
• datatype — the datatype of each buffer element (a handle)

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🧚 THREAD AND INTERRUPT SAFETY 🧚

Okay so the tea ☕: this routine is thread-safe. That means multiple threads can use it at the same time with zero need for any user-provided thread locks — no cap, you don't have to babysit it 💖

But — and this is the part where we lower our voices — it is NOT interrupt safe. Why? Typically it's because of memory allocation routines like malloc, or other non-MPICH runtime routines that are themselves not interrupt-safe. So like… threads slay, signals do not 🙈

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🦋 NOTES FOR FORTRAN 🦋

Every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has an extra argument called ierr chillin' at the end of the argument list. ierr is an integer and it means the same thing as the return value of the routine in C. Also — plot twist — in Fortran, MPI routines are subroutines, and you invoke them with the call statement ✨

And all MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. Periodt.

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💖 ERRORS 💖

Alright, everything MPI returns an error value (except MPI_Wtime and MPI_Wtick — main character energy, they don't play by the rules). C routines return it as the value of the function, and Fortran routines put it in that last argument.

Before the value comes back, the current MPI error handler gets called. By default that error handler aborts the MPI job — yes, the whole thing, dramatic much 😭. You can swap it out with:

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

There's also the MPI-1 routine MPI_Errhandler_set, but lowkey its use is deprecated, so maybe don't 👀

The predefined error handler MPI_ERRORS_RETURN can be used to make error values actually get returned instead of nuking your job.

Important: MPI does not guarantee an MPI program can continue past an error. That said, MPI implementations will attempt to continue whenever possible. So… maybe?

The error codes, bestie:

• MPI_SUCCESS — no error; the MPI routine completed successfully. Slay!! 🎉
• MPI_ERR_BUFFER — invalid buffer pointer. Usually a null buffer where a null buffer is not valid 😬
• MPI_ERR_COUNT — invalid count argument. Counts must be non-negative; a count of zero is often valid though! (zero is valid, we love a humble queen)
• MPI_ERR_TYPE — invalid datatype argument. This one can also pop off if you use an uncommitted MPI_Datatype (see MPI_Type_commit) in a communication call. Commit your types, bestie 🫶
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code

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🌈 friendly reminder that none of this is vibes-only — every flag, every error, every warning above is the real deal. Read it fr fr.

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okay bye bestie, go write something iconic 💅✨

this page came to you from the MPI manual, dated 4/10/2026, and it's the MPI_File_write_at_all_begin(3) page 🎀🌟