MPIX_Grequest_class_allocate(3)

💖 MPIX_Grequest_class_allocate 💖

okay bestie, so this one lives in the MPI manual, section 3, and it's called MPIX_Grequest_class_allocate. yep. it's a mouthful. we're gonna get through it together tho 🫶

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✨ NAME ✨

MPIX_Grequest_class_allocate — create and return a user-defined extended request based on a generalized request class. like, you made a class, now you're making an actual request out of it. iconic behavior honestly 👑

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🌸 SYNOPSIS 🌸

int MPIX_Grequest_class_allocate(MPIX_Grequest_class greq_class,
       void *extra_state, MPI_Request *request)

she takes three things and gives you back a request. simple as that 💅

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🎀 INPUT PARAMETERS 🎀

• greq_class — the generalized request class. that's it, that's the vibe. no default, no nothing (None).
• extra_state — your extra state. also just... nothing specified, no default (None). you bring whatever you wanna bring ✨

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🦋 OUTPUT PARAMETERS 🦋

• request — the generalized request. this is a handle, and it's how you walk away with your brand-new request. slay.

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🔥 THREAD AND INTERRUPT SAFETY 🔥

okay so this routine is thread-safe, periodt. that means multiple threads can use it at the same time and you do NOT need to bring your own user-provided thread locks. no cap.

BUT — and this is the important part, so pay attention bestie 😭 — it is not interrupt safe. typically that's because it calls memory allocation stuff like malloc, or other non-MPICH runtime routines that themselves are not interrupt-safe. so don't be interrupting her, she's busy.

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🌈 NOTES FOR FORTRAN 🌈

fr fr, Fortran girlies, listen up:

• Every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) gets an extra argument called 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. and in Fortran, MPI routines are subroutines — you invoke them with the call statement. cute, right?
• All MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. every single one. 💜

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💀 ERRORS 💀

okay so every MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value — C routines give it back as the value of the function, Fortran routines put it in that last argument.

and BEFORE that value comes back, the current MPI error handler gets called. by default, that error handler aborts the whole MPI job. yes, literally the entire job. so maybe know what you're doing before you yolo it 🙈

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 jsyk its use is deprecated. we don't do that anymore bestie, it's giving 1994.

The predefined error handler MPI_ERRORS_RETURN may be used to make error values actually get returned to you instead of everything exploding.

⚠️ Note that MPI does not guarantee that an MPI program can continue past an error. HOWEVER, MPI implementations will attempt to continue whenever possible. so, like, fingers crossed 🤞

Here are the error codes:

• MPI_SUCCESS — no error, the MPI routine completed successfully. we love to see it ⭐
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (like MPI_ERR_RANK). rude of you tbh.
• MPI_ERR_OTHER — other error. use MPI_Error_string to get more info about this error code. iykyk.

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and that's the whole page, bestie 🎀 go forth and allocate those generalized requests. you got this 👑

mwah 💋 this was the MPIX_Grequest_class_allocate(3) page, served up from the MPI manual, dated 4/10/2026. see you in the next one, love u, bye ✨