MPI_TYPE_LB(3)

👑 NAME 👑

MPI_Type_lb — returns the lower bound of a data type 🎀 and bestie, I need you to hear me when I say this routine is deprecated in favor of MPI_Type_get_extent.

okay so real talk: this is the Open MPI manual, section 3, and it's the whole lore on MPI_Type_lb. grab a boba, we're getting into datatype extents 💅

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✨ SYNTAX ✨

🧚 C Syntax

#include <mpi.h>

int MPI_Type_lb(MPI_Datatype datatype, MPI_Aint *displacement)

🧚 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_TYPE_LB(DATATYPE, DISPLACEMENT, IERROR)
     INTEGER DATATYPE, DISPLACEMENT, IERROR

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🍓 INPUT PARAMETER 🍓

• datatype — the datatype handle, your little ticket stub for the type you're asking about 🎫

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💖 OUTPUT PARAMETERS 💖

• displacement — the displacement of the lower bound from origin, in bytes (an integer, no decimals no games)
• ierror — Fortran only: error status (integer). C girlies get their error as the return value instead, iykyk

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🦋 DESCRIPTION 🦋

Heads up bestie: use of this routine is deprecated as of MPI-2. Please use MPI_Type_get_extent instead. fr fr, she's the main character now 👑

So what does MPI_Type_lb actually do? It returns the lower bound of a data type. And plot twist — that lower bound might not be zero, if the type was constructed using MPI_LB. Not me being surprised by my own datatypes 🙈

The "pseudo-datatypes" MPI_LB and MPI_UB can be used, respectively, to mark the lower bound (or the upper bound) of a datatype. These pseudo-datatypes occupy no space — that is, extent (MPI_LB) = extent (MPI_UB) =0. They don't affect the size or count of a datatype, and they do not affect the context of a message created with this datatype. However — and this is the part that matters — they do affect the definition of the extent of a datatype, and therefore affect the outcome of a replication of this datatype by a datatype constructor. so they're quiet but they're ✨influential✨

In general, if:

Typemap = {(type0, disp0), ..., (type(n-1), disp(n-1)}

then the lower bound of Typemap is defined to be:

                        (min(j) disp(j)                          if no entry has
          lb(Typemap) = (                                        basic type lb
                        (min(j) {disp(j) such that type(j) = lb} otherwise

Similarly, the upper bound of Typemap is defined to be:

                            (max(j) disp(j) + sizeof((type(j)) + e   if no entry has
              ub(Typemap) = (                                        basic type ub
                            (max(j) {disp(j) such that type(j) = ub} otherwise

Then:

              extent(Typemap) = ub(Typemap) - lb(Typemap)

And if type(i) requires alignment to a byte address that is a multiple of k(i), then e is the least nonnegative increment needed to round extent(Typemap) to the next multiple of max(i) k(i). math girlies stay winning 🧮💅

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🌈 ERRORS 🌈

Okay so, error tea ☕ — almost all MPI routines return an error value. C routines hand it back as the return result of the function, and Fortran routines tuck it into the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (like a communicator, window, or file) gets called. If no communication object is associated with the MPI call, then the call counts as attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized — that's 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. That initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or by the mpi_initial_errhandler CLI argument to mpiexec, or by an 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, 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 🫶

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, no promises 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_Type_get_extent

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📜 COPYRIGHT 📜

2003-2026, The Open MPI Community 💜

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okay that's the whole page bestieee 🌸 this was the Open MPI MPI_TYPE_LB(3) man page, dated Sep 10, 2026 — go forth and measure those lower bounds, you absolute icon 🎀✨🦋