MPI_Pack_external_size(3)

💖 MPI_Pack_external_size(3) 💖

okay bestie, sit down, hydrate, and let me tell you about the little function that's basically your pre-flight ✨vibe check✨ before you go packing anything external. This is part of the MPI manual, and yes it's giving "responsible friend who measures twice so you cut once." Periodt.

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🌸 NAME 🌸

MPI_Pack_external_size — returns the upper bound on the amount of space needed to pack a message using MPI_Pack_external. 🎀

Like... it doesn't pack anything itself, bestie. It just tells you how big your buffer needs to be so you don't look silly later. It's the calculator, not the packer. 💅

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✨ SYNOPSIS ✨

int MPI_Pack_external_size(const char *datarep, int incount,
MPI_Datatype datatype, MPI_Aint *size)

int MPI_Pack_external_size_c(const char *datarep, MPI_Count incount,
MPI_Datatype datatype, MPI_Count *size)

two flavors, same energy 🫶 — the classic one and the _c one with the big MPI_Count flex.

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🦋 INPUT PARAMETERS 🦋

• datarep — the data representation, and it's a string bestie, not an integer, keep up 💅
• incount — how many input data items you're bringing to the function (an integer)
• datatype — the datatype of each one of those input data items (a handle)

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🎀 OUTPUT PARAMETERS 🎀

• size — the output buffer size, in bytes, as an integer 🍓

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🔥 THREAD AND INTERRUPT SAFETY 🔥

ngl this part is lowkey iconic: this routine is thread-safe ✨. Which means, and I quote, it "may be safely used by multiple threads without the need for any user-provided thread locks." so you and your little thread besties can all hit it at once, no babysitting required 🫶

However. And this is a big however. It is not interrupt safe. 😭

Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. So like... thread-safe yes, interrupt-safe no. Read the room. 💀

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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 are invoked with the call statement. (Yes, call. Fortran said "I do things my way" and honestly, respect. 👑)

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

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😭 ERRORS 😭

Okay so here's the tea: 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 that last argument we just talked about. Before the value is returned, the current MPI error handler is called. ☎️

By default, this error handler aborts the MPI job. Yep. Straight up. 💀 The error handler can 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 — like, it still shows up to the party but everyone knows it shouldn't. 🙈

The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. 🫶

And here's your cast of characters, the error codes themselves:

• MPI_SUCCESS — no error; the MPI routine completed successfully, she ate and left no crumbs 💅
• 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_COUNT — invalid count argument. Count arguments must be non-negative; a count of zero is often valid. (Zero is fine bestie, negative is not. 😤)
• MPI_ERR_TYPE — invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used in a communication call. commit your types, I'm literally begging 🧚
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the mysterious one. the enigma. ask MPI_Error_string for the lore 🔮

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no cap: MPI_Pack_external_size gives you the upper bound, not the exact size. It's an estimate that's guaranteed to be enough. Don't come for it when your buffer is bigger than needed — that's literally the job description. 💖

stay packed and stay slay. 🌸✨

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signed off from the MPI manual, dated 4/10/2026, page MPI_Pack_external_size(3) — mwah, take care of your buffers bestie 🫶💅