MPI_GET_ELEMENTS(3)

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version 5.0.11 · 17 versions
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💅 MPI_Get_elements / MPI_Get_elements_x — count the actual basic elements, bestie

Okay so this is MPI_GET_ELEMENTS(3) from the Open MPI fam, and it's genuinely one of those little functions that saves your whole life when datatypes get weird. It returns the number of basic elements in a data type. Two flavors: the classic one and the _x one for when your counts are so big they need their own zip code. 🌈

✨ SYNTAX ✨

C Syntax 🎀

#include <mpi.h>

int MPI_Get_elements(const MPI_Status *status, MPI_Datatype datatype,
    int *count)

int MPI_Get_elements_x(const MPI_Status *status, MPI_Datatype datatype,
    MPI_Count *count)

Fortran Syntax 🦋

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

MPI_GET_ELEMENTS(STATUS, DATATYPE, COUNT, IERROR)
    INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR

MPI_GET_ELEMENTS_X(STATUS, DATATYPE, COUNT, IERROR)
    INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE
        INTEGER(KIND=MPI_COUNT_KIND) COUNT
        INTEGER IERROR

Fortran 2008 Syntax 💖

USE mpi_f08

MPI_Get_elements(status, datatype, count, ierror)
    TYPE(MPI_Status), INTENT(IN) :: status
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    INTEGER, INTENT(OUT) :: count
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Get_elements_x(status, datatype, count, ierror)
    TYPE(MPI_Status), INTENT(IN) :: status
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    INTEGER(KIND = MPI_COUNT_KIND), INTENT(OUT) :: count
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🌸 INPUT PARAMETERS 🌸

  • status — the return status of a receive operation, aka the receipt from your last RECV (a status). Keep it, you'll need it. 🧾
  • datatype — the datatype that your receive operation used (a handle). Yes, the same one. Don't switch it up on me. 👀

🫶 OUTPUT PARAMETERS 🫶

  • ierror — Fortran girly only: the error status (integer).

👑 DESCRIPTION 👑

Okay so here's the tea, and it's a big one: MPI_Get_elements and MPI_Get_elements_x do not behave like MPI_Get_count. Not even a little. MPI_Get_count gives you the number of "top-level entries" received — think of it as how many copies of datatype showed up. MPI_Get_count can hand you any integer value k where 0 =< k =< count. But if MPI_Get_count returns k, then the number of basic elements received — and that's literally the value MPI_Get_elements and MPI_Get_elements_x give back — is n k, where n is the number of basic elements in the type map of datatype. Math! Iconic. 🔥

But wait, it gets spicier: if the number of basic elements received is not a multiple of n — like, if your receive operation didn't get a nice clean integral number of datatype "copies" — then MPI_Get_count returns the value MPI_UNDEFINED. Yeah. It just gives up. 💀

And for both functions: if the count parameter can't express the value that needs to be returned (say, the parameter is too small to hold the output value), it gets set to MPI_UNDEFINED too. Rude but fair.

🍓 Example: how MPI_Get_count and MPI_Get_elements act in the wild

call MPI_TYPE_CONTIGUOUS(2, MPI_REAL, Type2, ierr)
call MPI_TYPE_COMMIT(Type2, ierr)
call MPI_COMM_RANK(comm, rank, ierr)

IF ( rank == 0 ) THEN
   CALL MPI_SEND(a, 2, MPI_REAL, 1, 0, comm, ierr)
   CALL MPI_SEND(a, 3, MPI_REAL, 1, 0, comm, ierr)
ELSE
   CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr)
   CALL MPI_GET_COUNT(stat, Type2, i, ierr) ! returns i=1
   CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr) ! returns i=2
   CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr)
   CALL MPI_GET_COUNT(stat, Type2, i, ierr) ! returns i=MPI_UNDEFINED
   CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr)  ! returns i=3
END IF

Look at that second round — MPI_GET_COUNT straight up says MPI_UNDEFINED and MPI_GET_ELEMENTS comes through with i=3. That's the whole vibe. 😭

Also good to know: MPI_Get_elements can be used after a probe too, to find the number of elements in the probed message. And ngl, MPI_Get_count and MPI_Get_elements return the same values when you use them with primitive data types — so if you're only doing basic types, they're giving twin energy. 👯

💀 ERRORS 💀

Almost all MPI routines return an error value — C routines as the return result of the function, and Fortran routines stick it in the last argument. Fr fr, always check it.

Before that error value comes back, the current MPI error handler associated with the communication object (e.g., 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 it'll 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. That initial error handler can be changed 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 the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. Phew. 🫠

If no other appropriate error handler has been set, then MPI_ERRORS_RETURN gets called for MPI I/O functions, and MPI_ERRORS_ABORT gets called for all other MPI functions.

Open MPI ships three predefined error handlers you can use, and they're honestly a whole mood:

  • MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. Nuclear option. 💥
  • 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. Situational awareness, love to see it. 👑
  • MPI_ERRORS_RETURN — returns an error code to the application. Chillest of the three. 🧚

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. So like. Don't count on bouncing back, 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_Get_count

2003-2026, The Open MPI Community

Okay that's the whole page, mwah 💋 — this was MPI_GET_ELEMENTS(3) from the Open MPI manual, dated Sep 11, 2026. Go forth and count your elements correctly, queen. 🦋✨

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