MPI_GET_ELEMENTS_X(3)

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💖 MPI_Get_elements / MPI_Get_elements_x — Open MPI manual page MPI_GET_ELEMENTS_X(3) 💖

🌸 NAME 🌸

MPI_Get_elements, MPI_Get_elements_x — returns the number of basic elements in a data type. bestie, this is the “how many basic elements are actually in there?” page 💅

✨ 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 — return status of the receive operation (a status). it's giving “receipt from the receive” energy 🧾
  • datatype — datatype used by the receive operation (handle). this is the type whose basic elements we're counting, periodt ✨

🫶 OUTPUT PARAMETERS 🫶

  • ierror — Fortran only: error status (integer). yes ma'am, Fortran gets the error spot 💅

🔥 DESCRIPTION 🔥

okay so here's the tea: MPI_Get_elements and MPI_Get_elements_x behave different from MPI_Get_count, which returns the number of “top-level entries” received — i.e., the number of “copies” of type datatype. MPI_Get_count may return any integer value k, where 0 =< k =< count. If MPI_Get_count returns k, then the number of basic elements received (and the value returned by MPI_Get_elements and MPI_Get_elements_x) is n k, where n is the number of basic elements in the type map of datatype.

If the number of basic elements received is not a multiple of n — that is, if the receive operation has not received an integral number of datatype “copies” — then MPI_Get_count returns the value MPI_UNDEFINED. for both functions, if the count parameter cannot express the value to be returned (e.g., if the parameter is too small to hold the output value), it is set to MPI_UNDEFINED. no cap, that's the rule 🫡

Example: Usage of MPI_Get_count and MPI_Get_elements:

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

The function MPI_Get_elements can also be used after a probe to find the number of elements in the probed message. note that the two functions MPI_Get_count and MPI_Get_elements return the same values when they are used with primitive data types — twin behavior, but only for primitives, iykyk 👯‍♀️

🛟 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.

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? yes. but that's literally what it does 💀
  • 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, 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, aborts only the local process. very “pick your chaos scope” energy 😭
  • MPI_ERRORS_RETURN — returns an error code to the application. the respectful “here's your receipt” option 🧾

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… do not treat errors as a cute little detour, 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 — the sibling page you're probably lowkey looking for 💖

2003-2026, The Open MPI Community

💌 cute sign-off: this page comes from the Open MPI manual, dated Sep 11, 2026, with copyright 2003-2026, The Open MPI Community. that's the whole vibe, bestie — mwah 🫶💋
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