MPI_PACK_SIZE(3)

💖 MPI_PACK_SIZE 💖

okay so bestie, welcome to the MPI_PACK_SIZE(3) page 🎀 — this one lives in the Open MPI manual, and it's literally here to answer one iconic question: how much room do I need for this packed message, fr fr? Spoiler: it gives you an upper bound, not an exact number, and honestly? that's the whole vibe. Let's get into it 🦋

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✨ NAME ✨

MPI_Pack_size — Returns the upper bound on the amount of space needed to pack a message. 🎁

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📜 SYNTAX 📜

C Syntax

#include <mpi.h>

int MPI_Pack_size(int incount, MPI_Datatype datatype, MPI_Comm comm,
     int *size)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PACK_SIZE(INCOUNT, DATATYPE, COMM, SIZE, IERROR)
     INTEGER INCOUNT, DATATYPE, COMM, SIZE, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Pack_size(incount, datatype, comm, size, ierror)
     INTEGER, INTENT(IN) :: incount
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🌸 INPUT PARAMETERS 🌸

these are the things you hand over, bestie, and they gotta match what you'd pass to the packing call:

• incount — Count argument to packing call (integer). 🔢
• datatype — Datatype argument to packing call (handle). 🏷️
• comm — Communicator argument to packing call (handle). 📡

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🎀 OUTPUT PARAMETERS 🎀

what you get back, and yes it's giving "receipts":

• size — Upper bound on size of packed message, in bytes (integer). 📏
• ierror — Fortran only: Error status (integer). 🚨

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💜 DESCRIPTION 💜

So MPI_Pack_size lets your application find out how much space is needed to pack a message. A call to MPI_Pack_size(incount, datatype, comm, size) returns in size an upper bound on the increment in position that would occur in a call to MPI_Pack, with the same values for incount, datatype, and comm. ✨

Rationale: the call returns an upper bound, rather than an exact bound, since the exact amount of space needed to pack the message may depend on the context (e.g., the first message packed in a packing unit may take more space). 🫶 Like, it's not being vague on purpose, that's just genuinely how it works — no cap.

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💀 ERRORS 💀

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument. 😭

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used: 🫧

• MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. 💥
• MPI_ERRORS_ABORT — An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process. 🛑
• MPI_ERRORS_RETURN — Returns an error code to the application. 📨

MPI applications can also implement their own error handlers by calling: 🧚

• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler

⚠️ Note that MPI does not guarantee that an MPI program can continue past an error. Like, that's a real caveat, bestie — keep it in mind. 🙈

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also: 🌈

• MPI_Pack
• MPI_Unpack

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👑 Copyright 👑

2003-2026, The Open MPI Community

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aaaand that's the whole page, bestie, you ate it 💅✨ This lil' cutie is brought to you by Open MPI, dated Sep 10, 2026, and it signs off as nobody other than MPI_PACK_SIZE(3) — go pack something iconic, ily, byeee 🦋💖