MPI_PACK_EXTERNAL_SIZE(3)

💅 MPI_PACK_EXTERNAL_SIZE 💅

okay so bestie, we're in the Open MPI manual, and today's main character is MPI_Pack_external_size — the little guy who tells you exactly how much room you're gonna need. no cap, it's giving "measure twice, pack once" energy 🎀

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✨ NAME ✨

MPI_Pack_external_size — literally calculates the upper bound on the space you need to write stuff out in a portable format 🦋

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🌸 SYNTAX 🌸

🩷 C Syntax

#include <mpi.h>

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

🩷 Fortran Syntax

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

     INTEGER         INCOUNT, DATATYPE, IERROR
     INTEGER(KIND=MPI_ADDRESS_KIND) SIZE
     CHARACTER*(*)   DATAREP

🩷 Fortran 2008 Syntax

USE mpi_f08
MPI_Pack_external_size(datarep, incount, datatype, size, ierror)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, INTENT(IN) :: incount
     CHARACTER(LEN=*), INTENT(IN) :: datarep
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💜 INPUT PARAMETERS 💜

• datarep — the data representation, and it's a string. yes bestie, a whole string about vibes and formats 🌈
• incount — how many input data items you're bringing to the party (an integer) 🔥
• datatype — the datatype of each one of those input data items (a handle) ✨

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🎀 OUTPUT PARAMETERS 🎀

• size — the upper bound on the size of the packed message, measured in bytes (an integer) 📏
• ierror — Fortran only, babes: the error status (an integer) 🙈

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🦋 DESCRIPTION 🦋

so MPI_Pack_external_size lets your application figure out how much space it needs to pack a message in the portable format defined by the MPI Forum. iconic behavior honestly 👑

it returns in size an upper bound on the increment in position that would happen in a call to MPI_Pack_external with the same values for datarep, incount, and datatype. matching set, we love to see it 💅

and why an upper bound instead of an exact bound? because the exact amount of space needed to pack the message may depend on context and alignment — like, the first message packed in a packing unit may take more space. context matters, always 💀

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⭐ NOTES ⭐

the datarep argument specifies the data format. the only valid value in the current version of MPI is external32 — that's the one, that's the vibe. the argument is provided for future extensibility, so it's basically dressed up for a party that hasn't happened yet 🧚

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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. know the difference, bestie 🫶

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. that's the whole flow, no notes ✨

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic? yes. effective? also yes 💀
• 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, it 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, it aborts only the local process. context-dependent queen 👑
• MPI_ERRORS_RETURN — returns an error code to the application. polite, respectful, hands you the receipt 🧾

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. so like… don't count on it, bestie 😭

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_external
• MPI_Unpack_external

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🌈 COPYRIGHT 🌈

2003-2026, The Open MPI Community

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and that's the whole page, periodt 💖 this was brought to you by the Open MPI girlies, dated Sep 11, 2026 — go pack something iconic, bestie, and remember: measure your upper bound first 🎀✨🦋