MPI_Pack(3)

💖 MPI_Pack(3) — the MPI manual, but make it a group chat 💖

okay bestie so this is MPI_Pack(3) from the MPI manual and it's literally the glow-up your scattered data has been waiting for 🫶

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✨ NAME ✨

MPI_Pack — packs a datatype into contiguous memory. it's giving "put all my little guys in one neat little bag" 💅

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✨ SYNOPSIS ✨

int MPI_Pack(const void *inbuf, int incount, MPI_Datatype datatype,
void *outbuf, int outsize, int *position, MPI_Comm comm)

int MPI_Pack_c(const void *inbuf, MPI_Count incount, MPI_Datatype datatype,
void *outbuf, MPI_Count outsize, MPI_Count *position,
MPI_Comm comm)

yes there are two of them and yes the _c one is the big boy version with MPI_Count instead of int. both are iconic, no competition 💜

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✨ INPUT PARAMETERS ✨

• inbuf — input buffer start (choice). this is where your lil data items are living rn 🌸
• incount — number of input data items (non-negative integer). count them. all of them. zero is fine too, no judgment 🎀
• datatype — datatype of each input data item (handle). handle, so like, one of those MPI object things, not a vibe 🦋
• outsize — output buffer size, in bytes (non-negative integer). bytes bestie, BYTES 📏
• comm — communicator for packed message (handle). also a handle. we love a handle 💖

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✨ INPUT/OUTPUT PARAMETERS ✨

• position — current position in buffer, in bytes (integer). it goes in and it comes out changed. main character energy fr fr 👑

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✨ OUTPUT PARAMETERS ✨

• outbuf — output buffer start (choice). this is where the packed lil bundle ends up ✨

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✨ NOTES (FROM THE SPECIFICATIONS) ✨ _{yes the big official ones}

okay so here's the tea ☕ — the input value of position is the first location in the output buffer to be used for packing. then position is incremented by the size of the packed message, and the output value of position is the first location in the output buffer following the locations occupied by the packed message. so it literally walks forward as it packs. obsessed with that for it 😭💖

and the comm argument is the communicator that will be subsequently used for sending the packed message. so like, keep that in mind — it's not just decoration, it's the future 📮

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✨ THREAD AND INTERRUPT SAFETY ✨

slay: this routine is thread-safe 💅 which means it may be safely used by multiple threads without the need for any user-provided thread locks. no locks, no stress, no cap 🔥

but bestie, lowkey listen: 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, not its fault, it's the friends it hangs out with 🫶

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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. in Fortran, MPI routines are subroutines, and they are invoked with the call statement. yes, call. that's just how they do it over there 💜

also all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. every single one. it's giving vintage 🌈

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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 job, gone. dramatic but that's the default.

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 🚫 (please 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 okay 😤

here's the error code roster 🎀:

• MPI_SUCCESS — no error; MPI routine completed successfully. the good ending 💖
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). it's giving mystery meat 🙈
• MPI_ERR_COMM — invalid communicator. a common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank). not even. NOT EVEN 😭
• MPI_ERR_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. zero is safe, negative is not ✋
• 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 your types bestie, they're not gonna commit to you first 💅
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the "we'll figure it out later" bucket 🍓

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👑 SEE ALSO 👑

MPI_Type_commit, MPI_Error_string, MPI_Comm_set_errhandler, MPI_File_set_errhandler, MPI_Win_set_errhandler, MPI_Errhandler_set, and honestly just go read the whole MPI manual, it's a journey 🦋

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that's the whole page bestie, no crumbs left 💖

reminder: MPI_Errhandler_set is deprecated, MPI_Pack is not interrupt safe, and the default error handler will abort your entire MPI job. choose your handlers wisely ✨

and that's a wrap on MPI_Pack(3) from the MPI manual, dated 4/10/2026 — mwah, periodt 💅🎀