MPI_TYPE_GET_TRUE_EXTENT(3)

💖 MPI_TYPE_GET_TRUE_EXTENT — bestie edition 💖

okay so this is a page from the Open MPI manual, section 3, and it's all about MPI_Type_get_true_extent and MPI_Type_get_true_extent_x. say it with me: true lower bound and true extent of a data type's corresponding typemap, ignoring MPI_UB and MPI_LB markers. that's the whole vibe. no cap. 🦋

---

✨ SYNTAX ✨

🌸 C Syntax 🌸

#include <mpi.h>

int MPI_Type_get_true_extent(MPI_Datatype datatype,
     MPI_Aint *true_lb, MPI_Aint *true_extent)
int MPI_Type_get_true_extent_x(MPI_Datatype datatype,
     MPI_Count *true_lb, MPI_Count *true_extent)

🌸 Fortran Syntax 🌸

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_TYPE_GET_TRUE_EXTENT(DATATYPE, TRUE_LB, TRUE_EXTENT, IERROR)
     INTEGER DATATYPE, IERROR
     INTEGER(KIND=MPI_ADDRESS_KIND) TRUE_LB, TRUE_EXTENT
MPI_TYPE_GET_TRUE_EXTENT_X(DATATYPE, TRUE_LB, TRUE_EXTENT, IERROR)
     INTEGER DATATYPE, IERROR
     INTEGER(KIND=MPI_COUNT_KIND) TRUE_LB, TRUE_EXTENT

🌸 Fortran 2008 Syntax 🌸

USE mpi_f08
MPI_Type_get_true_extent(datatype, true_lb, true_extent, ierror)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(OUT) :: true_lb, true_extent
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Type_get_true_extent_x(datatype, true_lb, true_extent, ierror)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER(KIND = MPI_COUNT_KIND), INTENT(OUT) :: true_lb, true_extent
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

---

🎀 INPUT PARAMETER 🎀

• datatype — the data type you want the tea on (handle). tell me everything bestie 💅

---

🫶 OUTPUT PARAMETERS 🫶

• true_lb — true lower bound of the data type (integer). ✨
• true_extent — true size of the data type (integer). ✨
• ierror — Fortran only: error status (integer). 🙈

---

🌈 DESCRIPTION 🌈

okay so first up, true_lb hands you the offset of the lowest unit of store that the data type actually addresses — that's the lower bound of the corresponding typemap, and it does not care about your MPI_LB markers. they simply do not exist to it. 💀

then true_extent gives you the true size of the data type, meaning the extent of the corresponding typemap, ignoring MPI_LB and MPI_UB markers, and performing no rounding for alignment. zero rounding. none. it's giving raw, unfiltered honesty. 🧚

for both functions: if either the true_lb or true_extent parameter literally cannot express the value that wants to be returned (like, the parameter is too small to hold the output value 😭), it gets set to MPI_UNDEFINED. that's the fallback, and it's iconic.

and the true_extent is the minimum number of bytes of memory necessary to hold a data type, uncompressed. minimum. we love a low-maintenance king. 👑

if you want the detailed definitions of these parameters in relation to the typemap, go read section 4.1.8 of the MPI-3 standard. iykyk. 📚

---

💀 ERRORS 💀

almost all MPI routines return an error value, bestie. C routines do it as the return result of the function, and Fortran routines do it in the last argument. main character split behavior, honestly. ✨

before that error value comes back to you, the current MPI error handler associated with the communication object (think: communicator, window, file) gets 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. and when MPI_COMM_SELF is not initialized (that means before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when you're 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 you're using the World model, or by the mpi_initial_errhandler CLI argument to mpiexec, or by the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. pick your fighter. 🌸

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 you can use:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. it's giving dramatic exit. 😭
• 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. very targeted, very slay. 🎯
• MPI_ERRORS_RETURN — returns an error code to the application. polite. respectful. 🫶

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

heads up bestie: note that MPI does not guarantee that an MPI program can continue past an error. not even a little. 🚨

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

and see the Error Handling section of the MPI-3.1 standard for more information. go read it, i believe in you. 📖

---

👑 Copyright 👑

2003-2026, The Open MPI Community

---

and that's the whole page, bb! this one's from the Open MPI manual, dated Sep 10, 2026, and it lives at MPI_TYPE_GET_TRUE_EXTENT(3). go get those true extents, bestie, you're gonna eat. 💖✨🎀🦋🌸💅🫶