MPI_TYPE_SIZE(3)

💅 MPI_TYPE_SIZE(3) — Open MPI

okay so bestie, welcome to the glow-up of MPI_TYPE_SIZE(3), one of the Open MPI manual pages ✨. this one's short and sweet and honestly? kind of iconic. let's get into it 🎀

🌸 NAME 🌸

MPI_Type_size, MPI_Type_size_x — returns the number of bytes occupied by entries in a data type. that's it. that's the whole vibe check. 💖

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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). that's your lil handle bestie, the thing you're asking about 💅

👑 OUTPUT PARAMETERS 👑

• size — the datatype size (an integer). the answer!! the moment we've all been waiting for ⭐
• ierror — Fortran only: error status (integer). classic Fortran, always gotta have the ierror at the end fr fr 🙈

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🔥 DESCRIPTION 🔥

okay so here's the tea ☕: MPI_Type_size 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 actually be created with this datatype. so if you were wondering "how much stuff is this thing gonna send," this is the function that tells you 💀✨

and here's the important part that people lowkey forget: entries that occur multiple times in the datatype are counted with their multiplicity. so like, no skipping duplicates, every instance counts, periodt 👑

for either function — and yes that means both MPI_Type_size and MPI_Type_size_x — if the size parameter cannot express the value to be returned (e.g., if the parameter is too small to hold the output value), then it gets set to MPI_UNDEFINED. not a value, not an error, just ✨ undefined ✨. so like, heads up on that 💖

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💀 ERRORS 💀

almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines stick it in the last argument. standard behaviour, no surprises there 🫶

before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) gets called. if there's no communication object associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. and 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. 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. yes that's a lot, no we're not skipping it, iykyk 😭

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

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. like, everyone. gone. 💀
• 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 matters bestie 💅
• MPI_ERRORS_RETURN — returns an error code to the application. the chill one 🌈

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, it might just be over. main character energy with a tragic ending 😭

see the MPI man page for a full list of MPI error codes, and 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 the whole page bestie!! slay 💅✨ this was MPI_TYPE_SIZE(3) from Open MPI, dated Sep 11, 2026 — go forth and count your bytes 🦋💖