MPI_TYPE_INDEXED(3)

💅 MPI_TYPE_INDEXED(3) — like, the Open MPI glow-up edition ✨

okay so bestie, today we're learning about MPI_Type_indexed and MPI_Type_create_hindexed — the two functions that create an indexed datatype. yes that's the whole NAME section, and yes it's giving main character energy already 🎀

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Type_indexed(int count, const int array_of_blocklengths[],
     const int array_of_displacements[], MPI_Datatype oldtype,
     MPI_Datatype *newtype)

int MPI_Type_create_hindexed(int count,
     const int array_of_blocklengths[],
     const MPI_Aint array_of_displacements[], MPI_Datatype oldtype,
     MPI_Datatype *newtype)

🌸 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_TYPE_INDEXED(COUNT, ARRAY_OF_BLOCKLENGTHS,
             ARRAY_OF_DISPLACEMENTS, OLDTYPE, NEWTYPE, IERROR)
     INTEGER COUNT, ARRAY_OF_BLOCKLENGTHS(*)
     INTEGER ARRAY_OF_DISPLACEMENTS(*), OLDTYPE, NEWTYPE
     INTEGER IERROR

MPI_TYPE_CREATE_HINDEXED(COUNT, ARRAY_OF_BLOCKLENGTHS,
             ARRAY_OF_DISPLACEMENTS, OLDTYPE, NEWTYPE, IERROR)
     INTEGER COUNT, ARRAY_OF_BLOCKLENGTHS(*)
     INTEGER OLDTYPE, NEWTYPE
     INTEGER(KIND=MPI_ADDRESS_KIND) ARRAY_OF_DISPLACEMENTS(*)
     INTEGER IERROR

🌸 Fortran 2008 Syntax

USE mpi_f08
MPI_Type_indexed(count, array_of_blocklengths, array_of_displacements,
             oldtype, newtype, ierror)
     INTEGER, INTENT(IN) :: count, array_of_blocklengths(count),
     array_of_displacements(count)
     TYPE(MPI_Datatype), INTENT(IN) :: oldtype
     TYPE(MPI_Datatype), INTENT(OUT) :: newtype
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Type_create_hindexed(count, array_of_blocklengths,
             array_of_displacements, oldtype, newtype, ierror)
     INTEGER, INTENT(IN) :: count, array_of_blocklengths(count)
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) ::
     array_of_displacements(count)
     TYPE(MPI_Datatype), INTENT(IN) :: oldtype
     TYPE(MPI_Datatype), INTENT(OUT) :: newtype
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🫶 INPUT PARAMETERS 🫶

• count — the number of blocks 💫 and, like, also the number of entries in array_of_displacements and array_of_blocklengths. it's a nonnegative integer, no cap.
• array_of_blocklengths — how many elements live in each block (array of nonnegative integers). slay.
• array_of_displacements — the displacement for each block 🦋. for MPI_Type_indexed it's measured in multiples of oldtype extent; for MPI_Type_create_hindexed it's in bytes, fr fr. (array of integer for MPI_TYPE_INDEXED, array of MPI_Aint for MPI_TYPE_CREATE_HINDEXED.)
• oldtype — the old datatype (handle). the OG 💜

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👑 OUTPUT PARAMETERS 👑

• newtype — the new datatype (handle). the glow-up ✨
• ierror — Fortran only: error status (integer). classic.

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🌈 DESCRIPTION 🌈

okay so MPI_Type_indexed lets you replicate an old datatype into a sequence of blocks — each block is a concatenation of the old datatype — where each block can hold a different number of copies and sit at a different displacement. all block displacements are multiples of the old data type's extent. it's giving customizable, honestly.

Example time, bestie 💖 Let oldtype have type map {(double, 0), (char, 8)}, with extent 16. Let B = (3, 1) and let D = (4, 0). A call to MPI_Type_indexed(2, B, D, oldtype, newtype) returns a datatype with type map

{(double, 64), (char, 72), (double, 80), (char, 88),
(double, 96), (char, 104),
(double, 0), (char, 8)}

that is, three copies of the old type starting at displacement 4 x 16 = 64, and one copy starting at displacement 0. iconic behaviour, no notes 😭

in general, assume that oldtype has type map

{(type(0), disp(0)), ..., (type(n-1), disp(n-1))},

with extent ex. let B be the array_of_blocklength argument and D be the array_of_displacements argument. the newly created datatype has

n x S ^count-1
    i = 0           B[i]  entries:

    {(type(0), disp(0) + D[0]* ex), ...,
    (type(n-1), disp(n-1) + D[0]* ex), ...,
    (type(0), disp(0) + (D[0] + B[0]-1)* ex), ...,
    (type(n-1), disp(n-1) + (D[0]+ B[0]-1)* ex), ...,
    (type(0), disp(0) + D[count-1]* ex), ...,
    (type(n-1), disp(n-1) + D[count-1]* ex), ...,
    (type(0), disp(0) +  (D[count-1] + B[count-1] -1)* ex), ...,
    (type(n-1), disp(n-1) + (D[count-1] + B[count-1] -1)* ex)}

and here's a cute little equivalence for you 🧚 a call to MPI_Type_vector(count, blocklength, stride, oldtype, newtype) is equivalent to a call to MPI_Type_indexed(count, B, D, oldtype, newtype) where

D[j] = j * stride, j = 0,..., count-1

and

B[j] = blocklength, j = 0, .., count-1

now MPI_Type_create_hindexed is literally identical to MPI_Type_indexed, except that block displacements in array_of_displacements are specified in bytes, rather than in multiples of the oldtype extent. same vibes, different units 💅

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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. yes, both. 🙈

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. 🫠

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, but make it fatal 💀
• 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. subtle.
• MPI_ERRORS_RETURN — returns an error code to the application. chill of it 🌸

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, at all. 💀

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. iykyk 📚

🔗 SEE ALSO

• MPI_Type_hindexed

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🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community

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and that's the whole page bestie 🫶 mwah 💋 — written by the Open MPI girlies, dated Sep 11, 2026, on the MPI_TYPE_INDEXED(3) page under Open MPI. go forth and index your types ✨💅