MPIL_Comm_id(3)

✨ MPIL_Comm_id — bestie's guide to peeking at your communicator's context ID ✨

okay so this is MPIL_Comm_id(3), a page from the LAM/MPI manual, section 3 (library calls, obviously 💅). It's a LAM-specific little function that hands you back the context ID of a communicator. Very niche, very demure, very much for debugging. Let's get into it 🫶

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🌸 NAME 🌸

MPIL_Comm_id — a LAM/MPI-specific function that returns the context ID of the communicator. That's it, that's the whole job. She's not complicated, she's just ✨specialized✨.

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🎀 SYNOPSIS 🎀

#include <mpi.h>
int
MPIL_Comm_id(MPI_Comm comm, int *pcid)

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💖 INPUT PARAMETER 💖

• comm — the communicator (handle). It's giving "which one, bestie?" energy.

💖 OUTPUT PARAMETER 💖

• pcid — the communicator's context id (an integer). She writes into your pointer, she does not return it. Noted. 💀

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🦋 NOTES 🦋

okay so here's the lore. In the LAM implementation of MPI, every communicator has an integer context ID attached to it, and that ID is what lets everything synchronize on 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? Iconic. They let processes exchange messages safely without other communicators' operations barging in and causing chaos. No interference, no drama, periodt. 🔥

The thing is, the MPI standard doesn't give you any way to access or view this implementation-dependent synchronization, because communicators are opaque objects. And honestly? For normal MPI operations you do not need that access. It's giving "you don't need to see behind the curtain, bestie."

BUT. When you're debugging MPI applications, that opaque nature is lowkey fighting against you. Especially on fully observable systems like LAM, where you can monitor the full state of processes and message queues — and that full state includes the context ID. See mpitask (1) and mpimsg (1) if you want to go stare at it directly 👀.

This is a LAM/MPI-specific function and it's intended mainly for debugging. If you're gonna use it, use it in conjunction with the LAM_MPI C preprocessor macro, like a responsible main character:

#if LAM_MPI
int cid;
MPIL_Comm_id(MPI_COMM_WORLD, &cid);
#endif

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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're invoked with the call statement. Yes bestie, different language, different etiquette. 💅

All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in Fortran. Every single one. That's just how it is over there.

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😭 ERRORS 😭

If an error occurs in an MPI function, the current MPI error handler is called to handle it. By default, this error handler aborts the MPI job. (Not a vibe. Not cute. But that's the default. 🙈) The error handler may be changed with MPI_Errhandler_set ; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned (in C and Fortran; this error handler is less useful in with the C++ MPI bindings. The predefined error handler MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value needs to be recovered). Note that MPI does not guarantee that an MPI program can continue past an error. So like... she said what she said.

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. The C++ bindings for MPI do not return error values; instead, error values are communicated by throwing exceptions of type MPI::Exception (but not by default). Exceptions are only thrown if the error value is not MPI::SUCCESS .

Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error, there will be no way to recover what the actual error value was. Yeah. That's the tradeoff. 💀

• MPI_SUCCESS
  • No error; MPI routine completed successfully. Slay. ✨
• MPI_ERR_COMM
  • Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank ). So don't do that, bestie.
• MPI_ERR_ARG
  • Invalid argument. Some argument is invalid and is not 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_Type_id

(go read those too, they're the friend group 🫶)

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🍓 LOCATION 🍓

mpil_id.c

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LAM/MPI version 7.1.4, dated 6/24/2006 — yeah she's an elder icon but she still eats 💅✨ Thanks for reading, bestie, now go forth and check your context IDs responsibly. mwah 🎀🦋💖