MPIL_Type_id(3)

💖 MPIL_Type_id — bestie edition 💖

okay so you're reading the LAM/MPI manual, page MPIL_Type_id(3), and honestly? slay. this is a LAM/MPI-specific function that hands you back the type ID of an MPI datatype. it's giving "peek behind the curtain" energy, and i'm obsessed. 🫶

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✨ NAME ✨

MPIL_Type_id — a LAM/MPI-specific function that returns the type ID of an MPI datatype. no cap, that's literally it. 🎀

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✨ SYNOPSIS ✨

#include <mpi.h>
int MPIL_Type_id(MPI_Datatype dtype, int *ptid)

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✨ INPUT PARAMETER ✨

• dtype — the MPI datatype, as a handle. that's your in, bestie. 💅

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✨ OUTPUT PARAMETER ✨

• ptid — the datatype ID, as an integer. this is what you came for. ✨

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✨ NOTES ✨

so here's the tea, and it's actually kind of iconic. ☕

in the LAM implementation of MPI, every communicator has an integer context ID attached to it, and that's what it uses to synchronize across different contexts. this ID is global to all the processes in the communicator's group, and it uniquely identifies that communicator for each process. those two properties together are what let processes safely exchange messages without other communicators' operations barging in and causing chaos. love that for them. 💖

the MPI standard, though? it gives you no way to access or view this implementation-dependent synchronization, because communicators are opaque objects. and honestly for normal MPI operations you don't need that access — it's fine, it's fine. BUT. 🚨 when you're debugging MPI applications, that whole opaque-object situation lowkey gets in your way. this is especially true on fully observable systems like LAM, where you can monitor the full state of the processes and message queues — which, yes, includes the context ID (go look at mpitask (1) and mpimsg (1), bestie, they're right there 👀).

heads up: this is a LAM/MPI-specific function and it's intended mainly for debugging. if you're gonna use it, use it together with the LAM_MPI C preprocessor macro, like so:

#if LAM_MPI
int tid
MPIL_Type_id(MPI_INT, &tid);
#endif

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✨ NOTES FOR FORTRAN ✨

bestie, fortran says "i have notes." 🌸

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

and all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. that's just how it is. 🦋

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✨ ERRORS ✨

okay so if an error happens in an MPI function, the current MPI error handler gets called to deal with it. by default, that error handler aborts the MPI job. 😭 you can swap the error handler with MPI_Errhandler_set; the predefined error handler MPI_ERRORS_RETURN can be used to make error values actually get returned (in C and Fortran — though ngl this error handler is less useful with the C++ MPI bindings. if you're in C++ and you need to recover the error value, use the predefined error handler MPI::ERRORS_THROW_EXCEPTIONS). and note that MPI does not guarantee that an MPI program can continue past an error. so like, be careful. 💀

all MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines return it as the value of the function, and Fortran routines put it in the last argument. the C++ bindings for MPI do not return error values — instead, error values get communicated by throwing exceptions of type MPI::Exception (but not by default, so don't just assume). exceptions are only thrown if the error value is not MPI::SUCCESS.

also, note that if the MPI::ERRORS_RETURN handler is set in C++, then while MPI functions will return upon an error, there will be no way to recover what the actual error value was. tragic. 🙈

• MPI_SUCCESS — no error; the MPI routine completed successfully. we love a clean run. 💖
• MPI_ERR_TYPE — invalid datatype argument. it may be an uncommitted MPI_Datatype (see MPI_Type_commit).
• MPI_ERR_ARG — invalid argument. some argument is invalid and it isn't identified by a specific error class. this is typically a NULL pointer or other such error.

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✨ SEE ALSO ✨

MPIL_Comm_gps, MPIL_Comm_id — go read those next, bestie, they're the sibling vibes. 🦋

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✨ LOCATION ✨

mpil_id.c

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okay that's the whole page, and if you made it this far you're genuinely the main character. 👑

LAM/MPI 7.1.4 · 6/24/2006 — yeah, this page is old enough to have its own vintage aesthetic and we respect it. mwah, love you, bye! 💅✨🎀