MPI_GET_COUNT(3)

💅 MPI_GET_COUNT(3) 💅

ok so this is an Open MPI page, bestie — MPI_GET_COUNT — and it's literally about finding out how many things you just received. buckle up 🎀

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✨ NAME ✨

MPI_Get_count — gets the number of top-level elements received. that's it, that's the whole vibe 🫶

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🌸 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

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🦋 INPUT PARAMETERS 🦋

• status — the return status from your receive operation, yeah that one (status). it's giving receipts of what just happened 💅
• datatype — the datatype of each element in the receive buffer (handle). important, don't skip it ✨

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🌈 OUTPUT PARAMETERS 🌈

• count — the number of received elements (integer). slay 🎀
• ierror — Fortran only, babes: error status (integer).

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⭐ DESCRIPTION ⭐

so what it does: returns the number of entries received. and yes — we count entries, each one of type datatype, not bytes. no cap 📏

the datatype argument should match the argument you handed to the receive call that set the status variable. this is all explained in the “Use of General Datatypes in Communication” section of the MPI Standard, iykyk. and heads up — MPI_Get_count may, in certain situations, return the value MPI_UNDEFINED. 😭

now, why is datatype even passed in there? performance, bestie. a message might get received without counting how many elements it contains, and honestly the count value is often not even needed. plus this way the same function works after a call to MPI_Probe. iconic design tbh 👑

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🎀 NOTES 🎀

• if the size of the datatype is zero, this routine will return a count of zero. easy.
• if the amount of data in status is not an exact multiple of the size of datatype (so count wouldn't be integral), then a count of MPI_UNDEFINED is returned instead. so like… no fractional counts, we don't do that here 💀

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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. classic 💜

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. and 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. that's the whole flowchart, bestie 🧚

Open MPI includes three predefined error handlers you can use:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic but effective 💀
• 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. selective chaos, we love to see it ✨
• MPI_ERRORS_RETURN — returns an error code to the application. polite 🫶

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. fr fr, don't count on it 😭

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. 📚

and one last thing — if the value to be returned is larger than can fit into the count parameter, an MPI_ERR_TRUNCATE error is raised. big number, small box, we've all been there 🙈

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👑 SEE ALSO 👑

• MPI_Get_elements

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💜 COPYRIGHT 💜

2003-2026, The Open MPI Community

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and that's the tea, bestie 🍓 page dated Sep 11, 2026, brought to you by Open MPI — now go count your elements and slay ✨💅🎀