MPI_GROUP_C2F(3)

💖 MPI_GROUP_C2F 💖

okay bestie, welcome to the Open MPI glowing-up-your-handles page 💅 this is MPI_GROUP_C2F(3) and it's literally the girlies who translate your handles back and forth, no cap.

Meet the whole squad (yes they all share this page, they're a package deal 🫶):

• MPI_Comm_f2c, MPI_Comm_c2f
• MPI_File_f2c, MPI_File_c2f
• MPI_Info_f2c, MPI_Info_c2f
• MPI_Message_f2c, MPI_Message_c2f
• MPI_Op_f2c, MPI_Op_c2f
• MPI_Request_f2c, MPI_Request_c2f
• MPI_Type_f2c, MPI_Type_c2f
• MPI_Win_f2c, MPI_Win_c2f

and what do they do?? Translates a C handle into a Fortran handle, or vice versa. iconic ✨

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✨ SYNTAX ✨

🌸 C Syntax 🌸

#include <mpi.h>

MPI_Comm MPI_Comm_f2c(MPI_Fint comm)
MPI_Fint MPI_Comm_c2f(MPI_Comm comm)

MPI_File MPI_File_f2c(MPI_Fint file)
MPI_Fint MPI_File_c2f(MPI_File file)

MPI_Group MPI_Group_f2c(MPI Fint group)
MPI_Fint MPI_Group_c2f(MPI Group group)

MPI_Info MPI_Info_f2c(MPI_Fint info)
MPI_Fint MPI_Info_c2f(MPI_Info info)

MPI_Message MPI_Message_f2c(MPI_Fint message)
MPI_Fint MPI_Message_c2f(MPI_Message message)

MPI_Op MPI_Op_f2c(MPI_Fint op)
MPI_Fint MPI_Op_c2f(MPI_Op op)

MPI_Request MPI_Request_f2c(MPI_Fint request)
MPI_Fint MPI_Request_c2f(MPI_Request request)

MPI_Datatype MPI_Type_f2c(MPI_Fint datatype)
MPI_Fint MPI_Type_c2f(MPI_Datatype datatype)

MPI_Win MPI_Win_f2c(MPI_Fint win)
MPI_Fint MPI_Win_c2f(MPI_Win win)

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🦋 DESCRIPTION 🦋

so here's the tea, bestie ☕ — Fortran and C are two different friend groups and they do NOT pass handles to each other directly. Handles are passed between Fortran and C by using an explicit C wrapper to convert Fortran handles to C handles. That's it. That's the whole system. There is no direct access to C handles in Fortran. 💀

The type definition MPI_Fint is provided in C for an integer of the size that matches a Fortran INTEGER; usually, MPI_Fint will be equivalent to int. Slay, sized correctly. 👑

And the handle translation functions? They're provided in C to convert from a Fortran handle (which is an integer) to a C handle, and vice versa. And vice versa!! both directions, we contain multitudes 💜

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🎀 an example, because we love a demonstration 🎀

if comm is a valid Fortran handle to a communicator, then MPI_Comm_f2c returns a valid C handle to that same communicator 🫶

if comm = MPI_COMM_NULL (Fortran value), then MPI_Comm_f2c returns a null C handle 🙈

if comm is an invalid Fortran handle, then MPI_Comm_f2c returns an invalid C handle 💀 (we don't fix it. we just hand it back. girl, we tried.)

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⚠️ NOTE ⚠️

okay listen up because this part is IMPORTANT 🔥

This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. Not "a little weird". Not "probably fine". Undefined. So don't do it. Do not be that girl. 😭

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📜 Copyright 📜

2003-2026, The Open MPI Community

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and that's the whole page bestie, mwah 💋 this was Open MPI's finest, dated Sep 10, 2026, filed under MPI_GROUP_C2F(3) — go forth and translate your handles responsibly, ily, bye 🦋✨💖