MPI_COMM_F2C(3)

✨ MPI_COMM_F2C(3) ✨

okay so this whole page is straight out of the Open MPI manual, and it's literally the ✨glue✨ that lets your Fortran code and your C code pass handles back and forth without anyone having a meltdown. 💅

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🎀 NAME 🎀

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 — translates a C handle into a Fortran handle, or vice versa. 🦋

yes bestie, the whole squad is here. it's giving translation bureau. 😌

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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 💫

okay so here's the tea ☕: handles get passed between Fortran and C by using an explicit C wrapper to convert Fortran handles into C handles. that's the whole move. 🫶

and notice there is no direct access to C handles in Fortran — nope, not a thing, don't even try it bestie. 🙈

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. 👑

so basically the handle translation functions are provided in C so you can convert from a Fortran handle (which is an integer) to a C handle, and vice versa. both directions, no cap. ✨

for example, 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. and if comm is an invalid Fortran handle, then MPI_Comm_f2c returns an invalid C handle. 🌈

like... it's giving honesty. you get back exactly the energy you put in. 🔥

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⚠️ NOTE ⚠️

big heads up bestie 🚨 — this function does not return an error value. consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. 😭

not me calling it at the wrong time and getting undefined behaviour... that's a whole situation, please don't. 💀

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📜 Copyright 📜

2003-2026, The Open MPI Community

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and that's the whole page, bestie! 💖 this came to you from the Open MPI crew, copyright 2003-2026, The Open MPI Community, and the last time anyone touched this vibe was Sep 10, 2026. slay, go convert those handles, and remember: if you call it outside MPI_Init / MPI_Finalize you're on your own. periodt. 👑✨🎀