MPI_TYPE_SIZE_X(3)

🎀 MPI_TYPE_SIZE_X — okay bestie, let's talk bytes ✨

Hiii so this page covers two icons: MPI_Type_size and MPI_Type_size_x — both living in the Open MPI manual, section 3 (that's the library-calls girlies 💅). Lowkey the whole job of these two is: "returns the number of bytes occupied by entries in a data type." That's it. That's the gig. 🫶

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Type_size(MPI_Datatype datatype, int *size)
int MPI_Type_size_x(MPI_Datatype datatype, MPI_Count *size)

🦋 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_TYPE_SIZE(DATATYPE, SIZE, IERROR)
     INTEGER DATATYPE, SIZE, IERROR
MPI_TYPE_SIZE_X(DATATYPE, SIZE, IERROR)
     INTEGER DATATYPE
        INTEGER(KIND=MPI_COUNT_KIND) SIZE
        INTEGER IERROR

💜 Fortran 2008 Syntax

USE mpi_f08
MPI_Type_size(datatype, size, ierror)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Type_size_x(datatype, size, ierror)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER(KIND=MPI_COUNT_KIND), INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💖 INPUT PARAMETER 💖

• datatype — the datatype handle, bestie. That's the thing you're measuring. 🔍

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🌈 OUTPUT PARAMETERS 🌈

• size — the datatype's size, as an integer. This is the number we came for. 💯
• ierror — Fortran only, bestie: the error status (an integer). 🧚

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👑 DESCRIPTION 👑

So <pink>MPI_Type_size</pink> returns the total size, in bytes, of the entries in the type signature associated with datatype — like, the total size of the data in a message that would be created with this datatype. Main character energy, truly.

And the way that multiplicity works? Entries that show up multiple times in the datatype are counted with their multiplicity. So no, you don't get to pretend the repeats aren't there. They count. Periodt. ✨

For either function — yes, either one — if the size parameter cannot express the value to be returned (e.g., if the parameter is too small to hold the output value), it is set to MPI_UNDEFINED. 🫠 Not a vibe, but forewarned is forearmed, iykyk.

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💀 ERRORS 💀

Okay so, almost all MPI routines return an error value, right? For C routines it comes back as the return result of the function, and for Fortran routines it's in the last argument. Noted. 📝

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. And 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. And bestie, you can change that initial error handler:

• by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or
• via the mpi_initial_errhandler CLI argument to mpiexec, or
• via the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. 🌟

And 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. No cap.

Open MPI ships three predefined error handlers you can use:

• 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. 🎯
• MPI_ERRORS_RETURN — returns an error code to the application. Chill of it honestly. 🍓

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. That's a real one, bestie. Don't assume it'll just walk it off. 😭

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. 🧠

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🌟 Copyright 🌟

2003-2026, The Open MPI Community

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And that's a wrap, my love 💖 this page came straight out of Open MPI and it's dated Sep 10, 2026 — go forth and measure ur datatypes, ur doing amazing, slay 🎀🦋💅