MPI_COMM_C2F(3)

🫶 NAME 🫶

hiii bestie, you just opened MPI_COMM_C2F(3) — that's the Open MPI manual page for the girlies who live in two languages at once 🌈✨

okay so this page is actually a whole squad, not just one function. everyone here does the exact same job, just for a different kind of handle, and their mission statement is literally: translates a C handle into a Fortran handle, or vice versa.

let me introduce you properly 💅

• 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

the naming is giving main character energy and also, lowkey, the whole plot: _f2c = Fortran ⮕ C, _c2f = C ⮕ Fortran. see? iconic. no cap. 🔥

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🎀 SYNTAX 🎀

C Syntax

okay so in C you gotta bring the header first, always:

#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)

not me typing the MPI_Group ones exactly as the page has them, MPI Fint and MPI Group and all 🙈 — bestie I do not edit source material, I just deliver it

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✨ DESCRIPTION ✨

so here's the tea, and it's honestly kind of a cute little bridge situation 🌸

handles get passed between Fortran and C by using an explicit C wrapper to convert Fortran handles to C handles. that's the whole move. and there is no direct access to C handles in Fortran — like, zero, none, that door does not open 💀

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. cute, right? a little matching-set moment.

and the handle translation functions are provided in C to convert from a Fortran handle (which is an integer, bestie, it's just an integer) to a C handle, and vice versa. back and forth. crosstalk. bilingual queen behaviour 👑

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🌟 the example (yes it's inside DESCRIPTION, stay with me) 🌟

say comm is a valid Fortran handle to a communicator. then MPI_Comm_f2c returns a valid C handle to that same communicator. same communicator!! she just changed outfits 💖

if comm = MPI_COMM_NULL (Fortran value), then MPI_Comm_f2c returns a null C handle. fair, consistent, slay.

if comm is an invalid Fortran handle, then MPI_Comm_f2c returns an invalid C handle. garbage in, garbage out, no notes 🫠

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⚠️ NOTE ⚠️

okay this is the serious part and I need you locked in, bestie:

this function does not return an error value. so if you call it before MPI_Init or after MPI_Finalize, the result is undefined. undefined!! not "probably fine", not "lol who cares" — undefined. do not do it, do not test it, do not "just see what happens" 🙈

🚨 warning bestie: no error value comes back from this one. before MPI_Init, after MPI_Finalize — undefined. periodt.

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💜 Copyright 💜

2003-2026, The Open MPI Community

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and that's the whole page, angel 🦋 source and version for the record: this is the Open MPI manual, dated Sep 10, 2026, and it lives at MPI_COMM_C2F(3). go translate some handles, you're gonna eat 💅💖