MPI_File_read_at_all_begin(3)

💖 NAME 💖

MPI_File_read_at_all_begin — ok so bestie, this is the one that begins a split collective read using explicit offset. Periodt. ✨

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🦋 SYNOPSIS 🦋

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

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

Two flavors, same vibe — the _c one just takes an MPI_Count for count instead of an int. Choose your fighter. 💅

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🌸 INPUT PARAMETERS 🌸

• fh — the file handle, aka which file we're being nosy about (handle)
• offset — file offset, like where in the file we're starting (integer)
• count — how many elements are chillin' in the buffer (integer)
• datatype — the datatype of each buffer element, so it knows what it's actually reading (handle)

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🎀 OUTPUT PARAMETERS 🎀

• buf — initial address of the buffer, the spot where the goods get dropped off (choice)

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✨ THREAD AND INTERRUPT SAFETY ✨

Ok so the good news first: this routine is thread-safe. That means multiple threads can use it at the same time and you do not need to bring your own thread locks. No cap. 🫶

But — and this is important bestie — it is NOT interrupt safe. 😭 Why? Typically because of memory allocation routines in there, like malloc, or other non-MPICH runtime routines that are themselves not interrupt-safe. So don't come for it with signals, it's not built like that.

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🌈 NOTES FOR FORTRAN 🌈

Every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK, they're the exceptions, iykyk) gets an extra argument ierr tacked on at the end of the argument list. ierr is an integer and it means exactly the same thing as the return value of the routine does in C. 💯

Also in Fortran, MPI routines are subroutines, so you invoke them with the call statement. That's just how it is.

And one more: all MPI objects — like MPI_Datatype, MPI_Comm — are of type INTEGER in Fortran. Slay.

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💀 ERRORS 💀

Every single MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value. In C routines that's the return value of the function; in Fortran routines it's in that last argument. Before the value comes back to you, the current MPI error handler gets called. And by default? That error handler aborts the entire MPI job. Like, full shutdown, no survivors. 🚨

You can change the error handler with:

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

The MPI-1 routine MPI_Errhandler_set may also be used, but heads up: its use is deprecated. 🙈

If you'd rather get error values back instead of getting your whole job ended, the predefined error handler MPI_ERRORS_RETURN is the one that does that.

Also note that MPI does not guarantee an MPI program can continue past an error. That said, MPI implementations will attempt to continue whenever possible. So like… fingers crossed? 🍓

Here's the full error lineup:

• MPI_SUCCESS — no error, it just worked, we love that; the MPI routine completed successfully ⭐
• MPI_ERR_BUFFER — invalid buffer pointer. Usually a null buffer where a null buffer is not valid, so like, don't do that
• MPI_ERR_COUNT — invalid count argument. Count arguments must be non-negative; a count of zero is often valid though, so zero isn't always the villain here
• MPI_ERR_TYPE — invalid datatype argument. This can ALSO happen if you use an uncommitted MPI_Datatype (see MPI_Type_commit) in a communication call. Commit your types bestie, that's the whole point 💅
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more info about this error code

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ok that's the whole page, you're officially educated, go read some files with explicit offsets 👑

This has been your girl, reporting live from the MPI manual, dated 4/10/2026 — same page, same MPI_File_read_at_all_begin(3) energy. Byeeeee! 💖✨