MPI_Unpack_external(3)

💖 MPI_Unpack_external(3) 💖

okay bestie, welcome to the MPI manual, and specifically to the page for MPI_Unpack_external. this one is the *un*packing bestie — it takes a buffer that was packed with MPI_Pack_external (using some datatype) and unpacks it into contiguous memory. it's giving reverse uno card, and it's iconic. ✨

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🌸 NAME 🌸

MPI_Unpack_external — Unpack a buffer (packed with MPI_Pack_external) according to a datatype into contiguous memory. no cap, that's literally the whole job. 🎀

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✨ SYNOPSIS ✨

int MPI_Unpack_external(const char datarep[], const void *inbuf,
MPI_Aint insize, MPI_Aint *position, void *outbuf,
int outcount, MPI_Datatype datatype)

int MPI_Unpack_external_c(const char datarep[], const void *inbuf,
MPI_Count insize, MPI_Count *position, void *outbuf,
MPI_Count outcount, MPI_Datatype datatype)

two flavors, periodt. the _c one is for when your counts and sizes are MPI_Count sized, the regular one is for the MPI_Aint/int girlies. 💅

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💜 INPUT PARAMETERS 💜

• datarep — the data representation, as a string. it's giving "how are we describing these bytes". 🌈
• inbuf — start of the input buffer (a choice type). your packed lil pile of bytes goes in here. 🍓
• insize — size of the input buffer, in bytes (integer). keep it honest. ⭐
• outcount — number of output data items (integer). how many things are we pulling out. 💖
• datatype — datatype of the output data item (a handle). this tells us the shape of what we're unpacking. 🦋

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🫶 INPUT/OUTPUT PARAMETERS 🫶

• position — current position in the buffer, in bytes (integer). it goes in as "where we're at" and comes back updated to where we ended up. main character energy, honestly. 👑

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🎀 OUTPUT PARAMETERS 🎀

• outbuf — start of the output buffer (a choice type). this is where the unpacked goodies land. 💅

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🧚 THREAD AND INTERRUPT SAFETY 🧚

this routine is thread-safe. that means the way that it can be safely used by multiple threads without you needing to bring your own user-provided thread locks? slay. ✨

but — and this is the important part — it is not interrupt safe. 😭 typically this is because of memory allocation routines like malloc, or other non-MPICH runtime routines that are themselves not interrupt-safe. so no interrupting, bestie. 💀

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🌺 NOTES FOR FORTRAN 🌺

all MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an extra argument ierr tacked onto the end of the argument list. ierr is an integer and it means the exact same thing as the return value of the routine in C. in Fortran, MPI routines are subroutines and you invoke them with the call statement. 🎀

all MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. yes, all of them. iconic behavior. 💖

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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 gets called. by default, this error handler aborts the MPI job. 😳

the error handler can 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 — do not be that girl. 🙈 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. 🫶

• MPI_SUCCESS — no error; the MPI routine completed successfully. we love a happy ending. ✨
• 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_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. ngl, zero is fine, 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!! 🔥
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. it's giving mystery, use the lookup. 🦋

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lowkey remember: this is the counterpart to MPI_Pack_external. whatever you packed, this unpacks. match your datarep and you're golden. 💖

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and that's the whole page, bestie!! this one came from the MPI manual, dated 4/10/2026, and it lives at MPI_Unpack_external(3). go unpack something iconic. 🎀✨💅