MPI_Get_count(3)

✨ NAME ✨

MPI_Get_count — gets the number of "top level" elements 🧮✨

like… it counts ur top level elements fr. that's the whole job. she's an icon, she's a legend, and she's in the MPI manual 💅

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🌸 SYNOPSIS 🌸

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

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

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💜 INPUT PARAMETERS 💜

• status — the return status of a receive operation (type: Status). it's the receipts from ur recv, bestie 🧾
• datatype — the datatype of each receive buffer entry (a handle). yes, each one. she's specific like that.

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🎀 OUTPUT PARAMETERS 🎀

• count — number of received entries (an integer). the number!! the moment we've all been waiting for 🫶

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🦋 NOTES 🦋

okay so two little plot twists u need to know about:

• if the size of the datatype is zero, this routine will return a count of zero. zero datatype = zero count, 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. like sorry bestie, no clean answer for u 🙈

ngl MPI_UNDEFINED is giving "it's not a number and we're not pretending otherwise" 💀

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🔥 THREAD AND INTERRUPT SAFETY 🔥

this routine is thread-safe 💪 that means it may be safely used by multiple threads without needing any user-provided thread locks. no lock drama, we love to see it.

however — the routine is not interrupt safe 😭 typically this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

so like… threads? slay. interrupts? not slay. periodt.

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🌈 NOTES FOR FORTRAN 🌈

all MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. yes, she's bilingual 💅

in Fortran, MPI routines are subroutines, and they're invoked with the call statement. very retro, very chic.

all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. that's just how Fortran do 🧚

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👑 ERRORS 👑

all MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. before the value is returned, the current MPI error handler is called. by default, this error handler aborts the MPI job — like, whole thing, gone, no survivors 💀

the error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). the MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated (old news, don't do it bestie 🙈). the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead of just dying.

note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. so it's giving hopeful but not promised 🫠

here's the lineup of errors:

⭐ MPI_SUCCESS

no error; MPI routine completed successfully ✨ the dream. the goal. we ate.

⭐ MPI_ERR_ARG

invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). something's wrong but the error code won't tell u which thing 🫣

⭐ MPI_ERR_TYPE

invalid datatype argument. additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used in a communication call. commit ur types first, bestie, i'm begging 😭

⭐ MPI_ERR_OTHER

other error; use MPI_Error_string to get more information about this error code. the catch-all. the miscellaneous drawer of errors 🍓

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💖 SEE ALSO 💖

MPI_Type_commit, MPI_Comm_set_errhandler, MPI_File_set_errhandler, MPI_Win_set_errhandler, MPI_Errhandler_set, MPI_ERRORS_RETURN, MPI_Error_string, MPI_WTIME, MPI_WTICK, MPI_Wtime, MPI_Wtick — everything we yelled about above, all linked up 🫶

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okay that's the whole page, bestie!! mwah 💋 this was MPI_Get_count(3) from the MPI manny, dated 4/10/2026. go forth and count ur top level elements, u absolute main character 👑✨🌈