MPI_GET_COUNT(3)

📦
version 5.0.10 · 7 versions
, in ubuntu stonking

💖 NAME 💖

MPI_Get_count — gets the number of top-level elements received. ✨

okay so bestie, this is from the Open MPI manual, section 3 (MPI_GET_COUNT(3), iykyk), and it's lowkey one of those functions you'll use CONSTANTLY once you start doing receives. Let's go 🫶

🌸 SYNTAX 🌸

💻 C Syntax 💻

#include <mpi.h>

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

🧚 Fortran Syntax 🧚

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

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

🦋 Fortran 2008 Syntax 🦋

USE mpi_f08

MPI_Get_count(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

🎀 INPUT PARAMETERS 🎀

  • status : return status of receive operation (status). 💅
  • datatype : datatype of each receive buffer element (handle). ✨

👑 OUTPUT PARAMETERS 👑

  • count : number of received elements (integer). ⭐
  • ierror : Fortran only: error status (integer). 🫶

🌈 DESCRIPTION 🌈

So here's the tea: this function returns the number of entries received. And not me being specific — we count entries, each one of type datatype, not bytes. Do not come for me with bytes, bestie, that is NOT the vibe. 🚫

The datatype argument should match the argument provided by the receive call that set the status variable. Like, they have to be the same datatype or the numbers won't make sense, periodt.

As explained in the "Use of General Datatypes in Communication" section of the MPI Standard (https://www.mpi-forum.org/docs/), MPI_Get_count may, in certain situations, return the value MPI_UNDEFINED. 😭 Yes, really. It's giving "the answer is undefined," and that's by design. 💀

Now, why does it even want the datatype argument? The datatype argument is passed to MPI_Get_count to improve performance. A message might be received without counting the number of elements it contains, and the count value is often not needed. Iconic efficiency behaviour honestly. Also, this allows the same function to be used after a call to MPI_Probe. 🔥

⭐ NOTES ⭐

  • If the size of the datatype is zero, this routine will return a count of zero. Yes, zero of zero, the math is mathing. 💅
  • If the amount of data in status is not an exact multiple of the size of datatype (so that count would not be integral), a count of MPI_UNDEFINED is returned instead. 🙈 bestie the count just... isn't a whole number, so it says nope.

💜 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, Fortran gets the little trailer argument at the end, we love her. 🎀)

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. 💀 (it's giving chaos, but it's documented chaos)
  • 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. 👑
  • MPI_ERRORS_RETURN — returns an error code to the application. 🫶 (the polite one, our fave)

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. ✨

If the value to be returned is larger than can fit into the count parameter, an MPI_ERR_TRUNCATE error is raised. 😭 (the number said "no thank you, I do not fit")

See also:

  • MPI_Get_elements

This page is © 2003-2026, The Open MPI Community, and the last time anybody touched this page was Sep 10, 2026.

so yeah bestie, that's MPI_Get_count(3) — go forth, count your elements, and don't forget your datatype argument or it's MPI_UNDEFINED o'clock for you. slay 💅✨🦋💖

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