MPI_Group_from_session_pset(3)

💖 MPI_Group_from_session_pset(3) — the MPI manual, but make it bestie-coded ✨

okay so bestie, we're in the MPI section of the manual (section 3, the library-calls girlies) and today's main character is MPI_Group_from_session_pset. It's giving "I make groups out of session process sets" and honestly? Obsessed. 🦋

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🌸 NAME 🌸

MPI_Group_from_session_pset — get a group from a session processes set 💅

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✨ SYNOPSIS ✨

int MPI_Group_from_session_pset(MPI_Session session, const char *pset_name,
MPI_Group *newgroup)

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🎀 INPUT PARAMETERS 🎀

• session — the session, which is a handle. That's it, that's the vibe 🫶
• pset_name — the name of the process set you wanna use to create the new group. It's a string 👑

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🦋 OUTPUT PARAMETERS 🦋

• newgroup — your brand new group, derived from the session and process set you handed over. Also a handle. She's fresh, she's new, she's yours ✨

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🔥 THREAD AND INTERRUPT SAFETY 🔥

This routine is thread-safe. What does that mean fr fr? It means multiple threads can use it at the same time with zero user-provided thread locks needed. No extra safety gear, no chaos. Slay. 👑

BUT — and this is the important part, no cap — the routine is not interrupt safe. Why? Typically because of memory allocation routines like malloc (or any other non-MPICH runtime routine) being used under the hood, and those little guys aren't interrupt-safe themselves. So don't be out here interrupting her mid-glow-up 😭

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💜 NOTES FOR FORTRAN 💜

Every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has one extra argument at the end of the argument list: ierr. It's 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. That's just how she operates 💅

And one more for the notebook: all MPI objects (e.g. MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. Periodt 📓

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💀 ERRORS 💀

Okay so every MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value. In C routines, that's the value of the function itself; in Fortran routines, it's the last argument. Before that value gets returned, the current MPI error handler gets called. By default? That error handler aborts the MPI job 😭 so like, dramatic.

The error handler can be changed with:

• MPI_Comm_set_errhandler — for communicators
• MPI_File_set_errhandler — for files
• MPI_Win_set_errhandler — for RMA windows

The MPI-1 routine MPI_Errhandler_set may be used, but its use is deprecated 🚫 (girl, retire her). And the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

⚠️ Heads up: MPI does not guarantee that an MPI program can continue past an error. However, MPI implementations will attempt to continue whenever possible. So like… maybe it works, maybe it doesn't. No promises, bestie 🙈

Here are the error codes, and yes, they're all iconic in their own way:

• MPI_SUCCESS — no error at all! The routine completed successfully. Clean, flawless, ate. ✨
• MPI_ERR_ARG — invalid argument. Some argument is invalid and is not identified by a specific error class (e.g. MPI_ERR_RANK). Rude, but ok 💅
• MPI_ERR_SESSION — invalid session handle 🚫
• MPI_ERR_OTHER — other error. Use MPI_Error_string to get more info about this error code. The "mystery box" of errors 🎁

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That's the whole page, gorgeous. Go forth and build groups responsibly 🫶

✧･ﾟ: ✧･ﾟ: M P I :･ﾟ✧:･ﾟ✧

okay and for the little sign-off: this page came to you from the MPI source/version, dated 4/10/2026 — and it's MPI_Group_from_session_pset(3), obviously, the icon herself 👑💖 bye bestie, go slay some parallel code! 🦋✨