MPI_COMM_F2C(3)

🌈 MPI_COMM_F2C(3) — the Open MPI glow-up ✨

ok so bestie, gather round, because today we're talking about the Open MPI manual page for all the handle-translation girlies 💅

🎀 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 — literally the whole squad whose entire job is translating a C handle into a Fortran handle, or vice versa. 🔄

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✨ SYNTAX ✨

💻 C Syntax

ok so first things first, you gotta include the header, no cap:

#include <mpi.h>

then it's just the whole roster, one f2c/c2f pair per handle type 🫶

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 vibe: handles get passed between Fortran and C by using an explicit C wrapper to convert Fortran handles into C handles. There is no direct access to C handles in Fortran — like, none, the door is locked, iykyk 🚪🔒

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

The handle translation functions are provided in C to convert from a Fortran handle (which is an integer) to a C handle, and vice versa. It's giving bilingual icon energy, fr fr 🌍💬

So for example — and this is the part where it clicks, bestie — 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 (the 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. Garbage in, garbage out, no judgment 💀

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⚠️ NOTE ⚠️

okay this is the important one so listen up 🧚 This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. Undefined! Like, we don't know, nobody knows, the vibes are simply unknowable 😭 So don't do it. Periodt. 💅

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community 🌸

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and that's the whole page, bestie — ty for coming to my TED talk 🫶 this was the Open MPI MPI_COMM_F2C(3) page, dated Sep 11, 2026, and honestly? iconic. go forth and translate your handles responsibly 💜🦋