MPI_Pack_size(3)

💖 MPI_Pack_size(3) — or, "how much room do I need for this snack" 💖

okay bestie, welcome to the MPI manual, page MPI_Pack_size(3). yes we're doing MPI lore today, grab a boba 🧋

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✨ NAME ✨

MPI_Pack_size — Returns the upper bound on the amount of space needed to pack a message

like. it tells u how big ur buffer needs to be. it's giving "measure twice, pack once" 📏

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🌸 SYNOPSIS 🌸

int MPI_Pack_size(int incount, MPI_Datatype datatype, MPI_Comm comm, int *size)

int MPI_Pack_size_c(MPI_Count incount, MPI_Datatype datatype, MPI_Comm comm,
MPI_Count *size)

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🎀 INPUT PARAMETERS 🎀

these are the little info morsels u gotta hand over:

• incount — the count argument to the packing call (non-negative integer, no negatives pls, that's not the vibe)
• datatype — the datatype argument to the packing call (a handle)
• comm — the communicator argument to the packing call (also a handle)

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🦋 OUTPUT PARAMETERS 🦋

• size — upper bound on the size of the packed message, in bytes (non-negative integer) 💅

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⭐ NOTES ⭐

so the MPI standard document describes this whole thing in terms of MPI_Pack, but lowkey it applies to both MPI_Pack and MPI_Unpack. which means that size value is the max that either MPI_Pack or MPI_Unpack could possibly need. one number, both jobs, iconic 👑

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🔥 THREAD AND INTERRUPT SAFETY 🔥

this routine is thread-safe, periodt. which means u can safely use it from multiple threads with zero user-provided thread locks. no locking arc for u 🫶

but bestie… it is not interrupt safe 😭 typically this is bc of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so like, don't get it twisted.

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🌈 NOTES FOR FORTRAN 🌈

all MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK, they're the exceptions, they always are) 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. and in Fortran, MPI routines are subroutines, invoked with the call statement. that's just the rules fr fr.

also, all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. yes, all of them. 💜

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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 that last argument. before the value is returned, the current MPI error handler is called. and by default, this error handler aborts the MPI job. like, goodbye, the whole thing is over 🚨

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 — so like, pls don't, it's giving 1994 🙈

the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. they're trying their best ok 😭

here's the error lineup:

• MPI_SUCCESS — no error; MPI routine completed successfully. we love to see it ✨
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).
• MPI_ERR_COMM — invalid communicator. a common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank). like, not even there. not allowed. nope.
• MPI_ERR_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. (zero ≠ invalid, iykyk)
• 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 bestie, it's a whole thing 💅
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the catch-all, the mystery box, the "idk go ask" 🎁

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okay that's the whole page!! no cap, u now know ur pack sizes, ur thread safety, and ur Fortran lore 🧚

sign-off time: this lil page is from the MPI manual, dated 4/10/2026, and it's the MPI_Pack_size(3) era. bye bestie, go pack something responsibly 🍓💜