MPI_Unpack(3)

💖 MPI_Unpack(3) 💖

okay bestie, welcome to the MPI_Unpack page — this is a manual from the MPI collection, and it's literally about taking a packed-up buffer and unpacking it back into contiguous memory. it's giving "unboxing video but for bytes". let's get into it 🎀

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✨ NAME ✨

MPI_Unpack — Unpack a buffer according to a datatype into contiguous memory

like, you have a squished-together blob of bytes (that's the packed buffer), and this routine un-squishes it into nice normal contiguous memory using a datatype as the instruction manual. no cap. 🦋

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🌸 SYNOPSIS 🌸

int MPI_Unpack(const void *inbuf, int insize, int *position, void *outbuf,
int outcount, MPI_Datatype datatype, MPI_Comm comm)

int MPI_Unpack_c(const void *inbuf, MPI_Count insize, MPI_Count *position,
void *outbuf, MPI_Count outcount, MPI_Datatype datatype,
MPI_Comm comm)

(yeah there's two of them — the _c one is the big-count twin, using MPI_Count instead of int for the sizes and counts 💅)

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👑 INPUT PARAMETERS 👑

• inbuf — input buffer start (choice) — this is where the packed-up stuff lives, the little blob we're about to un-blob
• insize — size of input buffer, in bytes (non-negative integer) — how big that blob is, measured in bytes, and it must not be negative fr fr
• outcount — number of items to be unpacked (integer) — how many items you want out of it
• datatype — datatype of each output data item (handle) — the blueprint for each thing you're unpacking
• comm — communicator for packed message (handle) — the communicator that the packed message belongs to

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🫶 INPUT/OUTPUT PARAMETERS 🫶

• position — current position in bytes (integer) — reads where you're at in the byte stream, and then it's the thing that moves forward as unpacking happens. she's an in-and-out girlie, main character energy honestly

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💜 OUTPUT PARAMETERS 💜

• outbuf — output buffer start (choice) — where the unpacked, now-contiguous data lands. this is the destination, the final form, the after photo 🌈

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🔥 THREAD AND INTERRUPT SAFETY 🔥

This routine is thread-safe. That means you can safely use it from multiple threads and you do not need to bring your own thread locks. no extra homework, we love that for us.

however — and this is the important part — the routine is not interrupt safe. Typically this is because it uses memory allocation routines like malloc, or other non-MPICH runtime routines, that are themselves not interrupt-safe. so like, don't yank the rug out from under it mid-call 💀

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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.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

(so yeah, Fortran besties get a bonus trailing argument. it's not a bug, it's a feature 🍓)

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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. 😳

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. 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.

here's the error code lineup, bestie:

• MPI_SUCCESS — No error; MPI routine completed successfully. 🎉
• 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). 🙈
• MPI_ERR_COUNT — Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
• 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. 👀
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code.

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⭐ SEE ALSO ⭐

• MPI_Pack
• MPI_Pack_size

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ngl if you're unpacking, you probably packed something first — go say hi to MPI_Pack, she's the one who put it all in there 💅

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and that's the whole page, bestie, no notes ✨ this one came from the MPI manual set, dated 4/10/2026, and it's filed under the name MPI_Unpack(3) — go unpack something iconic, periodt 💖🎀🦋