MPI_Type_get_envelope(3)

✨ NAME ✨

MPI_Type_get_envelope — get type envelope 💅

okay so real talk, this is a page from the MPI manual (section 3, the library-calls girlies), and it's all about peeking inside a datatype and finding out what it's made of. no cap.

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🦋 SYNOPSIS 🦋

int MPI_Type_get_envelope(MPI_Datatype datatype, int *num_integers,
int *num_addresses, int *num_large_counts,
int *num_datatypes, int *combiner)

int MPI_Type_get_envelope_c(MPI_Datatype datatype, MPI_Count *num_integers,
MPI_Count *num_addresses,
MPI_Count *num_large_counts,
MPI_Count *num_datatypes, int *combiner)

like… the _c version is the same vibe but with MPI_Count output pointers so the numbers can be big big. slay.

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🌸 INPUT PARAMETERS 🌸

• datatype — the datatype to access (handle). this is the one you're snooping on, bestie 🔍

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💖 OUTPUT PARAMETERS 💖

• num_integers — number of input integers used in call constructing combiner (non-negative integer) 🔢
• num_addresses — number of input addresses used in call constructing combiner (non-negative integer) 🏠
• num_large_counts — number of input large counts used in call constructing combiner (non-negative integer) 🔢
• num_datatypes — number of input datatypes used in call constructing combiner (non-negative integer) 🧬
• combiner — combiner (state) 🎛️

no fr, every single one of those is a non-negative integer except combiner, which is the state. periodt.

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🧚 THREAD AND INTERRUPT SAFETY 🧚

this routine is thread-safe. what that means: multiple threads can use it at the same time and you do not need to bring your own thread locks. she's independent, she's low-maintenance, she's that girl. 💅

however — and this is the part you cannot skip — she is not interrupt safe. typically that's because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so… yeah. interrupt her at your own risk 🫠

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🌈 NOTES FOR FORTRAN 🌈

every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has an extra argument ierr at the very 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. iykyk.

also: in Fortran, MPI routines are subroutines, and you invoke them with the call statement. iconic.

and all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. yes, even the fancy ones.

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💀 ERRORS 💀

okay so all MPI routines (except MPI_Wtime and MPI_Wtick) return an error value — C routines hand it back as the value of the function, Fortran routines put it in that last argument. and before the value even comes back to you, the current MPI error handler gets called. by default, that error handler aborts the MPI job. 💀 not a drill.

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 🚫 — like, use it if you must but she's old news.

the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

heads up bestie: MPI does not guarantee that an MPI program can continue past an error. but implementations will attempt to continue whenever possible. so, like, fingers crossed 🤞

here's the error lineup:

• 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_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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and that's the whole page, bestie!! 🌟 this was page MPI_Type_get_envelope(3) from the MPI manual, dated 4/10/2026. go forth and get those envelopes. mwah 💋